How do the DESTINY‑Breast06 (trastuzumab deruxtecan) and TROPION‑Breast01 (trastuzumab duocarmazine) trials compare regarding patient eligibility, efficacy (progression‑free and overall survival), and safety, and which drug is preferred for HER2‑low/ultralow versus HER2‑positive metastatic breast cancer after prior therapy?

DESTINY-Breast06 vs TROPION-Breast01: Critical Comparison

For HER2-low HR+ metastatic breast cancer after endocrine therapy, trastuzumab deruxtecan (T-DXd) from DESTINY-Breast06 is the preferred agent over datopotamab deruxtecan (Dato-DXd) from TROPION-Breast01, as T-DXd is studied in an earlier treatment line (no prior chemotherapy for metastatic disease) and is explicitly prioritized by ESMO guidelines. 1

Key Trial Design Differences

Patient Eligibility

DESTINY-Breast06 2:

• HER2 status: HER2-low (IHC 1+ or IHC 2+/ISH-) OR HER2-ultralow (IHC 0 with membrane staining)
• Prior therapy: ≥1 line of endocrine-based therapy; NO prior chemotherapy for metastatic disease
• Hormone receptor: HR+ only
• Sample size: 866 patients (713 HER2-low, 153 HER2-ultralow)

TROPION-Breast01 3, 4:

• HER2 status: HER2-negative (includes HER2-low and HER2-0)
• Prior therapy: Disease progression on endocrine therapy where endocrine therapy unsuitable; 1-2 prior lines of chemotherapy in metastatic setting
• Hormone receptor: HR+ only
• Sample size: 711 patients (360 Dato-DXd, 351 chemotherapy)

Critical distinction: DESTINY-Breast06 enrolled chemotherapy-naive patients in the metastatic setting, while TROPION-Breast01 required 1-2 prior chemotherapy lines—making T-DXd applicable earlier in the treatment algorithm.

Efficacy Outcomes

DESTINY-Breast06 (T-DXd)

HER2-low population 2:

• Median PFS: 13.2 months (T-DXd) vs 8.1 months (chemotherapy)
• Hazard ratio: 0.62 (95% CI 0.52-0.75; P<0.001)
• Confirmed ORR: 69.7% vs 29.2%
• Median DOR: 19.6 months vs 8.3 months
• OS data: Immature at primary analysis (39% events); HR 0.66 (95% CI 0.50-0.86; p=0.0021) 5

Consistent benefit across disease burden subgroups 6:

• Time to progression on prior CDK4/6i + ET <6 months: PFS 14.0 vs 6.5 months
• Time to progression 6-12 months: PFS 13.2 vs 6.9 months
• Time to progression >12 months: PFS 12.9 vs 8.2 months
• Primary endocrine resistance: Benefit maintained
• High disease burden (visceral, liver mets, ≥3 sites): Benefit maintained

TROPION-Breast01 (Dato-DXd)

Overall population 3, 4:

• Median PFS: Improved vs chemotherapy (specific numbers not provided in excerpts, but statistically significant)
• Activity across TROP-2 expression levels: Including heterogeneous expression
• Heavily pretreated population: 1-2 prior chemotherapy lines required

Important caveat: TROPION-Breast01 data shows activity in a more pretreated population, making direct comparison challenging.

Safety Profiles

T-DXd (DESTINY-Breast06) 5, 2

Most common adverse events (≥20%):

• Nausea (70%), decreased WBC/neutrophils, decreased hemoglobin, fatigue (59%), alopecia (46%)
• Grade ≥3 events: 52.8%

Critical safety concern:

• Adjudicated ILD/pneumonitis: 11.3% (49 patients)
  • Grade 5 (fatal): 3 events (0.7%)
  • Requires proactive monitoring with CT imaging
  • Exclusion criteria: History of ILD or current ILD at screening

Dose modifications 5:

• Permanent discontinuation: 14% (>2% due to ILD/pneumonitis)
• Dose interruptions: 48%
• Dose reductions: 25%

Dato-DXd (TROPION-Breast01) 3, 4

Most common treatment-related AEs:

• Nausea (51.9%, grade ≥3: 1.4%)
• Oral mucositis/stomatitis: 57.2% (grade ≥3: 7.2%)
• Ocular surface events: 43.9% (grade ≥3: 1.9%)
• Grade ≥3 events: Not specified in excerpts but hematologic toxicity noted as "relatively limited"

Critical safety concern:

• Adjudicated drug-related ILD/pneumonitis: 3.9% (mostly low-grade)
  • Grade ≥3: 0.8%
  • Grade 5 (fatal): 1 event
  • Lower incidence than T-DXd

Supportive care requirements 3:

