The drug is an inducer of microsomal liver enzymes. Chemical transformations of drugs in the body, the role of microsomal liver enzymes
Activity of microsomal monooxygenases, catalyzing the biotransformation of xenobiotics in the first phase of detoxification, as well as the activity of enzymes involved in the conjugation reactions that make up the second phase of detoxification, depends on many factors. For example, depending on the functional state of the body, on age and gender, on the diet, there are seasonal and daily fluctuations in activity, etc.
However, the most pronounced effect on functioning of biochemical systems, responsible for detoxification processes, have chemicals related to inducers and inhibitors of microsomal monooxygenases. The combined action of xenobiotics is often determined precisely by the inductive or inhibitory properties of the compounds involved in the combinations. Inducers or inhibitors of microsomal oxidation can serve as the basis for prevention and treatment of intoxication.
About 300 chemical compounds are currently known. compounds, causing an increase in the activity of microsomal enzymes, i.e. inductors. These are, for example, barbiturates, biphenyls, alcohols and ketones, polycyclic and halogenated hydrocarbons, some steroids and many others. They belong to various classes of chemical compounds, but have some common features. Thus, all inducers are lipid-soluble substances and are characterized by tropism in relation to the membranes of the endoplasmic reticulum.
Inductors are substrates microsomal enzymes. There is a direct correlation between the power of inductors and their half-life in the body. Inducers may also have a certain specificity with respect to foreign substances or have a wide spectrum of action. More details about all this and much more can be found in the following books and monographs.
Much of what has been said above applies to microsomal monooxygenase inhibitors, just like the references to the chapter by L.A. Tiunov et al. Inhibitors include substances from various classes of chemical compounds. On the one hand, these can be very complex organic compounds, and on the other, simple inorganic compounds such as heavy metal ions. In particular, we have described and put into practice, in order to increase the antitumor activity of known antitumor drugs, the xenobiotic metabolism inhibitor hydrazine sulphate.
The use of inhibitors to increase the activity is considered promising. pesticides. In both cases, the modifying effect of inhibitors is based on the delay or prevention of the metabolism of the parent compounds, which, when selecting the appropriate dose and regimen for the use of inhibitors, makes it possible to change the strength and quality of the effect.
According to the mechanism of action, metabolic inhibitors divided into 4 groups. The first of these includes reversible direct-acting inhibitors: these are esters, alcohols, lactones, phenols, antioxidants, etc. The second group consists of reversible indirect-acting inhibitors that affect microsomal enzymes through intermediate products of their metabolism by forming complexes with cytochrome P-450. This group includes benzene derivatives, alkylamines, aromatic amines, hydrazines, etc. The third group includes irreversible inhibitors that destroy cytochrome P-450 - these are polyhalogenated alkanes, olefin derivatives, acetylene derivatives, sulfur-containing compounds, etc.
Finally, the fourth group includes inhibitors, inhibiting the synthesis and / or accelerating the breakdown of cytochrome P-450. Typical representatives of the group are metal ions, inhibitors of protein synthesis and substances that affect heme synthesis.
So far, it has only been about microsomal mechanisms of metabolism xenobiotics. However, there are other non-microsomal mechanisms. This is the second type of metabolic transformations, it includes the reactions of non-microsomal oxidation of alcohols, aldehydes, carboxylic acids, alkylamines, inorganic sulfates, 1,4-naphthoquinones, sulfoxides, organic disulfides, some esters; hydrolysis of ester and amide bonds, as well as hydrolytic dehalogenation.The following are some of the enzymes involved in the extramicrosomal metabolism of xenobiotics: monoamine oxidase, diamine oxidase, alcohol dehydrogenase, aldehyde dehydrogenase, aldehyde oxidase, xanthine oxidase, esterases, amidases, peroxidases, catalase, etc. In this way, predominantly water-soluble xenobiotics Below are some examples.
Aliphatic alcohols and aldehydes are metabolized mainly in the liver of mammals. So, 90-98% of the ethanol that enters the body is metabolized in the liver cells and only 2-10% in the kidneys and lungs. In this case, part of the ethanol enters into glucuronide conjugation reactions and is excreted from the body; the other part undergoes oxidative transformations. The ratio of these processes depends on the type of animal, on the chemical structure of alcohol and on its concentration. Under the action of low concentrations of aliphatic alcohols, the main pathway for their biotransformation in the body is the oxidative pathway with the help of alcohol dehydrogenase.
Mostly extramicrosomal mechanism of metabolism used to detoxify cyanides. In this case, the main reaction is the displacement of the sulfite group from the thiosulfate molecule by the cyano group. The resulting thiocyanate is practically non-toxic.
Division of detoxification mechanisms on microsomal and extramicrosomal somewhat conditionally. The metabolism of a number of groups of chemical compounds can be mixed, as follows from the example with alcohols. As already briefly described above, the monooxygenase system containing cytochrome P-450 in the form of its various isoforms protects the internal environment of the body from the accumulation of toxic compounds in it. Taking part in the first phase of xenobiotic metabolism - converting low molecular weight xenobiotics with low water solubility into more soluble compounds - it facilitates their excretion from the body. However, this function of them can also pose a serious danger to the body, which is not so rare.
The fact is that mechanism of oxidation reactions provides for the formation in the body of intermediate reactive metabolites belonging to two types. First of all, these are the products of partial oxygen reduction: hydrogen peroxide and superoxide radicals, which are the sources of the most reactive hydrophilic radicals. The latter are able to oxidize a wide variety of molecules in the cell. Another type is reactive metabolites of oxidizable substances. Already in small quantities, these metabolites can have certain side effects: carcinogenic, mutagenic, allergenic, and others, which are based on their ability to covalently bind to biological macromolecules - proteins, nucleic acids, lipids of biomembranes. Attention to the circumstances indicated here was paid not so long ago and mainly due to the development of ideas about the molecular mechanisms of detoxification processes. But it was these ideas that made it possible to explain many facts that previously seemed incomprehensible of the high toxicity of certain compounds under certain conditions.
At the 16th European Workshop on Xenobiotic Metabolism (June 1998) presented numerous examples of xenobiotic toxicity modification. In particular, 2,6-dichloromethylsulfonylbenzene (2,6-DCB) forms toxic metabolites in the olfactory system of mice, while 2,5-DCB does not. The metabolism of benzene in the liver of some lines of mice leads to the formation of toxic metabolites, while others do not, and this depends on the activity of cytochrome P-450. Metabolic activation of anticancer compounds is different in different species; the difference may apply to different individuals. Cytochrome P-450 isozymes determine the difference in the kinetics of xenobiotic metabolism. Based on the developed concepts, an in vitro test system was proposed to determine the metabolism and toxicity of xenobiotics in relation to the liver, lungs, intestines, and kidneys of different human individuals. It is indicated that therapeutic monitoring is mandatory in the treatment of alcoholism with disulfiram: it is necessary to prescribe a therapeutic dose of the drug depending on the characteristics of its metabolism in different individuals, and not depending on the patient's body weight, as is customary. Examples can be seen in the three-volume Encyclopedia of Toxicol.
Biotransformation (metabolism) is a change in the chemical structure of drugs and their physicochemical properties under the influence of various enzymes.
As a result, as a rule, the structure of the drug changes and passes into a more convenient form for excretion - water.
For example: ethnolaprin (treat hypertension) - an ACE inhibitor, only after biotransformation does it become active ethnolaprilat, a more active form.
Most often, all this happens in the liver. Also in the intestinal wall, lungs, muscle tissue, blood plasma.
Stages of biotransformation:
1. Metabolic transformation - metabolites are formed. non-synthetic reactions. For example: oxidation (Aminazin, Codeine, Warforin), reduction (Nitrosipam, Levomycetin), hydrolysis (Novocaine, Lidocaine, Aspirin).
"Lethal synthesis" - metabolites are formed that are more toxic (Amidopyrine, led to cancer; Paracetamol, at an increased dosage).
2. Conjugation - synthetic reactions. Something joins, either to the drug or to the metabolites. Reactions such as: acetylation (Sulfadimezin); methylation (Histamine, Catecholamines); glucuronidation (Morphine, Paracetamol - adults); sulfation (Paracetamol - children).
Microsomal liver enzymes- localized in the sarcoplasmic reticulum of liver cells.
Inducers of microsomal enzymes: Phenobarbital, Griseofulvin, Rifampicin, etc. The action of inductors is ambiguous, because with an increase in the metabolism of vitamins, hypervitaminosis develops - this is a minus. And plus - phenobarbital induces microsomal enzymes, and thus helps with hyperbilirubinemia.
Inhibitors: Cimetidine, Erythromycin, Levomycetin, etc.
3. Excretion (excretion):
kidneys (diuretics);
Gastrointestinal tract (with bile), they can be reabsorbed and re-excreted into the intestine - enterodipathic circulation. For example: Tetracycline, Difinin.
· With the secrets of the sweat glands (Bromides, their overdose - acne), salivary (Iodides), bronchial, lacrimal (Rifampicin), milk (sleeping pills, analgesics - for nursing mothers) and others.
Elimination - biotransformation and excretion.
Quantitative characteristics of elimination processes:
· Elimination constant - what part of the substance as a percentage of the injected amount is eliminated per unit of time. Needed to calculate the maintenance dose.
Elimination half-life (T ½) - the time during which the concentration of a substance in the blood plasma is reduced by half.
Systemic (total) clearance - the volume of blood that is released from the substance per unit time (ml / min).
Non-narcotic analgesics
Difference from narcotic - for everyone!
Non-narcotic drugs do not have: psychotropic, hypnotic, antitussive action, euphoria does not cause and LZ. Does not depress the respiratory center. According to indications, they stop mainly pains of an inflammatory nature.
For example: toothache, headache, joint, muscle pain, pain associated with inflammatory diseases of the pelvic organs.
Main Effects
Analgesic effect
anti-inflammatory
Antipyretic
Classification
1. Non-selective COX inhibitors (cyclooxygenase)
Salicylic acid derivatives- salicylates: Acetylsalicylic acid (Aspirin), Aspirin Caardio, Thrombo ASS (aspirin at a reduced dosage, for the treatment of coronary artery disease), Salicylamide, Methyl salicylate, Acelisin, Otinum (contains choline salicylate).
Combined with Citramon: Citramon P, Citrapar, Citrapak, Askofen, Alka-Seltzer, Alka-prim, Aspirin UPSA with vitamin C.
Pyrozolon derivatives: 1. Matamizol (Analgin), combined (analgin + antispasmodics) - Baralgin, Spazgan, Trigan; 2. Butadion - more pronounced anti-inflammatory = inflammatory effect, can be used for gout (increases excretion).
Aniline derivatives(para-aminophenol, paracetamol): paracetamol; combined - Coldrex, Ferveks, Solpadein, Panadol extra, Citramon, Askofen.
NSAIDs - derivatives of acetic acid: indolacetic acid - Indomethacin (Metindol); phenylacetic acid - Diclofenac - sodium (Voltaren, Ortofen).
Propionic acid derivatives: phenylpropionic - Ibuprofen (Brufen, Nurofen); Naphthylpropionic - Naproxen (Naprosin).
Oxycams: Piroxicam: anthranilic acid derivatives - mefenamic acid; derivatives of pyrrolysine-carboxylic acid - Ketorolac (Ketaov, Ketorol).
2. Selective COX-2 inhibitors: Meloxicam (Movalis), Celecoxib (Celebrex), Nimesulide (Nise).
Pronounced analgesic activity:
Ketorolac
Ibuprofen
Naproxen
Paracetamol
Analgin
Mechanism of anti-inflammatory action
All inhibitors of cyclooxygenase (COX) à disrupt the formation of prostaglandins E2, I2 (they accumulate in the focus of inflammation), and potentiate the actions of other inflammatory mediators.
COX took:
Phospholipids + phospholipase A2, inhibited by HA àArachidonic acid + COX-1,2 (inhibited by NSAIDs) = prostaglandins - I2, and others, Thromboxanes are formed.
Arachidonic acid + Lipooxygenase = Leukotrienes.
*NSAIDs are non-steroidal anti-inflammatory drugs.
COX exists in the form of several isoenzymes:
COX-1 - an enzyme of blood vessels, gastric mucosa, kidneys. Participates in the formation of Pg (prostaglandins) that regulate physiological processes in the body.
COX-2 - is activated during inflammation.
· COX-3 - is involved in the synthesis of Pg CNS.
Influence on the phases of inflammation
o Alteration:
Stabilize lysosomes and prevent the release of hydrolytic enzymes - proteases, lipases, phosphatases
Inhibit (reduce) LPO (peroxidation) in the lysosomal membrane.
o Exudation:
The activity of inflammatory mediators (histamine, serotonin, bradykinin), hyaluronidase decreases.
The permeability of the vascular wall decreases à edema decreases, microcirculation improves, i.e. absorbing action.
o Proliferation:
They limit the activity of stimulators of fibroblast division (serotonin, bradykinin), i.e. reduced formation of connective tissue.
They disrupt energy production, which ensures proliferation (limit the bioenergetics of inflammation, reduce ATP synthesis).
Decreased connective tissue formation and collagen synthesis.
Mechanism of analgesic action
Peripheral (main) - due to the anti-inflammatory component: reduces swelling, and reduces irritation of pain receptors.
Central (not leading, and less pronounced) - limits the accumulation of Pg in the brain - inhibits COX-3 (paracetamol); reduces the conduction of pain impulses along the ascending fibers; reduces the transmission of pain impulses in the thalamus.
Mechanism of antipyretic action
Fever is protective.
Pg E1 and E2 of the preoptic area of the hypothalamus - accumulation of cAMP - violation of the ratio of Na and Ca - vessels narrow - heat production prevails.
COX block à reduction of Pg synthesis and à restoration of equilibrium between heat production and heat transfer.
Indications for use:
Rheumatoid arthritis, non-rheumatoid arthritis, ankylosing spondylitis, myalgia, neuralgia, toothache, headache, algodismenoria, postoperative pain.
Salicylates:
Salicylic acid: antiseptic, distracting, irritant, keratolytic (against calluses).
Acetylsalicylic acid:
In addition to 3 effects - inhibition of the formation of thromboxanes - antiaggregatory action. For the prevention of thrombosis in IHD (small doses).
Side effects of salicylates
o Ulcerogenic action - the ability to ulcerate mucous membranes, tk. indiscriminate action.
o Bleeding (gastric, nasal, uterine, intestinal)
o Bronchospasm (more for asthmatics)
o Reye's syndrome (under 12 years of age) - encephalopathy, liver necrosis against the background of viral diseases
o Neurological and psychiatric disorders
o Teratogenic effect
Pyrazolones
Side effects:
Inhibition of hematopoiesis
allergic reactions
Ulcerogenic action
Nephrotoxicity, hepatotoxicity - mainly for Butadione
Derivative of analgin - paracetamol - considered the safest analgesic
· No anti-inflammatory action, tk. inhibits COX-3 in the central nervous system; in peripheral tissues, the synthesis of prostaglandins is not disturbed.
