What Is Herceptin Used For and How Does It Work?

Herceptin is the brand name for trastuzumab, a targeted cancer drug designed to treat tumors that overproduce a protein called HER2. It is used primarily in HER2-positive breast cancer and HER2-positive stomach cancer, and it works by latching onto the HER2 protein on the surface of cancer cells, interfering with growth signals and flagging those cells for destruction by the immune system. Since its approval in the late 1990s, Herceptin has changed outcomes for a cancer subtype that was once among the most aggressive, but the drug’s story involves more nuance than the phrase “targeted therapy” suggests.

Why HER2 Matters in Cancer

HER2 stands for human epidermal growth factor receptor 2. It is a protein that sits on the surface of cells and helps regulate normal growth and repair. In about one in five breast cancers, the gene coding for HER2 is amplified, meaning the cell churns out far more HER2 protein than it should. When HER2 is overexpressed, the excess receptors pair up with each other or with related receptors, firing off constant signals that tell the cell to divide and migrate. This drives faster, more aggressive tumor growth.

HER2-positive tumors tend to grow quickly, are more likely to spread, and historically carried a worse prognosis than other breast cancer subtypes. HER2 overexpression also tends to show an inverse relationship with hormone receptor status, meaning HER2-positive tumors are less likely to also be estrogen- or progesterone-receptor positive.¹ That matters because it limits the treatment options available. Before Herceptin, the standard toolkit for these cancers was smaller and less effective.

Accurate testing for HER2 status is therefore critical. If a tumor scores high on tests measuring HER2 protein levels or gene copies, the patient becomes a candidate for HER2-targeted therapy. The benefit of trastuzumab is strongest in tumors with the clearest HER2 amplification. In clinical trials, patients whose tumors fell into the most definitively amplified category showed a significant improvement in both disease-free and overall survival with trastuzumab, while patients with borderline or equivocal HER2 results did not see the same benefit.²

How Herceptin Attacks Cancer Cells

Trastuzumab is a monoclonal antibody, which means it is a lab-made protein engineered to recognize and bind to one specific target. In this case, the target is a particular region on the HER2 protein called domain IV, located on the outside of the cell.³ By binding there, trastuzumab disrupts how HER2 pairs up with itself and with other receptors, which weakens the growth signals flowing into the cell.

But blocking growth signals is only part of the story. Trastuzumab also recruits the patient’s own immune system. When the antibody attaches to HER2 on a cancer cell, the tail end of the antibody sticks outward like a beacon. Immune cells called natural killer cells recognize that tail and destroy the tagged cancer cell in a process called antibody-dependent cell-mediated cytotoxicity (ADCC). Research has shown that trastuzumab triggers this immune-mediated killing just as effectively in patients with advanced, metastatic disease as in patients with early-stage cancer, and the effect does not seem to diminish over months of treatment.⁴ That finding supports the clinical practice of continuing trastuzumab even after the disease has progressed on other fronts.

There is also evidence that trastuzumab promotes the internalization of HER2 receptors, essentially pushing the receptors off the cell surface and into the cell interior where they can no longer send growth signals. Mathematical modeling of this process shows that trastuzumab triggers sustained receptor internalization, particularly in cells with active membrane ruffling.⁵ So the drug does not simply block the receptor in place; it actively reduces how much HER2 is available on the surface.

Herceptin in Breast Cancer

The strongest evidence for trastuzumab comes from early-stage (adjuvant) breast cancer, where the drug is given after surgery to reduce the risk of the cancer coming back. Multiple large trials have established that adding one year of trastuzumab to standard chemotherapy meaningfully improves outcomes. In the HERA trial, after eleven years of follow-up, patients who received one year of trastuzumab had roughly a 24% lower risk of their disease returning and a 26% lower risk of death compared with patients who received chemotherapy alone.⁶

A separate major trial found that at five years, estimated disease-free survival was about 84% in patients who received trastuzumab with chemotherapy, compared with 75% in patients who received chemotherapy alone. Overall survival reached roughly 92% versus 87%.⁷ These are meaningful differences when projected across thousands of patients.

