Triple-Negative Breast Cancer Treatment: Immunotherapy with Chemotherapy and Who Qualifies Based on PD-L1 Testing

Research-informed explainer — last updated April 12, 2026

Triple-negative breast cancer (TNBC) — the subtype defined by the absence of estrogen receptor, progesterone receptor, and HER2 overexpression — is the most aggressive common breast cancer subtype, but pembrolizumab combined with chemotherapy is now standard neoadjuvant treatment for early-stage disease regardless of PD-L1 status, and for metastatic disease in PD-L1-positive patients. Patients with BRCA mutations have an additional option in olaparib maintenance, and those with high tumor-infiltrating lymphocyte counts have a particularly favorable prognosis.

The research defining current TNBC treatment comes from oncologists at major academic cancer centers. Lajos Pusztai, Professor of Medicine and Scientific Co-Director of the Center for Breast Cancer at Yale Cancer Center, is a co-author on KEYNOTE-522 — the trial establishing pembrolizumab plus chemotherapy as neoadjuvant standard in early TNBC (2,947 citations) — and published the landmark paper showing that pathologic complete response (pCR) after neoadjuvant therapy predicts long-term survival in TNBC (2,876 citations). Melinda Telli, Director of the Breast Cancer Program at Stanford Cancer Institute, published the HRD score validation study for platinum sensitivity in TNBC (1,128 citations) and was first author of IMpassion031 testing neoadjuvant atezolizumab plus chemotherapy in TNBC (1,030 citations). Joseph Sparano, Chief of Hematology and Medical Oncology at Mount Sinai, has established the prognostic value of tumor-infiltrating lymphocytes (TILs) in TNBC (1,327 citations) and co-authored international TIL recommendations (3,065 citations). Antonio Wolff, at Johns Hopkins Bayview, has defined HER2 testing standards — the tests that establish the "triple negative" diagnosis by confirming HER2 is NOT overexpressed (4,462 citations for the initial ASCO/CAP guideline and 4,028 citations for the update). Daniel Haber at Massachusetts General Hospital contributed research on circulating tumor cell clusters in breast cancer (2,442 citations), relevant to how TNBC is monitored during and after treatment.

What triple-negative means and why it matters

Breast cancers are classified by whether they express three receptors:

• Estrogen receptor (ER): Drives cell growth in response to estrogen
• Progesterone receptor (PR): Drives cell growth in response to progesterone
• HER2 (human epidermal growth factor receptor 2): A growth factor receptor that when overexpressed drives rapid cell division

TNBC is negative for all three — which means hormone-targeted therapies (tamoxifen, aromatase inhibitors) and HER2-targeted therapies (trastuzumab, pertuzumab) do not work. This left cytotoxic chemotherapy as the only systemic treatment for decades.

Antonio Wolff's ASCO/CAP guidelines define the testing standards that establish HER2-negativity — critical to confirm TNBC diagnosis and avoid misclassifying a HER2-positive tumor. A technically adequate HER2 test is the starting point for the triple-negative designation.

TNBC accounts for approximately 10–15% of all breast cancers and disproportionately affects younger women, Black women, and BRCA1 mutation carriers.

PD-L1 testing in TNBC: which patients need it and when

TNBC tumors are more immunogenic than other breast cancer subtypes — they have more tumor mutations, more infiltrating T cells, and higher rates of PD-L1 expression. Joseph Sparano's TIL research showed that higher TIL counts in TNBC correlate with better responses to chemotherapy and better overall survival, even before immunotherapy was available.

For early-stage TNBC (stages II–III): KEYNOTE-522 — the trial Lajos Pusztai contributed to — enrolled 1,174 patients with stage II or III TNBC and randomized them to pembrolizumab plus chemotherapy or placebo plus chemotherapy as neoadjuvant treatment, followed by pembrolizumab or placebo as adjuvant therapy after surgery. The pembrolizumab group had significantly higher pCR rates (64.8% vs 51.2%) and significantly better event-free survival. Critically, this benefit was seen across PD-L1 subgroups — PD-L1 testing is not required to determine eligibility for pembrolizumab in early TNBC. Pembrolizumab plus standard chemotherapy is now the standard neoadjuvant approach for stage II–III TNBC.