Good tolerability
Small therapeutic latitude
Features of biotransformation ( adults):
~ 80% glucuronide conjugation
~ 17% hydroxyl (cytochrome P-450)
è As a result, an active metabolite is formed - N-acetyl-benzoquinoneimine (toxic!) à it also conjugates with glutathione (therapeutic doses)
Toxic doses - N-acetyl-benzoquinoneimine is partially inactivated
Overdose:
o Accumulation of N-acetyl-benzoquinoneimine - cell necrosis (hepato- and nephrotoxicity)
Treatment: (in the first 12 hours!)
§ Acetylcysteine - promotes the formation of glutathione
§ Methionine - activates conjugation - the addition of substances that form metabolites
Children under 12:
Deficiency of cyt R-450
Sulfate pathway of biotransformation
No toxic metabolites
Indomethacin - inside, into the muscle, rectally and locally
One of the most effective anti-inflammatory, promotes the excretion of uric acid (for gout).
High toxicity:
§ Ulcerogenic action
§ Inhibition of hematopoiesis
§ Edema, increased blood pressure
§ Neurological and mental disorders
§ May inhibit labor activity
It is contraindicated in children under 14 years of age, but it is prescribed even for newborns - once, a maximum of 1-2 times with an open arterial duct, accelerates the development of the closure of the arterial - Botal duct.
These are substances that selectively eliminate negative emotions - fear, anxiety, tension, aggression.
Synonyms:
Classification:
benzodiazepine derivatives
Diazepam (Relanium, Seduxen, Sibazon)
Phenazepam
Oxazepam (Nozepam, Tazepam)
Alprazolam (Alzolam, Zoldak)
Lorazepam
Tofisopam (Grandaxin)
Medazepam (Mezapam, Rudotel)
Derivatives of different chemical groups
Mechanism of action:
Anatomical substrate – limbic system, hypothalamus, brainstem RF, thalamic nuclei
GABAergic inhibition - "benzodizepine" receptors + GABA receptors
GABA - implements functions through the opening of channels for chloride ions in the neuron membrane
Figure look at sleeping pills
Pharmacological effects
Anxiolytic - reduction of fear, anxiety, tension
Sedative - soothing (not the main one, drugs with a sedative effect)
Sleeping pills - especially in violation of the process of falling asleep
Anticonvulsant
Antiepileptic
Muscle relaxant (test: why tranquilizers are contraindicated in myasthenia gravis. Myasthenia gravis is muscle weakness they have a muscle relaxant effect, the central component is muscle relaxant)
Potentiating
Amnestic - in high doses
Vegetotropic - a decrease in the activity of the sympathetic-adrenal system
Application
The main use, in contrast to neuroleptics (with psychosis) is neurosis (inadequate reaction to a non-standard situation)
Insomnia
Psychosomatic disorders (HA, angina pectoris, arrhythmias, GU, BA, etc.)
Premedication and ataralgesia (a kind of potentiation of anesthesia)
Seizures, epilepsy
Spastic states (with brain lesions), hyperkinesis
Withdrawal from alcoholism and drug addiction
Side effects
Violation of attention, memory
Drowsiness, muscle weakness, incoordination
addictive
drug addiction
Impotence
Not compatible with alcohol (potentiate their action)
"Daytime" tranquilizers
Afobazole is not a benzidiazepine. Membrane modulator of the GABA-receptor complex - brings to the physiological norm - the maximum physiological mechanism. It has membrane-producing properties. It does not cause addiction, lethargy or drowsiness.
Mezapam (Rudotel)
Grandaxin (Tofisopam)
Contraindications
myasthenia gravis
Diseases of the liver and kidneys
Drivers and persons performing precise activities
alcohol shared
Pregnancy - I trimester
3. Origin of insulin preparations:
Recombinant human insulin (INS) (genetic engineering method) – NM
From the pancreas (pancreas) of a pig (Suinsulin) - C
From the degree of purification - MP (monopig, monocomponent) or MK (MS)
Insulin is administered only parenterally - syringes, a syringe pen with a cartridge (Penfill).
Classification
Short action 30 min (beginning of action) - 2-4 hours. (after what time the peak of action must necessarily fall on a meal) - 6-8 hours (total duration of action) - s / c, / m, / v.
Medium duration (+ protamine, Zn) - 2h - 6-12h - 20-24h - s.c.
Protafan MS
Monotrad MS
Long-term action - 4 hours - 8-18 hours - 28 hours - p \ c.
Ultratard NM
Long-acting (24h) peak-free insulin - Insulin glargine (Lantus) - reduces the risk of nocturnal hypoglycemia
Indications for use:
Type I diabetes (IDDM - insulin dependent diabetes mellitus);
Hypotrophy, anorexia, furunculosis, long-term IZ (infectious diseases), poorly healing wounds;
As part of a polarizing mixture (K, Cl, glucose, insulin);
Sometimes, treatment of mental patients;
Principles of insulin therapy:
Combination of KD (short-acting) + DD (basal and stimulated secretion);
Primarily - in the hospital individually! (choice, dose - glycemia, glucosuria). 1 unit utilizes 4-5 g of sugar, half a unit per kg;
Dose selection for coma and precoma - only short-acting!
Maximum hypoglycemia = food intake;
Biphasic drugs (2 in 1, KD + DD):
Side effects:
Lipodymtrophy at the injection site, so the places change;
allergic reactions;
Overdose - hypoglycemia;
Synthetic oral antidiabetic agents:
Used in type II diabetes (InezDM).
The secretion of insulin decreases and the activity of β-cells decreases.
tissue resistance to insulin. Reducing the number of receptors or their sensitivity to insulin.
Classification:
Sulfonylureas derivatives:
Bucarban Chlorpropamide. They are rarely used, in large doses, short-acting.
Glibenclamide (Maninil), Glipizide (Minidiab), Gliquidone (Glurenorm), Gliclazide (Diabeton) + AAG
Glimepiride (Amaryl) - prolonged action.
Mechanism of action: stimulate the secretion of endogenous insulin, while reducing K atf β-cells depolarization opening of calcium channels increase in calcium in the cell degranulation with increased secretion of insulin.
Side effects: hypoglycemia, leukopenia and agranulocytosis, impaired liver function, thyroid gland, dyspepsia, taste disturbance, allergies.
Biguanides - Metmorphine (Gliformin), aka Siofor 500. Stimulates glucose uptake by peripheral tissues (PT) and inhibits gluconeogenesis (GNG) in the liver and glucose absorption in the intestine. Appetite decreases, lipolysis is activated, and lipogenesis is inhibited.
Side effects: metallic taste in the mouth, dyspepsia, malabsorption of vitamins (B12).
Glibomet = Glibenclamide + Metmorphine.
α-glucosidase inhibitors:
Decreased absorption of carbohydrates in the intestine.
Side effects: flatulence, diarrhea.
Prandial glycemic regulators - glymids:
Nateglinide (Starlix) - a derivative of AK FA
Рpeaglinide (Novonorm) - a derivative of benzoic acid
Block KATP-dependent β-cells. They act quickly and briefly.
Insulin sensitizers (thiazolidinediones):
Used for intolerance to conventional therapy.
Increase tissue sensitivity to insulin. Inhibit GNG in the liver. Apply 1 time per day.
Incretins (incretion - the entry of a product produced by the endocrine glands directly into the bloodstream):
Hormones that increase insulin secretion in response to food intake are produced in the intestine (up to 70% of postprandial insulin secretion in healthy people).
Significantly reduces in patients with DM II and with impaired glucose tolerance (IGT).
Effects of GLP-1:
Stimulation of glucagon-dependent secretion of INS (incretin effect) - the action depends on the concentration of glucose a SC, and stops when it decreases below 3.0 mmol / l - cannot cause the development of severe hypoglycemia
Whitoprotective - increase in the mass of β-cells, stimulation of neogenesis.
Apoptosis of β-cells is blocked
Mitotic effect on β-cells - an increase in the differentiation of new β-cells from cells - precursors of the epithelium of the pancreatic duct.
Inhibits the secretion of glucagon.
Gastric emptying is blocked - feeling of fullness - anorexigenic effect
GLP-1 inactivation:
GLP-1 agonists:
Liraglutide (Victoza) is a human GLP-1 analogue with a half-life of about 13 hours. 1 time per day s / c (+ weight loss, decrease in blood pressure)
Exenatide
DPP-4 inhibitor - Sitagliptin (Januvia) - prevents hydrolysis of incretins activates plasma concentrations of active forms of GLP-1 and GIP. 1 tab 1 time per day.
1. Biotransformation of medicinal substances. Reactions of stages I and II of metabolism. Inducers and inhibitors of microsomal enzymes (examples).
Biotransformation (metabolism) - a change in the chemical structure of medicinal substances and their physicochemical properties under the action of body enzymes. The main focus of this process is the conversion of lipophilic substances, which are easily reabsorbed in the renal tubules, into hydrophilic polar compounds, which are rapidly excreted by the kidneys (not reabsorbed in the renal tubules). In the process of biotransformation, as a rule, there is a decrease in the activity (toxicity) of the starting substances. Biotransformation of lipophilic drugs mainly occurs under the influence of liver enzymes localized in the membrane of the endoplasmic reticulum of hepatocytes. These enzymes are called microsomal because they are associated with small subcellular fragments of the smooth endoplasmic reticulum (microsomes), which are formed during homogenization of the liver tissue or tissues of other organs and can be isolated by centrifugation (precipitated in the so-called "microsomal" fraction). In blood plasma, as well as in the liver, intestines, lungs, skin, mucous membranes and other tissues, there are non-microsomal enzymes localized in the cytosol or mitochondria. These enzymes may be involved in the metabolism of hydrophilic substances. There are two main types of drug metabolism (stages): non-synthetic reactions (metabolic transformation); synthetic reactions (conjugation).
biotransformation (metabolic reactions of the 1st phase), occurs under the action of enzymes - oxidation, reduction, hydrolysis.
conjugation (metabolic reactions of the 2nd phase), in which residues of other molecules (glucuronic, sulfuric acids, alkyl radicals) are attached to the molecule of the substance, with the formation of an inactive complex that is easily excreted from the body with urine or feces.
Medicinal substances can undergo either metabolic biotransformation (where substances called metabolites are formed) or conjugation (conjugates are formed). But most drugs are first metabolized with the participation of non-synthetic reactions with the formation of reactive metabolites, which then enter into conjugation reactions. Metabolic transformation includes the following reactions: oxidation, reduction, hydrolysis. Many lipophilic compounds are oxidized in the liver by a microsomal system of enzymes known as mixed function oxidases, or monooxygenases. The main components of this system are cytochrome P450 reductase and cytochrome P450 hemoprotein, which binds drug molecules and oxygen in its active center. The reaction proceeds with the participation of NADPH. As a result, one oxygen atom is attached to the substrate (drug) with the formation of a hydroxyl group (hydroxylation reaction).
Under the influence of certain drugs (phenobarbital, rifampicin, carbamazepine, griseofulvin), induction (an increase in the rate of synthesis) of microsomal liver enzymes can occur. As a result, while prescribing other drugs (for example, glucocorticoids, oral contraceptives) with inducers of microsomal enzymes, the metabolic rate of the latter increases and their effect decreases. In some cases, the metabolic rate of the inductor itself may increase, as a result of which its pharmacological effects (carbamazepine) decrease. Some medicinal substances (cimetidine, chloramphenicol, ketoconazole, ethanol) reduce the activity (inhibitors) of metabolizing enzymes. For example, cimetidine is an inhibitor of microsomal oxidation and, by slowing down the metabolism of warfarin, may increase its anticoagulant effect and provoke bleeding. Known substances (furanocoumarins) contained in grapefruit juice that inhibit the metabolism of drugs such as cyclosporine, midazolam, alprazolam and, therefore, increase their action. With the simultaneous use of medicinal substances with inducers or inhibitors of metabolism, it is necessary to adjust the prescribed doses of these substances.
Source: StudFiles.netV.G. Kukes, D.A. Sychev, G.V. Ramenskaya, I.V. Ignatiev
A person is daily exposed to a variety of foreign chemicals called "xenobiotics". Xenobiotics enter the human body through the lungs, skin and from the digestive tract as part of air, food, drinks, and drugs. Some xenobiotics do not have any effect on the human body. However, most xenobiotics can induce biological responses. The body reacts to drugs in the same way as to any other xenobiotic. In this case, drugs become objects of various mechanisms of influence on the part of the body. This, as a rule, leads to the neutralization and elimination (removal) of drugs. Some, easily soluble in water, drugs are eliminated by the kidneys unchanged, other substances are previously exposed to enzymes that change their chemical structure. Thus, biotransformation is a general concept that includes all chemical changes that occur with drugs in the body. The result of the biological transformation of drugs: on the one hand, the solubility of substances in fats (lipophilicity) decreases and their solubility in water (hydrophilicity) increases, and on the other hand, the pharmacological activity of the drug changes.
Decreased lipophilicity and increased hydrophilicity of drugs
A small number of drugs can be excreted by the kidneys unchanged. Most often, these drugs are "small molecules" or they are able to be in an ionized state at physiological pH values. Most drugs do not have such physical and chemical properties. Pharmacologically active organic molecules are often lipophilic and remain non-ionized at physiological pH values. These drugs are usually associated with plasma proteins, are poorly filtered in the renal glomeruli and are simultaneously easily reabsorbed in the renal tubules. Biotransformation (or biotransformation system) is aimed at increasing the solubility of the drug molecule (increasing hydrophilicity), which contributes to its excretion from the body with urine. In other words, lipophilic drugs are converted into hydrophilic and, therefore, more easily excreted compounds.
Changes in the pharmacological activity of drugs
Directions of changes in the pharmacological activity of drugs as a result of biotransformation.
A pharmacologically active substance turns into a pharmacologically inactive one (this is typical for most drugs).
The pharmacologically active substance is first converted into another pharmacologically active substance (Table 5-1).
An inactive pharmacological drug is converted in the body into a pharmacologically active substance; such drugs are called "prodrugs" (Table 5-2).
Table 5-1. Drugs whose metabolites retain pharmacological activity
End of table 5-1
Table 5-2. Prodrugs
End of Table 5-2
* Phenacetin has been discontinued due to severe side effects, in particular nephrotoxicity (“phenacetin nephritis”).
It should be noted that the efficacy and safety of the use of drugs (listed in Table 5-1) with active metabolites depend not only on the pharmacokinetics of the drugs themselves, but also on the pharmacokinetics of their active metabolites.
5.1. PRODRUGS
One of the goals of creating prodrugs is to improve pharmacokinetic properties; this speeds up and increases the absorption of substances. Thus, esters of ampicillin (pivampicin p, talampicin p and bicampicin p) were developed, unlike ampicillin, they are almost completely absorbed when taken orally (98-99%). In the liver, these drugs are hydrolyzed by carboxyesterases to ampicillin, which has antibacterial activity.
The bioavailability of the antiviral drug valacyclovir is 54%, in the liver it turns into acyclovir. It should be noted that the bioavailability of acyclovir itself does not exceed 20%. The high bioavailability of valaciclovir is due to the presence of a valine amino acid residue in its molecule. That is why valaciclovir is absorbed in the intestine by active transport using the oligopeptide transporter PEPT 1.
