How Trastuzumab Targets HER2-Positive Cancers

Trastuzumab is a monoclonal antibody designed to latch onto HER2, a protein that sits on the surface of certain cancer cells and drives their growth. Approved by the FDA in 1998 as the first targeted therapy for HER2-positive metastatic breast cancer, it fundamentally changed survival outcomes for a cancer subtype that had been among the most aggressive. But the story of trastuzumab and HER2 stretches well beyond that initial approval, touching on how the drug actually kills cancer cells, why it sometimes stops working, and how newer HER2-targeted therapies have expanded the playing field.

What HER2 Does in Cancer

HER2 (short for human epidermal growth factor receptor 2) is a receptor protein encoded by the ERBB2 gene. In healthy tissue, it plays a role in cell growth and repair. But in roughly 15 to 20 percent of breast cancers, the gene is amplified, meaning cells carry too many copies and churn out far more HER2 protein than normal. That overabundance sends constant “grow and divide” signals, fueling aggressive tumor behavior. Decades of research have firmly linked amplification of the HER2 gene to cancer development and progression.¹

What makes HER2-positive tumors especially troublesome is that HER2 resists the normal cleanup process cells use after a growth signal fires. Most growth receptors get pulled inside the cell and broken down once they have done their job. HER2 tends to linger on the cell surface, prolonging the downstream signals that push cell growth and proliferation, which ultimately promotes tumor formation.² Before trastuzumab, HER2-positive breast cancer carried a worse prognosis than HER2-negative disease. That context is important for understanding why a drug that specifically blocks this one receptor was such a turning point.

How Trastuzumab Attacks HER2-Positive Cells

Trastuzumab does not work through a single mechanism. It operates on several fronts at once, which is part of why it is effective even in settings where one pathway might be partially blocked.

The most straightforward action is physical blockade. Trastuzumab binds to domain IV of the HER2 extracellular region, and recent structural work shows this binding locks HER2 into a shape that may prevent it from pairing with HER3, the partner receptor it normally teams up with to form a particularly cancer-promoting signaling unit.³ By interfering with that partnership, trastuzumab can dampen the growth signals HER2 would otherwise amplify.

The second major mechanism involves the immune system. When trastuzumab binds HER2 on a tumor cell, the tail end of the antibody sticks out and acts as a flag for natural killer (NK) cells. These immune cells recognize the antibody, latch on, and destroy the tagged cancer cell in a process called antibody-dependent cell-mediated cytotoxicity, or ADCC. Lab studies show trastuzumab increases this immune-mediated killing across a range of HER2-expressing breast cancer cell lines, including some where HER2 is present but not amplified to the extreme levels seen in classic HER2-positive tumors.⁴ Clinical evidence supports the idea that NK cell activation contributes meaningfully to the drug’s effectiveness, not just its direct signaling blockade.⁵

Whether trastuzumab also causes the cell to pull HER2 off its surface and degrade it remains debated. Some lab studies suggest it does, but a clinical study in breast cancer patients who received trastuzumab before surgery found no significant HER2 downregulation in those who responded to treatment.⁶ So the drug’s benefit appears to come primarily from blocking HER2 signaling and flagging cells for immune destruction, rather than from stripping HER2 off the cell surface entirely.

Survival Benefits in Breast Cancer

The clinical data behind trastuzumab in breast cancer is robust. In the metastatic setting, trastuzumab was the first targeted therapy to show real improvement for HER2-positive disease, and its favorable benefit-to-risk profile has been demonstrated both as a single agent and in combination with chemotherapy or hormonal therapy.⁷

In early-stage breast cancer given after surgery (the adjuvant setting), results from a landmark trial showed that adding trastuzumab to standard chemotherapy pushed the five-year disease-free survival rate from about 75 percent to 84 percent and overall survival from 87 percent to 92 percent at a median follow-up of about five and a half years.⁸ A meta-analysis pooling six randomized trials confirmed that adding trastuzumab to chemotherapy improved disease-free survival, overall survival, and rates of both local and distant recurrence.⁹

One finding from that meta-analysis deserves attention: while trastuzumab reduced recurrences overall, it was associated with a higher rate of recurrence in the central nervous system. This is not because the drug makes brain metastases worse in any biological sense. Rather, by controlling disease elsewhere in the body and extending survival, patients live long enough for brain metastases to emerge, and trastuzumab itself has difficulty crossing the blood-brain barrier. That limitation has spurred an entire line of research into getting the drug into the brain, which we will come back to.

