HER2-Positive Breast Cancer in 2026: TDXd Enters the First Line, Maintenance Gets Smarter, and Early Disease Is Changing Fast with TDxd neoadjuvant and adjuvant approvals

Introduction: The HER2 Landscape Is Shifting at Every Line

The treatment of HER2-positive metastatic breast cancer has undergone more transformation in the past two years than in the prior decade. Trastuzumab deruxtecan (TDXd) has now moved into the first-line setting, challenging a standard that held for over a decade. Maintenance strategies have become meaningfully more sophisticated. In early breast cancer, TDXd is recently approved in both the adjuvant and neoadjuvant settings, bringing new questions about sequencing, toxicity monitoring, and what to do when early disease treatment decisions affect later options.

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First-Line Metastatic HER2+Breast Cancer (MBC): 

The CLEOPATRA Legacy and the DB09 Challenge (FDA approved 12/15/2025)

For well over a decade, the first-line standard for metastatic HER2-positive breast cancer was trastuzumab plus pertuzumab plus taxane (THP), established by the landmark CLEOPATRA trial. The eight-year overall survival analysis from CLEOPATRA remains impressive: 57 months in the THP arm versus 40.8 months, more than a year of survival gained (16.3 months). Approximately 20% of patients sit on the long tail of the survival curve, alive at eight years. Clinically, we see patients even farther out with a potential cure and lack molecular tools to identify these durable responses. Circulating tumor DNA (ctDNA) dynamics may help as a predictor of treatment response in metastatic breast cancer (mBC)(Razavi et al).

Weekly taxane-based THP regimens show a consistent PFS of approximately 18 months as a control arm benchmark like CLEOPATRA PFS. 

The transition from induction THP to maintenance HP (trastuzumab plus pertuzumab, no chemotherapy) is a quality-of-life landmark for patients. Hair grows back; patients return to work up and possibly picking up children from school; life normalizes. That maintenance phase can persist for years, and preserving that quality of life in the metastatic setting matters.

DESTINY-Breast09 (DB09) now offers a compelling alternative. This phase 3 trial enrolled 1134 patients with previously untreated HER2-positive metastatic breast cancer (first line), randomizing to one of three arms; TDXd with pertuzumab, TDXd with pertuzumab and placebo, or the standard THP induction (6 cycles) followed by HP maintenance. The primary endpoint was progression-free survival.

Results: Primary endpoint PFS 40 months with TDXd versus 26 months with THP, a delta of more than a year (13.8 months) (Hazard ratio, HR=0.56 Ci 0.44-0.71 p<0.00001) The benefit was present regardless of hormone receptor status. As a result, the FDA approved Tdx on December 15, 2025.

Important caveats:

• Approximately half the patients were enrolled in Asia; about half had de novo metastatic disease (stratification factor).

• PIK3CA mutations were present in approximately 30% of patients (stratification factor.; these patients progress faster (visible as an early separation in the Cleopatra survival curves), though TDXd efficacy appears preserved regardless of PIK3CA status.

• Among triple-positive patients (HR+ HER2+), the THP arm had substantially more patients receiving concurrent endocrine therapy (38% vs 13.5% in the TDXd arm), creating an imbalance that may affect interpretation of the triple-positive subgroup.

• 8.7% (33/380) switched to trastuzumab (+P) after stopping T-DXd for reasons other than Disease progression (begs questions about tolerance or desire to de-escalate to HP)

• Unlike THP, there is no maintenance phase de-escalation in the DB09 regimen; patients continue TDXd until progression or toxicity. Whether this is sustainable long-term, and what to do with long responders who remain on TDXd for years, are open questions. The DEMETHER study (phase 2, 165 patients, one US site) is exploring an induction-then-maintenance approach: 6 cycles of TDXd induction followed by a maintenance strategy guided by ctDNA (Cortes J et al) which may minimize side effects like nausea, fatigue and ILD. Primary results are expected late 2026.

The sequencing question now confronting clinicians: with TDXd in the first line, what comes second? The rich options that followed THP ( TDXd, tucatinib combinations, TDM1) need to be reconsidered in a TDXd-first framework.

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Maintenance in first line Triple-Positive/HER2+ Disease: The PATINA Trial

For patients with HER2-positive and hormone receptor-positive ("triple-positive") metastatic breast cancer, the PATINA trial addresses a specific question: after induction THP and transition to HP maintenance, does adding the CDK 4/6 (cyclin-dependent kinase) inhibitor, palbociclib to HP plus endocrine therapy improve outcomes?

The trial randomized patients after their THP induction to HP with endocrine therapy alone versus HP with endocrine therapy plus palbociclib. The result: PFS 44 months with palbociclib versus 29 months without (Hazard ratio, HR=0.74 Ci 0.58-0.95 p=0.0074). This is an impressive clinically meaningful improvement from adding a CDK4/6 inhibitor to the maintenance phase. In addition, patients enjoy a chemotherapy free quality of life. Overall survival (OS) analysis remains immature, with only 119 of 247 planned events observed to presentation date. 

