Enhertu is neither traditional chemotherapy nor immunotherapy. It is an antibody-drug conjugate (ADC), a newer class of targeted cancer therapy that combines elements of both worlds: a precision-guided antibody attached to a powerful chemotherapy drug. If you had to pick one category, it’s closer to chemotherapy in how it kills cancer cells, but it delivers that killing power far more selectively than conventional chemo.
How Enhertu Actually Works
Enhertu has three distinct parts working together. The first is trastuzumab, a monoclonal antibody (the same one used in Herceptin) that seeks out and locks onto HER2 receptors on cancer cells. The second is a chemical linker that holds the drug together while it circulates through your bloodstream. The third is a cytotoxic payload called deruxtecan (DXd), which is a potent cell-killing agent derived from a class of chemotherapy drugs.
Once the antibody attaches to a HER2 receptor on a cancer cell, the entire complex gets pulled inside the cell. Enzymes inside the cell break the linker apart, releasing the chemotherapy payload directly into the cancer cell’s interior. There, DXd interferes with a protein that cancer cells need to copy their DNA, causing the DNA strands to break and the cell to die. This is the same basic mechanism used by traditional chemotherapy drugs in the topoisomerase inhibitor family.
What makes Enhertu especially effective is something called the bystander effect. The released payload can leak out of the dying cancer cell and kill neighboring cancer cells nearby, even ones that don’t have high levels of HER2 on their surface. This extends its reach beyond just the cells the antibody directly targets.
Why It’s Not Immunotherapy
Immunotherapy works by activating your own immune system to recognize and attack cancer. Checkpoint inhibitors, the most common type of immunotherapy, essentially take the brakes off your T cells so they can find and destroy tumor cells on their own. The drug itself doesn’t kill cancer. It empowers your immune system to do the killing.
Enhertu takes a fundamentally different approach. It delivers a cytotoxic chemical directly into cancer cells to destroy them. Your immune system isn’t the primary weapon here. The antibody component does involve immune system technology (monoclonal antibodies were originally developed from immune research), and there is some evidence that ADCs can trigger secondary immune responses. But the core mechanism is targeted chemical destruction of cancer cells, not immune activation. Researchers are now studying combinations of ADCs like Enhertu with checkpoint inhibitors, precisely because they work through different and potentially complementary pathways.
Why It’s Not Traditional Chemotherapy Either
Standard chemotherapy drugs flood your entire body and attack all rapidly dividing cells, cancerous or not. That’s why conventional chemo causes widespread side effects like hair loss, nausea, and immune suppression: it can’t distinguish between a cancer cell and a healthy cell that happens to be dividing quickly.
Enhertu’s antibody component acts like a guided missile system, directing the chemotherapy payload specifically toward cells displaying HER2 receptors. This means the cytotoxic drug concentrates at the tumor site rather than spreading evenly throughout the body. You still get side effects (the payload does affect some healthy tissue), but the targeting mechanism reduces the indiscriminate damage that defines traditional chemo. The official classification for Enhertu is “targeted therapy,” specifically an antibody-drug conjugate.
What Enhertu Treats
Enhertu is approved for several cancer types that express HER2. Its most prominent use is in HER2-positive breast cancer, where it was approved in December 2025 as a first-line treatment in combination with pertuzumab for adults with unresectable or metastatic disease. It has also expanded into HER2-low breast cancer, a category of tumors that have some HER2 expression but not enough to qualify as HER2-positive by traditional standards. This was a significant shift because it opened targeted HER2 therapy to a much larger group of breast cancer patients.
Beyond breast cancer, Enhertu has approvals for certain types of gastric (stomach) cancer and non-small cell lung cancer that express HER2.
What Treatment Looks Like
Enhertu is given as an intravenous infusion once every three weeks. The first session takes about 90 minutes. If you tolerate that well, subsequent infusions can be shortened to around 30 minutes. Treatment continues on this three-week cycle until the cancer progresses or side effects become unmanageable.
The side effect profile reflects its chemotherapy payload. Nausea, fatigue, and lowered blood counts are common. The most serious risk specific to Enhertu is interstitial lung disease (ILD), an inflammation of lung tissue that occurred in about 9% of patients in pooled clinical data. Most of those cases were mild (grade 1), but ILD requires close monitoring because severe cases can be dangerous. Your care team will typically watch for new or worsening respiratory symptoms like cough, shortness of breath, or fever throughout treatment.
The Bottom Line on Classification
Enhertu sits in its own category. It uses a chemotherapy drug as its weapon but delivers it through a targeted antibody, making it more precise than traditional chemo. It is not immunotherapy because it kills cancer cells directly rather than training your immune system to fight. The most accurate label is antibody-drug conjugate, a form of targeted therapy that borrows from chemotherapy’s playbook while minimizing its collateral damage.