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

Abstract

(−)-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 (pEC50MOR = 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 (pKBMOR = 6.12 and pKBKOR = 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.

LP1 and LP2: Dual-Target MOPr/DOPr Ligands as Drug Candidates for Persistent Pain Relief

“…Bulkier aromatic groups, such as naphthyl, quinoline and isoquinoline rings, at the N -propanamide spacer, were not tolerated and the bulkier size of the N -substituent moved the functional profile from agonism to antagonism versus MOPr [ 43 , 44 ]. In particular, in LP1, the phenyl replacement with the 1-naphthyl ring switched to a selective and potent MOPr antagonist ( 2 , Figure 2 ), with a K i of 38 ± 4 nM and an antagonist potency value (pA 2 ) of 8.6 in presence of MOPr agonist DAMGO.…”

Novel N-Substituted Benzomorphan-Based Compounds: From MOR-Agonist/DOR-Antagonist to Biased/Unbiased MOR Agonists

“…In particular, LP1 ( 2 , Figure ), with an N -phenylpropanamido substituent, resulted in vitro and in vivo a potent MOR agonist/DOR antagonist able to counteract nociceptive pain and behavioral signs of persistent pain with low tolerance-inducing capability. , The phenyl replacement with the bulkier N -naphthyl ring ( 3 , Figure ) switched the MOR efficacy profile from agonism to antagonism . Analogously, the increased steric hindrance of the aromatic moiety with an indoline, tetrahydroquinoline or diphenylamine group affected the shift from MOR agonism to antagonism . More recently, a dual MOR/DOR agonist, endowed of a significant long-lasting antinociceptive effect, − was developed through the introduction of the short and flexible 2 R / S -methoxy ethyl spacer as N -substituent (LP2 4 , Figure ).…”

“…Thus, methylation in ortho and meta is well tolerated while the para -methylation was unfavorable. In MOR-ligand interaction the negative influence of para substitution, with both electron-withdrawing or electron-donor groups, was outlined . A worse DOR and KOR binding profile was recorded for derivatives 7a – e .…”

“…21 Analogously, the increased steric hindrance of the aromatic moiety with an indoline, tetrahydroquinoline or diphenylamine group affected the shift from MOR agonism to antagonism. 22 short and flexible 2R/S-methoxy ethyl spacer as N-substituent (LP2 4, Figure 1). Moreover, the 2S diastereoisomer of LP2 was found to be a potent G-protein biased MOR/DOR agonist with a 3-times lower ED 50 value.…”

“…In MOR-ligand interaction the negative influence of para substitution, with both electron-withdrawing or electron-donor groups, was outlined. 22 A worse DOR and KOR binding profile was recorded for derivatives 7a−e. Indeed, in comparison to LP1 their DOR and KOR affinities were from 7-to 69-times and from 7-to 61-times lower, respectively.…”