Beyond Breast Cancer: Gastric and Gastroesophageal Tumors

HER2 overexpression is not exclusive to breast cancer. It shows up in a subset of gastric (stomach) and gastroesophageal junction cancers, and trastuzumab has been tested there as well. The pivotal ToGA trial found that adding trastuzumab to chemotherapy extended median overall survival from about 11 months to nearly 14 months in patients with HER2-positive advanced gastric or gastroesophageal junction cancer.¹⁰ Subgroup analyses, including among Japanese patients, confirmed the benefit held across different populations.¹¹

The story in gastric cancer is more complicated than in breast cancer, though. While first-line trastuzumab with chemotherapy provides a meaningful survival benefit, subsequent clinical trials targeting HER2 in gastric cancer after the first-line setting have largely failed to show additional benefit.¹² The reasons are still being worked out, but the tumor biology in gastric cancer appears less uniformly dependent on HER2, and HER2 expression can be more heterogeneous within a single tumor.

Testing for HER2 Status

Because trastuzumab only helps patients whose tumors are driven by HER2, accurate testing is critical. Two main methods are used in clinical practice. Immunohistochemistry (IHC) stains a tumor sample to see how much HER2 protein is on the cell surface, scoring it from 0 (no overexpression) to 3+ (strong overexpression). Fluorescence in situ hybridization (FISH) looks directly at the DNA to count how many copies of the HER2 gene each cell carries. Both methods have the advantage of being morphology-driven, meaning a pathologist can see exactly which cells are being evaluated under the microscope.¹³

Current guidelines use a tiered approach. An IHC score of 3+ or a positive FISH result (indicating gene amplification) qualifies a tumor as HER2-positive and eligible for trastuzumab. A score of 2+ is considered equivocal and requires reflex FISH testing to confirm. A score of 0 or 1+ is generally classified as HER2-negative. However, the boundaries are not always clean. A study of tumors scored 1+ by IHC found that when those tumors had aggressive features and were reflexed to FISH testing, roughly 15 percent turned out to harbor HER2 gene amplification, suggesting some patients at the borderline may benefit from targeted therapy who would otherwise be missed.¹⁴

Updated ASCO-College of American Pathologists guidelines have added nuance, breaking FISH results into five groups based on both the HER2-to-chromosome-17 ratio and the absolute HER2 gene copy number per cell. Different groups carry different levels of confidence that the tumor will respond to HER2-targeted therapy.¹⁵ The practical takeaway is that HER2 testing is more sophisticated than a simple positive-or-negative call, and borderline cases sometimes require repeat testing or additional methods.

Why Trastuzumab Stops Working

Resistance to trastuzumab is one of the biggest clinical challenges in HER2-positive cancer. Some tumors never respond, and many that initially shrink eventually find a way around the drug. Several distinct resistance mechanisms have been identified.

One involves a truncated version of the HER2 receptor called p95HER2. This shortened form lacks the extracellular domain that trastuzumab binds to, so the drug simply cannot attach. In a study of metastatic breast cancer patients, only about 11 percent of those whose tumors expressed p95HER2 responded to trastuzumab, compared with roughly 51 percent of those with full-length HER2.¹⁶

Another major pathway involves a cell survival route called PI3K/Akt. A large RNA interference screen found that loss of the tumor suppressor PTEN was the strongest single modulator of trastuzumab sensitivity. Mutations in the PIK3CA gene, which activates the same pathway, also conferred resistance. Analyzing both PTEN and PIK3CA together identified twice as many patients at risk for disease progression as looking at PTEN alone.¹⁷ Encouragingly, lab studies showed that PI3K inhibitors could rescue trastuzumab sensitivity in PTEN-deficient cells, pointing toward a potential clinical workaround.¹⁸

Signaling through the insulin-like growth factor 1 receptor (IGF-1R) represents yet another escape route. Increased IGF-1R activity has been linked to trastuzumab resistance, and research shows that its main biological effect in resistant cells is promoting invasion rather than pure growth. Blocking IGF-1R alongside HER2 produced modest growth inhibition but a more dramatic reduction in the cancer cells’ ability to invade surrounding tissue.¹⁹ Interestingly, some preclinical work has found that metformin, the widely used diabetes drug, disrupts HER2/IGF-1R complexes that appear specifically in trastuzumab-resistant cells, reducing the activity of downstream survival signals.²⁰ Whether metformin has clinical relevance in overcoming trastuzumab resistance remains an open question.

