MK-1903 is a potent and selective hydroxycarboxylic acid receptor 2 (HCA2, GPR109A) full agonist. Exhibits no binding at the GRP109B receptor. This drug had been in phase II clinical trial for the treatment of atherosclerosis and Dyslipidemia. But then, according to Merck, elevation of HDL cholesterol relative to placebo did not meet the trial's pre-specified primary objective for efficacy; no safety signals were implicated as drivers of the decision to discontinue development.

URB-597 (KDS-4103) is a highly potent and selective inhibitor of the enzyme fatty acid amide hydrolase (FAAH), which catalyzes the intracellular hydrolysis of the endocannabinoid anandamide. URB597 is one the best studied carbamate-based inhibitors. Early studies showed that the compound did not inhibit or bind related biological targets. Administration of URB597 to rats and subsequent in vivo evaluation of brain FAAH activity showed the compound elevated endogenous anandamide levels. Importantly, the compound did not produce catalepsy, hypothermia, or hyperphagia, three of the typical effects of exogenous cannabinoids. The compound did produce antinociceptive effects in the mouse hot-plate test, which were reversed by the CB1 antagonist rimonabant. These findings again support the expectation that inhibition of FAAH produces pharmacology distinct from an exogenous CB1 agonist. A more detailed study was published later showing time-course data in mice demonstrating elevation of anandamide, oleamide, and N-palmitoyl ethanolamine for 2–6 h after administration of URB597. In vivo administration of URB597 showed almost complete inhibition of FAAH by the compound, and the FAAH inhibition was still approximately 70% after 16 h. Complete recovery of CNS FAAH activity was observed 24 h after administration of URB597. In vivo administration of URB597 or the cannabinoid receptor agonist HU210 reduced both mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. Effects in the inflammatory model were partially reversed by CB1 and CB2 antagonists. In a related study, the compound produced analgesic effects in the mouse CCI model (neuropathic) when administered orally. These effects were also reversed by both CB1 and CB2 antagonists.