ADCs Explained: How Antibody-Drug Conjugates Work

You keep seeing “ADC” in biotech headlines. Pfizer paid $43 billion for Seagen. AbbVie paid $10 billion for ImmunoGen. Merck paid $4 billion upfront for three Daiichi Sankyo ADCs. $57 billion in acquisitions in under two years. Every biotech investor is suddenly talking about antibody-drug conjugates, and if you don’t understand what they are, you’re trading headlines instead of knowing what you own.

The problem

ADCs are a new drug class that combines an antibody, a chemical linker, and a cytotoxic payload into a single molecule. The science is genuinely different from standard chemotherapy, and the implications for how these drugs perform in trials, get priced, and move stocks are different too. If you can’t explain the three parts to yourself, you can’t evaluate an ADC company’s catalyst.

The solution

An ADC is a targeted drug delivery system: an antibody finds the cancer cell, a linker keeps the payload stable in the blood, and the payload kills the cell from inside. Understanding those three parts, and how they interact, is the framework for evaluating any ADC stock.

The three parts: homing device, fuse, and bomb

An antibody-drug conjugate has three components, and each one matters for how well the drug works and how safe it is.

The antibody is the homing device. It’s a protein engineered to bind a specific antigen on the surface of cancer cells, the same way your immune system targets a virus. The antibody is what makes an ADC selective. Instead of flooding the body with chemotherapy, the antibody steers the drug to the tumor. The target matters: HER2 (overexpressed in about 20% of breast cancers, targeted by ENHERTU), TROP2 (expressed on over 90% of breast and lung cancers, targeted by Trodelvy), CD30 (targeted by Adcetris in lymphoma), FRα (targeted by Elahere in ovarian cancer), and Nectin-4 (targeted by Padcev in bladder cancer). A good target is highly expressed on cancer cells and minimally expressed on healthy tissue.

The linker is the fuse. It’s a chemical bridge that connects the antibody to the payload, and it has a difficult job: stay completely stable while circulating in the blood, then break apart the moment the ADC gets inside a cancer cell. If the linker breaks too early, the payload leaks into the bloodstream and causes off-target toxicity. If it never breaks, the drug doesn’t work. Linkers come in two types: cleavable (breaks in response to a specific trigger inside the cell, like acidity or enzymes) and non-cleavable (the antibody itself gets digested, releasing the payload still attached to an amino acid). Cleavable linkers can produce a bystander effect, where the released payload diffuses into neighboring cancer cells that don’t have the target antigen. Non-cleavable linkers are more stable but have no bystander effect. Daiichi Sankyo’s cleavable DXd linker technology is the competitive moat behind ENHERTU, Datroway, and the rest of the DXd platform.

The payload is the bomb. It’s a cytotoxic agent, typically 100 to 1,000 times more potent than standard chemotherapy, sometimes active at picomolar concentrations. You don’t inject these drugs directly, because at that potency they’d kill the patient. The payload only becomes safe because the antibody delivers it exclusively to the tumor. Common payloads include MMAE (microtubule inhibitor, used in Adcetris, Padcev, Polivy), DXd (topoisomerase inhibitor, used in ENHERTU and Datroway), SN-38 (used in Trodelvy), and DM4 (used in Elahere). The payload’s mechanism and potency directly determine efficacy.

Step by step: how an ADC kills a cancer cell

1. The ADC enters the bloodstream. It circulates, stable and intact, because the linker holds.
2. The antibody binds its target antigen on a cancer cell. This is the targeting step. Only cells expressing the antigen get the drug.
3. The ADC is internalized. The cancer cell literally swallows the ADC through a process called receptor-mediated endocytosis, pulling the drug inside.
4. The linker cleaves in the lysosome. The lysosome is an acidic, enzyme-rich compartment inside the cell. Cleavable linkers break here. For non-cleavable linkers, the antibody itself is digested.
5. The payload is released and kills the cell. The cytotoxic agent disrupts microtubule function or DNA replication, and the cancer cell dies from inside.

