HER2-Positive Breast Cancer: Testing and Targeted Therapies

HER2-positive breast cancer is a subtype defined by abnormally high levels of a protein called HER2 (human epidermal growth factor receptor 2) on the surface of tumor cells. It accounts for roughly 15 to 20 percent of all breast cancers, and before targeted treatments existed, it carried a worse prognosis than most other subtypes. That picture has changed dramatically over the past two decades, with a succession of drugs designed to exploit HER2 as a therapeutic target turning one of the more aggressive forms of breast cancer into one of the more treatable.

What Makes a Breast Cancer HER2-Positive

Every normal breast cell carries two copies of the HER2 gene, which produces a receptor protein on the cell surface involved in growth signaling. In HER2-positive cancers, that gene is amplified, meaning the cell has too many copies. The result is an overabundance of HER2 protein studding the outside of the cell. Those extra receptors pair up with each other or with related receptors, forming clusters that fire off growth signals continuously, even without the external growth factors that would normally be required. Two major downstream signaling cascades are involved: one that promotes cell survival and another that drives cell division.¹ The net effect is a tumor that grows faster, divides more aggressively, and is more likely to spread.

Population-based estimates from the United States put the overall prevalence of HER2-positive disease at about 16 percent among women with early-stage breast cancer, with younger women (under 50) trending slightly higher at around 19 percent compared with roughly 15 percent in women over 50.² Broader reviews across cancer types put the figure for breast cancer at 15 to 30 percent, with variation depending on the population studied and the testing method used.³

How HER2 Status Is Determined

Two main laboratory tests are used to figure out whether a breast tumor is HER2-positive. The first, immunohistochemistry (IHC), stains a tissue sample and scores HER2 protein expression on a scale from 0 to 3+. A score of 3+ is considered positive. A score of 2+ is equivocal, and a score of 0 or 1+ is typically considered negative. The second test, fluorescence in situ hybridization (FISH), directly counts the number of HER2 gene copies in each cell. It is used to resolve equivocal IHC results and serves as a definitive confirmation. Studies comparing the two methods in large series of invasive breast cancers have found strong agreement between IHC and FISH results.⁴

Getting the classification right matters enormously because it determines whether a patient is eligible for HER2-targeted drugs. Current clinical guidelines define multiple FISH groups based on the ratio of HER2 gene copies to a reference chromosome and the average number of HER2 copies per cell. In a large screening analysis of more than 10,000 breast cancers, about 41 percent fell into the clearly amplified group, which was strongly correlated with IHC 3+ status, while roughly 54 percent landed in the clearly negative group.⁵ A small percentage of cases fell into borderline or equivocal categories, which is why guidelines now mandate reflex FISH testing when IHC gives a 2+ score.

An emerging area of interest is whether some tumors scored as IHC 1+ might harbor hidden HER2 amplification, particularly when other tumor features look aggressive. One study that applied FISH to IHC 1+ cases with unfavorable characteristics found that about 15 percent of those tumors actually showed gene amplification, with a strong association between amplification and a high rate of cell division.⁶ This raises questions about whether certain patients currently classified as HER2-negative might benefit from targeted therapy if further testing were done.

Targeted Treatments That Changed the Prognosis

Trastuzumab, first approved in 1998, was the drug that transformed HER2-positive breast cancer from one of the deadliest subtypes into one with markedly improved outcomes. It is a monoclonal antibody that binds to the extracellular portion of the HER2 receptor, interfering with its ability to signal. The drug also flags tumor cells for destruction by the immune system. Despite its success, researchers have debated the precise mechanisms behind trastuzumab’s effectiveness for over two decades, and not all patients respond.⁷ Some patients who initially respond eventually develop resistance, a problem that drove the search for additional HER2-targeting strategies.

Pertuzumab, a second monoclonal antibody, attacks a different part of the HER2 receptor than trastuzumab does. When the two are combined, they block HER2 from pairing up with its partner receptors more completely than either drug alone. This dual blockade approach, typically given alongside chemotherapy, became a standard first-line treatment for advanced HER2-positive disease after the FDA approved the combination.⁸

Antibody-Drug Conjugates and the Bystander Effect

One of the most significant advances in HER2-positive treatment has been the development of antibody-drug conjugates, or ADCs. These drugs are essentially guided missiles: a HER2-targeting antibody is chemically linked to a potent chemotherapy payload. The antibody delivers the toxic cargo directly to HER2-expressing cells, sparing much of the healthy tissue that conventional chemotherapy would damage.

The first such drug to reach patients was ado-trastuzumab emtansine (T-DM1). When it binds to HER2 on a cancer cell, the whole complex gets pulled inside the cell through a normal recycling process. Once inside, the antibody portion is broken down, releasing the chemotherapy agent within the cell.⁹ T-DM1 proved effective in patients whose cancer had progressed on trastuzumab and became a standard second-line option.

