Antibody-drug conjugates, widely known as ADCs, are a class of cancer therapy that works like a guided missile: a laboratory-made antibody seeks out a specific protein on cancer cells and delivers a potent cell-killing chemical directly to the tumor. The concept has been in development for decades, but the field has accelerated sharply in recent years, with more than a dozen ADCs now approved for various cancers and over a hundred more in clinical trials. What makes them appealing is the promise of concentrating toxic firepower at the tumor while sparing the rest of the body, though the reality turns out to be more complicated than that tidy metaphor suggests.
How an ADC Actually Works
Every ADC has three components: the antibody, the linker, and the payload. The antibody is engineered to recognize a specific protein, called a target antigen, that sits on the surface of cancer cells. Once the antibody locks onto that protein, the entire complex gets pulled inside the cell through a natural process the cell already uses to absorb material from its surface. Inside the cell, the linker connecting the antibody to its toxic cargo breaks apart, freeing the payload to do its damage. That payload is typically a small molecule far too toxic to give on its own as a systemic drug. By packaging it inside an antibody that homes in on cancer cells, the idea is to deliver a lethal dose right where it is needed.
The linker is a deceptively important piece of this puzzle. If it breaks apart too early, while the ADC is still circulating in the bloodstream, the toxic payload leaks out and causes side effects unrelated to the tumor. If it is too stable, the payload may never fully release even after the ADC reaches the cancer cell. Most ADCs in clinical development use cleavable linkers, which are designed to snap open in response to specific conditions inside the cell, such as acidic environments or particular enzymes found in cellular compartments called lysosomes. A smaller number use non-cleavable linkers that require the antibody itself to be broken down before the payload is freed.1PubMed Central. Antibody-drug conjugates: Recent advances in linker chemistry
The Bystander Effect
Tumors are not uniform. Even in a single mass, some cells may have high levels of the target protein on their surface, some may have low levels, and some may have none at all. This patchwork, called tumor heterogeneity, is one of the core challenges for any targeted therapy. ADCs have a partial answer: the bystander effect. Once the payload is released inside a target-positive cancer cell, some of it can diffuse out and kill neighboring cells that do not carry the target antigen at all.
Whether an ADC has a meaningful bystander effect depends on its design. Cleavable linkers and payloads that are hydrophobic enough to cross cell membranes tend to produce stronger bystander killing.2PubMed. Bystander effect in antibody-drug conjugates: Navigating the fine line in tumor heterogeneity The clinical significance is real: ADCs engineered with strong bystander effects, like trastuzumab deruxtecan, have shown superior results compared to earlier ADCs that lacked this property. Still, the tumor microenvironment can limit how well the bystander effect works. High internal pressure within tumors, poor blood supply, and low oxygen levels all restrict how far the freed payload can travel.
Researchers are actively trying to amplify this effect. One approach uses a novel linker that gets cleaved not only inside the first target cell but also by enzymes released when that cell dies, creating a chain reaction of payload release into the surrounding tumor tissue. In animal models of breast cancer, this design maintained its ability to kill tumor cells even after the antigen-positive cells had been eliminated, essentially continuing to work through bystander activity alone.3PubMed. Enhancing the bystander effect of antibody-drug conjugate by using a novel caspase-3 cleavable peptide linker to overcome tumor heterogeneity
Where ADCs Have Made the Biggest Clinical Impact
ADCs have found success across both solid tumors and blood cancers, but some of the most dramatic results have come in breast cancer. Trastuzumab deruxtecan, which targets HER2, produced what many oncologists considered a practice-changing moment when it showed benefit in patients whose tumors had only low levels of HER2, a group that had never responded to HER2-targeted therapies before. In the DESTINY-Breast04 trial, patients receiving trastuzumab deruxtecan lived a median of roughly 23 months compared to about 17 months for those on standard chemotherapy.4Nature Medicine. Trastuzumab deruxtecan in HER2-low metastatic breast cancer: long-term survival analysis of the randomized, phase 3 DESTINY-Breast04 trial The time before the disease worsened roughly doubled, from about five months to ten.5PubMed. Trastuzumab Deruxtecan in Previously Treated HER2-Low Advanced Breast Cancer Earlier phase studies had already hinted at this, with response rates around 37% in HER2-low breast cancer patients who had run out of standard options.6PubMed Central. Antitumor Activity and Safety of Trastuzumab Deruxtecan in Patients With HER2-Low-Expressing Advanced Breast Cancer: Results From a Phase Ib Study
