64.0 percent had a pathological complete response on pembrolizumab, 54.7 percent on placebo, in TNBC

In the Phase III KEYNOTE-522 trial of triple negative breast cancer (TNBC), 64.0 percent of participants given pembrolizumab plus chemotherapy before surgery had a pathological complete response, against 54.7 percent given placebo plus chemotherapy, the study record shows. The pathological complete response analysis covered the first 1002 participants randomly assigned. The record from Merck Sharp & Dohme LLC now holds the final event-free survival and overall survival results for all 1,174 randomized participants.

Why it matters

The DailyMed label for KEYTRUDA QLEX (pembrolizumab and berahyaluronidase alfa-pmph), version published Aug 12, 2026, lists adult patients with high-risk early-stage triple-negative breast cancer (TNBC). The label says the use is in combination with chemotherapy as neoadjuvant treatment, then continued as a single agent as adjuvant treatment after surgery. The study record lists pembrolizumab as an intravenous (IV) infusion. The record says the trial tested whether pembrolizumab is superior to placebo, with chemotherapy, on the rate of pathological complete response and/or event-free survival in participants with locally advanced TNBC.

Pathological complete response

The study record shows results first posted on October 22, 2026. The measure covered the first 1002 participants randomly assigned to study treatment, assessed at the time of definitive surgery, up to approximately 27-30 weeks.

The record defines pathological complete response (ypT0/Tis ypN0) as the percentage of participants without residual invasive cancer on hematoxylin and eosin evaluation of the complete resected breast specimen and all sampled regional lymph nodes following completion of neoadjuvant systemic therapy by current American Joint Committee on Cancer (AJCC) staging criteria, assessed by the local pathologist at the time of definitive surgery.

Pembrolizumab + Chemotherapy: 64.0 percent (95% confidence interval 60.2 to 67.6), 669 participants analyzed.
Placebo + Chemotherapy: 54.7 percent (95% confidence interval 49.1 to 60.1), 333 participants analyzed.
Difference in percentage: 9.2 (95% confidence interval 2.8 to 15.6), one-sided p-value 0.00221, stratified Miettinen & Nurminen method.

Event-free survival and overall survival

The record lists event-free survival and overall survival for all randomized participants, 784 on pembrolizumab plus chemotherapy and 390 on placebo plus chemotherapy, with data up to approximately 102 months.

Event-free survival: median not reached in either arm. Hazard ratio 0.68 (95% confidence interval 0.54 to 0.86).
For overall survival, the median was not reached in either arm. Hazard ratio 0.64 (95% confidence interval 0.49 to 0.85).
For overall survival in participants with PD-L1 positive tumors (CPS ≥ 1): 656 on pembrolizumab, 316 on placebo. Hazard ratio 0.69 (95% confidence interval 0.51 to 0.95).

The record defines event-free survival as the time from randomization to the first occurrence of progression of disease that precludes definitive surgery, local or distant recurrence, second primary malignancy, as assessed by Investigator, or death due to any cause. The record reports one-sided p-values for both analyses and adds: "No formal statistical testing was performed for this updated analysis, and therefore, the p-value is nominal."

Study design and participants

KEYNOTE-522 is a randomized, double-blind, parallel study. Participants received 200 mg pembrolizumab or placebo every 3 weeks with paclitaxel and carboplatin for 4 cycles, then with doxorubicin or epirubicin plus cyclophosphamide for 4 cycles, about 24 weeks in all. Surgery followed 3-6 weeks later. After surgery, participants received 9 cycles of pembrolizumab or placebo, about 27 weeks.

The record lists 1174 participants randomized and 1172 treated. Of those, 784 were assigned to pembrolizumab plus chemotherapy and 390 to placebo plus chemotherapy. The mean age was 49.2 years in the pembrolizumab arm and 49.1 years in the placebo arm. The record lists 1173 women and 1 man, 605 participants with positive nodal status and 304 with T3/T4 tumors. The study began on March 7, 2017, and the record lists completion on October 14, 2025.

Safety

The record counts participants with at least one event across the neoadjuvant and adjuvant phases combined, and it counts each of these measures over its own time window, which differs from measure to measure.

Adverse event: 777 of 783 participants on pembrolizumab plus chemotherapy and 389 of 389 on placebo plus chemotherapy.
Discontinued study treatment due to an adverse event: 233 of 783 and 60 of 389.
Serious adverse event: 341 of 783 and 111 of 389.
All-Cause Mortality: 120 of 784 and 90 of 390.