Another example: adenosine-converting enzyme inhibitors containing a carboxyl group (enalapril, perindopril, trandolapril, quinapril, spirapril, ramipril, etc.). So, enalapril is absorbed when taken orally by 60%, hydrolyzed in the liver under the influence of carboxyesterases to active enalaprilat. It should be noted that when administered orally, enalaprilat is only absorbed by 10%.
Another goal of prodrug development is to improve the safety of drugs. For example, scientists have created sulindak p - NSAIDs. This drug initially does not block the synthesis of prostaglandins. Only in the liver is sulindac p hydrolyzed to form the active sulindac p sulfide (it is this substance that has anti-inflammatory activity). It was assumed that sulindac p would not have an ulcerogenic effect. However, the ulcerogenicity of NSAIDs is due not to local, but to “systemic” action, therefore, studies have shown that the incidence of erosive and ulcerative lesions of the digestive organs when taking sulindac p and other NSAIDs is approximately the same.
Another goal of creating prodrugs is to increase the selectivity of the action of drugs; this increases the efficacy and safety of drugs. Dopamine is used to increase renal blood flow in acute renal failure, but the drug affects the myocardium and blood vessels. An increase in blood pressure, the development of tachycardia and arrhythmias are noted. The addition of a glutamic acid residue to dopamine made it possible to create a new drug, glutamyl-dopa p. Glutamyl-dopa p is hydrolyzed to dopamine only in the kidneys under the influence of glutamyl transpeptidase and decarboxylase of L-aromatic amino acids and thus has practically no undesirable effect on central hemodynamics.
Rice. 5-1. Phases of drug biotransformation (Katzung V., 1998)
5.2. PHASES OF DRUG BIOTRANSFORMATION
The processes of biotransformation of most drugs occur in the liver. However, the biotransformation of drugs can also occur in other organs, for example, in the digestive tract, lungs, and kidneys.
In general, all drug biotransformation reactions can be classified into one of two categories, referred to as biotransformation phase I and biotransformation phase II.
Phase I reactions (non-synthetic reactions)
In the process of non-synthetic reactions, drugs are converted into more polar and better water-soluble (hydrophilic) compounds than the starting material. Changes in the initial physico-chemical properties of drugs are due to the addition or release of active functional groups: for example, hydroxyl (-OH), sulfhydryl (-SH), amino groups (-NH 2). The main reactions of phase I are oxidation reactions. Hydroxylation is the most common oxidation reaction - the addition of a hydroxyl radical (-OH). Thus, it can be considered that in the first phase of biotransformation, the drug molecule is “hacked” (Table 5-3). The catalysts for these reactions are enzymes called "mixed-function oxidases". In general, the substrate specificity of these enzymes is very low, so they oxidize various drugs. Other, less frequent phase I reactions include reduction and hydrolysis processes.
Phase II reactions (synthetic reactions)
Reactions of the II phase of biotransformation, or synthetic reactions, represent the connection (conjugation) of a drug and / or its metabolites with endogenous substances, resulting in the formation of polar, highly water-soluble conjugates, easily excreted by the kidneys or with bile. To enter into a phase II reaction, a molecule must have a chemically active radical (group), to which a conjugating molecule can attach. If active radicals are present in the drug molecule initially, then the conjugation reaction proceeds bypassing phase I reactions. Sometimes a drug molecule acquires active radicals during phase I reactions (Tables 5-4).
Table 5-3. Phase I reactions (Katzung 1998; with additions)
Table 5-4. Phase II reactions (Katzung 1998; with additions)
It should be noted that the drug in the process of biotransformation can be converted only due to phase I reactions, or exclusively due to phase II reactions. Sometimes part of the drug is metabolized through phase I reactions, and part - through phase II reactions. In addition, there is the possibility of successive reactions of phases I and II (Fig. 5-2).
Rice. 5-2. Functioning of the mixed-function oxidase system
First pass effect through the liver
Biotransformation of most drugs is carried out in the liver. Drugs metabolized in the liver are divided into two subgroups: substances with high hepatic clearance and substances with low hepatic clearance.
For drugs with high hepatic clearance, a high degree of extraction (extraction) from the blood is characteristic, which is due to the significant activity (capacity) of the enzyme systems that metabolize them (Tables 5-5). Since such drugs are quickly and easily metabolized in the liver, their clearance depends on the size and speed of hepatic blood flow.
drugs with low hepatic clearance. Hepatic clearance does not depend on the rate of hepatic blood flow, but on the activity of enzymes and the degree of drug binding to blood proteins.
Table 5-5. Drugs with high hepatic clearance
With the same capacity of enzyme systems, drugs that are largely associated with proteins (difenin, quinidine, tolbutamide) will have a low clearance compared to drugs that are weakly associated with proteins (theophylline, paracetamol). The capacity of enzyme systems is not a constant value. For example, a decrease in the capacity of enzyme systems is recorded with an increase in the dose of drugs (due to saturation of enzymes); this can lead to an increase in the bioavailability of drugs.
When drugs with high hepatic clearance are taken orally, they are absorbed in the small intestine and enter the liver through the portal vein system, where they are actively metabolized (by 50-80%) even before they enter the systemic circulation. This process is known as presystemic elimination, or the "first pass" effect. ("first-pass effect"). As a result, such drugs have low oral bioavailability, while their absorption can be almost 100%. The first pass effect is characteristic of drugs such as chlorpromazine, acetylsalicylic acid, vera-
pamil, hydralazine, isoprenaline, imipramine, cortisone, labetolol, lidocaine, morphine. Metoprolol, methyltestosterone, metoclopramide, nortriptyline p, oxprenolol p, organic nitrates, propranolol, reserpine, salicylamide, moracizin (ethmosin), and some other drugs also undergo first-pass elimination. It should be noted that a slight biotransformation of drugs can also take place in other organs (the lumen and wall of the intestine, lungs, blood plasma, kidneys and other organs).
As studies of recent years have shown, the effect of the first passage through the liver depends not only on the processes of drug biotransformation, but also on the functioning of drug transporters, and, above all, glycoprotein-P and transporters of organic anions and cations (see "The role of drug transporters in pharmacokinetic processes").
5.3. ENZYMES OF PHASE I OF BIOTRANSFORMATION OF DRUGS
microsomal system
Many enzymes that metabolize drugs are located on the membranes of the endoplasmic reticulum (EPR) of the liver and other tissues. When isolating ER membranes by homogenizing and fractionating the cell, the membranes are converted into vesicles called "microsomes". Microsomes retain most of the morphological and functional characteristics of intact ER membranes, including the property of roughness or smoothness of the surface, respectively, rough (ribosomal) and smooth (nonribosomal) ER. While rough microsomes are mainly associated with protein synthesis, smooth microsomes are relatively rich in enzymes responsible for the oxidative metabolism of drugs. In particular, smooth microsomes contain enzymes known as mixed-function oxidases or monooxygenases. The activity of these enzymes requires the presence of both a reducing agent, nicotinamide adenine dinucleotide phosphate (NADP-H), and molecular oxygen. In a typical reaction, one oxygen molecule is consumed (reduced) per substrate molecule, while one oxygen atom is included in the reaction product, and the other forms a water molecule.
Two microsomal enzymes play a key role in this redox process.
Flavoprotein NADP-N-cytochrome P-450-reductase. One mole of this enzyme contains one mole of flavin mononucleotide and one mole of flavin adenine dinucleotide. Since cytochrome C can serve as an electron acceptor, this enzyme is often referred to as NADP-cytochrome C reductase.
hemoprotein, or cytochrome P-450 performs the function of the final oxidase. In fact, the microsomal membrane contains many forms of this hemoprotein, and this multiplicity increases with repeated administration of xenobiotics. The relative abundance of cytochrome P-450, compared to liver reductase, makes the process of cytochrome P-450 heme reduction the limiting step in the process of drug oxidation in the liver.
The process of microsomal oxidation of drugs requires the participation of cytochrome P-450, cytochrome P-450 reductase, NADP-H and molecular oxygen. A simplified diagram of the oxidative cycle is shown in the figure (Fig. 5-3). Oxidized (Fe3+) cytochrome P-450 combines with the drug substrate to form a binary complex. NADP-H is an electron donor for flavoprotein reductase, which, in turn, reduces the oxidized cytochrome P-450-drug complex. The second electron passes from NADP-H through the same flavoprotein reductase, which reduces molecular oxygen and forms the "activated oxygen"-cytochrome P-450-substrate complex. This complex transfers "activated oxygen" to the drug substrate to form an oxidized product.
Cytochrome P-450
Cytochrome P-450, often referred to in the literature as CYP, is a group of enzymes that not only metabolize drugs and other xenobiotics, but also participate in the synthesis of glucocorticoid hormones, bile acids, prostanoids (thromboxane A2, prostacyclin I2), and cholesterol. For the first time, cytochrome P-450 was identified Klingenberg And Garfincell in rat liver microsomes in 1958. Phylogenetic studies have shown that cytochromes P-450 appeared in living organisms about 3.5 billion years ago. Cytochrome P-450 is a hemoprotein: it contains heme. The name of cytochrome P-450 is associated with the special properties of this hemoprotein. In restored-
In this form, cytochrome P-450 binds carbon monoxide to form a complex with maximum light absorption at a wavelength of 450 nm. This property is explained by the fact that in the cytochrome P-450 heme, iron is bound not only to the nitrogen atoms of four ligands (while forming a porphyrin ring). There are also the fifth and sixth ligands (above and below the heme ring) - the nitrogen atom of histidine and the sulfur atom of cysteine, which are part of the polypeptide chain of the protein part of cytochrome P-450. The largest amount of cytochrome P-450 is located in hepatocytes. However, cytochrome P-450 is also found in other organs: in the intestines, kidneys, lungs, adrenal glands, brain, skin, placenta and myocardium. The most important property of cytochrome P-450 is the ability to metabolize almost all known chemical compounds. The most important reaction is hydroxylation. As already mentioned, cytochromes P-450 are also called monooxygenases, since they include one oxygen atom in the substrate, oxidizing it, and one in water, in contrast to dioxygenases, which include both oxygen atoms in the substrate.
Cytochrome P-450 has many isoforms - isoenzymes. Currently, more than 1000 cytochrome P-450 isoenzymes have been isolated. Isoenzymes of cytochrome P-450, according to the classification Nebert(1987), it is customary to divide the proximity (homology) of the nucleotide/amino acid sequence into families. Families are further subdivided into subfamilies. Cytochrome P-450 isoenzymes with an identity of amino acid composition of more than 40% are grouped into families (36 families have been identified, 12 of them have been found in mammals). Cytochrome P-450 isoenzymes with an identity of amino acid composition of more than 55% are grouped into subfamilies (39 subfamilies have been identified). Cytochrome P-450 families are usually denoted by Roman numerals, subfamilies - by Roman numerals and a Latin letter.
Scheme for the designation of individual isoenzymes.
The first character (at the beginning) is an Arabic numeral for the family.
The second character is a Latin letter denoting a subfamily.
At the end (third character) indicate the Arabic numeral corresponding to the isoenzyme.
For example, the cytochrome P-450 isoenzyme designated CYP3A4 belongs to family 3, subfamily IIIA. Cytochrome P-450 isoenzymes - representatives of various families of subfamilies -
differ in activity regulators (inhibitors and inductors) and substrate specificity 1 . For example, CYP2C9 metabolizes S-warfarin exclusively, while R-warfarin metabolizes CYP1A2 and CYP3A4 isoenzymes.
However, members of individual families, subfamilies, and individual isoenzymes of cytochrome P-450 may have cross-substrate specificity, as well as cross-inhibitors and inducers. For example, ritonavir (an antiviral drug) is metabolized by 7 isoenzymes belonging to different families and subfamilies (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4). Cimetidine simultaneously inhibits 4 isoenzymes: CYP1A2, CYP2C9, CYP2D6 and CYP3A4. The isoenzymes of cytochrome P-450 I, II and III families take part in the metabolism of drugs. CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2D6, CYP2C9, CYP209, CYP2E1, CYP3A4 are the most important and well-studied cytochrome P-450 isoenzymes for drug metabolism. The content of various isoenzymes of cytochrome P-450 in the human liver, as well as their contribution to the oxidation of drugs, are different (Tables 5-6). Medicinal substances - substrates, inhibitors and inducers of cytochrome P-450 isoenzymes are presented in application 1.
Table 5-6. The content of cytochrome P-450 isoenzymes in the human liver and their contribution to the oxidation of drugs (Lewis et al., 1999)
1 Some isoenzymes of cytochrome P-450 have not only substrate specificity, but also stereospecificity.
Until now, endogenous substrates for isoenzymes of the CYPI family are not known. These isoenzymes metabolize xenobiotics: some drugs and PAHs are the main components of tobacco smoke and fossil fuel combustion products. A distinctive feature of isoenzymes of the CYPI family is their ability to induce under the action of PAHs, including dioxin and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Therefore, the CYPI family is called “cytochrome, inducible PAH” in the literature; "dioxin-inducible cytochrome" or "TCDD-inducible cytochrome". In humans, the CYPI family is represented by two subfamilies: IA and IB. The IA subfamily includes isoenzymes 1A1 and 1A2. The IB subfamily includes the 1B1 isoenzyme.
Cytochrome P-450 isoenzyme 1A1 (CYP1A1) is found mainly in the lungs, to a lesser extent in lymphocytes and placenta. CYP1A1 is not involved in drug metabolism; however, this isoenzyme actively metabolizes PAHs in the lungs. At the same time, some PAHs, for example, benzopyrene and nitrosamines, are converted into carcinogenic compounds that can provoke the development of malignant neoplasms, primarily lung cancer. This process is called "biological activation of carcinogens". Like other cytochromes of the CYPI family, CYP1A1 is induced by PAH. At the same time, the mechanism of CYP1A1 induction under the influence of PAHs was studied. Having entered the cell, PAHs bind to the Ah receptor (a protein from the class of transcription regulators); the resulting PAH-An-receptor complex penetrates into the nucleus with the help of another protein, ARNT, and then stimulates the expression of the CYP1A1 gene by binding to a specific dioxin-sensitive site (site) of the gene. Thus, in smokers, the processes of CYP1A1 induction proceed most intensively; this leads to biological activation of carcinogens. This explains the high risk of lung cancer in smokers.
Cytochrome P-450 isoenzyme 1A2 (CYP1A2) is found mainly in the liver. Unlike cytochrome CYP1A1, CYP1A2 metabolizes not only PAHs, but also a number of drugs (theophylline, caffeine, and other drugs). Phenacetin, caffeine, and antipyrine are used as marker substrates for CYP1A2 phenotyping. While phenacetin is subjected to O-demethylation, caffeine - 3-demethylation, and antipyrine - 4-hydroxylation. Grade
caffeine clearance is an important diagnostic test to determine the functional state of the liver. Due to the fact that CYP1A2 is the main metabolizing enzyme of caffeine, in fact, this test determines the activity of this isoenzyme. The patient is offered to ingest caffeine labeled with a radioactive carbon isotope C 13 (C 13 -caffeine), then the air exhaled by the patient is collected in a special reservoir for an hour and analyzed. At the same time, the air exhaled by the patient contains radioactive carbon dioxide (C 13 O 2 - formed by radioactive carbon) and ordinary carbon dioxide (C 12 O 2). The ratio in exhaled air C 13 O 2 to C 12 O 2 (measured by mass spectroscopy) determines the clearance of caffeine. There is a modification of this test: the concentration of caffeine and its metabolites in blood plasma, urine and saliva taken on an empty stomach is determined by high performance liquid chromatography. In this case, cytochromes CYP3A4 and CYP2D6 make a certain contribution to the metabolism of caffeine. Assessment of caffeine clearance is a reliable test that allows assessing the functional state of the liver in case of severe liver damage (for example, with cirrhosis of the liver) and determining the degree of impairment. The disadvantages of the test include its lack of sensitivity with moderate liver damage. The test result is affected by smoking (CYP1A2 induction), age, the combined use of drugs that change the activity of cytochrome P-450 isoenzymes (inhibitors or inducers).
