The psychoactive constituent of cannabis, Δ⁹-tetrahydrocannabinol, produces in humans subjective responses mediated by CB1 cannabinoid receptors, indicating that endogenous cannabinoids may contribute to the control of emotion. But the variable effects of Δ⁹-tetrahydrocannabinol obscure the interpretation of these results and limit the therapeutic potential of direct cannabinoid agonists. An alternative approach may be to develop drugs that amplify the effects of endogenous cannabinoids by preventing their inactivation. Here we describe a class of potent, selective and systemically active inhibitors of fatty acid amide hydrolase, the enzyme responsible for the degradation of the endogenous cannabinoid anandamide. Like clinically used anti-anxiety drugs, in rats the inhibitors exhibit benzodiazepine-like properties in the elevated zero-maze test and suppress isolation-induced vocalizations. These effects are accompanied by augmented brain levels of anandamide and are prevented by CB1 receptor blockade. Our results indicate that anandamide participates in the modulation of emotional states and point to fatty acid amide hydrolase inhibition as an innovative approach to anti-anxiety therapy.

Acute stress suppresses pain by activating brain pathways that engage opioid or non-opioid mechanisms. Here we show that an opioid-independent form of this phenomenon, termed stress-induced analgesia¹, is mediated by the release of endogenous marijuana-like (cannabinoid) compounds in the brain. Blockade of cannabinoid CB₁ receptors in the periaqueductal grey matter of the midbrain prevents non-opioid stress-induced analgesia. In this region, stress elicits the rapid formation of two endogenous cannabinoids, the lipids 2-arachidonoylglycerol² (2-AG) and anandamide³. A newly developed inhibitor of the 2-AG-deactivating enzyme, monoacylglycerol lipase^4,5, selectively increases 2-AG concentrations and, when injected into the periaqueductal grey matter, enhances stress-induced analgesia in a CB₁-dependent manner. Inhibitors of the anandamide-deactivating enzyme fatty-acid amide hydrolase⁶, which selectively elevate anandamide concentrations, exert similar effects. Our results indicate that the coordinated release of 2-AG and anandamide in the periaqueductal grey matter might mediate opioid-independent stress-induced analgesia. These studies also identify monoacylglycerol lipase as a previously unrecognized therapeutic target.

Although anecdotal reports suggest that cannabis may be used to alleviate symptoms of depression, the psychotropic effects and abuse liability of this drug prevent its therapeutic application. The active constituent of cannabis, Δ ⁹ -tetrahydrocannabinol, acts by binding to brain CB ₁ cannabinoid receptors, but an alternative approach might be to develop agents that amplify the actions of endogenous cannabinoids by blocking their deactivation. Here, we show that URB597, a selective inhibitor of the enzyme fatty-acid amide hydrolase, which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide, exerts potent antidepressant-like effects in the mouse tail-suspension test and the rat forced-swim test. Moreover, URB597 increases firing activity of serotonergic neurons in the dorsal raphe nucleus and noradrenergic neurons in the nucleus locus ceruleus. These actions are prevented by the CB ₁ antagonist rimonabant, are accompanied by increased brain anandamide levels, and are maintained upon repeated URB597 administration. Unlike direct CB ₁ agonists, URB597 does not exert rewarding effects in the conditioned place preference test or produce generalization to the discriminative effects of Δ ⁹ -tetrahydrocannabinol in rats. The findings support a role for anandamide in mood regulation and point to fatty-acid amide hydrolase as a previously uncharacterized target for antidepressant drugs.

