Drug metabolism, also called biotransformation, is the process by which the body chemically modifies drug molecules to facilitate their elimination. Most drugs are lipophilic by design, allowing them to cross membranes and reach their targets. However, the body has evolved enzymatic systems that convert these lipid-soluble compounds into more water-soluble products that can be excreted in urine or bile. This metabolic conversion typically results in loss of pharmacological activity, though some drugs are administered as inactive prodrugs that require metabolic activation.
Phase I Reactions
Phase I reactions introduce or unmask a functional group on the drug molecule through oxidation, reduction, or hydrolysis. These reactions generally produce a small change in the drug’s structure and often result in only a modest increase in water solubility. The most important Phase I enzymes are the cytochrome P450 (CYP) family, a large group of heme-containing proteins located primarily in the liver endoplasmic reticulum. CYP3A4 is the most abundant isoform in the human liver and intestine and metabolizes approximately 50% of all marketed drugs. Other significant isoforms include CYP2D6, CYP2C9, CYP2C19, and CYP1A2.
Oxidation reactions catalyzed by CYP enzymes include aliphatic and aromatic hydroxylation, N-dealkylation, O-dealkylation, and sulfoxidation. Reduction reactions can occur through CYP enzymes or through other systems such as NADPH-cytochrome P450 reductase. Hydrolysis of ester and amide bonds is carried out by esterases and amidases found in plasma and tissues.
Phase II Reactions
Phase II reactions, or conjugation reactions, involve the covalent attachment of a polar endogenous molecule to the drug or its Phase I metabolite. The resulting conjugate is typically large, polar, and pharmacologically inactive, making it readily excretable. Glucuronidation is the most common Phase II pathway, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs) . Other Phase II reactions include sulfation, acetylation, methylation, and conjugation with glutathione or amino acids.
Phase II reactions may follow Phase I metabolism or occur directly if the drug already possesses a suitable functional group. Some conjugation reactions, particularly glucuronidation, can produce active metabolites or undergo enterohepatic recirculation when conjugates are hydrolyzed by gut bacteria.
First-Pass Metabolism
First-pass metabolism refers to drug metabolism that occurs before the drug reaches the systemic circulation. For orally administered drugs, the portal vein delivers absorbed drug directly to the liver, where extensive metabolism can occur before the drug enters the general circulation. The gut wall itself also contains CYP3A4 and other enzymes that contribute to presystemic elimination. First-pass metabolism can dramatically reduce the oral bioavailability of susceptible drugs, requiring larger oral doses compared to intravenous administration to achieve equivalent plasma concentrations.
Enzyme Induction and Inhibition
Drug-metabolizing enzymes are subject to regulation by induction and inhibition. Enzyme induction occurs when a drug stimulates the synthesis or reduces the degradation of metabolic enzymes, increasing the rate of metabolism of the inducer itself and of coadministered drugs. For example, rifampicin is a potent inducer of CYP3A4 and can reduce the plasma concentrations of oral contraceptives, warfarin, and many other drugs, potentially leading to therapeutic failure.
Enzyme inhibition is generally a more immediate phenomenon occurring when one drug competes for or irreversibly binds to a metabolic enzyme, reducing its activity. Coadministration of a CYP inhibitor can cause rapid accumulation of a substrate drug, leading to toxicity. Grapefruit juice contains furanocoumarins that irreversibly inhibit intestinal CYP3A4, increasing the bioavailability of numerous drugs.
Genetic Polymorphisms
Genetic variability in drug-metabolizing enzymes contributes significantly to interindividual differences in drug response. CYP2D6 exhibits particularly well-characterized polymorphisms, with individuals classified as poor, intermediate, extensive, or ultrarapid metabolizers. Poor metabolizers lack functional enzyme activity and are at risk of toxicity from drugs that are primarily eliminated by CYP2D6, while ultrarapid metabolizers may fail to achieve therapeutic concentrations. Similarly, polymorphisms in N-acetyltransferase 2 produce the well-known distinction between slow and fast acetylators, affecting the metabolism of isoniazid and other drugs.
The study of drug metabolism pathways is fundamental to understanding drug clearance, predicting drug-drug interactions, and personalizing pharmacotherapy to an individual’s metabolic capacity.