Cytochrome P-450 subfamily CYPIIA
Of the isoenzymes of the CYPIIA subfamily, the cytochrome P-450 2A6 isoenzyme (CYP2A6) plays the most important role in drug metabolism. A common property of the CYPIIA subfamily isoenzymes is the ability to induce under the influence of phenobarbital, therefore the CYPIIA subfamily is called phenobarbital-inducible cytochromes.
Cytochrome P-450 isoenzyme 2A6 (CYP2A6) is found mainly in the liver. CYP2A6 metabolizes a small number of drugs. With the help of this isoenzyme, nicotine is converted to cotinine, as well as cotinine to 3-hydroxycotinine; 7-hydroxylation of coumarin; 7-hydroxylation of cyclophosphamide. CYP2A6 contributes to the metabolism of ritonavir, paracetamol and valproic acid. CYP2A6 is involved in the biological activation of tobacco smoke components nitrosamines, carcinogens that cause lung cancer. CYP2A6 promotes bioactivation
powerful mutagens: 6-amino-(x)-rizena and 2-amino-3-methylmidazo-(4,5-f)-quanoline.
Cytochrome P450 subfamily CYPIIB
Of the isoenzymes of the CYPIIB subfamily, the CYP2B6 isoenzyme plays the most important role in drug metabolism. A common property of isoenzymes of the CYPIIB subfamily is the ability to induce under the influence of phenobarbital.
The cytochrome P-450 2B6 isoenzyme (CYP2B6) is involved in the metabolism of a small number of drugs (cyclophosphamide, tamoxifen, S-methadone p, bupropion p, efavirenz). CYP2B6 mainly metabolizes xenobiotics. The marker substrate for CYP2B6 is an anticonvulsant.
S-mephenytoin p while CYP2B6 undergoes S-mephenytoin p N-demethylation (determined metabolite - N-demethylmephenytoin). CYP2B6 is involved in the metabolism of endogenous steroids: catalyzes 16α-16β-hydroxylation of testosterone.
Cytochrome P-450 subfamily CYPIIU
Of all the isoenzymes of the CYPIIC cytochrome subfamily, the most important role in drug metabolism is played by cytochrome P-450 isoenzymes 2C8, 2C9, 2C19. A common property of the cytochromes of the CYPIIC subfamily is 4-hydroxylase activity in relation to mephenytoin p (an anticonvulsant drug). Mephenytoin p is a marker substrate of CYPIIC subfamily isoenzymes. That is why the isoenzymes of the CYPIIC subfamily are also called mephenytoin-4-hydroxylases.
Cytochrome P-450 2C8 isoenzyme (CYP2C8) is involved in the metabolism of a number of drugs (NSAIDs, statins and other drugs). For many drugs, CYP2C8 is an "alternative" pathway for biotransformation. However, for drugs such as repaglinide (a hypoglycemic drug taken by mouth) and taxol (a cytostatic), CYP2C8 is the main metabolic enzyme. CYP2C8 catalyzes the 6a-hydroxylation of taxol. The marker substrate for CYP2C8 is paclitaxel (a cytotoxic drug). During the interaction of paclitaxel with CYP2C8, 6-hydroxylation of the cytostatic occurs.
Cytochrome P-450 isoenzyme 2C9 (CYP2C9) is found mainly in the liver. CYP2C9 is absent from the fetal liver and is only detected one month after birth. CYP2C9 activity does not change throughout life. CYP2C9 metabolizes various drugs. CYP2C9 is the main metabolic enzyme
many NSAIDs, including selective cyclooxygenase-2 inhibitors, angiotensin receptor inhibitors (losartan and irbesartan), hypoglycemic drugs (sulfonylurea derivatives), phenytoin (diphenin ♠), indirect anticoagulants (warfarin 1, acenocoumarol 2), fluvastatin 3.
It should be noted that CYP2C9 has “stereoselectivity” and mainly metabolizes S-warfarin and S-acenocoumarol, while the biotransformation of R-warfarin and R-acenocoumarol occurs with the help of other cytochrome P-450 isoenzymes: CYP1A2, CYP3A4. CYP2C9 inducers are rifampicin and barbiturates. It should be noted that almost all sulfonamide antibacterial drugs inhibit CYP2C9. However, a specific inhibitor of CYP2C9, sulfafenazole r. Echinacea purpurea extract has been shown to inhibit CYP2C9 in studies in vitro And in vivo, and hydrolyzed soy extract (due to the isoflavones contained in it) inhibits this isoenzyme in vitro. The combined use of LS-substrates of CYP2C9 with its inhibitors leads to inhibition of the metabolism of substrates. As a result, unwanted drug reactions of CYP2C9 substrates (up to intoxication) may occur. For example, the combined use of warfarin (CYP2C9 substrate) with sulfa drugs (CYP2C9 inhibitors) leads to an increase in the anticoagulant effect of warfarin. That is why when combining warfarin with sulfonamides, it is recommended to perform strict (at least 1-2 times a week) control of the international normalized ratio. CYP2C9 has a genetic polymorphism. "Slow" allelic variants of CYP2C9*2 and CYP2C9*3 are single nucleotide polymorphisms of the CYP2C9 gene, which are currently most fully studied. Carriers of CYP2C9*2 and CYP2C9*3 allelic variants have a decrease in CYP2C9 activity; this leads to a decrease in the rate of biotransformation of drugs metabolized by this isoenzyme and to an increase in their plasma concentration
1 Warfarin is a racemic mixture of isomers: S-warfarin and R-vafrarin. It should be noted that S-warfarin has a greater anticoagulant activity.
2 Acenocoumarol is a racematic mixture of isomers: S-acenocoumarol and R-acenocoumarol. However, unlike warfarin, these two isomers have the same anticoagulant activity.
3 Fluvastatin is the only drug from the group of lipid-lowering drugs, HMG-CoA reductase inhibitors, whose metabolism occurs with the participation of CYP2C9, and not CYP3A4. At the same time, CYP2C9 metabolizes both fluvastatin isomers: the active (+)-3R,5S enantiomer and the inactive (-)-3S,5R enantiomer.
blood. Therefore, heterozygotes (CYP2C9*1/*2, CYP2C9*1/*3) and homozygotes (CYP2C9*2/*2, CYP2C9*3/*3, CYP2C9*2/*3) are “slow” CYP2C9 metabolizers. So, it is in this category of patients (carriers of the listed allelic variants of the CYP2C9 gene) that adverse drug reactions are most often observed when using drugs that are metabolized under the influence of CYP2C9 (indirect anticoagulants, NSAIDs, oral hypoglycemic drugs - sulfonylurea derivatives).
Cytochrome P-450 isoenzyme 2C18 (CYP2C18) is found mainly in the liver. CYP2Cl8 is absent from the fetal liver and is only detected one month after birth. CYP2Cl8 activity does not change throughout life. CYP2Cl8 makes a certain contribution to the metabolism of drugs such as naproxen, omeprazole, piroxicam, propranolol, isotretinoin (retinoic acid) and warfarin.
Cytochrome P-450 isoenzyme 2C19 (CYP2C19) is the main enzyme in the metabolism of proton pump inhibitors. At the same time, the metabolism of individual drugs from the group of proton pump inhibitors has its own characteristics. Thus, omeprazole was found to have two metabolic pathways.
Under the action of CYP2C19, omeprazole is converted to hydroxyomeprazole. Under the action of CYP3A4, hydroxyomeprazole is converted to omeprazole hydroxysulfone.
Under the action of CYP3A4, omeprazole is converted to omeprazole sulfide and omeprazole sulfone. Under the influence of CYP2C19, omeprazole sulfide and omeprazole sulfone are converted to omeprazole hydroxysulfone.
Thus, regardless of the pathway of biological transformation, the final metabolite of omeprazole is omeprazole hydroxysulfone. However, it should be noted that these metabolic pathways are primarily characteristic of the R-isomer of omeprazole (the S-isomer undergoes biotransformation to a much lesser extent). The understanding of this phenomenon allowed the creation of esoprazole p - a drug representing the S-isomer of omeprazole (inhibitors and inducers of CYP2C19, as well as the genetic polymorphism of this isoenzyme, to a lesser extent affect the pharmacokinetics of esoprazole p).
The metabolism of lansoprazole is identical to that of omeprazole. Rabeprazole is metabolized via CYP2C19 and CYP3A4 to dimethylrabeprazole and rabeprazole sulfone, respectively.
CYP2C19 is involved in the metabolism of tamoxifen, phenytoin, ticlopidine, psychotropic drugs such as tricyclic antidepressants, diazepam, and some barbiturates.
CYP2C19 is characterized by genetic polymorphism. Slow CYP2Cl9 metabolizers are carriers of "slow" allelic variants. The use of drugs that are substrates of this isoenzyme in slow CYP2CL9 metabolizers leads to a more frequent occurrence of adverse drug reactions, especially when using drugs with a narrow therapeutic latitude: tricyclic antidepressants, diazepam, some barbiturates (mephobarbital, hexobarbital). However, the largest number of studies is devoted to the effect of CYP2C19 gene polymorphism on the pharmacokinetics and pharmacodynamics of proton pump inhibitor blockers. As shown by pharmacokinetic studies conducted with the participation of healthy volunteers, the area under the pharmacokinetic curve, the values of the maximum concentration of omeprazole, lansoprazole and rabeprazole are significantly higher in heterozygotes and, especially, in homozygotes for "slow" allelic variants of the CYP2C19 gene. In addition, a more pronounced suppression of gastric secretion with the use of omeprazole, lansoprazole, rabeprazole was observed in patients (heterozygotes and homozygotes for "slow" allelic variants of CYP2C19) suffering from peptic ulcer and reflux esophagitis. However, the frequency of adverse drug reactions with proton pump inhibitors does not depend on the CYP2C19 genotype. Existing data suggest that lower doses of proton pump inhibitors are needed to achieve "targeted" suppression of gastric secretion in heterozygotes and homozygotes for "slow" allelic variants of the CYP2C19 gene.
Cytochrome P-450 subfamily CYPIID
The cytochrome P-450 CYPIID subfamily includes a single isoenzyme, 2D6 (CYP2D6).
The cytochrome P-450 2D6 isoenzyme (CYP2D6) is found mainly in the liver. CYP2D6 metabolizes about 20% of all known drugs, including antipsychotics, antidepressants, tranquilizers, and β-blockers. Proven: CYP2D6 is the main enzyme of biotransformation and tricyclic antidepressant amitriptyline. However, studies have shown that a small part of amitriptyline is also metabolized by other cytochrome P-450 isoenzymes (CYP2C19, CYP2C9, CYP3A4) to inactive metabolites. Debrisoquine p, dextromethorphan and spartein are marker substrates used for phenotyping of the 2D6 isoenzyme. CYP2D6, unlike other cytochrome P-450 isoenzymes, does not have inducers.
The CYP2D6 gene has a polymorphism. Back in 1977, Iddle and Mahgoub drew attention to the difference in the hypotensive effect in patients with arterial hypertension who used debrisoquine p (a drug from the group of α-blockers). At the same time, an assumption was formulated about the difference in the rate of metabolism (hydroxylation) of debrisoquine p in different individuals. In the "slow" metabolizers of debrisoquine p, the greatest severity of the hypotensive effect of this drug was registered. Later, it was proved that in “slow” metabolizers of debrisoquine p, the metabolism of some other drugs, including phenacetin, nortriptyline p, phenformin p, spartein, encainide p, propranolol, guanoxan p and amitriptyline, is also slowed down. As further studies have shown, "slow" CYP2D6 metabolizers are carriers (both homozygotes and heterozygotes) of functionally defective allelic variants of the CYP2D6 gene. The result of these variants is the absence of CYP2D6 synthesis (the allelic variant of CYP2D6x5), the synthesis of an inactive protein (the allelic variants of CYP2D6x3, CYP2D6x4, CYP2D6x6, CYP2D6x7, CYP2D6x8, CYP2D6x11, CYP2D6x12, CYP2D6x14, CYP2D6x15, CYP2D6x19, CYP2D6x20), synthesis of a defective protein with reduced activity (options CYP2D6x9, CYP2D6x10, CYP2D6x17,
CYP2D6x18, CYP2D6x36). Every year, the number of found allelic variants of the CYP2D6 gene is growing (their carriage leads to a change in the activity of CYP2D6). However, even Saxena (1994) pointed out that 95% of all "slow" metabolizers for CYP2D6 are carriers of the CYP2D6x3, CYP2D6x4, CYP2D6x5 variants, other variants are found much less frequently. According to Rau et al. (2004), the frequency of the CYP2D6x4 allelic variant among patients who experienced adverse drug reactions while taking tricyclic antidepressants (arterial hypotension, sedation, tremor, cardiotoxicity) is almost 3 times (20%) higher than that in patients treated with no complications were recorded with these drugs (7%). A similar effect of CYP2D6 genetic polymorphism was found on the pharmacokinetics and pharmacodynamics of antipsychotics, as a result, associations were demonstrated between the carriage of some allelic variants of the CYP2D6 gene and the development of extrapyramidal disorders induced by antipsychotics.
However, the carriage of "slow" allelic variants of the CYP2D6 gene may be accompanied not only by an increase in the risk of developing adverse drug reactions when using the drug.
rats metabolized by this isoenzyme. If the drug is a prodrug, and the active metabolite is formed precisely under the influence of CYP2D6, then the carriers of "slow" allelic variants note the low effectiveness of the drug. So, in carriers of "slow" allelic variants of the CYP2D6 gene, a less pronounced analgesic effect of codeine is recorded. This phenomenon is explained by a decrease in O-demethylation of codeine (during this process, morphine is formed). The analgesic effect of tramadol is also due to the active metabolite O-demethyltramadol (formed under the action of CYP2D6). Carriers of "slow" allelic variants of the CYP2D6 gene have a significant decrease in the synthesis of O-demethyltramadol; this can lead to an insufficient analgesic effect (similar to the processes that occur when using codeine). For example, Stamer et al. (2003), having studied the analgesic effect of tramadol in 300 patients who underwent abdominal surgery, found that homozygotes for "slow" allelic variants of the CYP2D6 gene did not "respond" to tramadol therapy 2 times more often than patients who did not carry these alleles ( 46.7% versus 21.6%, respectively, p=0.005).
Currently, many studies have been performed on the effect of CYP2D6 genetic polymorphism on the pharmacokinetics and pharmacodynamics of β-blockers. The results of these studies are of clinical importance for the individualization of pharmacotherapy of this group of drugs.
