DESTINY-Breast01: Trastuzumab Deruxtecan in HER2-Positive Metastatic Breast Cancer

Primary endpoint (Objective response rate): 60.9% (95% CI, 53.4 to 68.0) — 112 responses in 184 patients Statistical test: Single-arm; no formal hypothesis test against a predefined null reported Key secondary (Median PFS): 16.4 months (95% CI, 12.7 to not reached) Sample size: N=184 (treated and analyzed at the recommended dose) Safety signal: Interstitial lung disease in 13.6% of patients, including 4 fatal (grade 5) cases (2.2%)

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Clinical Bottom Line

Trastuzumab deruxtecan delivered a striking objective response rate in a population of patients with HER2-positive metastatic breast cancer who had been heavily pretreated — a median of six prior lines of therapy, all of whom had previously received trastuzumab emtansine. In a disease space where response rates in the later lines of therapy have historically been modest, the magnitude and durability of responses observed in this single-arm study established trastuzumab deruxtecan as a transformative option for patients who had exhausted standard HER2-directed therapies.

Since the publication of DESTINY-Breast01, the treatment landscape for HER2-positive metastatic breast cancer has evolved substantially. Trastuzumab deruxtecan has moved earlier in the treatment algorithm, with DESTINY-Breast03 demonstrating superiority over trastuzumab emtansine in the second-line setting and DESTINY-Breast02 confirming activity after prior T-DM1. DESTINY-Breast01 remains the foundational efficacy and safety dataset for this drug in heavily pretreated patients. It is this study that first defined the benefit-risk profile — including the critical interstitial lung disease signal — that shapes how clinicians use trastuzumab deruxtecan across all lines of therapy today.

The key concern that emerged from this trial, and that remains central to clinical practice, is interstitial lung disease. Fatal cases occurred, and the signal mandates proactive monitoring, patient education, and a low threshold for dose interruption and corticosteroid initiation. ILD remains the defining safety challenge of this drug regardless of the treatment line in which it is used.

Trial Overview

Study Design

• Trial name and registration: DESTINY-Breast01, NCT03248492
• Design: Phase 2, open-label, single-group, multicenter, primary analysis
• Randomization: Not applicable (single-arm study). Part 1 involved dose-finding randomization across multiple dose levels; the primary analysis population includes all 184 patients who received the recommended dose of 5.4 mg/kg across parts 1 and 2.
• Setting: HER2-positive metastatic breast cancer, previously treated with trastuzumab emtansine (median 6 prior lines)
• Enrollment period and sites: 72 sites in eight countries in North America, Asia, and Europe

Mechanism of Action

Trastuzumab deruxtecan (DS-8201, T-DXd; Enhertu) is an antibody-drug conjugate (ADC) composed of a humanized anti-HER2 monoclonal antibody linked to a topoisomerase I inhibitor payload (a derivative of exatecan, DXd) via a cleavable tetrapeptide-based linker [2]. The antibody component binds HER2 on the tumor cell surface, triggering internalization and lysosomal cleavage of the linker, which releases the cytotoxic payload intracellularly. The membrane-permeable nature of the released payload also enables a bystander killing effect on neighboring tumor cells, including those with lower HER2 expression [2].

Patient Population

• Key eligibility: Adults with pathologically documented HER2-positive (IHC 3+ or ISH-positive, centrally confirmed) unresectable or metastatic breast cancer; prior treatment with trastuzumab emtansine required; ECOG performance-status score of 0 or 1
• Key exclusions: Untreated or symptomatic brain metastases; history of noninfectious interstitial lung disease or pneumonitis requiring glucocorticoids; current or suspected interstitial lung disease or pneumonitis
• Biomarker selection: HER2-positive (IHC 3+ or ISH-positive), centrally confirmed on archival tissue
• Sample size flow: 353 patients assessed for eligibility → 253 enrolled and treated → 184 received the recommended dose of 5.4 mg/kg (primary analysis population)

Baseline Characteristics

Treatment Protocol

Treatment Regimen: Trastuzumab Deruxtecan 5.4 mg/kg (n=253 enrolled; n=184 treated at recommended dose; n=184 analyzed)

Trastuzumab deruxtecan (DS-8201) 5.4 mg per kilogram of body weight administered by intravenous infusion every 3 weeks.

