Does oxidation of liraglutide at 30 °C show up as a loss of content or of purity?

Conditions: oxidation · liraglutide · 30 °C.

I want to know whether this is a real physical effect or an artefact of how it is measured.

What prompted the question is an inconsistency between two sources I otherwise trust.

What is the causal chain, and where does it stop being established?

At 30 °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. Met and Trp take up oxygen sixteen daltons at a time, and the oxidised species is more polar, so on a reversed-phase column it elutes ahead of the parent rather than behind it. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to 30 °C is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.

Answering this needs the physical state, since a dry powder is protected from most of these and a solution is protected from none.

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.

Mechanically, 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.

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.

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 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.

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.

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

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

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.

Sequence determines which pathways apply, so general statements are general.

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

It helps to be literal here: this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.

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.

Apparent loss in a dilute preparation is usually adsorption rather than degradation and is worth ruling out first.

At dilute concentrations, suspect adsorption before you suspect chemistry.

Aggregation is a physical process and is the one most often caused by handling rather than by time.

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.

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.