BIOP BIOtransformation5.1

8/2/2019 BIOP BIOtransformation5.1

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Biotransformation of Drugs

The onset of pharmacological response depends upon twopharmacokinetic processes—

Drug absorption, andDrug distribution

The duration and intensity of action depend upon –

Tissue redistribution of drug, and

The rate of drug removal from the body/site of action, i.e. rateof elimination.

Elimination is the major process for removal of a drug fromthe body and termination of its action.

It is defined as the irreversible loss of drug from the body .Elimination occurs by two processes viz.

biotransformation and excretion.1



Biotransformation of drugs is defined as the chemical conversion of one form to another .

The term is used synonymously with metabolism.

The chemical changes are usually affected enzymatically inthe body and thus, the definition excludes chemical instabilityof a drug within the body; for

e.g. conversion of penicillin to penicilloic acid by the bacterialpenicillinase and mammalian enzymes is metabolism but itsdegradation by the stomach acid to penicillenic acid ischemical instability.

Need for Drug BiotransformationAll chemical substances that are not nutrients for the body and enter the body through, ingestion, inhalation or absorption are called as xenobiotics (Greek: xenos = foreign ) or exogenous

compounds. 2



Water-soluble agents undergo renal excretion

whereas lipid soluble substances are passively reabsorbedfrom the renal tubules into the blood after glomerularfiltration.

Thus, if such a phenomenon continues, drugs wouldaccumulate in the body and precipitate toxic reactions.

However, to prevent such a consequence, the body isarmed with the metabolic system which transforms thewater insoluble, lipophilic, nonpolar drugs into polar andwater-soluble products that can be easily excreted by the

kidneys and are poorly reabsorbed;Eg. hippuric acid, the metabolite of benzoic acid, is 2.5 times

more water-soluble.

Drug biotransformation is thus a detoxification process.

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Biotransformation Definitions:

Normally results in pharmacological inactivation of drugs, i.e. itresults in formation of metabolites with little or nopharmacological activity; e.g. conversion of phenytoin to p-hydroxy phenytoin.

Occasionally yields metabolites with equal activity; e.g.conversion of phenylbutazone to oxyphenbutazone.

Rarely leads to toxicological activation of drugs, i.e. it results information of metabolites with high tissue reactivity; e.g.conversion of paracetamol to reactive metabolites that causehepatic necrosis.

Inactive drugs (prodrugs) also depend upon biotransformation

for activation, the process being called as pharmacological activation ; e.g. conversion of enalapril to enalaprilat.

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Drug Metabolising Organs

Liver is the primary site for metabolism of almost all drugs because ofits relative richness in possessing a large variety of enzymes in largeamounts.

Metabolism by organs other than liver (called as extrahepaticmetabolism) is of minor importance

The decreasing order of drug metabolising ability of various organsis:

Liver > Lungs > Kidneys > Intestine > Placenta > Adrenals > Skin Brain, testes, muscles, spleen, etc. also metabolise drugs but to asmall extent.

Drug Metabolising Enzymes

The enzymes are broadly divided into 2 categories:Microsomal enzymes

Non-microsomal enzymes.

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Microsomal enzymes

The microsomal enzymes catalyse a majority of drugbiotransformation reactions.

The microsomes were first made to isolate endoplasmic reticulum of

the liver homogenate.

These vesicular fragments or microsomes are derived from roughendoplasmic reticulum

which shed their ribosomes to become smooth surfaced.

The large variety of microsomal enzymes catalyse a number ofoxidative, reductive and hydrolytic and glucuronidation reactions.

Non-microsomal enzymes

Include those that are present in soluble form in the cytoplasm and

those attached to the mitochondria but not to endoplasmic reticulum.These are also non-specific enzymes that catalyse few oxidativereactions, a number of reductive and hydrolytic reactions andconjugation reactions other than glucuronidation.

