PT-141 Melanocortin Research Guide 2026 — Molecular Profile, MC4R Receptor Mechanism & Verification

PT-141 is the research-market name for bremelanotide, a synthetic cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (α-MSH) that acts as a non-selective agonist across the melanocortin receptor family. Because it engages the central melanocortin receptors MC3R and MC4R — two of the most structurally and pharmacologically scrutinized G-protein-coupled receptors in neuroscience — PT-141 has become a recurring reference ligand in laboratories studying melanocortin signaling, GPCR activation mechanics, and cyclic-peptide receptor engagement. For research groups modeling MC3R/MC4R pharmacology, cAMP signal transduction, and structure-based ligand design, PT-141 connects a single, well-characterized molecule to a receptor system whose active-state structures were solved by cryo-electron microscopy only in the last few years.

This guide is written for qualified laboratory researchers sourcing PT-141 / bremelanotide as a research compound. It covers the molecular profile, the mechanism at the receptor and pathway level as described in the published literature, representative preclinical and structural research data, real citations you can verify yourself through PubMed and the Protein Data Bank, and how to evaluate the analytical documentation that should accompany a cyclic peptide of this complexity. It also situates PT-141 within the current U.S. regulatory context, which differs from that of the peptides on the July 2026 compounding-review docket.

For in-vitro and preclinical laboratory research use only. Not for human consumption. Not for veterinary use.

Section 1 — Molecular Profile

Bremelanotide is a compact, cyclic seven-residue peptide built on the conserved His-Phe-Arg-Trp melanocortin pharmacophore, with a lactam bridge that constrains the ring and confers markedly greater metabolic stability than the linear α-MSH parent.

• Compound name: Bremelanotide; marketed in the research market as PT-141
• CAS number: 189691-06-3 (free base); 1607799-13-2 (acetate salt)
• Molecular formula: C₅₀H₆₈N₁₄O₁₀ (free base)
• Molecular weight: ≈ 1,025.2 g/mol
• Length: Cyclic heptapeptide (7 residues)
• Sequence: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH
• Pharmacophore: the conserved melanocortin His-D-Phe-Arg-Trp core
• Appearance: White lyophilized powder; reconstituted in aqueous buffer for in-vitro work

Two structural details matter for sourcing. First, bremelanotide is cyclic: a side-chain lactam bridge links the Asp and Lys residues to form a constrained macrocycle. That ring is central to both its receptor affinity and its stability, and it makes analytical identity confirmation more important than for a simple linear peptide, because ring-opened or linear by-products can share a near-identical amino-acid composition while differing in mass and chromatographic behavior. Second, PT-141 is chemically related to the earlier melanocortin agonist melanotan II; bremelanotide is the ring-opened, C-terminal-deaminated derivative, which means a competent COA should distinguish the two by both mass and retention time.

At roughly 1,025 g/mol, PT-141 sits in the small-cyclic-peptide range, and its D-phenylalanine residue — a non-natural stereocenter at position 7 of the melanocortin core — is a defining feature that reversed-phase HPLC and chiral or tandem-MS methods are expected to resolve. The single acetylated N-terminus and the internal lactam give the molecule a distinctive, well-behaved fingerprint on LC-MS: a clean protonated-molecule envelope near 1,025 Da (plus the expected multiply-charged ions) is the identity signature to look for.

Section 2 — Mechanism (Pathway Language)

Bremelanotide is studied as a non-selective melanocortin receptor agonist. The published mechanistic literature describes it at the level of receptor binding, G-protein coupling, and downstream second-messenger signaling — not at the level of any clinical endpoint.

Melanocortin receptor engagement. The melanocortin system comprises five GPCR subtypes (MC1R–MC5R). Bremelanotide binds across the family but is studied primarily for its activity at the centrally expressed MC3R and MC4R. Its His-D-Phe-Arg-Trp core occupies the orthosteric pocket shared by the endogenous melanocortin agonists derived from proopiomelanocortin (POMC), and the D-phenylalanine substitution is the key modification that improves receptor engagement and proteolytic stability relative to native α-MSH.

Gs coupling and cAMP signaling. Agonist binding at MC4R stabilizes an active receptor conformation that couples to the stimulatory Gs protein, activating adenylyl cyclase and raising intracellular cyclic AMP (cAMP). Secondary signaling through phospholipase C (PLC) and the ERK/MAPK cascade has also been reported in neuronal receptor populations. In cell-based reporter assays, this cAMP accumulation is the canonical readout used to quantify melanocortin agonist potency and efficacy.

Central versus peripheral distinction. A defining feature of the melanocortin literature is that MC3R and MC4R are expressed principally in the central nervous system — in hypothalamic and limbic circuits — which distinguishes melanocortin-pathway signaling from peripheral vascular mechanisms studied for other compound classes. Researchers use PT-141 as a probe of this central receptor system in controlled cell and tissue models.