• Dose and schedule: 5.4 mg per kilogram IV every 3 weeks
• Treatment duration: Until disease progression, the occurrence of unacceptable toxic effects, or withdrawal of consent
• Median treatment duration: 10.0 months (range, 0.7 to 20.5 months)
• Median relative dose intensity: Not reported in this publication

There was no comparator arm in this study.

Treatment status at data cutoff: At the time of the data cutoff (August 1, 2019), 79 of 184 patients (42.9%) were continuing to receive trastuzumab deruxtecan, and 128 patients (69.6%) had continued treatment for more than 6 months.

Reasons for discontinuation (105 patients, 57.1%):

• Progressive disease: 53 (28.8%)
• Adverse event: 28 (15.2%)
• Withdrew consent: 8 (4.3%)
• Death: 7 (3.8%)
• Withdrawn by physician: 4 (2.2%)
• Other reason: 5 (2.7%)

Efficacy Outcomes

Primary Endpoint: Objective Response Rate (ORR)

Definition: Overall response (complete response plus partial response) to trastuzumab deruxtecan therapy, confirmed on the basis of an independent central review of imaging with the use of modified RECIST, version 1.1 Analysis population: Intention-to-treat (all 184 patients who received the recommended dose of 5.4 mg/kg) Statistical method: Clopper–Pearson method for two-sided 95% confidence intervals Median follow-up: 11.1 months (range, 0.7 to 19.9)

Result: 60.9% (95% CI, 53.4 to 68.0) Events: 112 responses in 184 patients - Complete response: 6.0%

• Partial response: 54.9%

Key Secondary Endpoints

Disease-control rate Definition: Response rate plus stable-disease rate Analysis population: Intention-to-treat (184 patients at 5.4 mg/kg) Result: 97.3% (95% CI, 93.8 to 99.1) This endpoint was not formally tested in a predefined statistical hierarchy.

Clinical-benefit rate Definition: Disease-control rate with stable disease lasting ≥6 months Analysis population: Intention-to-treat (184 patients at 5.4 mg/kg) Result: 76.1% (95% CI, 69.3 to 82.1) This endpoint was not formally tested in a predefined statistical hierarchy.

Duration of response Definition: Duration of response among patients with complete or partial response Analysis population: 112 patients who had a complete or partial response Result: Median 14.8 months (95% CI, 13.8 to 16.9) This endpoint was not formally tested in a predefined statistical hierarchy.

Progression-free survival Definition: Duration of progression-free survival assessed with the use of modified RECIST, version 1.1 Analysis population: Intention-to-treat (184 patients at 5.4 mg/kg) Result: Median 16.4 months (95% CI, 12.7 to not reached) This endpoint was not formally tested in a predefined statistical hierarchy.

PFS in patients with brain metastases Analysis population: 24 patients who were enrolled with treated and asymptomatic brain metastases Result: Median 18.1 months (95% CI, 6.7 to 18.1) This endpoint was not formally tested in a predefined statistical hierarchy.

Overall survival Analysis population: Intention-to-treat (184 patients at 5.4 mg/kg) - 6-month estimated OS: 93.9% (95% CI, 89.3 to 96.6) - 12-month estimated OS: 86.2% (95% CI, 79.8 to 90.7) - Median overall survival was not reached at the time of this analysis. This endpoint was not formally tested in a predefined statistical hierarchy.

Time until response Analysis population: Intention-to-treat (184 patients at 5.4 mg/kg) Result: Median 1.6 months (95% CI, 1.4 to 2.6) — this interval corresponded to the time until the first imaging after baseline. This endpoint was not formally tested in a predefined statistical hierarchy.

Exploratory Endpoints

ORR among patients who had progressive disease as their best response to trastuzumab emtansine: Among the 66 patients whose best response to prior T-DM1 was progressive disease, the confirmed response rate to trastuzumab deruxtecan was 61.1% (95% CI, 53.6 to 68.3). This is an exploratory finding and should be interpreted accordingly.

