Design, Synthesis, and Evaluation of Novel (-)-cis-N-Normetazocine Derivatives: In Vitro and Molecular Modeling Insights

Abstract

Suitable structural modifications of the functional groups at N-substituent of (-)-cis-N-normetazocine nucleus modulate the affinity and activity profile of related ligands toward opioid receptors. Our research group has developed several compounds and the most interesting ligands, LP1 and LP2, exhibited a dual-target profile for mu-opioid receptor (MOR) and delta-opioid receptor (DOR). Recent structure-affinity relationship studies led to the discovery of novel LP2 analogs (compounds 1 and 2), which demonstrated high MOR affinity in the nanomolar range. Here, we reported the synthesis of the new (-)-cis-N-normetazocine derivatives (3-8) characterized by the absence of the phenyl ring in the N-substituent compared to all previous reported ligands. Compounds 3 and 4, featuring a methyl ester functional group in the N-substituent, retained significant MOR affinity and exhibited similar affinity for the kappa-opioid receptor (KOR). In contrast, compounds 7 and 8, which contain a hydroxamic acid functionality, maintained affinity exclusively toward MOR. Neither of compounds (3-8) showed DOR affinity. Molecular modeling studies confirmed a similar docking pose in the MOR binding pocket for these compounds. Additionally, the in silico ADME profile of the most interesting ligands (3, 4, 7, and 8) was investigated revealing a favorable profile for compound 7 regarding the blood-brain barrier permeability, suggesting its potential as a peripherally restricted opioid ligand.

Keywords: ADME studies; cis‐N‐normetazocine; docking studies; mu opioid receptor; radioligand competition‐binding.

FIGURE 1

Structures of LP1 and its analogs.

FIGURE 2

Structures of LP2 and its analogs 1 and 2 (Costanzo et al. 2023) with the sites of modification and the novel synthesized compounds (3–8).

SCHEME 1

Synthesis of target compounds 3–8: (a) methyl 3‐bromopropanoate or methyl 4‐chlorobutanoate, NaHCO₃, KI, DMF, 55°C, 24 h; (b) NaOH 1 M, 110°C, 5 h; (c) NH₂OH hydrochloride, NaOMe, BuOH, r.t., 24 h.

FIGURE 3

Docking simulations 2D and 3D diagrams for newly synthesized compounds with MOR receptor (PDB ID: 5C1M). (a) 3@MOR; (b) 7@MOR; (c) 4@MOR; (d) 8@MOR. Colors represent the type of interaction: Hydrophobic (light pink and light green), π (pink and purple), hydrogen bond (green), and salt bridge (orange).

FIGURE 4

Docking simulations 2D diagrams for preceding synthesized compounds with MOR (PDB ID: 5C1M) and KOR (PDB ID: 4DJH) receptors as reference. (a) LP2@MOR; (b) 1@MOR; (c) 2@MOR; (d) LP2@KOR; (e) 1@KOR. Colors represent the type of interaction: Hydrophobic (light pink and light green), π (pink and purple), hydrogen bond (green), and salt bridge (orange).

FIGURE 5

Docking simulations 2D and 3D diagrams for newly synthesized compounds with KOR receptor (PDB ID: 4DJH). (a) 3@KOR; (b) 4@KOR. Colors represent the type of interaction: Hydrophobic (light pink and light green), π (pink and purple), hydrogen bond (green), and salt bridge (orange).

FIGURE 6

Total energy and ligand RMSD for 7@MOR complex.

FIGURE 7

BOILED‐Egg model: The yellow sphere represents the brain permeation region, and the oval white part represents the gastrointestinal absorption region. The grey area represents the minimal absorption and partial brain permeation region. The molecule is designed as a blue sphere. The sphere is colored blue if predicted as actively effluxes by P‐gp (PGP+) and red if predicted as non‐substrate of P‐gp (PGP−).