Beyond HER2, another target called TROP2 has opened doors for patients with triple-negative breast cancer, one of the hardest-to-treat subtypes. Sacituzumab govitecan, the TROP2-targeting ADC, has also been studied across lung, gastrointestinal, and genitourinary cancers. A meta-analysis pooling data from over 1,500 patients across various tumor types found a median overall survival of about ten months, with an overall response rate around 17%. The most common serious side effect was a drop in a type of white blood cell called neutrophils, which occurred at severe levels in roughly 37% of patients.7BJC Reports. Efficacy and safety of sacituzumab govitecan Trop-2-targeted antibody-drug conjugate in solid tumors and UGT1A1*28 polymorphism: a systematic review and meta-analysis
In blood cancers, ADCs arrived even earlier. Gemtuzumab ozogamicin was one of the first ADCs approved for acute myeloid leukemia, and brentuximab vedotin became a standard treatment in certain lymphomas. More recently, inotuzumab ozogamicin, which targets CD22, a protein found on the surface of about 90 to 100% of malignant B cells, has been approved for relapsed acute lymphoblastic leukemia.8PubMed Central. Role of inotuzumab ozogamicin in the treatment of relapsed/refractory acute lymphoblastic leukemia These hematologic ADCs often benefit from the fact that blood cancer cells circulate freely, making them easier for an antibody to reach than cells buried deep in a solid tumor.
Side Effects Are Not What You Might Expect
The “guided missile” metaphor implies that ADCs should cause fewer side effects than conventional chemotherapy. In practice, the picture is more nuanced. Many of the dose-limiting side effects of ADCs turn out to be the same toxicities caused by the free payload drug alone, affecting tissues that do not express the target antigen at all. Analysis of clinical data has shown that different ADCs carrying the same payload often share the same dose-limiting problems, regardless of which antigen the antibody targets or which cancer type is being treated.9PubMed Central. Mechanisms of ADC Toxicity and Strategies to Increase ADC Tolerability This suggests that a meaningful fraction of the payload escapes the tumor and ends up affecting normal tissue.
One side effect that has drawn particular scrutiny is interstitial lung disease, an inflammation of the lung tissue that can range from mild and manageable to life-threatening. Research into why this happens has pointed to alveolar macrophages, a type of immune cell in the lungs, as a key culprit. These macrophages appear to swallow circulating ADCs through receptors on their surface, regardless of whether the lung tissue expresses the ADC’s target antigen. The payload then accumulates inside the macrophages in a dose-dependent fashion and gets released into the surrounding lung tissue, potentially triggering inflammation.10Molecular Cancer Therapeutics. Potential Mechanisms of Interstitial Lung Disease Induced by Antibody–Drug Conjugates Based on Quantitative Analysis of Drug Distribution Blocking the receptor that macrophages use to grab ADCs significantly reduced intracellular payload accumulation in lab experiments, hinting at a potential strategy to mitigate this risk in the future.11Molecular Cancer Therapeutics. Potential Mechanisms of Interstitial Lung Disease Induced by Antibody–Drug Conjugates Based on Quantitative Analysis of Drug Distribution
The Therapeutic Window Is Narrower Than Advertised
For years, the prevailing logic behind ADC development was straightforward: by concentrating a toxic drug at the tumor site, you should be able to give higher effective doses to cancer cells while keeping doses low enough elsewhere to avoid serious harm. The gap between the dose that works and the dose that causes unacceptable harm, known as the therapeutic window, was expected to be much wider for ADCs than for conventional chemotherapy. Growing clinical evidence has challenged that assumption. The tolerated doses of many ADCs are not dramatically different from those of the related small-molecule drugs they carry.12PubMed. The therapeutic window of antibody drug conjugates: A dogma in need of revision
A more refined model suggests that ADCs do improve the therapeutic window, but not primarily by raising the maximum tolerated dose. Instead, they appear to lower the minimum dose needed to achieve tumor shrinkage, meaning they are effective at lower payload exposures than the free drug would be. When researchers calculate the therapeutic index using actual drug exposure levels rather than simple dose numbers, the advantage of ADCs over chemotherapy becomes clearer.13PubMed Central. Therapeutic index improvement of antibody-drug conjugates The implication for patients is that ADCs often do work better than plain chemotherapy, but the margin of safety is tighter than early enthusiasm suggested, and careful dose management remains essential.