Safety

Safety Population

The safety analysis set included 184 patients who were enrolled in part 1 or part 2 of the study and who received at least one dose of trastuzumab deruxtecan at the recommended dose of 5.4 mg/kg.

Safety Summary

Grade ≥3 Adverse Events of Clinical Significance

The source reports the most common grade 3 or higher adverse events as combined "grade ≥3" rates in the abstract and main text. Table 2 reports grade 3 and grade 4 separately for key events. Both levels of granularity are presented below.

Febrile neutropenia occurred in 3 patients (1.6%).

Adverse Events of Special Interest

⚠️ Interstitial Lung Disease: Critical Safety Signal

• Any grade: 25 (13.6%)

• Grade 1 or 2: 10.9%

• Grade 3: 1 (0.5%)

• Grade 4: 0

• Grade 5 (fatal): 4 (2.2%)

• Adjudication: All ILD events were adjudicated by an independent adjudication committee.

• Time to onset: Median 193 days (range, 42 to 535) for investigator-reported cases
• Duration to recovery: Median 34 days (range, 3 to 179) for investigator-reported cases
• Management: Of 20 patients with grade 2 or higher ILD, 13 received glucocorticoids and 7 were hospitalized.
• Clinical implication: ILD is the most clinically consequential toxicity of trastuzumab deruxtecan. Four fatal cases in 184 patients underscores the need for baseline and serial pulmonary imaging, patient education on respiratory symptoms (cough, dyspnea, fever), and immediate dose hold with prompt initiation of corticosteroids if ILD is suspected. Per the FDA prescribing information, trastuzumab deruxtecan should be permanently discontinued for grade 2 or higher ILD [2].

Decreased left ventricular ejection fraction:

• Any grade: 3 (1.6%)
• Grade 3: 1 (0.5%)

Prolonged QT interval:

• Any grade: 9 (4.9%)
• Grade 3: 2 (1.1%)

Infusion-related reaction:

• Any grade: 4 (2.2%)
• Grade ≥3: 0

Antidrug antibodies: 11 patients (6.0%) tested positive for antidrug antibodies.

Deaths

A total of 25 deaths were reported: - 7 deaths occurred during treatment, including: - 3 due to disease progression - 4 due to adverse events (hemorrhagic shock, general physical health deterioration, pneumonia, and acute organ failure — 1 patient each) - 4 deaths were attributed to interstitial lung disease by independent adjudication (these are included in the 25 total deaths). - Treatment-related deaths were not reported as a separate aggregate metric in this publication.

Subgroup Analyses

Subgroup analyses of ORR were conducted across prespecified clinical subgroups. Because this is a single-arm study, these analyses present response rates with 95% confidence intervals rather than hazard ratios.

These subgroup analyses were not powered for formal statistical comparisons. Results should be interpreted as hypothesis-generating.

Notable observations: Response rates were consistent across most subgroups, including patients with brain metastases, visceral disease, and those who received trastuzumab deruxtecan immediately after trastuzumab emtansine. Patients with IHC 3+ tumors showed a numerically higher ORR compared to those with IHC 1+ or 2+ (ISH-positive) tumors, though the latter subgroup was small (n=28) with wide confidence intervals.

Key Comparator Trials

Cross-trial comparisons are limited by differences in patient populations, trial designs, and endpoints. These data are presented for context, not for direct statistical comparison.

Contextual Notes

DESTINY-Breast01 was conducted in a more heavily pretreated population than most prior studies in this space — with a median of six prior lines and universal prior T-DM1 exposure. The response rate of 60.9% in this context was unprecedented for a single-agent therapy. Prior to this trial, later-line options in HER2-positive metastatic breast cancer (e.g., lapatinib-based combinations, physician's choice chemotherapy) offered response rates typically well below this threshold, and outcomes in the third-line and beyond were limited.

