Nonclinical Efficacy Testing » Modality determines the nonclinical package » Antibody-drug conjugates

Antibody-drug conjugates

What an ADC is, and why it is evaluated differently

An antibody-drug conjugate has three parts: an antibody that determines where it goes, a cytotoxic payload that determines what happens when it arrives, and a linker that holds them together until it should not. The intended sequence is binding to the antigen, internalization, trafficking to the lysosome, linker cleavage or antibody degradation, and release of the payload inside the cell. Where the released payload can cross membranes, it may also affect neighbouring cells, which is described as the bystander effect. For a nonclinical programme the consequence of this architecture is that three different substances circulate after a dose, and they do not behave the same way.

The FDA has guidance specific to this modality

Oncology products generally follow the anticancer pharmaceuticals guidance.^[S2] Antibody-drug conjugates have a further document of their own, on clinical pharmacology considerations for this modality, finalized in March 2024.^[S1] The existence of a separate guidance is itself informative: the agency treats these products as raising questions that neither an antibody nor a small molecule raises alone. A programme designed only against the general oncology framework will be missing the parts that document addresses.

What the approved products show about the design choices

A high drug-to-antibody ratio with a cleavable linker

Fam-trastuzumab deruxtecan targets HER2. Its prescribing information states that approximately 8 molecules of the payload are attached to each antibody, a high drug-to-antibody ratio, and describes a cleavable tetrapeptide linker and a topoisomerase I inhibitor payload.^[S3] It carries a boxed warning for interstitial lung disease and pneumonitis.^[S3] That combination illustrates the trade-off in this design: a high ratio with a cleavable linker delivers more payload and generates more free payload systemically, and the toxicity profile reflects where that free payload goes.

A non-cleavable linker on the same target

Ado-trastuzumab emtansine targets the same antigen with a different construction. Its prescribing information describes the payload, a maytansine derivative that inhibits microtubules, as covalently linked through a stable thioether linker, with an average drug-to-antibody ratio of 3.5.^[S4] A non-cleavable linker means the payload is released only when the antibody itself is degraded, and the released species remains attached to an amino acid residue. Two products against the same target, differing in linker chemistry and ratio, is the clearest available illustration that these are design variables rather than fixed features.

A protease-cleavable linker

Brentuximab vedotin targets CD30. Its prescribing information describes a protease-cleavable linker attaching the payload to the antibody, with a drug-to-antibody ratio of approximately 4.^[S5] The payload is a microtubule inhibitor. Peripheral neuropathy and haematological effects are among the toxicities associated with payloads of this class, and both have to be monitored in the nonclinical studies as well as clinically.

The problems specific to this modality

Release before arrival

Linkers are classified as cleavable or non-cleavable,^[S7] and the choice determines where the payload is released. If a cleavable linker releases payload in circulation rather than inside the target cell, the result is systemic exposure to a cytotoxic agent that was never intended to circulate freely. That is the mechanism behind the toxicities most often associated with this class. Linker stability in plasma is therefore not a chemistry detail but a determinant of the safety profile, and it is measured rather than assumed.

A drug-to-antibody ratio is a distribution, not a number

Conjugation by conventional methods produces a mixture rather than a single species, with molecules carrying different numbers of payload. The stated ratio is an average across that mixture, and the species within it do not behave identically: more heavily loaded species are cleared differently from lightly loaded ones, and the ratio in circulation therefore changes over time after dosing. A single concentration measurement cannot represent this.

Antigen expression is not uniform within a tumour

In solid tumours the target antigen is expressed unevenly, so positive and negative cells sit alongside each other. This is the situation the bystander effect is meant to address, and it is also where the effect becomes difficult to bound: payload that diffuses to antigen-negative tumour cells can also diffuse to normal tissue. A cell line overexpressing the antigen uniformly cannot show either side of this, which is why mixed-population systems are used.

The nonclinical work this modality requires

Multi-analyte bioanalysis

This is the settled part of the field rather than a proposal. A white paper from the working group on this modality at the pharmaceutical scientists association sets out quantitation strategies for total antibody, conjugated conjugate, free unconjugated payload and the drug-to-antibody ratio as the analytes to be measured.^[S6] Measuring total antibody alone establishes neither how much intact conjugate is present nor how much payload has been released. The methods combine chromatography with mass spectrometry, immunoaffinity capture and immunoassay, and which analyte each method reports should be stated explicitly in the study report.

