Highly potent and selective CYP2C19 inhibitors are not currently available. In the present study, N-3-benzyl derivatives of nirvanol and phenobarbital were synthesized, their respective (+)- and (-)enantiomers resolved chromatographically, and inhibitor potencies determined for these compounds toward CYP2C19 and other human liver cytochromes P450 (P450s). (-)-N-3-Benzyl-phenobarbital and (+)-N-3-benzyl-nirvanol were found to be highly potent, competitive inhibitors of recombinant CYP2C19, exhibiting K, values of 79 and 250 nM, respectively, whereas their antipodes were 20- to 60-fold less potent. In human liver preparations, (-)-N-3-benzyl-phenobarbital and (+)-N-3-benzyl-nirvanol inhibited (S)mephenytoin 4''-hydroxylase activity, a marker for native microsomal CYP2C19, with K-i values ranging from 71 to 94 nM and 210 to 280 nM, respectively. At single substrate concentrations of 0.3 muM [(-)-N-3-benzyl-phenobarbital] and 1 muM [(+)-N-3-benzyl-nirvanol] that were used to examine inhibition of a panel of cDNA-expressed P450 isoforms, neither CYP1A2, 2A6, 2C8, 2C9, 2D6, 2E1, nor 3A4 activities were decreased by greater than 16%. In contrast, CYP2C19 activity was inhibited similar to80% under these conditions. Therefore, (+)-N-3-benzyl-nirvanol and (-)-N-3-benzyl-phenobarbital represent new, highly potent and selective inhibitors of CYP2C19 that are likely to prove generally useful for screening purposes during early phases of drug metabolism studies with new chemical entities