Enhertu (trastuzumab deruxtecan, often abbreviated T-DXd) is neither traditional chemotherapy nor immunotherapy. It belongs to a newer class of cancer drugs called antibody-drug conjugates, or ADCs, which fuse the precision targeting of an antibody with the cell-killing power of a chemotherapy payload. That hybrid design is exactly why the question comes up so often: Enhertu acts partly like chemo and partly like a targeted therapy, and it even nudges the immune system into action. Understanding what it actually is matters because it changes what you expect from treatment, from side effects to how it gets administered to how long it might work.
What an Antibody-Drug Conjugate Actually Is
An ADC is built from three parts: an antibody that seeks out a specific protein on cancer cells, a toxic drug (the “payload”) that kills cells once inside them, and a chemical linker that holds the two together until the drug reaches its target. The antibody acts like a homing device, latching onto a marker on the surface of tumor cells. Once bound, the whole package gets pulled inside the cell, where the linker breaks apart and releases the payload to do its damage. ADCs combine the target specificity of antibodies with the high potency of cytotoxic payloads, making them a distinct class from either conventional chemotherapy or immunotherapy.1PubMed Central. Antibody-drug conjugates in cancer therapy: current landscape, challenges, and future directions
The reason people ask whether Enhertu is “chemo” or “immunotherapy” is that ADCs genuinely share characteristics of both. They carry a chemotherapy agent linked to a targeted antibody, and the overall package has been described as combining the principles of both chemotherapy and immunotherapy.2Frontiers in Immunology. Antibody-drug conjugates: the paradigm shifts in the targeted cancer therapy But in practice, oncologists classify Enhertu as an ADC, not as chemo and not as immunotherapy. It is its own thing.
The Chemotherapy Piece Inside Enhertu
The payload attached to Enhertu’s antibody is deruxtecan, a derivative of camptothecin. Camptothecin-based drugs are topoisomerase I inhibitors, a well-established class of chemotherapy agents. They work by trapping an enzyme called topoisomerase I during DNA replication, which stalls the replication process and causes DNA strand breaks that ultimately kill the cell.3PubMed Central. The Potential of Topoisomerase Inhibitor-Based Antibody-Drug Conjugates Research has confirmed that Enhertu triggers robust DNA damage in tumor cells expressing the HER2 protein, including tumors with relatively low levels of HER2.4Journal of Clinical Oncology. Topoisomerase 1 and DNA damage: Pharmacodynamic responses and mechanism of trastuzumab deruxtecan in HER2-expressing advanced solid tumors
So yes, part of what Enhertu does inside the body is chemotherapy in the classic sense: it poisons a critical enzyme that cancer cells need to divide. The difference from standard chemo is delivery. Traditional chemotherapy drugs flood the entire body, killing rapidly dividing cells wherever they are, which is why side effects like hair loss and immune suppression are so common. Enhertu’s antibody steers the payload preferentially toward cancer cells that display HER2 on their surface, concentrating the toxic hit where it is needed most.
How the Antibody Side Engages the Immune System
The antibody portion of Enhertu is based on trastuzumab, the same antibody used in Herceptin. It targets HER2, a protein that many breast cancers (and some gastric and lung cancers) overexpress. But the antibody does more than just serve as a delivery vehicle. Like other therapeutic antibodies, it can activate parts of the immune system on its own. When the antibody binds to HER2 on a cancer cell, immune cells such as natural killer cells recognize the antibody’s tail end and attack the coated cell, a process researchers call antibody-dependent cell-mediated cytotoxicity. Both Enhertu and the older ADC T-DM1 have been shown to trigger this natural killer cell response even after the cytotoxic payload has been attached.5JCI Insight. Antibody-drug conjugates in breast cancer: overcoming resistance and boosting immune response ADCs can also trigger complement-dependent cytotoxicity, another arm of the immune response.6PubMed Central. Antibody–Drug Conjugates and Beyond: Next-Generation Targeted Therapies for Breast Cancer
This immune activation is real, but it is not the primary way Enhertu kills cancer. The heavy lifting comes from the cytotoxic payload. The immune piece is a bonus, not the headline mechanism. That is why Enhertu is not classified as immunotherapy: drugs like checkpoint inhibitors (nivolumab, pembrolizumab) work principally by unleashing the immune system against cancer, whereas Enhertu works principally by delivering a chemical poison directly to cancer cells. The immune activation is secondary.
The Bystander Effect
One feature that sets Enhertu apart from older ADCs is its bystander effect. When the payload is released inside a HER2-positive cancer cell, some of it leaks out and enters neighboring cells, including cancer cells that may not express HER2 at all. Preclinical research demonstrated that Enhertu effectively eliminated HER2-negative cells within tumors that had a mix of HER2-positive and HER2-negative cells, though it did not affect purely HER2-negative tumors elsewhere in the body.7Critical Reviews in Oncology/Hematology. Bystander effect in antibody-drug conjugates: Navigating the fine line in tumor heterogeneity
This matters clinically because real tumors are rarely uniform. A single breast tumor can contain patches of cells with high HER2 and patches with none. The bystander effect allows Enhertu to reach some of those HER2-dark zones that an older, more tightly sealed ADC would miss. It also helps explain why Enhertu has shown benefit even in so-called HER2-low cancers, a category that barely registered as treatable with HER2-targeted therapy a few years ago.
