Structural Determinants of μ-Opioid Receptor Antagonism and Respiratory Liability in Phenylfentanyl Analogues

The μ-opioid receptor (MOR) plays a key role in opioid addiction and fentanyl-induced respiratory depression. Phenylfentanyl, previously reported as a MOR antagonist, paradoxically induces respiratory depression in vivo, revealing a dissociation between receptor antagonism and respiratory liability within this scaffold. To interrogate structural determinants underlying this activity profile, phenylfentanyl was redesigned through systematic modification of the acyl chain, N-alkyl group, and core ring scaffold, including exploration of a 6,6-ring extension to probe the MOR binding space. Seventy-eight compounds were synthesized and evaluated. The 6,6-ring series exhibited markedly reduced MOR activity relative to the 6-membered core analogues, likely due to suboptimal pharmacophore spacing. Among these, compound 5 demonstrated solid MOR antagonist activity, effectively blocking morphine- and fentanyl-induced antinociception in vivo and showing greater potency than phenylfentanyl in vitro. Importantly, compound 5 did not induce respiratory depression at the tested dose, indicating a lack of intrinsic respiratory liability. These findings delineate structural features governing MOR antagonism and respiratory effects within a fentanyl-derived scaffold.