MODULE No.26: Drug Metabolism

Transcription

1 SUBJECT Paper No. and Title Module No. and Title Module Tag PAPER No. 9: Drugs of Abuse MODULE No. 26: Drug Metabolism FSC_P9_M26

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Sites of Drug Metabolism 4. Pathways of Drug Metabolism 5. Phase- I Reaction 6. Phase- II Reaction 7. Summary

3 1. Learning Outcomes After studying this module, you will be able to know about- The significance of Drug Metabolism Various pathways for the metabolism of drugs in body Stages and processes of Drug metabolism 2. Introduction Xenobiotic metabolism, which also includes drug metabolism, has become a chief pharmacological science with actual applicability to biology, therapeutics and toxicology. Drug metabolism is also of pronounced significance in curative chemistry because it influences in qualitative, quantitative, and kinetic terms the deactivation, activation, detoxification, and toxification of the vast majority of drugs. As a result, medicinal chemists engaged in drug discovery and development should be able to integrate metabolic considerations into drug design. To do so, however, requires a fair or even good knowledge of xenobiotic metabolism. Drugs are considered xenobiotics and most are extensively metabolized in humans. Drug metabolism displays the biotransformation of an endogenous or exogenous molecule by some or added enzymes to moieties, which are more hydrophilic and can be effortlessly expelled. Metabolism of a drug usually lessens its capability to give out a pharmacological action, it can also consequence in to metabolite which has significantly greater potency, and thereby, contributes to the overall pharmacological effect of the drug. In the case of prodrugs, biotransformation is required to produce the pharmacologically active metabolite. Understanding the metabolism, or biotransformation, of xenobiotics has come to be regarded as fundamental to appreciating the toxic mechanisms of chemicals, be they drugs, industrial chemicals, pesticides, or other molecule foreign to the body. Metabolism is an integral part of drug elimination. As well as facilitating excretion of a drug, it may also affect the pharmacological response of a drug by modifying its potency and/or duration of action.

4 3. Sites of Drug Metabolism Xenobiotic metabolizing enzymes are present greatest in the tissues of the body with the maximum levels found in the tissues of the gastrointestinal tract (small and large intestines, liver). Drugs which are verbally directed, absorbed by the gut, and taken to the liver, can be comprehensively metabolized. Liver is known as the main Metabolic Clearing House for individually endogenous chemicals such as steroid hormones, cholesterol, proteins etc.), and xenobiotics. The high concentration of xenobioticmetabolizing enzymes located in the epithelial cells of the Gastro Intestinal tract is responsible for the initial metabolic processing of most oral medications. This should be considered the initial site for first pass metabolism of drugs. 4. Pathways of Drug Metabolism Metabolism of Drugs is accomplished in two distinct phases:- Phase - I reaction (Non- synthetic Phase): Phase- II metabolism is categorized as the functional phase of drug metabolism. The reactions carried out by Phase - I enzymes customarily may result in activation, change or inactivation of drug. This involves a transformation in drug molecule. It involves oxidation, reduction or hydrolysis. This Phase - II reaction (Synthetic Phase): Phase - II enzymes enable the elimination of drugs and the inactivation of electrophilic and potentially toxic metabolites produced during oxidation. This phase comprises of the formation of the conjugates with drugs and their metabolites made during the Phase I reaction. The formation of the conjugates takes place with the endogenous substances for example amino acids and carbohydrates. In case of Phase I reactions the resultant is the biological inactivation of the drug, a better water soluble metabolite is produced with increased molecular weight in the Phase II reaction, which functions to expedite the elimination of the drug from the tissue.