The HER2CLIMB trial [5] established tucatinib in combination with trastuzumab and capecitabine as an effective option in a similarly pretreated HER2-positive population, including patients with brain metastases. DESTINY-Breast01 and HER2CLIMB addressed overlapping but not identical patient populations, and the two regimens offer different mechanisms and toxicity profiles. DESTINY-Breast03, published subsequently, moved trastuzumab deruxtecan into the second-line setting by demonstrating superiority over T-DM1, reshaping the treatment sequence and rendering DESTINY-Breast01's later-line population somewhat narrower in current practice.

Grey Zones and Unanswered Questions

• Single-arm design and lack of comparator: Without a randomized control arm, it is impossible to determine the magnitude of benefit attributable to trastuzumab deruxtecan versus other therapies that could have been given in this setting. The response rate is compelling, but the contribution of patient selection and investigator enthusiasm for a novel agent cannot be excluded.

• Optimal sequencing relative to tucatinib-based therapy: DESTINY-Breast01 did not compare trastuzumab deruxtecan to tucatinib-trastuzumab-capecitabine. For patients who have progressed on T-DM1, the choice between these two regimens — and whether one should precede the other — remains unresolved by direct evidence.

• Brain metastases population limitations: Only 24 patients with treated, asymptomatic brain metastases were included. Patients with untreated or symptomatic brain metastases were excluded. The intracranial activity of trastuzumab deruxtecan in active brain disease was not assessed in this trial.

• ILD risk factors and predictors remain undefined: While ILD occurred in 13.6% of patients (including fatal cases), the trial did not identify clinical or biomarker predictors of ILD susceptibility. It remains unclear which patients are at highest risk and whether dose modifications, proactive monitoring strategies, or patient selection can mitigate the fatal ILD risk.

• Activity in HER2 IHC 1+/2+ ISH-positive patients: The subgroup with lower HER2 expression (IHC 1+ or 2+, ISH-positive; n=28) showed a numerically lower ORR of 46% compared to 63% in IHC 3+ patients. The small sample size limits interpretation, but this raises the question of whether the magnitude of benefit differs by HER2 expression level within the "HER2-positive" population — a question with implications for treatment selection as T-DXd has subsequently been studied in HER2-low populations.

• Long-term safety and late ILD events: Median time to ILD onset was 193 days with a range extending to 535 days, indicating that ILD can emerge late in the treatment course. With a median follow-up of only 11.1 months, the full late-onset ILD risk remains incompletely characterized.

Clinical Implications

Where This Fits in the Treatment Sequence

At the time of its publication, DESTINY-Breast01 established trastuzumab deruxtecan as the most active single agent available for patients with HER2-positive metastatic breast cancer who had progressed on T-DM1. Since then, DESTINY-Breast03 has moved T-DXd into the second-line setting (after progression on a taxane plus trastuzumab/pertuzumab), and current NCCN guidelines [3] recommend trastuzumab deruxtecan as a preferred second-line option. T-DXd remains a standard option in the third line and beyond, which corresponds more closely to the DESTINY-Breast01 population.

Practical Considerations

• Dosing: 5.4 mg/kg IV every 3 weeks. Treatment continues until progression or unacceptable toxicity.
• ILD monitoring: Baseline CT scan of the chest recommended. Clinicians should have a low threshold for CT imaging in any patient who develops new or worsening cough, dyspnea, or fever. Per the FDA prescribing information [2], T-DXd should be permanently discontinued for grade 2 or higher ILD.
• Nausea management: Nausea was the most common any-grade adverse event (77.7% in Table 2). Grade ≥3 nausea occurred in 7.6%. Proactive antiemetic prophylaxis is recommended.
• Hematologic monitoring: Decreased neutrophil count (grade ≥3: 20.7%) and anemia (grade ≥3: 8.7%) were the most common high-grade toxicities. Regular CBC monitoring is essential.
• Dose modifications: Dose reductions occurred in 23.4% and dose interruptions in 35.3% of patients. Clinicians should be familiar with the dose modification algorithm in the prescribing information [2].

