First-Line Treatment in HER2-Positive Metastatic Breast Cancer
Reviewed by Ann Dietrich, MD, FAAP, FACEP
The first-line treatment landscape in human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer changed meaningfully in the United States in December 2025, when the Food and Drug Administration (FDA) approved fam-trastuzumab deruxtecan-nxki plus pertuzumab for adults with unresectable or metastatic HER2-positive breast cancer. That approval was based on DESTINY-Breast09, a study that compared trastuzumab deruxtecan plus pertuzumab with taxane, trastuzumab, and pertuzumab (THP) in previously untreated advanced disease. For clinicians treating breast cancer in 2026, the first question no longer is just whether a patient is fit for THP. It is whether the patient is a good candidate for the newer antibody-drug conjugate-based combination or whether a taxane-based triplet still makes better clinical sense.
That does not make the older standard irrelevant. THP remains one of the most important regimens ever developed in metastatic HER2-positive breast cancer, with durable survival data from the CLEOPATRA study and years of familiarity in day-to-day oncology practice. The newer trastuzumab deruxtecan regimen has the strongest contemporary progression-free survival data in the first line, but THP still matters because it is well characterized, broadly usable, and supported by mature long-term outcomes. In practice, first-line decision-making now depends on three main issues: trial eligibility relative to prior HER2-directed therapy, the patient’s tolerance for distinct toxicity profiles, and whether hormone receptor status changes the role of maintenance endocrine therapy.
In metastatic HER2-positive breast cancer, this shift is important because the first-line choice shapes much more than the first few cycles. It influences when or whether a taxane is given, how aggressively the medical team must monitor for interstitial lung disease, which maintenance options are realistic, and how later-line sequencing will unfold after progression occurs. The best first-line regimen, therefore, is not just the drug with the best hazard ratio. It is the regimen that best matches the biology, prior exposure history, pace of disease, and toxicity tolerance of the patient you are treating.
Why the first-line landscape changed
DESTINY-Breast09 is the reason the field moved. In this Phase III study, patients with previously untreated unresectable or metastatic HER2-positive breast cancer were randomized to trastuzumab deruxtecan plus pertuzumab, THP, or T-DXd monotherapy investigational arm that remained blinded at the time of the primary readout. Prior neoadjuvant or adjuvant HER2-directed therapy was allowed only if it had been completed more than six months before the diagnosis of advanced or metastatic disease, and one prior line of endocrine therapy in the advanced setting was permitted. Those eligibility details matter because they define the population to whom the frontline approval most cleanly applies.
The efficacy signal was large enough to alter routine practice. The FDA reported a median progression-free survival of 40.7 months with trastuzumab deruxtecan plus pertuzumab vs. 26.9 months with THP, with a hazard ratio for progression or death of 0.56. Objective response rates also were high in both arms, 87% with the antibody-drug conjugate combination and 81% with THP. The New England Journal of Medicine report likewise concluded that trastuzumab deruxtecan plus pertuzumab significantly lowered the risk of progression or death compared with THP, without new safety signals beyond what already is known about the component drugs. Overall survival was immature at the time of the primary progression-free survival analysis, so the strongest first-line advantage at present is in disease control rather than mature survival.
Clinically, that magnitude of progression-free survival benefit is hard to ignore. A median progression-free survival beyond three years is unusual in metastatic breast cancer and suggests that the frontline conversation in HER2-positive disease now is less about preserving trastuzumab deruxtecan for second-line and more about deciding which patients should receive it immediately. At the same time, it still is important not to flatten the comparison into “antibody-drug conjugate (ADC) good, taxane obsolete.” THP remains a highly active regimen with deep survival follow-up, and some patients still will be better served by the older triplet.
