Triple Negative Breast Cancer in 2026: ADCs Move to the Front, Immunotherapy Stays Central, and Biomarkers Still Lag Behind

Triple Negative Breast Cancer in 2026: ADCs Move to the Front, Immunotherapy Stays Central, and Biomarkers Still Lag Behind

Triple negative breast cancer is no longer defined only by what it lacks. In 2026, antibody-drug conjugates are moving into the first-line metastatic setting, immunotherapy's role in early-stage disease is being recalibrated, and biomarker-driven treatment selection remains an urgent unmet need. Dr. Nikitha Vobugari reviews the latest evidence and what it means for clinical practice.

Introduction: From an IHC Definition to a Heterogeneous Disease

Triple negative breast cancer (TNBC) has undergone a fundamental reconceptualization in recent years. What began as a disease defined purely by what it lacks (no estrogen receptor, no progesterone receptor, no HER2 amplification) is now understood as a molecularly and genomically heterogeneous group of diseases with heterogeneous outcomes. Early-stage survival is improving. Metastatic disease, however, continues to carry very limited survival and remains one of the most urgent unmet needs in breast oncology.

The two most important developments of the last twelve months in TNBC are: (1) the emergence of antibody-drug conjugates (ADCs) in the first-line metastatic setting, and (2) ongoing refinement of immunotherapy's role in early-stage disease, particularly the still-unresolved question of what to do after pembrolizumab-based neoadjuvant therapy.

Biomarker Assessment: The Non-Negotiable Starting Point

Before making any treatment decision in metastatic TNBC, comprehensive biomarker assessment is mandatory:

• PD-L1 status (CPS score, using 22C3 assay): In the metastatic setting, CPS ≥10 is considered positive and CPS <10 is considered negative for pembrolizumab eligibility.
• Germline genetic testing: All patients with TNBC should have germline BRCA1, BRCA2, and PALB2 testing if not already done.
• Reconfirm receptor status from a metastatic site: HER2 status in particular can evolve, and HER2-low and HER2-ultralow classifications now carry treatment implications in second/later line setting that did not exist several years ag

First-Line Metastatic: A Rapidly Evolving Three-Option Landscape

Metastatic TNBC remains aggressive, with a substantial proportion of patients not receiving second-line therapy and a high risk of early mortality. Antibody–drug conjugates have demonstrated meaningful survival benefits in later treatment lines in TNBC and are now being incorporated earlier in treatment paradigms, reshaping management strategies.

Pembrolizumab Plus Chemotherapy (PD-L1 CPS ≥10)

Pembrolizumab plus chemotherapy (nab-paclitaxel or paclitaxel or gemcitabine/carboplatin) established a standard from KEYNOTE-355 trial for patients with CPS ≥10, producing meaningful improvements in both PFS and OS. This remains the current standard for PD-L1 positive disease.

ASCENT 04/ KEYNOTE-D19: Sacituzumab Govitecan Plus Pembrolizumab

This was among the most important breast cancer presentations at ASCO 2025, published in NEJM 2026. ASCENT 04 enrolled previously untreated locally advanced or metastatic TNBC patients — including those who relapsed more than six months from curative-intent therapy with PD-L1 positive (CPS ≥10 by 22C3 assay). The experimental arm used sacituzumab govitecan (SG) plus pembrolizumab; the control arm was standard chemotherapy plus pembrolizumab (essentially the KEYNOTE-355 regimen).

With a median follow-up of 14 months, the SG-plus-pembrolizumab arm produced a median PFS of 11.2 months versus 7.8 months with standard chemoimmunotherapy — a 35% risk reduction in disease progression. The median duration of response was 16.5 months, which is notably durable for metastatic TNBC. Overall survival data are still immature, but importantly, 81% of control arm patients who progressed received SG in the second line — meaning the OS analysis will need to account for significant crossover.