The advantage over chemotherapy is the delivery. Standard chemo floods the entire body, killing rapidly dividing cells regardless of whether they’re cancerous. That’s why patients lose hair, white blood cells, and intestinal lining. An ADC delivers the same class of cytotoxic agent directly to the tumor, sparing most healthy tissue. The result is higher efficacy at the tumor and lower systemic toxicity. For a deeper look at the deal-making and valuation side of this theme, see our ADC stocks analysis.

Why ADCs command premium pricing

ADCs are expensive drugs. ENHERTU costs $10,000 to $20,000 per month. Elahere and Trodelvy are in the same range. The pricing reflects manufacturing complexity that limits supply, combined with selectivity that beats chemo and a small patient population per indication.

Manufacturing is the hidden barrier. An ADC is a biological (the antibody) plus a chemical (the payload and linker), which means two separate production lines, a conjugation step to join them, and rigorous quality control to ensure the drug-antibody ratio is consistent. That complexity is why there are only 15 FDA-approved ADCs as of 2026 despite the modality existing since Mylotarg’s first approval in 2000, and it’s why pharma has paid $57 billion to acquire the companies that already have the manufacturing figured out.

For investors, the takeaway is that ADCs are a platform, not a single drug. Daiichi’s DXd linker technology produced ENHERTU, Datroway, I-DXd, and multiple pipeline assets from one chemical architecture. When you evaluate an ADC company, you’re betting on the platform, not one approval. Our guide to biotech investing fundamentals covers how to size positions for binary regulatory events like the PDUFA dates these catalysts generate.

Common mistakes

Treating all ADCs as the same drug. They’re not. The target, linker, and payload determine efficacy and safety. ENHERTU’s cleavable DXd linker produces a bystander effect that Kadcyla’s non-cleavable linker doesn’t. Interstitial lung disease is a class effect for DXd ADCs but not for MMAE ADCs. Read the trial data before assuming one ADC’s success predicts another’s.

Ignoring linker stability in safety data. When an ADC trial shows high rates of off-target toxicity (neutropenia, thrombocytopenia, peripheral neuropathy), the linker is often the culprit. Payload leaking into the blood before reaching the tumor causes the same side effects as chemotherapy. If the linker is unstable, the ADC loses its entire advantage.

Assuming Phase 2 ADC data guarantees Phase 3 success. This is the most common biotech investing mistake, and ADCs don’t get a pass. The Phase 2 to Phase 3 transition rate across therapeutic areas is roughly 30 to 45%, and as low as 27% in neurology. For a refresher on why Phase 2 is a hypothesis, not an approval signal, see our clinical trial phases explainer.

What to watch

If you want to track ADC catalysts, use the FDA’s accelerated approval program page to understand how many ADCs reach the market on surrogate endpoints and require confirmatory trials. Search ClinicalTrials.gov by drug name (e.g., “trastuzumab deruxtecan”) to find the trial IDs and primary endpoints behind any ADC catalyst. For a calendar of upcoming FDA decisions, BioPharmCatalyst lists PDUFA dates for free.

For the 2026 oncology catalyst calendar, including which ADC PDUFA dates are still ahead, see our oncology catalysts roundup. And if you’re new to the endpoints these trials use, our surrogate endpoint guide explains how the FDA measures benefit when survival data takes years to collect.

Final checklist

• You can name the three ADC components and what each does
• You know which target antigen the company’s ADC addresses
• You know whether the linker is cleavable or non-cleavable and what that means for safety
• You understand the payload’s mechanism and potency relative to chemo
• You’ve checked the trial phase and know the Phase 2 to Phase 3 transition risk
• You’re sizing the position for binary risk, not betting the portfolio on one catalyst

ADCs are the modality driving $57 billion in pharma acquisitions, and the science genuinely works. But the stocks are priced for success before the data arrives. Understand the mechanism, read the trial, and size for the binary outcome. The three-part smart bomb is a real innovation. It’s still a biotech catalyst, and catalysts cut both ways.