Unanswered Questions

The two most practice-relevant open questions from this trial remain: (1) the optimal sequence of trastuzumab deruxtecan relative to tucatinib-based therapy in patients who have progressed on second-line treatment, and (2) whether any biomarker or clinical characteristic can predict ILD risk and guide patient selection.

Post-Progression Options

Subsequent therapy rates were not reported in this publication. In the current treatment landscape, patients who progress on trastuzumab deruxtecan may be considered for tucatinib-trastuzumab-capecitabine, other HER2-directed therapy combinations, or clinical trials, depending on prior therapy and performance status.

Regulatory and Guideline Status

Regulatory

• FDA: Trastuzumab deruxtecan (Enhertu) received accelerated approval from the FDA in December 2019 for the treatment of adult patients with unresectable or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2-based regimens in the metastatic setting, based on the DESTINY-Breast01 results. The indication was subsequently expanded with regular approval in the second-line setting (May 2022) based on DESTINY-Breast03 and further expanded to HER2-low breast cancer (August 2022) based on DESTINY-Breast04.
• EMA: Conditional marketing authorization granted in January 2021 for HER2-positive metastatic breast cancer after two or more prior anti-HER2 regimens, with subsequent indication expansions.

⚠️ Regulatory status verified as of March 2026. Confirm current approval status and labeled indications before clinical use at FDA.gov.

Guidelines

• NCCN: Trastuzumab deruxtecan is listed as a preferred regimen for HER2-positive metastatic breast cancer in the second-line setting (Category 1) and as a recommended option in later lines of therapy in the NCCN Clinical Practice Guidelines in Oncology: Breast Cancer [3].

Companion Diagnostics

HER2-positive status (IHC 3+ or ISH-positive) must be confirmed using an FDA-approved assay. Central confirmation of HER2 status was required for trial enrollment.

About the Author

Andrew Stevenson is the founder and systems architect of kill-cancer.com, a clinical intelligence platform delivering structured, source-traced oncology trial analysis to practicing clinicians. He holds 17 Google technical certifications in data systems, automation, and applied AI — the engineering foundation behind the extraction and verification pipeline that produces every article on this platform. Every number traces directly to its source publication. Zero calculation. Zero editorializing. Zero hallucination.

Five siblings lost to cancer built the urgency. The engineering builds the trust.

📧 andrew@kill-cancer.com 🌐 kill-cancer.com 💬 kill-cancer.com/forum

Disclaimer

This article is intended for healthcare professionals only. It is not medical advice and should not be used as a substitute for professional clinical judgment. Treatment decisions should be made in consultation with qualified healthcare providers based on individual patient circumstances.

All trial data presented in this article are sourced directly from the published clinical trial report and its supplementary materials. Numbers are reproduced exactly as reported; no calculations, derivations, or estimates have been performed.

Trial results are presented as reported in the source publication. Updated data, label changes, or guideline revisions published after the source article may alter clinical applicability.

Comparator trial data presented in Section 8 are sourced from their respective published reports and are provided for contextual purposes only. Cross-trial comparisons have inherent limitations and should not be interpreted as direct statistical comparisons.

References

1. Modi S, Saura C, Yamashita T, et al. Trastuzumab deruxtecan in previously treated HER2-positive breast cancer. N Engl J Med. 2020;382(7):610-621. doi:10.1056/NEJMoa1914510
2. Enhertu (fam-trastuzumab deruxtecan-nxki) prescribing information. U.S. Food and Drug Administration. Accessed March 2026.
3. NCCN Clinical Practice Guidelines in Oncology: Breast Cancer. Version 1.2026. Accessed March 2026.
4. Krop IE, Kim SB, Martin AG, et al. Trastuzumab emtansine versus treatment of physician's choice in patients with previously treated HER2-positive metastatic breast cancer (TH3RESA): final overall survival results from a randomised open-label phase 3 trial. Lancet Oncol. 2017;18(6):743-754.
5. Murthy RK, Loi S, Okines A, et al. Tucatinib, trastuzumab, and capecitabine for HER2-positive metastatic breast cancer. N Engl J Med. 2020;382(7):597-609.