Bioassay Validation: The Question a Purity Number Never Answers
The Tirzepatide 10mg batch certificate behind this article was pulled and read directly. Report DBAV-TIRZ-10-052926, lot 5966, received 3 June 2026 and tested 22 June by Testides Analytical. Expected mass 10.00 mg, content mass 10.90 mg, fill accuracy 109.0%, purity 99.88% by HPLC-UV at 214 nm, appearance white lyophilized powder.
That is a good certificate by the standards of this market: batch-specific, laboratory named, content mass reported alongside purity rather than purity alone, and downloadable without an account.
It contains no potency section. No bioassay result, no receptor-activation measurement, no statement of biological activity anywhere on the document.
That absence is not a defect. It is a scope, and it is the scope of essentially every research-peptide certificate in the United States. What makes it worth an article is that the gap between “99.88% pure” and “biologically active” is a specific, well-characterized one, and the frameworks governing biological products treat the two as separate claims for documented reasons.
All material discussed here is supplied for research use only and is not intended for human use.
Two different questions, one number
A chromatographic purity figure answers a question about proportion: integrate the peaks, divide the main peak area by the total, and you have the fraction of ultraviolet absorbance at 214 nm attributable to the dominant species. It describes what is in the vial relative to everything else in the vial.
A potency measurement answers a question about behavior. Put the material into a biological system that responds to it, put a reference standard into the same system, and compare. The output is not a proportion of the sample but a ratio of activities against a defined comparator.
Neither reduces to the other. A batch can be 99.88% one chemical species and that species can carry the wrong stereochemistry or the wrong backbone connectivity. A batch can also carry 3% of something inert and lose nothing functionally. Purity constrains potency loosely; it does not determine it.
What ICH Q6B actually says
The governing text is ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products, adopted at ICH Step 4 on 10 March 1999 and issued by FDA as a guidance for industry in August 1999.
Its position is not ambiguous. Section 4.1.4 states that “a relevant, validated potency assay should be part of the specifications for a biotechnological or biological drug substance and/or drug product.” Section 2.1.2 states that “a valid biological assay to measure the biological activity should be provided by the manufacturer.”
The guideline then goes further and specifies the only circumstances under which physicochemical measurement may stand in. A biological assay “may be replaced by physicochemical tests only in those instances where: sufficient physicochemical information about the drug, including higher-order structure, can be thoroughly established by such physicochemical methods, and relevant correlation to biologic activity demonstrated; and there exists a well-established manufacturing history.”
Read those two conditions against a research-peptide certificate. The first requires a demonstrated correlation between the physicochemical result and biological activity — not an assumption of one, a demonstration. The second requires a manufacturing history that has been established. An HPLC-UV purity figure on a single batch from a contract synthesiser satisfies neither.
Q6B does not apply to laboratory reagents — its scope is biological products entering regulatory review, and research-grade synthetic peptides sit outside it entirely. That is exactly why it works as a reading tool rather than a compliance stick: it describes what a fully characterized potency claim looks like, which lets a reader see how far any given certificate sits from one.
Q6B is also clear about what a specification is for: it is chosen “to confirm the quality of the drug substance and drug product rather than to establish full characterization.” A certificate is a confirmation document, not a characterization document. Nobody in this market says that out loud often enough.
Why bioassays are their own discipline
The United States Pharmacopeia gives biological assays general chapters of their own rather than folding them into the analytical validation framework. General Chapter <1033>, Biological Assay Validation, notes that bioassays “are an integral part of the quality assessment required for the manufacturing and marketing of many biological and some non-biological drug products,” then draws the distinction that matters: bioassays “commonly used for drug potency estimation can be distinguished from chemical tests by their reliance on a biological substrate (e.g., animals, living cells, or functional complexes of target receptors),” and consequently “typically exhibit a greater variability than do chemically-based tests.”
That last clause is why bioassay is not simply a better version of HPLC. A chromatographic purity determination on a competent instrument is highly repeatable. A cell-based potency determination is not, because the substrate is alive and varies between passages, plates and days. Bioassay validation therefore has to demonstrate what chemical validation does not — relative accuracy against a reference standard, and precision partitioned across the sources of biological variation.
What a functional assay physically is
For a peptide acting at a G protein-coupled receptor, the standard functional readout is second-messenger accumulation in cells expressing the receptor. The Assay Guidance Manual, published by Eli Lilly & Company and the National Center for Advancing Translational Sciences, devotes a chapter to it: Wang, Li, Cvijic, Zhang and Sum, Measurement of cAMP for Gαs- and Gαi Protein-Coupled Receptors, last updated 20 November 2017. Cyclic AMP is described there as “an important intracellular second messenger in GPCR signal transduction,” with Gαs coupling raising intracellular cAMP and Gαi coupling lowering it through modulation of adenylate cyclase.
The chapter names the detection formats in general use — HTRF, LANCE, HitHunter, GloSensor — built variously on fluorescence resonance energy transfer, enzymatic complementation and split-luciferase reassembly. It also makes a point that reads as pedantry until it bites: raw signal ratios must be converted to cAMP concentrations against a standard curve, because reading potency off the raw ratio skews the estimate.
Tirzepatide illustrates the stakes precisely because it is a dual-receptor agonist. Its pharmacology is defined by activity at two receptors at once, and no chromatogram distinguishes a molecule that engages both from one that has lost activity at one of them. That distinction lives in a functional assay or it lives nowhere.
Binding affinity is not potency either
A common half-measure is to substitute a receptor-binding number — a radioligand displacement constant, a surface plasmon resonance affinity — for a potency number. Better than nothing, and still not the same measurement.
