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METABOLISM OR BIOTRANSFORMATION The conversion from one chemical form of a substance to another. The term metabolism is commonly used probably because products of drug transformation are called metabolites. Metabolism is an essential pharmacokinetic process, which renders lipid soluble and non-polar compounds to water soluble and polar compounds so that they are excreted by various processes. This is because only water-soluble substances undergo excretion, whereas lipid soluble substances are passively reabsorbed from renal or extra renal excretory sites into the blood by virtue of their lipophilicity. Metabolism is a necessary biological process that limits the life of a substance in the body. Biotransformation: It is a specific term used for chemical transformation of xenobiotics in the body/living organism.• a series of enzyme-catalyzed processes—that alters the physiochemical properties offoreign chemicals (drug/xenobiotics) from those that favor absorption across biologicalmembranes (lipophilicity) to those favoring elimination in urine or bile (hydrophilicity )
Metabolism : It is a general term used for chemical transformation of xenobiotics and endogenous nutrients (e.g., proteins, carbohydrates and fats) within or outside the body.Xenobiotics : These are all chemical substances that are not nutrient for body (foreign to body) and which enter the body through ingestion, inhalation or dermal exposure.They include : drugs, industrial chemicals, pesticides, pollutants, plant and animal toxins, etc.
Functions of BiotransformationIt causes conversion of an active drug to inactive or less active metabolite(s) called as pharmacological inactivation.It causes conversion of an active to more active metabolite(s) called as bioactivation or toxicological activation.• It causes conversion of an inactive to more active toxic metabolite(s) called as lethal synthesis
Functions of Biotransformation….contd• It causes conversion of an inactive drug (pro-drug) to active metabolite(s) called as pharmacological activation• It causes conversion of an active drug to equally active metabolite(s) (no change in pharmacological activity)• It causes conversion of an active drug to active metabolite(s) having entirely different pharmacological activity (change in pharmacological activity)
Site/Organs of drug metabolismThe major site of drug metabolism is the liver (microsomal enzyme systems of hepatocytes)Secondary organs of biotransformation• kidney (proximal tubule)• lungs (type II cells)• testes (Sertoli cells)• skin (epithelial cells); plasma. nervous tissue (brain); intestines
Sites of Biotransformation…contdLiver The primary site for metabolism of almost all drugs because it is relatively rich in a large variety of metabolising enzymes. Metabolism by organs other than liver (called as extra-hepatic metabolism) is of lesser importance because lower level of metabolising enzymes is present in such tissues. Within a given cell, most drug metabolising activity is found in the smooth endoplasmic reticulum and the cytosol. Drug metabolism can also occur in mitochondria, nuclear envelope and plasma membrane. A few drugs are also metabolised by non-enzymatic means called as non- enzymatic metabolism. For example, atracurium, a neuromuscular blocking drug, is inactivated in plasma by spontaneous non-enzymatic degradation (Hoffman elimination) in addition to that by pseudocholinesterase enzyme.
Drug Metabolising EnzymesA number of enzymes in animals are capable of metabolising drugs. These enzymes are located mainly in the liver, but may also be present in other organs like lungs, kidneys, intestine, brain, plasma, etc.Majority of drugs are acted upon by relatively non-specific enzymes, which are directed to types of molecules rather than to specific drugs.The drug metabolising enzymes can be broadly divided into two groups: microsomal and non-microsomal enzymes.
Microsomal enzymes: The endoplasmic reticulum (especially smooth endoplasmic reticulum) of liver and other tissues contain a large variety of enzymes, together called microsomal enzymes(microsomes are minute spherical vesicles derived from endoplasmic reticulum after disruption of cells by centrifugation, enzymes present in microsomes are called microsomal enzymes).They catalyse glucuronide conjugation, most oxidative reactions, and some reductive and hydrolytic reactions.The monooxygenases, glucuronyl transferase, etc are important microsomal enzymes.
