Late-Stage Functionalization with Cysteine Staples Generates Potent and Selective Melanocortin Receptor‑1 Agonists

In this work, cysteine staples were used as a late-stage
functionalization
strategy to diversify peptides and build conjugates targeting the
melanocortin G-protein-coupled receptors [melanocortin receptor-1
(MC1R) and MC3R–MC5R]. Monocyclic and bicyclic agonists based
on sunflower trypsin inhibitor-1 were used to generate a selection
of stapled peptides that were evaluated for binding (pKi) and functional activation (pEC50) of the
melanocortin receptor subtypes. Stapled peptides generally had improved
activity, with aromatic stapled peptides yielding selective MC1R agonists,
including a xylene-stapled peptide (2) with an EC50 of 1.9 nM for MC1R and >150-fold selectivity for MC3R
and
MC4R. Selected stapled peptides were further functionalized with linkers
and payloads, generating a series of conjugated peptides with potent
MC1R activity, including one pyridazine-functionalized peptide (21) with picomolar activity at MC1R (Ki 58 pM; EC50 < 9 pM). This work demonstrates
that staples can be used as modular synthetic tools to tune potency
and selectivity in peptide-based drug design.