Cytochrome P-450 subfamily CYPIIB
Of the isoenzymes of the cytochrome IIE subfamily, the cytochrome P-450 2E1 isoenzyme plays the most important role in drug metabolism. A common property of the CYPIIE subfamily isoenzymes is the ability to induce under the influence of ethanol. That is why the second name of the CYPIIE subfamily is ethanol-inducible cytochromes.
The cytochrome P-450 2E1 isoenzyme (CYP2E1) is found in the liver of adults. CYP2E1 accounts for about 7% of all cytochrome P-450 isoenzymes. CYP2E1 substrates - a small amount of drugs, as well as some other xenobiotics: ethanol, nitrosamines, "small" aromatic hydrocarbons such as benzene and aniline, aliphatic chlorohydrocarbons. CYP2E1 catalyzes the conversion of dapsone to hydroxylamindapsone, n1-demethylation and N7-demethylation of caffeine, dehalogenation of chlorofluorocarbons and inhalation anesthetics (halothane), and some other reactions.
CYP2E1, together with CYP1A2, catalyzes the important conversion of paracetamol (acetaminophen) to N-acetylbenzoquinone imine, which has a potent hepatotoxic effect. There is evidence of the involvement of cytochrome CYP2E1 in waterogenesis. For example, CYP2E1 is known to be the most important cytochrome P-450 isoenzyme that oxidizes low-density lipoprotein (LDL) cholesterol. Cytochromes and other isoenzymes of cytochrome P-450, as well as 15-lipoxygenase and NADP-H-oxidase, also take part in the process of LDL oxidation. Oxidation products: 7a-hydroxycholesterol, 7β-hydroxycholesterol, 5β-6β-epoxycholesterol, 5α-6β-epoxycholesterol, 7-ketocholesterol, 26-hydroxycholesterol. The process of LDL oxidation occurs in endotheliocytes, smooth muscles of blood vessels, macrophages. Oxidized LDL stimulates the formation of foam cells and thus contributes to the formation of atherosclerotic plaques.
Cytochrome P-450 subfamily CYPIIIA
The cytochrome P-450 CYPIIIA subfamily includes four isoenzymes: 3A3, 3A4, 3A5, and 3A7. Subfamily IIIA cytochromes account for 30% of all cytochrome P-450 isoenzymes in the liver and 70% of all isoenzymes of the digestive tract wall. At the same time, the isoenzyme 3A4 (CYP3A4) is predominantly localized in the liver, and the isoenzymes 3A3 (CYP3A3) and 3A5 (CYP3A5) are located in the walls of the stomach and intestines. Isoenzyme 3A7 (CYP3A7) is found only in the fetal liver. Of the IIIA subfamily isoenzymes, CYP3A4 plays the most important role in drug metabolism.
The cytochrome P-450 3A4 isoenzyme (CYP3A4) metabolizes about 60% of all known drugs, including slow calcium channel blockers, macrolide antibiotics, some antiarrhythmics, statins (lovastatin, simvastatin, atorvastatin), clopidogrel 1 and other drugs.
CYP3A4 catalyzes the 6β-hydroxylation of endogenous steroids, including testosterone, progesterone, and cortisol p. Marker substrates for determining CYP3A4 activity are dapsone, erythromycin, nifedipine, lidocaine, testosterone and cortisol p.
Metabolism of lidocaine occurs in hepatocytes, where monoethylglycine xylidide (MEGX) is formed via oxidative N-deethylation of CYP3A4.
1 Clopidogrel is a prodrug, under the action of CYP3A4 it is converted into an active metabolite with antiplatelet action.
Determination of CYP3A4 activity by MEGX (lidocaine metabolite) is the most sensitive and specific test to assess the functional state of the liver in acute and chronic liver diseases, as well as in systemic inflammatory response syndrome (sepsis). In cirrhosis of the liver, the concentration of MEGX correlates with the prognosis of the disease.
In the literature, there are data on the intraspecific variability of drug metabolism under the influence of CYP3A4. However, molecular evidence for a CYP3A4 genetic polymorphism has only recently emerged. So, A. Lemoin et al. (1996) described a case of intoxication with tacrolimus (CYP3A4 substrate) in a patient after liver transplantation (CYP3A4 activity could not be detected in liver cells). Only after treatment of transplanted liver cells with glucocorticoids (CYP3A4 inducers) can CYP3A4 activity be determined. There is an assumption that a violation of the expression of transcription factors of the gene encoding CYP3A4 is the cause of the variability in the metabolism of this cytochrome.
The cytochrome P-450 3A5 isoenzyme (CYP3A5), according to recent data, may play a significant role in the metabolism of certain drugs. It should be noted that CYP3A5 is expressed in the liver of 10-30% of adults. In these individuals, the contribution of CYP3A5 to the activity of all isoenzymes of the IIIA subfamily ranges from 33 (in Europeans) to 60% (in African Americans). Studies have shown that under the influence of CYP3A5, the processes of biotransformation of those drugs that are traditionally considered as substrates of CYP3A4 occur. It should be noted that CYP3A4 inducers and inhibitors have a similar effect on CYP3A5. The activity of CYP3A5 in different individuals varies more than 30 times. Differences in CYP3A5 activity were first described by Paulussen et al. (2000): they were watching in vitro significant differences in the rate of metabolism of midazolam under the influence of CYP3A5.
Dihydropyrimidine dehydrogenase
The physiological function of dihydropyrimidine dehydrogenase (DPDH) - the reduction of uracil and thymidine - is the first reaction of the three-stage metabolism of these compounds to β-alanine. In addition, DPDH is the main enzyme that metabolizes 5-fluorouracil. This drug is used as part of combined chemotherapy for cancer of the breast, ovaries, esophagus, stomach, colon and rectum, liver, cervix, vulva. Also
5-fluorouracil is used in the treatment of cancer of the bladder, prostate, tumors of the head, neck, salivary glands, adrenal glands, pancreas. At present, the amino acid sequence and the number of amino acid residues (1025 in total) that make up DPDH are known; the molecular weight of the enzyme is 111 kD. The DPDH gene located on chromosome 1 (locus 1p22) was identified. The cytoplasm of cells of various tissues and organs contains DPDH, especially a large amount of the enzyme is found in liver cells, monocytes, lymphocytes, granulocytes, and platelets. However, DPDH activity was not observed in erythrocytes (Van Kuilenburg et al., 1999). Since the mid-1980s, there have been reports of serious complications arising from the use of 5-fluorouracil (the cause of complications is the hereditary low activity of DPDH). As shown by Diasio et al. (1988), low DPDH activity is inherited in an autosomal recessive manner. Thus, DPDH is an enzyme with genetic polymorphism. In the future, apparently, the methods of phenotyping and genotyping of DPDH will be introduced into oncological practice to ensure the safety of chemotherapy with 5-fluorouracil.
5.4. ENZYMES OF THE II PHASE OF BIOTRANSFORMATION OF DRUGS
Glucuronyltransferase
Glucuronidation is the most important phase II reaction of drug metabolism. Glucuronation is the addition (conjugation) of uridine diphosphate-glucuronic acid (UDP-glucuronic acid) to a substrate. This reaction is catalyzed by a superfamily of enzymes called "UDP-glucuronyltransferases" and referred to as UGT. The superfamily of UDP-glucuronyltransferases includes two families and more than twenty isoenzymes localized in the endoplasmic system of cells. They catalyze the glucuronidation of a large number of xenobiotics, including drugs and their metabolites, pesticides, and carcinogens. Compounds undergoing glucuronidation include ethers and esters; compounds containing carboxyl, carbomoyl, thiol and carbonyl groups, as well as nitro groups. Glucuronidation
leads to an increase in the polarity of chemical compounds, which facilitates their solubility in water and elimination. UDP-glucuronyltransferases are found in all vertebrates, from fish to humans. In the body of newborns, a low activity of UDP-glucuronyltransferases is recorded, however, after 1-3 months of life, the activity of these enzymes can be compared with that in adults. UDP-glucuronyltransferases are found in the liver, intestines, lungs, brain, olfactory epithelium, kidneys, but the liver is the main organ in which glucuronidation occurs. The degree of expression of various isoenzymes of UDP-glucuronyltransferase in organs is not the same. Thus, the isoenzyme of UDP-glucuronyl transferase UGT1A1, which catalyzes the reaction of bilirubin glucuronidation, is expressed mainly in the liver, but not in the kidneys. UDP-glucuronyltransferase isoenzymes UGT1A6 and UGT1A9 responsible for phenol glucuronidation are expressed in the same way in the liver and kidneys. As mentioned above, according to the identity of the amino acid composition, the superfamily of UDP-glucuronyltransferases is divided into two families: UGT1 and UGT2. Isoenzymes of the UGT1 family are similar in amino acid composition by 62-80%, and isoenzymes of the UGT2 family - by 57-93%. Isoenzymes that are part of the human UDP-glucuronyltransferase families, as well as gene localization and marker substrates of isoenzymes for phenotyping, are presented in the table (Tables 5-7).
The physiological function of UDP-glucuronyltransferases is glucuronidation of endogenous compounds. The product of heme catabolism, bilirubin, is the best studied endogenous substrate for UDP-glucuronyltransferase. Glucuronidation of bilirubin prevents the accumulation of toxic free bilirubin. In this case, bilirubin is excreted in the bile in the form of monoglucuronides and diglucuronides. Another physiological function of UDP-glucuronyltransferase is participation in hormone metabolism. Thus, thyroxine and triiodothyronine undergo glucuronidation in the liver and are excreted in the form of glucuronides with bile. UDP-glucuronyltransferases are also involved in the metabolism of steroid hormones, bile acids, and retinoids, but these reactions are currently not well understood.
Drugs of different classes undergo glucuronidation, many of them have a narrow therapeutic latitude, for example, morphine and chloramphenicol (Tables 5-8).
Table 5-7. Composition of human UDP-glucuronyltransferase families, gene localization and marker substrates of isoenzymes
Table 5-8. Drugs, metabolites and xenobiotics undergoing glucuronidation by various isoenzymes of UDP-glucuronyltransferase
End of table 5-8
Drugs (representatives of different chemical groups) undergoing glucuronidation
Phenols: propofol, acetaminophen, naloxone.
Alcohols: chloramphenicol, codeine, oxazepam.
Aliphatic amines: ciclopiroxolamine p, lamotrigine, amitriptyline.
Carboxylic acids: ferpazone p, phenylbutazone, sulfinpyrazone.
Carboxylic acids: naproxen, somepiral p, ketoprofen. Thus, compounds undergo glucuronidation
containing different functional groups that act as acceptors for UDP-glucuronic acid. As mentioned above, as a result of glucuronidation, polar inactive metabolites are formed, which are easily excreted from the body. However, there is an example when an active metabolite is formed as a result of glucuronidation. Glucuronidation of morphine leads to the formation of morphine-6-glucuronide, which has a significant analgesic effect and is less likely than morphine to cause nausea and vomiting. Also, glucuronidation can contribute to the biological activation of carcinogens. Carcinogenic glucuronides include 4-aminobiphenyl N-glucuronide, N-acetylbenzidine N-glucuronide, 4-((hydroxymethyl)-nitrosoamino)-1-(3-pyridyl)-1-butanone O-glucuronide.
The existence of hereditary disorders of bilirubin glucuronidation has long been known. These include Gilbert's syndrome and Crigler-Najjar syndrome. Gilbert's syndrome is a hereditary disease inherited in an autosomal recessive manner. The prevalence of Gilbert's syndrome among the population is 1-5%. The reason for the development of this disease is point mutations (usually substitutions in the nucleotide sequence) in the UGT1 gene. In this case, UDP-glucuronyltransferase is formed, which is characterized by low activity (25-30% of the normal level). Changes in glucuronidation of drugs in patients with Gilbert's syndrome have been little studied. There is evidence of a decrease in the clearance of tolbutamide, paracetamol (acetaminophen ♠) and rifampin p in patients with Gilbert's syndrome. We studied the incidence of side effects of the new cytotoxic drug irinotecan in patients suffering from both colorectal cancer and Gilbert's syndrome and in patients with colorectal cancer. Irinotecan (STR-11) is a new highly effective drug with a cytostatic effect, inhibiting topoisomerase I and used in colorectal cancer in the presence of resistance to fluorouracil. Irinotecan in the liver, under the action of carboxyesterases, converts
Xia in the active metabolite 7-ethyl-10-hydroxycamptothekin (SN-38). The main pathway of SN-38 metabolism is glucuronidation by UGT1A1. In the course of studies, the side effects of irinotecan (in particular, diarrhea) were significantly more frequently recorded in patients with Gilbert's syndrome. Scientists have proven that the carriage of allelic variants UGT1A1x1B, UGT1A1x26, UGT1A1x60 is associated with a more frequent development of hyperbilirubinemia with the use of irinotecan, while registering low values of the area under the pharmacokinetic curve of SN-38 glucuronide. Currently, the US Food and Drug Administration (Food and drug administration- FDA) approved the determination of allelic variants of the UGT1A1 gene for the choice of irinotecan dosing regimen. There are data on the effect of carriage of allelic variants of genes encoding other UGT isoforms on the pharmacokinetics and pharmacodynamics of various drugs.
Acetyltransferases
Acetylation evolutionarily represents one of the earliest adaptation mechanisms. The acetylation reaction is necessary for the synthesis of fatty acids, steroids, and the functioning of the Krebs cycle. An important function of acetylation is the metabolism (biotransformation) of xenobiotics: drugs, household and industrial poisons. Acetylation processes are affected by N-acetyltransferase, as well as coenzyme A. The control of the intensity of acetylation in the human body occurs with the participation of β 2 -adrenergic receptors and depends on metabolic reserves (pantothenic acid, pyridoxine, thiamine, lipoic acid *) and genotype. In addition, the intensity of acetylation depends on the functional state of the liver and other organs containing N-acetyltransferase (although acetylation, like other phase II reactions, changes little in liver diseases). Meanwhile, acetylation of drugs and other xenobiotics occurs mainly in the liver. Two N-acetyltransferase isoenzymes have been isolated: N-acetyltransferase 1 (NAT1) and N-acetyltransferase 2 (NAT2). NAT1 acetylates a small number of arylamines and has no genetic polymorphism. Thus, the main acetylation enzyme is NAT2. The NAT2 gene is located on chromosome 8 (locuses 8p23.1, 8p23.2, and 8p23.3). NAT2 acetylates various drugs, including isoniazid and sulfonamides (Tables 5-9).
