Synthesis and Structure–Activity Relationships of FAAH Inhibitors: Cyclohexylcarbamic Acid Biphenyl Esters with Chemical Modulation at the Proximal Phenyl Ring

Abstract

Fatty acid amide hydrolase (FAAH) is a serine hydrolase that catalyzes the intracellular hydrolysis of fatty acid ethanolamides such as anandamide and oleoylethanolamide. Targeting this enzyme may have important therapeutic potentials owing to the multiple physiological roles of these amides. Cyclohexylcarbamic acid biphenyl‐3‐yl ester (URB524) was one of the most promising FAAH inhibitors so far described. We report the modulation of the electronic and steric features of the proximal phenyl ring of this compound by introducing a series of substituents at the ortho and para positions. pIC50 values were found to correlate with molecular features thought to be involved in the recognition step such as steric hindrance and hydrogen‐bonding ability. Derivatives with small polar groups at the para position of the proximal phenyl ring were slightly better FAAH inhibitors than the parent compound URB524.

Understanding the role of carbamate reactivity in fatty acid amide hydrolase inhibition by QM/MM mechanistic modelling

“…The water molecule (Wat627, Fig. 3), involved in the recognition of p-OH and p-NH 2 substituents, 7 has a negligible contribution to TS stabilization (DTSS of À0.4 and À0.7 kcal mol À1 for URB694 and URB618, respectively).…”

“…1), can be docked in two possible orientations (called orientations I and II) within the FAAH catalytic site. 6,7 Recently, hybrid quantum mechanical/molecular mechanics (QM/MM) modelling, 8 using the B3LYP/6-31G(d)//PM3-CHARMM22 potential, showed that the inhibitory process is energetically preferred in orientation II. 9 This orientation allows the catalytic nucleophile, Ser241, to efficiently attack the carbonyl group of URB524 (Fig.…”

“…In vitro potency of FAAH inhibitors depends on a combination of covalent binding to the catalytic Ser241 and noncovalent interactions with specific recognition elements in the active site of FAAH, 7 while in vivo potency is also affected by plasmatic stability. 12 Structure-activity relationship (SAR) studies showed that conjugated, electron-donor groups on the biphenyl scaffold of URB524 (which increase electron density around the carbamate carbon) are associated with enhanced carbamate stability both in alkaline buffer and in rat plasma.…”

“…13 Conversely, these groups do not affect FAAH inhibitor potency in vitro. 7 To understand this apparent discrepancy, we here apply a QM/MM approach to model the carbamoylation reaction of FAAH by the reference inhibitor URB524 (IC 50 = 25.6 nM) and by its p-OH (URB694, IC 50 = 30.0 nM) and p-NH 2 (URB618, IC 50 = 27.2 nM) derivatives (Fig. 1).…”

Correlation between energetics of collisionally activated decompositions, interaction energy and biological potency of carbamate FAAH inhibitors

“…formation of a noncovalent complex (recognition step), and nucleophile attack to the carbamate by Ser241 10,11 leading to its carbamoylation and to irreversible inactivation of the enzyme (inactivation step). The recognition step is related to stereoelectronic complementarity between the inhibitor and the active site of the enzyme and it was rationalized by molecular modeling studies, 8,9 whereas the inactivation reaction may be related to the propensity of the C(O)-O bond to be cleaved and it may be described by suitable indicators of bond reactivity. In this particular case, the second step was studied by breakdown curves, 12 relative to ESI-generated protonated molecules, which, taking place under collisional conditions during resonance excitation in an ion trap, exclusively gave fragments related to the C(O)-O bond cleavage.…”