The regimen that built the benchmark
Before DESTINY-Breast09, first-line treatment in HER2-positive metastatic breast cancer was anchored by CLEOPATRA. In the original Phase III study, adding pertuzumab to trastuzumab and docetaxel improved median progression-free survival from 12.4 months to 18.5 months. Later follow up showed a median overall survival of 56.5 months with the pertuzumab-containing regimen vs. 40.8 months in the control group, and end-of-study follow-up reported eight-year overall survival rates of 37% vs. 23%. Those are the data that turned dual HER2 blockade plus taxane into the benchmark against which every new first-line regimen has been measured.
The FDA label for pertuzumab still reflects that historical backbone. Pertuzumab (Perjeta) is indicated in combination with trastuzumab and docetaxel for adults with HER2-positive metastatic breast cancer who have not received prior anti-HER2 therapy or chemotherapy for metastatic disease. That labeling language is narrower than the newer trastuzumab deruxtecan plus pertuzumab approval, which matters when clinicians are evaluating patients who relapse after perioperative HER2-directed therapy.
THP also remains familiar because its toxicity profile is distinct and predictable. Diarrhea, alopecia, neutropenia, nausea, fatigue, rash, and peripheral neuropathy are common, and cardiac monitoring remains part of routine use because HER2-directed antibodies can reduce left ventricular ejection fraction. In CLEOPATRA, left ventricular dysfunction occurred in a minority of patients, and symptomatic left ventricular systolic dysfunction or congestive heart failure was uncommon. For many oncologists, that combination of long-term efficacy data and well-understood toxicity still makes THP a reassuring first-line choice when a taxane-based regimen is preferred.
Trastuzumab deruxtecan plus pertuzumab in the first line
Who fits the approval population
The most straightforward candidate for frontline trastuzumab deruxtecan plus pertuzumab is a patient with untreated unresectable or metastatic HER2-positive breast cancer, confirmed as immunohistochemistry (IHC) 3+ or in situ hybridization (ISH)+, who either presents with de novo metastatic disease or relapses more than six months after completing neoadjuvant or adjuvant HER2-directed therapy. That is the population studied in DESTINY-Breast09 and the population reflected in the FDA approval. The trial did allow one prior line of endocrine therapy in advanced disease, so hormone receptor-positive patients are not excluded from this strategy because an endocrine-based approach was attempted first.
The key limitation is early relapse after perioperative therapy. DESTINY-Breast09 did not enroll patients whose disease recurred during neoadjuvant or adjuvant anti-HER2 therapy or within six months of completing it. That matters because many difficult first-line metastatic cases arise exactly in that setting. For those patients, it is better to think in terms of the separate trastuzumab deruxtecan monotherapy indication rather than assuming that the new combination approval automatically applies.
Why the efficacy matters clinically
The practical appeal of the trastuzumab deruxtecan plus pertuzumab regimen is obvious. It outperformed THP on progression-free survival, produced a very high response rate, and did so without requiring upfront taxane exposure. For some patients, especially those in whom a prolonged first remission is a priority, this is a meaningful advantage. It also raises the possibility of reaching later lines of therapy with less cumulative neuropathy than a docetaxel-heavy course might produce. That said, this is not a chemotherapy-free strategy. Trastuzumab deruxtecan is an antibody-drug conjugate with its own cytotoxic burden and its own monitoring demands.
The other important point is that the strongest evidence at present is progression-free survival, not mature overall survival. Therefore, clinicians should resist overstating what is known. The regimen has become a major first-line option because it clearly delayed progression compared with THP, but long-term survival comparisons still are evolving. That nuance matters in patient counseling, especially for patients who value mature overall survival benchmarks and for clinicians who still view THP as the regimen with the deepest long-term record in metastatic HER2-positive breast cancer.
Safety monitoring changes frontline practice
What truly differentiates trastuzumab deruxtecan plus pertuzumab from THP in clinic not only is efficacy, but also the kind of vigilance it requires. The label also states that trastuzumab deruxtecan is highly emetogenic, so prophylactic antiemetics should be built into routine first-line use rather than added reactively after early nausea.
