Evaluation of Icotinib as a Potent and Selective Inhibitor of Aldehyde Oxidase for Reaction Phenotyping in Human Hepatocytes.

Aldehyde oxidase (AO) is a molybdenum cofactor-containing cytosolic enzyme that has gained prominence due to its involvement in the developmental failure of several drug candidates in first-in-human trials. Unlike cytochrome P450s (P450) and glucuronosyltransferase, AO substrates have been plagued by poor in vitro to in vivo extrapolation, leading to low systemic exposures and underprediction of human dose. However, apart from measuring a drug's AO clearance rates, it is also important to determine the relative contribution to metabolism by this enzyme (f m,AO ). Although hydralazine is the most well-studied time-dependent inhibitor (TDI) of AO and is frequently employed for AO reaction phenotyping in human hepatocytes to derive f m,AO , multiple studies have expressed concerns pertaining to its utility in providing accurate estimates of f m,AO values due to its propensity to significantly inhibit P450s at the concentrations typically utilized for reaction phenotyping. In this study, we characterized icotinib, a cyclized analogue of erlotinib, as a potent TDI of AO - inactivating human liver cytosolic zoniporide 2-oxidation equipotently with erlotinib with a k inact / K I ratio of 463 and 501 min -1 mM -1 , respectively. Moreover, icotinib also exhibits selectivity against P450 and elicits significantly weaker inhibition against human liver microsomal UGT1A1/3 as compared to erlotinib. Finally, we evaluated icotinib as an inhibitor of AO for reaction phenotyping in cryopreserved human hepatocytes and demonstrated that it can yield more accurate prediction of f m,AO compared to hydralazine and induce sustained suppression of AO activity at higher cell densities - which will be important for reaction phenotyping endeavors of low clearance drugs. Significance Statement In this study, we characterized icotinib as a potent time-dependent inhibitor of AO with ample selectivity margins against the P450s and UGT1A1/3 and demonstrated its utility for reaction phenotyping in human hepatocytes to obtain accurate estimates of f m,AO for victim DDI risk predictions. We envisage the adoption of icotinib in place of hydralazine in AO reaction phenotyping.