Design, Synthesis, and <i>In Vitro</i> Characterization of Proteolytically-Stable Opioid-Neurotensin Hybrid Peptidomimetics

Linking an opioid to a nonopioid pharmacophore represents a promising approach for reducing opioid-induced side effects during pain management. Herein, we describe the optimization of the previously reported opioid-neurotensin hybrids (OPNT-hybrids), <b>SBL-OPNT-05</b> & <b>-10</b>, containing the μ-/δ-opioid agonist H-Dmt-d-Arg-Aba-β-Ala-NH₂ and NT(8-13) analogs optimized for NTS2 affinity. In the present work, the constrained dipeptide Aba-β-Ala was modified to investigate the optimal linker length between the two pharmacophores, as well as the effect of expanding the aromatic moiety within constrained dipeptide analogs, via the inclusion of a naphthyl moiety. Additionally, the <i>N</i>-terminal Arg residue of the NT(8-13) pharmacophore was substituted with β³ <i>h</i>Arg. For all analogs, affinity was determined at the MOP, DOP, NTS1, and NTS2 receptors. Several of the hybrid ligands showed a subnanomolar affinity for MOP, improved binding for DOP compared to <b>SBL-OPNT-05</b> & <b>-10</b>, as well as an excellent NTS2-affinity with high selectivity over NTS1. Subsequently, the G_αi1 and β-arrestin-2 pathways were evaluated for all hybrids, along with their stability in rat plasma. Upon MOP activation, <b>SBL-OPNT-13</b> and <b>-18</b> were the least effective at recruiting β-arrestin-2 (<i>E</i> _max = 17 and 12%, respectively), while both compounds were also found to be partial agonists at the G_αi1 pathway, despite improved potency compared to DAMGO. Importantly, these analogs also showed a half-life in rat plasma in excess of 48 h, making them valuable tools for future <i>in vivo</i> investigations.