AstraZeneca and Daiichi Sankyo have secured European Union approval for Enhertu, or trastuzumab deruxtecan, in combination with pertuzumab as first-line treatment for adults with unresectable or metastatic HER2-positive breast cancer, potentially replacing a treatment backbone that has dominated this setting for more than a decade. The European Commission decision is based on the Phase 3 DESTINY-Breast09 trial, where the Enhertu combination reduced the risk of disease progression or death by 44% compared with a taxane, trastuzumab and pertuzumab regimen, commonly known as THP. Median progression-free survival reached 40.7 months with Enhertu plus pertuzumab compared with 26.9 months for THP, creating a difference of nearly 14 months in how long patients remained alive without disease progression.
The approval represents a meaningful shift because first-line therapy can determine the trajectory of metastatic HER2-positive breast cancer for years. THP became a deeply established standard after demonstrating substantial survival benefits, and subsequent therapeutic innovation often focused on what physicians should use after patients progressed. Enhertu’s move into the first-line setting means the antibody-drug conjugate can now be used before patients have exhausted the long-standing trastuzumab-based regimen, potentially moving one of oncology’s most successful ADC platforms much earlier into the treatment sequence.
How much better was Enhertu plus pertuzumab than the existing THP regimen?
DESTINY-Breast09 enrolled 1,157 patients globally and randomized them between Enhertu plus pertuzumab, Enhertu monotherapy and standard THP treatment. The European approval is supported by the combination arm, where blinded independent central review found median progression-free survival of 40.7 months compared with 26.9 months for THP, corresponding to a hazard ratio of 0.56. Confirmed objective response rates were also higher, at 85.1% with Enhertu plus pertuzumab compared with 78.6% under the established regimen.
The magnitude of the progression-free survival difference is particularly important because metastatic HER2-positive breast cancer remains incurable for most patients despite dramatic treatment advances. Extending first-line disease control can postpone progression, delay the need for subsequent therapies and potentially preserve quality of life before cumulative treatment toxicity becomes more difficult to manage. Overall survival remains an important longer-term measure, but the existing progression-free survival result was sufficiently compelling to support regulatory approval and inclusion in European Society for Medical Oncology guidelines as a Category IA first-line option.
Why does moving Enhertu into first-line treatment matter so much?
Treatment sequencing is becoming one of the defining competitive issues in HER2-positive breast cancer. Enhertu was initially established as a treatment for patients whose disease had progressed after previous HER2-directed therapy, but successive trials have steadily moved the medicine toward earlier stages and wider HER2 expression categories. First-line approval represents a particularly consequential step because it puts trastuzumab deruxtecan in front of many patients before exposure to other metastatic regimens.
That creates downstream questions for oncologists. If Enhertu is used first, physicians will eventually need stronger evidence determining what therapies work best after progression on a topoisomerase-I ADC. HER2-directed treatment will therefore not simply become more effective; the entire sequence could be reorganized around earlier exposure to ADC therapy, creating new opportunities for alternative payloads, bispecific antibodies and other mechanisms after resistance develops.
What is different about Enhertu compared with traditional HER2 antibodies?
Enhertu is an antibody-drug conjugate designed to combine HER2 targeting with delivery of a cytotoxic payload. Its trastuzumab-based antibody component recognizes HER2-expressing cancer cells, while a cleavable linker attaches the antibody to multiple molecules of a topoisomerase-I inhibitor payload derived from exatecan. After the ADC reaches a tumor cell and is internalized, the payload can be released to damage DNA and kill the cancer cell, while its membrane permeability can also create a bystander effect in neighboring tumor cells.
Pertuzumab works differently by binding another region of the HER2 receptor and interfering with receptor dimerization. Combining the two therefore creates complementary HER2-directed mechanisms: pertuzumab disrupts signaling while Enhertu delivers cytotoxic therapy directly toward HER2-expressing tumor tissue. DESTINY-Breast09 provides the pivotal evidence that this biological combination can outperform a taxane-based first-line regimen rather than merely providing theoretical mechanistic appeal.
What safety issues remain important with Enhertu?
The safety profile of Enhertu plus pertuzumab was consistent with the known profiles of the individual medicines, but that does not mean the combination is free of serious toxicity. Grade 3 or 4 adverse reactions included neutropenia, hypokalemia, anemia, diarrhea, fatigue and thrombocytopenia, among other events. Grade 5 adverse reactions occurred in 1.6% of patients in the pooled safety analysis, including pneumonia, interstitial lung disease or pneumonitis, dyspnea and febrile neutropenia.
Interstitial lung disease remains one of the particularly important risks associated with trastuzumab deruxtecan and requires early recognition and management. Cardiac monitoring also remains relevant because HER2-targeted therapies can affect left ventricular function in susceptible patients. The stronger efficacy therefore needs to be evaluated within a treatment program capable of managing these established risks, especially as patients may now receive the medicine for a longer period in the first-line setting.
How large is the patient population affected by the European approval?
Breast cancer remains the most commonly diagnosed cancer among women worldwide, and HER2-positive disease accounts for roughly 15% to 20% of metastatic cases. AstraZeneca and Daiichi Sankyo estimate that approximately 540,000 breast cancers were diagnosed in Europe in 2024, with more than 140,000 deaths. While outcomes are generally strong for patients diagnosed with early disease, the five-year survival rate remains substantially lower after breast cancer becomes metastatic.
The combination has already gained first-line approval in more than 40 countries and regions, so the European Commission decision expands an increasingly global shift rather than creating an isolated national treatment option. AstraZeneca will also pay Daiichi Sankyo a $100 million milestone following the European approval, illustrating the commercial significance of moving the jointly developed ADC into this major first-line population.
What should oncologists watch after the Enhertu first-line approval?
The most important question will be whether longer follow-up shows that the substantial progression-free survival benefit translates into a durable overall survival advantage. The Enhertu monotherapy arm of DESTINY-Breast09 also remains relevant because investigators continue evaluating whether some patients can obtain comparable disease control without pertuzumab, potentially reducing treatment complexity. Additional studies across early breast cancer could meanwhile move trastuzumab deruxtecan even further forward in the disease course.
For now, the European approval marks a clear change in clinical positioning. More than a decade after THP established itself as a first-line benchmark, Enhertu plus pertuzumab has produced a 40.7-month median progression-free survival and secured authorization to challenge that standard directly. The next phase of HER2-positive breast cancer treatment will increasingly revolve around what happens when an ADC becomes the starting point rather than the rescue therapy.
