Discrepancy in Reported and Observed Monoacylglycerol Lipase Inhibition of Aryl Formyl Piperidine Containing Potent and Highly Selective Monoacylglycerol Lipase Inhibitor.

Zhi et al. reported compound 36 as a potent reversible inhibitor of human recombinant monoacylglycerol lipase (hrMAGL; IC50 = 15 nM) with excellent selectivity over other targets. To develop potential PET tracers for imaging MAGL, we synthesized compound 36 as a reference compound. Surprisingly, we observed a significant discrepancy between the reported potency and our findings in two well-established in vitro assays. Compound 36 exhibited no inhibition of endocannabinoid hydrolysis in HEK293 lysates overexpressing human MAGL in a glycerol-based assay at 1 μM. In the competitive ABPP assay, it appeared completely inactive inhibiting MAGL in mouse whole-brain proteome even at 100 μM. In addition, eight structural analogs (2-9) of compound 36 also failed to show any significant MAGL inhibition. Interestingly, when the synthetic substrate 4-nitrophenyl acetate, also used by Zhi et al., was employed, compound 36 inhibited MAGL more effectively than in glycerol-based assay, with an IC50 value of 143 nM, being almost 10-fold less potent than reported. In our reversibility assay, compound 36 exhibited measurable MAGL inhibition only at a concentration of 100 μM. In summary, this study contradicts the earlier report and therefore, compound 36 should be used with caution as a research tool in studies exploring MAGL inhibition.