Preoperative radiation therapy added to anti–PD-1 immune checkpoint inhibition led to a significant increase in T-cell infiltration in patients with hormone receptor–positive/HER2-negative early-stage breast cancer, according to findings from the phase II P-RAD trial presented at the 2025 San Antonio Breast Cancer Symposium (Abstract GS2-05).
"This trial has shown that 24 Gy preoperative radiation with pembrolizumab significantly increases T-cell infiltration in node-positive, higher-risk hormone receptor–positive/HER2-negative early-stage breast cancer. 24 Gy preoperative radiation with pembrolizumab also increases the abundance of tertiary lymphoid structures containing B cells, T cells, and dendritic cells," said presenting author Gaorav Gupta, MD, PhD, Associate Professor of Radiation Oncology and Co-leader of the Breast Cancer Research Program at the University of North Carolina Lineberger Comprehensive Cancer Center, during a press briefing at the symposium. "Despite having a high burden of disease and including non–grade 3 tumors, we've observed encouraging rates of pathologic complete response in the radiation plus pembrolizumab arms."
Rationale and Study Methods
T-cell infiltration is a biomarker of response to anti–PD-1 immune checkpoint inhibition and chemotherapy in breast cancer, which is a newer approach to neoadjuvant therapy that has not yet been approved, possibly due to efficacy findings varying by PD-L1 expression and immune infiltration. However, in patients with hormone receptor–positive/HER2-negative breast cancer specifically, T-cell infiltration is typically found at lower levels.
"That was really the motivation to ask whether localized radiation could have this effect in these tumors, perhaps leading to better immunotherapy response," Dr. Gupta explained. Preoperative radiation therapy has been found in preclinical studies to enhance T-cell infiltration and anti-tumor immunity when combined with immune checkpoint inhibitors.
Researchers conducted the first randomized trial of preoperative radiation therapy in combination with anti–PD-1 immune checkpoint inhibition to determine if various levels of radiation could increase T-cell infiltration and improve responses to immunotherapy in patients with hormone receptor–positive/HER2-negative breast cancer or triple-negative breast cancer with nonirradiated metastatic lymph nodes. Dr. Gupta further explained that "boost, immune-priming" radiation was targeted to intact tumors and regional lymphatics were shielded during radiation to allow for an immune response and assessment of nonirradiated lymph nodes. His presentation focused on the hormone receptor–positive/HER2-negative cohort.
The study enrolled 51 patients in this cohort across 10 centers with cT1c to T4c, cN1 to N3 disease and at least one of the following criteria: grade 3 or grade 1–2 with cN2-N3, Ki67 > 20%, or high genomic assay score. Dr. Gupta clarified that the studied required a higher level of estrogen receptor or progesterone receptor positivity than similar clinical trials, including KEYNOTE-756. Patients were randomly assigned to receive no radiation, low-dose radiation (9 Gy), or high-dose radiation (24 Gy) to the primary tumor with concurrent pembrolizumab. Two weeks later, patients underwent tumor biopsy. Subsequently, patients received pembrolizumab plus 12 weeks of paclitaxel, then 4 cycles of doxorubicin/cyclophosphamide and pembrolizumab, and then surgery and adjuvant therapy.
Dual co-primary endpoints were 2-week tumor T-cell infiltration in the primary breast tumor, as assessed by multiplex immunofluorescence, and nodal pathologic complete response at the time of surgery. Secondary endpoints included pathologic complete response and residual cancer burden.
Key Findings
At the time of the analysis, 49 patients were evaluable for T-cell infiltration and 48 were evaluable for nodal pathologic complete response.
Two weeks posttreatment, tumor T-cell infiltration increased to the upper quartile in 31% of patients treated with no radiation, in 40% treated with low-dose radiation, and in 53% treated with high-dose radiation. Baseline T-cell infiltration scores were 0.50 from reference samples, and increased at 2 weeks to median scores of 0.60 with 0 Gy, 0.56 with 9 Gy, and 0.82 with 24 Gy. The difference in T-cell infiltration increase between the high-dose radiation group and the no-radiation group was considered statistically significant (P = .027).
"When we analyzed the primary endpoint, we found that it was only the pembrolizumab plus 24 Gy arm that led to a statistically significant increase in T-cell infiltration in the breast primary tumor. Patients that had high T-cell infiltration also had higher levels of PD-L1 expression relative to those that did not have high T-cell infiltration," Dr. Gupta said. "This showed us that 24 Gy is able to both increase T-cell infiltration and increase PD-L1 expression, which has been associated with better response to chemoimmunotherapy."
Surgery involved mastectomy for 70.8% of patients and immediate reconstruction for 39.6%. Clearance of disease in the lymph nodes was 33% in the 24 Gy arm vs 23% in the no-radiation arm. Eight percent had grade 2/3 wound complications, with rates balanced between the arms. The median number of removed lymph nodes was 6.5 (range = 1-42).
The overall nodal pathologic complete response rate was 29%; the rate in the no-radiation arm was 24%, 29% in the low-dose radiation arm, and 33% in the high-dose radiation arm.
Rates of pathologic complete response were 18% in the overall cohort, 6% in the no-radiation group, 29% in the low-dose radiation group, and 19% in the high-dose radiation group. Rates for residual cancer burden levels of 0 to 1 were 27% in the overall group and 18%, 29%, and 33% for the no, low, and high radiation dose groups, respectively. Analysis found that patients with non–luminal A breast cancers were more likely to achieve a complete response.
Gene expression analysis with RNA sequencing showed that infiltration could also be seen with B cells, dendritic cells, immunostimulatory macrophages, and NK cells. Spatial transcriptomics found that tertiary lymphoid structures were being induced more significantly with the 24 Gy radiation, which has also been associated with improved immunotherapy responses in other cancer types.
Disclosure: The study was funded by Merck, the Breast Cancer Research Foundation, the Translational Breast Cancer Research Consortium, the Susan G. Komen Foundation, and the Department of Defense Breast Cancer Research Program. Gupta has received research funds from both Merck and Breakpoint Therapeutics, and he also receives royalty payments from Naveris, Inc. Gupta discloses an ownership stake in Naveris, Inc.