Non-microsomal enzymes: Enzymes occurring in organelles/sites other than endoplasmic reticulum (microsomes) are called non-microsomal enzymes.These are usually present in the cytoplasm, mitochondria, etc. and occur mainly in the liver, Gl tract, plasma and other tissues.They are usually non-specific enzymes that catalyse few oxidative reactions, a number of reductive and hydrolytic reactions, and all conjugative reactions other than glucuronidation.None of the non-microsomal enzymes involved in drug biotransformation is known to be inducible.
Factors Affecting Drug Metabolism1. Species differences : eg in phenylbutazone, procaine and barbiturates.2. Genetic differences – variation exist with species3. Age of animal –feeble in fetus,aged, newborn.4.sex: under the influence of sex hormones.5. Nutrition: starvation and malnutrition6. Patholigical conditions: Liver/Kidney dysfunction
TYPES OF BIOTRANSFORMATION Phase 1 reaction. (Non synthetic phase). Phase II reaction. (Synthetic phase)• a change in drug molecule. generally • Last step in detoxification reactions results in the introduction of a and almost always results in loss of functional group into molecules or the biological activity of a compound. exposure of new functional groups of • May be preceded by one or more of molecules phase one reaction : Phase I (non-synthetic or non- • Involves conjugation of functional conjugative phase) includes reactions groups of molecules with hydrophilic which catalyse oxidation, reduction and endogenous substrates- formation hydrolysis of drugs. of conjugates - is formed with (an endogenous substance such as carbohydrates and amino acids. )with In phase I reactions, small polar drug or its metabolites formed in functional groups like-OH, -NH2. -SH, - phase 1 reaction. COOH, etc. are either added or unmasked (if already present) on the Involve attachment of small polar lipid soluble drugs so that the resulting endogenous molecules like glucuronic products may undergo phase II acid, sulphate, methyl, amino acids, reactions. etc., to either unchanged drugs or phase I products.• result in activation, change or inactivation of drug. Products called as conjugates are water-soluble metabolites, which are readily excreted from the body.
• Phase I metabolism is sometimes called a • Phase II metabolism includes what are known “functionalization reaction,” as conjugation reactions.• Results in the introduction of new hydrophilic functional groups to compounds. • Generally, the conjugation reaction with• Function: introduction (or unveiling) of endogenous substrates occurs on the functional group(s) such as –OH, –NH2, –SH, metabolite( s) of the parent compound after –COOH into the compounds. phase I metabolism; however, in some cases,• Reaction types: oxidation, reduction, and the parent compound itself can be subject to hydrolysis phase II metabolism. • Function: conjugation (or derivatization) of• Enzymes: functional groups of a compound or its• Oxygenases and oxidases: Cytochrome P450 (P450 metabolite(s) with endogenous substrates. or CYP), flavincontaining • Reaction types: glucuronidation, sulfation,• monooxygenase (FMO), peroxidase, monoamine oxidase(MAO), alcohol dehydrogenase, aldehyde glutathione-conjugation, Nacetylation, dehydrogenase, and xanthine 0xidase. Reductase: methylation and conjugation with amino acids Aldo-keto reductase and quinone reductase. (e.g., glycine, taurine, glutamic acid).• Hydrolytic enzymes: esterase, amidase, aldehyde oxidase, and alkylhydrazine • Enzymes: Uridine diphosphate-Glucuronosyltransferase (UDPGT): sulfotransferase (ST), N-acetyltransferase,• oxidase. glutathione S-transferase (GST),methyl transferase, and• Enzymes that scavenge reduced oxygen: amino acid conjugating enzymes. Superoxide dismutases, catalase, • Glucuronidation by uridine diphosphate-• glutathione peroxidase, epoxide hydrolase, y- glucuronosyltransferase; Sulfation by sulfotransferase glutamyl transferase, • 3. Acetylation by N-acetyltransferase; Glutathione• dipeptidase, and cysteine conjugate β-lyase conjugation by glutathione S-transferase;. Methylation by methyl transferase; Amino acid conjugation
PHASE I BIOTRANSFORMATION Oxidation• Oxidation by cytochrome P450 isozymes (microsomal mixed- functionoxidases).• Oxidation by enzymes other than cytochrome P450s—most of these• (a) oxidation of alcohol by alcohol dehydrogenase,• (b) oxidation of aldehyde by aldehyde dehydrogenase,• (c) N-dealkylation by monoamineoxidase.