The PATINA regimen is now incorporated into NCCN guidelines (version 3/2026). Important context: because randomization occurs after induction, the true PFS measured from the start of treatment would be considerably longer than what the trial reports. Early progressors during chemotherapy THP induction are not captured in the PATINA analysis. The real PFS from start of chemo would be even longer in PATINA. These data pre-date DB-09 which has no “maintenance” phase. ****

A CNS signal from PATINA updates presented at San Antonio in December 2025 is intriguing: at 3 years, 13% of palbociclib patients had first CNS progression versus 20% of controls. Brain metastasis imaging was not mandated, so the absolute numbers may underestimate true differences. Whether palbociclib offers CNS protection or simply delays the appearance of detected events is unclear, but worth tracking prospectively.

The HER2-CLIMB-05 trial explored a parallel maintenance strategy: adding tucatinib (a HER2-selective small molecule inhibitor) to HP maintenance following induction. The result; PFS 24.9 versus 16 months (8.6-month delta) favoring tucatinib (Hazard ratio, HR=0.64 Ci 0.51-0.79 p<0.00001). The control arm PFS of 16 months is somewhat lower than the typical 18-month PFS benchmark from Cleopatra, which may reflect patient population differences. Only 45% of triple-positive patients received concurrent endocrine therapy. The CNS signal with the addition of tucatinib remains numerically promising (4.2 month delta) but not yet definitive. patients with brain metastases, should T-DXd or tucatinib-based therapy be preferred in the first-line setting?

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Early Breast Cancer (EBC): TDXd Moves Up the Treatment Ladder (FDA approved 5/15/2026)

DESTINY-Breast05 (adjuvant setting): In patients with HER2-positive early breast cancer (N=818) who did not achieve pathological complete response after neoadjuvant therapy (residual disease at surgery), T-DM1 has been the adjuvant standard per the KATHERINE trial. DESTINY-Breast05 compared to adjuvant TDXd versus T-DM1 for 14 cycles in this residual disease population. Concomitant adjuvant endocrine therapy was allowed in patients with “triple positive” early breast cancer. 

The efficacy result is impressive: 3-year invasive disease-free survival (iDFS) (Hazard ratio, HR= 0.47 95% Ci 0.34-0.66; P < .0001), a 53% reduction in recurrence risk, with an absolute delta of approximately 8% at 36 months (92.4% vs 83.7%).

The toxicity concern, however, deserves careful attention. These patients are receiving curative-intent radiation concurrently, and TDXd's risk of interstitial lung disease (ILD) becomes particularly significant in that context. Any-grade ILD occurred in more than 10% of patients in the TDXd arm. There were three grade 5 ILD events, deaths from lung toxicity in patients who were otherwise on a path to cure which requires serious individualized discussion in a shared-decision making context.

DESTINY-Breast11 (neoadjuvant setting): A total of 927 patients were randomly assigned 1:1:1 to receive:

• 8 cycles of T-DXd monotherapy (n = 286),

• 4 cycles of T-DXd followed by 4 cycles of THP, or

• 4 cycles of ddAC followed by 4 cycles of THP

The primary end point was pathologic complete response (pCR) rate. Secondary end points included event-free survival, iDFS, overall survival, and safety.

Enrollment to the T-DXd monotherapy arm was closed early based on a prior efficacy evaluation, as recommended by an independent data monitoring committee.

Pathological complete response rates were 67% overall and importantly 61% in patients with “triple-positive” breast cancer. The anthracycline-containing control arm with AC-based regimens limits direct applicability to US practice, where anthracyclines have largely been abandoned in HER2-positive breast cancer given the cardiac risk and the availability of taxane-pertuzumab-trastuzumab regimens with or without carboplatin. The TDXd-alone neoadjuvant arm was closed due to inadequate pCR rates. ILD rates in the 4-cycle neoadjuvant setting were approximately 4%, lower than in the adjuvant setting, as expected with shorter exposure. The unresolved question: if a patient receives TDXd in the neoadjuvant setting and has residual disease, what is the appropriate adjuvant strategy?

TCHP versus THP in the neoadjuvant setting: In HER2+ EBC, the carboplatin role has been challenged with THP as non-inferior to TCHP in pathologic complete response rates in the deescalation neoCARHP study (Hong-Fei Gao et al ASCO 2025) and we await event free survival from Compass HER2. 

Current practice remains individualized: younger patients with node-positive disease often receive TCHP, while older patients or those with lower disease burden may receive THP. Carboplatin continues to have a stronger role in triple-negative than in HER2-positive breast cancer.

There is also some enthusiasm for neoadjuvant HP without chemo (PHERGAIN phase 2 trial) with a FDG-PET response-adapted approach to further de-escalate and minimize side effects in a highly selected favorable risk population with small, lymph node negative tumors. 