Mixed-population and patient-derived models

Co-cultures combining antigen-positive and antigen-negative cells, and patient-derived models retaining native heterogeneity, are used to assess penetration into a tumour mass and the extent of bystander activity. Mass spectrometry imaging can locate payload within the tissue rather than reporting a homogenate concentration, which is what distinguishes payload that reached the tumour interior from payload that stayed at its margin.

Off-target uptake and human cell systems

Two sources of toxicity have to be separated. Payload released anywhere acts on any dividing cell, which produces the effects characteristic of the payload class. Antigen expressed on normal tissue directs the conjugate there specifically, which produces effects characteristic of the target. Tissue cross-reactivity work addresses the second. Human cell systems, including those derived from induced pluripotent stem cells, are used where a species difference in the target or in payload sensitivity would make an animal result uninformative. This article does not assert a role for Fc receptor mediated uptake, because no source for that could be located in preparing this page.

Internalization and linker cleavage

Binding is necessary but not sufficient: a conjugate that binds well and internalizes poorly delivers little payload. Live cell imaging follows internalization and trafficking to the lysosome, and cell-free assays measure cleavage rate under the pH and enzyme conditions found there. Together these establish whether the antibody and linker chosen work as a pair, which is a question neither binding affinity nor plasma stability answers on its own.

What to establish with the laboratory

Four questions separate a laboratory that runs conjugates from one that runs antibodies. Which analytes its bioanalytical methods report, named individually rather than as ADC concentration.^[S6] Whether it can measure free payload at the concentrations that matter, which is usually far below the antibody concentration. Whether it holds, or can obtain, the antigen-negative counterpart of the cell line for bystander work. And whether the reference standards for each analyte exist or have to be made, which is often the item that sets the timeline. Whether a given study falls under the good laboratory practice regulation is a separate question to settle alongside these.^[S9]

About this article

This is an independent editorial article for people who commission nonclinical work in the United States. Product characteristics are taken from the prescribing information as published, and approvals are recorded as facts; this site does not discuss the efficacy of any product or compare products. Guidance describes the agency current thinking and is not binding. It does not recommend any laboratory. Last reviewed: September 3, 2026.

References

FDA — Clinical Pharmacology Considerations for Antibody-Drug Conjugates, Guidance for Industry（2024/3）. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/clinical-pharmacology-considerations-antibody-drug-conjugates (accessed 2026-09-03)
FDA — ICH S9 Nonclinical Evaluation for Anticancer Pharmaceuticals, March 2010. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/s9-nonclinical-evaluation-anticancer-pharmaceuticals (accessed 2026-09-03)
FDA — ENHERTU (fam-trastuzumab deruxtecan-nxki) full prescribing information（BLA 761139）. https://www.accessdata.fda.gov/scripts/cder/daf/ (accessed 2026-09-03)
FDA — KADCYLA (ado-trastuzumab emtansine) full prescribing information（BLA 125427）. https://www.accessdata.fda.gov/scripts/cder/daf/ (accessed 2026-09-03)
FDA — ADCETRIS (brentuximab vedotin) full prescribing information（BLA 125388）. https://www.accessdata.fda.gov/scripts/cder/daf/ (accessed 2026-09-03)
Updated Recommendations for the Bioanalysis of Antibody-Drug Conjugates (ADC) from the ADC working group of the AAPS Bioanalytical Community. https://pubmed.ncbi.nlm.nih.gov/ (accessed 2026-09-03)
Linkers: An Assurance for Controlled Delivery of Antibody-Drug Conjugate. https://pubmed.ncbi.nlm.nih.gov/ (accessed 2026-09-03)
21 CFR Part 58 — Good Laboratory Practice for Nonclinical Laboratory Studies (§58.1 scope). https://www.ecfr.gov/current/title-21/chapter-I/subchapter-A/part-58 (accessed 2026-09-03)

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Antibody-drug conjugates

What an ADC is, and why it is evaluated differently

3 Recommended Contract Research Organizations for Non-Clinical Studies — by Target goal and Expertise

SMC Laboratories, Inc.

Charles River Laboratories

Inotiv

3 Recommended Contract Research Organizations
for Non-Clinical Studies
— by Target goal and Expertise