1. What Is PT-141 (Bremelanotide)
PT-141, known by its generic name bremelanotide, is a synthetic cyclic heptapeptide derived from Melanotan II. It is an analog of alpha-melanocyte-stimulating hormone (α-MSH), specifically corresponding to the α-MSH(4-10) fragment with several modifications: a norleucine substitution at position 4, a D-phenylalanine at position 7, and a lactam bridge between the aspartate and lysine side chains that forms its cyclic structure. The N-terminus is acetylated and the C-terminus retains a free carboxylic acid, distinguishing it from Melanotan II, which carries a C-terminal amide.
In June 2019, bremelanotide was approved by the U.S. FDA under the brand name Vyleesi for the treatment of generalized hypoactive sexual desire disorder (HSDD) in premenopausal women. It was designated a first-in-class medication — the first melanocortin receptor agonist approved for this indication. It is administered as a subcutaneous injection. In the research peptide market, PT-141 is widely available, making verification of identity and purity an important consideration.
2. Mechanism of Action
Bremelanotide acts as a non-selective agonist of the melanocortin receptor family. Its activity spans MC1R, MC3R, MC4R, and MC5R, with no meaningful activity at MC2R (the ACTH receptor). The clinical effect is believed to be mediated primarily through central MC3R and MC4R activation, though the precise mechanism underlying its effect on sexual desire is not fully established.
Melanocortin Receptor Activation
Bremelanotide binds to melanocortin receptors with a potency order of MC1R > MC4R > MC3R > MC5R. MC3R and MC4R are expressed in the hypothalamus, a brain region involved in regulating food intake, energy homeostasis, and sexual behavior. Unlike PDE5 inhibitors (such as sildenafil), which act on peripheral vasculature, bremelanotide acts centrally in the nervous system.
Central Nervous System Pathway
The proposed mechanism involves stimulation of dopaminergic pathways in the medial preoptic area, a region implicated in sexual motivation and behavior. However, the FDA label notes that the exact mechanism by which bremelanotide improves sexual desire in humans is unknown. Melanocortin signaling is also involved in skin pigmentation (via MC1R), which accounts for the hyperpigmentation observed as a side effect.
3. Clinical Evidence
Bremelanotide has an extensive clinical development history spanning multiple phases, formulations, and indications. Development was led by Palatin Technologies, with the approved product ultimately marketed by AMAG Pharmaceuticals.
Phase III trials (HSDD). Two pivotal Phase III trials using a subcutaneous autoinjector were launched in late 2014, with results announced in November 2016. Pooled data included 1,247 premenopausal women (627 bremelanotide, 620 placebo) at the 1.75 mg dose. The trials demonstrated statistically significant improvements in desire and reductions in distress, leading to the June 2019 FDA approval. Independent reviewers have noted that the absolute magnitude of benefit was modest.
Earlier clinical development. An intranasal formulation was tested in Phase II for both female sexual dysfunction and erectile dysfunction. In 2007, the FDA placed a clinical hold on intranasal trials due to blood pressure elevation concerns. Palatin discontinued intranasal development in 2008. Four trials in erectile dysfunction were conducted, including a Phase IIb published in 2008, but this indication was not pursued to approval.
FDA approval and regulatory status. Bremelanotide (Vyleesi) was approved on June 21, 2019, for generalized HSDD in premenopausal women. The approval specifies that the condition must not be attributable to a co-existing medical or psychiatric condition, relationship problems, or medication effects. It is a prescription-only medication in the United States. It has not been approved by the EMA or other major regulatory bodies outside the U.S.
4. Limitations and Safety Considerations
Despite FDA approval, there are important considerations and limitations to understand:
- The FDA approval is narrow in scope. Vyleesi is approved only for generalized HSDD in premenopausal women. It is not approved for use in men, postmenopausal women, or for any other indication. Off-label and research use falls outside the evaluated safety and efficacy profile.
- Nausea is a significant side effect. In Phase III trials, 40% of bremelanotide-treated participants reported nausea (versus 1.3% with placebo). Flushing (20.3%), headache (11.3%), and injection site reactions (5.4%) were also common.