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CHEMICAL PATHWAYS OF DRUG BIOTRANSFORMATION

R.T.Williams , the leading pioneer in drug biotransformation research,divided the pathways of drug metabolism reactions into two general

categories—

Phase I reactions, and

Phase II reactions.

Phase I Reactions

These reactions generally precede phase II reactions and includeoxidative, reductive and hydrolytic reactions.

By way of these reactions, a polar functional group is either introduced orunmasked if already present on the otherwise lipid soluble substrate, e.g. -OH, -COOH, -NH2 and -SH.

Thus, phase I reactions are also called as functionalisation reactions.

These transformations are also called as asynthetic reactions, oppositeto the synthetic phase II reactions.

The resulting product of phase I reaction is susceptible to phase II

reactions. 7



Phase II Reactions

These reactions generally involve covalent attachment of small polarendogenous molecules such as glucuronic acid, sulphate, glycine,etc. to either unchanged drugs or phase I products having suitablefunctional groups viz. -OH, -COOH, -NH2 and -SH and form highlywater-soluble conjugates which are readily excretable by the kidneys(or bile). Thus, these reactions are called as conjugation reactions

Since the outcome of such processes are generally products with

increased molecular size they are also called as syntheticreactions.

Quite often, a phase I reaction may not yield a metabolite that issufficiently hydrophilic or pharmacologically inert

but conjugation reactions generally result in products with total lossof pharmacological activity and high polarity.

Hence, phase II reactions are better known as true detoxificationreactions. Since these reactions generally involve transfer ofmoieties to the substrate to be conjugated, the enzymes responsible

are called as transferases . 8



9

Sequence of phase I and phase II reactions yielding a range of products



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FACTORS AFFECTING BIOTRANSFORMATION OF DRUGS

The therapeutic efficacy, toxicity and biological half-life of a druggreatly depend upon its metabolic rate.

A number of factors may influence the rate of drug metabolism.

They are:

1. Physicochemical properties of the drug

2. Chemical factors:

a. Induction of drug metabolising enzymes

b. Inhibition of drug metabolising enzymes

c. Environmental chemicals3. Biological factors:

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3. Biological factors:

a. Species differences

b. Strain differences

c. Sex differences

d. Age

e. Diet

f. Altered physiologic factors:i. Pregnancy

ii. Hormonal imbalance

iii. Disease states

g. Temporal factors:i. Circadian rhythm

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PHYSICOCHEMICAL PROPERTIES OF THEDRUG

Molecular size and shape, pKa, acidity/basicity,lipophilicity and steric and electroniccharacteristics of a drug influence its interactionwith the active sites of enzymes and the

biotransformation processes to which it issubjected.

Biotransformation requires interaction of a drug

with an enzyme, Many drugs that undergometabolism exhibit stereo selective hepaticclearance.

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CHEMICAL FACTORS

Induction of Drug Metabolising Enzymes

The phenomenon of increased drug metabolising ability of the enzymes by several drugs and chemicals is called as enzyme induction and the agents which bring about such an effect are known

as enzyme inducers.

Most enzyme inducers have following properties –

1. They are lipophilic compounds.

2. They are substrate for the inducted enzymesystem.

3. They have long elimination half-lives.

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Mechanisms involved in enzyme induction are –

1. Increase in both liver size and liver blood flow.2. Increase in both total and microsomal proteincontent.

3. Increased stability of enzymes.

4. Increased synthesis of cytochrome P-450.

5. Decreased degradation of cytochrome P-450.

6. Proliferation of smooth endoplasmic reticulum.

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Two categories of inducers have been defined –

1. Phenobarbital type inducers: includes several drugsand pesticides which increase the rate of metabolism of

a large number of drugs.phenobarbital which can increase enzyme activity up to4 times.

2. Polycyclic hydrocarbon type inducers: such as 3-methyl cholanthrene and cigarette smoke whichstimulate the metabolic rate of few drugs.

