Stimulation of the Melanocortin-1-Receptor (MC1R) by its endogenous ligands, α-MSH and ACTH, has been found to increase MC1R expression, promote the biosynthesis of melanin, increase melanosome formation, and increase melanocyte dendricity. All of these factors contribute to the overall survival of human melanocytes and result in a decrease in the incidence of melanoma, the most dangerous type of skin cancer. LK-184 and LK-394, synthetic analogs of α-MSH, are potent peptide agonists of hMC1R (EC50 = 0.009, 5.0 nM)88-90, but lack any selectivity for MC1R over the other melanocortin receptors, most notably MC5R.
In order to develop a more comprehensive SAR of the N-terminus, a series of N-capped tripeptides was synthesized and screened for MC1R-MC5R activity with the cloned mouse melanocortin receptors. The tripeptides were also assayed on cultured human melanocytes to determine tyrosinase activity, a marker for stimulation of MC1R. In all, 24 tripeptides were synthesized and tested with EC50 values for mMC1R ranging from 83-8200 nM. Contrary to LK-184, these tripeptides showed selectivity form mMC1R over all other mMCRs including MC5R.
In addition to the N-capping study, a number of C-terminally modified and arginine modified tripeptides were synthesized and assayed in order to map these regions of the receptor. The other goal was to develop tripeptide analogs of LK-394 that would be more hydrophobic and lack the positively charged arginine residue in order to potentially improve penetration through the skin. These compounds had mMC1R EC50 values ranging from 319-45000 nM and also showed selectivity for mMC1R over the other melanocortin receptors. In all, over 30 tripeptide analogs of LK-394 were synthesized and evaluated for melanocortin receptor activity resulting in a number of compounds that were found to be potent and selective MC1R agonists.