METABOLISM OF DRUGS

METABOLISM

Biotransformation means chemical alteration of the drug in the body. It is needed to render nonpolar (lipid-soluble) compounds polar (lipidinsoluble) so that they are not reabsorbed in the renal tubules and are excreted. Most hydrophilic drugs, e.g. streptomycin, neostigmine, pancuronium, etc. are little biotransformed and are largely excreted unchanged. Mechanisms which metabolize drugs (essentially foreign substances) have developed to protect the body from ingested toxins.

The primary site for drug metabolism is liver; others are—kidney, intestine, lungs and plasma. 

Biotransformation of drugs may lead to the following. 

(i) Inactivation Most drugs and their active metabolites are rendered inactive or less active, e.g. ibuprofen, paracetamol, lidocaine, chloramphenicol, propranolol and its active metabolite 4-hydroxypropranolol. 

(ii) Active metabolite from an active drug Many drugs have been found to be partially converted to one or more active metabolite; the effects observed are the sum total of that due to the parent drug and its active metabolite(s). 

e.g; codeine → morphine

(iii) Activation of inactive drug Few drugs are inactive as such and need conversion in the body to one or more active metabolites. Such a drug is called a prodrug. The prodrug may offer advantages over the active form in being more stable, having better bioavailability or other desirable pharmacokinetic properties or less side effects and toxicity. Some prodrugs are activated selectively at the site of action.

e.g; levodopa → dopamine

Biotransformation reactions can be classified into: 

(a) Nonsynthetic/PhaseI/Functionalization reactions: a functional group is generated or exposed— metabolite may be active or inactive. 

(b) Synthetic/Conjugation/ Phase II reactions: an endogenous radical is conjugated to the drug— metabolite is mostly inactive; except few drugs, e.g. glucuronide conjugate of morphine and sulfate conjugate of minoxidil are active.

Nonsynthetic reactions 

(i) Oxidation: This reaction involves addition of oxygen/negatively charged radical or removal of hydrogen/positively charged radical. Oxidations are the most important drug metabolizing reactions. Various oxidation reactions are: 

a) Hydroxylation

Phenytoin → hydroxyphenytoin

b) Dealkylation

Codeine → morphine

c) S oxidation

Cimetidine → cimetidine sulfoxide

Oxidative reactions are mostly carried out by a group of monooxygenases in the liver, which in the final step involve a cytochrome P-450 haemoprotein, NADPH, cytochrome P-450 reductase and molecular O2. More than 100 cytochrome P-450 isoenzymes differing in their affinity for various substrates (drugs), have been identified.

(ii) Reduction: This reaction is the converse of oxidation and involves cytochrome P-450 enzymes working in the opposite direction. Alcohols, aldehydes, quinones are reduced. Drugs primarily reduced are chloralhydrate, chloramphenicol, halothane, warfarin. 

(iii) Hydrolysis: This is cleavage of drug molecule by taking up a molecule of water

 

Similarly, amides and polypeptides are hydrolysed by amidases and peptidases. In addition, there are epoxide hydrolases which detoxify epoxide metabolites of some drugs generated by CYP oxygenases. Hydrolysis occurs in liver, intestines, plasma and other tissues. Examples of hydrolysed drugs are choline esters, procaine, lidocaine, procainamide, aspirin, carbamazepine-epoxide, pethidine, oxytocin. 

(iv) Cyclization: This is formation of ring structure from a straight chain compound, e.g. proguanil.

(v) Decyclization: This is opening up of ring structure of the cyclic drug molecule, e.g. barbiturates, phenytoin. This is generally a minor pathway.

Synthetic reactions: 

These involve conjugation of the drug or its phase I metabolite with an endogenous substrate, usually derived from carbohydrate or amino acid, to form a polar highly ionized organic acid, which is easily excreted in urine or bile. Conjugation reactions have high energy requirement. 

(i) Glucuronide conjugation: This is the most important synthetic reaction carriedout by a group of UDP-glucuronosyl transferases (UGTs). Compounds with a hydroxyl or carboxylic acid group are easily conjugated with glucuronic acid which is derived from glucose. Examples are— chloramphenicol, aspirin, paracetamol, diazepam, lorazepam, morphine, metronidazole. Not only drugs but endogenous substrates like bilirubin, steroidal hormones and thyroxine utilize this pathway. Glucuronidation increases the molecular weight of the drug which favours its excretion in bile. Drug glucuronides excreted in bile can be hydrolysed by bacteria in the gut—the liberated drug is reabsorbed and undergoes the same fate. This enterohepatic cycling of the drug prolongs its action, e.g. phenolphthalein, oral contraceptives. 

(ii) Acetylation Compounds having amino or hydrazine residues are conjugated with the help of acetyl coenzyme-A, e.g. sulfonamides, isoniazid, PAS, dapsone, hydralazine, clonazepam, procainamide. Multiple genes control the N-acetyl transferases (NATs), and rate of acetylation shows genetic polymorphism (slow and fast acetylators). 

(iii) Methylation The amines and phenols can be methylated by methyl transferases (MT); methionine and cysteine acting as methyl donors, e.g. adrenaline, histamine, nicotinic acid, methyldopa, captopril, mercaptopurine. 

(iv) Sulfate conjugation The phenolic compounds and steroids are sulfated by sulfotransferases (SULTs), e.g. chloramphenicol, methyldopa, adrenal and sex steroids. 

