ten weeks is 70 days and, on a weekly schedule, 10 stopper punctures out of one vial at 8 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 70 days is 2.5 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 8 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 10 withdrawals do add is 10 opportunities to introduce air, 10 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 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.
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.
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.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Sequence determines which pathways apply, so general statements are general.
Swirl, never shake. Aggregation is a handling problem more than a time problem.
edited 15 Jul 2025 by e_dziedzic — added the method parameters
4Does the same reasoning apply to material already in solution, or is that a different curve? – Dr_Ilse_Vandenberg 4 months ago 3The desiccant point is under-appreciated and costs nothing to act on. – amara_nwachukwu 3 months ago add a comment