Is 2.5 mg/mL a sensible working concentration for cagrilintide, or should I go lower?

What I am working with: 2.5 mg/mL · cagrilintide.

I suspect the honest answer is that it depends, in which case I would like to know on what.

Assume I can obtain either option without difficulty, so availability is not the deciding factor.

Under what conditions does the answer flip?

The answer depends on exactly which dose and which vial you are asking about, but the method is always the same.

On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.

The relevant detail is that the rounding error accumulates if you round too many times — rounding concentration to 5.0, rounding the dose volume to 0.1 mL, rounding the unit reading to 10 — and the safest approach is to work the full precision and round only the final answer.

The limitation is that technique reduces risk, it does not remove it, and nothing you can do outside a controlled environment makes a non-sterile preparation sterile.

Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.

The common error is getting the concentration right but then misreading the syringe scale, which is why checking the barrel marking rather than your memory matters.

Do not use the same needle to pierce the stopper and to administer. The tip is blunted by the stopper, and the hub now contains a dose you are about to lose to dead space anyway.

Specifically, room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and condensation on a cold barrel makes it harder to read the meniscus.

Do the arithmetic twice, ideally with someone else doing it independently.

This is one of those calculations where checking your work takes two minutes and prevents a very consequential error.

Dead space quantified: a fixed-needle insulin syringe holds roughly 3 to 5 µL in the hub and needle after the plunger bottoms out. A luer-lock syringe with a detachable needle holds 35 to 100 µL depending on the hub design. At 5 mg/mL that is 15 to 25 µg lost per draw on the insulin syringe and 175 to 500 µg on the luer-lock — which over ten draws is the difference between losing a rounding error and losing half a milligram.

The caveat is that this assumes the vial contains what the label says, and if the content assay has not been done, the arithmetic is precise about an unknown quantity.

If in doubt, use more diluent and accept the shorter usable window.

edited 29 Oct 2025 by tabular_nums — expanded the table to cover the lower concentration

The part that matters: two people working through the same arithmetic independently should get the same answer, and if they do not, someone has made a unit error.

The concentration you actually work with is label claim times content fraction divided by actual diluent volume, which is usually not the same as the nominal concentration because content is usually not 100 per cent and you rarely measure the diluent volume to 0.1 mL precision.

The content assay results from major testing services show that nominal vial claim and measured content differ by one to ten per cent, making content a driver of dose error.

Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.