Where Enhertu Is Used
Enhertu has approvals across several cancer types, with the strongest evidence in breast cancer. In HER2-positive metastatic breast cancer, the DESTINY-Breast03 trial compared Enhertu head-to-head against the older ADC T-DM1 (Kadcyla). Long-term follow-up showed a median progression-free survival of about 29 months with Enhertu versus roughly 7 months with T-DM1, and median overall survival reached nearly 53 months versus about 43 months.8Nature Medicine. Trastuzumab deruxtecan versus trastuzumab emtansine in HER2-positive metastatic breast cancer: long-term survival analysis of the DESTINY-Breast03 trial The confirmed response rate was about 79% for Enhertu compared with roughly 37% for T-DM1, with responses lasting a median of over 30 months.9The Lancet. Trastuzumab deruxtecan versus trastuzumab emtansine in patients with HER2-positive metastatic breast cancer: safety and overall survival results of the randomized phase 3 DESTINY-Breast03 study
In HER2-low metastatic breast cancer, the DESTINY-Breast04 trial showed that Enhertu produced significantly longer progression-free and overall survival than standard chemotherapy, opening an entirely new treatment lane for patients whose tumors had previously been considered too low in HER2 to benefit from HER2-targeted drugs.10PubMed. Trastuzumab Deruxtecan in Previously Treated HER2-Low Advanced Breast Cancer
Beyond breast cancer, Enhertu is approved for HER2-positive gastric and gastroesophageal junction cancers in adults who have already been treated with a trastuzumab-based regimen. Phase II trials in Japanese, South Korean, and Western populations showed durable responses and improved overall survival compared with standard chemotherapy options.11PubMed. Trastuzumab Deruxtecan: A Review in Gastric or Gastro-Oesophageal Junction Adenocarcinoma Approvals in certain HER2-mutant non-small-cell lung cancers and other solid tumors have followed or are in progress.
What Treatment Feels Like
Enhertu is given as an intravenous infusion once every three weeks. The dose is calculated based on body weight. The first infusion runs over about 90 minutes; if well tolerated, later infusions can be shortened to about 30 minutes. Because the drug tends to cause nausea and vomiting, anti-nausea medications are given beforehand.12Cancer Research, Statistics, and Treatment. Fam-trastuzumab deruxtecan-nxki Enhertu A narrative drug review
Patient-reported outcome data from the DESTINY-Breast02 trial offer a concrete picture of quality of life during treatment. Compared with physician’s choice of standard therapy, Enhertu delayed the point at which patients reported meaningful worsening of their overall health status by roughly eight months. It also delayed worsening of pain, physical functioning, and breast and arm symptoms.13The Lancet Oncology. Patient-reported outcomes and hospitalisation from DESTINY-Breast02 That does not mean treatment is easy. Nausea, fatigue, and hair thinning are common. But the data suggest that, on average, people feel better for longer on Enhertu than on the chemotherapy alternatives they would otherwise receive.
The Side Effect That Gets the Most Attention
The side effect that oncologists watch most carefully with Enhertu is interstitial lung disease, or ILD, a type of lung inflammation that can become serious. ILD is a known risk with several cancer drugs, but it has drawn particular scrutiny with Enhertu because it can progress to irreversible lung scarring if caught late.14PubMed Central. Real-World Perspectives and Practices for Pneumonitis/Interstitial Lung Disease Associated With Trastuzumab Deruxtecan Use in Human Epidermal Growth Factor Receptor 2-Expressing Metastatic Breast Cancer
When caught early, at the asymptomatic stage (grade 1 on the severity scale), ILD can be monitored and treated, and it may even be possible to continue Enhertu. But if it progresses to grade 2 or higher, the drug must be permanently stopped.15PubMed Central. Managing the Risk of Lung Toxicity with Trastuzumab Deruxtecan (T-DXd): A Canadian Perspective That is why treatment centers have protocols for regular lung monitoring, and patients are typically told to report new or worsening cough, shortness of breath, or fever immediately. Pharmacovigilance analyses have also flagged febrile neutropenia and certain other blood-count changes as side effects more specific to Enhertu compared with the older ADC T-DM1.16PubMed. High risks adverse events associated with trastuzumab emtansine and trastuzumab deruxtecan for the treatment of HER2-positive/mutated malignancies: a pharmacovigilance study based on the FAERS database
Combining Enhertu with Immunotherapy
Because Enhertu’s antibody can stimulate immune responses, researchers have been testing whether adding a true immunotherapy drug on top of it might boost results. Early-phase data from a trial combining Enhertu with nivolumab (an immune checkpoint inhibitor) in HER2-positive and HER2-low metastatic breast cancer found the combination was well tolerated at full therapeutic doses of both drugs. Among evaluable patients, the confirmed response rate was about 59% in the HER2-positive group and 38% in the HER2-low group, with disease control rates of 91% and 75%, respectively.17Daiichi Sankyo. Results from First Combination Trial of ENHERTU and Immune Checkpoint Inhibitor in Patients with HER2 Expressing Metastatic Breast Cancer Presented at the 2020 San Antonio Breast Cancer Symposium
These are preliminary numbers from a small trial, not definitive proof that the combination is better than Enhertu alone. But they illustrate an important conceptual point: Enhertu and immunotherapy are different enough mechanistically that they can be layered together. If Enhertu were already an immunotherapy drug, combining it with another immunotherapy would be a much harder sell biologically. The fact that the combination is being explored underscores that Enhertu’s primary mechanism is cytotoxic, not immune-based.
