Retatrutide + Tirzepatide

1 Identity

Fixed-ratio two-component article: two acylated lipopeptides of closely related design, co-lyophilized. Both components are 39-residue C-terminally amidated single chains carrying a C20 fatty diacid on a lysine side chain, and their differences are small, specific and entirely invisible to a residue count. The two components are retatrutide and tirzepatide, and each component's full identity lives on its own record and is cross-referenced from here rather than restated. As molecules: tirzepatide is the active moiety of approved products under NDA 217806 and is the analog originated by Eli Lilly, while retatrutide is an investigational molecule of the same acylated-lipopeptide class. market titles carried as found and never adopted as identity statements. No sponsor, monograph or published specification names a fixed combination of these two molecules, so there is not even a proprietary article of this composition for such a title to be mistaken for. Neither component name fixes a salt; both circulate as the free base, the acetate and the trifluoroacetate.

Sequence

Per component, cross-referenced. Both 39 residues, both C-terminally amidated, both acylated on a lysine side chain to a C20 diacid. The three differences that matter: retatrutide carries one AEEA unit in its linker and tirzepatide carries two; retatrutide has alpha-methyl-leucine at position 13 where tirzepatide has Aib; and the acylation sites differ, Lys17 on retatrutide against Lys20 on tirzepatide, each chain carrying a second unmodified lysine that is the positional-isomer risk. The linker asymmetry is stated on the face of this record because reversing it is the commonest error in circulating descriptions of these two molecules.

Molecular formula

Per component; a mixture has no combined formula and none is written. Retatrutide C221H342N46O68; tirzepatide C225H348N48O68 (free base), with the des-acyl tirzepatide backbone C188H283N45O56 carried as a named impurity formula. Neither molecule contains sulfur, so the isotope-envelope route that separates the cagrilintide pairing is unavailable here and separation rests on mass and retention time alone. Counterion is determined per component and independently.

Average mass

Per component, on IUPAC 2021 abridged conventional atomic weights: retatrutide 4731.421 Da; tirzepatide 4813.527 Da - 82.106 Da apart, or 1.7 percent. AT A nominal 10 mg plus 10 mg fill, 20 mg total, that is 2.1135 and 2.0775 micromol, a molar ratio of 1.0174 : 1.0000 - the closest to equimolar-at-equal-mass of any blend in this set, and still not equimolar. Tirzepatide des-acyl backbone 4069.591, a des-acylation interval of 743.936 Da. No single blend molecular weight is printed.

Monoisotopic mass

Per component. Retatrutide 4728.4718 Da neutral; tirzepatide 4810.52486 Da neutral. Both are observed only multiply charged; at 5+ the two protonated envelopes sit about 16.4 m/z apart and are trivially resolved. The separations that are not trivial are internal to each component: C-terminal amide against free acid is 0.984 Da and is invisible below about 30,000 resolving power, and tirzepatide des-acylation is 743.4568 Da monoisotopic and must be reported as a named attribute rather than inferred from a purity number.

Salt / variant note

(1) ratio: no standard has been located. (2) salt, per component and independently: free base, acetate, trifluoroacetate. (3) des-acyl of either component, a specified impurity; the tirzepatide interval is 743.936 Da average. (4) C-terminal state: the amide is the article on both sides and the free acid is a 0.984 Da impurity, not a synonym. (5) positional acylation isomers: each chain carries a second unmodified lysine, so an isomer acylated at the wrong lysine is exactly isobaric with the article and no mass measurement at any resolution will find it.

2 Class & testing panel

Form

Fixed-ratio blend

Testing panel

P2 — panel definitionwith mandatory P3 additions for both components, run per component and never once for the vial. Both chains carry non-proteinogenic residues - two Aib each, plus alpha-methyl-leucine in retatrutide only - so the P3 element is not optional on either side, and the acylation attributes of P2 are determined separately for each component because the linkers differ. Aib is a common internal standard in amino acid analysis and is a residue of both chains, so that 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; tirzepatide additionally carries an extensive registered trial program, and one registered phase 3 study places retatrutide against tirzepatide as active comparator. That is literature about the two molecules separately or head to head, and none of it is about a fixed-ratio mixture of them. No 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 at -20 degrees C plus or minus 5 degrees C, desiccated and protected from light, in low-adsorption containers with recovery demonstrated on the actual container-closure system. Both components are surface-active C20-diacid conjugates, and their adsorptive loss to glass and plastic is a real quantitation error at low concentration rather than a theoretical one, which is why the container is a specification attribute here and not packaging.

Shelf life

Provisional 24 months at -20 degrees C plus or minus 5 degrees C is the starting interval for either component alone; the blend runs its own protocol and inherits neither component study, pending the company's own data. The stability-indicating attributes are per-component content, the measured ratio, des-acylation on either chain at the 743.936 Da average interval on the tirzepatide side, and hydrolysis of either C-terminal amide at 0.984 Da.