In accordance with research-context framing, this article describes bremelanotide strictly at the receptor-binding and signal-transduction level. It is studied for its interaction with MC3R/MC4R, Gs/cAMP coupling, and melanocortin-pathway signaling in controlled systems; no human outcome, therapeutic, or physiological benefit is claimed or implied.

Section 3 — Preclinical & Structural Research Data

The bremelanotide and MC4R literature is unusually rich for a peptide of this size, spanning receptor pharmacology, animal models, and — most notably in recent years — high-resolution structural biology.

Receptor pharmacology. Foundational work characterized PT-141 as a synthetic α-MSH analogue with agonist activity at MC3R and MC4R and improved stability over linear melanocortins, establishing the cAMP reporter assay as the standard in-vitro potency readout. This receptor-pharmacology framework is the reason PT-141 remains a common comparator ligand in melanocortin assay development.

Structural biology of the receptor. The MC4R was among the GPCRs whose active-state structures were resolved by cryo-electron microscopy in 2021. Cryo-EM structures of the human MC4R–Gs complex were determined with bremelanotide bound, at approximately 3.1 Å resolution, alongside companion structures with the agonists NDP-α-MSH and setmelanotide. These structures revealed how the melanocortin pharmacophore sits in the orthosteric pocket, the agonist-induced role of transmembrane helix 6 (TM6) in receptor activation, and — strikingly — the essential role of a bound calcium ion that bridges the ligand to TM2 and TM3. This structural work turned PT-141 from a functional probe into a defined reference ligand for structure-based melanocortin research.

Continued structural refinement. Structure-based melanocortin work has continued past the initial cryo-EM reports, including 2024 work characterizing an MC4R-specific orthosteric nanobody agonist — an example of how the receptor’s now-known active-state architecture is being used to design new, more selective molecular tools.

Across these studies, the recurring experimental thread is a measurable molecular endpoint — cAMP accumulation, receptor-binding affinity, or atomic-resolution structural coordinates deposited in the Protein Data Bank — quantified in a defined cell-based or biophysical system.

Section 4 — Published Literature (Verifiable Citations)

The following are real, published references retrievable through PubMed, PubMed Central (PMC), and the RCSB Protein Data Bank. Researchers are encouraged to read the primary sources directly.

• Molinoff PB, Shadiack AM, Earle D, Diamond LE, Quon CY. “PT-141: a melanocortin agonist for the treatment of sexual dysfunction.” Annals of the New York Academy of Sciences. 2003;994:96–102. PubMed: 12851303 — foundational description of PT-141 as an α-MSH analogue with MC3R/MC4R agonist activity.
• Diamond LE, Earle DC, Heiman JR, Rosen RC, Perelman MA, Harning R. “An effect on the subjective sexual response in premenopausal women with sexual arousal disorder by bremelanotide (PT-141), a melanocortin receptor agonist.” Journal of Sexual Medicine. 2006. PubMed: 16839319 — early controlled clinical pharmacology of the melanocortin agonist.
• Clayton AH, Kingsberg SA, Portman D, et al. “Safety Profile of Bremelanotide Across the Clinical Development Program.” Journal of Women’s Health (Larchmt). 2022 — pooled safety analysis across the bremelanotide development program.
• Zhang H, Chen LN, Yang D, et al. “Structures of active melanocortin-4 receptor–Gs-protein complexes with NDP-α-MSH and setmelanotide.” Cell Research. 2021;31(11):1163–1175. PMC: PMC8563958 — cryo-EM active-state MC4R structures defining the melanocortin activation mechanism.
• “Structural insights into ligand recognition and activation of the melanocortin-4 receptor.” 2021. PMC: PMC8563965 — companion cryo-EM study reporting the MC4R–Gs complex bound to bremelanotide (~3.1 Å) and the calcium-ion binding role.
• “Structure elucidation of a human melanocortin-4 receptor specific orthosteric nanobody agonist.” Nature Communications. 2024. — structure-based design of a selective MC4R molecular tool, illustrating ongoing melanocortin structural research.

Citing real primary literature is a core part of the PYXAX research-context standard. Any source — vendor or publication — that references “studies” without traceable identifiers should be treated with caution. Because bremelanotide has both a defined receptor-pharmacology literature and a separate pharmaceutical development history under an approved drug pathway, the strongest evidentiary practice is to read the primary structural and pharmacology reports directly and to keep clear the distinction between approved-drug clinical data and research-market PT-141 material.

