Identification, structure-activity relationships and molecular modeling of potent triamine and piperazine opioid ligands

Abstract

Opioid receptors are important targets for pain management. Here, we report the synthesis and biological evaluation of three positional scanning combinatorial libraries, consisting of linear triamines and piperazines. A highly potent (14 nM) and selective (IC(50(mu))/IC(50(kappa))=71; IC(50(delta))/IC(50(kappa))=714) triamine for the kappa-opioid receptor was found. In addition, non-selective mu-kappa binders were obtained, with binding affinities of 54 nM and 22 nM for mu- and kappa-opioid receptors, respectively. Structure-activity relationships of each subset are described. 3D molecular alignments based on shape similarity to internal and external query molecules were carried out. For the combinatorial chemistry dataset studied here a 1.3 similarity cut-off value was observed to be efficient in the rocs-based alignment method. Interactions from the overlays analyzed in the binding sites of homology models of the receptors revealed specific substitution patterns for enhancing binding affinity in the piperazine series. Pharmacophore modeling of the compounds found from the three combinatorial libraries was also performed. The pharmacophore model indicated that the important feature for receptor binding activity with the mu-receptor was the presence of at least one hydrogen bond acceptor and one aromatic hydrophobic group. Whereas for the kappa-receptor two binding modes emerged with one set of compounds employing the hydrogen bond acceptor and aromatic hydrophobic group, and a second set possibly via interactions with the receptor by hydrophobic and ionic salt-bridges.

Figure 1

Structures of receptor-selective query molecules employed in the 3D similarity search: (i) JOM6; (ii) morphiceptin; (iii) fentanyl; (iv) morphine; (v) μ-internal query, iq(μ); (vi) 5′-guanidinium naltrindole; (vii) bicyclic guanidine; (viii) κ-internal query iq(κ).

Figure 2

Multi-fusion similarity maps: A) μ-queries, and C) κ-queries. B (μ-queries) and D (κ-queries) use only the max score towards two query molecules each (B: iq(μ) = Y axis; Fentanyl = X axis and D: iq(κ) = Y axis; BCG531 = X axis). Black: active (< 500 nM); gray: inactive ((> 500 nM)). The compound classes are: Triangles: derived from 760; Squares: derived from 761; Circles: derived from 762.

Figure 3

ROCS overlays between two μ-queries and compounds derived from the three libraries in the database. First row: iq(μ); Second row: fentanyl. The parent libraries are, from left to right: 762, 761 and 760. The Average combo score similarities are shown in parentheses.

Figure 4

ROCS overlays between three κ-queries and compounds derived from the three libraries in the database. First row: iq(κ); Second row: BCG531. The parent libraries are, from left to right: 762, 761 and 760. The Average combo score similarities are shown in parentheses.

Figure 5

Possible binding interactions of the two orientations of compound 64 with the μ-opioid receptor. The resulting orientations indicate when the compounds were overlaid with iq(μ) (Left) and fentanyl (Right).

Figure 6

A A six site-point pharmacophore model with no site features placed at R2. B. Overlay of the R2 S-4-hydroxybenzyl of compounds 11 and 14, with R1 S-4-hydroxybenzyl of compound 59. Green: hydrophobic (H); blue: positively ionizable (P); orange: aromatic (R); red: hydrogen bond acceptor (A).

Figure 7

The efficiency of the μ-hypotheses (A) and κ-hypothesis (B) in terms of the percent of all compounds as well as actives retrieved from the database with increasing restrictions on the volume cutoff variable during the database search.

Scheme 1

Solid-phase synthesis of triamines and piperazines from N-acylated dipeptides (1). Reduction of the amide groups of the resin-bound N-acylated dipeptide yielded a polyamine. Cleavage of the polyamine with HF yielded the N-methyl triamine (X=Methyl). Treatment of the resin-bound polyamine with oxalyldiimidazole yielded the diketopiperazine, and further treatment with diborane in THF followed by HF cleavage yielded the N-methylated and N-benzylated 1,3,4-trisubstituted piperazines (3, X=Methyl or Benzyl).