Frakefamide is a potent analgesic that acts as a peripheral active μ-selective receptor agonist. Frakefamide is unable to penetrate the blood-brain-barrier and enter the central nervous system^[1][2]. In Vivo: Frakefamide (LEF576) yields a dose dependent increase in morphine appropriate responding to 50% at the highest dose tested (10 μmol/kg) after infusion durations of 2 min (Fig. 6, left panel), whereas after 15 min infusions a maximum of 25% morphine appropriate responding was occasioned at 17.5 μmol/kg^[1][2].