5 5. Phase - I Reactions Phase- I reactions are also called as non-synthetic reactions. These reactions may occur due to reduction, oxidation, hydrolysis, cyclization and decyclization, removal/addition of hydrogen, carried out by mixed function oxidases, repeatedly in the liver. 5.1 Oxidation Cytochrome P450 monooxygenase system The cytochrome P450 superfamily (CYP) is a large and diverse group of enzymes. The oxidation of the organic substances is catalyzed by most of the CYP enzymes. The substrates of CYP enzymes comprise mainly the metabolic intermediates for example, steroidal hormones and lipids and also the xenobiotic substances for example, drugs and other toxic chemicals. The P450- dependent monooxygenase system is central to the metabolism of most xenobiotics. Not only is it the crucial enzymatic system for metabolism of several xenobiotics, but it is also involved as the preliminary functionalization step in the further metabolism of many others. Accordingly, P450 plays essential roles in a number of areas of research, including biochemistry, pharmacology, toxicology, physiology, and medicine. On primary assessment, it seems that P450 can catalyze an incomprehensible number of reactions. However, on closer inspection, a degree of commonality exists among these reactions. The first area of commonality is that most of the reactions represent oxidations. Second, the reactions transform lipophilic substrates to more hydrophilic products. Third, many of the reactions can be understood as hydroxylations Flavin-containing monooxygenase system Tertiary amines such as trimethylamine and dimethylamine had long been recognized to be metabolized to N-oxides by a microsomal amine oxidase that was not dependent on CYP. This enzyme, now known as the microsomal flavin-containing monooxygenase

6 (FMO), is also dependent on NADPH and O2, and has been purified to homogeneity from a number of species.

7 Isolation and characterization of the enzyme from liver and lung samples provided evidence of undoubtedly distinct physicochemical properties and substrate specificities proposing the manifestation of at least two different isoforms. Subsequent studies have substantiated the presence of multiple forms of the enzyme. Most FMO substrates are also substrates for CYP. Since both enzymes are microsomal and need NADPH and oxygen, it is challenging to differentiate which enzyme is responsible for oxidation without the use of methods involving specific inactivation or inhibition or one or the other of these enzymes while concurrently examining the metabolic involvement of the other Alcohol dehydrogenase and Aldehyde dehydrogenase Alcohol dehydrogenases catalyze the conversion of alcohols to aldehydes or ketones. The alcohol dehydrogenase reaction is reversible, with the carbonyl compounds being reduced to alcohols. This enzyme is found in the soluble fraction of the liver, kidney, and lung and is perhaps the most important enzyme involved in the metabolism of foreign alcohols. Alcohol dehydrogenase is a dimer whose subunits can occur in several forms under genetic control, as a consequence giving rise to a large number of variants of the enzyme. RCH2OH + NAD + RCHO + NADH + H + Aldehydes are produced from a variety of endogenous and exogenous substrates. Endogenous aldehydes may be formed during metabolism of amino acids, lipids, carbohydrates, vitamins, steroids, and biogenic amines. Aldehyde dehydrogenases are vital in facilitating to improve some of the toxic effects of aldehyde generation. This enzyme catalyzes the formation of acids from aliphatic and aromatic aldehydes; the acids are then available as substrates for conjugating enzymes. RCHO + NAD + RCOOH + NADH + H +

8 5.1.4 Amine oxidase The most significant role of amine oxidases seems to be the oxidation of amines formed during normal processes. Two types of amine oxidases are concerned with oxidative deamination of both endogenous and exogenous amines. The monoamine oxidases are a family of flavoproteins found in the mitochondria of a wide-ranging variety of tissues such as brain, liver, intestine, kidney and blood platelets. They are a set of analogous enzymes with coinciding specificities and inhibition. Although the enzyme in the central nervous system is concerned chiefly with neurotransmitter turnover, that in the liver will deaminate primary, secondary, and tertiary aliphatic amines, reaction rates with the primary amines being more rapidly. Electron withdrawing substitutions on an aromatic ring increase the reaction rate, whereas compounds with a methyl group on the α-carbon such as amphetamine and ephedrine are not metabolized. Diamine oxidases are enzymes that also oxidize amines to aldehydes. Secondary and tertiary amines are not metabolized. Diamine oxidases are characteristically soluble pyridoxal phosphate-containing proteins that also contain copper. They have been found in a number of tissues, including liver, kidney, intestine, and placenta. 5.2 Reduction Several functional groups, such as nitro, carbonyl, disulfide sulfoxide, diazo, alkene, and pentavalent arsenic, are prone to reduction, although in many cases it is difficult to tell whether the reaction proceeds enzymatically or non-enzymatically by the action of such biologic reducing agents as reduced flavins or reduced pyridine nucleotides. In certain circumstances, such as the reduction of the double bound in Cinnamic acid, the reaction has been credited to the intestinal microflora.

