SAR investigation and synergistic optimization of setmelanotide yields a potent, selective, and soluble MC4R agonist.

The bioactive peptide setmelanotide is a validated MC4R agonist, yet its clinical utility is constrained by poor aqueous solubility and dose-limiting, off-target hyperpigmentation. To overcome these dual liabilities, we executed a synergistic optimization strategy guided by detailed SAR investigation. This approach unveiled two critical design principles: a C-terminal "cationic imperative", where lysine uniquely conferred a > 20-fold solubility enhancement while retaining potency, and rational manipulation of the core pharmacophore, which imparted >100-fold selectivity over MC1R/MC3R. This synergy yielded the lead compound SC19, which integrates these features into a balanced profile of sub-nanomolar potency (EC₅₀ = 0.12 nM; pEC₅₀ = 9.93), exceptional selectivity, and high aqueous solubility. In a diet-induced obesity model, SC19 demonstrated robust efficacy comparable to setmelanotide in reducing weight gain and improving lipid profiles, affirming its therapeutic potential. This work not only presents a promising lead compound but also validates a synergistic optimization blueprint for concurrently enhancing the pharmacological and drug-like properties of therapeutic peptides.