Section 5 — Research Applications (In-Vitro Use Cases)

Within qualified laboratory settings, bremelanotide is used as a reference and probe compound in several categories of in-vitro and preclinical work:

• Melanocortin receptor pharmacology — as a reference agonist in MC3R and MC4R binding and functional assays.
• cAMP signal-transduction research — as a Gs-coupled agonist in cell-based cAMP reporter and second-messenger accumulation assays.
• GPCR structural biology — as a defined orthosteric ligand for cryo-EM and structure-based studies of melanocortin receptor activation.
• Comparative melanocortin studies — as a non-selective comparator against subtype-selective agonists such as setmelanotide in receptor-selectivity panels.
• Structure–activity relationship (SAR) research — as a cyclic-peptide scaffold reference in melanocortin analogue design.
• Analytical method development — as a defined cyclic heptapeptide standard for validating reversed-phase HPLC and LC-MS identity methods, including resolution of the D-Phe stereocenter and the intact lactam ring.

Each of these applications is an in-vitro or preclinical research use. Compounds supplied for research are not intended for, and must not be used in, any human or veterinary context.

Section 6 — How to Evaluate a Source

Bremelanotide is a cyclic peptide with a non-natural stereocenter and a close structural relative in melanotan II, which makes analytical documentation more — not less — important. The following steps separate verifiable sourcing from marketing claims.

Step 1 — Confirm the exact molecule and salt form. The COA and product page should state that the material is bremelanotide (free base or acetate), give the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, and report a molecular weight near 1,025 g/mol. Ambiguity between PT-141 and melanotan II is a documentation gap, not a detail.

Step 2 — Confirm the testing laboratory is named. “Third-party tested” is meaningless without a named, accredited laboratory. Look for an ISO 17025-accredited facility or equivalent recognized accreditation, and a COA that names the lab that tested the specific batch.

Step 3 — Confirm identity by mass spectrometry. A genuine bremelanotide sample should present a protonated-molecule envelope consistent with a ~1,025 g/mol cyclic heptapeptide on LC-MS. A COA that reports only “purity” without an identity mass is incomplete — and for a cyclic peptide, mass confirms that the ring is intact.

Step 4 — Confirm chromatographic purity and check for related peptides. HPLC should show a single dominant peak. The relevant impurities to resolve are ring-opened (linear) by-products and any melanotan II carry-over; a well-resolved reversed-phase method plus an accompanying mass spectrum will reveal them.

Step 5 — Verify lot specificity and a complete panel. The batch number on the COA must match the vial label and correspond to the production lot — not a single historical testing event applied across an entire catalog. A COA you can verify independently, through a QR-linked laboratory portal or a searchable community database, is the standard to hold. For sensitive cell-based signaling work, purity and identity should be accompanied by endotoxin and heavy-metal data, both of which can confound receptor-assay systems.

Section 7 — Regulatory Context (July 2026)

Researchers sourcing bremelanotide in the United States should understand how it sits in the current landscape. Bremelanotide is the active ingredient in an FDA-approved prescription drug product (Vyleesi), which places it on a different footing from the peptides now under compounding review. The FDA’s Pharmacy Compounding Advisory Committee (PCAC) is scheduled to meet July 23–24, 2026 to consider a set of peptides — BPC-157, KPV, TB-500, MOTS-c, emideltide, epitalon, and semax, in both free-base and acetate-salt forms — for potential inclusion on the Section 503A Bulk Drug Substances List; bremelanotide is not among the substances on that docket. Separately, in April 2026 the Department of Health and Human Services confirmed the removal of a group of peptides from the compounding “Category 2” list following withdrawal of their nominations. Across the docketed peptides, the FDA’s briefing materials leaned toward not adding the substances, citing incomplete characterization and limited human safety and effectiveness data; PCAC recommendations are advisory only and are not final until the FDA issues its own determination. None of this process changes the status of bremelanotide as a compound supplied strictly for laboratory research. Researchers remain responsible for compliance with all applicable regulations in their jurisdiction. (See the PYXAX peptide-compliance landscape guide for the full regulatory picture.)

Section 8 — PYXAX Verification Standard

Every PYXAX batch is independently third-party tested by accredited laboratories including ILS Labs, Krause Analytical, and Janoshik. Batch-specific COAs are published for every lot, naming the accredited lab that tested that batch.

Testing panel:

• Chromatographic purity by HPLC
• Molecular identity by LC-MS
• Endotoxin (USP <85> LAL method)
• Heavy metals by ICP-MS
• QR-verified, batch-specific COA published for every lot

Lot-specific documentation. Every production lot receives its own batch number. The batch number on the vial matches the batch number on the COA in the PYXAX COA Library, so researchers can confirm identity — including the exact cyclic sequence and salt form supplied — before ordering.

Community verification. Select lots are submitted to Janoshik Analytical for community verification, with results publicly searchable by batch number — no vendor contact required. Founding batches were verified through Krause Analytical (accredited US laboratory), and ongoing production lots are tested across the accredited-laboratory network described above, with a batch-specific COA published for every lot.

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All PYXAX compounds are supplied strictly for in-vitro and preclinical laboratory research use only. Not for human consumption. Not for veterinary use. Not for diagnostic procedures. These statements have not been evaluated by the FDA. Researchers are responsible for compliance with all applicable laws and regulations governing the use of research compounds in their jurisdiction.