How much of the gross mass of a 20 mg cagrilintide vial is actually peptide?

What I am working with: 20 mg · cagrilintide.

I have worked this out and I would like someone to find the error, because I suspect there is one.

My working so far, for the record, is below, and I am fairly sure the error is in the unit conversion rather than the algebra.

Is my approach right even if my number is wrong?

Concretely, start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.

The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.

If a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.

Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.

The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.

edited 20 Aug 2025 by tobias_maartens — removed a claim I could not source

More usefully, the short answer is that purity says "what fraction of detected material is the target" while content says "how many milligrams of the target are present," and those are two different things.

Water content and counter-ion content are part of the gross mass but not part of the content assay result, which is why the two do not sum to label claim.

Running multiple independent aliquots of the same sample should give results that agree to within the method precision, which is usually one to three per cent.

The caveat is that content assay costs more than purity, so most people do not do it, which is exactly why it is valuable on the first lot from a new supplier.

If a supplier gives you content without the standard's purity, ask them to provide it.

Put another way, the honest answer is that you cannot know for certain what you have without a content assay, and the purity number alone is not enough.

For peptides at 214 nanometres the response is roughly proportional to the number of peptide bonds, so truncation impurities have lower response factors and overestimate content.

Where content data have been published from testing services on common peptides, the spread between services on identical material is typically a few per cent.

I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.

Ask for both the purity and the content, and do not accept purity alone.

On the detail: content assay and purity are orthogonal measurements answering orthogonal questions, and the confusion between them is one of the most expensive misreadings in this space.

Comparing content results from different laboratories requires knowing whether they both used certified reference materials or whether one used an in-house standard of unknown provenance.

Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.

Worth noting that the standard certificate carries its own uncertainty, usually on the order of two to three per cent, which the sample result inherits.

The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.