The objective is to evaluate methoxsalen as an in vitro phenotyping tool in comparison to ABT as a nonspecific inactivator of P450 mediated metabolism. The reversible inhibition of methoxsalen and ABT against the P450, FMO, AO, MAO-A and -B, enzymes were evaluated using standard marker probe reactions. The time-dependent inhibition of P450 enzymes was evaluated in human liver microsomes. CES1 activities were determined by monitoring the depletion of known substrate, the clopidogrel. The metabolism of P450 substrates in the presence and absence of methoxsalen or ABT was evaluated in human liver microsomes. Methoxsalen is a direct inhibitor and inhibited the activities (>90%) of all enzymes at a concentration of 300 µM except for CYP2C9. Methoxsalen is also a potent time-dependent inhibitor of all P450 enzymes except for CYP2C19 (moderate) at a concentration of 300 µM. Methoxsalen inhibited the metabolism of P450 substrates in the pre-incubation mode. ABT is a potent TDI of several P450 except for CYP2C19 (47%) and CYP2C9 (27%). The results indicate that methoxsalen is a potent pan P450 inhibitor than ABT and can be a better tool in distinguishing P450 mediated metabolism form non-P450 metabolism in human liver microsomes.
Keywords: Aminobenzotriazole; aldehyde oxidase; carboxylesterases; cytochrome P450; flavin monooxygenase; methoxsalen; monoamine oxidase; phenotyping; time-dependent inhibition.