How an overlooked pathology finding helped redefine a disease, expand a market, and accelerate one of the most competitive areas of innovation in oncology.
When Apple introduced the iPhone in 2007, I remember thinking of it as an impressive an expensive piece of technology. Like millions of other people, I eventually bought one. At the time, I certainly did not realize I was witnessing the beginning of a sequence of events that would reshape entire industries over the following decade.
The phone itself was only part of the story. GPS devices gradually disappeared. Digital cameras became increasingly niche products. Companies such as Uber, Instagram, Airbnb, and countless others built businesses around capabilities that simply did not exist before smartphones became ubiquitous. Looking back, the device was important, but the ecosystem that emerged around it proved far more influential.
Scientific advances sometimes follow a similar pattern. A study is published and a result is reported. The immediate implications appear relatively clear. Only years later does it become apparent that the most important consequences had little to do with the original headline.
DESTINY-Breast04 is one of those stories. When the results appeared in the New England Journal of Medicine in 2022, attention naturally focused on trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate developed by Daiichi Sankyo and AstraZeneca. The trial demonstrated substantial clinical benefits in patients with HER2-low metastatic breast cancer and quickly became one of the most discussed studies in oncology.
The more interesting story had been sitting quietly in pathology reports for years. To understand why, it helps to go back several decades. In the late 1980s, researchers including Dennis Slamon helped establish the connection between HER2 overexpression and aggressive breast cancer. That work eventually led to the development of trastuzumab and transformed outcomes for patients with HER2-positive disease. By the early 2000s, HER2 had become one of the most important biomarkers in oncology and one of the clearest examples of precision medicine in practice.
The framework appeared straightforward. Tumors were classified as either HER2-positive or HER2-negative. The distinction influenced treatment decisions, clinical trial design, pathology workflows, and commercial strategy throughout the industry.
Over time, the classification became so familiar that few people questioned it. The biology, however, was always more complicated. Many tumors expressed low levels of HER2. Not enough to qualify as HER2-positive, but not completely absent either. Pathologists reported these findings every day. The information was available. It simply did not influence treatment decisions.
In retrospect, that may be one of the most interesting aspects of the entire story.
HER2-low was not discovered in 2022. It had been there all along. The category existed long before anyone knew what to do with it. DESTINY-Breast04 changed that. The study enrolled 557 patients with HER2-low metastatic breast cancer who had previously received one or two lines of chemotherapy. Participants received either trastuzumab deruxtecan or physician’s choice chemotherapy.
The results immediately attracted attention. Among patients with hormone receptor-positive disease, median progression-free survival increased from 5.4 months to 10.1 months. Median overall survival improved from 17.5 months to 23.9 months. Objective response rates exceeded 50%, substantially higher than those observed with standard chemotherapy.
For many oncologists, however, the surprise was not simply the magnitude of the benefit. It was where the benefit appeared. For years, HER2-targeted therapies had been associated almost exclusively with HER2-positive disease. DESTINY-Breast04 suggested that the biological boundary separating HER2-positive and HER2-negative tumors might be less rigid than previously assumed. Almost overnight, HER2-low evolved from a descriptive pathology finding into a clinically actionable category. That shift created opportunities, but it also created questions. One question quickly emerged across pathology departments: how low is low?
A distinction that had rarely influenced treatment decisions suddenly became central to patient selection. Laboratories revisited testing practices. Clinicians sought greater consistency in reporting. Discussions that had once been largely academic acquired immediate practical relevance.
Not every important consequence of DESTINY-Breast04 occurred in hospitals. Some occurred in boardrooms. One reason the study attracted so much attention was that it arrived at a time when antibody-drug conjugates were generating increasing interest but had not yet fully proven themselves as a platform.
The concept behind ADCs had always been appealing. Use an antibody to guide a highly potent payload toward cancer cells while limiting exposure to healthy tissue. The challenge was execution. Over the years, multiple programs struggled with efficacy, toxicity, manufacturing complexity, or some combination of the three. Success was far from guaranteed.
Trastuzumab deruxtecan changed the conversation. Importantly, pharmaceutical companies do not invest billions of dollars because a single product succeeds. They invest when a successful product suggests that an entire technological approach may be scalable across multiple targets and future programs.
That is how many observers interpreted DESTINY-Breast04. In 2019, AstraZeneca agreed to a collaboration with Daiichi Sankyo for trastuzumab deruxtecan valued at up to $6.9 billion. It was a striking number at the time and raised more than a few eyebrows across the industry. Following the publication of DESTINY-Breast04 and subsequent studies, the strategic logic became considerably easier to understand.
It did not take long for the rest of the industry to notice. Between 2022 and 2024, major pharmaceutical companies expanded their ADC capabilities through acquisitions, partnerships, licensing agreements, and internal development programs. What had once been viewed as a promising but specialized modality became one of the most competitive areas in oncology.
Interestingly, none of this required the discovery of a new biological target. HER2 was the same receptor researchers had been studying for decades. What changed was not the biology itself, but the tools available to act on it.
And once the technology changed, an overlooked biological observation suddenly became valuable. Looking back, what stands out is how ordinary the underlying observation once seemed. HER2-low tumors had been appearing in pathology reports for years. What changed was not what physicians could see, but what they could do with what they saw.
There is something almost ironic about that. One of the most influential developments in breast cancer over the past decade did not begin with the discovery of a new gene, a new pathway, or a new disease. It began with a category that had been sitting in front of us all along.
We often think of innovation as the discovery of something entirely new. Yet some of the most consequential innovations emerge from a different process: finding new ways to create value from knowledge that already exists.
Perhaps that is a topic for another article.
