PT-141 (bremelanotide) is one of the most-studied research peptides in the melanocortin family. This guide covers its mechanism (MC3R/MC4R activation), how it differs from its parent compound Melanotan II, what to look for in a COA, and the storage + reconstitution protocol Canadian research labs typically use.
What PT-141 is
PT-141 is a 7-residue cyclic heptapeptide (Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH) developed in the mid-1990s as a metabolic byproduct of Melanotan II research. Where Melanotan II is a near-pan-agonist at the melanocortin receptors (MC1R/MC3R/MC4R/MC5R), PT-141 has the lactam-bridged backbone of MT-II without the C-terminal amide — a modification that shifts its selectivity profile toward MC3R/MC4R and away from MC1R (the receptor primarily responsible for pigmentation responses in MT-II research).
Mechanism: MC3R and MC4R activation
MC3R and MC4R are G-protein-coupled melanocortin receptors expressed primarily in central nervous system tissue, particularly the hypothalamus and limbic regions. Binding produces downstream signaling via the cAMP/PKA pathway and, in mammalian research models, modulates appetite regulation, sexual response circuitry, and inflammatory pathways. PT-141 binds MC3R and MC4R with low-nanomolar affinity per published radioligand displacement studies; the MC1R activity is roughly an order of magnitude weaker, which is the basis for the catalog distinction between PT-141 (selective MC3/4) and Melanotan II (broad melanocortin).
PT-141 vs Melanotan II at a glance
| Property | PT-141 | Melanotan II |
|---|---|---|
| Sequence | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH | Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂ |
| Selectivity | MC3R / MC4R preferred | Broad: MC1R, MC3R, MC4R, MC5R |
| Molecular weight | ~1025 Da | ~1024 Da |
| Research focus | Central MC3/4 signaling, appetite-axis, sexual response circuitry | Pigmentation models + broad melanocortin |
| Pure North Peptides | Not currently stocked — see catalog | Not currently stocked — see catalog |
Reading the COA for a PT-141 lot
A research-grade PT-141 lot should ship with an independent third-party COA (Janoshik Analytical is a common reference lab). The three items to check before any research run:
- HPLC purity floor. Research-grade PT-141 lots typically run 99.2–99.5%. Anything below 98.5% should be considered unfit for comparative research.
- Mass-spec identity. Confirm the observed mass matches expected (~1025 Da for the protonated [M+H]⁺ ion). Discrepancies of more than 1 Da suggest a synthesis fragment, not the target.
- LAL endotoxin. Should be below 0.5 EU/mg for in-vitro research. Cell culture work is especially endotoxin-sensitive.
The lot number printed on the vial cross-references to the chromatogram archive at /lab-results/. See the COA reading guide for full field-by-field detail.
Storage and reconstitution
PT-141 storage follows the standard cyclic-peptide profile:
- Lyophilized: −20 °C, desiccated, protected from light. Stable 24+ months.
- Reconstituted: bacteriostatic water (0.9% benzyl alcohol). At 4 °C in solution, stable ~28 days. PT-141 is more stable in solution than many shorter linear peptides — the lactam ring contributes meaningful resistance to backbone proteolysis.
- Avoid: repeated freeze-thaw cycles. Aliquot before freezing if you need multiple access events. The cyclic backbone is robust, but ice-crystallization stress still degrades solution integrity over many cycles.
Cross-batch reproducibility
A retain-sample programme is one of the most useful things to look for in any supplier. If a vendor holds a retain on every lot and your assay produces unexpected results, the retain can be run against your in-hand material to localize whether variance is in the peptide itself, the lot-to-lot synthesis range, or upstream in your reagent or cell-culture pipeline. This retain practice is standard at pharmaceutical-grade suppliers but rare at research-chemical retailers — it's worth asking any supplier whether they hold retains before you commit to a long-running protocol.
Where to source PT-141 in Canada
PT-141 and Melanotan II are not currently part of the Pure North Peptides catalog. For the reference standards we do stock — HPLC-verified, dispatched within 2 business days and shipped across Canada via Canada Post Xpresspost (~4 days transit) — see the current catalog. Independent COAs for every shipped lot are published at /lab-results/.
FAQ
What's the difference between PT-141 and bremelanotide?
They are the same molecule. Bremelanotide is the INN (international non-proprietary name) used in clinical literature; PT-141 is the developmental code name from the original Palatin Technologies research program. In research-supply catalogs the “PT-141” designation is more common.
How does PT-141 differ from Melanotan II?
Both are cyclic heptapeptides with nearly identical scaffolds. The key difference is the C-terminus: Melanotan II has a C-terminal amide; PT-141 has a free carboxylic acid. That one modification shifts the receptor-selectivity profile substantially — PT-141 prefers MC3R/MC4R, while Melanotan II also engages MC1R (the pigmentation receptor).
Is PT-141 stable in solution longer than other peptides?
The cyclic backbone gives it better protease resistance than typical linear peptides, but it's not indefinitely stable. Standard 28-day in-solution storage at 4 °C still applies. The improvement is mostly in tolerating freeze-thaw cycles better than equivalent linear sequences.
What purity should I expect from a research-grade PT-141 lot?
≥99% HPLC purity. Pure North Peptides lots typically test 99.2–99.5% on independent Janoshik verification. Anything below 98.5% is a red flag.
Can I compare PT-141 and Melanotan II in the same protocol?
Yes — this is a common comparative-pharmacology design. Order both as separate reference vials, document both COAs, and ensure equimolar dosing rather than equimass.