• Anti-emetics: 76.7%
• Prophylactic mouthwash (steroid strongly recommended): 77.2%
• Artificial tears: 71.7%

Guideline-Based Treatment Algorithm

For HR+ HER2-low Metastatic Breast Cancer

ESMO Expert Consensus 2023 1 provides clear sequencing:

1. First-line after endocrine therapy failure (no prior chemotherapy for metastatic disease):

   • T-DXd is standard of care for HER2-low disease
   • Requires: Prior CDK4/6 inhibitor + ≥1 line chemotherapy in metastatic setting (OR progression within 6 months of [neo]adjuvant chemotherapy)
   • When both T-DXd and SG (sacituzumab govitecan) available: T-DXd prioritized as studied in less pretreated population

2. Second-line or beyond (after 1-2 prior chemotherapy lines):

   • Dato-DXd becomes an option (TROPION-Breast01 population)
   • SG also valid option for HER2-low disease

3. HER2-0 disease:

   • SG is only approved ADC
   • T-DXd shows some activity but less robust evidence 1

For HER2-low Triple-Negative Breast Cancer

ESMO guidance differs 1:

• SG should be considered first (more robust evidence in TNBC)
• T-DXd may be considered after SG
• T-DXd showed benefit in exploratory TNBC cohort (n=58): PFS HR 0.46, OS HR 0.48 1

Patient-Reported Outcomes

T-DXd (DESTINY-Breast06) 7

Quality of life preserved:

• Similar global health status/QOL vs chemotherapy
• Delayed deterioration in physical/role/emotional functioning, pain, fatigue
• Less pain (adjusted mean difference -7.2)
• Fewer skin/mucosal symptoms (adjusted mean difference -9.5)
• More GI symptoms: Nausea/vomiting (+7.2), appetite loss (+6.8), constipation (+5.5)

Dato-DXd (TROPION-Breast01) 3

Delayed time to deterioration:

• GHS/QOL, pain, physical functioning all favored Dato-DXd vs chemotherapy
• Patient-reported symptomatic AEs consistent with clinician-reported profile

Clinical Decision Framework

Choose T-DXd (DESTINY-Breast06 paradigm) when:

• HR+ HER2-low metastatic breast cancer
• Post-endocrine therapy (including CDK4/6i)
• No prior chemotherapy for metastatic disease (or progression within 6 months of adjuvant chemotherapy)
• No contraindications (active ILD, significant cardiac disease, ECOG >1)
• Patient can tolerate more intensive monitoring for ILD

Choose Dato-DXd (TROPION-Breast01 paradigm) when:

• HR+ HER2-negative (including HER2-low and HER2-0) metastatic breast cancer
• After 1-2 prior chemotherapy lines in metastatic setting
• Endocrine therapy unsuitable
• Patient requires lower ILD risk profile (3.9% vs 11.3%)
• Willing to implement prophylactic measures (steroid mouthwash, artificial tears)

HER2-ultralow considerations:

• T-DXd showed consistent benefit in HER2-ultralow subgroup (n=153) 2
• Dato-DXd studied in HER2-0 population (no specific ultralow designation)
• For HER2-ultralow specifically: T-DXd has more direct evidence

Common Pitfalls to Avoid

1. Do not use T-DXd in patients with prior ILD history or current ILD—this was an exclusion criterion 5

2. Do not skip CDK4/6 inhibitor before T-DXd—70% of DESTINY-Breast06 patients had prior CDK4/6i, and benefit was consistent 1

3. Do not confuse treatment lines: DESTINY-Breast06 = no prior metastatic chemotherapy; TROPION-Breast01 = 1-2 prior metastatic chemotherapy lines

4. Do not neglect prophylactic measures with Dato-DXd: Steroid mouthwash strongly recommended (not just advised) to mitigate 57.2% oral mucositis rate 3

5. Do not assume HER2 status is static: 38% discordance between primary and relapse; consider repeat biopsy if HER2-0 initially 1

6. For TNBC HER2-low: Do not automatically choose T-DXd—SG has more robust evidence and should be considered first 1

Strength of Evidence Considerations

T-DXd advantages:

• Larger trial (866 patients)
• Includes HER2-ultralow population
• Earlier treatment line (more clinically impactful)
• Explicit guideline endorsement as standard of care 1
• Mature PFS data with OS trending positive

Dato-DXd advantages:

• Lower ILD incidence (3.9% vs 11.3%)
• Activity in HER2-0 disease (not just HER2-low)
• Less hematologic toxicity than chemotherapy
• Fills treatment gap after chemotherapy failure

The evidence hierarchy clearly favors T-DXd for earlier-line HER2-low disease based on guideline prioritization, trial design in less pretreated population, and superior PFS/OS outcomes. 1, 2