Behavioral pharmacology of the μ/δ opioid glycopeptide MMP2200 in rhesus monkeys

H₂N-Tyr-D-Thr-Gly-Phe-Leu-Ser-(O-β-D-lactose)-CONH ₂ (MMP2200) is a novel glycopeptide opioid agonist with similar affinities for μ and δ receptors. Glycosylation promoted brain penetration and production of centrally mediated behavioral effects in mice; however, it is unknown whether the magnitude of enhanced brain penetration is sufficient to permit central mediation of drug effects and production of synergistic μ/δ antinociceptive interactions after systemic administration in primates. To address this issue, the present study compared the effects of MMP2200 and the μ-agonist morphine in four behavioral procedures in rhesus monkeys. In an assay of thermal nociception, morphine (1.0 -5.6 mg/kg) produced dose-dependent antinociception, whereas MMP2200 (10-56 mg/kg) was ineffective. In an assay of capsaicin-induced thermal allodynia, both morphine (0.01-1.0 mg/kg) and MMP2200 (0.032-3.2 mg/kg) produced dose-dependent antiallodynic effects. MMP2200-induced antiallodynia was blocked by the moderately μ-selective antagonist naltrexone (0.01 mg/kg), the δ-selective antagonist naltrindole (1.0 mg/kg), and the peripherally selective opioid antagonist quaternary naltrexone (0.32 mg/kg). In an assay of schedule-controlled behavior, both morphine (0.01-1.0 mg/kg) and MMP2200 (10-56 mg/kg) decreased response rates. Morphine effects were antagonized by naltrexone (0.001-0.01 mg/kg); however, the effects of MMP2200 were not antagonized by either naltrexone (0.01 mg/kg) or naltrindole (1.0 mg/kg). In an assay of drug self-administration, morphine (0.0032-0.32 mg/kg/injection) produced reinforcing effects, whereas MMP2200 (0.032-0.32 mg/kg/injection) did not. These results suggest that systemically administered MMP2200 acted as a peripheral, μ/δ-opioid agonist with limited distribution to the central nervous system in rhesus monkeys. These results also suggest the existence of species differences in the pharmacokinetics and brain penetration of glycopeptides.