A series of studies were designed and carried out in order to explore the potential for the major human hepatic hydrolase, carboxylesterase 1 (hCES1), to serve as a target of metabolic inhibition by a variety of medications. The risk of adverse drug-drug interaction(s) is present when metabolic inhibitors are combined with known or suspected substrates of a given enzyme. In the present report the abundantly expressed hepatic enzyme, hCES1, was examined as a potential target of metabolic inhibition by a number of routinely prescribed medications. hCES1 has been seldom assessed in this regard despite its role in the metabolism and detoxification of many compounds. The psychostimulant methylphenidate (MPH) was chosen as an hCES1 selective substrate. In vitro studies were performed using previously developed cell lines which overexpress hCES1 with both p-nitrophenyl acetate and d-MPH serving as known substrates. Aripiprazole, perphenazine, thioridazine, and fluoxetine were determined to be the potent hCES1 inhibitors. A complementary animal study followed in vitro screening studies to further evaluate the inhibitory effect of aripiprazole on CES1 activity in FVB mice. The results suggest that the concurrent administration of racemic (i.e. dl-) MPH with aripiprazole significantly increased the plasma concentrations of both total MPH as well as the less active l-isomer. The ratio of d-MPH and l-MPH plasma concentrations was significantly decreased in the mice treated with aripiprazole compared to the control animals, indicating an overall decrease of CES1 catalytic activity in aripiprazole treated animals. Additionally, a quantitative structure-activity relationship based analysis identified a number of structural similarities of CES1 inhibitors. In conclusion, drug-drug interactions with MPH are likely mediated via CES1 inhibition as a result of concomitant drug therapies. CES1 inhibition represents an overlooked and little studied source of variability in MPH disposition, tolerability, and response.
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