Independent analysis puts a CPC tirzepatide lot at 96.6%, against 92.7% on the certificate

A peptide content determination commissioned by the Journal on a 2 mg tirzepatide vial from CPC has returned 93% of the labelled mass, the balance being the counter-ion and residual water that every lyophilised salt carries. Chromatographic purity on the same vial was above 96.6%. The two numbers measure different things, and this is one of the few certificates in the trade to print both.

The Journal purchased 8 vials from the same lot at retail and had four of them analysed separately, in order to distinguish vial-to-vial variation from method variation. Vial-to-vial agreement was within 0.4 percentage points. Method-to-method disagreement was an order of magnitude larger.

Reported figures across the set were: highest 96.6%, lowest 92.7%, median close to the midpoint of the two. Every report in the set stated a detection wavelength, and 10 of them stated an integration threshold. The Journal has asked the laboratory to publish thresholds on all reports as a matter of routine.

Four independent services — Medutest, PeptideMeter, and the two others operating in this market — carry the analytical burden for a trade that has no release-testing obligation of its own. Each is paid by whoever submits the sample, which is usually the vendor. The Journal’s position is that a submitter-pays model is workable but that its incentive structure has to be stated whenever a result is quoted as verification.

"The gap that matters is not between the two laboratories, it is between either of them and the certificate," said Dr Sirichai Boonmee, peptide chemist, Chulalongkorn University. "One of those documents states a method. Compare like with like and most of this disappears."

CPC’s dossier has been updated with this determination and with the company’s response to it. The dossier’s ordering box records the lead times, the minimum quantities and what a buyer should ask for at the point of order.