How a sun-tanning peptide accidentally led to sexual pharmacology
In the 1980s, a pharmacologist in Arizona tried to develop a melanotropin analog to prevent skin cancer through stimulation of endogenous tanning. An unexpected side effect during self-experiments — spontaneous erections — led two decades later to the approval of bremelanotide (PT-141) as a therapy for hypoactive sexual desire. A story of research serendipity, self-experimentation and the long path to approval.
An idea against skin cancer
In the late 1970s and early 1980s, melanoma diagnoses in the US rose dramatically. A group around Victor Hruby (peptide chemist) and Mac Hadley (endocrinologist) at the University of Arizona pursued an elegant idea: endogenous α-melanocyte-stimulating hormone (α-MSH) stimulates melanocytes to produce melanin — that is, endogenous tanning. Pharmacological stimulation of this pathway could theoretically reduce UV-induced DNA damage because more pigment protects the skin.
The challenge: native α-MSH has a half-life of a few minutes and is clinically unusable. Hruby's group developed stabilised analogs through the 1980s. One — a modified cyclic heptapeptide designated MT-II (melanotan-II) — showed the desired pigment stimulation in animal models and a substantially extended plasma half-life.
The self-experiment and the surprise
Before the formal toxicity phase, Hruby, Hadley and colleagues conducted a series of self-experiments — a historically not unusual practice in pharmacology, today ethically acceptable only in very narrow frameworks. Reported effects included expected (skin darkening) but also unexpected observations: nausea, appetite suppression, and — in the male subjects — spontaneous erections lasting several hours.
This observation was initially a curiosity. But it was systematically documented and led to the hypothesis that the central melanocortin receptor pathway — particularly MC4R and MC3R in the brain — could modulate sexual arousal, independent of the peripheral vascular mechanism on which sildenafil (Viagra, approved 1998) was being developed at the same time.
From tanning peptide to sexual pharmacology
In the mid-1990s the erection observation was taken pharmacologically seriously. Palatin Technologies licensed the substance pool from Arizona and developed a further shortened and more stable variant: PT-141 (bremelanotide), a cyclic heptapeptide with more selective MC4R affinity than melanotan-II. Clinical trials began in the late 1990s for erectile dysfunction and later evolved to the indication 'Hypoactive Sexual Desire Disorder' (HSDD) in pre-menopausal women.
The trial career was long and not linear. An early nasal formulation failed phase 3 in 2008 due to blood pressure increases. A lower subcutaneous dose was newly developed; the RECONNECT trials (phase 3, completed 2016) in pre-menopausal women with HSDD led in 2019 to FDA approval of Vyleesi (bremelanotide 1.75 mg s.c., as needed before sexual activity).
„We were originally looking for a sun protection. What we found was a signal transduction pathway for sexual arousal — and it kept us busy for 30 years."
The shadow life — melanotan-II as a research chemical
While bremelanotide went through the lengthy clinical path, melanotan-II — the original, less selective substance — spread from the mid-2000s in bodybuilding and tanning communities as an unapproved injectable tanning agent. Users reported skin darkening, but also the same side effects academic research had documented: nausea, sexual effects, appetite suppression. A PubMed-listed case series from Australia documented from 2009 onwards the appearance of new pigmented naevi and changes in existing moles in melanotan-II users — a clinically relevant safety signal largely ignored in user forums.
EMA and national authorities have repeatedly publicly warned against the use of melanotan-II. The FDA has classified it as an unapproved substance. Nevertheless, melanotan-II remains available on research-chemical markets to this day.
What the story shows pharmacologically
Three observations are interesting for the scientific assessment of this line. First: the separation between desired and undesired effect of a substance depends on the indication context. What is a side effect in a tanning peptide (erection) is the desired endpoint in an HSDD therapy. Second: the path from a self-experiment observation in the 1980s to regulatory approval in 2019 took nearly 40 years. Third: the parallel black-market story shows how hard it is to control a pharmacologically active substance after first publication — even when the approved form is not the original substance.
Open questions
- What is the clinical effect size of bremelanotide in HSDD compared with non-pharmacological therapies?
- What long-term safety signals (cardiovascular, naevi, pigmentation) emerge with continued use?
- Can MC4R-more-selective ligands with better side-effect profiles be developed?
- How should authorities deal with the persistent melanotan-II black market — education, prosecution, or both?