PT-141: The Tanning Experiment That Led to a Desire Drug

Mac Hadley was studying peptides that could darken skin when he injected an experimental compound called Melanotan II. He later described spending about eight hours with an erection, feeling nauseated, with a basin beside his bed.

When his wife found him, his response was: “I think we may become rich.” This was a dangerous self-experiment, recorded in his own account of the discovery. Hadley, Peptides, 2005

That unexpected effect opened a line of research into sexual function. A related peptide, bremelanotide, became known as PT-141. In 2019, the FDA approved an injected version called Vyleesi for certain premenopausal women whose low sexual desire causes distress.

The path from tanning research to that approval contains a question worth asking before anyone buys a libido treatment: what change would make the treatment worthwhile to the person taking it?

Peptides are short chains of amino acids. Some carry messages between cells by attaching to receptors, proteins that receive a chemical signal and start a response.

Bremelanotide activates several members of the melanocortin receptor family. A substance that activates a receptor is called an agonist. Two of its relevant targets are MC1 and MC4, which have different jobs in the body.

MC1 helps regulate pigment production in skin cells. MC4 receptors occur in multiple parts of the central nervous system, which includes the brain. These targets help explain how research that began with pigmentation could lead scientists toward sexual function. The FDA's review describes both targets. FDA risk review, 2019

A receptor map gives us a starting point for understanding a drug. It leaves much of the human experience unexplained. The prescribing information says the mechanism by which Vyleesi improves low sexual desire remains unknown. Claims that it fixes a particular person's dopamine problem go beyond that evidence. Vyleesi prescribing information

The clinical trial therefore had to answer a practical question: after taking the medicine, did women report a worthwhile change in their symptoms?

The condition behind the approval is hypoactive sexual desire disorder, usually shortened to HSDD. For Vyleesi, the approved indication is acquired, generalized HSDD in premenopausal women.

“Acquired” means the loss of desire developed after a period without that problem. “Generalized” means it occurs across situations or partners, rather than being confined to one circumstance. The low desire must cause marked distress or interpersonal difficulty. A clinician also has to consider whether another medical condition, a medication or a relationship problem explains it. FDA benefit-risk assessment

Those definitions give the conversation a place to begin. Someone can describe when their desire changed, how broadly the change appears, and what it has cost them. The history matters because people can use the same phrase, “low libido,” for experiences that need different care.

The FDA heard some of those differences directly at a 2014 public meeting. Participants described loss of interest, difficulty becoming physically aroused and the effect on how they felt about themselves. Some had begun avoiding ordinary affection because they worried it would lead to a sexual encounter they did not want.

The meeting included a range of views about causes and treatment. Its accounts were not a representative survey. They do show why asking only how often someone has sex can miss the problem they came for help with. FDA, The Voice of the Patient

The pivotal RECONNECT program used two randomized trials. Together, they assigned 1,267 women to bremelanotide or placebo, with a 24-week treatment period. Participants used an injection before anticipated sexual activity. The studies enrolled women with acquired, generalized HSDD who were in stable relationships. The average age was about 39.

The drug improved both main measures compared with placebo. The published paper also reports substantial withdrawals, and the participants were mostly white. Those details affect how confidently we can apply the results to a different patient. The studies were funded by the drug's developer. Kingsberg et al., Obstetrics & Gynecology, 2019

The first measure asked about the frequency and strength of sexual desire over the preceding four weeks. Its score runs from 1.2 to 6.0, with higher scores indicating more desire. In the label's first trial, the mean increase was 0.5 points on bremelanotide and 0.2 on placebo. In the second, the increases were 0.6 and 0.2.

The other measure asked how often the participant felt bothered by low desire. That score runs from 0 to 4, with lower scores indicating less distress. The mean decrease was 0.7 points on bremelanotide and 0.4 on placebo in each trial. These are changes in questionnaire scores, not percentages of women cured. Vyleesi clinical-study tables

The FDA described the average benefit as modest. That judgment leaves room for differences between patients. An average can combine women who notice a useful improvement with others who gain little or stop treatment. It cannot tell one reader what their own experience will be.

The choice of measurement also matters. A questionnaire asking about a month includes days when the participant took no drug. FDA reviewers noted that this recall period could make an intermittent effect harder to detect. That is a limitation of the measurement; it does not establish a larger benefit than the trial found. FDA multidisciplinary review

The trials also counted satisfying sexual events associated with treatment. That comparison did not show a significant advantage over placebo.

FDA reviewers explained why the medicine could still qualify for approval: the defining symptoms of HSDD are low desire and the associated distress. A count of satisfying events is a separate outcome. The agency required evidence that treatment improved the defining symptoms. FDA benefit-risk assessment

The patient meeting helps make that distinction concrete. Some participants valued feeling desire even when it did not lead to a sexual event. Views also differed on what made an encounter satisfying. Closeness mattered to some; others placed more weight on orgasm. A single event count could not express all of those preferences. FDA patient report

For me, the useful way to read these findings is to keep the outcomes visible. Someone hoping to regain interest can learn from the desire result. A claim that the treatment reliably creates more satisfying encounters would need evidence beyond what these trials showed.

A benefit also has to be worth the experience of taking the medicine. In the FDA risk review, nausea occurred in about 40% of bremelanotide recipients and about 1% of placebo recipients. Roughly 8% stopped bremelanotide because of nausea.

Across all adverse reactions, 17.5% of the drug group stopped treatment, compared with 2.3% of the placebo group. These figures answer different questions: experiencing nausea is more common than stopping because of it. FDA risk review

Bremelanotide can temporarily raise blood pressure. It is contraindicated in people with uncontrolled high blood pressure or known cardiovascular disease. It can also cause patches of darker pigmentation, which did not resolve in every affected participant during follow-up. The original pigmentation biology remains relevant to its clinical use. FDA review

The current label includes medication-interaction and pregnancy precautions. It also directs patients to stop after eight weeks if symptoms have not improved. Approval applies to Vyleesi's injected product and defined population. The trials do not establish equivalent results for every spray, troche or vial sold as PT-141. Current prescribing information

If a loss of desire is bothering you, write down when it began and what else changed around that time. Include medicines you take and any pain or other symptoms. A clinician can use that history to assess what may be contributing and discuss treatment options.

Before starting a treatment, describe what a useful improvement would feel like in your own life. That gives the follow-up visit something specific to examine. Side effects and the effort of using the treatment belong in the same conversation as benefit.

Reply with the question about libido-treatment evidence you want me to cover next. No personal health details needed.

For entertainment and education only. Not medical advice. Talk to your doctor before starting anything.