Details up front: cagrilintide · 6.67 mg/mL · two weeks.
I am asking for verification rather than opinion, ideally with something I can read myself.
It is possible the evidence exists and I am searching for the wrong term.
Is this actually true, and what is the evidence?
two weeks is 14 days and, on a weekly schedule, 2 stopper punctures out of one vial at 6.67 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 14 days is 0.5 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 6.67 mg/mL is high enough that adsorption to the glass is a rounding error and low enough that it is not protecting you from anything. What 2 withdrawals do add is 2 opportunities to introduce air, 2 coring events on the same stopper, and a headspace that grows with every draw — none of which show up on a certificate and all of which are avoided by splitting into aliquots at reconstitution.
The short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.
Adsorption onto glass and plastic is significant at low concentrations — micrograms per millilitre — and negligible at milligrams per millilitre. It is the usual explanation for an apparent loss in a dilute preparation.
| Pathway | Dominant when | Detected by |
|---|---|---|
| Deamidation | Solution, neutral to alkaline pH | RP-HPLC, +1 Da on MS |
| Oxidation | Light, trace metals, peroxides | RP-HPLC, +16 Da on MS |
| Hydrolysis | Solution, extremes of pH | RP-HPLC, fragment masses |
| Aggregation | Agitation, interfaces, high concentration | SEC, visual haze; often invisible on RP-HPLC |
| Freeze-concentration damage | Freeze-thaw of buffered solution | SEC, loss of recovered content |
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.
Deamidation via the succinimide intermediate is well characterised, with sequence-dependent rates highest for asparagine-glycine motifs.
The caveat is that none of these pathways can be seen by looking at a vial, and a clear solution can be substantially degraded.
Cold, dry, dark, still. Those four words cover most of the mitigation.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemThe relevant detail is that this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
Light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.
Concretely, 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.
Aggregation at air-liquid interfaces is established from surface-tension and particle-count studies and is the basis for anti-agitation handling guidance.
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.
Answer first: the degradation pathways worth knowing are hydrolysis, deamidation, oxidation, aggregation and adsorption, and each has a different trigger and a different mitigation.
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.
Aggregation is physical: peptides unfold at air-liquid interfaces and associate. Shaking maximises that interface, which is why swirling and shaking produce visibly different outcomes on the same vial.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Sequence determines which pathways apply, so general statements are general.
At dilute concentrations, suspect adsorption before you suspect chemistry.
edited 3 Feb 2025 by halvard_ness — corrected a unit error in the worked example
The relevant point is that a mass shift of plus one dalton is deamidation and plus sixteen is oxidation, so degradation is often visible in a mass spectrum if anyone looks.
Deamidation converts asparagine or glutamine to the corresponding acid via a succinimide intermediate, adding one dalton. It is base-catalysed, accelerates above neutral pH and is the dominant aqueous pathway for many peptides.
Nothing here is medical advice, and research-use compounds are not approved for human use.
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.