Substrate-dependent modulation of the catalytic activity of CYP3A by erlotinib

Aim: To ascertain the effects of erlotinib on CYP3A, to investigate the amplitude and kinetics of erlotinib-mediated inhibition of seven major CYP isoforms in human liver microsomes (HLMs) for evaluating the magnitude of erlotinib in drug-drug interaction in vivo. Methods: The activities of 7 major CYP isoforms (CYP1A2, CYP2A6, CYP3A, CYP2C9, CYP2D6, CYP2C8, and CYP2E1) were assessed in HLMs using HPLC or UFLC analysis. A two-step incubation method was used to examine the time-dependent inhibition of erlotinib on CYP3A. Results: The activity of CYP2C8 was inhibited with an IC50 value of 6.17 +/- 2.0 mu mol/L. Erlotinib stimulated the midazolam 1'-hydroxy reaction, but inhibited the formation of 6 beta-hydroxytestosterone and oxidized nifedipine. Inhibition of CYP3A by erlotinib was substrate-dependent: the IC50 values for inhibiting testosterone 6 beta-hydroxylation and nifedipine metabolism were 31.3 +/- 8.0 and 20.5 +/- 5.3 mu mol/L, respectively. Erlotinib also exhibited the time-dependent inhibition on CYP3A, regardless of the probe substrate used: the value of K-I and k(inact) were 6.3 mu mol/L and 0.035 min(-1) for midazolam; 9.0 mu mol/L and 0.045 min(-1) for testosterone; and 10.1 mu mol/L and 0.058 min(-1) for nifedipine. Conclusion: The inhibition of CYP3A by erlotinib was substrate-dependent, while its time-dependent inhibition on CYP3A was substrate-independent. The time-dependent inhibition of CYP3A may be a possible cause of drug-drug interaction, suggesting that attention should be paid to the evaluation of erlotinib's safety, especially in the context of combination therapy.