Antibody-Drug Conjugates: Treatment and Clinical Trial Questions

Key Takeaways

• An antibody-drug conjugate (ADC) combines a targeting antibody, a chemical linker and a potent payload. Changing any one component can change efficacy, resistance and toxicity.
• The word “targeted” does not mean the payload remains only inside cancer cells or that ordinary chemotherapy-type adverse effects disappear.
• Eligibility may depend on the exact tumour type, stage, treatment history, target assay, specimen, scoring system and threshold—not simply “HER2 positive” or “TROP-2 present.”
• Approved treatment, off-label treatment and a clinical trial are different routes. Verify the current Chinese indication for the exact generic product, regimen and line of therapy.
• Monitoring must follow the product rather than a generic ADC checklist: lung, eye, nerve, skin, blood-count, liver, cardiac or glucose risks vary markedly across agents and combinations.

Content

An ADC is sometimes described as “chemotherapy with a homing device.” That image is useful only up to a point. An ADC is a single engineered product in which an antibody is chemically linked to a drug, usually a highly potent cytotoxic payload [1]. Its behaviour depends on the whole construct: what the antibody binds, how stable the linker is in circulation, where the payload is released, how many drug molecules are attached, and what that payload does after release.

For a patient, the most important question is therefore not “Are ADCs good?” It is: “Why does this exact ADC, at this dose and in this regimen, fit my current tumour and treatment history?”

Read the drug name as three components

Component · What it controls · Questions worth asking

Antibody · Target binding, distribution and sometimes immune activity · What antigen? How much expression is required? Is the assay validated for this use?

Linker · Stability and the timing/location of payload release · Cleavable or non-cleavable? Could payload be released outside the target cell?

Payload · How cells are killed and which organs may be vulnerable · Topoisomerase inhibitor, microtubule inhibitor or another class? What prior payload exposure matters?

Drug-to-antibody ratio · Average number of payload molecules attached · How was the clinical dose selected, and which analytes were measured?

FDA’s ADC clinical-pharmacology guidance treats the intact conjugate, total antibody and free or unconjugated payload as related but distinct analytes. It also highlights organ impairment, drug interactions, immunogenicity, dose/exposure-response and QT assessment as product-development questions [2]. This is why safety cannot be predicted from the antibody target alone.

How an ADC can work

A conventional pathway is:

1. the antibody binds an antigen on a tumour cell;
2. the ADC-antigen complex is internalised;
3. intracellular trafficking brings it to a compartment where the linker or antibody is processed;
4. the payload is released;
5. the payload disrupts DNA, topoisomerase activity, microtubules or another essential process.

Some released payloads can also diffuse into nearby cells. This “bystander effect” may help when antigen expression is uneven, but it can also broaden exposure. NCI’s discussion of trastuzumab deruxtecan explains both target-directed internalisation and killing of neighbouring cells with low or absent HER2 expression [3]. That mechanism belongs to that construct; it should not be assumed for every ADC.

Target testing needs exact language

The pathology report should identify the specimen, collection date, tumour site, method, antibody clone or platform where relevant, scoring system, result and threshold. Ask whether the study or label accepts archival tissue, requires a new biopsy, or mandates central review.

Target rules differ. A product may require high protein expression by immunohistochemistry, a defined “low” category, a gene alteration, or a validated companion diagnostic. Some approved indications may not require routine target selection even though the ADC has a named antigen. Past results can also be misleading after treatment or at a different metastatic site.

Avoid collapsing results into a simple positive/negative label. “HER2-positive,” “HER2-low” and “HER2-mutant,” for example, describe different biological and regulatory contexts. The exact current product label or protocol should decide what result is usable.

Approved product or trial product?

For treatment in China, record all of the following:

• Chinese generic and trade name;
• marketing authorisation holder and NMPA approval number;
• target, antibody, linker and payload if disclosed;
• exact approved tumour type, disease setting and treatment line;
• single-agent or combination regimen;
• dose, schedule and required dose modifications;
• current Chinese prescribing information.

An approval in the United States or another country does not automatically create a Chinese approval. Nor does one approval for a molecule authorise every tumour, biomarker level, treatment line or combination. If the proposed use falls outside the current Chinese label, the centre should say whether it is off-label clinical care or research and explain the evidence, alternatives, consent and payment responsibility.

