An assessment of human liver-derived in vitro systems to predict the in vivo metabolism and clearance of almokalant

The ability of various human derived in vitro systems to predict various as pects of the in vivo metabolism and kinetics of almokalant have been invest igated in a multicenter collaborative study. Although almokalant has been w ithdrawn from further clinical development, its metabolic and pharmacokinet ic properties have been well characterized. Studies with precision-cut live r slices, primary hepatocyte cultures, and hepatic microsomal fractions for tified with UDP-glucuronic acid all suggested that almokalant is mainly glu curonidated to the stereoisomers M18a and M18b, which is in good agreement with the results in vivo. Both in vivo and in vitro studies indicate that t he formation of M18b dominates over that of M18a, although the difference i s more pronounced with the in vitro systems. Molecular modeling, cDNA-expre ssed enzyme analysis, correlation analysis, and inhibition studies did not clearly indicate which P450 enzymes catalyze the oxidative pathways, which may indicate a problem in identifying responsible enzymes for minor metabol ic routes by in vitro methods. All of the in vitro systems underpredicted t he metabolic clearance of almokalant, which has previously been reported to be a general problem for drugs that are cleared by P450-dependent metaboli sm. Although few studies on in vivo prediction of primarily glucuronidated drugs have appeared, in vitro models may consistently underpredict in vivo metabolic clearance. We conclude that in vitro systems, which monitor phase II metabolism, would be beneficial for prediction of the in vivo metabolis m, although all of the candidate liver-derived systems studied here, within their intrinsic limitations, provided useful information for predicting me tabolic routes and rates.