AR-M 1000390 hydrochloride is an exceptionally selective, potent δ opioid receptor agonist with an EC₅₀ of 7.2±0.9 nM for δ agonist potency. IC50 & Target: EC50: 7.2±0.9 nM (δ opioid receptor)^[1] In Vitro: AR-M 1000390 (Compound 6a) exhibits the binding affinities (IC₅₀) of 0.87±0.23 nM for the δ opioid receptor and extremely high selectivity over the µ receptor (IC₅₀=3800±172 nM) and the κ receptor (IC₅₀=7470±606 nM)^[1]. RINm5F cells are treated with AR-M 1000390 (AR-M100390) and Cyclizine for 16-24 h before measurement of intracellular and secreted insulin levels. AR-M 1000390 mediates a dose-dependent decrease in insulin content with a maximal inhibition of ~90% at the highest concentration tested (10 μM)^[2]. In Vivo: Rats are treated with 5, 100, and 600 μmol/kg of AR-M 1000390 (AR-M100390) for 3 and/or 7 days; another group of rats treated with 600 μmol/kg of compound are allowed to recover for 14 days. AR-M 1000390 (600 μmol/kg) causes vacuolation in the β-cell of the rat pancreas that is associated with depletion of insulin and hyperglycemia after 7 days of dosing. Treatment of rats with 600 μmol/kg of AR-M 1000390 results in vacuolation of the β-cell of the rat pancreas that is similar to that reported for cyclizine and cyproheptadine^[2].