Interstitial lung disease or pneumonitis is the toxicity that most clearly distinguishes this regimen from THP. In the trastuzumab deruxtecan plus pertuzumab population described in the label, interstitial lung disease or pneumonitis occurred in 12% of patients, with fatal outcomes in 0.5%. The median time to onset was eight months, which means clinicians cannot treat pulmonary monitoring as just an early-cycle issue. The label instructs clinicians to interrupt trastuzumab deruxtecan for grade 1 interstitial lung disease, consider corticosteroids, and permanently discontinue the drug for grade 2 or higher events while promptly starting systemic steroids. That monitoring burden is central to frontline selection, especially in patients with baseline lung disease or in patients who may not reliably report early respiratory symptoms.
Myelosuppression and cardiac monitoring also remain important. In the combination-treated population, decreased neutrophil counts were common. Grade 3 or 4 neutropenia occurred in roughly 29% of patients, febrile neutropenia occurred in 2.6% of patients, and left ventricular ejection fraction decreases were reported as well. The label recommends cardiac assessment before treatment and at regular intervals during therapy. Put simply, frontline trastuzumab deruxtecan plus pertuzumab is not a low-maintenance regimen. It is a high-efficacy regimen that demands disciplined surveillance.
When THP still makes sense
The approval of trastuzumab deruxtecan plus pertuzumab does not eliminate reasonable use cases for THP. The most obvious is a patient in whom clinicians want to avoid the pulmonary risk profile of trastuzumab deruxtecan. A history of prior drug-related pneumonitis, significant underlying interstitial lung disease, or serious concern about the ability to detect and manage early pulmonary toxicity may tilt the balance toward THP even when the newer regimen is otherwise available.
THP also may remain attractive when clinicians and patients value the maturity of the long-term dataset. CLEOPATRA is one of the few metastatic breast cancer trials with truly deep survival follow-up, and those data continue to matter when discussing expectations for disease course. A regimen that has shown eight-year survival in more than one-third of patients remains clinically meaningful, even after a newer option improves progression-free survival. In that sense, THP has moved from being the only default to being the most established alternative.
There also is a practical taxane question. The pertuzumab label is docetaxel-based, but the PERUSE study provides reassurance that paclitaxel is a valid alternative backbone in routine first-line use. Mature PERUSE results, with almost six years of median follow-up, were consistent with CLEOPATRA and suggested that paclitaxel is a reasonable substitute for docetaxel, with median overall survival exceeding five years across the study population. That is useful because many clinicians prefer paclitaxel when they want a taxane-based regimen with different tolerability tradeoffs than docetaxel.
Hormone receptor-positive disease adds a second layer to first-line planning
Where endocrine therapy still fits
Hormone receptor-positive, HER2-positive metastatic breast cancer has always forced clinicians to answer two questions at once: how much anti-HER2 intensity is needed, and how much endocrine sensitivity still can be leveraged in the first line. The older pre-ADC literature still matters here. In PERTAIN, adding pertuzumab to trastuzumab plus an aromatase inhibitor improved progression-free survival to 20.6 months vs.15.8 months with trastuzumab plus aromatase inhibitor alone in previously untreated hormone receptor-positive, HER2-positive metastatic or locally advanced breast cancer. That supports a chemotherapy-sparing first-line option in selected patients whose disease biology and tempo make endocrine therapy plausible.
SYSUCC-002 adds another important layer. In this randomized Phase III study, trastuzumab plus endocrine therapy was noninferior to trastuzumab plus chemotherapy as first-line treatment in hormone receptor-positive, HER2-positive metastatic breast cancer, with median progression-free survival of 19.2 months vs. 14.8 months and substantially less toxicity in the endocrine-therapy arm. Taken together, PERTAIN and SYSUCC-002 support the idea that selected patients with indolent, endocrine-responsive metastatic breast cancer still can begin with an endocrine-based anti-HER2 approach. The newer frontline trastuzumab deruxtecan data do not erase that option, but they do make it a more selective strategy rather than the obvious default.