The next-generation ADC, trastuzumab deruxtecan (T-DXd), pushed the concept further. Its payload is designed to be membrane-permeable, meaning that once released inside one cancer cell, it can leak out and kill neighboring cells that may express little or no HER2 on their surface.¹⁰ This “bystander effect” was directly visualized in preclinical models, which showed T-DXd distributing to HER2-positive tumor areas first, then spreading to adjacent HER2-negative zones.¹¹

The bystander effect has huge practical implications because real tumors are rarely uniform. Even within a single HER2-positive cancer, patches of cells may have high HER2 expression while others have low or absent expression. Conventional HER2-targeted antibodies largely miss those low-expressing cells. Research has shown that an enzyme called cathepsin L, released by tumor cells in the surrounding tissue, can clip the chemical linker on T-DXd outside of cells, freeing the payload to kill even tumor cells that have lost HER2 expression or resist being pulled into the cell.¹² This mechanism helps explain why T-DXd has shown activity not just in HER2-positive cancers but also in tumors with very low HER2 levels, a category increasingly referred to as “HER2-low.”

When Cancer Reaches the Brain

HER2-positive breast cancer has a particular tendency to spread to the brain. Brain metastases have historically been one of the most difficult complications to treat because many drugs cannot cross the blood-brain barrier effectively. Large antibodies like trastuzumab and pertuzumab penetrate brain tissue poorly, which means that even when systemic disease is well controlled, the brain can become a sanctuary site for tumor growth.

Tucatinib, a small-molecule drug that inhibits HER2 signaling, changed the equation. Unlike antibodies, tucatinib is small enough to get into the brain. In the HER2CLIMB trial, adding tucatinib to trastuzumab and the chemotherapy drug capecitabine cut the risk of brain disease progression or death by 68 percent, and the risk of death by 42 percent, compared with trastuzumab and capecitabine alone. Median survival for patients with brain metastases went from 12 months to over 18 months, and almost half of patients with measurable brain lesions saw those lesions shrink.¹³ Among the broader population with brain metastases in the same trial, about a quarter of patients on the tucatinib combination were progression-free at one year compared with none in the control group.¹⁴

Tucatinib is now being studied in combination with stereotactic radiosurgery, a precise form of radiation that targets individual brain lesions, to see whether the drug and radiation together can improve control further.¹⁵ The broader family of small-molecule HER2 inhibitors, including neratinib and lapatinib, also cross the blood-brain barrier to varying degrees. Resistance patterns differ among them: the most common acquired HER2 mutation that emerges under treatment with lapatinib and tucatinib, known as L755S, appears to remain sensitive to neratinib, offering a potential sequencing strategy when one drug stops working.¹⁶

Neoadjuvant Therapy and Pathological Complete Response

Many patients with HER2-positive breast cancer now receive treatment before surgery rather than after it. This neoadjuvant approach, which typically combines HER2-targeted therapy with chemotherapy, serves two purposes: it can shrink a large tumor to make surgery less extensive, and it provides a real-time test of how well the drugs are working. The most meaningful measure of success is whether the tumor disappears entirely from the breast and lymph nodes by the time surgery happens, a result called pathological complete response.

A meta-analysis pooling data from 78 studies confirmed that achieving a complete response predicts significantly better long-term outcomes for patients with HER2-positive breast cancer.¹⁷ A separate meta-analysis quantified the benefit: patients who achieved a complete response had roughly a 63 percent lower risk of disease recurrence compared with those who had residual tumor, and the benefit was even more pronounced in hormone receptor-negative cancers.¹⁸ This information helps guide post-surgical decisions: patients who don’t achieve a complete response can be switched to T-DM1 as adjuvant therapy, a strategy shown in trials to reduce recurrence risk further.

Heart Health During HER2-Targeted Treatment

HER2 is not only found on cancer cells. Heart muscle cells also express the receptor at low levels, where it plays a role in maintaining cardiac function. Because of this, drugs that block HER2 carry a risk of cardiotoxicity, particularly a decline in the heart’s pumping ability. Trastuzumab-related heart problems differ from the damage caused by older chemotherapy drugs like doxorubicin: they tend to be reversible once the drug is stopped, rather than cumulative and permanent. Still, regular heart monitoring with echocardiograms or similar imaging is a standard part of any HER2-targeted treatment regimen.¹⁹

In practice, most patients tolerate trastuzumab-based therapy without significant cardiac events, but the risk increases when HER2-targeted drugs are given alongside or after heart-stressing chemotherapy agents. If a patient’s heart function drops below a certain threshold during treatment, the oncology team will typically pause or stop the HER2-targeted drug and reassess. The reversibility of trastuzumab-related cardiac effects means that many patients can resume treatment after their heart function recovers, though this decision is always individualized.

