Iso- and Benzisothiazolinone Inhibitors of Monoacylglycerol Lipase: Exploring On-Target Activity Through Scaffold Decoration and In Situ Warhead Generation.

Monoacylglycerol lipase (MGL) is an intracellular serine hydrolase that regulates endocannabinoid signaling by hydrolyzing 2-arachidonoylglycerol (2-AG), thereby controlling cannabinoid receptor activity and related physiological processes. Targeting MGL through allosteric mechanisms is a promising strategy to modulate 2-AG levels while avoiding limitations associated with active site-directed inhibitors. In this study, we aimed to develop selective MGL inhibitors based on isothiazolinone (ITZ) and benzisothiazolinone (BTZ) scaffolds that covalently target the regulatory cysteine residues Cys201 and Cys208 forming disulfide adducts. Two complementary approaches were explored. First, a mechanism-based strategy was designed to exploit MGL-mediated hydrolysis of O-substituted (benz)isothiazol-3-ol derivatives, enabling localized release of the reactive warhead in proximity to the target cysteines. Second, to enhance molecular recognition, tailored substituents were introduced on the BTZ scaffold to promote selective interactions with residues surrounding the allosteric site. Although several compounds displayed nanomolar inhibitory activity, structure-activity relationship analysis indicated that inhibitory potency is primarily driven by intrinsic warhead reactivity rather than specific protein-ligand interactions. Overall, these findings highlight both the potential and the challenges of developing selective covalent allosteric MGL inhibitors, emphasizing the need for strategies that better balance reactivity and molecular recognition.