Phase I ReactionsOxidation :• Oxidative reactions are most important metabolic reactions, asenergy in animals is derived by oxidative combustion of organicmolecules containing carbon and hydrogen atoms.• The oxidative reactions are important for drugs because theyincrease hydrophilicity of drugs by introducing polar functionalgroups such as -OH.• Oxidation of drugs is non-specifically catalysed by a number ofenzymes located primarily in the microsomes. Some of theoxidation reactions are also catalysed by non-microsomal enzymes(e.g., aldehyde dehydrogenase, xanthine oxidase and monoamineoxidase).
The most important group of oxidative enzymes are microsomalmonooxygcnases or mixed function oxidases (MFO).These enzymes are located mainly in the hepatic endoplasmicreticulum and require both molecular oxygen (02) and reducingNADPH to effect the chemical reaction.Mixed function oxidase name was proposed in order tocharacterise the mixed function of the oxygen molecule, which isessentially required by a number of enzymes located in themicrosomes.
The term monooxygenses for the enzymes was proposed as theyincorporate only one atom of molecular oxygen into the organic substratewith concomitant reduction of the second oxygen atom to water.The overall stoichiometry of the reaction involving the substrate RH whichyields the product ROH, is given by the following reaction: MFORH+02+NADPH+H+ ----------------► R0H+H20+NADP+The most important component of mixed function oxidases is thecytochrome P-450 because it binds to the substrate and activates oxygen.The wide distribution of cytochrome P-450 containing MFOs in varyingorgans makes it the most important group of enzymes involved in thebiotransformation of drugs.
The cytochrome P-450 ENZYMES• Superfamily of haem-thiolate proteins that are widely distributedacross all living creatures.• The name given to this group of proteins because their reducedform binds with carbon monoxide to form a complex, which hasmaximum absorbance at 450 nm.• Depending upon the extent of amino acid sequence homology, thecytochrome P-450 (CYP) enzymes superfamily contains a number ofisoenzymes, the relative amount of which differs among species andamong individuals of the same species.• These isoenzymes are grouped into various families designated byArabic numbers 1, 2, 3 (sequence that are greater than 40% identicalbelong to the same family), each having several subfamiliesdesignated by Capital letter A, B, C, while individual isoenzymes areagain allotted Arabic numbers 1.2,3 (e.g., CYP1A1, CYP1A2, etc.).
ROLE OF CYP ENZYMES IN HEPATIC DRUG METABOLISMIn human beings, of the 1000 currently known cytochrome P-450s, about 50 are functionallyactive. These are categorised into 17 families, out of which the isoenzymes CYP3A4 and CYP2D6carry out biotransformation of largest number of drugs.RELATIVE HEPATIC CONTENT % DRUGS METABOLIZED OF CYP ENZYMES BY CYP ENZYMES CYP2E1 CYP2D6 7% 2% CYP 2C19 11% CYP 2C9 CYP 2C 14% CYP2D6 17% 23% OTHER 36% CYP 1A2 CYP 1A2 14% 12% CYP 3A4-5 CYP2E1 CYP 3A4-5 26% 5% 33%
Participation of the CYP Enzymes in Metabolism of SomeClinically Important DrugsCYP Enzyme Examples of substrates1A1 Caffeine, Testosterone, R-Warfarin1A2 Acetaminophen, Caffeine, Phenacetin, R-Warfarin2A6 17 -Estradiol, Testosterone2B6 Cyclophosphamide, Erythromycin, Testosterone2C-family Acetaminophen, Tolbutamide (2C9); Hexobarbital, S- Warfarin (2C9,19); Phenytoin, Testosterone, R- Warfarin, Zidovudine (2C8,9,19);2E1 Acetaminophen, Caffeine, Chlorzoxazone, Halothane2D6 Acetaminophen, Codeine, Debrisoquine3A4 Acetaminophen, Caffeine, Carbamazepine, Codeine, Cortisol, Erythromycin, Cyclophosphamide, S- and R- Warfarin, Phenytoin, Testosterone, Halothane, Zidovudine
2. Reduction :ReductionEnzymes responsible for reduction of xenobiotics require NADPH as a cofactor.Substrates for reductive reactions include azo- or nitrocompounds, epoxides,heterocyclic compounds, and halogenated hydrocarbons:(a) Azo or nitroreduction by cytochrome P450;(b) Carbonyl (aldehyde or ketone) reduction by aldehyde reductase, aldose reductase, carbonyl reductase, quinone reductase(c) other reductions including disulfide reduction, sulfoxide reduction, and reductive dehalogenation.