The Cardiotoxicity Trade-Off

HER2 is not found only on cancer cells. It also plays a role in normal heart muscle, where it helps cardiomyocytes (heart cells) handle stress and maintain function. Knockout mouse models lacking HER2 in the heart ventricle developed dilated cardiomyopathy and poor contractility, demonstrating that the heart genuinely depends on this receptor for maintenance. When trastuzumab blocks HER2 on heart cells, it reduces that protective signaling and leaves the heart more vulnerable to damage.²¹

The cardiotoxicity appears to involve multiple hits. One proposed model describes a “dual-hit” mechanism: trastuzumab both shuts down the neuregulin-1 survival signaling pathway in heart cells and triggers activation of an enzyme (NADPH oxidase) that increases the production of damaging reactive oxygen species.²² Separate research has uncovered a third layer: trastuzumab dysregulates HER2 signaling in heart cells in a way that impairs autophagy, the cellular housekeeping process that clears damaged components. Specifically, the drug activates a pathway that suppresses the cell’s baseline recycling machinery, leading to an accumulation of cellular debris and oxidative stress. Pertuzumab, another HER2-targeting antibody, did not trigger this same effect, which may help explain why cardiotoxicity rates differ between the two drugs.²³

In practice, the heart effects of trastuzumab are usually reversible once the drug is stopped, unlike the more permanent damage caused by anthracycline chemotherapy. Patients receiving trastuzumab undergo regular cardiac monitoring, typically with echocardiograms or MUGA scans, and treatment may be paused or discontinued if heart function drops below a certain threshold.

Combining Trastuzumab with Pertuzumab

Trastuzumab and pertuzumab both target HER2, but they bind to different parts of the receptor. Trastuzumab attaches to domain IV, while pertuzumab binds domain II, which is the region HER2 uses to pair up with other receptors. Lab studies found that combining the two antibodies produced very strong synergistic killing of breast cancer cells, well beyond what either achieved alone. The combination enhanced the disruption of HER2’s partnerships with other growth receptors, leading to reduced survival signaling and synergistic cancer cell death.²⁴ This dual blockade strategy has since been validated in clinical trials and is now a standard regimen for many HER2-positive breast cancer patients.

Antibody-Drug Conjugates Built on Trastuzumab

One of the most consequential developments in HER2-targeted therapy has been the creation of antibody-drug conjugates (ADCs) that use trastuzumab as a delivery vehicle. The idea is straightforward: attach a potent chemotherapy agent to the antibody so that it gets carried directly to HER2-expressing cells, sparing the rest of the body from much of the toxicity.

Trastuzumab emtansine (T-DM1) was the first of these to reach the clinic. It links trastuzumab to DM1, a powerful antimicrotubule agent, via a linker that does not break apart in the bloodstream. Once the conjugate binds HER2 and gets pulled inside the cancer cell, it is broken down in lysosomes, releasing DM1 to disrupt the cell’s internal scaffolding. T-DM1 retains trastuzumab’s own anti-tumor effects, including ADCC and signaling inhibition, while adding the direct cell-killing power of the chemotherapy payload.²⁵ Research into how the drug enters cells has shown that T-DM1 is internalized through a lipid raft pathway rather than the standard clathrin-mediated route, and that this internalization depends on a protein called caveolin-1.²⁶ The cytotoxic effect scales with how much drug accumulates inside the cell: high levels trigger rapid cell death, moderate levels cause the cell’s internal logistics to break down, and the lowest levels leave the cell relatively unaffected.²⁷

A newer ADC, trastuzumab deruxtecan (T-DXd), carries a different payload and a cleavable linker that allows the drug to diffuse to neighboring cells, a feature known as a bystander effect. This design is particularly relevant for tumors where HER2 expression varies from cell to cell within the same mass. In a phase Ib study, T-DXd showed a confirmed objective response rate of 37 percent in patients with HER2-low breast cancer, a group that had not previously been considered treatable with HER2-targeted agents.²⁸ That finding essentially expanded the universe of patients who can benefit from HER2-directed therapy and has reshaped how oncologists think about HER2 status as a spectrum rather than a strict binary.