Some drugs such as carbamazepine, meprobamate,cyclophosphamide, rifampicin, etc. stimulate their own metabolism, the phenomenon being called as auto-

induction or self-induction. 17



Consequences of enzyme induction includes –

Decrease in pharmacological activity of drugs

Increased activity where the metabolites are

active, and

Altered physiologic status due to enhancedmetabolism of endogenous compounds such

as sex hormones.

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Drugs Commonly

Affected by Them

Inducers

Drugs with Enhanced Metabolism

Barbiturates Coumarins, phenytoin, cortisol, testosterone,

oral contraceptives

Alcohol Pentobarbital, coumarins, phenytoin

Phenytoin Cortisol, coumarins, oral contraceptives,

tolbutamide

Rifampicin Coumarins, oral contraceptives, tolbutamide,

rifampicin

Cigarette Smoke Nicotine, amino azo dyes

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Inhibition of Drug Metabolising Enzymes

A decrease in the drug metabolising ability of an enzyme is called as enzyme inhibition. Theprocess of inhibition may be direct or indirect.

1. Direct Inhibition: may result from interactionat the enzymic site, the net outcome being achange in enzyme activity.

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Di t i hibiti b f th 3



Direct enzyme inhibition can occur by one of the 3mechanisms –

a. Competitive Inhibition : results when structurally similarcompounds compete for the same site on an enzyme. e.g.methacholine inhibits metabolism of acetylcholine by competingwith it for cholinesterase.

b. Non-competitive Inhibition : results when a structurallyunrelated agent interacts with the enzyme and prevents themetabolism of drugs.

Since the interaction is not structure-specific,

metals like lead, mercury and arsenic and organophosphorusinsecticides inhibit the enzymes non-competitively.

Isoniazid inhibits the metabolism of phenytoin by the samemechanism.

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P d t I hibiti lt h th t b li



c. Product Inhibition : results when the metabolicproduct competes with the substrate for the sameenzyme. The phenomenon is also called as

autoinhibition .

Certain specific inhibitors such as xanthine oxidaseinhibitors (e.g. allopurinol) and MAO inhibitors (e.g.phenelzine) also inhibit the enzyme activity directly.

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2. Indirect Inhibition: is brought about by one of the twomechanisms –

a. Repression : is defined as the decrease in enzyme content.

It may be due to a fall in the rate of enzyme synthesis asaffected by ethionine, puromycin and actinomycin D orbecause of rise in the rate of enzyme degradation such as bycarbon tetrachloride, carbon disulphide, disulphiram, etc.

b. Altered Physiology : due to nutritional deficiency orhormonal imbalance.

Enzyme inhibition is more important clinically than enzyme induction , especially for drugs with narrowtherapeutic index, e.g. anticoagulants, antiepileptics,hypoglycaemics, since it results in prolonged pharmacologicalaction with increased possibility of precipitation of toxic

effects. 23



Inhibitors Drugs with Decrea

Metabolism

MAO inhibitors Barbiturates, tyramine

Coumarins Phenytoin

Allopurinol 6-Mercaptopurine

PAS Phenytoin, hexobarbital

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Environmental Chemicals

Several environmental agents influence the drugmetabolising ability of enzymes.

Halogenated pesticides such as DDT(dichlorodiphenyltrichloroethane) and polycyclicaromatic hydrocarbons contained in cigarette smokehave enzyme induction effect.

Organophosphate insecticides and heavy metals suchas mercury, tin, nickel, cobalt and arsenic inhibit drugmetabolising ability of enzymes.

Other environmental factors that may influence drugmetabolism are temperature, altitude, pressure,atmosphere, etc.

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BIOLOGICAL FACTORS

Species DifferencesScreening of new therapeutic molecules toascertain their activity and toxicity requires studyin several laboratory animal species.

Differences in drug response due to speciesdifferences are taken into account whileextrapolating the data to man.

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Strain Differences/Pharmacogenetics

Enzymes influencing metabolic reactions are under the

genetic control.