The acceptance of one or more electron(s) or their equivalent from anothersubstrate.Reductive reactions, which usually involve addition of hydrogen to the drugmolecule, occur less frequently than the oxidative reactions.Biotransformation by reduction is also capable of generating polar functionalgroups such as hydroxy and amino groups, which can undergo furtherbiotrans-formation. Many reductive reactions are exact opposite of the oxidative reactions (reversible reactions) catalysed cither by the same enzyme (true reversible reaction) or by different enzymes (apparent reversible reactions). Such reversible reactions usually lead to conversion of inactive metabolite into active drug, thereby delaying drug removal from the body.
3. Hydrolysis :Esters, amides, hydrazides, and carbamates can be hydrolyzed byvariousenzymes. The hydrolytic reactions, contrary to oxidative or reductive reactions, do not involve change in the state of oxidation of the substrate, but involve the cleavage of drug molecule by taking up a molecule of water.The hydrolytic enzymes that metabolise drugs are the ones that act on endogenous substances, and their activity is not confined to liver as they are found in many other organs like kidneys, intestine, plasma, etc.A number of drugs with ester, ether, amide and hydrazide linkages undergo hydrolysis. Important examples are cholinesters, procaine, procainamide, and pethidine.
PHASE II REACTIONS Phase II or conjugation (Latin, conjugatus = yoked together) reactions involve combination of the drug or its phase I metabolite with an endogenous substance to form a highly polar product, which can be efficiently excreted from the body. In the biotransformation of drugs, such products or metabolites have two parts: Exocon, the portion derived from exogenous compound or xenobiotic, Endocon, the portion derived from endogenous substance. Conjugation reactions have high energy requirement and they often utilise suitable enzymes for the reactions.
The endogenous substances (endocons) for conjugation reactions are derived mainly from carbohydrates or amino acids and usually possess large molecular size. They are strongly polar or ionic in nature in order to render the substrate water-soluble. The molecular weight of the conjugate (metabolite) is important for determining its route of excretion.High molecular weight conjugates are excreted predominantly in bile (e.g., glutathione exclusively, glucuronide mainly),while low molecular weight conjugates are excreted mainly in the urine.As the availability of endogenous conjugating substance is limited, saturation of this process is possible and the unconjugated drug/metabolite may precipitate toxicity.
1. Conjugation with glucuronic a./ Glucuronidation Conjugation with glucuronic acid (glucuronide conjugation or glucuroni-dation) is the most common and most important phase II reaction in vertebrates, except cats and fish. The biochemical donor (cofactor) of glucuronic acid is uridine diphosphate«-D-glucuronicacid (UDPGA) and the reaction is carried out by enzyme uridine diphosphate-glucuronyl transferase (UDP-giucuronyl transferase; glucuronyl transferase). Glucuronyl transferase is present in microsomes of most tissues but liver is the most active site of glucuronide synthesis.