How Cancers Fight Back
Like nearly every cancer treatment, ADCs eventually face resistance. Tumors find ways to survive, and the mechanisms they use tend to fall into a few recognizable categories.14PubMed Central. Mechanisms of Resistance to Antibody-Drug Conjugates
- Antigen loss: Cancer cells can reduce or completely stop producing the surface protein the ADC is designed to find. With fewer targets on the cell surface, fewer ADC molecules get internalized, and less payload reaches the interior.
- Impaired internalization: Even when the target antigen is still present, some tumor cells become less efficient at pulling the ADC inside. If the ADC stays stuck on the cell surface, the linker may never encounter the conditions it needs to release the payload.
- Drug efflux pumps: Cancer cells can ramp up molecular pumps that actively eject the payload back out of the cell before it can do damage. Several commonly used ADC payloads, including monomethyl auristatin E and calicheamicin, have been identified as substrates of these efflux pumps.15PubMed Central. Identification of antibody-drug conjugate payloads that are substrates of ATP-binding cassette drug efflux transporters
These resistance mechanisms can occur alone or in combination, and they apply broadly across ADC classes. For breast cancer specifically, resistance to major ADCs like trastuzumab emtansine, trastuzumab deruxtecan, and sacituzumab govitecan has been documented, raising important questions about how to sequence these therapies and what to try next when one fails.16PubMed Central. Antibody-drug conjugates in breast cancer treatment: resistance mechanisms and the role of therapeutic sequencing
Picking the Right Patient
The obvious starting point for deciding who should receive an ADC is whether their tumor expresses the target antigen. But this turns out to be deceptively complicated. Antigen expression is not static. It can change as the cancer progresses, as prior treatments reshape the tumor, or simply because different parts of the same tumor express different levels of the protein. A biopsy taken from one spot at one moment may not reflect what the rest of the tumor looks like.17PubMed. Antibody-Drug Conjugates: Patient and Treatment Selection
The question of how much antigen expression is “enough” also remains unsettled. Some ADCs clearly work better when expression is high, and the development of a patient selection strategy tied to expression levels has been called critically important for identifying who will actually benefit.18PubMed Central. Antibody-Drug Conjugates (ADCs) for Personalized Treatment of Solid Tumors: A Review Yet the success of trastuzumab deruxtecan in HER2-low patients upended the assumption that strong expression was always necessary, suggesting that newer ADC designs with potent bystander effects may work even when the target is scarce. This has pushed the field to reconsider what “targetable” really means.
Next-Generation Designs
The ADC field is not standing still. Several engineering approaches aim to address the limitations of current drugs.