A study of inter-subject variability in drug response (due to differences in, for example, rate of biotransformation) is called as pharmacogenetics.

The inter-subject variations in drug biotransformation mayeither be monogenically or polygenically controlled.

A polygenic control has been observed in studies in twins.

In identical twins (monozygotic), very little or no differencein the metabolism of phenylbutazone, and antipyrine wasdetected but large variations were apparent in fraternaltwins for the same drugs.

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Sex Differences

sex related differences in the rate of metabolismcould be attributed to regulation of such processes bysex hormones since variations between male andfemale.

Such sex differences are widely studied in rats; the

male rats have greater drug metabolising capacity.In humans, women metabolise benzodiazepinesslowly than men and

several studies show that women on contraceptivepills metabolise a number of drugs at a slow rate.

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AgeDifferences in the drug metabolic rate in different age groups are mainly dueto variations in the enzyme content, enzyme activity and haemodynamics

In neonates (upto 2 months), the microsomal enzyme system is not fullydeveloped and many drugs are biotransformed slowly;

for example, caffeine has a half-life of 4 days in neonates in comparison to 4hours in adults.

sulphonamides cause renal toxicity and paracetamol causes hepatotoxicity.

Children (between one year and 12 years) and older infants metaboliseseveral drugs much more rapidly than adults

for example, the theophylline half-life in children is two-third of that in adults.

In very elderly persons, the liver size is reduced, the microsomal enzymeactivity is decreased and hepatic blood flow also declines as a result ofreduced cardiac output all of which contribute to decreased metabolism ofdrugs. 29



Diet

The enzyme content and activity is altered by a number ofdietary components. In general –

Low protein diet decreases and high protein diet increases thedrug metabolising ability.

This is because the enzyme synthesis is promoted by proteindiet which also raises the level of amino acids for conjugation

with drugs.The protein-carbohydrate ratio in the diet is also important; ahigh ratio increases the microsomal mixed function oxidaseactivity.

Fat free diet depresses cytochrome P-450 levels

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Dietary deficiency of vitamins (e g vitamin A B2 B3



Dietary deficiency of vitamins (e.g. vitamin A, B2, B3,C and E) and minerals such as Fe, Ca, Mg, Cu and Znretard the metabolic activity of enzymes.

Starvation results in decreased amount of

glucuronides formed than under normal conditions.

Malnutrition in women results in enhancedmetabolism of sex hormones.

Alcohol ingestion results in a short-term decreasefollowed by an increase in the enzyme activity.

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Altered Physiological Factors



Altered Physiological Factors

Pregnancy: Studies in animals have shown that thematernal drug metabolising ability (of both phase I

and phase II reactions) is reduced during the laterstages of pregnancy. This was suggested as due tohigh levels of steroid hormones in circulation duringpregnancy.

Hormonal Imbalance: The effect of other hormonesis equally complex. Higher levels of one hormonemay inhibit the activity of few enzymes while inducing

that of others. Adrenolectomy, thyroidectomy andalloxan induced diabetes in animals showedimpairment in the enzyme activity with a subsequentfall in the rate of metabolism.

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Disease States: As liver is the primary site for



Disease States: As liver is the primary site formetabolism of most drugs, all pathologicconditions associated with it result in enhanced

half-lives of almost all drugs.

Thus, a reduction in hepatic drug metabolisingability is apparent in conditions such as hepatic

carcinoma, hepatitis, cirrhosis, obstructive jaundice, etc.

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Temporal Factors

Circadian Rhythm: variations in the enzyme activity with light cycle is called as circadian rhythm in

drug metabolism .

It has been observed that the enzyme activity ismaximum during early morning (6 to 9 a.m.) and

minimum in late afternoon (2 to 5 p.m.)which was suggested to correspond with the highand low serum levels of corticosterone (the serumcorticosterone level is dependent upon the light-

dark sequence of the day).

The half-life of metyrapone was shown to be 2.5times longer during the night than in the day, in

rats 34

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