Two peptides. Fifty carbon atoms each. Both built from the same seven amino acids, closed into the same ring, capped the same way at the front. Put their formulas next to each other and the entire difference is one nitrogen swapped for one oxygen.
One of them is a prescription drug approved by the FDA. The other has been the subject of enforcement letters and public health warnings on three continents.
That gap is worth understanding, because it is not an accident of regulation. It follows directly from the chemistry.
For research use only
Melanotan II and PT-141 are supplied by Validated Peptides as reference material for laboratory research. They are not for human or animal consumption, and nothing on this page is a protocol, a recommendation, or a description of what either compound does in people. Where an approved drug product is mentioned, it is mentioned as a regulatory fact.
The short version
PT-141, known generically as bremelanotide, is Melanotan II with the C-terminal amide replaced by a free carboxylic acid. Everything else is identical: the acetylated norleucine at the front, the six-residue ring closed by a lactam bridge between aspartic acid and lysine, and the D-phenylalanine that makes the whole family resistant to ordinary enzymatic breakdown.
Bremelanotide is also a metabolite of Melanotan II. Strip the terminal amide, which is something the body does on its own, and you get the other compound. That relationship is why the two are studied together and why comparing them is more interesting than comparing two unrelated peptides.
Both act on the melanocortin receptor family. Neither one is selective for a single receptor. If you have read that Melanotan II is the MC1R compound and PT-141 is the MC4R compound, that is a simplification, and the section below on receptor selectivity explains exactly where it breaks down.
Side by side
| Melanotan II | PT-141 (bremelanotide) | |
|---|---|---|
| Sequence | Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-NH₂ | Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-OH |
| Molecular formula | C₅₀H₆₉N₁₅O₉ | C₅₀H₆₈N₁₄O₁₀ |
| Molecular weight | 1024.2 g/mol | 1025.2 g/mol (free base) |
| C-terminus | Amide, neutral | Carboxylic acid, negatively charged at neutral pH |
| Relationship | Parent compound | Metabolite of Melanotan II, developed as its own agent |
| Receptor family | Melanocortin receptors (MC1R through MC5R), nonselective for both | |
| Regulatory status (US) | Never approved. Treated as an unapproved new drug in FDA enforcement. | Approved 2019 as Vyleesi (prescription drug product) |
| Research focus in the literature | Pigmentation biology, melanocortin signaling, appetite and feeding models | Central melanocortin signaling, MC3R and MC4R pathways |
Note the molecular weights. Bremelanotide is the heavier of the two by about one mass unit, despite being described almost everywhere as the smaller, stripped-down version. Swapping an amide (NH₂) for a hydroxyl (OH) loses a nitrogen and a hydrogen but gains an oxygen, and oxygen is heavier than the pair it replaces. It is a small detail that happens to be a fast way to check whether a vendor understands what they are selling.
The whole difference is one end of the molecule
Peptides are chains of amino acids joined head to tail. Every chain has an N-terminus at one end and a C-terminus at the other. What sits at those ends changes how the molecule behaves, sometimes dramatically.
Melanotan II ends in an amide group. Bremelanotide ends in a carboxylic acid. At the pH of a physiological buffer, an amide stays neutral and a carboxylic acid gives up a proton and carries a negative charge.
So the practical difference is not a missing chunk of molecule. It is a change in charge at one end. Think of two otherwise identical plugs where one has been given a magnet. The appliance is the same. Which sockets it prefers, and how readily it moves across a surface, is not.
Charge at a terminus affects several things at once. It changes how the peptide is solvated, how tightly it associates with membranes, how quickly it is cleared, and how it seats itself in a receptor binding pocket. A negatively charged tail can form new contacts with positively charged residues in a receptor, or it can be repelled by negative ones. Which of those happens depends on the receptor.
Why the ring matters as much as the tail
Both compounds share a lactam bridge that closes Asp through Lys into a ring. Cyclization is a standard trick in peptide chemistry: an open chain flops between thousands of shapes, while a ring holds a much smaller set. Locking the shape usually improves resistance to enzymes and sharpens receptor preference. The D-phenylalanine does related work, because most peptidases are built for L-amino acids and stumble on the mirror-image form. This is why both peptides are far more stable than a naturally occurring melanocortin like α-MSH.
What the melanocortin receptors actually do
There are five melanocortin receptors, MC1R through MC5R. All five are G protein-coupled receptors and all five respond to the same small family of natural ligands derived from proopiomelanocortin. They differ in where they are expressed and what they control.
