Carboxylesterases (CE) are ubiquitous enzymes responsible for the detoxification of xenobiotics, including numerous clinically used drugs. Therefore, the selective inhibition of these proteins may prove useful in modulating drug half-life and bioavailability. Recently, we identified 1,2-diones as potent inhibitors of CEs, although little selectivity was observed in the inhibition of either human liver CE (hCE1) or human intestinal CE (hiCE). In this paper, we have further examined the inhibitory properties of ethane-1,2-diones toward these proteins and determined that, when the carbonyl oxygen atoms are cis-coplanar, the compounds demonstrate specificity for hCE1. Conversely, when the dione oxygen atoms are not planar (or are trans-coplanar), the compounds are more potent at hiCE inhibition. These properties have been validated in over 40 1,2-diones that demonstrate inhibitory activity toward at least one of these enzymes. Statistical analysis of the results confirms the correlation (P < 0.001) between the dione dihedral angle and the preferential inhibition of either hiCE or hCE1. Overall, the results presented here define the parameters necessary for small molecule inhibition of human CEs.
Benzil has been identified as a potent selective inhibitor of carboxylesterases (CEs). Essential components of the molecule required for inhibitory activity include the dione moiety and the benzene rings, and substitution within the rings affords increased selectivity toward CEs from different species. Replacement of the benzene rings with heterocyclic substituents increased the K(i) values for the compounds toward three mammalian CEs when using o-nitrophenyl acetate as a substrate. Logarithmic plots of the K(i) values versus the empirical resonance energy, the heat of union of formation energy, or the aromatic stabilization energy determined from molecular orbital calculations for the ring structures yielded linear relationships that allowed prediction of the efficacy of the diones toward CE inhibition. Using these data, we predicted that 2,2'-naphthil would be an excellent inhibitor of mammalian CEs. This was demonstrated to be correct with a K(i) value of 1 nM being observed for a rabbit liver CE. In addition, molecular simulations of the movement of the ring structures around the dione dihedral indicated that the ability of the compounds to inhibit CEs was due, in part, to rotational constraints enforced by the dione moiety. Overall, these studies identify subdomains within the aromatic ethane-1,2-diones, that are responsible for CE inhibition.