Table 5-9. Acetylated drugs
The most important property of NAT2 is genetic polymorphism. The acetylation polymorphism was first described by Evans in the 1960s; he isolated the slow and fast acetylators of isoniazid. It was also noted that in "slow" acetylators, due to the accumulation (cumulation) of isoniazid, polyneuritis occurs more often. So, in "slow" acetylators, the half-life of isoniazid is 3 hours, while in "fast" acetylators it is 1.5 hours. for myelin synthesis. It was assumed that in "fast" acetylators, the use of isoniazid is more likely to lead to the development of a hepatotoxic effect due to the more intense formation of acetylhydrazine, but this assumption has not received practical confirmation. The individual rate of acetylation does not significantly affect the daily dosing regimen, but may reduce the effectiveness of therapy with intermittent use of isoniazid. After analyzing the results of isoniazid treatment of 744 patients with tuberculosis, it was found that the "slow" acetylators close the cavities in the lungs faster. As shown by a study conducted by Sunahara in 1963, "slow" acetylators are homozygotes for the "slow" NAT2 allele, and "fast" metabolizers are homozygotes or heterozygotes for the "fast" NAT2 allele. In 1964, Evans published evidence that acetylation polymorphism is characteristic not only for isoniazid, but also for hydralazine and sulfonamides. Then the presence of acetyl-
studies have also been proven for other drugs. The use of procainamide and hydralazine in "slow" acetylators causes liver damage (hepatotoxicity) much more often, thus, these drugs are also characterized by acetylation polymorphism. However, in the case of dapsone (which also undergoes acetylation), no differences were found in the incidence of lupus-like syndrome when using this drug with "slow" and "fast" acetylators. The prevalence of "slow" acetylators varies from 10-15% among the Japanese and Chinese to 50% among the Caucasians. Only at the end of the 1980s did they begin to identify allelic variants of the NAT2 gene, the carriage of which causes slow acetylation. Currently, about 20 mutant alleles of the NAT2 gene are known. All of these allelic variants are inherited in an autosomal recessive manner.
The type of acetylation is determined using NAT2 phenotyping and genotyping methods. Dapsone, isoniazid, and sulfadimin (sulfadimezin *) are used as marker substrates for acetylation. The ratio of the concentration of monoacetyldapsone to the concentration of dapsone less than 0.35 in blood plasma 6 hours after the administration of the drug is typical for "slow" acetylators, and more than 0.35 - for "fast" acetylators. If sulfadimin is used as a marker substrate, then the presence of less than 25% sulfadimin in blood plasma (analysis is performed after 6 hours) and less than 70% in urine (collected 5-6 hours after drug administration) indicates a "slow" acetylation phenotype.
Thiopurine S-methyltransferase
Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the reaction of S-methylation of thiopurine derivatives - the main pathway for the metabolism of cytostatic substances from the group of purine antagonists: 6-mercaptopurine, 6-thioguanine, azathioprine. 6-mercaptopurine is used as part of combined chemotherapy for myeloid and lymphoblastic leukemia, chronic myeloid leukemia, lymphosarcoma, and soft tissue sarcoma. In acute leukemia, 6-thioguanine is usually used. At present, the amino acid sequence and the number of amino acid residues that make up TPMT are known - 245. The molecular weight of TPMT is 28 kDa. The TPMT gene located on chromosome 6 (locus 6q22.3) was also identified. TPMT is located in the cytoplasm of hematopoietic cells.
In 1980, Weinshiboum studied TPMT activity in 298 healthy volunteers and found significant differences in TPMT activity in humans: 88.6% of those examined had high TPMT activity, 11.1% intermediate. Low TPMT activity (or complete absence of enzyme activity) was registered in 0.3% of the examined volunteers. Thus, the genetic polymorphism of TPMT was described for the first time. As shown by later studies, people with low TPMT activity are characterized by increased sensitivity to 6-mercaptopurine, 6-thioguanine and azathioprine; at the same time, life-threatening hematotoxic (leukopenia, thrombocytopenia, anemia) and hepatotoxic complications develop. Under conditions of low TPMT activity, the metabolism of 6-mercaptopurine follows an alternative path - to the highly toxic compound 6-thioguanine nucleotide. Lennard et al. (1990) studied plasma concentration of 6-thioguanine nucleotide and TPMT activity in erythrocytes of 95 children treated with 6-mercaptopurine for acute lymphoblastic leukemia. The authors found that the lower the activity of TPMT, the higher the concentration of 6-TGN in blood plasma and the more pronounced the side effects of 6-mercaptopurine. It has now been proven that low TPMT activity is inherited in an autosomal recessive manner, with low TPMT activity recorded in homozygotes, and intermediate in heterozygotes. Genetic studies in recent years, carried out by the polymerase chain reaction method, made it possible to detect mutations in the TPMT gene, which determine the low activity of this enzyme. Safe doses of 6-mercaptopurine: with high TPMT activity (normal genotype), 500 mg/(m 2 × day) are prescribed, with intermediate TPMT activity (heterozygotes) - 400 mg/(m 2 × day), with slow activity TRMT (homozygotes) - 50 mg / (m 2 × day).
Sulfotransferases
Sulfation is the reaction of addition (conjugation) to the substrate of the sulfuric acid residue, with the formation of sulfuric acid esters or sulfomates. Exogenous compounds (mainly phenols) and endogenous compounds (thyroid hormones, catecholamines, some steroid hormones) undergo sulfation in the human body. 3"-phosphoadenyl sulfate acts as a coenzyme for the sulfation reaction. Then 3"-phosphoadenyl sulfate is converted to adenosine-3",5"-bisphosphonate. The sulfation reaction is catalyzed by over-
a family of enzymes called "sulfotransferases" (SULT). Sulfotransferases are located in the cytosol. Three families have been found in the human body. Currently, about 40 sulfotransferase isoenzymes have been identified. Sulfotransferase isoenzymes in the human body are encoded by at least 10 genes. The greatest role in the sulfation of drugs and their metabolites belongs to the isoenzymes of sulfotransferase family 1 (SULT1). SULT1A1 and SULT1A3 are the most important isoenzymes of this family. SULT1 isoenzymes are localized mainly in the liver, as well as in the large and small intestines, lungs, brain, spleen, placenta, and leukocytes. SULT1 isoenzymes have a molecular weight of about 34 kDa and consist of 295 amino acid residues; the SULT1 isoenzyme gene is localized on chromosome 16 (locus 16p11.2). SULT1A1 (thermostable sulfotransferase) catalyzes the sulfation of "simple phenols", including drugs of a phenolic structure (minoxidil r, acetaminophen, morphine, salicylamide, isoprenaline, and some others). It should be noted that sulfation of minoxidil p leads to the formation of its active metabolite, minoxidil sulfate. SULT1A1 sulfates the metabolites of lidocaine: 4-hydroxy-2,6-xylidine(4-hydroxyl) and ropivacaine: 3-hydroxyropivacaine, 4-hydroxyropivacaine, 2-hydroxymethylropivacaine. In addition, SULT1A1 sulfates 17β-estradiol. The marker substrate of SULT1A1 is 4-nitrophenol. SULT1A3 (thermolabile sulfotransferase) catalyzes the reactions of sulfation of phenolic monoamines: dopamine, norepinephrine, serotonin. The marker substrate for SULT1A3 is dopamine. Sulfotransferase family 2 isoenzymes (SULT2) provide sulfation of dihydroepiandrosterone, epiandrosterone, androsterone. SULT2 isoenzymes are involved in the biological activation of carcinogens, for example, PAH (5-hydroxymethylchrysene, 7,12-dihydroxymethylbenz[a]anthracene), N-hydroxy-2-acetylaminofluorene. Sulfotransferase family 3 (SULT3) isoenzymes catalyze the N-sulfation of acyclic arylamines.
Epoxide hydrolase
Water conjugation plays an important role in the detoxification and biological activation of a large number of xenobiotics, such as arenes, aliphatic epoxides, PAHs, aflotoxin B1. Water conjugation reactions are catalyzed by a special enzyme - epoxide hydrolase
(ERNH). The largest amount of this enzyme is found in the liver. Scientists have isolated two isoforms of epoxide hydrolase: EPHX1 and EPHX2. EPNH2 consists of 534 amino acid residues, has a molecular weight of 62 kDa; the EPNH2 gene is located on chromosome 8 (locus 8p21-p12). EPNH2 is localized in the cytoplasm and peroxisomes; this epoxide hydrolase isoform plays a minor role in xenobiotic metabolism. Most of the water conjugation reactions are catalyzed by EPPH1. EPNH1 consists of 455 amino acid residues and has a molecular weight of 52 kDa. The EPRNX1 gene is located on chromosome 1 (locus 1q42.1). The significance of EPNH1 in aqueous conjugation of toxic metabolites of medicinal substances is great. The anticonvulsant phenytoin is oxidized by cytochrome P-450 to two metabolites: parahydroxylated and dihydrodiol. These metabolites are active electrophilic compounds capable of covalently binding to cell macromolecules; this leads to cell death, the formation of mutations, malignancy, and mitotic defects. In addition, parahydroxylated and dihydrodiol, acting as haptens, can also cause immunological reactions. Gingival hyperplasia, as well as teratogenic effects - toxic reactions of phenytoin have been reported in animals. It has been proven that these effects are due to the action of phenytoin metabolites: parahydroxylated and dihydrodiol. As shown by Buecher et al. (1990), low EPNH1 activity (less than 30% of normal) in amniocytes is a serious risk factor for the development of congenital fetal anomalies in women taking phenytoin during pregnancy. It has also been proven that the main reason for the decrease in EPNH1 activity is a point mutation in exon 3 of the EPNH1 gene; as a result, a defective enzyme is synthesized (tyrosine in position 113 is replaced by histidine). The mutation is inherited in an autosomal recessive manner. A decrease in EPNH1 activity is observed only in homozygotes for this mutant allele. Data on the prevalence of homozygotes and heterozygotes for this mutation are not available.
Glutathione transferase
Xenobiotics with different chemical structures undergo conjugation with glutathione: epoxides, arene oxides, hydroxylamines (some of them have a carcinogenic effect). Among medicinal substances, ethacrynic acid (uregit ♠) and the hepatotoxic metabolite of paracetamol (acetaminophen ♠) - N-acetylbenzoquinone imine, are conjugated with glutathione, converting
resulting in a non-toxic compound. As a result of the conjugation reaction with glutathione, cysteine conjugates are formed, called "thioesters". Glutathione conjugation is catalyzed by glutathione SH-S-transferase (GST) enzymes. This group of enzymes is localized in the cytosol, although microsomal GST has also been described (however, its role in the metabolism of xenobiotics has been little studied). The activity of GST in human erythrocytes in different individuals differs by 6 times, however, there is no dependence of the activity of the enzyme on gender). However, studies have shown that there is a clear correlation between GST activity in children and their parents. According to the identity of the amino acid composition in mammals, 6 GST classes are distinguished: α- (alpha-), μ- (mu-), κ- (kappa-), θ- (theta-), π- (pi-) and σ- (sigma -) GST. In the human body, GSTs of the μ (GSTM), θ (GSTT and π (GSTP) classes are mainly expressed. Among them, GSTs of the μ class, designated as GSTM, are of the greatest importance in the metabolism of xenobiotics. Currently, 5 GSTM isoenzymes have been isolated: GSTM1, GSTM2, GSTM3, GSTM4 and GSTM5 The GSTM gene is localized on chromosome 1 (locus 1p13.3) GSTM1 is expressed in the liver, kidneys, adrenal glands, stomach, weak expression of this isoenzyme is found in skeletal muscle, myocardium GSTM1 is not expressed in the fetal liver, fibroblasts, erythrocytes, lymphocytes and platelets.GSTM2 ("muscle" GSTM) is expressed in all of the above tissues (especially in muscle), except fibroblasts, erythrocytes, lymphocytes, platelets and fetal liver.Expression of GSTM3 ("brain" GSTM) is carried out in all tissues of the body, GSTM1 plays an important role in the inactivation of carcinogens, which is indirectly confirmed by a significant increase in the incidence of malignant diseases among carriers of null alleles of the GSTM1 gene, who lack GSTM1 expression. Harada et al. (1987), having studied liver samples taken from 168 corpses, found that the null allele of the GSTM1 gene is significantly more common in patients with hepatocarcinoma. Board et al. (1987) for the first time put forward a hypothesis: in the body of carriers of null alleles of GSTM1, inactivation of some electrophilic carcinogens does not occur. According to Board et al. (1990), the prevalence of the null GSTM1 allele among the European population is 40-45%, while among the representatives of the Negroid race it is 60%. There is evidence of a higher incidence of lung cancer in carriers of the GSTM1 null allele. As shown by Zhong et al. (1993)
70% of patients with colon cancer are carriers of the GSTM1 null allele. Another GST isoenzyme belonging to the π class, GSTP1 (located mainly in the liver and blood-brain barrier structures), is involved in the inactivation of pesticides and herbicides widely used in agriculture.
5.5. FACTORS AFFECTING DRUG BIOTRANSFORMATION
Genetic Factors Influencing the Biotransformation System and Drug Transporters
Genetic factors representing single nucleotide polymorphisms of genes encoding biotransformation enzymes and drug transporters can significantly affect the pharmacokinetics of drugs. Interindividual differences in the rate of drug metabolism, which can be assessed by the ratio of the concentration of the drug substrate to the concentration of its metabolite in plasma or urine (metabolic ratio), make it possible to distinguish groups of individuals that differ in the activity of one or another metabolic isoenzyme.
"Extensive" metabolizers (extensive metabolism, EM) - persons with a "normal" metabolic rate of certain drugs, as a rule, homozygotes for the "wild" allele of the gene of the corresponding enzyme. The majority of the population belongs to the group of "extensive" metabolizers.
"Slow" metabolizers (poor metabolism, RM) - persons with a reduced metabolic rate of certain drugs, as a rule, homozygotes (with an autosomal recessive type of inheritance) or heterozygotes (with an autosomal dominant type of inheritance) for the "slow" allele of the gene of the corresponding enzyme. In these individuals, the synthesis of a “defective” enzyme occurs, or there is no synthesis of a metabolic enzyme at all. The result is a decrease in enzymatic activity. Quite often find complete absence of enzymatic activity. In this category of persons, high rates of the ratio of the concentration of the drug to the concentration of its metabolite are recorded. Consequently, in "slow" metabolizers, drugs accumulate in the body in high concentrations; this leads to the development
Tyu expressed adverse drug reactions, up to intoxication. That is why such patients (slow metabolizers) need to carefully select the dose of drugs. "Slow" metabolizers are prescribed lower doses of drugs than "active" ones. "Overactive" or "fast" metabolizers (ultraextensive metabolism, UM) - persons with an increased metabolic rate of certain drugs, as a rule, homozygotes (with an autosomal recessive type of inheritance) or heterozygotes (with an autosomal dominant type of inheritance) for the "fast" allele of the gene of the corresponding enzyme or, which is more often observed, carrying copies of functional alleles. In this category of persons, low values of the ratio of the concentration of the drug to the concentration of its metabolite are recorded. As a result, the concentration of drugs in the blood plasma is insufficient to achieve a therapeutic effect. Such patients (“overactive” metabolizers) are prescribed higher doses of drugs than “active” metabolizers. If there is a genetic polymorphism of one or another biotransformation enzyme, then the distribution of individuals according to the rate of metabolism of drug substrates of this enzyme acquires a bimodal (if there are 2 types of metabolizers) or trimodal (if there are 3 types of metabolizers) character.
Polymorphism is also characteristic of genes encoding drug transporters, while the pharmacokinetics of drugs may vary depending on the function of this transporter. The clinical significance of the most important biotransformation enzymes and transporters is discussed below.
