An Innovative Auto-Antagonistic Multi-Target Strategy: MOR/DOR/NPFFR2 Agonists Achieve Separation of Analgesia from Side Effects

Long-term opioid analgesia is limited by severe adverse effects. We propose an auto-antagonistic multitarget strategy incorporating NPFFR2-mediated counter-regulation to limit excessive opioid effects while preserving analgesia. Structure-guided optimization of MCRT yielded analogs 1, 4, and 9, which function as MOR/DOR/NPFFR2 triple agonists. Analog 1 provided pharmacological proof-of-concept, exhibiting characteristic bell-shaped antinociceptive and gastrointestinal dose−response profiles sensitive to RF-9 blockade. Together with its NPFFR2 agonism and no detectable NPFFR1 agonism in vitro, these findings support a contribution of NPFFR2-mediated counter-regulation to its self-limiting profile. In contrast, analogs 4 and 9 displayed dose-dependent analgesia and distinct in vivo profiles. All three analogs produced robust antinociception in CFA-induced inflammatory pain and showed no detectable tolerance or persistent gastrointestinal inhibition during repeated treatment, while analogs 1 and 4 showed no detectable withdrawal, conditioned place preference, or motor impairment. These findings support the auto-antagonistic strategy as a framework for developing safer analgesics.