Synthesis and Structure–Activity Relationship (SAR) of 2-Methyl-4-oxo-3-oxetanylcarbamic Acid Esters, a Class of PotentN-Acylethanolamine Acid Amidase (NAAA) Inhibitors

N-Acylethanolamine acid amidase (NAAA) is a lysosomal cysteine hydrolase involved in the degradation of saturated and monounsaturated fatty acid ethanolamides (FAEs), a family of endogenous lipid agonists of peroxisome proliferator-activated receptor-alpha, which include oleoylethanolamide (OEA) and palmitoylethanolamide (PEA). The beta-lactone derivatives (S)-N-(2-oxo-3-oxetanyl)-3-phenylpropionamide (2) and (S)-N-(2-oxo-3-oxetanyl)-biphenyl-4-carboxamide (3) inhibit NAAA, prevent FAE hydrolysis in activated inflammatory cells, and reduce tissue reactions to pro-inflammatory stimuli. Recently, our group disclosed ARN077 (4), a potent NAAA inhibitor that is active in vivo by topical administration in rodent models of hyperalgesia and allodynia. In the present study, we investigated the structure activity relationship (SAR) of threonine-derived beta-lactone analogues of compound 4. The main results of this work were an enhancement of the inhibitory potency of beta-lactone carbamate derivatives for NAAA and the identification of (4-phenylpheny1)-methyl-N-[(2S,3R)-2-methyl-4-oxo-oxetan-3-yl]carbamate (14q) as the first single-digit nanomolar inhibitor of intracellular NAAA activity (IC50 = 7 nM on both rat NAAA and human NAAA).