Induction and inhibition of the biotransformation system and transporters
The induction of a biotransformation enzyme or transporter is understood as an absolute increase in its quantity and (or) activity due to the action of a certain chemical agent, in particular, a drug. In the case of biotransformation enzymes, this is accompanied by ER hypertrophy. Both enzymes of phase I (cytochrome P-450 isoenzymes) and phase II of biotransformation (UDP-glucuronyl transferase, etc.), as well as drug transporters (glycoprotein-P, transporters of organic anions and cations) can undergo induction. Drugs that induce biotransformation enzymes and transporters do not have obvious structural similarity, but they are characterized by
thorns are some common features. Such substances are soluble in fats (lipophilic); serve as substrates for the enzymes (which they induce) and have, most often, a long half-life. The induction of biotransformation enzymes leads to an acceleration of biotransformation and, as a rule, to a decrease in pharmacological activity, and, consequently, to the effectiveness of drugs used together with the inducer. The induction of drug transporters can lead to various changes in the concentration of drugs in the blood plasma, depending on the functions of this transporter. Different substrates are able to induce drug biotransformation enzymes and drug transporters with different molecular weight, substrate specificity, immunochemical and spectral characteristics. In addition, there are significant interindividual differences in the intensity of induction of biotransformation enzymes and drug transporters. The same inducer can increase the activity of an enzyme or transporter in different individuals by 15-100 times.
Main types of induction
"Phenobarbital" type of induction - the direct effect of the inductor molecule on the regulatory region of the gene; this leads to the induction of the biotransformation enzyme or drug transporter. This mechanism is most characteristic of autoinduction. Autoinduction is understood as an increase in the activity of an enzyme that metabolizes a xenobiotic under the influence of the xenobiotic itself. Autoinduction is considered as an adaptive mechanism developed in the course of evolution for the inactivation of xenobiotics, including those of plant origin. So, autoinduction in relation to cytochromes of the subfamily IIB has garlic phytoncide - dialyl sulfide. Barbiturates (inducers of isoenzymes of cytochrome P-450 3A4, 2C9, subfamily IIB) are typical autoinducers (among medicinal substances). That is why this type of induction is called "phenobarbital".
"Rifampicin-dexamethasone" type - the induction of cytochrome P-450 isoenzymes 1A1, 3A4, 2B6 and glycoprotein-P is mediated by the interaction of the inducer molecule with specific receptors; receptor, CAR receptor. Connecting with these receptors, LS-inducers form a complex, which, penetrating into the cell nucleus, affects
Regulatory region of a gene. As a result, the induction of the drug biotransformation enzyme, or transporter, occurs. According to this mechanism, rifampins, glucocorticoids, St. John's wort and some other substances induce cytochrome P-450 isoenzymes and glycoprotein-P. "Ethanol" type - stabilization of the drug biotransformation enzyme molecule due to the formation of a complex with some xenobiotics (ethanol, acetone). For example, ethanol induces cytochrome P-450 isoenzyme 2E1 at all stages of its formation: from transcription to translation. It is believed that the stabilizing effect of ethanol is associated with its ability to activate the phosphorylation system in hepatocytes through cyclic AMP. According to this mechanism, isoniazid induces cytochrome P-450 isoenzyme 2E1. The process of induction of cytochrome P-450 isoenzyme 2E1 during starvation and diabetes mellitus is associated with the “ethanol” mechanism; in this case, ketone bodies act as inducers of cytochrome P-450 isoenzyme 2E1. Induction leads to an acceleration of the biotransformation of drug substrates of the corresponding enzymes, and, as a rule, to a decrease in their pharmacological activity. Among the inducers, rifampicin (inducer of isoenzymes 1A2, 2C9, 2C19, 3A4, 3A5, 3A6, 3A7 of cytochrome P-450; glycoprotein-P) and barbiturates (inducers of isoenzymes 1A2, 2B6, 2C8, 2C9, 2C19, 3A4) are most often used in clinical practice. , 3A5, 3A6, 3A7 cytochrome P-450). It takes several weeks for the inducing effect of barbiturates to develop. Unlike barbiturates, rifampicin, as an inductor, acts quickly. The effect of rifampicin can be detected after 2-4 days. The maximum effect of the drug is recorded after 6-10 days. Induction of enzymes or drug transporters caused by rifampicin and barbiturates sometimes leads to a decrease in the pharmacological efficacy of indirect anticoagulants (warfarin, acenocoumarol), cyclosporine, glucocorticoids, ketoconazole, theophylline, quinidine, digoxin, fexofenadine and verapamil (this requires correction of the dosing regimen of these drugs t .e. dose increase). It should be emphasized that when the inducer of drug biotransformation enzymes is canceled, the dose of the combined drug should be reduced, since its concentration in the blood plasma increases. An example of such an interaction can be considered a combination of indirect anticoagulants and phenobarbital. Studies have shown that in 14% of cases of bleeding during treatment
indirect anticoagulants develop as a result of the abolition of drugs that induce biotransformation enzymes.
Some compounds can inhibit the activity of biotransformation enzymes and drug transporters. Moreover, with a decrease in the activity of enzymes that metabolize drugs, the development of side effects associated with the long-term circulation of these compounds in the body is possible. Inhibition of drug transporters can lead to various changes in the concentration of drugs in blood plasma, depending on the functions of this transporter. Some medicinal substances are able to inhibit both the enzymes of the first phase of biotransformation (cytochrome P-450 isoenzymes) and the second phase of biotransformation (N-acetyltransferase, etc.), as well as drug transporters.
Main mechanisms of inhibition
Binding to the regulatory region of the biotransformation enzyme or drug transporter gene. According to this mechanism, drug biotransformation enzymes are inhibited under the action of a large amount of the drug (cimetidine, fluoxetine, omeprazole, fluoroquinolones, macrolides, sulfonamides, etc.).
Some drugs with a high affinity (affinity) for certain cytochrome P-450 isoenzymes (verapamil, nifedipine, isradipine, quinidine) inhibit the biotransformation of drugs with a lower affinity for these isoenzymes. This mechanism is called competitive metabolic interaction.
Direct inactivation of cytochrome P-450 isoenzymes (gastoden r). Inhibition of the interaction of cytochrome P-450 with NADP-N-cytochrome P-450 reductase (fumarocoumarins of grapefruit and lime juice).
A decrease in the activity of drug biotransformation enzymes under the action of appropriate inhibitors leads to an increase in the plasma concentration of these drugs (substrates for enzymes). In this case, the half-life of drugs is prolonged. All this causes the development of side effects. Some inhibitors affect several biotransformation isoenzymes simultaneously. Large inhibitor concentrations may be required to inhibit multiple enzyme isoforms. Thus, fluconazole (an antifungal drug) at a dose of 100 mg per day inhibits the activity of the 2C9 isoenzyme of cytochrome P-450. With an increase in the dose of this drug to 400 mg, inhibition is also noted.
activity of isoenzyme 3A4. In addition, the higher the dose of the inhibitor, the faster (and the higher) its effect develops. Inhibition generally develops faster than induction, usually it can be registered as early as 24 hours after the administration of inhibitors. The rate of inhibition of enzyme activity is also affected by the route of administration of the drug inhibitor: if the inhibitor is administered intravenously, then the interaction process will occur faster.
Inhibitors and inducers of biotransformation enzymes and drug transporters can serve not only drugs, but also fruit juices (Tables 5-10), and herbal remedies (appendix 2)- all this is of clinical importance when using drugs that act as substrates for these enzymes and transporters.
Table 5-10. Effect of fruit juices on the activity of the biotransformation system and drug transporters
5.6. EXTRAHEPATIC BIOTRANSFORMATION
The role of the gut in drug biotransformation
The intestine is considered the second most important organ (after the liver) that performs the biotransformation of drugs. In the intestinal wall, both phase I reactions and phase II reactions of biotransformation are carried out. Biotransformation of drugs in the intestinal wall is of great importance in the effect of the first pass (presystemic biotransformation). The essential role of biotransformation in the intestinal wall in the effect of the first passage of drugs such as cyclosporine A, nifedipine, midazolam, verapamil has already been proven.
Phase I enzymes of drug biotransformation in the intestinal wall
Among the enzymes of phase I of drug biotransformation, cytochrome P-450 isoenzymes are mainly localized in the intestinal wall. The average content of cytochrome P-450 isoenzymes in the human intestinal wall is 20 pmol/mg of microsomal protein (in the liver - 300 pmol/mg of microsomal protein). A clear pattern has been established: the content of cytochrome P-450 isoenzymes decreases from the proximal to the distal intestines (Tables 5-11). In addition, the content of cytochrome P-450 isoenzymes is maximal at the top of the intestinal villi and minimal in the crypts. The predominant intestinal cytochrome P-450 isoenzyme, CYP3A4, accounts for 70% of all intestinal cytochrome P-450 isoenzymes. According to different authors, the content of CYP3A4 in the intestinal wall varies, which is explained by interindividual differences in cytochrome P-450. Also important are the methods of purification of enterocytes.
Table 5-11. The content of cytochrome P-450 isoenzyme 3A4 in the intestinal wall and human liver
Other isoenzymes have also been identified in the intestinal wall: CYP2C9 and CYP2D6. However, compared with the liver, the content of these enzymes in the intestinal wall is insignificant (100-200 times less). The conducted studies demonstrated insignificant, in comparison with the liver, metabolic activity of cytochrome P-450 isoenzymes of the intestinal wall (Tables 5-12). As shown by studies devoted to the study of the induction of cytochrome P-450 isoenzymes of the intestinal wall, the inducibility of intestinal wall isoenzymes is lower than that of cytochrome P-450 isoenzymes of the liver.
Table 5-12. Metabolic activity of cytochrome P-450 isoenzymes in the intestinal wall and liver
Phase II enzymes of drug biotransformation in the intestinal wall
UDP-glucuronyltransferase and sulfotransferase are the most well-studied phase II enzymes of drug biotransformation located in the intestinal wall. The distribution of these enzymes in the intestine is similar to cytochrome P-450 isoenzymes. Cappiello et al. (1991) studied the activity of UDP-glucuronyltransferase in the human intestinal wall and liver by the metabolic clearance of 1-naphthol, morphine and ethinyl estradiol (Tables 5-13). Studies have shown that the metabolic activity of UDP-glucuronyltransferase in the intestinal wall is lower than that of the liver UDP-glucuronyltransferase. A similar pattern is also characteristic of bilirubin glucuronidation.
Table 5-13. Metabolic activity of UDP-glucuronyltransferase in the intestinal wall and in the liver
Cappiello et al. (1987) also studied the activity of sulfotransferase in the intestinal wall and liver by the metabolic clearance of 2-naphthol. The data obtained indicate the presence of differences in metabolic clearance indicators (moreover, the clearance of 2-naphthol in the intestinal wall is lower than in the liver). In the ileum, the value of this indicator is 0.64 nmol/(minhmg), in the sigmoid colon - 0.4 nmol/(minhmg), in the liver - 1.82 nmol/(minhmg). However, there are drugs whose sulfation occurs mainly in the intestinal wall. These include, for example, β 2 -agonists: terbutaline and isoprenaline (Table 5-14).
Thus, despite a certain contribution to the biotransformation of medicinal substances, the intestinal wall is significantly inferior to the liver in terms of its metabolic capacity.
Table 5-14. Metabolic clearance of terbutaline and isoprenaline in the intestinal wall and liver
The role of the lungs in drug biotransformation
Human lungs contain both phase I biotransformation enzymes (cytochrome P-450 isoenzymes) and phase II enzymes.
(epoxide hydrolase, UDP-glucuronyl transferase, etc.). In human lung tissue, it was possible to identify various cytochrome P-450 isoenzymes: CYP1A1, CYP1B1, CYP2A, CYP2A10, CYP2A11, CYP2B, CYP2E1, CYP2F1, CYP2F3. The total content of cytochrome P-450 in human lungs is 0.01 nmol/mg of microsomal protein (this is 10 times less than in the liver). There are cytochrome P-450 isoenzymes that are predominantly expressed in the lungs. These include CYP1A1 (found in humans), CYP2B (in mice), CYP4B1 (in rats) and CYP4B2 (in cattle). These isoenzymes are of great importance in the biological activation of a number of carcinogens and pulmonotoxic compounds. Information on the involvement of CYP1A1 in the biological activation of PAHs is presented above. In mice, the oxidation of butylated hydroxytoluene by the CYP2B isoenzyme leads to the formation of a pneumotoxic electrophilic metabolite. The CYP4B1 isoenzymes of rats and CYP4B2 of cattle promote the biological activation of 4-ipomenol (4-ipomenol is a potent pneumotoxic furanoterpenoid of the raw potato fungus). It was 4-impomenol that caused the mass mortality of cattle in the 70s in the USA and England. At the same time, 4-ipomenol, oxidized by the CYP4B2 isoenzyme, caused interstitial pneumonia, which led to death.
Thus, the expression of specific isoenzymes in the lungs explains the selective pulmonotoxicity of some xenobiotics. Despite the presence of enzymes in the lungs and other parts of the respiratory tract, their role in the biotransformation of medicinal substances is negligible. The table shows the drug biotransformation enzymes found in the human respiratory tract (Tables 5-15). Determining the localization of biotransformation enzymes in the respiratory tract is difficult due to the use of lung homogenizate in studies.
Table 5-15. Biotransformation enzymes found in the human respiratory tract
The role of the kidneys in drug biotransformation
Studies performed over the past 20 years have shown that the kidneys are involved in the metabolism of xenobiotics and drugs. In this case, as a rule, there is a decrease in biological and pharmacological activity, however, in some cases, the process of biological activation is also possible (in particular, the bioactivation of carcinogens).
In the kidneys, both enzymes of the first phase of biotransformation and enzymes of the second phase were found. Moreover, biotransformation enzymes are localized both in the cortex and in the medulla of the kidneys (Tables 5-16). However, as studies have shown, a greater number of cytochrome P-450 isoenzymes contains exactly the cortical layer of the kidneys, and not the medulla. The maximum content of cytochrome P-450 isoenzymes was found in the proximal renal tubules. Thus, the kidneys contain the CYP1A1 isoenzyme, previously considered specific for the lungs, and CYP1A2. Moreover, these isoenzymes in the kidneys are subject to PAH induction (for example, by β-naphthovlavone, 2-acetylaminoflurin) in the same way as in the liver. CYP2B1 activity was found in the kidneys, in particular, the oxidation of paracetamol (acetaminophen ♠) in the kidneys under the action of this isoenzyme was described. It was later demonstrated that it is the formation of the toxic metabolite N-acetibenzaquinoneimine in the kidneys under the influence of CYP2E1 (similar to the liver) that is the main reason for the nephrotoxic effect of this drug. With the combined use of paracetamol with CYP2E1 inducers (ethanol, testosterone, etc.), the risk of kidney damage increases several times. CYP3A4 activity in the kidneys is not always recorded (only in 80% of cases). It should be noted that the contribution of renal cytochrome P-450 isoenzymes to the biotransformation of medicinal substances is modest and, apparently, in most cases has no clinical significance. However, for some drugs, biochemical transformation in the kidneys is the main route of biotransformation. Studies have shown that tropisetron p (an antiemetic drug) is mainly oxidized in the kidneys under the action of CYP1A2 and CYP2E1 isoenzymes.
