In vivo evaluation of the dual MOP/NOP agonist AT121: a potent analgesic that delays opioid tolerance and offers a superior withdrawal suppression profile to methadone.

To evaluate the in vivo analgesic efficacy, tolerance liability, and dependence-modulating potential of AT121, a dual μ-opioid (MOP)/nociceptin (NOP) partial agonist, compared with morphine in Wistar rats. Male Wistar rats (n = 264) underwent tail-flick thermal nociception assays to assess dose-response and duration of analgesia following intraperitoneal AT121 (0.0025-0.02 mg/kg) or morphine (1 mg/kg). Tolerance was evaluated after 17 days of repeated administration. Dopamine levels were quantified in brain tissue. Additional experiments assessed naloxone antagonism, haloperidol (D2 blockade) interaction, Complete Freund's Adjuvant (CFA)-induced hyperalgesia, withdrawal reversal compared with methadone, and co-administration effects with morphine. Statistical analyses included ANOVA and nonparametric tests (p ≤ 0.05). AT121 produced dose-dependent antinociception, with 0.02 mg/kg exceeding morphine's maximal effect and prolonging analgesia (11 vs. 6 h). An equivalent analgesic dose (mg/kg) was 100-fold lower than morphine. Repeated AT121 did not induce tolerance, whereas morphine tolerance developed by day 17. Morphine significantly elevated brain dopamine levels, while AT121 caused minimal change. Naloxone completely blocked morphine but only partially attenuated AT121 analgesia. AT121 retained efficacy in CFA-induced hyperalgesia and during D2 receptor blockade. In morphine-tolerant rats, AT121 reversed withdrawal faster and more effectively than methadone. Co-administration with morphine enhanced analgesic magnitude without shortening duration. AT121 demonstrates superior potency, prolonged analgesia, and sustained efficacy without tolerance development compared with morphine. Dual MOP/NOP partial agonism with AT121 offers a promising strategy for effective analgesia while significantly delaying the onset of tolerance, and managing acute opioid withdrawal manifestations, while producing reduced dopaminergic activation that may indicate potentially lower abuse-related effects compared with morphine.