Received for publication December 2, 2005.
Revised June 7, 2006.
Accepted for publication June 8, 2006.

Minimizing polymorphic metabolism in drug discovery: evaluation of the utility of in vitro methods for predicting pharmacokinetic consequences associated with CYP2D6 metabolism

Abstract

Minimizing inter-individual 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 EM/PM exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 µM substrate and 0.5 µM P450 monitoring substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 µM quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A non-linear relationship between the contribution of CYP2D6 and decreased oral clearance for PM relative to EM 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 (3.5-53-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 utilizing 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 cut-off of >60% metabolism by CYP2D6.

Key words: CYP2D, drug clearance, drug discovery, human CYP enzymes, in vitro-in vivo prediction, liver microsomes, pharmacokinetics

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