In metastatic TNBC, immunotherapy plus chemo is now the baseline—but a new ADC is challenging it
The evidence standard for first-line treatment of metastatic triple-negative breast cancer (mTNBC) has moved from chemotherapy alone to immunotherapy combined with chemotherapy. The phase III KEYNOTE-355 trial established pembrolizumab (an anti-PD-1 immune checkpoint inhibitor) plus chemotherapy as a standard, showing a median progression-free survival of 9.7 months versus 5.6 months with chemo alone in PD-L1–positive tumors—a gain of about 4 months [5][6]. This is the benchmark against which new options are measured.
But a 2026 phase III trial of izalontamab brengitecan (iza-bren), a bispecific antibody-drug conjugate targeting EGFR and HER3, reported a median progression-free survival of 8.5 months versus 3.1 months with physician's choice of chemo (eribulin, capecitabine, gemcitabine, or vinorelbine) in patients who had already progressed after 1–2 prior lines—a hazard ratio of 0.29, meaning an approximately 71% reduction in the risk of progression [1]. Overall survival was also longer: 15.9 months versus 12.5 months [1]. This suggests that for patients who progress on first-line immunotherapy, a targeted ADC may outperform standard chemo, potentially shifting the second-line standard.
However, the two studies are not directly comparable: KEYNOTE-355 was first-line, while iza-bren was tested in later lines. The iza-bren trial included patients who had or had not received prior immunotherapy, and its benefit was seen across that mixed population [1]. So while immunotherapy plus chemo remains the first-line standard, the evidence is moving toward a more personalized sequence: immunotherapy first, then an ADC like iza-bren for those who progress.
In early-stage TNBC, neoadjuvant immunotherapy plus chemo is the new standard—but toxicity is a concern
For early or locally advanced TNBC, the standard has shifted to neoadjuvant (pre-surgery) immunotherapy combined with chemotherapy, based on the KEYNOTE-522 trial, which showed improved pathological complete response (pCR) rates and event-free survival [5]. A recent phase 2 trial (NeoTOP) tested a de-escalated regimen of low-dose carboplatin, docetaxel, and the PD-1 inhibitor toripalimab, and reported a pCR rate of 58.0%—meaning that in 29 of 50 patients who went to surgery, no cancer was found in the breast or lymph nodes [2]. The objective response rate was 90.2% [2].
But the toxicity of these regimens is a real issue. In the NeoTOP trial, all 51 patients experienced some treatment-related adverse event, and 45.1% had grade 3 or higher toxicity [2]. The standard KEYNOTE-522 regimen is known to be more intensive, and a 2026 review highlights the need to optimize timing, duration, and dose to reduce side effects while maintaining benefit [5]. This is an active area of research: the NeoTOP trial is one attempt to de-escalate chemo-immunotherapy without compromising outcomes, and it suggests that lower doses might be feasible, but a phase III comparison is still needed [2].
Real-world data show the standard doesn't work equally for everyone—especially rare subtypes
Clinical trial results don't always translate to routine practice, and a real-world study of metaplastic TNBC (a rare, aggressive subtype) found that pembrolizumab plus chemotherapy produced a median progression-free survival of only 6.0 months and overall survival of 14.1 months—markedly shorter than the 9.7 and 23 months seen in KEYNOTE-355 [3]. This underscores that the evidence standard is based on unselected mTNBC, but certain subtypes respond worse, and the authors call for more research in rare subtypes [3].
Another real-world challenge is patient selection. A 2022 study using PET/CT imaging found that tumor heterogeneity—measured by a novel index (IETH)—was an independent predictor of progression-free survival in mTNBC patients receiving first-line immunotherapy plus chemo: patients with low heterogeneity had a median PFS of 9.4 months versus 4.9 months for high heterogeneity [4]. This suggests that not all patients benefit equally, and tools like imaging biomarkers could help tailor treatment decisions.
The evidence overall points to a clear shift: immunotherapy plus chemo is the new standard in both early and metastatic TNBC, but the field is actively refining who gets it, at what dose, and what comes next. The new ADC data [1] and de-escalation efforts [2] are part of that evolution, but the standard is not a one-size-fits-all answer.
About These Sources
This answer is built on 6 peer-reviewed studies — published from 2022 to 2026, 4 from 2024 or later, 4 in Q1 journals, collectively cited 162 times — selected as the most relevant from 7 studies that passed quality screening, drawn from 62 papers retrieved from a database of over 500 million.
Sources used in this answer
Izalontamab brengitecan (iza-bren) versus physician’s choice of chemotherapy in patients with unresectable locally advanced or metastatic triple-negative breast cancer (TNBC): A randomized phase III study.
In a randomized phase III trial of 418 patients with advanced TNBC who had progressed after 1–2 prior lines, iza-bren (a bispecific ADC) improved median PFS to 8.5 months vs 3.1 months with chemo (HR 0.29) and median OS to 15.9 vs 12.5 months (HR 0.60), supporting it as a new standard in this later-line setting.
Neoadjuvant low-dose carboplatin and docetaxel in combination with toripalimab for early or locally advanced triple-negative breast cancer (NeoTOP): A single-arm phase 2 trial.
In a single-arm phase 2 trial (NeoTOP) of 51 patients with early-stage TNBC, neoadjuvant low-dose carboplatin, docetaxel, and toripalimab achieved a pathological complete response rate of 58.0% and an objective response rate of 90.2%, with grade ≥3 toxicity in 45.1% of patients.
Treatment outcomes with pembrolizumab and chemotherapy in metastatic metaplastic triple-negative breast cancer: Data from a central European cohort.
In a real-world cohort of 14 patients with metastatic metaplastic TNBC treated with first-line pembrolizumab plus chemotherapy, median PFS was 6.0 months and median OS was 14.1 months—shorter than KEYNOTE-355 results—highlighting the aggressive nature of this subtype.
Heterogeneity derived from <sup>18</sup>F‐FDG PET/CT predicts immunotherapy outcome for metastatic triple‐negative breast cancer patients
In a cohort of 32 mTNBC patients receiving first-line immunotherapy plus chemo, baseline tumor heterogeneity measured by 18F-FDG PET/CT (IETH) was an independent predictor of PFS: low heterogeneity was associated with 9.4 months median PFS vs 4.9 months for high heterogeneity.
Revisiting the standard of care for immune checkpoint inhibitors in early-stage triple-negative breast cancer: timing, duration, dose, combination, and patient selection
A 2026 review of early-stage TNBC immunotherapy concludes that neoadjuvant pembrolizumab plus chemo (KEYNOTE-522 regimen) is the current standard, but highlights the need to optimize timing, duration, dose, and patient selection to reduce toxicity and improve outcomes.
Progress and Prospect of Immunotherapy for Triple-Negative Breast Cancer
A 2022 review of TNBC immunotherapy notes that a subset of TNBC is immunogenic and that atezolizumab plus nab-paclitaxel was FDA-approved for PD-L1-positive metastatic TNBC, marking a new era, but also discusses challenges and future directions including ADCs and combination strategies.