Glucuronidation can take place in most body tissues because the glucuronic acid donor UDPGA is present in abundant quantity in body, unlike donors involved in other phase II reactions. In cats, there is reduced glucuronyl transferase activity, while in fish there is deficiency of endogenous glucuronic acid donor. The limited capacity of this metabolic pathway in cats may increase the duration of action, pharmacological response and potential of toxicity of several lipid-soluble drugs (e.g., aspirin) in this species.
A large number of drugs undergo glucuronidation including morphine, paracetamol and desipramine. Certain endogenous substances such as steroids, bilirubin, catechols and thyroxine also form glucuronides. Deconjugaiion process: Occasionally some glucuronide conjugates that are excreted in bile undergo deconjugation process in the intestine mainly mediated by β glucuronidase enzyme. This releases free and active drug in the intestine, which may be reabsorbed and undergo entero-hepatic cycling. Deconjugation is an important process because it often prolongs the pharmacological effects of drugs and/or produces toxic effects.
2. Conjugation with sulphate/ Sulphation: Conjugation with sulphate (sulphate conjugation, sulphoconjugation orsulphation) is similar to glucuronidation but is catalysed by non-microsomal enzymes and occurs less commonly. The endogenous donor of the sulphate group is 3- phosphoadenosine-5-phosphosulphate (PAPS), and enzyme catalysing the reaction is sulphotransferase
The conjugates of sulphate are referred to as sulphate esterconjugates or ethereal sulphates. Unlike glucuronideconjugation, sulphoconjugation in mammals is less importantbecause the PAPS donor that transfers sulphate to the substrateis easily depleted. Capacity for sulphate conjugation is limited in pigs. Howeverin cats, where glucuronidation is deficient, sulphate conjugationis important. Functional groups capable of forming sulphateconjugates include phenols, alcohols, arylamines, N-hydroxylamines and N-hydroxyamides. Drugs undergoing sulphate conjugation includechloramphenicol, phenols, and adrenal and sex steroids.
3. Conjugation with methyl group/ Methylation : Conjugation with methyl group (methyl conjugation or methylation) involves transfer of a methyl group (-CH3) from the cofactor S-adenosyl methionine (SAM) to the acceptor substrate by various methyl transferase enzymes. Methylation reaction is of lesser importance for drugs, but is more important for biosynthesis (e.g., adrenaline, melatonin) and | Inactivation (e.g., histamine) of endogenous amines. Occasionally, the metabolites formed are not polar or water- soluble and may possess equal or greater activity than the parent compound (e.g., adrenaline synthesised from noradrenaline).
4. Conjugation with glutathione and mercapturic acid formation. Conjugation with glutathione (glutathione conjugation) and mercapturic acid formationis a minor but important metabolic pathway in animals. Glutathione (GSH, G=glutathione and SH = active-SH group) is a tripeptide havingglutamic acid, cysteine and glycine. It has a strong nucleophilic character due to the presence of a -SH (thiol) group in itsstructure. Thus, it conjugates with electrophilic substrates, a number of which arepotentially toxic compounds, and protects the tissues from their adverse effects. The interaction between the substrate and the GSH is catalysed by enzyme glutathione- S-transferase, which is located in the soluble fraction of liver homogenates. The glutathione conjugate either due to its high molecular weight is excreted as such in the bile or is further metabolised to form mercapturic acid conjugate that is excreted in the urine.
5. Conjugation with acetyl group/ Acetylation : Conjugation with acetyl group (acetylation) is an important metabolic pathway for drugs containing the amino groups. The cofactor for these reactions is acetyl coenzyme A and the enzymes are non-microsomal N-acetyl transferases, located in the soluble fraction of cells of various tissues. Acetylation is not a true detoxification process because occasionally it results in decrease in water solubility of an amine and. thus, increase in its toxicity (e.g., sulphonamides).
