Are antibody-drug conjugates the same as chemotherapy?

Not exactly. ADCs carry a cytotoxic drug, so they are related to chemotherapy, but they use an antibody to direct that drug toward cells with a specific target such as HER2 or TROP2. They are often called targeted chemotherapy because they aim to improve selectivity, not because they are free of chemotherapy-like side effects.

How are ADCs different from regular monoclonal antibodies?

A regular monoclonal antibody may block a receptor or recruit the immune system, but it does not necessarily carry a cell-killing drug. An ADC has 3 parts: the antibody, a linker, and a potent payload. That payload is what allows the ADC to directly deliver a toxic drug into or near the cancer cell.

Do I need a biomarker test before getting an ADC?

Usually yes. Most ADCs are used only when the tumour shows the relevant target, such as HER2, CD33, or TROP2, on pathology testing. The exact test depends on the cancer type and drug, and in breast cancer HER2 testing commonly uses immunohistochemistry, sometimes with in situ hybridisation.

What side effects should patients watch for during ADC therapy?

The side effects depend on the specific ADC, but important ones can include low blood counts, neuropathy, liver problems, infusion reactions, diarrhoea, and fatigue. Some ADCs can also cause interstitial lung disease or pneumonitis, so new cough or breathlessness should be reported urgently. Eye symptoms such as blurred vision or dryness are also important with certain products.

Are ADCs available in India?

Some ADCs are available in India through oncology centres, but availability varies by drug, indication, and hospital system. Access may depend on regulatory approval, pathology testing, infusion facilities, and cost. Patients should ask their oncologist whether a specific ADC is approved and appropriate for their cancer subtype.

Antibody-drug conjugates — Glossary

What it is

Antibody-drug conjugates are targeted cancer medicines that join an antibody to a potent anticancer drug in 3 parts: a monoclonal antibody, a chemical linker, and a cytotoxic payload. They are designed to deliver chemotherapy-like treatment more selectively to cancer cells that carry a chosen surface target, such as HER2, TROP2, or CD33. The basic idea dates back to Paul Ehrlich's 1913 "magic bullet" concept, but modern ADCs became clinically useful only after advances in antibody engineering, linker chemistry, and payload design.

An ADC is not simply "chemotherapy attached to an antibody." Its effect depends on whether the target antigen is present on the tumour, whether the ADC is stable in the bloodstream, whether it is taken up into the cancer cell, and whether the payload is then released in an active form. Because of this, two ADCs aimed at the same target can behave differently in practice.

A simple way to think about ADCs is:

Component	Role
Antibody	Finds and binds a specific antigen on cancer cells
Linker	Holds the drug to the antibody until the right time/place
Payload	Kills the cell after delivery

In cancer care in India, ADCs are increasingly relevant in breast cancer, some blood cancers, and selected other solid tumours, but access depends on tumour testing, regulatory approval, cost, and the ability to monitor side effects.

How it works

After infusion, the antibody part of the ADC circulates in the blood and binds to its target antigen on the cancer cell surface. In many cases, the bound ADC is then internalised into the cell. Inside the cell, the linker is broken down by enzymes, acidity, or other intracellular conditions, releasing the payload. The payload then damages DNA or disrupts microtubules, leading to cell death.

Several design features matter:

Target selection: The antigen should be present on cancer cells and limited on normal tissues.
Linker stability: The linker should stay intact in blood but release the drug in or near the tumour.
Payload potency: ADC payloads are usually far more potent than standard chemotherapy drugs because only a small amount may reach each cell.
Drug-to-antibody ratio (DAR): This is the average number of payload molecules attached to each antibody. DAR affects potency, stability, and toxicity.

There are also important differences between ADCs:

Design feature	Main options	Why it matters
Linker type	Cleavable or non-cleavable	Influences where and how the drug is released
Payload class	Microtubule inhibitor or DNA-damaging agent	Changes activity and side-effect pattern
Bystander effect	Present or limited	Some released drugs can diffuse into nearby tumour cells, which may help in tumours with uneven target expression

The "bystander effect" is one reason some newer ADCs may work even when not every cancer cell strongly expresses the target. At the same time, this can also contribute to off-target toxicity.