The Assay Guidance Manual chapter by Sum, Murphy, Li, Wang, Zhang and Cvijic, dated 1 November 2019, sets out why. Where measured affinity is better than measured potency, receptor reserve is likely present, and under high receptor reserve “compounds with vastly different pharmacological properties can yield exactly the same potency and efficacy.” As receptor expression rises the functional response strengthens; as it falls it weakens. The number depends on the cell line as much as on the ligand.
The chapter also covers biased agonism: ligands that “can preferentially stimulate/suppress one signaling pathway over another pathway,” behavior it calls “rather complex” and requiring characterization across multiple pathways. A molecule that binds normally and signals abnormally is invisible to a binding assay by construction. Its conclusion is the sensible one: a binding assay “is also valuable in providing affinity and kinetic assessments” alongside cell-based work — complementary, not substitutable.
Which degradation products a chromatogram sees, and which it does not
A claim circulates widely on peptide vendor sites that methionine oxidation is chromatographically invisible. It is backwards, and correcting it sharpens the real point.
Methionine sulfoxide is more polar than methionine, so it elutes earlier on a reversed-phase column. Mautz, Larraillet, König and Mølhøj put it plainly in LCGC International (37(11), November 2019): “Given that MetO is more hydrophilic relative to methionine, it will elute earlier on a reversed-phase column.” Oxidation is among the modifications reversed-phase HPLC is best at seeing. Their paper concerns the opposite failure mode — artefactual on-column oxidation from trace metal ions in aging column frits, which after roughly 700 to 800 runs began inflating measured oxidation beyond what was in the sample.
The modifications that genuinely evade both HPLC-UV and intact-mass spectrometry are the isomerizations. Geiger and Clarke, working in the Journal of Biological Chemistry in 1987, characterized the succinimide pathway in detail: asparaginyl and aspartyl residues in small peptides cyclize to a succinimide intermediate whose hydrolysis has a half-time near 2.3 hours and whose racemization has a half-time near 19.5 hours at physiological pH and temperature, yielding a mixture of L- and D- forms of both normal-aspartyl and isoaspartyl products.
Consider what a certificate sees there. An isoaspartyl residue has the same mass as the aspartyl residue it came from — the backbone was rearranged, not added to. A D-residue has the same mass and very nearly the same hydrophobicity as its L- counterpart. Both alter three-dimensional structure, which is what a receptor actually reads. A method that separates by hydrophobicity and detects by mass is blind to the changes that matter most for function, while being sharp-eyed about the one vendor copy claims it cannot see. That is the real argument for functional testing, and it survives without any exaggeration about oxidation.
What to ask a US supplier, and what we can answer
The US domestic market has consolidated around a short list of trust signals: a batch certificate, a named third-party laboratory, a purity figure, a domestic ship-from address. Ours ship same day from Santa Barbara under a Wyoming entity, and every certificate is public. Those are real, and they are not potency data.
Useful questions, in ascending order of how few suppliers can answer them:
- Is the certificate batch-specific, and does the lot number on it match the vial?
- What method produced the purity figure, at what wavelength?
- Is there any orthogonal identity confirmation — mass spectrometry, amino acid analysis?
- Is there any functional or potency data on the batch, and against what reference standard?
- If not, is there functional data on the compound from published literature that the supplier can point to?
Question four gets a “no” from us, and from every supplier in this category we are aware of. Per-batch cell-based potency testing would cost more than the material and would require a validated assay and a qualified reference standard that, for most of these compounds, does not exist.
Question five is where the honest version of this conversation lives. Published preclinical literature tells you what the molecule does when it is intact. The certificate tells you whether this batch is chemically what it claims to be. Neither is potency data on this vial, and a supplier who blurs the two is selling an inference as a measurement.
Our certificates carry a sample-identification block, an HPLC-UV purity-and-content section, an appearance line and a digital verification block. They carry no potency assay, no bioassay, and no method validation statement. We would rather write that sentence than let a figure with two decimal places imply otherwise.
All compounds referenced are supplied for research use only. They are laboratory reagents intended for in vitro and preclinical investigation by qualified researchers, are not for human use, are not foods or drugs, and nothing here describes or endorses administration to a person.
References
- ICH Q6B — specifications, test procedures and acceptance criteria for biotechnological and biological products. Adopted ICH Step 4, 10 March 1999; issued as FDA guidance for industry, August 1999. Sections 1.2, 2.1.2, 4.1.2, 4.1.3 and 4.1.4.
- United States Pharmacopeia — General Chapter <1033>, Biological Assay Validation. Cited for the distinction between bioassays and chemical tests and for the observation on relative variability.
- Wang T, Li Z, Cvijic ME, Zhang L, Sum CS — measurement of cAMP for Gαs- and Gαi protein-coupled receptors. In Assay Guidance Manual, Eli Lilly & Company and the National Center for Advancing Translational Sciences; chapter dated 20 November 2017.
- Sum CS, Murphy BJ, Li Z, Wang T, Zhang L, Cvijic ME — pharmacological characterization of GPCR agonists, antagonists, allosteric modulators and biased ligands from HTS hits to lead optimization. In Assay Guidance Manual, Eli Lilly & Company and the National Center for Advancing Translational Sciences; chapter dated 1 November 2019.
- Mautz B, Larraillet V, König M, Mølhøj M — monitoring of on-column methionine oxidation as part of a system suitability test during UHPLC–MS/MS peptide mapping. LCGC International, 2019; 37(11).
- Geiger T, Clarke S — deamidation, isomerization and racemization at asparaginyl and aspartyl residues in peptides via succinimide-linked reactions. Journal of Biological Chemistry, 1987. (Volume and page numbers were not present on the accessible full-text page and are deliberately omitted rather than guessed.)
- Certificate of analysis DBAV-TIRZ-10-052926, lot 5966, Testides Analytical — read directly from mapleresearchlabs.co on 26 August 2026.