Fatty acid amide hydrolase (FAAH) is an intracellular serine enzyme that catalyzes the hydrolysis of bioactive fatty acid ethanolamides such as anandamide and oleoylethanolamide (OEA). Genetic deletion of the faah gene in mice elevates brain anandamide levels and amplifies the effects of this endogenous cannabinoid agonist. Here, we show that systemic administration of the selective FAAH inhibitor URB597 (cyclohexyl carbamic acid 3′-carbamoyl-biphenyl-3-yl ester; 0.3 mg/kg i.p.) increases anandamide levels in the brain of rats and wild-type mice but has no such effect in FAAH-null mutants. Moreover, URB597 enhances the hypothermic actions of anandamide (5 mg/kg i.p.) in wild-type mice but not in FAAH-null mice. In contrast, the FAAH inhibitor does not affect anandamide or OEA levels in the rat duodenum at doses that completely inhibit FAAH activity. In addition, URB597 does not alter the hypophagic response elicited by OEA (5 and 10 mg/kg i.p.), which is mediated by activation of peroxisome proliferator-activated receptor type-α. Finally, exogenously administered OEA (5 mg/kg i.p.) was eliminated at comparable rates in wild-type and FAAH^-/- mice. Our results indicate that URB597 increases brain anandamide levels and magnifies anandamide responses by inhibiting intracellular FAAH activity. The results also suggest that an enzyme distinct from FAAH catalyzes OEA hydrolysis in the duodenum, where this lipid substance acts as a local satiety factor.

In the present article, we review the pharmacological properties of KDS‐4103 (URB597), a highly potent and selective inhibitor of the enzyme fatty‐acid amide hydrolase (FAAH), which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide. In vitro, KDS‐4103 inhibits FAAH activity with median inhibitory concentrations (IC₅₀) of 5 nM in rat brain membranes and 3 nM in human liver microsomes. In vivo, KDS‐4103 inhibits rat brain FAAH activity after intraperitoneal (i.p.) administration with a median inhibitory dose (ID₅₀) of 0.15 mg/kg. The compound does not significantly interact with other cannabinoid‐related targets, including cannabinoid receptors and anandamide transport, or with a broad panel of receptors, ion channels, transporters and enzymes. By i.p. administration to rats and mice KDS‐4103 elicits significant, anxiolytic‐like, antidepressant‐like and analgesic effects, which are prevented by treatment with CB1 receptor antagonists. By contrast, at doses that significantly inhibit FAAH activity and substantially raise brain anandamide levels, KDS‐4103 does not evoke classical cannabinoid‐like effects (e.g., catalepsy, hypothermia, hyperphagia), does not cause place preference, and does not produce generalization to the discriminative effects of the active ingredient of cannabis, Δ9‐tetrahydrocannabinol (Δ9‐THC). These findings suggest that KDS‐4103 acts by enhancing the tonic actions of anandamide on a subset of CB₁ receptors, which may normally be engaged in controlling emotions and pain. KDS‐4103 is orally available in rats and cynomolgus monkeys. Sub‐chronic repeated dose studies (1500 mg/kg, per os) in these two species have not demonstrated systemic toxicity. Likewise, no toxicity was noted in bacterial cytotoxicity tests in vitro and in the Ames test. Furthermore, no deficits were observed in rats on the rotarod test after acute i.p. treatment with KDS‐4103 at doses up to 5 mg/kg or in a functional observation battery after oral doses up to 1500 mg/kg. The results suggest that KDS‐4103 will offer a novel approach with a favorable therapeutic window for the treatment of anxiety, depression and pain.

The authors have developed a new inhibitor of the enzyme that degrades the endocannabinoid anandamide. This new drug does not cross the blood-brain barrier. It elevates peripheral anandamide only, which, acting on peripheral CB1 receptors, attenuates pain responses in rodents. Peripheral cannabinoid receptors exert a powerful inhibitory control over pain initiation, but the endocannabinoid signal that normally engages this intrinsic analgesic mechanism is unknown. To address this question, we developed a peripherally restricted inhibitor (URB937) of fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide. URB937 suppressed FAAH activity and increased anandamide levels outside the rodent CNS. Despite its inability to access brain and spinal cord, URB937 attenuated behavioral responses indicative of persistent pain in rodent models of peripheral nerve injury and inflammation and prevented noxious stimulus–evoked neuronal activation in spinal cord regions implicated in nociceptive processing. CB1 cannabinoid receptor blockade prevented these effects. These results suggest that anandamide-mediated signaling at peripheral CB1 receptors controls the access of pain-related inputs to the CNS. Brain-impenetrant FAAH inhibitors, which strengthen this gating mechanism, might offer a new approach to pain therapy.