For a trial, use China’s Drug Trial Registration and Information Publication Platform to confirm the registration number, sponsor, sites, status and study information [4]. Request the exact cohort because the same protocol may contain several tumour types, biomarker thresholds, doses or combinations.

Trial questions that expose the real uncertainty

Early ADC studies may change both dose and schedule. “The highest dose” is not necessarily the best dose; modern oncology development aims to balance activity with tolerability rather than accept avoidable toxicity [5]. Ask:

• Which dose level or expansion cohort is open?
• Is dosing based on actual body weight, adjusted weight, a cap or another rule?
• What dose-limiting-toxicity window is used, and what later cumulative toxicities are tracked?
• What evidence supports the target threshold and companion assay?
• Is the control group standard treatment, physician’s choice, placebo plus background therapy, or another ADC?
• If combined with immunotherapy or chemotherapy, which risks overlap and which component may be held first?
• Are mandatory biopsies, pharmacokinetic sampling or anti-drug-antibody tests required?
• What event ends treatment: progression, clinical deterioration, toxicity, maximum cycles or another rule?

Cross-trial response rates should not be compared without checking disease, line of therapy, biomarker distribution, dose, follow-up, endpoint definitions and whether the study was randomised.

Toxicity is product-specific, not a single class list

ADCs can cause infusion reactions, nausea, fatigue, cytopenias, infection, liver-test abnormalities, gastrointestinal symptoms and embryo-fetal harm, but the dominant risks differ substantially. Current US labels illustrate that difference:

• trastuzumab deruxtecan carries a boxed warning for interstitial lung disease (ILD)/pneumonitis and requires prompt investigation of new cough, shortness of breath, fever or other respiratory change [6];
• mirvetuximab soravtansine has boxed ocular warnings and scheduled visual-acuity and slit-lamp examinations [7];
• enfortumab vedotin warns about serious skin reactions, hyperglycaemia, ILD/pneumonitis, peripheral neuropathy and ocular disorders [8];
• sacituzumab govitecan has boxed warnings for severe neutropenia and diarrhoea, and reduced UGT1A1 activity can increase haematological risk [9].

These US examples are not substitutes for the current Chinese label or trial protocol. They demonstrate why a centre must not hand every ADC patient the same generic warning sheet.

Before the first infusion, establish a product-relevant baseline: complete blood count, liver and renal function, pregnancy status where applicable, medicines and interactions, performance status and infection review. Depending on the ADC, baseline may also include chest symptoms/imaging, eye examination, neurological findings, glucose, cardiac function or electrolyte assessment.

Between cycles, report symptoms early rather than waiting for the next scheduled scan. New breathlessness, persistent cough, fever, visual change, eye pain, extensive rash or blistering, weakness or numbness, uncontrolled diarrhoea, dehydration, bleeding or signs of infection may require urgent assessment. The protocol or label—not a travel coordinator—should define holding, reducing or discontinuing treatment.

“Targeted delivery” does not eliminate systemic exposure

Linker instability, deconjugation, payload diffusion, target expression in normal tissue and ordinary drug clearance can expose non-tumour tissues. A systematic review found associations between linker cleavability, drug-to-antibody ratio, free payload and systemic toxicity across approved ADC studies, while also showing that design variables interact [10]. This does not allow a patient to rank drugs from linker type alone; it explains why the complete molecule and clinical data matter.

Why resistance can occur despite target expression

Failure may arise before or after initial benefit. Tumours can reduce or vary antigen expression, internalise the complex poorly, reroute intracellular trafficking, alter lysosomal processing, pump out the payload or become resistant to the payload’s cellular target. These ADC-specific resistance pathways are summarised in peer-reviewed literature [11].

Consequently, a repeat biopsy or new biomarker test may sometimes answer a real question, but it is not automatically necessary or sufficient. The oncologist should state what decision the new tissue would change and whether the procedure risk is justified.

Sequencing and combinations deserve their own discussion

Two ADCs directed at the same antigen may carry different payloads; two ADCs with similar payloads may target different antigens. Prior exposure may matter through residual organ toxicity or payload resistance even when the next antibody is different. Evidence for sequencing one ADC after another is disease- and product-specific.