Acetylation is the primary route of biotransformation of sulphonamide compounds. Dogs and foxes do not acetylate the aromatic amino groups due to deficiency of arylamine acetyltransferase enzyme. Conjugation with amino acids : Conjugation with amino acids occurs to a limited extent in animals because of limited availability of amino acids. The most important reaction involves conjugation with glycine. Conjugation with other amino acids like glutamine in man and ornithine in birds is also seen. Examples of drugs forming glycine or glutamine conjugates are salicylic acid, nicotinic acid and cholic acid.
Conjugation with thiosulphate : Conjugation with thiosulphate isan important reaction in the detoxification of cyanide. Conjugationof cyanide ion involves transfer of sulphur atom from thethiosulphate to the cyanide ion in presence of enzyme rhodancseto form inactive thiocyanate.Thiocyanate formed is much less toxic than the cyanide (truedetoxification) and it is excreted in urine.
INDUCTION OF METABOLISM Administration of certain xenobiotics sometimes results in aselective increase in the concentration of metabolizing enzymes inboth phase I and II metabolism, and thereby in their activities Enzyme induction becomes important especially whenpolypharmacy involves drugs with narrow therapeutic windows, sincethe induced drug metabolism could result in a significant decrease inits exposure and therapeutic effects. In addition, enzyme induction may cause toxicity, associated withincreased production of toxic metabolites. Mechanisms of Induction Stimulation of transcription of genes and/or translation of proteins,and/or stabilization of mRNA and/or enzymes by inducers, resulting inelevated enzyme levels.
Stimulation of preexisting enzymes resulting in apparentenzyme induction without an increase in enzyme synthesis (thisis more common in vitro than in vivo). In many cases, the details of the induction mechanisms areunknown. TWO receptors have been identified for CYPlA1/2 andCYP4A1/2induction:(a) Ah (aromatic hydrocarbon) receptor in cytosol, whichregulates enzyme (CYP1 A1 and 1A2) induction by polycyclicaromatic hydrocarbon (PAH)-type inducers;(b) Peroxisome proliferator activated receptor (PPAR), where hypolipidemic agents cause peroxisome proliferation in rats(CYP4A1 and 4A2);-humans have low PPAR and show noeffects from hypolipidemic agents.
Characteristics of Induction Induction is a function of intact cells and cannot be achieved by treatingisolated cell fractions such as microsomes with inducers. Evaluation of enzyme induction is usually conducted in ex vivo experiments,ie., treating animals in vivo with potential inducers and measuring enzymeactivities in vitro or in cell-based in vitro preparations such as hepatocytes, liverslices, or cell lines. Recent studies have demonstrated that primary cultures of hepatocytes canbe used for studying the inducibility of metabolizing enzymes such as P450 undercertain incubation conditions Enzyme induction is usually inducer-concentration–dependent. The extent ofinduction increases as the inducer concentration increases; however, abovecertain values, induction starts to decline. In general, inducers increase the content of endoplasmic reticulum withinhepatocytes as well as liver weight. In some cases, an inducer induces enzymes responsible for its ownmetabolism (so-called “autoinduction”).
Induction of Drug Metabolising Enzymes Several drugs and chemicals have ability to increase the drug metabolising activity of enzymes called as enzyme induction. These drugs known as enzyme inducers mainly interact with DNA and increase the synthesis of microsomal enzyme proteins, especially cytochrome P-450 and glucuronyl transferase. As a result, there is enhanced metabolism of endogenous substances (e.g., sex steroids) and drugs metabolised by microsomal enzymes. Some drugs (e.g., carbamazepine and rifampicin) may stimulate their own metabolism, the phenomenon being called as auto-induction or self induction.
