The Cold Open
Bremelanotide, sold as Vyleesi, is one of the few peptides in this newsletter that cleared every regulatory hurdle: Phase I, II, III, FDA approval. It arrived in 2019 as the first prescription drug for low sexual desire in premenopausal women. Then it had a commercial life that was, let's say, underwhelming. This month: what it does, what the trials found, and what the gap between regulatory success and real-world uptake tells us.
Peptide of the Month: Bremelanotide
What it is. Bremelanotide (also called PT-141) is a cyclic seven-amino-acid peptide, an active metabolite of Melanotan II with the tanning and worst side effects trimmed away. Think of MT-II as a shotgun and Bremelanotide as a rifle pointed at the same pathway. Both hit melanocortin 4 receptor (MC4R). Only Bremelanotide does it selectively enough to be a drug.
What it's supposedly for. FDA-approved for Hypoactive Sexual Desire Disorder (HSDD) in premenopausal women. Off-label, clinics prescribe it for male ED, especially in men who don't respond to Viagra or Cialis.
The evidence as of today. The case for Bremelanotide is unusually strong by this newsletter's standards. A 2004 Phase II intranasal study found dose-dependent erectile effects in men. A 2008 Phase II SC trial in women with desire and arousal disorders beat placebo. The pivotal 2019 RECONNECT trials (two Phase III studies, 1,267 women) showed significant increases in desire and decreases in desire-related distress. Nausea hit about 40 percent of users. That Phase III data got the FDA approval. Peptide evidence the way regulators want it: modern, large, double-blind, placebo-controlled.
One thing to know: it works centrally, not peripherally. Viagra and Cialis dilate blood vessels downstream. Bremelanotide acts on brain arousal circuits upstream. That's why it works in people who don't respond to PDE5 inhibitors, and why it helps with desire, not just erection.
The rare peptide in this newsletter that has a Phase III trial, an FDA letter, and a brand name you can walk into a pharmacy and ask for.
The catch. Even with approval, Vyleesi is not a blockbuster. Uptake since 2019 has been weak, partly due to nausea, partly due to dosing caps (one injection per 24 hours, eight per month), partly because it competes with Addyi, an oral HSDD drug. Off-label use in men means grey-market PT-141 vials with the usual purity concerns.
Research Radar: What Vyleesi's trajectory tells us
Three developments worth tracking.
Slow commercial adoption. Despite being the only approved peptide for female HSDD, Vyleesi has not captured the market. Prescription numbers remain modest. FDA approval is necessary but not sufficient to become a clinical default.
The off-label men's market. PT-141 has quietly become one of the most-prescribed compounded peptides in men's wellness clinics, used exactly as the Phase II male ED trials suggested. The evidence base for male use is real but pre-2019.
Next-generation melanocortins. Several programs are developing MC4R-selective compounds that preserve sexual-function effects while reducing nausea and pigmentation. The Bremelanotide file may look different in five years.
The rule: An FDA approval is a ceiling on credibility, not a floor on efficacy. Strong trials and a green light still need the drug to work well enough for real patients to keep using it.
Not medical advice. Most peptides discussed in this newsletter are investigational or research chemicals. Talk to a clinician before starting anything.