Inhibitory modulation of gastrointestinal motility in mice by Tyr-c[D-Lys-Gly-p-F-Phe-Asp]-D-Pro-NH2, a novel cyclic hexapeptide with multifunctional opioid agonism.

Opioid analgesics are critical for managing moderate-to-severe pain，yet are limited by adverse gastrointestinal (GI) effects, notably constipation. This necessitates developing novel opioid agonists with robust analgesia and reduced GI side effects. The cyclic hexapeptide Tyr-c[D-Lys-Gly-p-F-Phe-Asp]-D-Pro-NH2 (analog 15), a recently characterized multifunctional agonist of μ-opioid receptor (MOR), κ-opioid receptor (KOR), and δ-opioid receptor (DOR), exhibits potent antinociception following subcutaneous (s.c.) administration with constipation observed only at high doses. To further evaluate its GI impact, we assessed the effects of analog 15 on intestinal motility using in vivo upper GI transit and colonic bead expulsion assays. Our results indicated that fentanyl, analog 15, and its parent peptide analog 0 dose-dependently slowed upper GI transit and colonic expulsion after s.c. administration, with the upper GI tract exhibiting greater sensitivity. Mechanistically, fentanyl inhibited the GI motility via both central and peripheral opioid receptors, whereas analog 15 inhibited upper GI transit exclusively through the peripheral MOR, KOR, and DOR, and suppressed colonic transit via the peripheral MOR and KOR, both effects were independent of the central opioid receptor pathway. In conclusion, we demonstrated that the high doses of analog 15 inhibited GI motility through peripherally restricted activation of multiple opioid receptors. This finding aligns with analog 15's limited blood-brain barrier (BBB) permeability, which explains its reduced constipating effects while preserving potent analgesia, thereby supporting the therapeutic potential of multi-target peripheral opioid agonists.