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 321,910 new cases of female breast cancer will be diagnosed in 2026, and about 42,140 women will die of the disease, largely due to metastatic recurrence.
The Persistent Therapeutic Gap in Metastatic TNBC
Triple-Negative Breast Cancer (TNBC) which accounts for roughly 10-15% of breast cancers remains one of the most biologically aggressive breast cancer subtypes, defined by the absence of estrogen and progesterone receptor expression and lack of HER2 overexpression. Despite therapeutic advances, metastatic TNBC carries a dismal prognosis, with 5-year relative survival rates near 15%.
Approximately 40% of TNBC tumors express PD-L1 (combined positive score [CPS] ≥10). Based on results from KEYNOTE-355, the combination of Pembrolizumab (KEYTRUDA®) and chemotherapy (Taxane or Gemcitabine–Carboplatin backbone) became the standard first-line regimen for PD-L1–positive metastatic TNBC, improving both Progression-Free Survival (PFS) and Overall Survival (OS), compared with chemotherapy alone.
However, Real-World Data suggest that nearly half of patients with metastatic TNBC do not receive therapy beyond first line due to rapid progression or early mortality. This underscores the importance of maximizing disease control early in the treatment course.
The Rationale for ADC–Immunotherapy Combination
Sacituzumab govitecan (TRODELVY®) is a Trop-2–directed Antibody–Drug Conjugate (ADC) delivering the topoisomerase I inhibitor SN-38 via a hydrolyzable linker. It previously demonstrated significant survival benefit over chemotherapy in heavily pretreated metastatic TNBC (ASCENT), as well as significantly prolonged Progression-Free Survival compared with standard chemotherapy in the first-line treatment of advanced or metastatic TNBC among patients who are not candidates for PD-1/PD-L1 inhibitors (ASCENT-03).
Given the complementary mechanisms of cytotoxic payload delivery and immune checkpoint inhibition, combining Sacituzumab govitecan with Pembrolizumab represents a biologically compelling strategy to deepen and prolong response in earlier lines of therapy.
ASCENT-04/KEYNOTE-D19: Trial Design
ASCENT-04/KEYNOTE-D19 is a global, randomized, open-label Phase 3 trial conducted across 186 sites in 28 countries.
Eligible patients included:
- Adults with locally advanced unresectable or metastatic TNBC
- No prior therapy for advanced disease
- PD-L1–positive tumors (CPS ≥10, centrally confirmed using PD-L1 IHC 22C3 pharmDx assay)
A total of 443 patients were randomized 1:1:
- Sacituzumab govitecan (10 mg/kg IV days 1 and 8, q21 days) plus Pembrolizumab (200 mg IV q3 weeks)
- Physician’s choice chemotherapy plus Pembrolizumab
Stratification factors included:
- Disease status (de novo metastatic vs. early relapse)
- Geographic region
- Prior exposure to anti–PD-1/PD-L1 therapy in curative-intent setting
The Primary endpoint was PFS by Blinded Independent Central Review. Secondary endpoints included OS, Objective Response Rate (ORR), Duration of Response (DoR), and Safety.
Efficacy Outcomes
At a median follow-up of 14 months, the study met its Primary endpoint.
- Median PFS:
- 11.2 months with Sacituzumab govitecan plus Pembrolizumab
- 7.8 months with chemotherapy plus Pembrolizumab
- Hazard Ratio for progression or death: 0.65 (95% CI, 0.51–0.84; P<0.001)
Investigator-assessed PFS results were concordant. The PFS benefit was consistent across predefined subgroups, including patients with liver metastases and those with early relapse (6–12 months after curative-intent therapy). Notably, median PFS approached one year with the ADC–immunotherapy combination, exceeding historical benchmarks from chemo-immunotherapy trials in this setting.
- ORR:
- 60% with Sacituzumab govitecan plus Pembrolizumab
- 53% with chemotherapy plus Pembrolizumab
- Complete Response rate: 13% vs. 8%
- Median Duration of Response:
- 16.5 months vs. 9.2 months
Although formal hypothesis testing was not performed for secondary endpoints, responses with the ADC combination were notably more durable. Importantly, despite a higher-than-expected ORR in the control arm, responses with chemotherapy were less sustained, likely explaining the PFS separation.
OS data remain immature at this analysis (26% event rate), with medians not yet reached in either arm.
Safety and Treatment Adherence
Grade ≥3 adverse events occurred at similar rates in both groups (about 70%). However, treatment discontinuation due to adverse events was substantially lower with the ADC combination (12% vs. 31%). This difference may reflect the nature of toxicities. ADC-associated events such as diarrhea are generally manageable with supportive care, whereas chemotherapy-related toxicities such as neuropathy may be cumulative and less reversible. Adverse events leading to death occurred in 3% of patients in each arm.
Clinical Interpretation
ASCENT-04/KEYNOTE-D19 establishes the first positive Phase 3 data evaluating an ADC combined with immunotherapy in the frontline treatment of PD-L1–positive metastatic TNBC.
The magnitude of PFS benefit (HR 0.65), prolonged response durability, and lower discontinuation rate suggest that Sacituzumab govitecan plus Pembrolizumab may represent an evolution of first-line therapy in this population.
Given that many patients will not reach subsequent lines of treatment, intensifying effective therapy upfront is clinically meaningful. The ADC backbone may offer sustained tumor control beyond what is achievable with chemotherapy-based combinations.
Limitations and Ongoing Questions
Key limitations include:
- Open-label design
- Crossover to Sacituzumab govitecan in the control arm
- Immature OS data
- Limited representation of patients previously treated with adjuvant immunotherapy
Future analyses will clarify survival impact and optimal sequencing, particularly as additional ADCs enter earlier lines of therapy. Ongoing trials such as ASCENT-05 (NCT05633654) and SASCIA (NCT04595565) are evaluating Sacituzumab govitecan in high-risk early-stage settings, potentially broadening its therapeutic footprint.
Bottom Line for Practice
For patients with previously untreated, PD-L1–positive, locally advanced unresectable or metastatic TNBC, Sacituzumab govitecan plus Pembrolizumab significantly prolongs Progression-Free Survival compared with chemotherapy plus Pembrolizumab, with durable responses and manageable toxicity.
These findings position ADC–immunotherapy combinations as a potential new backbone strategy in frontline TNBC management, pending maturation of Overall Survival data.
Sacituzumab Govitecan plus Pembrolizumab for Advanced Triple-Negative Breast Cancer. Tolaney SM, de Azambuja E, Kalinsky K, et al. for the ASCENT-04/KEYNOTE-D19 Clinical Trial Investigators. N Engl J Med 2026;394:354-366.
