GLP-3 (RT) / retatrutide research peptide UK: chemistry, purity and handling
Research-grade retatrutide, listed by Alphex as GLP-3 (RT): the 39-residue lipidated peptide's chemistry, why 99+% purity is hard at this size.
GLP-3 (RT) is the Alphex catalogue name for retatrutide (LY3437943), a 39-residue lipidated peptide developed as a single-molecule ligand at three receptors: GIP, GLP-1 and glucagon. It is the largest and most chemically complex compound in our catalogue, the most demanding to synthesise to a 99+% specification, and the one where the difference between suppliers is widest. This guide covers what it is, its chemistry, where it appears in the literature, how to handle it, and how Alphex GLP-3 (RT) is verified. All material is supplied for laboratory and in-vitro research only.
What GLP-3 (RT) is
Retatrutide is a synthetic peptide built on a glucagon-family backbone and engineered to bind the receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1) and glucagon. It is often described as a "triple agonist" for that reason. Chemically it carries a C20 fatty diacid attached through a linker to a lysine side chain, the same design principle used in semaglutide and tirzepatide to promote albumin binding, and it includes non-natural residues such as alpha-aminoisobutyric acid (Aib) at positions chosen to resist enzymatic cleavage. The full sequence is stated on the batch Certificate of Analysis.
The compound is the subject of a large, ongoing pharmaceutical development programme with published clinical trial results. That programme is entirely separate from research-grade material: Alphex GLP-3 (RT) is a laboratory reagent, is not the pharmaceutical product, is not licensed as a medicine anywhere, and is supplied only for in-vitro and laboratory research. Alphex adopted the "GLP-3 (RT)" catalogue name in August 2026; the product was previously listed as Retatrutide and the specification is unchanged.
Specification
- Structure: 39-residue lipidated peptide (LY3437943); full sequence on the COA
- Molecular formula: C221H342N46O68
- Molecular weight: 4731.3 g/mol
- CAS number: 2381089-83-2
- Synonyms: Retatrutide, LY3437943
- Form supplied: lyophilised powder in a vacuum-sealed glass vial
- Purity specification: 99+% by RP-HPLC at 220 nm; identity by ESI-MS within 1 Da of theoretical mass
- Sizes: 5 mg, 10 mg, 20 mg and 30 mg vials
Where retatrutide appears in the research literature
Unlike most compounds in this catalogue, retatrutide's literature is dominated by pharmaceutical development publications rather than academic exploratory work. Themes include:
- Receptor pharmacology. In-vitro potency and selectivity at the three target receptors, and comparisons with single- and dual-receptor ligands.
- Preclinical models. Rodent and non-human primate metabolic models used during development.
- Clinical trial reports. Phase 1 through phase 3 publications from the developer, which are public and describe the pharmaceutical product, not research-grade material.
- Analytical chemistry. Method papers on characterising lipidated glucagon-family peptides by HPLC and mass spectrometry, relevant to anyone verifying material.
The size of the clinical literature makes this compound uniquely prone to online claims that go far beyond what a research supplier can support. Alphex makes none: the material is for laboratory research, and any question about the pharmaceutical programme belongs with the published trials.
Why 99+% is hard for a 39-residue lipidated peptide
Purity is more difficult to achieve for GLP-3 (RT) than for a short peptide, and the reason is arithmetic. Solid-phase synthesis couples one residue at a time, and each step is slightly less than perfect. Over 39 couplings plus lipidation, small per-step losses compound into a meaningful fraction of deletion, truncation and incompletely-lipidated species. Reaching 99+% area purity requires either exceptional synthesis or an additional preparative purification pass, and either way the impurity profile should be visible as small resolved peaks on the chromatogram. A COA that shows a single perfect peak for a peptide this size, with no minor species at all, is more suspicious than reassuring. Our note on what 99+% HPLC purity actually tells you covers how to read the trace.
Handling, storage and reconstitution
Lyophilised: -20 C long term, 2-8 C short term, protected from light and moisture. Warm the sealed vial to room temperature before opening; a large peptide with a hydrophobic lipid chain is particularly sensitive to condensation.
Reconstitution: the fatty-diacid modification makes GLP-3 (RT) considerably less water-soluble than short peptides, and it can aggregate if reconstituted carelessly. Add bacteriostatic water or buffer slowly down the vial wall, leave the vial to stand rather than agitating it, and swirl very gently only if needed. Do not shake or vortex. Slightly alkaline buffers generally give better solubility than acidic ones for this class of peptide; if a solution is cloudy, it has aggregated and should not be used for quantitative work.
In solution: refrigerate at 2-8 C in the dark and use within a few weeks; aliquot and freeze at -20 C for longer storage; avoid repeated freeze-thaw. Because the lipid chain drives adsorption to plastics, use low-binding tubes and pipette tips for dilute solutions. Guidance in full: storage, reconstitution and shelf life.
How Alphex verifies GLP-3 (RT)
Every batch is run on reverse-phase HPLC at 220 nm against a reference standard with a 99+% area purity threshold. Both results are issued on a Certificate of Analysis carrying the batch number printed on the vial, with the chromatogram and the testing laboratory named. Stock is held in the UK and dispatched the same day for orders before 2pm Monday to Friday. Method: how Alphex tests every batch.
For a peptide of this size the mass-spec result is worth reading closely: the observed mass should be within 1 Da of 4731.3, and a genuine spectrum for a 4.7 kDa peptide will show a multiply-charged envelope rather than a single ion. Both are on the COA. Because a verified batch of this compound is expensive to hold, Alphex vials it in four sizes and lists a size only when it is physically in UK stock; a sold-out size means the next verified batch has not yet been vialled, not that it is being back-ordered from abroad.
Buying retatrutide / GLP-3 (RT) in the UK
This is the compound in the UK research market with the most counterfeit and under-specified material in circulation. Before ordering from anyone, insist on a batch-matched COA available before purchase, an observed mass on the certificate, an HPLC chromatogram with the minor peaks visible, a named independent laboratory and UK-held stock. Then run the 12-point supplier checklist. Our comparison of supplier types is in where to buy research peptides in the UK.
Frequently asked questions
Is GLP-3 (RT) the same as retatrutide?
Yes. GLP-3 (RT) is the Alphex catalogue name for research-grade retatrutide (LY3437943), CAS 2381089-83-2. It is not the pharmaceutical product and is not a medicine.
Why does the price per mg fall with vial size?
Because the cost of vialling, sealing and testing is largely fixed per vial. The 20 mg and 30 mg sizes carry the lowest per-mg cost. Live prices and stock are on the product page.
Can I request the COA before ordering?
Yes: request the in-stock batch COA and we reply with the certificate and chromatogram.
Is retatrutide legal to buy in the UK for research?
Yes, for laboratory research. It is not a controlled substance and can be purchased and possessed for research use; it has no UK marketing authorisation as a medicine. See UK research peptide compliance.
Alphex GLP-3 (RT) is supplied for laboratory and in-vitro research by qualified researchers only. It is not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 18 February 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.