Melanotan 2, CAS 121062-08-6, is a cyclic analog of alpha melanocyte stimulating hormone built around a lactam bridge, catalogued in our dataset as a cyclic alpha-MSH analog. PT-141, CAS 189691-06-3 and catalogued as bremelanotide, was identified as the deamidated derivative of that cyclic analog and then developed as a compound in its own right. The ring system, the bridge and the core message residues are shared between them. Only the carboxy terminus differs.
The recorded formulas make the change visible. Melanotan 2 is C50H69N15O9 with an average mass of 1024.18. PT-141 is C50H68N14O10 with an average mass of 1025.16. The carbon count is identical. PT-141 carries one fewer nitrogen, one fewer hydrogen and one more oxygen, exactly the exchange expected when a terminal amide is hydrolysed to a carboxylic acid, and the mass rises by roughly one dalton. This is one of the rare cases where two distinct research compounds are separated by a single unit of mass, which makes identity confirmation against the theoretical mass a necessary step rather than a formality.
The consequence described in the literature is a shift in melanocortin receptor emphasis. The cyclic amide is reported as a broad agonist across melanocortin 1, 3, 4 and 5 receptors, with the melanocortin 1 receptor arm accounting for the pigment cell activity that dominates published work on it. The carboxylic acid form is characterised with the melanocortin 4 receptor as its principal focus, and most of the preclinical literature on it concerns central melanocortin 4 receptor signaling in rodent models rather than pigment cell responses. The selectivity is relative rather than absolute, since both engage multiple subtypes.
That difference determines what each compound is used to model. Studies that need a broad melanocortin agonist, or that examine melanocortin 1 receptor driven pigment cell behaviour in culture, use the cyclic amide. Studies aimed at the central melanocortin 4 receptor arm, including receptor binding assays and rodent behavioural neuroscience, use the acid form. A design that wants to attribute an effect to melanocortin 4 receptor signaling specifically will often run both, since a pair differing by one functional group makes an unusually clean structure activity comparison.
Handling is nearly identical, with one caveat that follows directly from the relationship between them. Because PT-141 is the hydrolysis product of melanotan 2, conditions that favour amide hydrolysis, meaning extended storage in solution, elevated temperature and non neutral pH, will slowly convert one into the other, which appears as a second peak on HPLC. Both are supplied lyophilized, held at minus 20 degrees Celsius protected from light and moisture, reconstituted by gentle swirling, refrigerated at 2 to 8 degrees Celsius once in solution, and aliquoted so that repeated freeze thaw is avoided. Neither is kept in solution longer than the assay requires.
What remains unsettled is the quantitative selectivity picture. Reported affinity ratios across melanocortin subtypes vary with the receptor construct, the species and the assay format, so the melanocortin 4 receptor preference of the acid form is best treated as a tendency described in the literature rather than as a fixed number.