A useful way to picture it: five locks that accept similar keys, installed on five very different doors. The key does not decide what happens next. The door does.
- MC1R sits mainly on melanocytes and immune cells. In pigmentation biology it is the receptor that drives the switch toward eumelanin production, which is why it is the receptor of interest in research on coat color and skin pigmentation.
- MC2R is the odd one out. It responds to ACTH rather than the MSH peptides and sits in the adrenal cortex, where it governs steroid production. Neither compound discussed here has meaningful activity at it.
- MC3R and MC4R are the central pair, expressed largely in the brain. Published research examines them in energy balance, feeding behavior, and autonomic and sexual function pathways. MC4R in particular is one of the better characterized targets in the genetics of body weight regulation.
- MC5R is associated with exocrine tissue, notably sebaceous glands.
This spread is the reason melanocortin pharmacology is difficult. A compound that activates the family broadly will touch pigmentation, feeding, and central signaling at the same time. Getting an effect at one receptor without the others has been the central problem in the field for decades, and it is the problem both of these peptides illustrate rather than solve.
Where most comparisons get the receptor story wrong
Search for these two compounds and you will find the same summary repeated across dozens of vendor pages: Melanotan II is the MC1R peptide, PT-141 is the MC4R peptide, the tanning one versus the central one.
The FDA prescribing information for bremelanotide says something different. It describes bremelanotide as a melanocortin receptor agonist that nonselectively activates several receptor subtypes with the following order of potency: MC1R, MC4R, MC3R, MC5R, MC2R, and notes that MC1R and MC4R binding predominates at therapeutic exposures.
Read that order again. MC1R is first, not fourth. The regulatory document for the approved drug does not describe an MC4R-selective compound. It describes a nonselective one that happens to be most potent at the receptor the popular summary says it avoids.
Both statements can be partly reconciled. Bremelanotide does show reduced MC1R activity relative to its parent, and the pharmacology relevant to its approved indication is attributed to central MC3R and MC4R signaling. But “less MC1R activity than Melanotan II” is a comparative statement, and it has been widely flattened into “MC4R-selective,” which is a different claim and not one the label supports.
On the affinity numbers you will see quoted
Specific binding constants for these peptides vary considerably between publications, and the variation is usually methodological rather than a real disagreement. Values depend on the cell line, the receptor species, whether the readout is competitive displacement or cyclic AMP accumulation, and which reference ligand anchored the assay. A number lifted from one paper and dropped next to a number from another is frequently not a valid comparison. We have quoted the rank order of potency from the regulatory label rather than a single figure, because the rank order is the more durable claim.
The honest summary is less tidy than the one that ranks well. Two nonselective agonists, both hitting several receptors in the same family, differing in degree rather than in target. The tail chemistry shifts the balance. It does not install a new lock.
One is an approved drug. The other collects warnings.
Bremelanotide was approved by the FDA in 2019 and is marketed as Vyleesi. The approved indication is the treatment of premenopausal women with acquired, generalized hypoactive sexual desire disorder. That is a specific, regulated indication attached to a specific drug product, manufactured under pharmaceutical controls and dispensed by prescription.
Melanotan II has no approval anywhere. It has been marketed online for years as an injectable tanning product, and regulators have responded to that consistently. The FDA has treated Melanotan II as an unapproved new drug in enforcement proceedings, on the basis that it has no approved application and no established recognition as safe and effective for its marketed uses. Australia's Therapeutic Goods Administration maintains a public consumer warning against products containing melanotan.
The clinical literature that exists on unsupervised Melanotan II use is a case report literature, which is the pattern you see when a compound reaches people outside a trial structure. Published reports include systemic toxicity with rhabdomyolysis, melanoma occurring in association with use, and changes to oral mucosal pigmentation. Case reports cannot establish causation. They are, however, the signal that a compound is being used in a setting where nobody is collecting systematic safety data.
We are stating this plainly rather than working around it, for a straightforward reason. A supplier that sells a compound with an active regulatory warning attached and does not mention the warning is telling you something about how it handles inconvenient information.
What we sell, and what we do not
Validated Peptides supplies Melanotan II and PT-141 as laboratory reference material, tested and sold for research use. Neither is a drug product. Neither is Vyleesi. Research-grade material and an approved pharmaceutical are different categories of thing, held to different standards, and intended for different purposes. Anyone blurring that line is not doing you a favor.
