Metabolism of Drugs and Xenobiotics

Enzyme Induction and Inhibition

Variability in enzymatic metabolism profoundly affects drug bioavailability and may result from one of three mechanisms: enzyme induction, enzyme inhibition, and enzyme polymorphisms. Enzyme induction occurs when an inducer promotes synthesis of additional enzyme protein, reduces inactivation of the formed protein, or both. Inhibition of enzyme activity occurs when a drug reversibly binds to the substrate binding site, acting as a competitive inhibitor. In other cases, an inhibitor may render an enzyme nonfunctional by covalent binding to a substrate site. These are known as suicide inhibitors or mechanism-based inhibitors . Several drugs may act as enzyme inducers and inhibitors, thus affecting the bioavailability of other drugs metabolized by the same enzyme. In addition, adverse reactions caused by drug interactions are more likely to occur from an alteration in pharmacokinetic profiles by either induction or inhibition of drug-metabolizing enzymes. They usually occur at the level of phase I metabolism because it is the rate limiting step. However, drug–drug interactions may not necessarily result in adverse events and may even be used to therapeutic advantage as in the successful use of ritonavir, a weak antiviral agent. Ritonavir effectively inhibits the drug-metabolizing enzyme CYP3A (see subsequent discussion of nomenclature), thus increasing the blood concentrations of concomitantly administered protease inhibitors in the treatment of human immunodeficiency virus.

Enzyme Polymorphisms

Enzyme polymorphisms have been best studied in the context of the human CYP superfamily that contains 57 functional genes and 58 pseudogenes ( www.cypalleles.ki.se ). Each enzyme is considered an isoform because each derives from a different gene and is designated by the letters “CYP” followed by an Arabic numeral indicating family, a letter indicating subfamily, and another Arabic numeral indicating the individual gene; for example, CYP3A4 or CYP2D6. Proteins included in a family have at least 40% amino acid sequence homology and proteins within subfamilies have at least 55% homology.

Genetic polymorphisms from single base pair changes in the DNA sequence are common in drug metabolizing enzymes and contribute to interindividual variability in enzyme activity. The alleles or gene variants that occur with the lowest frequency at a locus observed in a particular population are termed single-nucleotide polymorphisms (SNPs). Based on the location, they are categorized as intragenic or extragenic. Intragenic SNPs that occur in the coding region may result in translation of a different amino acid or give rise to a premature codon resulting in a truncated protein. These are known as a nonsynonymous SNP, and the changes may influence enzyme activity. As a consequence of the redundancy of the genetic code, SNPs may result in the production of the same polypeptide. These are known as synonymous SNPs and are not of any functional consequence. Extragenic SNP are functionally important when they occur in regions that regulate gene expression. In general, the first alleles sequenced is termed “wild-type” and is designated as ∗1 . Thus, the wild-type allele may not necessarily be the major allele in every ethnic group.

The in vivo activity of a drug metabolizing enzyme is assessed by studying systemic clearance of a probe drug that is exclusively metabolized by the target enzyme. The metabolic profile of a specific population may be classified as ultrarapid metabolizers, extensive metabolizers, intermediate metabolizers, and poor metabolizers. This interindividual variability in enzyme activity is attributed to the polymorphic expression of the gene. Subjects with ultrarapid metabolizer phenotype often carry more copies of the functional gene, resulting in up to a 1000-fold increase in enzymatic activity. The poor metabolizer phenotype is because of the presence of two nonfunctional (null) alleles or deletion of both alleles, whereas an intermediate phenotype is typically found in individuals carrying one null allele. Subjects with extensive metabolizer phenotype have normal enzyme activity with one or two functional alleles.

Disease States

The activity of drug metabolizing enzymes and transporters, in general, may be downregulated in disease states such as infection, chronic liver disease, and cancer. These changes in drug clearance occur because of altered gene expression at the transcriptional and posttranscriptional level mediated by proinflammatory cytokines such as interleukin-6, interleukin-1β, and tumor necrosis factor-α ( Fig. 22.2 ). These alterations in the metabolism and excretion of drugs thus introduce the risk of drug interactions and toxicity in disease states.

In advanced liver disease such as cirrhosis, CYP activity may be reduced in a selective and sequential manner depending on the etiology and the severity of the liver disease. Studies evaluating CYP activity in cirrhosis have also shown that loss of CYP activity is selective and dependent on the etiology (cholestatic versus noncholestatic) and severity of liver disease. George et al. observed CYP3A activity and protein reduction only in livers of patients with cirrhosis because of noncholestatic liver disorders. In patients with nonalcoholic fatty liver disease without advanced fibrosis, our group has showed a significant negative relationship between severity of steatosis and hepatic CYP3A activity. Increased hepatic CYP2E1 activity has been observed in steatohepatitis from both alcoholic liver disease and nonalcoholic liver disease from metabolic syndrome ( Fig. 22.3 ).