Does ecnoglutide adsorb measurably to the vial wall at 5 mg/mL?

Concretely: ecnoglutide · 5 mg/mL.

I want the working, not the result — I need to be able to redo it with different numbers.

I care about the precision as well as the value — I want to know how many figures are real.

How many significant figures are actually justified here?

5 mg/mL is 5000 µg/mL — roughly 500 times the concentration at which surface adsorption is measurable. Losses to glass and plastic matter in the low microgram-per-millilitre range, where a monolayer on the wall is a real fraction of what is in solution. At 5 mg/mL that same monolayer is a rounding error. If you see an apparent loss at this concentration, suspect the dilution step or the assay before you suspect the wall.

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.

Reported and extrapolated stability by condition

Windows for the solid state are chemical; windows for solution are microbiological and usually shorter than the chemical limit.

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

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

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

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

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 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.

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

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.

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.

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.

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

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

edited 10 Jan 2025 by triple_agonist_q — added a caveat about sampling

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

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 decides which pathways are even available. Check the residues.

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

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.

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.

At dilute concentrations, suspect adsorption before you suspect chemistry.