Synthesis and Structure-Activity Relationships of (-)- cis - N -Normetazocine-Based LP1 Derivatives.

(−)- cis - N -Normetazocine represents a rigid scaffold able to mimic the tyramine moiety of endogenous opioid peptides, and the introduction of different N -substituents influences affinity and efficacy of respective ligands at MOR (mu opioid receptor), DOR (delta opioid receptor), and KOR (kappa opioid receptor). We have previously identified LP1, a MOR/DOR multitarget opioid ligand, with an N -phenylpropanamido substituent linked to (−)- cis - N -Normetazocine scaffold. Herein, we report the synthesis, competition binding and calcium mobilization assays of new compounds 10 ⁻ 16 that differ from LP1 by the nature of the N -substituent. In radioligand binding experiments, the compounds 10 ⁻ 13 , featured by an electron-withdrawing or electron-donating group in the para position of phenyl ring, displayed improved affinity for KOR (Ki = 0.85⁻4.80 μM) in comparison to LP1 (7.5 μM). On the contrary, their MOR and DOR affinities were worse (Ki = 0.18⁻0.28 μM and Ki = 0.38⁻1.10 μM, respectively) with respect to LP1 values (Ki = 0.049 and 0.033 μM). Analogous trends was recorded for the compounds 14 ⁻ 16 , featured by indoline, tetrahydroquinoline, and diphenylamine functionalities in the N -substituent. In calcium mobilization assays, the compound 10 with a p -fluorophenyl in the N -substituent shared the functional profile of LP1 (pEC50 MOR = 7.01), although it was less active. Moreover, the p -methyl- ( 11 ) and p -cyano- ( 12 ) substituted compounds resulted in MOR partial agonists and DOR/KOR antagonists. By contrast, the derivatives 13 ⁻ 15 resulted as MOR antagonists, and the derivative 16 as a MOR/KOR antagonist (pKB MOR = 6.12 and pKB KOR = 6.11). Collectively, these data corroborated the critical role of the N -substituent in (−)- cis - N -Normetazocine scaffold. Thus, the new synthesized compounds could represent a template to achieve a specific agonist, antagonist, or mixed agonist/antagonist functional profile.