N-alkylated dipeptide amides and related structures as imitations of the melanocortins' active core.

Thirty-three low molecular mass structures combining both peptide and peptoid features were prepared and tested on human melanocortin receptors MC1,3-5R. Most of them displayed low micromolar activity with preference for diamines, guanidino and 2-naphthyl derivatives compared to monoacetylated, amino and 3-indolyl counterparts. Some contained L- or D-histidine residues, but the change did not influence affinity. QSAR modelling yielded excellent models for the MC3-5 receptors explaining R2Y=0.89-0.91 and predicting Q2=0.77-0.80 of the affinity variations. One compound displayed MC1R selectivity (13-fold and more). An NMR study of showed that it exists as a mixture of four rotamers at its tertiary amide bonds. Comparisons with earlier data for melanocortin core tetrapeptide analogues indicate that the novel peptide-peptoids interact with the melanocortin receptors in a different way.

New substituted piperazines as ligands for melanocortin receptors. Correlation to the X-ray structure of "THIQ".

A series of piperazine analogues of the melanocortin 4 receptor (MC4R) specific small-molecule agonist "THIQ" was synthesized and characterized structurally and pharmacologically. First, several THIQ imitations lacking the triazole moiety were prepared. Syntheses included acylation of 4-phenylpiperazine or 4-cyclohexylpiperazine. In two cases the tertiary amine function was replaced by the corresponding N-oxide. To obtain more complex structures, a 4-substituted piperazine ring was formed by alkylation of the primary amino group of cyclohexane-derived amino alcohols with N,N-bis(2-chloroethyl)benzylamine. The hydroxylic group of the intermediate was first activated with methanesulfonyl chloride, and the sulfonic ester formed in situ was introduced into the reaction with the sodium salt of 1,2,4-triazole. In one case (i.e., preparation of 23c) introduction of the 1,2,4-triazole moiety was performed at a carbon of the cyclohexane ring. In addition, this intermediate contained a piperazine moiety connected via its nitrogen atom to a cyclohexane ring carbon neighboring the reaction center. As established in NMR and X-ray investigations herein, this substitution proceeded with retention of the initial trans configuration of 1,2-disubstituted cyclohexane. To obtain pure enantiomers of 23c, its precursor 21c was subjected to chiral chromatography on a Chirobiotic V column. The derivatives (R,R)-21cand (S,S)-21c obtained were introduced into further syntheses steps, giving (R,R)-23c and (S,S)-23c, respectively. Melanocortin MC(1,3-5) receptor binding studies showed that all tested piperazine derivatives were active. Several compounds showed clear selectivity for MC4R, with submicromolar affinities being obtained. Among them, one substance, (R,R)-23c, displayed a biphasic curve in displacement of [125I]NDP-MSH on MC4R [K(i)high = 1 nM and K(i)low = 260 nM]. This biphasic competition curve was similarly biphasic to the competition curve obtained herein using THIQ. An X-ray study performed on crystals of the THIQ sulfate salt revealed two closely related conformations, which resemble the shape of the letter "Y", where piperidine and 4-chlorophenyl groups are situated close to each other, but the 1,2,3,4-tetrahydroisoquinoline residue is remote, the triazole function being highly exposed to the environment. The crystals of the dinitrate salt of (R,R)-23c showed a different conformation, where parts of the molecule are spread out almost symmetrically around the central section. Molecular modeling, based on the THIQ crystal structure and the functional similarity of THIQ and (R,R)-23c, allowed us to suggest a possible "bioactive" conformation of (R,R)-23c that is similar to the crystal conformation of THIQ.