Evidence and uses

ADCs are an established drug class in oncology, not an experimental concept. Different ADCs are approved for different cancers, and each has its own evidence base. Examples include HER2-directed ADCs in breast and gastric cancers, CD33-directed ADCs in acute myeloid leukaemia, CD22-directed ADCs in acute lymphoblastic leukaemia, and TROP2-directed ADCs in breast and some other solid tumours.

In breast cancer, ADCs have changed treatment for some patients with HER2-positive disease and, more recently, HER2-low disease. Reviews of the field show that ADCs can improve outcomes compared with older treatment options in selected settings, but the benefit depends on the exact drug, line of therapy, and tumour biology. They are also being studied across lung, gynecologic, gastrointestinal, and genitourinary cancers.

ADCs are often described as "targeted chemotherapy." That is useful shorthand, but it can be misleading if it suggests they spare all normal tissue. They usually have a narrower and more tumour-focused delivery than standard chemotherapy, yet they can still cause serious adverse effects. Their benefit comes from improving the therapeutic index, not from eliminating toxicity.

Use of an ADC usually requires biomarker testing on the tumour. For example, HER2-directed ADCs depend on HER2 assessment by pathology methods such as immunohistochemistry, and sometimes in situ hybridisation, depending on the cancer type and treatment context. This makes accurate pathology especially important.

In India, the practical questions are often as important as the science: whether the tumour has the right target, whether the ADC is approved and available, whether infusion facilities and supportive care are in place, and whether the patient can be monitored for lung, liver, eye, nerve, or blood-related toxicity depending on the product.

Safety and interactions

ADCs can cause side effects from both the antibody and the payload. Common or important toxicities vary by product but may include low blood counts, nausea, fatigue, hair loss, neuropathy, liver test abnormalities, infusion reactions, mouth sores, diarrhoea, and eye problems. Some ADCs carry a risk of interstitial lung disease/pneumonitis, which can be serious and requires urgent evaluation of new cough, breathlessness, or fever.

Safety is product-specific. For example, one ADC may be more associated with neuropathy, another with eye toxicity, and another with lung inflammation. This is why patients should not assume all ADCs have the same side-effect profile just because they are in the same class.

General safety points:

ADCs are given under oncology supervision, usually by intravenous infusion.
Blood counts and liver tests are commonly monitored.
Some ADCs need baseline and follow-up assessment for lung symptoms or eye symptoms.
Pregnancy should be avoided during treatment because payloads are cytotoxic.
Vaccines, other anticancer drugs, and supportive medicines may need timing or monitoring adjustments.

Drug interaction data for ADCs are less straightforward than for many oral cancer drugs. Interactions may involve the monoclonal antibody, the released payload, or both. Patients should tell their oncologist and pharmacist about all prescription medicines, over-the-counter drugs, and supplements. This includes Ayurvedic or herbal products, because some may affect liver function, bleeding risk, or symptom interpretation during cancer treatment.

When to see a clinician

Anyone receiving an ADC should contact their oncology team promptly for:

New cough, shortness of breath, chest tightness, or unexplained fever
Severe diarrhoea, vomiting, or dehydration
Yellowing of the eyes, dark urine, or right upper abdominal pain
New numbness, tingling, or weakness
Eye pain, blurred vision, or marked dryness
Mouth ulcers that limit eating or drinking
Unusual bruising, bleeding, or signs of infection

Emergency assessment may be needed if breathing symptoms are significant, because pneumonitis and severe infection can worsen quickly.

Limitations and open questions

ADCs are one of the most active areas in cancer drug development, but several limits remain. Not all tumours express a suitable target, and even target-positive cancers can become resistant by reducing antigen expression, changing internalisation, altering drug processing, or pumping out the payload. Tumour heterogeneity is another challenge, because different parts of the same cancer may not express the target equally.

Evidence is strong for some approved ADCs in specific cancers, but weaker or still emerging for many newer agents and combinations. It is also not yet clear which patients benefit most from sequencing one ADC after another, or how best to combine ADCs with immunotherapy and other targeted drugs while keeping toxicity acceptable.

Another open question is access. ADCs are complex biologic medicines and can be expensive. In real-world care, especially in low- and middle-income settings including parts of India, cost, testing availability, and supportive monitoring can limit use even when a drug is scientifically appropriate. For patients and families, the key point is that an ADC is a precise but still potent anticancer treatment, and decisions about its use should be individualised by an oncology team.

Antibody-drug conjugates