Reversible Inhibitors of Fatty Acid Amide Hydrolase That Promote Analgesia: Evidence for an Unprecedented Combination of Potency and Selectivity

Abstract

Fatty acid amide hydrolase (FAAH) is the primary catabolic regulator of several bioactive lipid amides in vivo, including the endogenous cannabinoid anandamide and the sleep-inducing substance oleamide. Inhibitors of FAAH are considered a potential therapeutic approach for the treatment of several nervous system disorders, including pain, anxiety, and insomnia. However, for FAAH inhibitors to achieve clinical utility, they must not only display efficacy in vivo but also selectivity for this enzyme relative to the numerous other serine hydrolases present in mammalian proteomes. Here, we report a general strategy for evaluating the pharmacological activity and target specificity of FAAH inhibitors and its implementation to develop the first class of selective reversible inhibitors of this enzyme that are highly efficacious in vivo. Using a series of functional proteomics, analytical chemistry, and behavioral pharmacology assays, we have identified a class of ␣-keto-heterocycles that show unprecedented selectivity for FAAH relative to other mammalian hydrolases, and, when administered to rodents, raise central nervous system levels of anandamide and promote cannabinoid receptor 1-dependent analgesia in several assays of pain sensation. These studies provide further evidence that FAAH may represent an attractive therapeutic target and describe a general route by which inhibitors of this enzyme can be optimized to achieve exceptional potency, selectivity, and efficacy in vivo.

Endocannabinoid system and pain: an introduction

“…Further studies have therefore focused on the use of FAAH and MAGL inhibitors to prolong the effects of endogenous EC actions. Systemic inactivation of FAAH via compounds such as URB597, OL135 and PF-3845 has been shown to be anti-nociceptive in models of acute and inflammatory pain (75)(76)(77)(78)(79)(80) . Elevations in both AEA and 2-AG have been shown, as well as reduced carrageenan-induced hyperalgesia (81) and expansion of peripheral receptive field size of wide dynamic range neurons (a marker of central sensitisation) following intra-plantar URB597 (82) .…”

Inhibition of Fatty-Acid Amide Hydrolase Accelerates Acquisition and Extinction Rates in a Spatial Memory Task

“…We selected the reversible inhibitor OL-135 because it is both highly selective for inhibiting FAAH and active in vivo (Lichtman et al, 2004a). In contrast, URB597 binds irreversibly to FAAH and AM404 is not selective.…”

“…URB-597 was shown to elicit a CB 1 receptor-mediated hypoalgesic response in the hot plate test (Kathuria et al, 2003) as well as a CB 2 receptor mediated anti-edema effect in the carrageenan test (Holt et al, 2005). Similarly, OL-135 produced CB 1 receptor mediated hypoalgesic effects in several pain assays including tail immersion, hot plate, and formalin tests (Lichtman et al, 2004a). OL-135 has also been found to block mechanical allodynia in a rat spinal nerve ligation model and this effect was blocked with either the CB 2 receptor antagonist SR144528 or naloxone, but not by SR141716 (Chang et al, 2006).…”

Anxiolytic-Like Properties of the Anandamide Transport Inhibitor AM404

“…These include substrate saturation at 371C, stereoselective substrate recognition, independence from ion gradients, and pharmacological inhibition by agents such as N-(4-hydroxyphenyl)-arachidonamide (AM404), 5-biphenyl-4-ylmethyl-tetrazole-1-carboxylic acid dimethylamide (LY2183240), and N-(3-furylmethyl)-arachidonamide (UCM707) (Beltramo et al, 1997;Piomelli et al, 1999;de Lago et al, 2002;Moore et al, 2005). After internalization, anandamide is hydrolyzed by fatty-acid amide hydrolase (FAAH) (Désarnaud et al, 1995;Hillard et al, 1995;Ueda et al, 1995;Cravatt et al, 1996), an intracellular membrane-bound enzyme whose activity is selectively blocked by the compounds cyclohexylcarbamic acid 3 0 -carbamoylbiphenyl-3-yl ester (URB597) and 1-oxo-1-[5-(2-pyridyl)oxazol-2-yl]-7-phenylheptane (OL-135) (Kathuria et al, 2003;Lichtman et al, 2004) as well as by other less selective inhibitors (for review, see Piomelli, 2005).…”

“…Moreover, the hypoalgesic phenotype of FAAH-deficient mice (Cravatt et al, 2001) and the ability of FAAH inhibitors to alleviate pain, anxiety, and depression in rodent models (Kathuria et al, 2003;Lichtman et al, 2004;Hohmann et al, 2005;Gobbi et al, 2005) suggest that anandamide modulates the activity of neural circuits involved in the control of nociception, stress and emotion. These findings raise two clinically relevant questions.…”