Accepted answer
six weeks is 42 days and, on a weekly schedule, 6 stopper punctures out of one vial at 3.33 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, 3.33 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.
More usefully, this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
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.
It helps to be literal here: 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.
Nothing here is medical advice, and research-use compounds are not approved for human use.
At dilute concentrations, suspect adsorption before you suspect chemistry.
4Small correction: it is the number of cycles rather than the freezer temperature that does the damage. – yuki_morishita 38 days ago 5Thank you — this is the answer I was looking for. – Dr_Elias_Weiss 3 months ago add a comment