In practical terms, endocrine-based first-line therapy remains most appealing when the disease burden is not immediately threatening, the patient strongly prefers to avoid cytotoxic therapy, or comorbidity makes an antibody-drug conjugate or a taxane-based regimen less attractive. In a rapidly progressive presentation, many clinicians still will prefer a regimen with the strongest disease-control data up front. That judgment is an inference from the available trials rather than a direct head-to-head comparison between endocrine-based therapy and frontline trastuzumab deruxtecan plus pertuzumab, which does not yet exist.
Maintenance after induction is evolving quickly
Maintenance therapy in hormone receptor-positive, HER2-positive disease now is one of the most interesting parts of metastatic breast cancer care. The PATINA trial enrolled patients with hormone receptor-positive, HER2-positive metastatic breast cancer whose disease had not progressed after four to eight cycles of chemotherapy plus HER2-targeted therapy, then randomized them to maintenance anti-HER2 therapy plus endocrine therapy with or without palbociclib. In the New England Journal of Medicine report, median progression-free survival was 44.3 months with palbociclib vs. 29.1 months without it, confirming that CDK4/6 inhibition can extend disease control meaningfully in this subgroup.
The regulatory caveat is important. Palbociclib remains FDA-approved for hormone receptor-positive, HER2-negative advanced or metastatic breast cancer, not HER2-positive metastatic disease. PATINA is highly relevant scientifically and increasingly relevant clinically, but it is not yet a labeled HER2-positive first-line maintenance standard in the United States. For now, it is best viewed as a compelling emerging strategy rather than a universally adopted default.
That means routine maintenance planning still starts with the basics. In hormone receptor-positive disease that begins with THP, maintenance trastuzumab plus pertuzumab plus endocrine therapy remains a common evidence-based framework. In patients who start with frontline trastuzumab deruxtecan plus pertuzumab, the role of later endocrine integration is less standardized and must be individualized. This is one of the places where real-world practice likely will evolve faster than formal guidelines.
Prior early-stage HER2 therapy changes the first-line answer
One of the most clinically important details in first-line metastatic breast cancer is not the metastatic presentation itself, but what happened before it. Patients who relapse more than six months after completing neoadjuvant or adjuvant HER2-directed therapy fit the DESTINY-Breast09 population and therefore fit the frontline trastuzumab deruxtecan plus pertuzumab approval cleanly. That group generally can be approached the same way as de novo metastatic disease, with the usual safety caveats.
Patients who recur during perioperative HER2-directed therapy or within six months after completing it are different. The frontline combination approval did not study them, but trastuzumab deruxtecan monotherapy already carries an FDA indication for unresectable or metastatic HER2-positive breast cancer after prior anti-HER2-based therapy in the metastatic setting or after recurrence during or within six months of completing neoadjuvant or adjuvant therapy. For that patient, trastuzumab deruxtecan monotherapy often is the most label-concordant systemic starting point, rather than assuming that THP or the new combination must be used first.
This is an important distinction because “first-line” in metastatic breast cancer does not always mean “treatment-naive HER2 biology.” Early relapse after curative-intent HER2-directed therapy usually signals a more treatment-exposed disease state, and the labeling now reflects that reality. Therefore, clinicians should separate truly untreated metastatic disease from early metastatic recurrence after modern early stage HER2 therapy, because the best starting regimen may differ even though both situations technically are first-line metastatic breast cancer.
Brain metastases and other special situations
Central nervous system (CNS) involvement remains a major issue in HER2-positive metastatic breast cancer, but it does not yet create a single automatic first-line answer. Patients with brain metastases at baseline still require individualized sequencing that integrates local therapy, neurologic symptoms, extracranial disease burden, and the broader systemic plan. What is most important in 2026 is not to confuse later-line CNS-active standards with upfront maintenance concepts that still are emerging.