- Blood pressure elevation occurs transiently. Bremelanotide causes a transient increase of approximately 6 mmHg systolic and 3 mmHg diastolic. It is contraindicated in patients with uncontrolled hypertension or cardiovascular disease.
- Skin hyperpigmentation may occur and may not resolve. Particularly with use exceeding eight doses per month, darkening of the face, gingiva, or breasts has been reported. This is consistent with MC1R activation and the compound's relationship to the tanning peptide Melanotan II.
- Long-term safety data beyond clinical trial duration is limited. The prescribing information recommends discontinuation after eight weeks if no improvement in desire or distress is observed.
5. Purity and Lab Testing
PT-141 is a cyclic heptapeptide with a free-base molecular weight of approximately 1025.18 Da (molecular formula C₅₀H₆₈N₁₄O₁₀). The acetate salt form (CAS 1607799-13-2) has a formula weight of approximately 1085.2 Da. When evaluating PT-141 from the research peptide market, the following analytical considerations are relevant.
HPLC Purity
Research-grade PT-141 from reputable suppliers typically tests at 98% or above via reverse-phase HPLC. Purity below 95% suggests inadequate synthesis or purification. HPLC chromatograms should show a single dominant peak with minimal secondary peaks. Common impurities include truncated sequences, deletion peptides, and residual protecting groups from solid-phase synthesis.
Mass Spectrometry
The observed molecular weight should match approximately 1025.18 Da for the free base or 1085.2 Da for the monoacetate salt. LC-MS or MALDI-TOF confirmation is essential for identity verification, particularly because PT-141 can be confused with or substituted by Melanotan II (MW ~1024.18 Da), which differs by only approximately 1 Da in the free base. High-resolution mass spectrometry is the definitive method to distinguish these two compounds.
Melanotan II Distinction
PT-141 is a metabolite of Melanotan II, differing only in the C-terminal group (free acid vs. amide). Their molecular weights differ by approximately 1 Da, making low-resolution mass spectrometry insufficient to distinguish them. The structural difference — a free carboxylic acid on PT-141 versus a C-terminal amide on Melanotan II — can be confirmed by high-resolution MS, MS/MS fragmentation analysis, or enzymatic digestion patterns. This is a known substitution risk in the research peptide market.
Stability and Storage
Like most peptides, PT-141 is sensitive to heat, moisture, and repeated freeze-thaw cycles. Lyophilized powder should be stored at -20°C. Once reconstituted, solutions should be refrigerated and used within a reasonable timeframe. Degradation products may include hydrolyzed (ring-opened) forms resulting from cleavage of the lactam bridge, which would alter bioactivity. Exposure to light and oxidative conditions can also lead to tryptophan and methionine oxidation products.
6. What to Look For in a PT-141 COA
When evaluating a Certificate of Analysis for a PT-141 product, confirm the following:
HPLC purity of 98% or higher from a named, independent third-party laboratory
Mass spectrometry confirming molecular weight near 1025.18 Da (free base) — high-resolution MS is preferred to differentiate from Melanotan II
Correct CAS number: 189691-06-3 for the free base or 1607799-13-2 for the acetate salt
A batch or lot number that matches the product you received
A lab reference number you can verify directly with the testing laboratory
Test date — a COA from years ago may not reflect current product quality
For a full walkthrough of evaluating a COA, see our guide to reading a peptide COA.
Summary
PT-141 (bremelanotide) is a cyclic heptapeptide melanocortin receptor agonist and the first drug in its class to receive FDA approval, marketed as Vyleesi for hypoactive sexual desire disorder in premenopausal women. Its well-characterized molecular profile — a cyclic peptide of 1025.18 Da with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH — provides clear benchmarks for analytical verification. When evaluating PT-141 from the research market, high-resolution mass spectrometry is particularly important to distinguish it from the closely related Melanotan II, which differs by only ~1 Da and a single C-terminal modification.