Since different cytochrome P450 isoenzymes are involved in themetabolism of different drugs, enzyme induction by one drug affectsmetabolism of only those drugs, which are substrate for the inducedisoenzyme.However, some drugs like Phenobarbitone may affect metabolism of alarge number of drugs because they induce isoenzymes like CYP3A4 andCYP2D6 which act on many drugs.Enzyme inducers are generally lipid-soluble compounds with relatively longplasma half-lives.Repeated administration of inducers for a few days (3 to 10 days) is oftenrequired for enzyme induction, and on stoppage of drug administration,the enzymes return to their original value over 1 to 3 weeks.Non-microsomal enzymes are not known to be induced by any drug orchemical.
Clinical importance of enzyme induction It reduces efficacy and potency of drugs metabolised by these enzymes. It reduces plasma half-life and duration of action of drugs. It enhances drug tolerance. It increases drug toxicity by enhancing concentration of metabolite, if metabolite is toxic. It increases chances of drug interactions. It alters physiological status of animal due to altered metabolism of endogenous compounds like sex steroids.
Inhibition of Drug Metabolising Enzymes Contrary to metabolising enzyme induction, several drugs orchemicals have the ability to decrease the drug metabolising activity ofcertain enzymes called as enzyme inhibition. Enzyme inhibition can be either non-specific of microsomal enzymesor specific of some non-microsomal enzymes (e.g., monoamine oxidase,cholinesterase and aldehyde dehydrogenase). The inhibition of hepatic microsomal enzymes mainly occurs due toadministration of hepatotoxic agents,which cause either rise in the rate of enzyme degradation (e.g., carbontetrachloride and carbon disulphide) or fall in the rate of enzyme synthesis(e.g., Puromycin and Dactinomycin).
Nutritional deficiency, hormonal imbalance or hepaticdysfunction, etc.also inhibit microsomal enzymes indirectly. Inhibition of non-microsomal enzymes with specific functionusually results when Structurally similar compounds compete forthe active site on the enzymes. Such an inhibition is usually rapid (a single dose of inhibitormay be sufficient) and clinically more important than the non-specific microsomal enzyme inhibition. Enzyme inhibition generally results in depressed metabolismof drugs.As a result, the plasma hall-life, duration of action, and efficacyas well toxicity of the object drug (whose metabolism has beeninhibited) are significantly enhanced.
In case the drug undergoes hepatic first-pass effect, thebioavailability and toxicity Of the drug will be markedly increasedin presence of enzyme inhibition. Enzyme inhibition may alsoproduce undesirable drug interactions. In therapeutics, some specific enzyme inhibitors likemonoamine oxidase inhibitors, cholinesterase inhibitors andangiotensin converting enzyme (ACE) inhibitors are purposelyused for producing desirable pharmacological actions
Inducing Agents In general, enzyme inducers are lipophilic at physiological pH andexhibit relatively long t 1/2 with high accumulation in the liver.Different classes of enzyme inducers.1. Barbiturates: Phenobarbitone, Phenobarbital.2. Polycyclic aromatic hydrocarbons (PAH): 3-methylcholanthrene (3-MC),2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD), β-naphthoflavone β ( -NF).3. Steroids: Pregnenalone 16-α -carbonitrile (PCN), Dexamethasone.4. Simple hydrocarbons with aliphatic chains: Ethanol (chronic), Acetone,5. Hypolipidemic agents: Clofibrate, lauric acids.6. Macrolide antibiotics: Triacetyloleandomycin (TAO).7. A wide variety of structurally unrelated compounds: e.g., Antipyrine,Carisoniazid. Bamazepine, Phenytoin, and Rifampicin
EXTRAHEPATIC METABOLISM Most tissues have some metabolic activity; however,quantitatively the liver is by far the most important organ for drugmetabolism.Important organs for extrahepatic metabolism include theintestine (enterocytes and intestinal microflora), kidney, lung,plasma, blood cells, placenta, skin, and brain. In general, the extent of metabolism in the major extrahepaticdrug-metabolizing organs such as the small intestine, kidney, andlung is approximately 10–20% of the hepatic metabolism. Less than 5% of extrahepatic metabolism compared to hepaticmetabolism can be considered low with negligiblepharmacokinetic implications
First-Pass Effect/First-Pass Metabolism First-pass effect (first-pass metabolism or pre-systemic metabolism) may be defined as the loss of drug through biotransformation before it enters systemic circulation. This may occur during passage of drug for first time (therefore called first-pass effect/metabolism) through intestine or liver after oral administration. Intestinal first-pass effect: In this type, drugs are metabolised in the gastrointestinal tract by enzymes present in either gut mucosa or gut lumen before they are absorbed Recent studies have indicated that P450 isoforms such as CYP2C19 and 3A4 in enterocytes might play an important role in the presystemic intestinal metabolism of drugs and the large interindividual variability in systemic exposure after oral administration The cytochrome P450 content of the intestine is about 35% of the hepatic content in the rabbit, but accounts for only 4% of the hepatic content in the mouse. Cytochrome P450 levels and activities are highest in the duodenum near the pyrolus, and then decrease toward the colon A similar trend in regional activity levels along the intestine has been observed for glucuronide, sulfate, and glutathione conjugating enzymes.