Tucatinib is the clearest example. The FDA-approved tucatinib regimen remains tucatinib plus trastuzumab and capecitabine for unresectable or metastatic HER2-positive breast cancer after one or more prior anti-HER2-based regimens in the metastatic setting, including patients with brain metastases. That remains the label-supported CNS-relevant role for tucatinib today. It is not a frontline metastatic approval.
At the same time, first-line maintenance is moving. HER2CLIMB-05 reported that adding tucatinib to trastuzumab plus pertuzumab as first-line maintenance after induction therapy improved progression-free survival, with a manageable safety profile. Those data are important because they suggest that maintenance intensification may become a real part of frontline strategy in metastatic breast cancer, especially in subgroups where CNS control is a major concern. But as of April 2026, this still is an evolving evidence base rather than an FDA-labeled first-line standard.
Practical toxicity monitoring in the first line
The choice between trastuzumab deruxtecan plus pertuzumab and THP partly is a choice between two different toxicity cultures. With trastuzumab deruxtecan plus pertuzumab, the key disciplines are pulmonary vigilance, antiemetic prophylaxis, neutropenia monitoring, and serial cardiac assessment. Every visit should include active questioning about cough, dyspnea, fever, or reduced exercise tolerance, because interstitial lung disease is easier to manage early rather than after symptoms have progressed. The combination also is sufficiently emetogenic that nausea prevention should be planned, not improvised.
With THP, the emphasis is different. Clinicians need to anticipate diarrhea, alopecia, taxane-related neuropathy, myelosuppression, and cardiac effects of dual HER2 blockade. The experience base is broad, which makes supportive care pathways familiar, but the regimen still is intensive and can be difficult in frail patients. In that sense, the first-line choice is less about “easy vs. hard” and more about “which set of toxicities is more acceptable and more manageable for this patient.”
A clinic-ready way to think about first-line therapy
For most patients with newly diagnosed unresectable or metastatic HER2-positive breast cancer who truly are treatment-naive in the advanced setting and who did not relapse within six months of perioperative HER2 therapy, trastuzumab deruxtecan plus pertuzumab now is the newest FDA-approved frontline option and the regimen with the strongest progression-free survival data. That will make it the leading starting choice for many eligible patients.
THP remains a very reasonable first-line regimen when clinicians want the most established long-term survival dataset, when they prefer a taxane-based platform, or when pulmonary risk makes trastuzumab deruxtecan less attractive. Paclitaxel is a reasonable substitute for docetaxel in many real-world settings, even though the original label and pivotal trial used docetaxel.
In hormone receptor-positive disease, endocrine-based first-line therapy still has a place in carefully selected patients with less aggressive biology, and endocrine therapy remains central to maintenance planning after induction of HER2-directed therapy. PATINA suggests that CDK4/6-based maintenance likely will become more important, but palbociclib is not yet FDA-approved for HER2-positive metastatic breast cancer.
For patients whose disease recurs during or within six months of completing neoadjuvant or adjuvant HER2-directed therapy, clinicians should not simply apply the findings of the DESTINY-Breast09 study reflexively. Those patients fall outside the main frontline combination population, and trastuzumab deruxtecan monotherapy already has a label-supported role in that setting.
Conclusion
First-line treatment in HER2-positive metastatic breast cancer no longer is a single-regimen conversation. The historical standard, THP, still matters because it is durable, familiar, and backed by exceptional long-term survival data. But the field changed when trastuzumab deruxtecan plus pertuzumab showed a substantial progression-free survival advantage and gained frontline FDA approval. For many patients with untreated metastatic breast cancer, that combination now is the most compelling starting point. The harder and more important work is deciding who does not fit that default because of early relapse after perioperative therapy, pulmonary risk, hormone receptor-positive disease biology, or the need for a different maintenance strategy. Therefore, the best first-line decision in HER2-positive breast cancer is not just about what is newest. It is about choosing the regimen whose evidence base, toxicity profile, and sequencing logic best match the clinical reality of the case.