In metastatic breast cancer, where the disease has spread beyond the breast, trastuzumab combined with chemotherapy has been the standard first-line approach for HER2-positive patients. Taxane-based chemotherapy drugs are the most common partners, though other agents including vinorelbine, gemcitabine, and platinum compounds have been studied in combination with trastuzumab. For patients whose tumors also express hormone receptors, combining trastuzumab with hormonal therapies like aromatase inhibitors is an option, though trials suggest this combination is somewhat less effective than trastuzumab plus chemotherapy.⁸

Beyond Breast Cancer

HER2 overexpression is not unique to breast tumors. It also occurs in a subset of gastric (stomach) and gastroesophageal junction cancers. The ToGA trial established trastuzumab as a standard option for these patients, showing that adding trastuzumab to chemotherapy extended median overall survival from about 11 months to nearly 14 months, a roughly 26% reduction in the risk of death.⁹ A phase II trial using trastuzumab with capecitabine and oxaliplatin in first-line HER2-positive gastric cancer found a response rate of about 67% and median overall survival of roughly 19.5 months, with patients whose tumors had especially high HER2 amplification ratios faring better.¹⁰

Research into HER2-targeted therapy continues to expand to other tumor types, including colorectal, bladder, and certain lung cancers, though these applications are newer and the evidence base is still developing compared with breast and gastric cancer.

How Herceptin Is Given

Herceptin was originally available only as an intravenous (IV) infusion, with the first dose typically taking about 90 minutes and subsequent doses around 30 minutes. A subcutaneous formulation was later developed that can be administered as a fixed-dose injection in about five minutes. The subcutaneous version uses an enzyme called recombinant human hyaluronidase, which temporarily opens up the tissue under the skin so the antibody can be absorbed properly.¹¹

Phase I trials confirmed that a fixed subcutaneous dose of 600 mg produced blood levels comparable to the standard weight-based IV dose. The subcutaneous version offers practical advantages: shorter chair time, no need for IV access, and the potential for home administration in some healthcare systems.¹² For patients spending a year or more on the drug, those time savings add up.

The Cardiac Side Effect

The most significant safety concern with Herceptin is its effect on the heart. Trastuzumab can cause a decline in heart-pumping function, sometimes leading to heart failure. This was initially classified as “Type II” cardiotoxicity, thought to be reversible once the drug was stopped, in contrast to the permanent heart damage sometimes caused by anthracycline chemotherapy drugs. The picture turned out to be more complicated. Retrospective studies have shown that trastuzumab-related heart problems can persist for years after treatment ends, challenging the original assumption that the damage always resolves.¹³

The risk is higher when trastuzumab is given alongside or after anthracycline chemotherapy, a combination that is common in clinical practice. Research into the mechanism suggests that both drug classes may share a target in heart cells, a protein called DNA topoisomerase IIB, which could explain why the combination is harder on the heart than either drug alone.¹⁴

Guidelines call for regular heart monitoring, usually with echocardiograms, throughout trastuzumab treatment. In practice, though, monitoring does not always happen as recommended. One study of older breast cancer patients found that only about 36% received adequate cardiac monitoring during adjuvant trastuzumab treatment. Patients were more likely to be monitored appropriately if they were diagnosed more recently, if they also received anthracyclines, or if their prescribing physician had graduated more recently from medical school. The presence of pre-existing heart conditions, which you would think would trigger closer attention, was not associated with better monitoring.¹⁵ That gap between guidelines and real-world practice is worth knowing about if you or someone you care about is on this drug.

When Herceptin Stops Working

Not all HER2-positive cancers respond to trastuzumab, and some that initially respond eventually develop resistance. This is one of the central challenges in HER2-targeted therapy. Resistance can arise through multiple routes: cancer cells can activate alternative growth pathways that bypass HER2, they can alter the structure of the HER2 protein itself so the antibody no longer binds well, or they can suppress the immune response that trastuzumab depends on for some of its killing power. More recently, researchers have identified additional escape mechanisms including changes in non-coding RNA, the development of cancer stem cells, and metabolic reprogramming.¹⁶