Key toxicities with this regimen: neutropenia (approximately 40%) and diarrhea are prominent — both known from prior SG experience. Proactive G-CSF prophylaxis and a low threshold for antibiotic use are essential in clinical management of this combination. The combination regimen was associated with manageable toxicities similar to each of the drug separately.

This regimen is not yet FDA approved as of this writing, though it is now included in NCCN guidelines. It provides longer PFS, more durable responses, and lower discontinuation rates versus standard chemoimmunotherapy. The OS data maturation and FDA review will be watched closely.

PD-L1 Low/Negative Disease: ADCs Moving Up the Ladder

For patients with CPS <10, immunotherapy has not been beneficial, and chemotherapy has been the standard of care — producing a dismal median PFS of approximately 5 months. Two ADC trials presented in the last year are now reshaping this landscape.

ASCENT-03 (SG vs. chemotherapy, first-line, including patients who relapsed ≥6 months from curative intent): Significant PFS improvement with SG with 38% reduction of disease progression compared to chemotherapy. Crossover to SG in the second line was allowed from the control arm — important context for OS interpretation.

Tropion Breast-02 (datopotamab deruxtecan/Dato-DXd vs. chemotherapy, first-line): Dato-DXd targets TROP2, a different target from Trop-2 directed SG. Significant PFS improvement with Dato-Dxd with 43% reduction of disease progression compared to chemotherapy. Median OS was a co-primary endpoint and was 23 months versus 18.7 months with chemotherapy. This is a meaningful difference in a disease where prior first-line chemotherapy-only approaches produced far shorter survival.

Both trials demonstrate significant improvement over chemotherapy. Neither is FDA approved yet in this frontline setting, but both are appearing in NCCN guidelines as emerging options.

Side-by-side toxicity considerations: Dato-DXd produced grade 3+ toxicities in approximately 33% of patients — impressive compared to historical chemotherapy data. Important specific toxicities include stomatitis (grade 3 approximately 10%, requiring prophylactic steroid mouthwash) and ocular toxicities including keratitis, which requires ophthalmology baseline evaluation, avoidance of contact lenses, and prophylactic eye drops. ILD rates were lower than seen with trastuzumab deruxtecan. SG's primary toxicities remain neutropenia and diarrhea.

When both options become available, how do I choose? Treatment selection is driven largely by differences in toxicity profiles, dosing schedules, and patient-specific factors. I would present all three options once FDA-approved and individualize based on patient toxicity profile preferences, comorbidities, and PD-L1 status. The ADC-after-ADC sequencing question — what to do when a patient progresses after a first-line ADC — is a major unanswered question that will require prospective data.

Early-Stage TNBC: Neoadjuvant Pembrolizumab and the Residual Disease Problem

The current standard for stage 2 and stage 3 TNBC is neoadjuvant chemoimmunotherapy (pembrolizumab plus chemotherapy per KEYNOTE-522 trial regimen) followed by adjuvant pembrolizumab for up to one year. This approach improves pathological complete response (pCR) rates, event-free survival, and overall survival.

The Adjuvant Pembrolizumab Question: Who Needs It After pCR?

An exploratory analysis of KEYNOTE-522 suggests that adjuvant pembrolizumab adds disease-free survival benefit primarily in patients with residual cancer burden (RCB 2 and 3) rather than those who achieved pCR (RCB 0). A meta-analysis of neoadjuvant immunotherapy trials across nine RCTs confirmed improved pCR rates with immunotherapy addition, but EFS benefit in the adjuvant continuation was concentrated in patients with residual disease.

The OptimICE-PCR trial is currently evaluating whether observation is non-inferior to adjuvant pembrolizumab in patients who achieve pCR — a clinically important question given the toxicity, cost, and inconvenience of a full year of pembrolizumab in patients who are potentially cured with surgery.