(v) Glycine conjugation Salicylates, nicotinic acid and other drugs having carboxylic acid group are conjugated with glycine, but this is not a major pathway of metabolism. 

(vi) Glutathione conjugation  This is carried out by glutathione-S-transferase (GST) forming a mercapturate. It is normally a minor pathway. However, it serves to inactivate highly reactive quinone or epoxide intermediates formed during metabolism of certain drugs, e.g. paracetamol. When large amount of such intermediates are formed (in poisoning or after enzyme induction), glutathione supply falls short—toxic adducts are formed with tissue constituents → tissue damage. 

(vii) Ribonucleoside/nucleotide synthesis This pathway is important for the activation of many purine and pyrimidine antimetabolites used in cancer chemotherapy.

ENZYMES FOR METABOLISM:

Microsomal enzymes: These are located on smooth endoplasmic reticulum (a system of microtubules inside the cell), primarily in liver, also in kidney, intestinal mucosa and lungs. The monooxygenases, cytochrome P450, UGTs, epoxide hydrolases, etc. are microsomal enzymes. 

They catalyse most of the oxidations, reductions, hydrolysis and glucuronide conjugation. Microsomal enzymes are inducible by drugs, diet and other agencies. 

Nonmicrosomal enzymes: These are present in the cytoplasm and mitochondria of hepatic cells as well as in other tissues including plasma. The esterases, amidases, some flavoprotein oxidases and most conjugases are nonmicrosomal. Reactions catalysed are: Some oxidations and reductions, many hydrolytic reactions and all conjugations except glucuronidation.

The nonmicrosomal enzymes are not inducible but many show genetic polymorphism (acetyl transferase, pseudocholinesterase). 

Both microsomal and nonmicrosomal enzymes are deficient in the newborn, especially premature, making them more susceptible to many drugs, e.g. chloramphenicol, opioids. This deficit is made up in the first few months, more quickly in case of oxidation and other phase I reactions than in case of glucuronide and other conjugations which take 3 or more months. 

FACTORS AFFECTING METABOLISM

1. Age: Neonates and infants donot have well developed complexes, i.e. phase II reaction is not well developed, whereas in adults phase II reaction is well developed. For example, in infants chloramphenicol results in grey baby syndrome. In old patients low hepatic enzyme activity due to lack of enzyme in liver or due to low hepatic blood flow results in improper kidney function and prevent drug excretion. 

2. Body temperature: increase in body temperature increases drug metabolism.

3. Chemical properties of the drug: certain drugs stimulates or inhibits the metabolism of other drugs. For example, phenobarbitone stimulates the metabolism of phenytoin.

4. Diet: Starvation can deplete enzymes (like glycine storage) and alter glycine conjugation reactions. For example, protein malnutrition prolongs the phenobarbitone (sleeping time).

5. Dose: Toxic dose can deplete enzymes necessary for detoxification reactions.

6. Enzyme induction: Certain drugs on chronic administration increases the activity of microsomal enzymes by an increased enzyme synthesis.

7. Enzyme inhibition: Azole antifungal drugs, macrolide antibiotics and some other drugs binds to heme iron in CYP450 and inhibits the metabolism of many drugs, as well as some endogenous substances like steroids, bilirubin. One drug can competitively inhibit the metabolism of another if it utilizes the same enzyme or cofactors. However, such interactions are not as common as one would expect, because often different drugs are substrates for different CYP-450 isoenzymes. It is thus important to know the CYP isoenzyme(s) that carry out the metabolism of a particular drug. A drug may inhibit one isoenzyme while being itself a substrate of another isoenzyme.

8. Genetic disorder: Abnormal development of enzymes responsible for metabolism give genetic disorder. For example, a typical choline esterase deficiency, metabolism of succcinyl choline becomes slow and respiratory muscle paralysis develops. 

9. Routes of drug administration: Oral route of administration can result in effective hepatic metabolism of some drugs i.e. first pass metabolism. 

FIRST PASS (PRESYSTEMIC) METABOLISM

 This refers to metabolism of a drug during its passage from the site of absorption into the systemic circulation. All orally administered drugs are exposed to drug metabolizing enzymes in the intestinal wall and liver (where they first reach through the portal vein). Presystemic metabolism in the gut and liver can be avoided by administering the drug through sublingual, transdermal or parenteral routes. However, limited presystemic metabolism can occur in the skin (transdermally administered drug) and in lungs (for drug reaching venous blood through any route). The extent of first pass metabolism differs for different drugs and is an important determinant of oral bioavailability. 

A drug can be excreted as such into bile. The hepatic extraction ratio (ERLiver) of a drug Is fraction of the absorbed drug prevented by the liver from reaching systemic circulation. Both presystemic metabolism as well as direct excretion into bile determine ERLiver, which is given by equation 

Accordingly the systemic bioavailability (F) of an orally administered drug will be:

F = fractional absorption x (1-ER)

Attributes of drugs with high first pass metabolism: 

(a) Oral dose is considerably higher than sublingual or parenteral dose. 

(b) There is marked individual variation in the oral dose due to differences in the extent of first pass metabolism. 

(c) Oral bioavailability is apparently increased in patients with severe liver disease. 

(d) Oral bioavailability of a drug is increased if another drug competing with it in first pass metabolism is given concurrently, e.g. chlorpromazine and propranolol.