Combination trials add another layer. An immune checkpoint inhibitor can introduce immune-mediated pneumonitis; an ADC may also have an ILD signal. Chemotherapy may compound cytopenia or diarrhoea. Ask for an attribution and management plan rather than assuming investigators can always identify the responsible drug immediately.

Plan repeated treatment, not just the first visit

Most ADC regimens involve recurring intravenous infusions and laboratory checks. A cross-border plan should specify:

• cycle dates and permitted delays;
• which tests can be completed in the home country and how results reach the Chinese team;
• infusion duration, observation and premedication;
• local emergency access between cycles;
• medicine supply for nausea, diarrhoea or other expected effects;
• criteria for flying, especially when respiratory, ocular, neurological or blood-count toxicity is possible;
• who decides dose holds and who records cumulative dose;
• costs for biomarker testing, central review, study-only procedures, routine care and complications;
• the treatment summary required if care transfers to another hospital.

An international patient should leave each cycle with the exact drug name, dose, date, lot or administration record if available, adverse events, dose changes, current medicines, next required tests and a 24-hour clinical contact.

Seven questions to take to the oncologist

1. Is this exact ADC approved for my current disease and treatment line in China, or is it off-label or investigational?
2. Which specimen and assay establish my target result, and does the protocol require central confirmation?
3. What do the antibody, linker and payload each contribute to expected activity and risk?
4. Which toxicities are most important for this product, dose and combination?
5. What baseline and cycle-by-cycle monitoring will detect them early?
6. What evidence supports using this ADC after my prior therapies, especially another ADC or a similar payload?
7. If treatment is held or stopped abroad, who will manage the cancer and the toxicity at home?

Medical disclaimer: This guide provides education, not an ADC recommendation, eligibility decision or dose instruction. Treatment requires review of the current Chinese label or approved protocol, pathology, prior therapy, organ function and alternatives by the treating oncology team.

FAQ

Is an ADC simply a monoclonal antibody plus ordinary chemotherapy?

It is a chemically defined antibody-linker-payload product with its own pharmacology. The payload may be highly potent, and release, distribution and toxicity depend on the full construct.

Does a target-positive result guarantee benefit?

No. Assay method, threshold, tumour heterogeneity, internalisation, intracellular processing and payload sensitivity all matter, and clinical benefit still remains uncertain.

Are all ADC side effects similar?

No. Some products are especially associated with lung, eye, skin, nerve, blood-count, gastrointestinal, cardiac or metabolic risks. Use the exact current label or protocol.

Can I use an older pathology result for trial screening?

Possibly, but the protocol may require a particular assay, enough remaining tissue, central review or a recent biopsy. The site should confirm this before tissue or travel costs are incurred.

If an ADC is approved overseas, can a Chinese hospital provide it as approved care?

Not on that fact alone. Verify the current NMPA approval, Chinese indication, product and regimen. Otherwise clarify whether the proposal is off-label care or a registered clinical trial.

Sources

1. US National Cancer Institute — Definition of Antibody-Drug Conjugate
2. US Food and Drug Administration — Clinical Pharmacology Considerations for ADCs
3. US National Cancer Institute — Trastuzumab Deruxtecan and the Bystander Effect
4. China Drug Trials — Drug Trial Registration and Information Publication Platform
5. US Food and Drug Administration — Project Optimus and Oncology Dose Optimisation
6. US Food and Drug Administration — Trastuzumab Deruxtecan Prescribing Information
7. US Food and Drug Administration — Mirvetuximab Soravtansine Prescribing Information
8. US Food and Drug Administration — Enfortumab Vedotin Prescribing Information
9. US Food and Drug Administration — Sacituzumab Govitecan Prescribing Information
10. Journal of Pharmacokinetics and Pharmacodynamics — Linker, Drug-to-Antibody Ratio and Systemic Toxicity
11. Cancers — Mechanisms of Resistance to Antibody-Drug Conjugates

Image Review

• Decision: Replaced with a topic-specific ImageGen hero and visually reviewed for medical relevance, obvious generation artifacts and bilingual reuse.
• Editorial note: The existing image shows a general medication discussion and several bottles. It does not depict the defining antibody-linker-payload structure, tumour-target testing, intravenous administration or product-specific monitoring, so it is not sufficiently specific for this guide.