My current approach: For patients with stage II–III triple-negative breast cancer who achieve a pCR after neoadjuvant pembrolizumab-based chemotherapy, I continue to discuss completing adjuvant pembrolizumab, as the trial evidence for de-escalation in this setting is not yet mature; however, if tolerance is an issue, it is reassuring that the available data suggest adjuvant pembrolizumab may not be adding substantial benefit in pCR patients, and discontinuation can be considered. For those with residual disease (RCB-II or RCB-III), the evidence clearly supports continuing adjuvant pembrolizumab.

Residual Disease: Capecitabine, Olaparib, and Pembrolizumab

For patients with residual disease after neoadjuvant chemoimmunotherapy, capecitabine (CREATE-X) and olaparib (OlympiA for BRCA1/2 carriers) have demonstrated survival benefits. The optimal sequencing strategies of these agents are not yet well defined. The phase 3 OptimICE-RD/ASCENT -05 clinical trial is evaluating SG with pembrolizumab versus pembrolizumab with or without capecitabine in this setting.

My current approach is individualized:

• For BRCA1/2-positive patients with residual disease: I prioritize olaparib, and consider continuing pembrolizumab, with capecitabine at the end.
• For non-BRCA patients: I would discuss either sequencing or concurrent capecitabine with continuing pembrolizumab, though combination data are limited.

Patients Who Cannot Receive Anthracyclines

For patients with contraindications to anthracyclines, carboplatin-based regimens have demonstrated reasonable pCR rates. In my practice, I would consider docetaxel/carboplatin or weekly paclitaxel/carboplatin for four-six cycles in this group. The phase 3 SCARLET trial is evaluating these regimens with immunotherapy in a phase 3 setting, and the Tropion Breast 04 trial is evaluating Dato-DXd completely replacing the chemotherapy backbone — results anticipated in the next few years.

The ER-Low and HER2-Ultralow Subcategories: Evolving Recognition

Two evolving subcategories deserve recognition:

ER-low breast cancer (ER 1–10%) is increasingly recognized as an aggressive phenotype that more closely resembles ER-negative disease in its biology. Oncotype scores are systematically higher in ER-low versus ER >10% tumors. Immunotherapy improved pCR rates in ER-low disease. The clinical implication: treat ER-low tumors more like TNBC than like luminal B disease.

HER2-ultralow (IHC 1+ with no staining in >0% and <10% of cells, or faint incomplete membrane staining in ≤10% of cells): Emerging data suggest that trastuzumab deruxtecan may be active in this population, expanding the potential ADC benefit beyond HER2-low. This is a growing area of clinical trial investigation.

The TIL Question in Stage 1 Disease

Stage 1 TNBC is among the most challenging groups because patients are largely excluded from large RCTs. A recent systematic review and meta-analysis of approximately 16,000 non-randomized stage 1 TNBC patients where about 12000 patients did not receive chemotherapy found that overall survival was significantly higher with chemotherapy treatment in this group — but with the effect concentrated in T1b (tumor 5-10mm) rather than T1a (≤5mm). In T1a tumors, chemotherapy did not appear to significantly improve outcomes. This is concordant with current international guidelines for chemotherapy recommendations.

Tumor-infiltrating lymphocyte (TIL) percentage is emerging as a potential biomarker in T1 disease: in T1b tumors, TIL ≥50% was predictive of significantly better event-free survival without chemotherapy. This is not yet a validated clinical tool but is being incorporated into prospective trials and is informing increasingly nuanced discussions about T1 management.