Among the enzymes of the II phase of biotransformation in the kidneys, UDP-glucuronyl transferase and β-lyase are most often determined. It should be noted that the activity of β-lyase in the kidneys is higher than in the liver. The discovery of this feature made it possible to develop some "prodrugs", the activation of which produces active meta-
pain, selectively acting on the kidneys. So, they created a cytostatic drug for the treatment of chronic glomerulonephritis - S-(6-purinyl)-L-cysteine. This compound, initially inactive, is converted in the kidneys by the action of β-lyase into active 6-mercaptopurine. Thus, 6-mercuptopurine has an effect exclusively in the kidneys; this significantly reduces the frequency and severity of adverse drug reactions.
Drugs such as paracetamol (acetaminophen ♠), zidovudine (azidothymidine ♠), morphine, sulfamethasone p, furosemide (lasix ♠) and chloramphenicol (levomycetin ♠) undergo glucuronidation in the kidneys.
Table 5-16. Distribution of drug biotransformation enzymes in the kidneys (Lohr et al., 1998)
* - the content of the enzyme is significantly higher.
Literature
Kukes V.G. Drug metabolism: clinical and pharmacological aspects. - M.: Reafarm, 2004. - S. 113-120.
Seredenin S.B. Lectures on pharmacogenetics. - M.: MIA, 2004. -
Diasio R.B., Beavers T.L., Carpenter J.T. Familial deficiency of dihydropyrimidine dehydrogenase: biochemical basis for familial pyrimidinemia and severe 5-fluorouracil-induced toxicity // J. Clin. Invest. - 1988. - Vol. 81.-
Lemoine A., Daniel A., Dennison A., Kiffel L. et al. FK 506 renal toxicity and lack of detectable cytochrome P-450 3A in the liver graft of a patient undergoing liver transplantation // Hepatology. - 1994. - Vol. 20. - P. 1472-1477.
Lewis D.F.V., Dickins M., Eddershaw P.J. et al. Cytochrome-P450 Substrate Specificities, Substrate structural Templates and Enzyme Active Site Geometries // Drug Metabol. drug interaction. - 1999. - Vol. 15. - P. 1-51.
The interaction of a number of medicinal substances in the process of their distribution in the body can be considered as one of the important pharmacokinetic stages that characterizes their biotransformation, leading in most cases to the formation of metabolites.
Metabolism (biotransformation) - the process of chemical modification of medicinal substances in the body.
Metabolic reactions are divided into non-synthetic(when medicinal substances undergo chemical transformations, undergoing oxidation, reduction and hydrolytic cleavage or several of these transformations) - I phase of metabolism and synthetic(conjugation reaction, etc.) - II phase. Usually, non-synthetic reactions are only the initial stages of biotransformation, and the resulting products can participate in synthetic reactions and then be eliminated.
Products of non-synthetic reactions may have pharmacological activity. If the activity is possessed not by the substance itself introduced into the body, but by some metabolite, then it is called a prodrug.
|
Some medicinal substances whose metabolic products have therapeutically important activity |
|
|
medicinal substance |
Active metabolite |
|
Allopurinol |
Alloxanthin |
|
Amitriptyline |
Nortriptyline |
|
Acetylsalicylic acid* |
Salicylic acid |
|
Acetohexamide |
Hydroxyhexamide |
|
Glutethimide |
4-hydroxyglutethimide |
|
Diazelam |
Desmethyldiazepam |
|
Digitoxin |
Digoxin |
|
Imipramine |
Desipramine |
|
Cortisone |
Hydrocortisone |
|
Lidocaine |
Desethyllidocaine |
|
Methyldopa |
Methylnorepinephrine |
|
Prednisone* |
Prednisolone |
|
propranolol |
4-hydroxyprolranolol |
|
Spironolactone |
canrenon |
|
Trimeperidine |
Normeperidine |
|
Phenacetin* |
Acetaminophen |
|
Phenylbutazone |
Oxyphenbutazone |
|
Flurazepam |
Desethylflurazepam |
|
Chloral Hydrate* |
Trichloroethanol |
|
Chlordiazepoxide |
Desmethylchlordiazepoxide |
|
* prodrugs, the therapeutic effect is mainly the products of their metabolism. |
|
Non-synthetic metabolic reactions of medicinal substances are catalyzed by microsomal enzyme systems of the endoplasmic reticulum of the liver or non-microsomal enzyme systems. These substances include: amphetamine, warfarin, imipramine, meprobamate, procainamide, phenacetin, phenytoin, phenobarbital, quinidine.
In synthetic reactions (conjugation reactions), a drug or metabolite is a product of a non-synthetic reaction, combining with an endogenous substrate (glucuronic, sulfuric acids, glycine, glutamine) to form conjugates. As a rule, they do not have biological activity and, being highly polar compounds, they are well filtered, but poorly reabsorbed in the kidneys, which contributes to their rapid excretion from the body.
The most common conjugation reactions are: acetylation(the main route of metabolism of sulfonamides, as well as hydralazine, isoniazid and procainamide); sulfation(reaction between substances with phenolic or alcohol groups and inorganic sulfate. The source of the latter can be sulfur-containing acids, such as cysteine); methylation(certain catecholamines, niacinamide, thiouracil are inactivated). Examples of various types of reactions of metabolites of medicinal substances are given in the table.
|
Types of drug metabolism reactions |
|
|
Reaction type |
medicinal substance |
|
I. NON-SYNTHETIC REACTIONS (catalyzed by endoplasmic reticulum or non-microsomal enzymes) |
|
|
Oxidation |
|
|
Aliphatic hydroxylation, or oxidation of the side chain of a molecule |
Thiolenthal, methohexital, pentazocine |
|
Aromatic hydroxylation, or hydroxylation of an aromatic ring |
Amphetamine, lidocaine, salicylic acid, phenacetin, phenylbutazone, chlorpromazine |
|
O-dealkylation |
phenacetin, codeine |
|
N-dealkylation |
Morphine, codeine, atropine, imipramine, isoprenaline, ketamine, fentanyl |
|
S-dealkylation |
Barbituric acid derivatives |
|
N-oxidation |
Aminazine, imipramine, morphine |
|
S-oxidation |
Aminazin |
|
Deamination |
Phenamine, hisgamine |
|
Desulfurization |
thiobarbiturates, thioridazine |
|
Dehalogenation |
Halothane, methoxyflurane, enflurane |
|
Recovery |
|
|
Restoration of the azo group |
Sulfanilamide |
|
Recovery of the nitro group |
Nitrazepam, chloramphenicol |
|
Recovery of carboxylic acids |
Prednisolone |
|
Reduction catalyzed by alcohol dehydrogenase |
Ethanol, chloral hydrate |
|
Ether hydrolysis |
Acetylsalicylic acid, norzpinephrine, cocaine, procainamide |
|
Amide hydrolysis |
Lidocaine, pilocarpine, isoniazid novocainamide fentanyl |
|
II. SYNTHETIC REACTIONS |
|
|
Conjugation with glucuronic acid |
Salicylic acid, morphine, paracetamol, nalorphine, sulfonamides |
|
conjugation with sulfates |
Isoprenaline, morphine, paracetamol, salicylamide |
|
Conjugation with amino acids: |
|
|
salicylic acid, nicotinic acid |
|
Isonicotinic acid |
|
Paracetamol |
|
Acetylation |
Novocainamide, sulfonamides |
|
Methylation |
Norepinephrine, histamine, thiouracil, nicotinic acid |
The transformation of some medicinal substances taken orally depends significantly on the activity of enzymes produced by the intestinal microflora, where unstable cardiac glycosides are hydrolyzed, which significantly reduces their cardiac effect. Enzymes produced by resistant microorganisms catalyze hydrolysis and acetylation reactions, due to which antimicrobial agents lose their activity.
There are examples when the enzymatic activity of the microflora contributes to the formation of medicinal substances that exhibit their activity. Thus, phthalazole (phthalylsulfathiazole) outside the body practically does not show antimicrobial activity, but under the influence of enzymes of the intestinal microflora it is hydrolyzed with the formation of norsulfazole and phthalic acid, which have an antimicrobial effect. With the participation of enzymes of the intestinal mucosa, reserpine and acetylsalicylic acid are hydrolyzed.
However, the main organ where the biotransformation of medicinal substances is carried out is the liver. After absorption in the intestine, they enter the liver through the portal vein, where they undergo chemical transformations.
Drugs and their metabolites enter the systemic circulation through the hepatic vein. The combination of these processes is called the "first pass effect", or presystemic elimination, as a result of which the amount and effectiveness of a substance entering the general circulation may change.
|
Medicinal substances with a "first pass effect" through the liver |
||
|
Alprenolol |
Cortisone |
Oxprenolol |
|
Aldosterone |
Labetalol |
organic nitrates |
|
Acetylsalicylic acid |
Lidocaine |
Pentazocine |
|
Verapamil |
metoprolol |
Prolranolol |
|
Hydralazine |
Moracizin |
Reserpine |
|
Isoprenaline |
Phenacetin |
|
|
Imipramine |
metoclopamid |
Fluorouracil |
|
Isoprenaline |
Methyltestosterone |
|
It should be borne in mind that when drugs are taken orally, their bioavailability is individual for each patient and varies for each drug. Substances that undergo significant metabolic transformations during the first passage in the liver may not have a pharmacological effect, for example, lidocaine, nitroglycerin. In addition, first pass metabolism can be carried out not only in the liver, but also in other internal organs. For example, chlorpromazine is more extensively metabolized in the intestine than in the liver.
The course of presystemic elimination of one substance is often influenced by other medicinal substances. For example, chlorpromazine reduces the “first pass effect” of propranolol, as a result, the concentration of β-blocker in the blood increases.
Absorption and presystemic elimination determine the bioavailability and, to a large extent, the effectiveness of drugs.
The leading role in the biotransformation of medicinal substances is played by enzymes of the endoplasmic reticulum of liver cells, which are often called microsomal enzymes. More than 300 drugs are known that can change the activity of microsomal enzymes.. Substances that increase their activity are called inductors.
Liver enzyme inducers are: sleeping pills(barbiturates, chloral hydrate), tranquilizers(diazepam, chlordiazepoxide, meprobamate), antipsychotics(chlorpromazine, trifluoperazine), anticonvulsants(phenytoin) anti-inflammatory(phenylbutazone), some antibiotics(rifampicin), diuretics(spironolactone), etc.
Food additives, small doses of alcohol, coffee, chlorinated insecticides (dichlorodiphenyltrichloroethane (DDT), hexachloran) are also considered active inducers of liver enzyme systems. In small doses, some drugs, such as phenobarbital, phenylbutazone, nitrates, can stimulate their own metabolism (autoinduction).
With the joint appointment of two medicinal substances, one of which induces liver enzymes, and the second is metabolized in the liver, the dose of the latter must be increased, and when the inducer is canceled, reduced. A classic example of such an interaction is the combination of indirect anticoagulants and phenobarbital. Special studies have shown that in 14% of cases, the cause of bleeding in the treatment of anticoagulants is the abolition of drugs that induce microsomal liver enzymes.
The antibiotic rifampicin has a very high inducing activity of microsomal liver enzymes, and somewhat less - phenytoin and meprobamate.
Phenobarbital and other inducers of liver enzymes are not recommended for use in combination with paracetamol and other drugs whose biotransformation products are more toxic than the parent compounds. Sometimes liver enzyme inducers are used to accelerate the biotransformation of compounds (metabolites) that are foreign to the body. So phenobarbital, which promotes the formation of glucuronides, can be used to treat jaundice with impaired conjugation of bilirubin with glucuronic acid.
The induction of microsomal enzymes often has to be considered as an undesirable phenomenon, since the acceleration of drug biotransformation leads to the formation of inactive or less active compounds and a decrease in the therapeutic effect. For example, rifampicin can reduce the effectiveness of glucocorticosteroid treatment, which leads to an increase in the dose of a hormonal drug.
Much less frequently, as a result of the biotransformation of the medicinal substance, more active compounds are formed. In particular, during treatment with furazolidone, dihydroxyethylhydrazine accumulates in the body for 4-5 days, which blocks monoamine oxidase (MAO) and aldehyde dehydrogenase, which catalyzes the oxidation of aldehydes into acids. Therefore, patients taking furazolidone should not drink alcohol, since the concentration in the blood of acetaldehyde, which is formed from ethyl alcohol, can reach a level at which a pronounced toxic effect of this metabolite (acetaldehyde syndrome) develops.
Medicinal substances that reduce or completely block the activity of liver enzymes are called inhibitors.
Drugs that inhibit the activity of liver enzymes include narcotic analgesics, some antibiotics (actinomycin), antidepressants, cimetidine, etc. As a result of the use of a combination of drugs, one of which inhibits liver enzymes, the metabolic rate of another drug is slowed down, its blood and the risk of side effects. Thus, the histamine H 2 receptor antagonist cimetidine dose-dependently inhibits the activity of liver enzymes and slows down the metabolism of indirect anticoagulants, which increases the likelihood of bleeding, as well as β-blockers, which leads to severe bradycardia and arterial hypotension. Possible inhibition of the metabolism of anticoagulants of indirect action by quinidine. The side effects that develop with this interaction can be severe. Chloramphenicol inhibits the metabolism of tolbutamide, diphenylhydantoin and neodicumarin (ethyl biscumacetate). The development of hypoglycemic coma in combination therapy with chloramphenicol and tolbutamide has been described. Fatal cases are known with the simultaneous appointment of patients with azathioprine or mercaptopurine and allopurinol, which inhibits xanthine oxidase and slows down the metabolism of immunosuppressive drugs.
The ability of some substances to disrupt the metabolism of others is sometimes specially used in medical practice. For example, disulfiram is used in the treatment of alcoholism. This drug blocks the metabolism of ethyl alcohol at the stage of acetaldehyde, the accumulation of which causes discomfort. Metronidazole and antidiabetic agents from the group of sulfonylurea derivatives also act in a similar way.
A kind of blockade of enzyme activity is used in case of poisoning with methyl alcohol, the toxicity of which is determined by formaldehyde formed in the body under the influence of the alcohol dehydrogenase enzyme. It also catalyzes the conversion of ethyl alcohol to acetaldehyde, and the affinity of the enzyme for ethyl alcohol is higher than for methyl alcohol. Therefore, if both alcohols are in the medium, the enzyme catalyzes mainly the biotransformation of ethanol, and formaldehyde, which has a much higher toxicity than acetaldehyde, is formed in a smaller amount. Thus, ethyl alcohol can be used as an antidote (antidote) for methyl alcohol poisoning.
Ethyl alcohol changes the biotransformation of many medicinal substances. Its single use blocks the inactivation of various drugs and can enhance their action. In the initial stage of alcoholism, the activity of microsomal liver enzymes may increase, which leads to a weakening of the action of drugs due to the acceleration of their biotransformation. On the contrary, in the later stages of alcoholism, when many liver functions are impaired, it should be borne in mind that the effect of drugs whose biotransformation is impaired in the liver may noticeably increase.
The interaction of drugs at the level of metabolism can be realized through a change in hepatic blood flow. It is known that the factors limiting the metabolism of drugs with a pronounced effect of primary elimination (propranolol, verapamil, etc.) are the amount of hepatic blood flow and, to a much lesser extent, the activity of hepatocytes. In this regard, any medicinal substances that reduce regional hepatic circulation, reduce the intensity of metabolism of this group of drugs and increase their content in blood plasma.