Microorganisms present in the GI tract also inactivate some drugs. Such drugs are not suitable by oral administration due to poor bioavailability (e.g., catecholamines). Hepatic first-pass effect: In this type, drugs are suitably absorbed across the GI tract and enter portal circulation, but they are rapidly and significantly metabolised during the first passage through the liver. (Normally, when a drug is absorbed across GI tract, it first enters the portal vein and passes through liver before it reaches the systemic circulation). Such drugs are also not/less suitable by oral administration due to their poor bioavailability. Examples of drugs undergoing significant hepatic first-pass effect include Propranolol, Lignocaine and Nitro-glycerine.
The rate and extent of first-pass intestinal metabolism of a drugafter oral administration are dependent on various physiologicalfactors1. Site of absorption: If the absorption site in the intestine is differentfrom the metabolic site, first-pass intestinal metabolism of a drugmay not be significant.2. Intracellular residence time of drug molecules in enterocytes: Thelonger the drug molecules stay in the enterocytes prior to enteringthe mesenteric vein, the more extensive the metabolism.3. Diffusional barrier between splanchnic bed and enterocytes: Thelower the diffusibility of a drug from the enterocytes to themesenteric vein, the longer its residence time.4. Mucosal blood flow: Blood in the splanchnic bed can act as a sinkto carry drug molecules away from the enterocytes, which reducesintracellular residence time of drug in the enterocytes
Renal Metabolism In addition to physiological functions of homeostasis in water andelectrolytes and the excretion of endogenous and exogenous compoundsfrom the body, the kidneys are the site of significant biotransformationactivities for both phase I and phase II metabolism. The renal cortex, outer medulla, and inner medulla exhibit differentprofiles of drug metabolism, which appears to be due to heterogeneousdistribution of metabolizing enzymes along the nephron. Most metabolizing enzymes are localized mainly in the proximal tubules,although various enzymes are distributed in all segments of the nephron The pattern of renal blood flow, pH of the urine, and the urinaryconcentrating mechanism can provide an environment that facilitates theprecipitation of certain compounds, including metabolites formed within thekidneys. The high concentration or crystallization of xenobiotics and/or theirmetabolites can potentially cause significant renal impairment in specificregions of the kidneys.
Metabolism in Blood Blood contains various proteins and enzymes. As metabolizing enzymes, esterases, including cholinesterase,arylesterase, and carboxylesterase, have the most significant effectson hydrolysis of compounds with ester, carbamate, or phosphatebonds in blood . Esterase activity can be found mainly in plasma, with less activity inred blood cells. Plasma albumin itself may also act as an esterase under certainconditions. For instance, albumin contributes about 20% of the total hydrolysisof aspirin to salicylic acid in human plasma. The esterase activity in blood seems to be more extensive in smallanimals such as rats than in large animals and humans. Limited, yetsignificant monoamine oxidase activities can be also found in blood.