The Immune Landscape of HER2-Positive Tumors

HER2-positive breast cancers tend to be more “immunologically hot” than some other subtypes, meaning they attract more immune cells into the tumor. The density of tumor-infiltrating lymphocytes, or TILs, within and around the tumor has emerged as a meaningful prognostic marker. Higher TIL counts are associated with better outcomes across multiple studies.²⁰

TIL levels also appear to predict how well patients respond to HER2-targeted therapy. In large trial analyses, higher levels of stromal TILs independently predicted a better chance of achieving pathological complete response with neoadjuvant treatment. Patients with the highest TIL levels derived the greatest benefit from adding pertuzumab to trastuzumab.²¹ This connection makes biological sense: trastuzumab works partly by tagging cancer cells for immune destruction, and a tumor already infiltrated by active immune cells provides the cellular machinery to carry that out. TIL assessment is inexpensive and can be done on the same tissue samples used for standard diagnosis, making it an attractive addition to existing prognostic tools.

Tracking Treatment Response Through Blood Tests

Traditional monitoring of HER2-positive breast cancer relies on imaging scans and, when feasible, tissue biopsies. But tumors evolve over time, particularly under the selective pressure of treatment, and a single biopsy can only capture what is happening at one location and one moment. Liquid biopsy, which analyzes fragments of tumor DNA circulating in the bloodstream, offers a way to track those changes in something close to real time.

Researchers have found that changes in circulating tumor DNA can signal whether treatment is working or whether resistance is developing, sometimes before imaging shows visible progression. Longitudinal sequencing of ctDNA panels has been used to identify emerging resistance mutations in patients on anti-HER2 therapy, providing a potential window to switch treatments earlier.²² Separately, tracking HER2 amplification itself in ctDNA has shown over 80 percent concordance with what is found in the primary tumor, suggesting that blood-based monitoring could reliably substitute for repeat tissue biopsies in many situations.²³ Liquid biopsy is not yet a routine replacement for tissue testing in clinical guidelines, but it is increasingly used in practice, especially when obtaining a fresh tissue sample would be invasive or risky.

New Drugs on the Horizon

The treatment landscape for HER2-positive breast cancer continues to expand. Among the newer approaches under investigation, bispecific antibodies represent a fresh engineering concept. Unlike trastuzumab and pertuzumab, which are separate drugs that each grab one spot on HER2, a bispecific antibody is a single molecule designed to bind two different parts of the HER2 receptor at once. Zanidatamab is one such agent in clinical development, and early-phase trial results combining it with chemotherapy as a first-line treatment for advanced HER2-positive breast cancer have shown promising anti-tumor activity with a manageable safety profile.²⁴ Whether bispecific antibodies will replace or complement existing dual-antibody regimens remains an open question that larger trials will need to answer.

Drug resistance remains the central unsolved problem. Even with the growing arsenal of targeted drugs, most patients with metastatic HER2-positive breast cancer will eventually develop resistance to one or more therapies. The mechanisms behind resistance are varied, ranging from mutations in the HER2 gene itself to activation of alternative signaling pathways that bypass HER2 altogether.²⁵ Understanding these escape routes is what drives the sequencing strategies oncologists use, moving from one line of therapy to the next in an order designed to stay ahead of the tumor’s adaptations as long as possible.

Biosimilars and Treatment Access

The financial burden of HER2-targeted treatment is not trivial. Trastuzumab was one of the first targeted cancer therapies to reach patients, and for years it was available only as a single branded product at premium pricing. As the original patents expired, biosimilar versions of trastuzumab began entering the market. A biosimilar is not a generic in the traditional sense: because antibodies are complex biological molecules, biosimilars must demonstrate equivalent safety and effectiveness through their own clinical testing rather than simply matching a chemical formula. Multiple trastuzumab biosimilars have now been approved and adopted into routine use.

The cost implications matter beyond individual patients. As the number of cancer patients worldwide grows and newer biological drugs carry high price tags, shifting to biosimilars where they exist frees up healthcare budgets that can be directed toward those newer agents. Researchers have argued that encouraging biosimilar adoption in oncology is an important lever for controlling rising treatment costs without sacrificing patient outcomes.²⁶ For patients in countries without universal healthcare, the availability of a less expensive but clinically equivalent version of trastuzumab can mean the difference between receiving treatment and going without it.

HER2 Beyond Breast Cancer

HER2 overexpression is not unique to breast tissue. The same receptor amplification occurs in a substantial fraction of stomach and gastroesophageal junction cancers, estimated at 10 to 30 percent.²⁷ HER2 has also been studied in colorectal, bladder, and other digestive system cancers, though with less consistent results.²⁸ Testing and treatment guidelines originally developed for breast cancer are now being adapted for these other tumor types, though the biology is not identical. HER2 expression in gastric cancers, for instance, is more heterogeneous within a single tumor than it typically is in breast cancers, which complicates both testing and treatment decisions. The expansion of HER2-directed drugs into multiple cancer types reflects a broader shift in oncology toward treating cancers based on their molecular features rather than solely by where in the body they originated.