What happens to oral semaglutide after ten weeks at minus 20 °C in solution?

For reference: oral semaglutide · ten weeks · minus 20 °C.

I would like the mechanism, because I want to be able to reason about the cases nobody has written about.

I have tried to reason it out from first principles and got to two contradictory conclusions.

So what is the mechanism, and how well established is it?

ten weeks is 70 days at a temperature where the chemistry all but stops and the physics does not. minus 20 °C is 25 kelvin below a refrigerator, and below the glass transition of a lyophilised cake the ten-degree rule of thumb stops applying at all — solid-state chemistry is not slow liquid chemistry, it is a different regime, and the failure modes that survive it are mechanical rather than chemical. In a frozen solution the solute is excluded from the growing ice, so the unfrozen fraction concentrates and the buffer's pH moves as one salt crystallises before the other. The damage is done at the transitions, and 70 days of stable hold between them contributes very little. Reconstituted material has no certificate; the one in the box describes the powder.

The short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.

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.

Degradation pathway by condition

The relevant detail is that 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.

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

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

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.

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.

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

At dilute concentrations, suspect adsorption before you suspect chemistry.

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.

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.

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

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

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