One major thrust is site-specific conjugation. Traditional manufacturing methods attach payloads somewhat randomly to the antibody, producing a mixture of molecules with different numbers of drug molecules per antibody. This heterogeneity can lead to variable behavior in the body. Newer methods attach the payload at defined, predetermined locations on the antibody, producing a more uniform product with more predictable properties.19PubMed. Advances in the Development of Site-Specific Antibody-Drug Conjugation Manufacturing ADCs remains inherently challenging, requiring precise control of the drug-to-antibody ratio, management of aggregation, and safe handling of extremely toxic payload chemicals.20PubMed. Continuous Processing for Manufacturing of Antibody-Drug Conjugates (ADCs)
Bispecific ADCs represent another frontier. These use antibodies engineered to recognize two different target proteins simultaneously. The idea is that by requiring two antigens to be present on the same cell, the ADC becomes more selective, reducing the chance of harming normal tissue that expresses only one of the two targets. In preclinical work, a bispecific ADC targeting both pCAD and CDH17 showed tumor-killing activity only in tumors expressing both proteins, sparing tumors that expressed just one.21PubMed Central. A bispecific antibody-drug conjugate targeting pCAD and CDH17 has antitumor activity and improved tumor-specificity Separately, researchers have explored loading bispecific ADCs with two different payloads rather than one, potentially attacking cancer cells through two independent mechanisms at once and limiting the chance of resistance. In animal models, a dual-payload bispecific ADC outperformed its single-payload counterparts and even showed activity against non-dividing cells that are typically resistant to traditional payloads.22PubMed Central. A Dual-Payload Bispecific ADC Improved Potency and Efficacy over Single-Payload Bispecific ADCs
Beyond Cell-Killing Payloads
Not every next-generation conjugate carries a traditional cytotoxic drug. A newer class called immune-stimulating antibody conjugates replaces the cell-killing payload with molecules that activate the immune system. Instead of poisoning cancer cells directly, these conjugates deliver immune-activating signals to the tumor site, essentially flagging it for destruction by the patient’s own immune cells. One recent approach combined tumor-targeted antibodies with innate immune agonists, using specialized engineering to attach the payload at precise locations on the antibody while also modifying the antibody’s sugar structures to enhance immune activation.23PubMed Central. Antibody glyco-optimization and site-specific conjugation enhance the immune-stimulating activity of antibody-TLR ligand conjugates This approach is still early in development, but it represents a conceptual expansion of what an antibody conjugate can do, moving from a pure drug-delivery vehicle toward a tool for reprogramming the immune response at the tumor.
Combining ADCs with Other Treatments
ADCs are increasingly being tested alongside other therapies rather than as standalone agents. Pairing them with immune checkpoint inhibitors has been a particularly active area, based on the logic that the cell death caused by the ADC payload may release signals that make the tumor more visible to the immune system. Combinations with conventional chemotherapy and with molecularly targeted agents have also advanced, with several reaching late-stage clinical trials or gaining regulatory approval.24PubMed Central. Strategic Combinations of Antibody-Drug Conjugates from 2023 to 2025: From Dual Therapies to Innovative ADC-Based Regimens Clinical studies of these combinations have shown improvements in response rates and time to disease progression across multiple cancer types.25PubMed Central. Antibody-drug conjugate combinations in cancer treatment: clinical efficacy and clinical study perspectives
One intriguing preclinical finding involves combining ADCs with drugs that interfere with DNA repair. When cancer cells are already struggling to fix DNA damage caused by the ADC’s payload, adding a second agent that blocks a repair pathway can tip the balance decisively. Research has shown that combining a drug that blocks a specific DNA-damage checkpoint with ADCs carrying topoisomerase-inhibitor payloads led to significantly more DNA damage and cell death than either agent alone. The same drug also enhanced the effects of ADCs carrying microtubule-disrupting payloads through a different mechanism, by worsening errors during cell division.26PubMed Central. Azenosertib is a broadly applicable enhancer of cytotoxic-payload antibody-drug conjugates The fact that this synergy worked across different payload types suggests the strategy could be broadly applicable.
Manufacturing Complexity
Building an ADC is substantially harder than making either an antibody or a small-molecule drug on its own. The conjugation step, where the payload is attached to the antibody via the linker, introduces variability that must be tightly controlled. Small differences in the number of drug molecules attached to each antibody, in where those molecules attach, or in how the product behaves during storage can all affect how the final drug performs in patients.27PubMed. Antibody-Drug Conjugate Overview: a State-of-the-art Manufacturing Process and Control Strategy Handling the highly toxic payloads also requires specialized containment facilities that most conventional pharmaceutical plants do not have. These manufacturing challenges contribute to the high cost of ADC therapies, which can run to tens of thousands of dollars per treatment cycle. As the number of approved ADCs grows and manufacturing processes mature, there is hope that costs will come down, but for now, the complexity of production remains a practical barrier to broader access.