Is cagrilintide at 6.67 mg/mL stable enough for eight weeks of multi-withdrawal use?

Details up front: cagrilintide · 6.67 mg/mL · eight weeks.

I would rather be corrected now than propagate something wrong.

I am specifically not interested in a testimonial; I am interested in a measurement.

What would count as evidence here, and does it exist?

eight weeks is 56 days and, on a weekly schedule, 8 stopper punctures out of one vial at 6.67 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 56 days is 2 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 6.67 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 8 withdrawals do add is 8 opportunities to introduce air, 8 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 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

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.

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.

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

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.

It helps to be literal here: light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.

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.

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

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.

More usefully, 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.

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

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