How much of the gross mass of a 5 mg cagrilintide vial is actually peptide?

What I am working with: 5 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?

On a 5 mg vial every percentage point of water and counter-ion is 0.05 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 5 mg on the label is gross fill or net peptide, because the two differ by exactly the quantity being asked about. If the certificate quotes net peptide content of 85 per cent, a 5 mg gross fill holds 4.25 mg of peptide; at 78 per cent it holds 3.9 mg. That 0.35 mg gap is 7 per cent of the label, larger than any purity difference anybody argues about, and it is invisible to a purity figure because water and acetate are not impurity peaks. Amino acid analysis or a nitrogen determination gives you the number; an HPLC area per cent never will.

Most research-grade certificates report purity without content, which is exactly backwards from what users actually need.

A content assay is always paired with a purity assay because purity tells you what fraction of the measured mass is the target and content tells you the total measured mass.

What each test answers

Specifically, if the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.

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.

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

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

Quantitation against a certified reference material assumes the sample and standard are treated identically through the analytical method, which is why the method for calibration matters as much as the method for measurement.

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

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.

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.

The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.

Specifically, 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.

The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.

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

edited 26 Jul 2024 by h_pergande — added a caveat about sampling

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.

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.

One qualification: a single result from a single vial is a point estimate, and repeating the assay on a second aliquot is worth doing if the first result is surprising.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

More usefully, two measures of the same vial can agree on purity and disagree on content by a few per cent, which usually means the content assay used a different standard.

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.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.