One well-characterized escape route involves HER3, another member of the same receptor family. When trastuzumab blocks HER2, some tumors respond by cranking up HER3, which then pairs with HER2 to reactivate the growth signals that trastuzumab was designed to shut down. This particular problem led to the development of pertuzumab, a second antibody that blocks a different region of HER2 and specifically prevents the HER2-HER3 pairing. Used together, trastuzumab and pertuzumab provide what oncologists call “dual HER2 blockade,” covering both escape routes simultaneously.¹⁷ The combination of trastuzumab, pertuzumab, and taxane chemotherapy is now FDA-approved as a first-line treatment for metastatic HER2-positive breast cancer and in the pre-surgery (neoadjuvant) setting.¹⁸

Antibody-Drug Conjugates Built on Trastuzumab

One of the more exciting developments in HER2 treatment is the creation of antibody-drug conjugates, or ADCs. These are essentially trastuzumab molecules with a potent chemotherapy payload chemically attached. The antibody delivers the drug directly to HER2-expressing cancer cells, concentrating the toxic agent where it is needed and sparing more of the healthy tissue. Two trastuzumab-based ADCs have been approved by the FDA: ado-trastuzumab emtansine (marketed as Kadcyla) and fam-trastuzumab deruxtecan (marketed as Enhertu).¹⁹

Trastuzumab deruxtecan (T-DXd, or Enhertu) has attracted particular attention because of a property called the “bystander effect.” The chemotherapy payload it carries can cross cell membranes after being released, meaning it can kill neighboring cancer cells even if those neighbors have low or absent HER2 expression. This has opened the door to treating tumors classified as “HER2-low,” a category that previously had no HER2-targeted options.²⁰ In gastric cancer, a trial of trastuzumab deruxtecan in patients who had already been treated with standard trastuzumab found that about half responded, compared with 14% of patients receiving physician’s choice chemotherapy, and median survival was extended from roughly 8 months to 12.5 months.²¹

Biosimilars and the Access Problem

The original Herceptin patent has expired in most markets, and multiple trastuzumab biosimilars are now available. Biosimilars are not generic drugs in the traditional sense; they are independently manufactured versions of a biologic medicine shown to be equivalent in efficacy, safety, and quality. Phase III trials of several biosimilars have demonstrated response rates and safety profiles that match the original Herceptin within pre-defined equivalence margins.²² Another biosimilar trial confirmed equivalence in the neoadjuvant setting, with comparable rates of pathologic complete response.²³

Biosimilars matter enormously for access. In many low- and middle-income countries, the cost of branded Herceptin has been a barrier so steep that HER2-positive breast cancer patients receive only standard chemotherapy, a less effective and often more toxic alternative.²⁴ A global survey found that trastuzumab was entirely unavailable for about 8% of providers, and where it was available, between 15% and 21% of respondents reported that patients could access it only by paying out of pocket. When the drug was not reimbursed by insurance or public health systems, only about 10% of providers could meaningfully offer it to patients.²⁵ Biosimilar competition has already brought prices down in many regions, but gaps in access remain wide.

Getting the Diagnosis Right

Herceptin only helps if the tumor actually overexpresses HER2, which makes accurate diagnostic testing a gatekeeper for the entire treatment pathway. Standard testing involves immunohistochemistry (IHC), which measures how much HER2 protein is present, and fluorescence in situ hybridization (FISH), which counts the number of HER2 gene copies. In routine practice, tumors scoring 3+ on IHC or showing clear gene amplification on FISH are considered HER2-positive and eligible for trastuzumab.

But the boundary is not always clean. Some tumors that score only 1+ on IHC, a category generally considered negative, actually harbor HER2 gene amplification when tested by FISH. One study of such borderline cases found that about 15% of tumors scoring 1+ by IHC showed HER2 amplification on FISH testing, particularly when those tumors had a high rate of cell division.²⁶ This raises the question of whether some patients who could benefit from HER2-targeted therapy are being missed by initial screening. It also underscores why getting a second opinion on pathology or requesting additional testing can matter, especially for aggressive tumors with borderline initial results.

The emergence of trastuzumab deruxtecan, with its ability to benefit even HER2-low tumors, is shifting how oncologists think about HER2 testing categories. What was once a binary question of positive or negative is becoming more of a spectrum, with treatment implications at multiple points along it.