Immunotherapy for Triple-Negative Breast Cancer: What the Evidence Shows

Triple-negative breast cancer (TNBC) lacks the estrogen, progesterone, and HER2 receptors that make most breast cancers treatable with targeted hormonal or anti-HER2 drugs, which has historically left chemotherapy as the only systemic option. That changed when the checkpoint inhibitor pembrolizumab (Keytruda) became the first immunotherapy FDA-approved for TNBC, first in the metastatic setting and then, more consequentially, in early-stage disease. This article walks through what the pivotal trials actually found, who currently qualifies for treatment, what the real risks and failure rates look like, and where the field is moving next — including a newly approved combination that reached patients in 2026.

Why TNBC Needed a Different Approach

TNBC accounts for roughly 10-15% of breast cancers but a disproportionate share of early relapses and deaths. Because it lacks the receptors that drugs like tamoxifen or trastuzumab target, oncologists were long limited to chemotherapy regimens that could shrink tumors but often failed to prevent recurrence in high-risk patients. TNBC tumors do tend to carry more genetic mutations and immune-cell infiltration than other breast cancer subtypes, making them a plausible — though unproven until trial data arrived — candidate for checkpoint inhibitors, the class of drugs that unleashes the immune system's own T-cells against cancer.

How Checkpoint Inhibitors Fit Into TNBC Treatment

Pembrolizumab is a PD-1 checkpoint inhibitor: it blocks a signal tumors use to make nearby immune cells stand down, removing their camouflage so T-cells can attack. Critically, pembrolizumab is not a substitute for chemotherapy in TNBC — it is added on top of it. In every approved setting, patients still receive standard chemotherapy; the immunotherapy is layered in alongside it and, in early-stage disease, continued alone afterward. This reflects exactly what the trials tested — chemotherapy plus pembrolizumab versus chemotherapy alone — not immunotherapy as a chemo-free alternative.

The Pivotal Trials: KEYNOTE-522 and KEYNOTE-355

KEYNOTE-522 tested pembrolizumab in patients with high-risk, early-stage TNBC (stage II-III), given before surgery alongside chemotherapy and continued afterward as a single agent. The headline result was a pathologic complete response (pCR) rate of 63% in the pembrolizumab arm versus 56% with chemotherapy alone — meaning more patients had no detectable invasive cancer left in their breast or lymph nodes at surgery. Longer follow-up strengthened the case further: in the trial's final analysis, event-free survival at seven years was 78.3% with pembrolizumab versus 69.8% with chemotherapy alone (hazard ratio 0.68), and overall survival was 85.1% versus 77.2% (hazard ratio 0.64) — an absolute overall survival gain of roughly 8 percentage points, reported regardless of PD-L1 status, nodal involvement, or disease stage, according to data highlighted at ASCO's 2026 meeting and published in the New England Journal of Medicine.

KEYNOTE-355 tested the combination in advanced, metastatic, or locally recurrent TNBC, and here the benefit was concentrated in patients whose tumors expressed PD-L1 at a Combined Positive Score (CPS) of 10 or higher. In that biomarker-selected group, median progression-free survival was 9.7 months with pembrolizumab plus chemotherapy versus 5.6 months with chemotherapy alone, a 35% reduction in risk of progression or death. A later survival analysis showed median overall survival of 23.0 months versus 16.1 months in the CPS≥10 population — about a 27% reduction in risk of death (hazard ratio 0.73). Unlike KEYNOTE-522, this benefit was not seen consistently across all comers; it was specifically the PD-L1-high subgroup driving the result, which is why PD-L1 testing became a gatekeeping requirement for this indication.

Current FDA-Approved Indications and Who Qualifies

As of the current KEYTRUDA prescribing information, pembrolizumab carries three related TNBC indications: (1) for high-risk, early-stage TNBC, in combination with chemotherapy as neoadjuvant treatment and then continued alone as adjuvant treatment after surgery — approved July 26, 2021, with no PD-L1 testing requirement; (2) for locally recurrent unresectable or metastatic TNBC whose tumors express PD-L1 (CPS≥10), in combination with chemotherapy — originally granted accelerated approval on November 13, 2020, and later supported by confirmatory survival data; and (3), newly, for first-line treatment of unresectable locally advanced or metastatic PD-L1-positive (CPS≥10) TNBC in combination with the antibody-drug conjugate sacituzumab govitecan-hziy (Trodelvy) rather than traditional chemotherapy, approved by the FDA on June 24, 2026, based on the ASCENT-04/KEYNOTE-D19 trial (median progression-free survival 11.2 vs. 7.8 months; hazard ratio 0.65). In short: early-stage patients can qualify regardless of PD-L1 status if their disease is high-risk, while anyone with advanced or metastatic TNBC needs a PD-L1 CPS≥10 result on an FDA-authorized test to be eligible for either pembrolizumab-containing regimen.

Real Limitations and Risks

This is not a treatment that works for everyone, and it is not free of harm. In the metastatic setting, PD-L1-low or -negative tumors — a substantial share of TNBC cases — simply do not qualify for pembrolizumab under the approved label, because the trial data did not support a benefit in that group. Even among eligible patients, response is not universal, and disease that progresses on immunotherapy still carries a poor prognosis. Toxicity is a real consideration: pembrolizumab combined with chemotherapy carries the class-wide risk of immune-related adverse events — inflammation of the thyroid, lungs, colon, liver, or adrenal and pituitary glands — some of which require corticosteroids and can become permanent, particularly certain endocrine effects. In KEYNOTE-522, serious adverse reactions occurred in 44% of patients receiving pembrolizumab, and treatment was discontinued due to adverse reactions in 20% — figures reflecting the combined chemo-immunotherapy regimen, not immunotherapy alone, but ones patients and clinicians weigh seriously against the survival benefit. It's also worth noting that a related checkpoint inhibitor, atezolizumab (Tecentriq), briefly held accelerated approval for metastatic TNBC based on the IMpassion130 trial, but its manufacturer voluntarily withdrew that indication in 2021 after a confirmatory trial failed to verify the benefit — a reminder that not every checkpoint inhibitor in this space has panned out, and that accelerated approvals are conditional for good reason.

Where Research Is Heading

The most active current direction pairs checkpoint inhibitors with newer antibody-drug conjugates rather than traditional chemotherapy backbones, exemplified by the 2026 approval of pembrolizumab plus sacituzumab govitecan for PD-L1-positive first-line metastatic TNBC. Researchers are also studying biomarkers beyond PD-L1 CPS to better predict who benefits, testing pembrolizumab-based combinations in PD-L1-negative populations who currently have no immunotherapy option, and exploring whether shorter, biomarker-guided courses of adjuvant immunotherapy could preserve benefit while reducing cumulative toxicity. Pembrolizumab remains the only checkpoint inhibitor with a durable, non-withdrawn FDA approval in TNBC as of this writing.

Bottom Line

Pembrolizumab is a genuine, evidence-backed advance for a cancer subtype that badly needed one: in high-risk early-stage TNBC it meaningfully improves both event-free and overall survival when added to standard chemotherapy, regardless of PD-L1 status, and in PD-L1-positive metastatic disease it extends survival by several months on average. But it is an addition to chemotherapy, not a replacement for it; it does not help patients with PD-L1-negative metastatic tumors under current approvals; and it carries real, sometimes lasting immune-related side effects that require monitoring. Patients considering it should discuss PD-L1 testing, eligibility, and the specific trial data behind their regimen with an oncologist experienced in current TNBC treatment guidelines.