How researchers check what is actually in the vial
Everything above assumes the powder in the vial is the compound on the label. For these two in particular, that assumption deserves scrutiny, because they are unusually easy to confuse.
Consider the analytical problem. Melanotan II and bremelanotide have the same seven residues in the same order, the same ring, and molecular weights one unit apart. A crude identity check that only looks at approximate mass will not reliably tell them apart. Worse, because bremelanotide is a hydrolysis product of Melanotan II, a poorly stored or poorly purified batch of one can genuinely contain some of the other.
This is why identity confirmation matters as much as a purity percentage. A certificate that reports 99% purity without showing what the 99% is has answered the less important question. Mass spectrometry with adequate resolution distinguishes a one-dalton difference. A purity figure alone does not.
Every batch we sell is tested by an independent laboratory and the report is published rather than described. You can open the reports on our Certificate of Analysis page and check them against the lab's own records. Two companion articles cover the mechanics: how to actually read a peptide COA walks through every section of a report, and how to verify a COA shows how to confirm a certificate against the issuing laboratory rather than taking a vendor PDF at face value.
Both compounds are supplied lyophilized and are reconstituted in the laboratory before analytical work. If bacteriostatic water is unfamiliar as a laboratory diluent, we have a separate explainer on what bacteriostatic water is and how it differs from sterile water.
Common questions
Is PT-141 the same as Melanotan II?
No, though they are closely related. PT-141 has the same seven amino acids, the same lactam ring, and the same acetyl cap. It differs only at the C-terminus, where Melanotan II carries an amide and PT-141 carries a free carboxylic acid. PT-141 is also a known metabolite of Melanotan II, which is how it was identified in the first place.
What receptors does each one bind?
Both are agonists across the melanocortin receptor family rather than at a single subtype. The FDA prescribing information for bremelanotide gives the order of potency as MC1R, MC4R, MC3R, MC5R, MC2R and describes the compound as nonselective. Melanotan II is likewise described in the literature as a broad melanocortin agonist.
Is bremelanotide FDA approved?
Yes, as the prescription drug Vyleesi, approved in 2019 for premenopausal women with acquired, generalized hypoactive sexual desire disorder. The approved product is not the same thing as research-grade reference material, and the approval does not extend to research chemicals sold under the name PT-141.
Is Melanotan II approved anywhere?
No. It has no marketing approval in the United States, Australia, or the United Kingdom. The FDA has treated it as an unapproved new drug in enforcement actions, and the Australian Therapeutic Goods Administration publishes a consumer warning about melanotan products.
Why do the receptor affinity numbers differ between sources?
Because the assays differ. Binding and potency values depend on the cell line, the receptor species, the readout, and the reference ligand. Two numbers from two papers are often not measuring quite the same thing, which is why the rank order of potency is a more reliable basis for comparison than any individual constant.
Can the two be told apart analytically?
Yes, but not casually. Their molecular weights differ by roughly one unit, so identity confirmation needs mass spectrometry with sufficient resolution rather than an approximate mass check. This is one of the clearer cases where a purity percentage on its own is not a sufficient certificate.
References
Primary sources for the factual claims above. Where this article disagrees with the common summary of these compounds, the disagreement rests on the first reference.
- Vyleesi (bremelanotide injection) prescribing information. DailyMed, National Library of Medicine. View label
- U.S. Food and Drug Administration. NDA 210557 approval package, bremelanotide, 2019. FDA approval documents
- Melanotan II, PubChem CID 92432. National Center for Biotechnology Information. Compound record
- Bremelanotide, PubChem CID 9941379. National Center for Biotechnology Information. Compound record
- Dores RM, Liang L, Davis P, et al. 60 YEARS OF POMC: Melanocortin receptors: evolution of ligand selectivity for melanocortin peptides. J Mol Endocrinol. 2016. PMID 26792827
- Pathophysiology of melanocortin receptors and their accessory proteins. Best Pract Res Clin Endocrinol Metab. 2018. PMID 29678289
- Bremelanotide for the treatment of female hypoactive sexual desire disorder. PMC8788464. Full text
- U.S. Food and Drug Administration. Notice of Opportunity for Hearing, Docket No. FDA-2015-N-4169, addressing Melanotan II as an unapproved new drug. FDA record
- Therapeutic Goods Administration (Australia). Do not risk using tanning products containing melanotan. TGA warning
- Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clin Toxicol (Phila). 2012. PMID 23121206
- Melanoma associated with the use of melanotan-II. Dermatology. 2014. PMID 24355990