Oxytocin + PT-141 - Love and Happiness

1 Identity

Fixed-ratio two-component article: one cyclic disulfide-bridged nonapeptide amide and one cyclic lactam-bridged heptapeptide carrying a D-residue and a non-proteinogenic residue. Two cyclic peptides closed by two different chemistries, which is the whole of the analytical difficulty. Each component's full identity lives on its own record and is cross-referenced from here rather than restated. As molecules, both components are named in approved product documentation: bremelanotide is the active moiety of Vyleesi, NDA 210557, approved June 2019, and oxytocin is the active moiety of approved oxytocin injection products. Those are identity facts about the two molecules and they fix the spelling and the reference naming, nothing more. and Oxytocin/PT-141; the PT-141 component is bremelanotide under its INN. Those blend titles are market titles in circulation and none of them is an identity statement: none names a molecule, none fixes a counterion and none states a ratio. Neither component name fixes a salt either, and both molecules circulate as the acetate and as the trifluoroacetate.

Sequence

Per component, cross-referenced. Oxytocin H-Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, Cys1-Cys6 disulfide, C-terminal glycinamide. PT-141 Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, head-to-side-chain lactam between the Asp2 beta-carboxyl and the Lys7 epsilon-amine, free C-terminal acid. The two rings are closed differently: a disulfide that can reduce or scramble against an amide bond that cannot, so a single cyclization-confirmed statement on a certificate covers neither ring properly and each is confirmed by its own method.

Molecular formula

Per component; a mixture has no combined formula and none is written. Oxytocin C43H66N12O12S2 (oxidized, bridged), reduced dithiol C43H68N12O12S2. PT-141 C50H68N14O10 free base, mono-acetate C52H72N14O12 (CAS 1607799-13-2). The sulfur asymmetry is the clean discriminator: oxytocin carries two sulfurs and PT-141 none, so the isotope envelope assigns the peaks independently of retention time. Counterion is determined per component and independently.

Average mass

Per component, on IUPAC 2021 abridged conventional atomic weights: oxytocin 1007.193 Da; PT-141 1025.182 Da free base, 1085.234 mono-acetate. The two averages differ by 17.99 Da, which is the mass of water, so a nominal-mass report of 1025 on this vial is ambiguous between PT-141 and an oxytocin-derived hydrolytic artifact until the component, the ion and the charge state are named. At a nominal 10 mg plus 10 mg fill, 20 mg total, that is 9.9286 and 9.7544 micromol - a molar ratio of 1.0179 : 1.0000. No single blend molecular weight is printed, because a mixture does not have one.

Monoisotopic mass

Per component. Oxytocin 1006.4365 Da neutral, [M+H]+ 1007.4437. PT-141 1024.5243 Da neutral, [M+H]+ 1025.5316. Two collisions belong on the face of this record: oxytocin's disulfide reduction is exactly +2.0157, and deamidation at Asn5 or Gln4 and conversion of the C-terminal amide to the free acid are each exactly +0.9841, giving one mass at 1007.4205 for three chemically different related substances; separately, the unqualified number 1024.5 is both PT-141's monoisotopic neutral and melanotan II's protonated ion.

Salt / variant note

(1) ratio: none standard. (2) oxytocin reduced form, disulfide open, 1009.209 average - a named attribute and not a grade. (3) PT-141 salt: free base 1025.182 against mono-acetate 1085.234, separately registered; the trifluoroacetate circulates alongside the acetate on both components. (4) confusable by name: melanotan II moves in the same channel and its protonated ion collides with PT-141's neutral mass at an unqualified 1024.5. (5) oxytocin des-amido and deamidated species, all three at +0.9841 and all three requiring chromatographic rather than mass separation.

2 Class & testing panel

Form

Fixed-ratio blend

Testing panel

P1 — panel definitionfor oxytocin and P3 for PT-141, run per component and never once for the vial. The union is P1 plus P3, and the P3 element is not decorative: D-phenylalanine at position 4 and norleucine at position 1 of PT-141 require a chiral method and an amino acid analysis standard that a P1 panel does not carry, while oxytocin's disulfide requires a reduction and scrambling assessment PT-141 does not need. One method constraint belongs on the face of the record rather than in a footnote: norleucine is the customary internal standard in amino acid analysis and is also PT-141's residue 1, so the default configuration is changed rather than accepted - the collision is silent and would not surface as an out-of-specification result

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

Published literature exists for each component and is graded on that component's record rather than restated here; both components additionally carry approved-product labeling documentation, which is a different class of document from research literature and is indexed as such. None of it is literature about this fixed two-component mixture. No published study of the two components co-formulated at a fixed ratio, and no published specification for such a mixture, has been located - an absence of located evidence rather than a demonstration that none exists.

4 Storage & specification

Storage

Co-lyophilized solid of the two components, stored at -20 degrees C plus or minus 5 degrees C, desiccated and protected from light, with a nitrogen headspace set by the oxytocin disulfide and the PT-141 tryptophan. Reducing conditions and elevated pH are excluded by specification rather than by habit, because the disulfide is the attribute they reach first.

Shelf life

The retest interval is set by the company's own stability protocol run on the blend itself, which inherits neither component study, and it is dated on the ring chemistries rather than on gross purity: free thiol, scrambled disulfide, deamidation and tryptophan oxidation are the stability-indicating attributes, each reported individually rather than inside a total-impurity sum.