Effect of ASP8062 on morphine self-administration and morphine-induced respiratory suppression in monkeys

ASP8062 is an orally available GABA_B receptor positive allosteric modulator (PAM). This study assessed the potential of ASP8062 for treating opioid use disorder (OUD). Three rhesus monkeys were pretreated with ASP8062 (0.3, 1 or 3 mg/kg) by oral administration 1 h prior to a 2-h morphine self-administration session (0.03 mg/kg, iv, per injection) under a fixed-ratio 5 schedule. We further examined the potential worsening of morphine-induced respiratory suppression by ASP8062 after coadministration of morphine (10 mg/kg, sc) and ASP8062 (10 mg/kg, po) in cynomolgus monkeys using a custom-made whole-body plethysmograph. Plasma concentrations of ASP8062 (3 or 10 mg/kg, po) were assessed in cynomolgus monkeys using liquid chromatography-tandem mass spectroscopy (LC-MS/MS). ASP8062 at 3 mg/kg, po decreased the morphine self-administrations with significant differences from the vehicle-treated group (IC₅₀ = 0.97 ± 0.36 mg/kg). Exposure levels at 3 mg/kg observed in monkeys were comparable to the clinical exposure levels which positive pharmacodynamic effects were previously shown. Further, ASP8062 did not potentiate morphine-induced respiratory suppression up to exposure levels higher than the clinically relevant dose. ASP8062 may reduce opioid use in OUD patients without affecting respiratory system, providing justification for further ASP8062 development as a potential treatment option for OUD.