¹H‐NMR studies on a potent and selective antagonist at human melanocortin receptor 4 (hMC‐4R)

Abstract

Melanocortin receptor 4 (MC-4R) is involved in the regulation of energy balance and body weight, and recognizes alpha-, beta-, and gamma-melanocyte stimulating hormones (alpha-, beta-, gamma-MSH). In the search for compounds that regulate food intake and body weight, two synthetic lactam-derivative ligands of alpha-MSH were discovered, MTII and SHU9119. MTII is an agonist and reduces food intake in rats, whereas SHU9119 is an antagonist, and increases food intake and body weight in rats. MTII and SHU9119 are nonselective compounds to MC-4R. To enhance the potency and selectivity at the human MC-4R (hMC-4R), we recently synthesized analogs of SHU9119 (M. A. Bednarek, T. MacNeil, R. N. Kalyani, R. Tang, Van der L. H. T. Ploeg, and D. H. Weinberg, Journal of Medicinal Chemistry, 2001, Vol. 44, pp. 401-409), wherein compound 1 was the most selective for hMC-4R. Replacement of D-Nal by L-Nal in compound 1 made compound 2 weakly active. Comparison of the structures by NMR and molecular modeling of compounds 1 and 2 vs SHU9119 and MTII indicated that, even though they existed as an average of several conformations in solution, there were distinctions in their structures. The gamma-methylene protons of Arg in compound 1 were nonequivalent and shielded probably by the aromatic ring of Nal. The NHi-NHi+1 NOE cross peaks and the temperature coefficients of the amide protons around the "essential core" Nal/Phe7-Arg8-Trp9, required for high affinity and high selectivity at hMC-4R, were indicative of a possible turn structure for these compounds but with differences in their NOE strengths and temperature coefficient values. Molecular modeling of these compounds based on their NMR data showed that the essential core appeared as a "V" shape with two different orientations, one for compound 1 and some of the conformers of SHU9119 and MTII, and the other for compound 2 and some other conformers of SHU9119 and MTII. The remaining conformers of SHU9119 and MTII, which did not map to compound 1 or 2, suggested that they were outside of the hMC-4R binding envelop. These observations may lead to conjectures as to why compound 1 is highly active and selective toward hMC-4R.

Vector-Mediated Transport Producing Drug-like Peptides

“…5S), with two conformational populations. This is consistent with the evidence for multiple conformations in solution for SHU9119 (Victoria Silva Elipe et al, 2003).Thus, the His for Pro substitution may have a global disruptive impact on the secondary structure of this peptide. This lack of structural stability could be related to the absence of transport activity in TCMCB03.…”

Structure−Activity Relationships of the Unique and Potent Agouti-Related Protein (AGRP)−Melanocortin Chimeric Tyr-c[β-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH2 Peptide Template

“…These data are consistent with previous reports of NMR-based structural studies of the cyclic synthetic melanocortin peptides MTII and SHU9119 in addition to linear R-MSH analogues that observed similar findings. 35,36 Peptide…”

Structure–Activity Relationships of Peptides Incorporating a Bioactive Reverse-Turn Heterocycle at the Melanocortin Receptors: Identification of a 5800-fold Mouse Melanocortin-3 Receptor (mMC3R) Selective Antagonist/Partial Agonist versus the Mouse Melanocortin-4 Receptor (mMC4R)

“…These results are consistent with previous biophysical studies on various potent melanocortin peptide ligands that identified a reverse turn structure in His-DPhe-Arg-Trp region of the ligands. 87–90 …”