Clinical Implications and Practice Takeaways

• Reconfirm receptor status and HER2 from a metastatic site; PD-L1 CPS and germline testing are mandatory.
• In PDL-L1 positive metastatic TNBC, SG plus pembrolizumab (ASCENT-04/KEYNOTE-D19) provides superior first-line PFS (11.2 vs. 7.8 months) over standard chemoimmunotherapy in TNBC. Pending OS results and FDA approval, both SG or chemo + pembro remain options.
• In PD-L1 low/negative metastatic TNBC, both ASCENT-03 (SG) and Tropion Breast 02 (Dato-DXd) demonstrated first-line ADC superiority over chemotherapy in terms of PFS with mature OS results in Dato-Dxd.
• Dato-DXd requires ophthalmology baseline, contact lens avoidance, and prophylactic eye drops; proactive stomatitis management with steroid mouthwash is essential.
• Adjuvant pembrolizumab continuation adds DFS benefit primarily in patients with residual disease (RCB 2/3) after neoadjuvant therapy; OptimICE-PCR trial is evaluating observation versus continuation in pCR patients.
• In patients with residual disease, capecitabine and olaparib have demonstrated survival benefits, although optimal sequencing strategies are not well defined.
• ER-low tumors (ER 1–10%) behave more like TNBC and should be treated accordingly.

For Patients: What This Means for Your Care

If you have triple negative breast cancer, the treatment options have expanded dramatically in the last few years. Antibody-drug conjugates — targeted therapies that deliver chemotherapy directly to cancer cells — are now moving into earlier lines of treatment, and immunotherapy plays a central role in early-stage disease. Whether you have early-stage or metastatic TNBC, ask your oncologist about biomarker testing including germline BRCA status, PD-L1 CPS, and HER2 status from your most recent tumor sample. These results will directly determine which treatments are most appropriate for your specific cancer.

Key Takeaways

• Antibody–drug conjugates are reshaping metastatic TNBC treatment. SG plus pembrolizumab represents a new first-line standard-in-waiting for metastatic TNBC, with superior PFS over standard chemoimmunotherapy.
• In PD-L1 low/negative disease, both SG and Dato-DXd demonstrate first-line superiority over chemotherapy; FDA approval pending for both indications.
• Biomarker-driven treatment selection (PD-L1, BRCA, HER2-low) is essential at every decision point.
• Sequencing ADC after ADC remains an unanswered question awaiting prospective studies.
• Immunotherapy remains central in majority of stage II-III TNBC. Adjuvant pembrolizumab continuation has clearest benefit in patients with residual disease after neoadjuvant therapy; OptimICE-PCR will help clarify its role in pCR patients. BRCA1/2 carriers with residual disease should receive olaparib with or without other adjuvant options.
• ER-low (1–10%) tumors should be treated as TNBC rather than luminal disease.
• Given the heterogeneous outcomes in TNBC, there remains a critical need for biomarkers to guide both treatment escalation and de-escalation strategies.
• TIL percentage and circulating tumor deoxyribonucleic acid (ctDNA) assays as prognostic and predictive biomarkers are promising but not yet approved decision tools.

References

1. KEYNOTE-522. Pembrolizumab plus chemotherapy for early TNBC. New England Journal of Medicine, 2022; 5-year update 2025.
2. ASCENT-04/KEYNOTE-D19. Sacituzumab govitecan plus pembrolizumab vs. chemoimmunotherapy in first-line metastatic TNBC. New England Journal of Medicine, 2026.
3. ASCENT-03. Sacituzumab govitecan vs. chemotherapy in first-line metastatic TNBC. Presented at ASCO, 2025.
4. Tropion Breast 02. Datopotamab deruxtecan vs. chemotherapy in first-line HER2-negative metastatic breast cancer. Lancet, 2025.
5. OlympiA trial. Olaparib in BRCA1/2 mutated early breast cancer. New England Journal of Medicine, 2021; OS update 2023.
6. CREATE-X trial. Capecitabine in TNBC with residual disease. New England Journal of Medicine, 2017.

About the Author

Nikitha Vobugari, MD, is an Assistant Professor in the Division of Hematology, Oncology and Transplantation at the University of Minnesota in Minneapolis. She is a medical oncologist specializing in breast cancer with a research focus on patient-centered, evidence-based, and outcomes-based approaches to improving cancer care. Her work integrates artificial intelligence, machine learning, and clinical informatics to address disparities in cancer treatment and survivorship. She has published research in cardio-oncology, cancer genomics, and the application of predictive modeling in medical oncology. She is a faculty member of the University of Minnesota Masonic Cancer Center.