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TDXd Toxicity Management: Nausea and ILD Are the Priorities

TDXd is a highly emetogenic regimen. The most intense nausea occurs in the first few cycles and can be severe enough to affect treatment compliance.

Four-drug Nausea management: Olanzapine 2.5 mg daily for a full week following each infusion, combined with a standard antiemetic backbone (NK1 receptor antagonist, 5-HT3 antagonist, and dexamethasone). Delayed nausea, occurring 6 to 7 days after infusion, may benefit from intravenous hydration with additional dexamethasone at that time point, and/or an additional long-acting antiemetic. Proactive management in the first two cycles establishes the pattern for the remainder of treatment.

ILD monitoring and management (the “5 S’s”); Screen, Scan, Synergy, Suspend treatment, Steroids:

• Screening: CT scans approximately every third cycle (clinical trials used every 2 months; every third cycle is practical in routine care).

• Grade 1 (asymptomatic): stop TDXd for one month; consider steroids (.5 mg/kg); rechallenge at same dose if fully resolved, dose reduction if not fully resolved. Some published real-world experience supports rechallenge in grade 1.

• Grade 2: hard stop; promptly initiate systemic steroids (1 mg/kg) with a long 4–6-week taper; involve pulmonology.

• High-risk populations: Asian descent (Daiichi Sankyo is a Japanese company, and ILD rates are anecdotally higher in Asian patients as seen with other TKIs including in lung cancer patients), age ≥65, pre-existing lung comorbidities, and baseline renal insufficiency. Patients with serum creatinine 2.0 to 2.5 mg/dL can develop severe acute kidney injury on TDXd; monitoring renal function closely before and during treatment is essential.

• Breast cancer dose: 5.4 mg/kg.

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For Patients

HER2-positive cancer treatment, especially breast cancer, is one of the success stories of modern oncology. Prior to Cleopatra, the median survival in metastatic HER2-positive breast cancer was approximately 3-4 years. It then exceeded 5 years with THP and HP maintenance with a meaningful proportion of patients alive at 8 or more years at the last landmark analysis. The recent approval of trastuzumab deruxtecan in the first-line setting has further extended progression-free survival by more than a year compared to prior standards. In early breast cancer, new neoadjuvant and adjuvant approaches are available. Talk to your oncologist about what the current options mean for your specific situation and ask about clinical trials — the HER2-positive breast cancer space is complex. 

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Key Takeaways

• DB09: TDXd + pertuzumab is now FDA-approved as first-line for HER2+ metastatic breast cancer; PFS 40 vs 26 months; benefit regardless of HR status; caveat around triple-positive patients and ET imbalance.

• PIK3CA mutations predict faster progression on THP but do not appear to affect TDXd efficacy.

• PATINA: adding palbociclib to HP + ET maintenance in triple-positive disease improves PFS from 29 to 44 months; now in NCCN guidelines; early CNS signal worth watching.

• DESTINY-Breast05: adjuvant TDXd vs T-DM1 in residual disease shows iDFS HR 0.47 but 10%+ any-grade ILD and 3 grade-5 events; benefit-risk discussion required. FDA-approved

• DESTINY-Breast11: neoadjuvant TDXd + THP, pCR 67%; TDXd-alone arm closed; ILD 4% in 4-cycle neoadjuvant setting. FDA-approved

• TDXd toxicity: proactive multiday antiemetic management; CT ILD screening every third cycle; hard stop at grade 2; renal insufficiency is an underrecognized risk factor.

Related Reading:

1. Top 5 Takeaways from Dr. Paolo Tarantino on the Future of Antibody-Drug Conjugates in Solid Oncology

2. AI in Breast Diagnostics: Why What We Do With the Tool Matters More Than the Tool Itself

3. The Hidden Barrier to Precision Oncology: How Insurance Policies Delay Access to Genomic Testing

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References

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8. Metzger O, et al. PATINA trial results. Presented at: San Antonio Breast Cancer Symposium; December 2025. https://doi.org/10.1158/1538-7445.SABCS25-GS2-05

9. Hamilton E, et al. HER2-CLIMB-05: Tucatinib in HER2+ maintenance. Presented at: San Antonio Breast Cancer Symposium; December 2025. Abstract GS1-01. https://doi.org/10.1158/1538-7445.SABCS25-GS1-01

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11. Harbeck N, et al. DESTINY-Breast11: Neoadjuvant T-DXd + THP in HER2+ early breast cancer. Presented at: ESMO Congress; 2025. Abstract 291O. https://doi.org/10.1016/j.annonc.2025.08.291

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16. Llombart Cussac A, et al. Chemotherapy-free, pathological complete response (pCR)-guided strategy with trastuzumab-pertuzumab (HP) and T-DM1 in HER2+ early breast cancer (EBC): PHERGain-2. ESMO Breast Cancer 2026. Abstract 214O