How KEYNOTE-522 Permanently Changed Triple-Negative Breast Cancer Care

The landmark phase 3 KEYNOTE-522 trial (NCT03036488) established a new standard of care for patients with previously untreated, high-risk, early-stage triple-negative breast cancer (TNBC).¹ The study evaluated a perioperative approach by combining pembrolizumab (Keytruda; an anti–PD-1 therapy) with neoadjuvant chemotherapy, followed by adjuvant pembrolizumab monotherapy after definitive surgery.

A total of 1174 patients were randomized in a 2:1 ratio to receive either pembrolizumab (n = 784) or a placebo (n = 390). In the neoadjuvant phase, both arms received a robust backbone consisting of 4 cycles of paclitaxel plus carboplatin, followed by 4 cycles of an anthracycline-based regimen (doxorubicin or epirubicin plus cyclophosphamide). Following surgical resection, patients continued their assigned treatment (adjuvant pembrolizumab or placebo) for an additional 9 cycles to complete approximately one year of immunotherapy. The trial's dual primary end points were pathologic complete response (pCR)—defined as the absence of invasive cancer in the breast and lymph nodes (ypT0/Tis ypN0)—and event-free survival (EFS). Overall survival (OS) served as the crucial secondary end point.

Final Long-Term Analysis Results (7.8-Year Follow-up)

At the definitive data cutoff on October 14, 2025, the trial reached a median follow-up of 93.8 months (approximately 7.8 years), providing highly mature survival metrics.² The final analysis confirmed that the clinical benefits of adding pembrolizumab are both durable and statistically profound:

• Event-Free Survival: The 7-year EFS rate reached 78.3% in the pembrolizumab cohort compared to 69.8% in the placebo group. This represents a 32% reduction in the risk of disease progression, local/distant recurrence, second primary cancer, or death (HR, 0.68; 95% CI, 0.54-0.86).
• Overall Survival: The 7-year OS rate was 85.1% for the pembrolizumab group vs 77.2% for the placebo group. This demonstrates a clear, long-term survival advantage, reducing the risk of death by 36% (HR, 0.64; 95% CI, 0.49-0.85).

The survival advantages spanned across most pre-specified subgroups, demonstrating that therapeutic efficacy is independent of nodal involvement, clinical disease stage, or PD-L1 expression levels.

Regarding safety, the addition of pembrolizumab slightly increased severe toxicity, with grade ≥3 treatment-related adverse events (AEs) occurring in 77.1% of the pembrolizumab group compared WITH 73.3% of the placebo group. Treatment-related fatalities were rare (0.5% vs 0.3%). However, immune-mediated AEs of any grade were substantially higher in the pembrolizumab arm (35.0% vs 13.1%), reflecting the expected safety profile of checkpoint inhibitors.

Historical Context and Evolution of KEYNOTE-522 Data

The final 7.8-year results are the culmination of a series of interim analyses that progressively shaped international clinical guidelines. Tracking the historical data underscores how early surrogate endpoints translated into long-term survival benefits.

1. Initial Pathologic Complete Response (pCR) Discoveries

In the trial's first formal publication, the addition of pembrolizumab demonstrated a statistically significant and unprecedented surge in pCR. The early evaluation revealed a pCR rate of 64.8% in the chemoimmunotherapy arm compared with 51.2% in the chemotherapy-alone control arm—a definitive absolute improvement of 13.6 percentage points.³ Later evaluations settled the final trial-wide pCR comparison at 63.0% vs 55.6%.

2. The 3-Year (Fourth Interim) Analysis

At a median follow-up of 39.1 months, the trial proved that improvements in pCR directly translated to a reduction in clinical recurrences. The 3-year EFS rate was 84.5% for the pembrolizumab cohort compared with 76.8% for the placebo cohort (HR, 0.63; 95% CI, 0.48–0.82), which triggered full FDA approval in July 2021.

Crucially, exploratory analyses highlighted the prognostic importance of residual disease via the residual cancer burden (RCB) index:

• With pCR (RCB-0): Patients achieving a pCR had excellent 3-year outcomes across both arms (94.4% with pembrolizumab vs. 92.5% with placebo) (Santa-Maria et al., 2022).
• Without pCR (Residual Disease): For patients who failed to achieve a pCR, pembrolizumab acted as a vital buffer. The 3-year EFS for non-responders was 67.4% in the pembrolizumab arm versus a dismal 56.8% in the placebo arm, proving that perioperative immunotherapy remains highly beneficial even when residual tumor cells are found at surgery (Santa-Maria et al., 2022).

3. The 5-Year (75-Month) Analysis

Presented at the 2024 ESMO Congress, the 5-year data offered the first definitive look at overall survival. At a median follow-up of 75.1 months, the 5-year EFS rate maintained a stark gap at 81.2% vs 72.2% (HR, 0.65). More importantly, it established a statistically significant OS advantage, showing a 5-year survival rate of 86.6% for pembrolizumab vs 81.7% for the control group (HR, 0.66; P =.00150).

This trajectory confirms that the absolute benefit of pembrolizumab remains stable over nearly 8 years, cementing the perioperative regimen's curative potential in high-risk early TNBC.

REFERENCES

1. Hu H, Kaklamani V. Updates on the preoperative immunotherapy for triple-negative breast cancer. Transl Breast Cancer Res. 2023 Apr 30;4:17. doi: 10.21037/tbcr-23-16. PMID: 38751476; PMCID: PMC11092992.

2. Schmid P et al. Neoadjuvant pembrolizumab or placebo plus chemotherapy followed by adjuvant pembrolizumab or placebo for high-risk early-stage TNBC: Final analysis results from the phase 3 KEYNOTE-522 study. J Clin Oncol. 44, 2026 (suppl 16; abstr 507)
3. Schmid P, Cortes J, Pusztai L, et al. Pembrolizumab for Early Triple-Negative Breast Cancer. N Engl J Med. 2020 Feb 27;382(9):810-821. doi: 10.1056/NEJMoa1910549. PMID: 32101663.