The Blood-Brain Barrier Problem

Trastuzumab is a large antibody molecule, and like most antibodies, it does not cross the blood-brain barrier effectively. This is a real clinical problem because HER2-positive breast cancer has a higher-than-average tendency to metastasize to the brain, and patients whose systemic disease is well controlled by trastuzumab can still develop brain lesions that progress unchecked.²⁹

Several experimental strategies are being pursued. MRI-guided focused ultrasound combined with microbubbles can temporarily and locally open the blood-brain barrier, allowing trastuzumab to reach brain tumors. Early clinical work has shown this approach is safe and can deliver spatially targeted antibody across the barrier in patients with brain tumors.³⁰ Another approach uses engineered nanoparticles designed to ferry trastuzumab across the barrier through a two-step targeting strategy, which has shown promise in preclinical models of HER2-positive brain metastases.³¹ Neither method has yet become standard of care, but the focused ultrasound work in particular has moved from animal models to human trials, which is a meaningful step.

Subcutaneous Formulation and Convenience

Standard trastuzumab is given intravenously, typically every three weeks, with infusions that can last from 30 to 90 minutes depending on the cycle. A subcutaneous formulation was developed to simplify this process. The subcutaneous version uses recombinant human hyaluronidase, an enzyme that temporarily loosens tissue under the skin so the drug can be absorbed more easily.³² Pharmacokinetic studies showed that a fixed subcutaneous dose of 600 mg every three weeks achieves blood levels comparable to the standard intravenous regimen.³³

In the PrefHer trial, which compared the two routes in early breast cancer, patients overwhelmingly preferred the subcutaneous injection. The efficacy was non-inferior, the safety profile was similar, and the administration time dropped to about five minutes.³⁴ This matters for quality of life during what can be a year or more of treatment, and it reduces the burden on infusion centers.

Pathological Complete Response as a Measuring Stick

When trastuzumab is given before surgery (in the neoadjuvant setting), oncologists use pathological complete response (pCR), meaning no residual invasive cancer is found in the surgical specimen, as a key measure of how well the treatment worked. In HER2-positive breast cancer, achieving pCR carries a strong link to long-term outcomes. A large meta-analysis of 78 studies involving over 25,000 patients found that those who achieved pCR had markedly better event-free survival and overall survival, with the association growing stronger at longer follow-up periods.³⁵ A separate comprehensive meta-analysis confirmed that pCR is a strong surrogate endpoint specifically in HER2-positive and triple-negative breast cancer subtypes, supporting the FDA’s use of pCR rates as a marker of efficacy in neoadjuvant treatment trials.³⁶

Data from the HannaH trial, which tested both subcutaneous and intravenous trastuzumab in the neoadjuvant-adjuvant setting, found that achieving total pCR was associated with a roughly 60 to 70 percent reduction in the risk of a disease event in each treatment arm.³⁷ This strong predictive value means that pCR is increasingly used not just as a trial endpoint but as a practical clinical tool: patients who achieve it may be spared additional aggressive therapy, while those who do not can be escalated to alternative regimens.

Biosimilars and Access

Trastuzumab’s original patent has long expired, and multiple biosimilar versions have entered the market. Unlike generic small-molecule drugs, biosimilars for antibodies must demonstrate analytical, pharmacokinetic, and clinical similarity through rigorous comparative trials. Several have cleared this bar convincingly. A phase III trial of the biosimilar EG12014 showed equivalent pathological complete response rates to the reference product, with the difference between treatment arms falling well within the predefined equivalence margin required by both the European Medicines Agency and the FDA.³⁸ Pharmacokinetic bioequivalence studies of other biosimilars, including BP02, have demonstrated that blood levels of the drug fall within the standard 80-to-125 percent bioequivalence window across all pairwise comparisons with EU- and US-approved reference trastuzumab.³⁹

The clinical relevance of biosimilars is mostly about access and cost. Trastuzumab is a drug that many patients take for a full year in the adjuvant setting, and that price tag adds up. Biosimilar competition has driven down costs in markets where they are available, potentially putting treatment within reach for patients in healthcare systems that might otherwise struggle to fund it. Safety profiles, including immunogenicity (whether the body develops antibodies against the drug), have been comparable across the reference product and biosimilars in the trials conducted to date.⁴⁰