PT-141 (bremelanotide): the tanning experiment that became a desire drug

A libido drug that started as a suntan

Erectile-dysfunction pills such as sildenafil act on blood vessels. PT-141 is different: it acts in the brain, on the same hormone system that darkens skin. That odd pairing is not a marketing story. It is the direct result of a laboratory surprise three decades ago, when men testing an experimental tanning injection noticed something nobody had planned for.

Today bremelanotide is an approved medicine in the US, used as needed rather than daily ^[11,13]. It is also a good case study in reading evidence carefully (see our guide to levels of evidence), because its benefits are real but modest, and its side effects are common.

From Tucson tanning lab to the FDA

At the University of Arizona, chemist Victor Hruby, pharmacologist Robert Dorr and cell biologist Mac Hadley set out to build better versions of alpha-melanocyte-stimulating hormone (alpha-MSH), the body's own tanning signal ^[1,14]. The goal was protective: a tan made without ultraviolet light might lower the risk of skin cancer ^[14].

One candidate was a linear peptide, melanotan I, whose first human trial results were published in 1991 ^[1,14]. By 1989 the team had also designed potent, long-acting ring-shaped versions of the hormone's active core, held in shape by a chemical bridge called a lactam ^[2]. One of these became melanotan II, a cyclic analogue that resists breakdown by enzymes ^[1,3].

In a 1996 pilot study of melanotan II in three healthy men, two developed a tan, and the injections were also followed by spontaneous erections lasting one to five hours ^[3].

The researchers followed the lead. In a 1998 double-blind crossover study of 10 men with psychogenic erectile dysfunction, 8 developed clinically apparent erections on melanotan II, with an average of 38 minutes of strong rigidity against 3 minutes on placebo ^[4]. Nausea, yawning and stretching were common ^[4].

Palatin Technologies then developed PT-141 as a new melanotan II analogue ^[1]. The two share the same seven-residue ring; melanotan II ends in an amide group, while bremelanotide ends in a free carboxylic acid ^[3,11]. Palatin first tested PT-141 in men with erectile dysfunction, where it produced rapid, dose-dependent erections ^[5].

The turn towards women came from animal work. In a 2004 study in female rats, PT-141 selectively increased "solicitation", the rat equivalent of initiating sex, without changing reflexive mating postures or general activity ^[6]. That pointed to an effect on desire rather than on genital mechanics, and the clinical programme moved to women with HSDD ^[7,8].

The melanotan story had a second ending. Melanotan I, renamed afamelanotide, won European approval in 2014 for erythropoietic protoporphyria, a rare disorder in which sunlight triggers painful skin reactions ^[14]. Neither Arizona molecule became the mass-market sunless tan its inventors first had in mind.

How a skin-tanning signal reaches desire

The melanocortin system is a family of five receptors, MC1R to MC5R, that respond to alpha-MSH and related hormones. MC1R sits on pigment cells in the skin ^[11]. MC3R and MC4R are expressed mainly in the central nervous system ^[5].

Bremelanotide is not picky. According to its FDA label, it activates several of these receptors, with the highest potency at MC1R, followed by MC4R, MC3R, MC5R and MC2R ^[11]. At the doses used clinically, MC1R and MC4R binding is thought to matter most ^[11]. The MC1R action explains the occasional darkening of skin and gums ^[11]; the MC4R action is thought to be the one that matters for sexual function ^[10].

A useful way to picture the difference from erectile-dysfunction pills: sildenafil works on the plumbing, widening blood vessels once arousal has begun. Bremelanotide works on the switchboard, nudging brain circuits that decide whether sexual cues get noticed at all. In rats, PT-141 activated neurons in the hypothalamus, a control centre for hormones and drives ^[5].

Still, the label is blunt: "The mechanism by which VYLEESI improves HSDD in women is unknown" ^[11]. Other peptides studied in sexual and social behaviour, such as kisspeptin-10 and oxytocin, work through entirely different hormone systems.

What the research shows

Early human work in men

Before the focus moved to women, small Palatin studies in healthy men and men with erectile dysfunction showed rapid, dose-dependent increases in erectile activity ^[5]. Bremelanotide was never approved for men, and the US label states that it is not indicated for them ^[11].

