SUMMARY: Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. It is estimated that in the US, approximately 316,950 new cases of female breast cancer will be diagnosed in 2025, and about 42,170 women will die of the disease, largely due to metastatic recurrence.
The HER or erbB family of receptors consist of HER1, HER2, HER3 and HER4. Approximately 15%-20% of invasive breast cancers overexpress HER2/neu oncogene, which is a negative predictor of outcomes without systemic therapy. Trastuzumab (HERCEPTIN®) is a humanized monoclonal antibody targeting HER2. Trastuzumab binds to subdomain IV of the HER2 extracellular domain and blocks the downstream cell signaling pathways (PI3K-AKT pathway) and induces Antibody Dependent Cellular Cytotoxicity (ADCC). Pertuzumab (PERJETA®) is a recombinant humanized monoclonal antibody that binds to the HER2 at a different epitope of the HER2 extracellular domain (subdomain II) compared to Trastuzumab, and prevents the dimerization of HER2 with HER3 receptor. Pertuzumab stimulates ADCC similar to Trastuzumab. By combining Trastuzumab and Pertuzumab, a more comprehensive blockade of HER2 signaling can be accomplished, as these two agents bind to different HER2 epitopes and may complement each other and improve efficacy.
Trastuzumab deruxtecan (T-DXd) (ENHERTU®) is a next-generation Antibody-Drug Conjugate (ADC) composed of a humanized monoclonal antibody specifically targeting HER2, with the amino acid sequence similar to Trastuzumab, a cleavable tetrapeptide-based linker, and a potent cytotoxic Topoisomerase I inhibitor as the cytotoxic drug (payload). T-DXd has a favorable pharmacokinetic profile and the tetrapeptide-based linker is stable in the plasma and is selectively cleaved by cathepsins that are up-regulated in tumor cells. Unlike ado-Trastuzumab emtansine, another ADC targeting HER2, T-DXd has a higher drug-to-antibody ratio (8 versus 4), released payload easily crosses the cell membrane with resulting potent cytotoxic effect on neighboring tumor cells regardless of target expression, and the released cytotoxic agent (payload) has a short half-life, thus minimizing systemic exposure.
Background and Clinical Rationale
Trastuzumab deruxtecan (T-DXd) has demonstrated potent antitumor activity in HER2-positive breast cancer across multiple treatment lines. However, prior to the DESTINY-Breast09 study, all approved indications for T-DXd required patients to have received prior systemic therapy in either the metastatic or adjuvant setting. With the longstanding CLEOPATRA regimen, Docetaxel, Trastuzumab, and Pertuzumab (THP) established over a decade ago as the first-line standard of care, the oncology community has been eager to evaluate whether T-DXd could improve frontline outcomes.
Study Design and Patient Population
DESTINY-Breast09 (NCT04784715) is a randomized, global Phase 3 study designed to evaluate the efficacy and safety of first-line T-DXd with or without Pertuzumab, versus Taxane plusTrastuzumab plus Pertuzumab (THP), in patients with HER2-positive advanced/metastatic breast cancer. A total of 1,157 patients were enrolled across 283 sites worldwide. Eligible patients had centrally confirmed HER2-positive disease (IHC 3+ or ISH+), no prior chemotherapy or HER2-targeted therapy in the metastatic setting, and ≤1 prior line of endocrine therapy. Patients were stratified by Hormone Receptor (HR) status, PIK3CA mutation status, and de novo vs recurrent disease, and randomized 1:1:1 to:
- T-DXd + placebo – N=387
- T-DXd + pertuzumab (T-DXd + P) – N=383
- THP (control arm) – N=387
The interim analysis presented at ASCO 2025 focused on the comparison between T-DXd + P and THP. The T-DXd monotherapy arm remains blinded until the final PFS analysis.
The Primary endpoint was Progression-Free Survival (PFS) by Blinded Independent Central Review (BICR) in the intent-to-treat population. Secondary endpoints included Overall Survival (OS), PFS by investigator (INV), Objective Response Rate (ORR), Duration of Response (DOR), and Safety
Efficacy Outcomes: Progression-Free Survival and Response
At a median follow-up of 29 months, T-DXd + P demonstrated a statistically significant and clinically meaningful improvement in PFS compared to THP:
- Median PFS by BICR:
- T-DXd + P: 40.7 months
- THP: 26.9 months
- HR: 0.56; P <0.00001
- PFS by Investigator Assessment:
- Median: 40.7 months vs 20.7 months
- HR: 0.49 (95% CI: 0.39–0.61)
- Overall Response Rate (ORR):
- T-DXd + P: 85.1%
- THP: 78.6%
- Complete Response Rate:
- T-DXd + P: 15.1%
- THP: 8.5%
- Median Duration of Response:
- T-DXd + P: 39.2 months
- THP: 26.4 months
The PFS benefit was consistent across all patient subgroups, including HR status and PIK3CA mutation.
Safety Profile and Adverse Events
The safety profile of T-DXd + P in the frontline setting was consistent with known toxicities of T-DXd, with no new safety signals. Adjudicated drug-related Interstitial Lung Disease/pneumonitis occurred in 12.1% of patients receiving T-DXd + P (mostly grade 1 and 2) in contrast to only 1.0% among patients receiving THP. Other treatment-related toxicities such as nausea, vomiting, and constipation were more common with T-DXd + P, possibly due to longer median treatment exposure (~3.5 years).
Clinical Implications and Emerging Questions
The marked 13.8-month PFS improvement positions T-DXd + P as a strong candidate to replace THP as the first-line standard for HER2-positive advanced metastatic breast cancer. These results mirror the transformative impact of T-DXd seen in the second-line DESTINY-Breast03 trial comparing T-DXd with ado-Trastuzumab emtansine, where it yielded a median PFS of 28.8 months.
However, while efficacy is unquestionable, questions remain around treatment sequencing, duration, and long-term quality of life:
- Could T-DXd be reserved for second-line therapy in select patients with less aggressive disease?
- Might a strategy of T-DXd + P induction followed by de-escalation to maintenance Trastuzumab/Pertuzumab reduce toxicity?
- Can biomarker-driven personalization refine who should receive first-line T-DXd?
The researchers of this study emphasized that these results represent a paradigm shift in first-line treatment of advanced HER2-positive breast cancer.
Conclusion
DESTINY-Breast09 demonstrates that T-DXd + Pertuzumab significantly improves PFS compared to THP, with durable responses and manageable toxicity. The findings suggest a potential new first-line standard for HER2-positive metastatic breast cancer. While overall survival and long-term safety data are still maturing, the study sets a new benchmark in the frontline treatment landscape and invites critical dialogue on optimizing sequencing, duration, and patient-centered outcomes.
Trastuzumab deruxtecan (T-DXd) + pertuzumab (P) vs taxane + trastuzumab + pertuzumab (THP) for first-line (1L) treatment of patients (pts) with human epidermal growth factor receptor 2–positive (HER2+) advanced/metastatic breast cancer (a/mBC): Interim results from DESTINY-Breast09. Tolaney S, Jiang Z, Zhang Q, et al. J Clin Oncol 43, 2025 (suppl 17; abstr LBA1008)
