Another strategy adopted to enhance

Another strategy adopted to enhance the action of endocannabinoids is indirectly through blocking their metabolism. Indeed, amplifying the action of anandamide and 2-AG by inhibiting their enzymatic degradation has emerged to develop the endocannabinoid system for medicinal purposes. It is interesting that AEA, at higher concentrations, activates TRPV1 channels, which are recognized to be implicated in nociception, and that the enzymes cyclooxygenase (COX) pathway represents a different metabolic itinerary for AEA. Thus, the COX inhibitors, and TRPV1 antagonists appear to be potential second targets to combine with FAAH inhibitors [5], [6], [7]. In the nervous system, anandamide and 2-AG are degraded primarily by the serine hydrolase enzymes FAAH and MAGL, respectively. The active site of FAAH contains a catalytic triad refers to the three amino papain inhibitor residues Ser241-Ser217-Lys142 in which Ser241 plays a key role [8], [9]. The extensive biochemical characterization of its mechanism revealed that unlike most serine hydrolases, which use a histidine residue as a catalytic base, FAAH recruits a lysine for this function, a distinction that enables FAAH to hydrolyze both amides and esters at equivalent rates [9], [10]. At the present time, FAAH inhibitors have gained attention as potential therapeutic targets in the management of neuropathic pain. The chemical structures of a number of FAAH inhibitors are shown in Fig. 4. Compound 1 (OL-135, Fig. 4) belongs to the α-ketoheterocycle chemical class which reversibly blocks FAAH activity. OL-135 is a highly potent and selective reversible inhibitor of FAAH devoid of CB1, CB2 and μ and δ opioid receptors activities that increases the analgesic and hypothermic activity of anandamide [11]. The compound was found to significantly elevate brain anandamide levels (approximately 5-fold) and produce antinociceptive effects for over 9 h [12]. Several carbamate inhibitors of FAAH have been described in the scientific literature [13]. The O-aryl carbamate 2 (URB597, Fig. 4) was found to have FAAH inhibitor activity through irreversible carbamylation of the enzyme's catalytic serine nucleophile [14], [15]. Recently published studies also disclosed the enol carbamates as novel reversible FAAH inhibitors with high selectivity against other endocannabinoid targets [16]. The 1-biphenyl-4-ylethenyl piperidine-1-carboxylate 3 (ST4070, Fig. 4) showed anxiolytic effects in mice and the administration of it generated long-lasting neuropathic pain relief compared with the FAAH inhibitors OL-135 and URB597; moreover it presented antiallodynic effects in several rodent models of neuropathic pain [17]. The urea-based derivative 4 (LY-2183240, Fig. 4) is a potent FAAH inhibitor which was found to block brain serine hydrolases and has been shown to produce both analgesic and anxiolytic effects in animal models [18], [19]. Nevertheless, an irreversible urea FAAH inhibitor 5 (PF-04457845), failed to induce analgesia in patients with knee osteoarthritis in a randomised placebo and active-controlled clinical trial [20]. Several recently published studies also disclosed the phenylheterocyclic scaffold [21], [22] exemplified by compound 6 (Fig. 4) as extremely potent, noncompetitive, and reversible FAAH inhibitors. We recently described the medicinal chemistry of a new series of heteroaryl-4-oxopyridine/7-oxopyrimidines as CB2 receptor partial agonists [23]. In this context, we have also discovered a series of 7-oxopyrazolo[1,5-a]pyrimidine derivatives [24], among which we identified very potent and selective CB2 inverse agonists. The design of both series was based on the replacement of the condensed benzene ring of the known 4-oxo-quinoline-based CB2 receptor agonists [25] with the pyrazolo nucleus. These consequences demonstrated that the phenyl and the pyrazole rings may occasionally work as bioisosters. Based upon these results and into a pain management program, we designed a series of pyrazolo phenylcyclohexylcarbamate derivatives with general structure 7 as a new scaffold to address FAAH enzyme (Fig. 5). Standing on the known carbamoyl derivative URB597 and on the previously reported phenylheterocyclic compounds, the specific pyrazolo heterocycle was introduced to evaluate the effect of replacing the phenyl ring of URB597 on the enzymatic activity. Aliphatic or aromatic moieties in the N-position of pyrazole skeleton were introduced, while position 3(5) of the pyrazole ring was investigated bearing mainly a cyclohexylcarbamoyl chain in the para or meta-phenyl ring. Finally, structural variations on the N-phenylpyrazole and 3-ethoxycarbonyl of the most active compound, obtained during this study, was carried out, and structure activity relationships on inhibition of FAAH were examined.