Independent analysis puts a Homopeptide tirzepatide lot at 99.3%, against 95.3% on the certificate

An independent purity determination on tirzepatide material sold by Homopeptide has returned 99.3%, a figure 3.7 percentage points below the number printed on the accompanying certificate. The analysis was performed by VendorInvestigate on a vial the Journal bought as an ordinary retail customer in November 2024 and posted unopened.

The Journal purchased 1800 vials from the same lot and had 8 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.

Content by elemental nitrogen determination returned 90% of the labelled nominal mass, which on a 15 mg vial corresponds to roughly 14.46 mg of peptide rather than the labelled figure. The remainder is accounted for by counter-ion, residual water and residual solvent, none of which is visible chromatographically.

Four independent services — VendorInvestigate, Medutest, 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 figure is not wrong and neither is theirs," said Lorcan Feeney, analytical services director at an independent laboratory. "They are answers to two different questions and the trade has decided to pretend there is only one question. Print the gradient and the argument disappears."

Homopeptide’s dossier has been updated with this determination and with the date of the request for comment. The dossier’s "what we could not verify" box records the questions still outstanding.