Evaluation of Cytochrome P450 Selectivity for Hydralazine as an Aldehyde Oxidase Inhibitor for Reaction Phenotyping

Hydralazine has been reported as a selective mechanism-based inactivator of aldehyde oxidase (AO) and it is widely used in the pharmaceutical industry for reaction phenotyping to estimate fraction metabolized by AO and to identify AO substrates. In this study, however, hydralazine was found to inhibit CYP1A2, 2B6, 2D6, and 3A in human suspension hepatocytes under reaction phenotyping assay conditions, at concentrations that chemically knocked out most of the AO activities (≥50 μM). Furthermore, hydralazine is a time-dependent inhibitor of CYP1A2. Based on these findings, precautions need to be taken when using hydralazine as an AO inhibitor for in vitro studies because fraction metabolized by AO is likely to be overestimated and the likelihood of false positives in identifying AO substrates increases.