Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist
Pfaus JG, Shadiack A, Van Soest T, Tse M, Molinoff P
Proceedings of the National Academy of Sciences (2004)
PT-141 selectively increased appetitive sexual behaviors — solicitations and hops-and-darts — in ovariectomized female rats without affecting lordosis, pacing, locomotion, or place preference, the first pharmacological evidence that central melanocortin agonism modulates sexual desire specifically rather than reflexive sexual response.
This is the foundational preclinical paper for PT-141 / bremelanotide as a "desire" drug rather than a "performance" drug, and it is the mechanism citation that supports the entire clinical-development case that culminated in the 2019 FDA approval of Vyleesi for hypoactive sexual desire disorder. James Pfaus's group at Concordia, in collaboration with the Palatin Technologies team, administered PT-141 subcutaneously at 50, 100, or 200 μg/kg to ovariectomized female rats primed with estradiol-benzoate plus progesterone or with estradiol alone, then assayed sexual behavior in bilevel and unilevel testing chambers. At 100-200 μg/kg, PT-141 produced large and dose-related increases in solicitations and in hops-and-darts, the rat behaviors that ethologists classify as appetitive (desire-driven) approach behavior toward males. Critically, the drug did not alter lordosis (the reflexive consummatory posture), did not alter pacing behavior, did not increase generalized locomotor activity in an open-field test, and did not produce conditioned place preference (an index of generalized reward). The dissociation is the heart of the paper: PT-141 acts on the appetitive, desire-oriented dimension of sexual behavior specifically, without confounding effects on motor activation or reward. The authors propose central melanocortin 4 receptor activation in hypothalamic circuits as the mechanism, building on prior c-Fos imaging that had localized PT-141's CNS effects to the medial preoptic area and other hypothalamic nuclei.
This is a rodent behavioral pharmacology paper; the inferential bridge from rat solicitation behavior to human "hypoactive sexual desire disorder" is substantial and is built on the assumption that the ethological categories of appetitive vs. consummatory sexual behavior translate across species — an assumption that is reasonable but not formally validated. The dose range (50-200 μg/kg subcutaneous in rats) does not directly translate to the human dose (1.75 mg subcutaneous as-needed); cross-species pharmacology is handled by the subsequent clinical-development program rather than by this paper. The receptor-subtype selectivity story — MC4R as the primary target — is supported by the broader Palatin/Pfaus literature but is not directly demonstrated in this paper by receptor-subtype-knockout or selective-antagonist controls. Industry involvement is direct: Shadiack, Van Soest, Tse, and Molinoff are Palatin Technologies employees, and the paper is the foundational mechanism paper of the commercial development program. Pfaus is a long-standing academic collaborator with Palatin who has published extensively on melanocortin sexual behavior. The translation from this animal model to the eventual FDA-approved indication required a substantial clinical program; the rodent paper alone is necessary but not sufficient evidence for the human indication.
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