Trastuzumab

A receptor that was also a vulnerability

The story begins with the biology, not the pharmacy. HER2 — human epidermal growth factor receptor 2 — is a protein that sits in the cell membrane and transmits growth signals inward. In normal breast tissue it is present but unremarkable. In roughly 15–20 percent of breast tumours, the gene encoding it is amplified: many extra copies are made, the receptor is vastly overexpressed, and the cell receives a near-constant stimulus to divide. When researchers began correlating this overexpression with patient outcomes in the late 1980s, the picture was bleak. High HER2 expression was associated with faster recurrence, resistance to some chemotherapy regimens, and shorter survival — a marker, as the oncologist Dennis Slamon and colleagues showed in a landmark 1987 paper in Science, not of a rare subtype but of a predictably worse disease within what had been called breast cancer.

That same feature — the receptor's extreme abundance on the tumour cell surface — also made it a target. If an antibody could be designed to bind HER2, it would find the tumour efficiently and might interfere with the growth signal. Slamon, working at the University of California Los Angeles, partnered with the biotechnology company Genentech, and through the early 1990s the monoclonal antibody that became trastuzumab was developed and tested in laboratory and animal models. The antibody bound the extracellular domain of the HER2 protein, blocked downstream signalling, and appeared to flag the cell for destruction by the immune system. The mechanism was at that point partly understood and partly inferred — what was clear was that it worked in models; what remained was whether it would work in people.

From phase I to the pivotal trials

Early clinical testing showed the antibody was tolerable and produced responses in some patients with heavily pretreated, HER2-positive metastatic disease. The decisive evidence came from two large randomised trials reported in 1998 and 1999. The first, published in the New England Journal of Medicine, compared chemotherapy alone with chemotherapy plus trastuzumab in women with HER2-positive metastatic breast cancer. Adding the antibody increased response rates, prolonged the time before the disease progressed, and extended overall survival — a finding sufficiently clear that the independent data monitoring committee recommended stopping the control arm early. The US Food and Drug Administration approved trastuzumab in September 1998, specifically and only for patients whose tumours overexpressed HER2.

That restriction was the novelty. Not all women with breast cancer; only those with a particular molecular characteristic, confirmed by a test. Eligibility for the drug required either immunohistochemistry scoring the protein at its highest level or, where the result was ambiguous, fluorescence in situ hybridisation to count the gene copies directly. The drug and the test arrived together, and neither made sense without the other. This was, in practice, the first treatment in routine oncological use where the test was constitutive of the treatment — where the diagnosis of HER2-positive disease was what the drug was for, not simply a subgroup analysis performed after the fact.

A serious safety signal emerged during the metastatic trials and became important at the planning stage for adjuvant use: trastuzumab caused cardiac dysfunction, including reductions in left ventricular ejection fraction, and the risk was substantially higher when it was combined with anthracycline chemotherapy. This was not trivial, and it shaped how subsequent trials were designed — separating trastuzumab temporally from anthracyclines, building in regular cardiac monitoring, and excluding patients with pre-existing heart disease.

Adjuvant use and the four trials

The metastatic results created immediate pressure to test trastuzumab in the adjuvant setting — after surgery, when the aim was to prevent recurrence in women whose primary tumour had been removed. Four large randomised trials reported between 2005 and 2006 produced a remarkably consistent answer. The HERA trial, run across multiple European centres, the North American NSABP B-31, the North Central Cancer Treatment Group N9831, and the BCIRG 006 trial each showed, with varying chemotherapy backbones, that one year of adjuvant trastuzumab roughly halved the relative risk of recurrence in HER2-positive early breast cancer. The reduction in distant recurrence was particularly striking. The HERA trial results, published in the New England Journal of Medicine in 2005, reported a hazard ratio for disease-free survival of approximately 0.54 after the first interim analysis — a magnitude of benefit that accelerated regulatory approval in most jurisdictions before the trials had reached their planned primary endpoints.

The Early Breast Cancer Trialists' Collaborative Group subsequently incorporated trastuzumab trials into their meta-analytic framework, confirming that the proportional reductions in recurrence and mortality were real and consistent across the pooled data. That confirmation, from the Oxford-based Collaborative Group, carried the same methodological weight it had carried for earlier treatments: pooled individual patient data, correcting for the underpowered estimates that any single trial carries.

What it settled and what it opened

Trastuzumab did several things simultaneously. It improved outcomes for a group of patients who had previously faced a particularly poor prognosis. It validated HER2 as a therapeutic target, not merely a prognostic marker. And it demonstrated, in clinical practice rather than in theory, that dividing breast cancer by molecular subtype rather than anatomy could determine treatment choice. The receptor, not the anatomy, was what the drug addressed.

The success opened questions as well as closing them. Some patients with HER2-positive disease did not respond, and the mechanisms of resistance — co-expression of other receptors, downstream mutations bypassing HER2 — became a productive area of research. The drug also opened a class: pertuzumab, a second anti-HER2 antibody binding a different domain, was later shown to add further benefit in combination with trastuzumab in both early and metastatic settings. Trastuzumab emtansine, an antibody linked to a cytotoxic payload, followed — a drug that used the antibody as a delivery vehicle rather than as the agent itself. None of those developments would have been intelligible without the original finding that a specific test could match a specific treatment to a specific receptor, and that matching it could change survival.