Michael acceptor approach to the design of new salvinorin A-based high affinity ligands for the kappa-opioid receptor

Abstract

The neoclerodane diterpenoid salvinorin A is a major secondary metabolite isolated from the psychoactive plant Salvia divinorum. Salvinorin A has been shown to have high affinity and selectivity for the κ-opioid receptor (KOR). To study the ligand–receptor interactions that occur between salvinorin A and the KOR, a new series of salvinorin A derivatives bearing potentially reactive Michael acceptor functional groups at C-2 was synthesized and used to probe the salvinorin A binding site. The κ-, δ-, and μ-opioid receptor (KOR, DOR and MOR, respectively) binding affinities and KOR efficacies were measured for the new compounds. Although none showed wash-resistant irreversible binding, most of them showed high affinity for the KOR, and some exhibited dual affinity to KOR and MOR. Molecular modeling techniques based on the recently-determined crystal structure of the KOR combined with results from mutagenesis studies, competitive binding, functional assays and structure–activity relationships, and previous salvinorin A–KOR interaction models were used to identify putative interaction modes of the new compounds with the KOR and MOR.

Synthesis and κ-Opioid Receptor Activity of Furan-Substituted Salvinorin A Analogues

“… 45 This has led to the proposal of several binding models including one based on the recently solved KOR crystal structure. 42 , 46 − 49 …”

Opioid receptors: Structural and mechanistic insights into pharmacology and signaling

“…A number of investigators have used various computational tools, including molecular docking, molecular dynamics (MD), free-energy perturbations, and ab initio calculations (e.g., see (Goldfeld et al, 2015; Leonis et al, 2014; Martinez-Mayorga et al, 2013; Polepally et al, 2014; Vardy et al, 2013; Wu et al, 2012)) to provide a molecular description of both the binding and function of selective κ receptor ligands at the κ receptor crystal structure, in relation to the crystallographic binding pose of the highly selective antagonist JDTic ((3R)-1,2,3,4-tetrahydro-7-hydroxy-N-[(1S)-1-[[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl]-2-methylpropyl]-3-isoquinolinecarboxamide) (Wu et al, 2012). For instance, comparison of the predicted poses of the κ receptor selective morphine-based antagonists nor-binaltorphimine (nor-BNI) and 5’-guanidinonaltrindole (GNTI) with the crystallographic pose of JDTic emphasized the role of κ receptor residues V2.53 (A in μ and δ receptors), V2.63 (N in μ receptor and K in δ receptor), I6.55 (V in μ and δ receptors), and Y7.35 (W in μ receptor and L in δ receptor) as molecular determinants of the binding selectivity of JDTic for κ receptor (Wu et al, 2012).…”

“…The κ receptor crystal structure was also recently used to rationalize the binding of several different derivatives of the selective non-nitrogenous diterpene κ receptor agonist salvinorin A (Polepally et al, 2014), including 22-thiocyanatosalvinorin A (RB-64) (Wu et al, 2012). A model of the latter was based on previous experimental evidence suggesting that the agonist 2-acetoxy moiety forms an irreversible, covalent bond with the κ receptor residue C7.38.…”

“…According to this docking hypothesis, the fused tricyclic scaffold of salvinorin A is oriented towards TM2, while the C-2 acetoxy group of the ligand is accommodated in a pocket surrounded by residues D3.32, Y3.33 and M3.36. Different was the case for the recently synthesized series of salvinorin A derivatives containing potentially reactive Michael acceptor functional groups at the C-2 position (Polepally et al, 2014), which cannot form an irreversible bond with C7.38. While the predicted binding was somehow similar to that of salvinorin A, the bulky substitutions at the C-2 position could not be accommodated in the same pocket, and led to a decreased affinity of the ligands for κ receptor.…”

“…Notably, the polarity introduced by a substitution with a heterocyclic nitrogen atom made the ligand loose affinity for the κ receptor while gaining it at the μ receptor. A possible explanation for this κ/μ receptor selectivity is that the ligand adopts an alternative binding mode in the μ receptor stabilized by an interaction with the non-conserved residue N2.63 (V in κ receptor and K in δ receptor) (Polepally et al, 2014). MD simulations, free energy, and ab initio calculations of the κ receptor crystal structure in complex with salvinorin A or JDTic showed that the ligands are stabilized within the binding pocket through interactions with different residues (Leonis et al, 2014).…”