The dose-finding trial

A phase 2b trial randomised premenopausal women with sexual dysfunction to placebo or one of three bremelanotide doses for 12 weeks; 327 were analysed for efficacy ^[7]. In the pooled higher-dose arms, satisfying sexual events rose by 0.7 per month against 0.2 on placebo, a small but statistically significant difference ^[7].

RECONNECT: the two phase 3 trials

The approval rests on two identical 24-week trials, studies 301 and 302, known together as RECONNECT. They randomised 1,267 premenopausal women with HSDD, mean age 39, almost all at US sites ^[8]. Women self-injected the drug as needed before anticipated sexual activity.

Both co-primary endpoints improved significantly over placebo ^[8]. Desire, measured on a 1.2 to 6.0 scale, rose by an extra 0.30 and 0.42 points in the two studies; distress about low desire, on a 0 to 4 scale, fell by an extra 0.37 and 0.29 points ^[8].

Sources: FDA press release ^[12] and FDA label ^[11].

Two findings deserve emphasis. First, the number of satisfying sexual events did not differ from placebo in either study ^[11]. Second, many more women on the drug left the trials early: 40% against 13% in study 301 and 39% against 25% in study 302 ^[11].

Side effects: nausea, blood pressure and pigmentation

Nausea was the most frequent problem: 40% of women on bremelanotide reported it, 13% needed anti-nausea medication and 8% quit because of it ^[11]. It was most common after the first dose (21%) and much rarer afterwards ^[11]. Flushing affected about 20% and headache 11% ^[11].

Each dose briefly raises blood pressure, by up to 6 mmHg systolic and 3 mmHg diastolic, peaking two to four hours after injection, while heart rate falls by up to 5 beats per minute; values usually return to baseline within 12 hours ^[11]. For that reason the drug is contraindicated in uncontrolled hypertension or known cardiovascular disease ^[11].

Focal darkening of the face, gums or breasts occurred in 1% of trial participants. In a separate study with daily dosing for eight days, 38% developed it, and resolution was not confirmed in all cases ^[11].

Inside the brain

A 2022 Imperial College London crossover study scanned the brains of 31 premenopausal women with HSDD after bremelanotide or placebo ^[13]. The drug increased self-reported desire for up to 24 hours and changed activity in brain regions that process erotic images, which the authors read as reduced self-consciousness and greater sensitivity to sexual cues ^[13].

Beyond desire: appetite

MC4R is also a target in obesity research, and Palatin has tested bremelanotide there. In an eight-week phase 2 study announced in March 2025, 96 people were randomised; those given bremelanotide plus low-dose tirzepatide lost 4.4% of body weight against 1.6% on placebo ^[15]. This is a company press release about a small, short study, not yet a peer-reviewed paper.

What we don't know yet

• How it works in HSDD. The label calls the mechanism unknown ^[11]. The brain-imaging work is a start, but it is one 31-person study ^[13].
• How large the benefit is in daily life. The gap over placebo was about 8 percentage points for meaningful desire gains and about 4 points for distress ^[12], and the number of satisfying sexual events did not change ^[11].
• Who it works for. Trial participants were 86% white and almost all from the US ^[8,11]. Postmenopausal women and men were not part of the approval ^[11].
• Dropout. The high early discontinuation rate on the drug makes the results harder to interpret ^[11].
• Long-term skin effects. Whether pigment changes always fade after stopping is not established ^[11].

Regulatory status

• United States: approved by the FDA on 21 June 2019 as Vyleesi for acquired, generalised HSDD in premenopausal women, where the low desire is not explained by another medical or psychiatric condition, relationship problems or a medication ^[11,12]. It is not indicated for postmenopausal women, for men, or to enhance sexual performance ^[11]. Palatin licensed North American rights to AMAG Pharmaceuticals, which filed the application ^[10].
• European Union: we found no EU marketing authorisation for bremelanotide; Vyleesi's approval is a US one.

In the lab: handling and storage

Research-grade PT-141 (product page) is typically supplied as a lyophilised (freeze-dried) powder. Long-term storage is usually at −20 °C, with 2–8 °C acceptable for shorter periods, protected from light and moisture. Its ring contains tryptophan and histidine, residues that are sensitive to oxidation, so vials are best kept sealed until use. For comparison, the finished US medicine is a solution labelled for storage at or below 25 °C, protected from light and not frozen ^[11]. Once reconstituted, common lab practice is refrigeration and use within a short window; our guide to storing research peptides covers the general principles.

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