For metastatic TNBC: PD-L1 testing using the CPS (combined positive score) is required. In the KEYNOTE-522-derived metastatic setting (KEYNOTE-355), pembrolizumab plus chemotherapy improved progression-free survival and overall survival specifically in patients with CPS ≥ 10. Patients with CPS below 10 did not derive significant benefit. PD-L1 CPS testing on the biopsy specimen is therefore essential before initiating immunotherapy for metastatic TNBC.

Pathologic complete response: the short-term goal with long-term implications

Pusztai's 2008 Journal of Clinical Oncology paper established that patients who achieve a pathologic complete response (pCR) — meaning no viable tumor cells remain in the breast or lymph nodes at surgery after neoadjuvant chemotherapy — have dramatically better long-term survival than those with residual disease. In TNBC, the difference is stark: pCR correlates with approximately 90% 5-year disease-free survival versus approximately 50–60% in those with residual disease.

This means the neoadjuvant response — what happens at surgery — is a meaningful early read on long-term prognosis. Patients who achieve pCR after pembrolizumab plus chemotherapy have an excellent long-term outlook. Patients with residual disease after neoadjuvant treatment are candidates for adjuvant capecitabine (CREATE-X trial), which further reduces recurrence risk.

BRCA testing and olaparib: an additional targeted option

Approximately 15–20% of TNBC patients carry germline BRCA1 or BRCA2 mutations. For these patients, the OlympiAD and EMBRACA trials demonstrated that PARP inhibitors olaparib and talazoparib, respectively, outperformed single-agent chemotherapy in metastatic TNBC. Germline BRCA testing is now recommended for all metastatic TNBC patients.

Melinda Telli's HRD score work extends this: even BRCA-wildtype TNBC patients may have HRD from other causes, and HRD-positive tumors respond better to platinum-based chemotherapy (carboplatin) — making HRD testing relevant for treatment selection in neoadjuvant regimens.

Sacituzumab govitecan: an ADC for pretreated metastatic TNBC

Antibody-drug conjugates (ADCs) have reshaped the metastatic TNBC treatment landscape. Sacituzumab govitecan (Trodelvy) — an anti-Trop-2 ADC — produced overall survival of 12.1 months versus 6.7 months for physician's choice chemotherapy in the ASCENT trial for pretreated metastatic TNBC. It is now FDA-approved for second-line+ metastatic TNBC. This is not immunotherapy, but it represents a major advance for patients who progress after pembrolizumab plus chemotherapy.

Monitoring TNBC after treatment: circulating tumor cells

Daniel Haber's research on circulating tumor cell (CTC) clusters in breast cancer showed that CTC clusters — multiple circulating tumor cells traveling together — are oligoclonal precursors of metastasis with higher metastatic efficiency than single CTCs. CTC monitoring is used in some settings to track treatment response in metastatic TNBC, though its role in clinical decision-making beyond research contexts is still evolving.

Questions to ask your doctor

• Has my tumor been definitively confirmed as triple-negative with proper ER, PR, and HER2 testing according to ASCO/CAP guidelines?
• For my stage (early vs. metastatic), which chemotherapy backbone is planned alongside pembrolizumab, and for how long?
• Has my tumor been tested for PD-L1 CPS if I have metastatic disease — and what is the score?
• Have I been offered germline BRCA testing? If my tumor is BRCA-mutated, am I eligible for olaparib?
• If I achieve a pathologic complete response after neoadjuvant treatment, what is my long-term prognosis? If I don't, what adjuvant options — such as capecitabine — are available?

The bottom line

TNBC treatment has changed significantly in recent years, moving from pure chemotherapy to immunotherapy-chemotherapy combinations for both early and metastatic disease. For patients with early-stage TNBC, pembrolizumab plus standard neoadjuvant chemotherapy is now the standard of care, regardless of PD-L1 status. For metastatic disease, PD-L1 CPS testing determines immunotherapy eligibility. BRCA testing opens the door to PARP inhibitor therapy. Newly diagnosed TNBC patients should confirm that all relevant biomarker testing — HER2, PD-L1, germline BRCA — has been completed before finalizing their treatment plan.