Is retatrutide at 20 mg/mL stable enough for six weeks of multi-withdrawal use?

Stated plainly: retatrutide · 20 mg/mL · six weeks.

This has the shape of a fact but I cannot find its origin.

What I found instead were three secondary sources all citing each other.

Is there data behind this, or is it received wisdom?

six weeks is 42 days and, on a weekly schedule, 6 stopper punctures out of one vial at 20 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 42 days is 1.5 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 20 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 6 withdrawals do add is 6 opportunities to introduce air, 6 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 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.

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

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

The underlying point is that 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.

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.

Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.

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

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.

Concretely, 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.

At dilute concentrations, suspect adsorption before you suspect chemistry.

edited 6 Sept 2024 by per_haugen — added the method parameters

This is answerable from the chemistry rather than from anecdote, which is unusual and welcome.

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.

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

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

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

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.

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.

The caveat is that none of these pathways can be seen by looking at a vial, and a clear solution can be substantially degraded.

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