Bremelanotide is a synthetic cyclic heptapeptide analogue of α-melanocyte-stimulating hormone. Cyclisation is achieved through a side-chain lactam bridge between the aspartate at position 2 and the lysine at position 7, which constrains the backbone and confers resistance to exopeptidase degradation. It retains the His-Phe-Arg-Trp melanocortin message sequence with a D-phenylalanine substitution, and carries an N-terminal acetyl group and a norleucine at position 1.

The relationship to Melanotan II is the single most important identity point in this listing, and the two are separate products. Bremelanotide differs from Melanotan II at one position only: it terminates in a free carboxylic acid where Melanotan II terminates in a carboxamide. It is the des-amido metabolite of Melanotan II. The verified formulas confirm this directly — C50H68N14O10 against C50H69N15O9, exactly the loss of one nitrogen and one hydrogen and the gain of one oxygen. The masses are close enough (1025.2 versus 1024.2) that a careless catalogue can transpose them, and analytical confusion between the two is a real risk.

Pharmacologically it is described as a non-selective melanocortin receptor agonist with activity at MC1R, MC3R and MC4R; centrally mediated MC4R activity is the stated basis for its investigation in sexual desire disorders. The FDA approved bremelanotide in June 2019 as Vyleesi for acquired, generalised hypoactive sexual desire disorder in premenopausal women. Two honest limitations attach to that approval. First, the approved labelling itself states that the mechanism by which the drug acts on sexual desire is unknown. Second, the approved product carries labelled warnings including transient increases in blood pressure and a high rate of nausea. Research-grade material is not the approved product and has not been through its controls.