Question

How does metabolic ratio determine the activity of CYTOCHROME P450?

Answer 1

Xenobiotics, including therapeutic agents, typically undergo chemical modification in the body to aid their elimination. The cytochrome P450 (P450) monooxygenase enzymes are the predominant enzyme system involved in human drug metabolism, accounting for about 75% of the total reactions for drug metabolism in the human liver, intestine, and kidney. The rate at which drugs and xenobiotics are metabolized by P450s affects the pharmacokinetics of the compound and, consequently, may also affect the pharmacodynamic response. Drug interactions involving either induction or inhibition of P450 enzymes can alter rates of P450-mediated metabolism. However, significant interindividual variation in basal rates of P450-mediated drug metabolism have been observed, including up to 30- to 40-fold variation for CYP3A enzymes ,100-fold variation for CYP2D6 , 50- to 60-fold variation for CYP2B6 , and 40- to 50-fold variation for CYP2C9.

Pharmacogenetic variation associated with changes in the amino acid sequence of the coding region accounts for some of the basal interindividual variation in P450-mediated metabolism in humans. Clinically relevant examples of this pharmacogenetic variation have been observed with CYP2C9 (for warfarin), CYP2C19 (for clopidogrel and omeprazole), CYP2D6 (for tamoxifen and codeine), and CYP3A5 (for tacrolimus) . In some instances, the resulting protein is still functional but exhibits reduced activity (e.g., CYP2C9; ; in other cases, the resulting variant protein may be completely devoid of activity or is not expressed (e.g., , such as CYP2D6, CYP2C19, and CYP3A5; ). Depending on whether metabolism produces an active (e.g., clopidogrel) or inactive metabolite (e.g., warfarin), the consequences of these polymorphisms can result in significant alternations in therapeutic effect.

Beyond differences in rates of P450 metabolism as a result of coding region changes, investigators have explored whether other factors contribute to interindividual variation in basal P450 activity. These factors include genetic variation in promoter regions, altered expression of microRNA that affect P450 expression, genetic variation in transcriptional regulators, and the influence of modulating agents early in development, among others. Genetic variation in the promoter region of P450 enzymes can result in altered levels of expression, which in turn lead to alterations in rates of drug metabolism . Likewise, variability in expression of microRNA has been demonstrated to alter expression of P450s . Although less well studied, genetic variations in the noncoding region play an important role in the interindividual variation of human drug metabolism. Finally, the ontogeny of drug-metabolizing enzymes has been established by several investigators. These studies have demonstrated that P450s mature at different rates throughout development, with some reaching adult activity shortly after birth, some taking several years before reaching full activity, and some even peaking and then diminishing after birth . However, little is known about the influence of environment or exposure to drugs or other xenobiotics on their long-term expression and activity.

Regardless of the cause of interindividual variation, it is desirable to determine an individual’s phenotype in the most rapid and least invasive manner. Typically, this has been accomplished by administering a “probe” compound and measuring the pharmacokinetics of the probe compound. Probe compounds have been studied for most P450s, and multiple probe compounds have been identified for some P450s. To assess the phenotype of several drug-metabolizing enzymes simultaneously, a “cocktail” of probe compounds can be used. However, ideally one would not have to administer an exogenous compound (or compounds), but be able to evaluate an individual’s phenotype through measurement of endogenous compounds and their pharmacokinetics. Cortisol and its metabolite, 6β-hydroxycortisol, are examples of endogenous compounds that have been used for CYP3A phenotyping.

This symposium describes the work spanning genetic variation, regulation, development, and activity of P450s of four laboratories. The roles of transcriptional regulators, their genetic variation, xenobiotic exposure during the early developmental period in interindividual variation in human drug metabolism, as well as metabolomic approaches to predicting individual rates of human drug metabolism are described.