Setup, so nobody has to ask: aggregation · a GLP-1 receptor agonist · room temperature.
I keep seeing this stated as a fact with no explanation attached, and unexplained facts make me suspicious.
My background is quantitative but not chemical, so I can follow an equation more easily than a hand-wave.
Why does this happen, and what would falsify the usual explanation?
At room temperature the question is which route is fastest, not whether aggregation happens — and the routes do not share an activation energy, so their ranking changes with temperature. Room temperature is not a number, so take the pharmacopoeial 20–25 °C and its 22.5 °C midpoint: 17.5 kelvin above the 5 °C middle of a 2–8 °C refrigerator. The ten-degree rule of thumb — degradation rate roughly doubling per 10 K — puts that at about 3.4 times the refrigerated rate. It is an order-of-magnitude statement about a rate, not a shelf life, and the top of the 20–25 °C band runs about 1.4 times faster than the bottom of it. That multiplier is an average over every route at once, which is exactly why it cannot tell you which one wins. Molecules associate without any covalent change, so the mass is unchanged and a reversed-phase run — which is performed in organic solvent — mostly dissolves the evidence before it can be measured. So the way to answer it for your vial is to pick the method that sees aggregation specifically and run it against a control held cold, rather than to infer a mechanism from a purity number that averages all of them.
Start with the sequence, because which pathways are available depends on which residues are present.
Oxidation targets methionine, cysteine and tryptophan, adding sixteen daltons per oxygen. It is catalysed by trace metals and promoted by dissolved oxygen and by light.
| State | Condition | Usable window | Basis |
|---|---|---|---|
| Lyophilised solid | −20 °C, sealed, dry | 24–36 months | Supplier guidance |
| Lyophilised solid | 2–8 °C, sealed | 12–24 months | Supplier guidance |
| Lyophilised solid | 25 °C, sealed | 4–8 weeks | Extrapolated (Arrhenius) |
| Lyophilised solid | 40 °C, sealed | 1–2 weeks | Extrapolated |
| Solution, preserved | 2–8 °C | 28 days | USP microbiological convention |
| Solution, preserved | 25 °C | 3–7 days | Extrapolated |
| Solution, unpreserved | 2–8 °C | 24 hours | USP microbiological convention |
Windows for the solid state are chemical; windows for solution are microbiological and usually shorter than the chemical limit.
A mass spectrum resolves most of this: minus eighteen is dehydration or succinimide, plus one is deamidation, plus sixteen is oxidation, and an unchanged mass with a shifted retention time is an isomer.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Apparent loss in a dilute preparation is usually adsorption rather than degradation and is worth ruling out first.
Swirl, never shake. Aggregation is a handling problem more than a time problem.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.
Freeze-thaw cycling drives aggregation through concentration at the ice interface and pH shifts as buffer components crystallise out at different rates. Each cycle costs something.
Hydrolysis cleaves the backbone, most readily at aspartate-proline and aspartate-glycine sequences, and is acid-catalysed. In a dry solid it barely proceeds at all.
The caveat is that none of these pathways can be seen by looking at a vial, and a clear solution can be substantially degraded.
A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.