Herceptin (trastuzumab) is a targeted cancer drug used to treat HER2-positive breast cancer and HER2-positive stomach cancer. It works by attaching to a specific protein on cancer cells called HER2, which drives aggressive tumor growth in roughly 15 to 20 percent of breast cancers. If your tumor tests positive for HER2 overexpression, Herceptin is likely part of your treatment plan.
How HER2 Testing Determines Eligibility
Not every cancer patient is a candidate for Herceptin. The drug only works on tumors that produce too much of the HER2 protein, so your cancer cells need to be tested first. This is done through a biopsy sample, using two main methods: an immunohistochemistry (IHC) test that scores protein levels on a scale from 0 to 3+, and a FISH test that checks whether the HER2 gene itself is amplified.
A score of IHC 3+ means your tumor clearly overexpresses HER2, making you eligible for Herceptin. A score of IHC 2+ is considered borderline and triggers a follow-up FISH test. If FISH confirms gene amplification (a ratio of 2 or higher), you qualify. Scores of 0 or 1+ have traditionally been classified as HER2-negative, though newer antibody-drug conjugates related to trastuzumab can now target some of these lower-expression tumors as well.
Breast Cancer: The Primary Use
Herceptin is used across multiple stages of HER2-positive breast cancer. In early-stage disease, it’s given after surgery (adjuvant therapy) to reduce the risk of the cancer returning. It can also be given before surgery (neoadjuvant therapy) to shrink tumors and make them easier to remove. In metastatic breast cancer, where the disease has spread to other parts of the body, Herceptin forms the backbone of first-line treatment.
For early-stage breast cancer, the standard course of Herceptin is typically one year. In metastatic disease, treatment continues for as long as it keeps working and the patient tolerates it. The current clinical standard is to maintain some form of HER2-targeted therapy even after switching regimens, because HER2-positive tumors tend to remain dependent on that protein for growth.
Stomach and Esophageal Cancer
Herceptin is also approved for advanced HER2-positive cancers of the stomach and the junction where the esophagus meets the stomach. These tumors are tested using the same IHC and FISH scoring system. To qualify, the tumor needs to score IHC 3+, or IHC 2+ with a positive FISH result (ratio of 2 or higher).
In this setting, Herceptin is combined with chemotherapy as a first-line treatment for patients whose cancer is too advanced for surgery or has already spread. More recently, adding an immunotherapy drug to the Herceptin-chemotherapy combination has become a recommended approach for patients whose tumors also express a marker called PD-L1.
How Herceptin Works
Herceptin is a monoclonal antibody, meaning it’s an engineered protein designed to lock onto one specific target. It binds to a particular region on the outside of the HER2 protein, sitting close to where the protein enters the cell membrane. This binding triggers several effects.
The most clearly established mechanism is that Herceptin flags cancer cells for destruction by the immune system. Once the drug attaches to a cancer cell, immune cells (primarily natural killer cells) recognize the antibody’s tail end and attack. This process is called antibody-dependent cell-mediated cytotoxicity. Herceptin also blocks the HER2 protein from being cut and shed from the cell surface, a process that would otherwise create a shortened, hyperactive form of the protein that fuels tumor growth. It also appears to interfere with the tumor’s ability to build new blood vessels and to repair its own DNA.
What Treatment Looks Like
Herceptin was originally given only as an intravenous (IV) infusion. The first infusion takes longer (about 90 minutes), and if well tolerated, subsequent infusions are shorter. A subcutaneous (under-the-skin) injection is now also available and dramatically cuts down time in the treatment chair. In a head-to-head comparison, the subcutaneous version took an average of about 7 minutes to administer compared to roughly 62 minutes for the IV infusion. Total time spent at the clinic, including waiting, dropped from nearly 3 hours to about an hour and a half with the subcutaneous option. Both forms deliver equivalent results.
Herceptin is rarely used alone. In early breast cancer, it’s paired with chemotherapy drugs like taxanes. In metastatic breast cancer, the standard first-line combination is Herceptin plus pertuzumab (another HER2-targeting antibody) plus a taxane. A taxane is typically given for at least six cycles if tolerated, after which patients continue on just the two antibodies as maintenance therapy. For patients whose tumors are also hormone receptor-positive, an anti-estrogen medication may be added to this maintenance phase.
If the cancer progresses on first-line treatment, newer antibody-drug conjugates like trastuzumab deruxtecan have become the preferred next step. These drugs use the same HER2-targeting mechanism but carry a chemotherapy payload directly into the cancer cell. If those aren’t available, trastuzumab emtansine (T-DM1) remains an effective second-line option.
Heart-Related Side Effects
The most significant risk with Herceptin is its effect on the heart. Unlike the permanent heart damage sometimes caused by certain chemotherapy drugs, Herceptin-related heart problems are generally reversible if caught early. The concern centers on a decline in the heart’s pumping ability, measured by a number called the left ventricular ejection fraction (LVEF).
Your heart function will be tested before starting Herceptin and monitored regularly throughout treatment. If LVEF drops 16 or more percentage points from your baseline, or falls below normal limits with a drop of 10 or more points, treatment is paused for 4 to 8 weeks to allow recovery. If your heart function doesn’t bounce back within 8 weeks, or if treatment has to be paused more than three times for this reason, Herceptin is permanently discontinued. The risk of cardiac problems is higher when Herceptin is given alongside or after certain chemotherapy drugs that also stress the heart.
Pregnancy and Fertility Considerations
Herceptin can cause serious harm to a developing fetus. Reports have documented dangerously low levels of amniotic fluid during pregnancy, leading to underdeveloped lungs, skeletal problems, and in some cases neonatal death. Pregnancy status is verified before starting treatment, and effective contraception is recommended during therapy and for 7 months after the last dose, since the drug takes that long to fully clear the body.
Data on breastfeeding is limited. Animal studies found the drug in breast milk but didn’t show toxicity to offspring. The 7-month washout period is a factor to consider when planning to breastfeed after completing treatment.
Biosimilar Options
The original Herceptin brand is no longer the only version available. Several biosimilars, essentially near-identical copies of the original drug, have been approved. These include Kanjinti, Ogivri, Ontruzant, Herzuma, and Trazimera. Each has been tested to confirm it works the same way and produces equivalent outcomes.
Some insurance plans now require patients to use a biosimilar rather than the brand-name drug, similar to how generic medications are substituted for brand-name pills. While biosimilars are costly to produce because of the complexity of manufacturing biological drugs, having multiple options on the market has created more access for patients.