Selective, high affinity peptide antagonists of alpha-melanotropin action at human melanocortin receptor 4: Their synthesis and biological evaluationin vitro

Peptide Ae-Nle(4)-cyclo(5 beta --> 10 epsilon)(Asp(5)-His(6)-D-(2')Nal(7)-A rg(8)-Trp(9)-Lys(10))-NH2, compound 1, a cyclic derivative of a-melanotropi n, is a nonselective high affinity antagonist at human melanocortin recepto rs 3 and 4, and an agonist at melanocortin receptors 1 and 5. To differenti ate between the physiological functions of these receptors, antagonists wit h improved receptor selectivity are needed. In this study, analogues of com pound 1 without Ac-Nle(4) or His(6) and/or the amino group of Asp(5) were p repared and tested in binding assays and in functional assays on CHO cells expressing hMC3-5R. Several of these peptides were to be selective, high af finity hMC-4R antagonists. The most interesting was compound 10, named MBP1 0, cyclo(6 beta --> 10 epsilon)-(succinyl(6)-D-(2')Nal(7)-Arg(8)-Trp(9)-Lys (10))-NH2, an antagonist (IC50 = 0.5 nM) with 125-fold selectivity over hMC -3R (and of > 300-fold selectivity over MC-1RB). This compound had no agoni st activity at hMC-3R or hMC-4R and only weak agonist activity at hMC-5R. E xamination of the sequences of these new peptides revealed that the D-(2')N al(7)-Arg(8)-Trp(9) segment of peptide 1 forms the "essential core" require d for high affinity and high selectivity of analogues of peptide 1 at hMC-4 R, but the "extended core", His(6)-D-(2')Nal(7)-Arg(8)-Trp(9), is necessary for the maximum affinity for hMC-3R and hMC-5R.