Potent mechanism-based inhibition of CYP3A4 by imatinib explains its liability to interact with CYP3A4 substrates

BACKGROUND AND PURPOSE Imatinib, a cytochrome P450 2C8 (CYP2C8) and CYP3A4 substrate, markedly increases plasma concentrations of the CYP3A4/5 substrate simvastatin and reduces hepatic CYP3A4/5 activity in humans. Because competitive inhibition of CYP3A4/5 does not explain these in vivo interactions, we investigated the reversible and time-dependent inhibitory effects of imatinib and its main metabolite N-desmethylimatinib on CYP2C8 and CYP3A4/5 in vitro. EXPERIMENTAL APPROACH Amodiaquine N-deethylation and midazolam 1'-hydroxylation were used as marker reactions for CYP2C8 and CYP3A4/ 5 activity. Direct, IC50-shift, and time-dependent inhibition were assessed with human liver microsomes. KEY RESULTS Inhibition of CYP3A4 activity by imatinib was pre-incubation time-, concentration-and NADPH-dependent, and the time-dependent inactivation variables K-I and k(inact) were 14.3 mM and 0.072 min(-1) respectively. In direct inhibition experiments, imatinib and N-desmethylimatinib inhibited amodiaquine N-deethylation with a K-i of 8.4 and 12.8 mM, respectively, and midazolam 1'-hydroxylation with a K-i of 23.3 and 18.1 mM respectively. The time-dependent inhibition effect of imatinib was predicted to cause up to 90% inhibition of hepatic CYP3A4 activity with clinically relevant imatinib concentrations, whereas the direct inhibition was predicted to be negligible in vivo. CONCLUSIONS AND IMPLICATIONS Imatinib is a potent mechanism-based inhibitor of CYP3A4 in vitro and this finding explains the imatinib-simvastatin interaction and suggests that imatinib could markedly increase plasma concentrations of other CYP3A4 substrates. Our results also suggest a possibility of autoinhibition of CYP3A4-mediated imatinib metabolism leading to a less significant role for CYP3A4 in imatinib biotransformation in vivo than previously proposed.