9 5.3 Hydrolysis Enzymes with carboxylesterase and amidases activity are widely distributed in the body, occurring in many tissues and in both microsomal and soluble fractions. Many xenobiotics and their Phase - I metabolites contain a carboxyl ester, an amide bond, or an epoxide that masks hydrophilic functional groups, such as alcohols, carboxylic acids, and amines. Hydrolysis generally contests with other detoxification reactions, but esterases are in very high content in many tissues, especially liver, and their affinity is low enough such that esterase /amidases mediated hydrolysis typically predominate. 6. Phase- II Metabolism Products of Phase - I metabolism and other xenobiotics containing functional groups such as amino, hydroxyl, epoxide, carboxyl, or halogen can undergo conjugation reactions with endogenous metabolites, these conjugations being jointly termed as Phase - II reactions. Conjugation reactions typically consist of metabolite activation by some high-energy intermediate and have been broadly categorized into two types, first, the Type I, in which an activated conjugating agent combines with the substrate to produce the conjugated product, and second, the Type II, in which the substrate is activated and then combines with an amino acid to yield a conjugated product. The formation of sulfates and glycosides are examples of type I, whereas type II consists primarily of amino acid conjugation. 6.1 Glucuronide Conjugation The glucuronidation reaction is one of the major pathways for elimination of many lipophilic xenobiotics and endobiotics from the body. The mechanism for this conjugation involves the reaction of one of many possible functional groups with the sugar derivative, uridine 5 -diphosphoglucuronic acid.

10 Glucuronide conjugation by and large results in the formation of products that are less biologically and chemically reactive. This combined with their better polarity and greater susceptibility to excretion, contributes prominently to the detoxication of most xenobiotics. However, there are at present many models where glucuronide conjugation results in greater toxicity. 6.2 Sulfate Conjugation Sulfation and sulfate conjugate hydrolysis, catalyzed by many members of the sulfotransferases and sulfatase enzyme super families, play crucial roles in the metabolism and disposition of many xenobiotics and endogenous substrates. Reactions of the sulfotransferase enzyme with various xenobiotics, including alcohols, arylamines, and phenols, results in the production of water soluble sulfate esters that often are easily eliminated from the organism. Although commonly these reactions are important in detoxication, they have also been shown to be involved in carcinogen activation, cellular signaling pathways, prodrug processing and the regulation of several potent endogenous chemicals including steroids, catechols, and thyroid hormones. 6.3 Glutathione S-Transferases (GSTs) The GSTs, the family of enzymes that catalyzes the preliminary step, are extensively distributed, being found in fundamentally all clusters of alive organisms. Although the best known examples have been described from the soluble fraction of mammalian liver, these enzymes have also been described in microsomes. The preliminary reaction is the conjugation of xenobiotics having electrophilic substituents with glutathione, a reaction catalyzed by one of the various forms of GST. The complete sequence, mainly the preliminary reaction is exceptionally important in toxicology because, by removing reactive electrophiles, vital nucleophilic groups in macromolecules such as proteins and nucleic acids are protected.

11 6.4 Methyltransferases The most common methyl donor is Sadenosyl methionine (SAM), which is formed from methionine and ATP. Even though these reactions may involve a decrease in water solubility, they are generally detoxication reactions. 7. Summary Drug metabolism is also of pronounced significance in curative chemistry because it influences in qualitative, quantitative, and kinetic terms the deactivation, activation, detoxification, and toxification of the vast majority of drugs. Metabolism is an integral part of drug elimination. As well as facilitating excretion of a drug, it may also affect the pharmacological response of a drug by modifying its potency and/or duration of action. The high concentration of xenobiotic-metabolizing enzymes located in the epithelial cells of the Gastro Intestinal tract is responsible for the initial metabolic processing of most oral medications. This should be considered the initial site for first pass metabolism of drugs. Two types of amine oxidases are concerned with oxidative deamination of both endogenous and exogenous amines. The monoamine oxidases are a family of flavoproteins found in the mitochondria of a wide-ranging variety of tissues such as brain, liver, intestine, kidney and blood platelets. Catalysts for xenobiotic transformation are incorporated into Phase- I and Phase- II reactions. Although occurring primarily in the liver, other organs such as kidneys, lungs, and dermal tissues have large capacities for these reactions. Phase- I reactions involve hydrolysis, reduction, and oxidation of chemicals to more hydrophilic, usually smaller, entities. Phase- II reactions follow with glucuronidation, sulfation, acetylation, methylation, and conjugation with amino acids of the Phase I metabolites.