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 (K-i = 0.85-4.80 M) in comparison to LP1 (7.5 M). On the contrary, their MOR and DOR affinities were worse (K-i = 0.18-0.28 M and K-i = 0.38-1.10 M, respectively) with respect to LP1 values (K-i = 0.049 and 0.033 M). 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 (pEC(50)(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 (pK(B)(MOR) = 6.12 and pK(B)(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.