Carboxylesterases are
enzymes that hydrolyze a broad suite of endogenous and exogenous
ester-containing compounds to the corresponding alcohol and
carboxylic acid. These
enzymes metabolize a number of
therapeutics including the anti-
tumor agent
CPT-11, the anti-viral drug
oseltamivir, and the anti-thrombogenic agent
clopidogrel as well as many
agrochemicals. In addition,
carboxylesterases are involved in
lipid homeostasis, including
cholesterol metabolism and transport with a proposed role in the development of
atherosclerosis. Several different scaffolds capable of inhibiting
carboxylesterases have been reported, including
organophosphates,
carbamates, trifluoromethyl
ketone-containing structures (TFKs), and aromatic ethane-1,2-diones. Of these varied groups, only the 1,2-diones evidence
carboxylesterase isoform-selectivity, which is an important characteristic for therapeutic application and probing biological mechanisms. This study constructed a series of classical and 3D-QSAR models to examine the physiochemical parameters involved in the observed selectivity of three mammalian
carboxylesterases: human intestinal
carboxylesterase (hiCE), human
carboxylesterase 1 (hCE1), and rabbit
carboxylesterase (rCE). CoMFA-based models for the
benzil-analogs described 88%, 95% and 76% of observed activity for hiCE, hCE1 and rCE, respectively. For TFK-containing compounds, two distinct models were constructed using either the
ketone or gem-diol form of the inhibitor. For all three
enzymes, the CoMFA
ketone models comprised more biological activity than the corresponding gem-diol models; however the differences were small with described activity for all models ranging from 85-98%. A comprehensive model incorporating both
benzil and TFK structures described 92%, 85% and 87% of observed activity for hiCE, hCE1 and rCE, respectively. Both classical and 3D-QSAR analysis showed that the observed
isoform-selectivity with the
benzil-analogs could be described by the volume parameter. This finding was successfully applied to examine substrate selectivity, demonstrating that the relative volumes of the alcohol and
acid moieties of
ester-containing substrates were predictive for whether hydrolysis was preferred by hiCE or hCE1. Based upon the integrated
benzil and TFK model, the next generation inhibitors should combine the A-ring and the 1,2-dione of the
benzil inhibitor with the long alkyl chain of the TFK-inhibitor in order to optimize selectivity and potency. These new inhibitors could be useful for elucidating the role of
carboxylesterase activity in
fatty acid homeostasis and the development of
atherosclerosis as well as effecting the controlled activation of
carboxylesterase-based
prodrugs in situ.