How Antibody-Drug Conjugates Target Cancer Cells

The Problem With Traditional Chemotherapy

For decades, chemotherapy worked by poisoning cells that divide rapidly — a blunt approach that kills cancer but also damages the gut lining, hair follicles, bone marrow, and other fast-dividing healthy tissue. The side effects are often brutal, and the collateral damage limits how much drug doctors can safely give.

Antibody-drug conjugates, or ADCs, were invented to solve this problem. The idea is deceptively simple: attach a powerful toxin directly to an antibody that is designed to seek out cancer cells, and let biology do the rest. Scientists sometimes call them "biological missiles" or "smart chemo."

Three Components, One Weapon

Every ADC has three essential parts: an antibody, a linker, and a payload. Together they function like a guided delivery system.

The Antibody

Monoclonal antibodies are lab-engineered proteins that bind with high specificity to a target molecule — called an antigen — on the surface of cancer cells. Different cancers overexpress different antigens. Breast cancer cells, for example, often carry large amounts of a protein called HER2 or TROP2. The antibody acts like a key cut for a specific lock, attaching only to cells that display the right surface marker.

The Linker

The linker is the chemical bridge connecting the antibody to the drug payload. Its job is to keep the payload locked in place while the ADC circulates harmlessly through the bloodstream, and then release the drug precisely inside the cancer cell. Some linkers are cleavable — they break apart in the acidic environment of a cancer cell's lysosomes or respond to enzymes found mainly in tumors. Others are non-cleavable and rely on the entire antibody being broken down inside the lysosome before the drug is freed. The stability of the linker is critical: a premature release in the bloodstream would cause the same toxic side effects as conventional chemotherapy.

The Payload

The cytotoxic payload is the actual killing agent — typically many times more potent than standard chemotherapy drugs. Common payloads include tubulin inhibitors like monomethyl auristatin E (MMAE), which disrupt cell division, and DNA-damaging agents like calicheamicin or topoisomerase inhibitors such as SN-38. Many of these compounds would be far too toxic to inject directly; they are only safe to use because the ADC system delivers them in minuscule, targeted doses.

How the Delivery Works, Step by Step

Once an ADC is infused into the bloodstream, the antibody navigates to the tumor site and binds to its target antigen on the cancer cell surface. The cell then does something it naturally does with proteins stuck to its membrane: it pulls the whole complex inside through a process called receptor-mediated endocytosis. The ADC ends up in a lysosome — a cellular compartment packed with digestive enzymes and acidic conditions. There, the linker breaks and releases the cytotoxic payload, which floods the interior of the cancer cell and triggers cell death.

Some ADCs also produce a "bystander effect": the released drug can diffuse into neighboring tumor cells, killing cancer cells that may not even express the target antigen — a useful property when tumors are heterogeneous.

From Concept to Clinic: A 60-Year Journey

The idea of targeting drugs to tumors dates to the early 20th century, when Paul Ehrlich coined the phrase "magic bullet." Practical ADC research began in the 1960s, but progress was slow because early antibodies triggered immune reactions and linker chemistry was unreliable.

The field transformed in the 1970s when Georges Köhler and César Milstein developed a method to produce monoclonal antibodies — identical antibodies cloned from a single immune cell — work for which they won the Nobel Prize. This made consistent, highly specific antibodies available at scale.

The first ADC approved by the U.S. Food and Drug Administration was gemtuzumab ozogamicin (Mylotarg) in 2000, targeting acute myeloid leukemia. It was briefly withdrawn over safety concerns, then re-approved with a revised dosing schedule in 2017. Since then, the field has exploded: as of 2025, more than 19 ADCs are approved worldwide, targeting cancers ranging from triple-negative breast cancer and bladder cancer to lung cancer, cervical cancer, and multiple myeloma, according to data compiled by Biopharma PEG.

Why ADCs Matter Now

The oncology community is experiencing what Dana-Farber Cancer Institute researchers have called an ADC "revolution." More than 100 ADC candidates are currently in clinical trials, and drug combinations pairing ADCs with immunotherapy checkpoint inhibitors are showing results that neither treatment achieves alone.

A landmark 2026 New England Journal of Medicine trial showed that sacituzumab govitecan combined with pembrolizumab significantly improved progression-free survival in metastatic triple-negative breast cancer compared to standard chemotherapy — a cancer type that previously had very limited targeted options.

ADCs are not without challenges. Engineering a molecule stable enough in the blood but reactive enough inside a tumor is enormously difficult. Side effects still occur, particularly when payloads leak prematurely or when target antigens appear on some healthy tissues. Drug resistance is also an active area of research. But as linker chemistry improves and new tumor targets are identified, ADCs are increasingly seen as one of the most promising platforms in all of cancer medicine.