Minimizing Polymorphic Metabolism in Drug Discovery: Evaluation of the Utility of in Vitro Methods for Predicting Pharmacokinetic Consequences Associated with CYP2D6 Metabolism

Abstract

ABSTRACT:Minimizing interindividual variability in drug exposure is an important goal for drug discovery. The reliability of the selective CYP2D6 inhibitor quinidine was evaluated in a retrospective analysis using a standardized approach that avoids laboratory-to-laboratory variation. The goal was to evaluate the reliability of in vitro metabolism studies for predicting extensive metabolizer (EM)/poor metabolizer (PM) exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 M substrate and 0.5 M P450 to monitor substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 M quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A nonlinear relationship between the contribution of CYP2D6 and decreased oral clearance for PMs relative to EMs was evident. For drugs having <60% CYP2D6 involvement in vivo, a modest difference between EM and PM exposure was observed (<2.5-fold). For major CYP2D6 substrates (>60%), more dramatic exposure differences were observed (3.5-to 53-fold). For compounds primarily eliminated by hepatic P450 and with sufficient turnover to be evaluated in vitro, the fraction metabolized by CYP2D6 in vitro compared favorably with the in vivo data. The in vitro estimation of fraction metabolized using quinidine as a specific inhibitor provided an excellent predictive tool. Results from microsomal substrate depletion experiments can be used with confidence to select compounds in drug discovery using a cutoff of >60% metabolism by CYP2D6.Minimizing interindividual variability in drug exposure is an important goal in drug discovery. One major source of interindividual variability in drug exposure involves clearance. The variable expression of drug-metabolizing enzymes can result in profound differences in drug exposure across a patient population. The genetic basis for interindividual variability has been described for numerous drugmetabolizing enzymes. Some common examples of polymorphic drug-metabolizing enzymes include thiopurine methyltransferase, glutathione S-transferase M1-1, CYP2C9, CYP2C19, CYP2D6, CYP3A5, N-acetyltransferase 1, UGT1A1, and flavin monooxygenase 3 (Haining and Yu, 2003).The consequences of polymorphic enzymes on drug metabolism in relation to efficacy and side effects has been the focus of numerous studies (Vandel et al., 1999;Dandara et al., 2001). For instance, individuals lacking the expression of a polymorphic drug-metabolizing enzyme (commonly referred to as "poor metabolizers" or PMs) will have higher drug exposure if those drugs are metabolized by those polymorphic enzymes, which could lead to exaggerated pharmacology or enhanced side effects relative to the intermediate metabolizer and extensive metabolizer (EM) subjects given the same dose (Mahgoub et al., 1977). Alternatively, if a polymorphic enzyme forms a p...

Integrated in Vitro Analysis for the in Vivo Prediction of Cytochrome P450-Mediated Drug-Drug Interactions

“…The f m estimates for CYP2D6 determined using rP450s were in broad agreement with those recently calculated from independent in vitro and in vivo methods, thus providing additional confidence to this approach Gibbs et al, 2006).…”

CYP2D6-Mediated Metabolism of a Novel Acyl Coenzyme A:Cholesterol Acyltransferase Inhibitor, Pactimibe, and Its Unique Plasma Metabolite, R-125528

“…On the other hand, on the basis of the equation above, small changes in percentage contribution can result in large pharmacokinetic differences when the fraction metabolized exceeds 60%, because of the nonlinear shape of the relationship between f m CYP2D6 and AUC PM /AUC EM (Gibbs et al, 2006). Therefore, the effect of CYP2D6 polymorphism on the pharmacokinetics of R-125528 is expected to be more significant than that of pactimibe.…”

“…It was reported elsewhere (Ito et al, 2005;Gibbs et al, 2006) that the AUC increase in CYP2D6 poor metabolizers can be calculated from the following equation: AUC PM /AUC EM ϭ 1/(1 Ϫ f m CYP2D6 ), where AUC EM is the area under the plasma concentration time curve in CYP2D6 extensive metabolizers, AUC PM is the area under the plasma concentration time curve in CYP2D6 poor metabolizers, and f m CYP2D6 is the fraction of metabolism via CYP2D6. Pactimibe was suggested to have multiple metabolic pathways (i.e., similar contributions of indolin oxidation and -1 oxidation beside glucuronidation to some extent) ( Table 1).…”

New Insights into the Structural Features and Functional Relevance of Human Cytochrome P450 2C9. Part II

“…Minimizing interindividual variability in drug exposure is an important goal in drug development and discovery [118]. CYP2C9 is considered one of the most important CYPs, with substrate specificity typical of many new chemical entities (i.e.…”