Does dimerisation of cagrilintide at 4 °C show up as a loss of content or of purity?

Stated plainly: dimerisation · cagrilintide · 4 °C.

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.

Is the standard explanation correct, and if so, what is the evidence for it?

At 4 °C it can show up as either, and which one depends entirely on whether the product still elutes under the main peak. Purity is a ratio of areas, so a degradant only costs purity if the method resolves it. Content is a mass against a standard, so a degradant costs content whenever the parent is consumed — resolved or not. Two chains join, usually through a disulfide, so the product is roughly twice the mass and shows up as a late peak — or as nothing, if it never comes off the column. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to 4 °C is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.

Start with the sequence, because which pathways are available depends on which residues are present.

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.

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.

Swirl, never shake. Aggregation is a handling problem more than a time problem.

Mechanically, asparagine and glutamine are the deamidation risk, and methionine is the oxidation risk.

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.

The relevant detail is that 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.

Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.

At dilute concentrations, suspect adsorption before you suspect chemistry.

The honest answer is that most reported "degradation" is adsorption and dilution error rather than chemistry.

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.

Specifically, 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.

Aggregation at air-liquid interfaces is established from surface-tension and particle-count studies and is the basis for anti-agitation handling guidance.

A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.

Answer first: the degradation pathways worth knowing are hydrolysis, deamidation, oxidation, aggregation and adsorption, and each has a different trigger and a different mitigation.

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.

Nothing here is medical advice, and research-use compounds are not approved for human use.

Cold, dry, dark, still. Those four words cover most of the mitigation.

edited 22 Jun 2025 by mz_4113 — corrected a unit error in the worked example

To be exact about it, aggregation is a physical process and is the one most often caused by handling rather than by time.

Light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.

Deamidation via the succinimide intermediate is well characterised, with sequence-dependent rates highest for asparagine-glycine motifs.

Sequence decides which pathways are even available. Check the residues.

edited 7 Jul 2025 by Dr_Colm_Fitzhenry — expanded the table to cover the lower concentration