When Enhertu Stops Working
Like all cancer drugs, Enhertu can lose effectiveness over time as tumors adapt. The emerging picture of resistance involves several routes. Some tumors reduce HER2 expression on their surface, removing the target the antibody needs to find. Others ramp up drug efflux pumps, molecular transporters that shove the payload back out of the cell before it can do damage. Mutations that affect DNA repair pathways, such as loss-of-function changes in a gene called SLX4, have also been identified as early evidence of resistance to Enhertu specifically.18PubMed Central. Resistance to Antibody-Drug Conjugates Targeting HER2 in Breast Cancer: Molecular Landscape and Future Challenges
Understanding these resistance mechanisms is an active area of research. It also highlights why the “is it chemo or immunotherapy” framing matters practically: if resistance is driven by loss of the antibody’s target (a targeted-therapy problem) versus efflux of the payload (a chemotherapy problem), the strategies for overcoming it would be very different. A patient whose tumor downregulates HER2 might benefit from switching to a drug with a different target, while a patient whose tumor pumps out the payload might respond to agents that block efflux.
What Enhertu Costs
Enhertu is expensive, and cost-effectiveness analyses tell a complicated story. In HER2-positive metastatic breast cancer in the United States, one analysis estimated total treatment costs for Enhertu at roughly $1.27 million compared with about $820,000 for T-DM1, with Enhertu delivering more quality-adjusted life years but at an incremental cost-effectiveness ratio of about $230,000 per quality-adjusted life year gained. At a commonly used willingness-to-pay threshold, Enhertu had only about an 11% probability of being considered cost-effective.19PubMed. Cost-Effectiveness Analysis of Trastuzumab Deruxtecan Versus Trastuzumab Emtansine for Patients With Human Epidermal Growth Factor Receptor 2 Positive Metastatic Breast Cancer in the United States
In the HER2-low setting, where the comparison is against standard chemotherapy rather than another ADC, a U.S.-based analysis found an even higher cost per quality-adjusted life year.20Frontiers in Public Health. Cost-effectiveness analysis of trastuzumab deruxtecan in patients with HER2-low advanced breast cancer based on DESTINY-Breast04 However, conclusions differ by country. A Danish analysis found Enhertu cost-effective for HER2-low metastatic breast cancer under Denmark’s willingness-to-pay threshold, partly reflecting differences in drug pricing, healthcare systems, and how subsequent treatments are valued.21Advances in Therapy. Cost-Effectiveness of Trastuzumab Deruxtecan in Patients with Unresectable or Metastatic HER2-Low Breast Cancer Who Have Received Prior Chemotherapy
For patients, these numbers translate into real access barriers. Insurance coverage, patient assistance programs, and country-specific reimbursement policies all affect whether Enhertu is a practical option. The clinical benefits are large by oncology standards, but so is the price tag, and the debate over value is far from settled.
Why the Label Matters Beyond Semantics
Calling Enhertu “chemo” or “immunotherapy” is not just a vocabulary question. The classification affects insurance coding and reimbursement. It shapes which clinical trials a patient qualifies for. It influences what side effects a treatment team monitors for: traditional chemo drugs come with one set of expected toxicities, checkpoint inhibitors come with another, and ADCs come with their own profile that borrows from both but adds unique risks like ILD. When a patient hears “chemotherapy,” they bring a set of expectations about nausea, hair loss, and immune suppression that may not map cleanly onto the ADC experience. When they hear “immunotherapy,” they might expect autoimmune-type side effects that are not the primary concern with Enhertu. Getting the category right helps set appropriate expectations and, frankly, reduces unnecessary anxiety about problems that are unlikely or unnecessary indifference to problems that are real.
ADCs like Enhertu sit in a space that the old two-bucket classification of cancer drugs never anticipated. They carry a chemotherapy warhead, wrapped in a targeting system borrowed from antibody therapy, and they happen to kick the immune system along the way. As more ADCs enter the clinic and combination strategies mature, the boundaries between these categories will continue to blur. For now, the most accurate way to describe Enhertu is as an antibody-drug conjugate, a targeted therapy that delivers chemotherapy with precision and recruits the immune system as a secondary ally.