An endogenous sleep-inducing compound is a novel competitive inhibitor of fatty acid amide hydrolase

2-Octyl γ-bromoacetoacetate (OγBr), an endogenous compound originally isolated from human cerebrospinal fluid (CSF), has previously been demonstrated to increase REM sleep duration in cats. Based on the chemical structure of OγBr and its reported sleep-inducing effects, we synthesized OγBr along with chemically related analogs and tested these compounds as inhibitors of fatty acid amide hydrolase (FAAH), a brain enzyme that degrades neuromodulatory fatty acid amides. OγBr was found to competitively inhibit FAAH activity with IC50 and Ki values of 2.6 μM and 0.8 μM, respectively [for the (R)-enantiomer of OγBr (1)]. A set of synthetic analogs of OγBr was examined to define the structural features required for FAAH inhibition and inhibitor potencies were assessed at pH 9.0 (near the pH optimum of FAAH) and pH 7.0. Interestingly, at pH 7.0 the γ-halo β-keto ester inhibitors proved to be significantly more potent than the trifluoromethyl ketone of oleic acid, one of the most potent FAAH inhibitors described to date. This study supports the possibility that OγBr may be a physiological regulator of FAAH activity and fatty acid amide levels in vivo. Additionally, the characterization of γ-halo β-keto esters as powerful FAAH inhibitors near physiological pH may aid in future studies of the enzymology and biological properties of FAAH.