Selective inhibition of human cytochrome P450 3A4 by N-[2(R)-hydroxy-1(S)-indanyl]-5-[2(S)-(1,1-dimethylethylaminocarbonyl)-4-[(furo[2,3-B]pyridin-5-yl) methyl]piperazin-1-yl]-4(S)-hydroxy-2(R)-phenylmethylpentanamide and P-glycoprotein by valspodar in gene transfectant systems

Our previous report showed that L754.394 and valspodar (PSC833) are potent inhibitors of midazolam hydroxylation in human jejunum microsomes and vecto rial transport of vinblastine in Caco-2 cells, respectively. In the present study, to directly examine the interactions of these compounds as well as other substrates with CYP3A4 and P-glycoprotein (P-gp), we performed in vit ro inhibition studies using recombinant CYP3A4-expressed microsomes and an MDR1-transfected cell line, LLC-MDR1, respectively. In CYP3A4-expressed mic rosomes, both L754.394 and ketoconazole, at a concentration less than 0.5 m u M, are the most potent inhibitors of the formation of 1'-hydroxymidazolam , a major metabolite of midazolam formed by CYP3A4. The greatest inhibitory effect on the transcellular transport of digoxin in LLC-MDR1 cells was obs erved in the presence of valspodar (< 0.1 mu M), followed by verapamil. Fro m a comparison of the IC50 values, it was shown that L754.394 and valspodar exhibited the highest selectivity for CYP3A4 and P-gp, respectively. To de monstrate such specificity, both midazolam hydroxylation and digoxin transp ort were observed in CYP3A4 transfected Caco-2 cells, which coexpress both P-gp and CYP3A4, in the presence or absence of L754.394 (0.5 mu M) and vals podar (1.0 mu M). L754.394 almost completely inhibited midazolam hydroxylat ion, but not digoxin transport, whereas almost complete inhibition of digox in transport was observed in the presence of valspodar, but inhibition of t he hydroxylation was minimal. Thus, the present study has demonstrated that L754.394 has a specific inhibitory effect on CYP3A4, whereas valspodar is specific for P-gp.