Olorinab (APD-371)

Olorinab (APD-371) is a small-molecule, orally bioavailable, peripherally acting full agonist of the cannabinoid type 2 receptor (CB2) discovered at Arena Pharmaceuticals and advanced through Phase 2b clinical development for the treatment of visceral abdominal pain associated with Crohn's disease and irritable bowel syndrome. The compound exhibits single-digit nanomolar potency at human CB2 (EC50 6.2 nM in functional assays), greater than 1000-fold selectivity for CB2 over cannabinoid receptor type 1 (CB1), and minimal off-target activity across a broad panel of noncannabinoid receptors, ion channels, and transporters. The peripheral selectivity, confirmed by low brain penetration in preclinical species, was designed to dissociate the analgesic properties of cannabinoid receptor activation from the psychoactive and abuse-liability concerns that accompany CB1 engagement in the central nervous system.

Structurally, olorinab contains a fused cyclopropa[4,5]cyclopenta[1,2-c]pyrazole core bearing a pyrazine N-oxide substituent and a (2S)-1-hydroxy-3,3-dimethylbutan-2-yl carboxamide side chain, with three defined stereocenters conferring the (4aS,5aS,2'S) absolute configuration. The molecular formula is C18H23N5O3, corresponding to a molecular weight of 357.41 g/mol. Arena Pharmaceuticals identified and optimized the compound from an internal medicinal chemistry program focused on heterocyclic CB2-selective scaffolds and secured United States Patent No. 8,778,950 covering the compound class.

Clinical pharmacology studies established rapid oral absorption (median time to maximum plasma concentration approximately 1 to 2 hours), dose-proportional pharmacokinetics across the 10 to 400 mg single-dose range, minimal accumulation on repeated dosing at three-times-daily regimens, and hepatic biotransformation to multiple metabolites (M1, M2, M4) with 10- to 200-fold lower CB2 affinity than the parent compound. In a Phase 1 single-ascending dose study in 56 healthy volunteers and a Phase 1b multiple-ascending dose study in 36 healthy volunteers (50, 100, 200 mg in the published literature for 10 days), olorinab was well tolerated with no dose-limiting adverse events, no psychoactive effects, and plasma concentrations exceeding those required for CB2 activation across all tested doses.

Preclinical pharmacology demonstrated that olorinab reduced visceral hypersensitivity in trinitrobenzene sulfonic acid-induced colitis in rats and 2,4-dinitrobenzene sulfonic acid-induced colitis in mice, with efficacy at 3 and 30 mg/kg that was abolished by the selective CB2 antagonist SR144528, confirming receptor-mediated mechanism. Notably, olorinab produced no analgesic effect in healthy (non-inflamed) animals, consistent with the hypothesis that CB2-mediated analgesia requires inflammatory or nociceptive sensitization context. In single-fiber nociceptor recordings, olorinab produced concentration-dependent inhibition of action potential firing from colonic afferents in both acute colitis and chronic visceral hypersensitivity states.

A Phase 2a open-label proof-of-concept study in 14 patients with quiescent to mildly active Crohn's disease and chronic abdominal pain (baseline average abdominal pain score 4 or greater) reported statistically significant pain reduction at both 25 mg and 100 mg three-times-daily dosing over 8 weeks, with 100 percent of evaluable subjects achieving 30 percent or greater pain improvement at week 8, no requirement for rescue pain medication, and no central nervous system adverse events. A subsequent Phase 2b randomized, double-blind, placebo-controlled trial (CAPTIVATE) in 273 patients with irritable bowel syndrome evaluated 10, 25, and 50 mg doses against placebo over 12 weeks. The trial did not meet its primary efficacy endpoint of statistically significant improvement in abdominal pain scores in the overall population at any dose; however, a pre-specified subgroup analysis of patients with moderate-to-severe baseline pain (average abdominal pain score 6.5 or greater) showed significant pain reduction at the highest dose. The compound was well tolerated across all clinical studies with no serious adverse events attributed to treatment.

Following the Phase 2b outcome, Arena Pharmaceuticals evaluated strategic options for the olorinab program. Pfizer acquired Arena in March 2022 for $6.7 billion, primarily for the etrasimod (sphingosine 1-phosphate receptor modulator) pipeline; olorinab development was subsequently discontinued. The compound is currently available as a research-grade preparation from chemical suppliers and remains of investigational interest as a tool compound for CB2 receptor pharmacology research, visceral pain mechanism studies, and gastrointestinal nociceptor biology. This monograph reviews the chemistry, molecular pharmacology, pharmacokinetics, preclinical and clinical evidence base, sourcing, handling, comparative positioning, and safety profile of olorinab in the context of the broader CB2 agonist therapeutic class.