Nature Communications | Application of PD-L1 Inhibitors Combined with Stereotactic Ablative Radiotherapy in Triple-Negative Breast Cancer

This study provides key clinical evidence for immunotherapy strategies in triple-negative breast cancer, indicating the safety of combining SABR with atezolizumab and the potential for durable disease control in oligometastatic patients, offering direction for future optimization of combination regimens and biomarker-driven research.

 

Literature Overview

The article titled 'Single-fraction or multi-fraction stereotactic ablative body radiotherapy followed by atezolizumab in advanced triple-negative breast cancer: a randomized phase II trial,' published in the journal Nature Communications, systematically investigates the efficacy and safety of single-fraction or multi-fraction stereotactic ablative body radiotherapy (SABR) combined with the anti-PD-L1 antibody atezolizumab in patients with advanced triple-negative breast cancer (TNBC). The study employs a multicenter randomized design to evaluate the impact of different radiotherapy fractionation schedules on immunotherapy response, further exploring the clinical potential of synergy between radiotherapy and immune checkpoint inhibitors.

Background Knowledge

Triple-negative breast cancer (TNBC) is associated with limited treatment options and poor prognosis due to the lack of expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). Although PD-L1 is highly expressed in some TNBC cases and tumor mutational burden is elevated—making these tumors theoretically suitable for immunotherapy—the objective response rate to single-agent immune checkpoint inhibitors (ICIs) remains suboptimal, possibly due to an immunosuppressive tumor microenvironment, Treg infiltration, and tumor heterogeneity. The current challenge for PD-1/PD-L1 inhibitors in TNBC lies in identifying the patient populations who truly benefit and in overcoming primary or acquired resistance. Radiotherapy is believed to induce immunogenic cell death (ICD), releasing tumor antigens and enhancing antigen presentation, thereby 'priming' an anti-tumor immune response. Thus, combining radiotherapy as an 'in situ vaccine' with ICIs has emerged as a promising strategy to improve therapeutic efficacy. This study is grounded in that rationale, aiming to determine whether different SABR fractionation regimens can more effectively activate systemic anti-tumor immunity and thereby enhance the effects of atezolizumab.

 

 

Research Methods and Experiments

The study enrolled 54 patients with advanced TNBC, who were randomly assigned to receive either single-fraction 20Gy (n=27) or three-fraction 24Gy (n=27) SABR, followed by sequential atezolizumab treatment. The primary endpoint was progression-free survival (PFS), with secondary endpoints including best overall response rate, clinical benefit rate, overall survival (OS), and safety. Predefined exploratory biomarker analyses were also conducted, including PD-L1 expression, tumor-infiltrating lymphocytes (TILs), and flow cytometry of peripheral blood mononuclear cells (PBMCs). All patients received SABR, and the vast majority initiated atezolizumab within 5 days of radiotherapy, ensuring treatment adherence and the rigor of the sequential design.

Key Conclusions and Perspectives

• No significant difference in PFS was observed between the two SABR fractionation regimens (20Gy×1 vs 24Gy×3; 2.5 vs 3.1 months), with neither regimen meeting the prespecified PFS target of 3.4 months. The null hypothesis could not be rejected, indicating no significant clinical benefit improvement in this population.
• Despite the lack of significant PFS improvement, a subset of patients (22%) achieved clinical benefit (disease stability ≥24 weeks), and all such patients were in an oligometastatic state, suggesting that SABR combined with atezolizumab may enable durable disease control in TNBC patients with low tumor burden.
• Response was independent of PD-L1 expression status, indicating that this combination may activate anti-tumor immunity through PD-L1–independent pathways, offering a potential therapeutic opportunity for PD-L1–negative patients.
• Peripheral blood analysis revealed that patients with higher baseline and on-treatment levels of CD8+ TPEX (progenitor-like exhausted T cells, i.e., TCF1+) were more likely to achieve clinical benefit, suggesting CD8+ TPEX as a potential predictive biomarker to guide future immunotherapy strategies.
• The combination was well tolerated, with a low incidence of grade ≥3 treatment-related adverse events (TRAEs) (14%), and no grade 5 toxicities were observed, confirming the tolerability of SABR combined with atezolizumab and supporting further clinical exploration.

Research Significance and Prospects

Although the study did not meet its primary endpoint, it reveals the potential of combining SABR with ICIs in specific TNBC subpopulations, such as those with oligometastatic disease. Future research should focus on enriching for patients most likely to benefit, potentially using dynamic immune monitoring markers such as TPEX cells. Additionally, exploring triplet regimens combining SABR, ICIs, and chemotherapy, or intervening at earlier oligometastatic stages, may further enhance efficacy.

 

 

Conclusion

This study systematically evaluates the efficacy and safety of SABR combined with atezolizumab in advanced triple-negative breast cancer. Although PFS was not significantly prolonged, durable disease control was observed in a subset of oligometastatic patients, and responses were independent of PD-L1 expression. These findings underscore the importance of tumor microenvironment modulation in immunotherapy and suggest that local radiotherapy may overcome the limitations of ICI monotherapy by activating systemic immune responses. Dynamic changes in CD8+ TPEX cells in peripheral blood were identified as a potential predictive biomarker, offering new insights for personalized immunotherapy strategies. From bench to bedside, this study lays the foundation for integrated treatment approaches in TNBC, supporting further exploration of radiotherapy-immunotherapy combinations in precisely selected patient populations to shift TNBC from being 'difficult to treat' to 'controllable.' Validating the functional role of TPEX cells in animal models will accelerate the translational application of related biomarkers.

 

Steven David, Peter Savas, Shankar Siva, Paul J Neeson, and Sherene Loi. Single-fraction or multi-fraction stereotactic ablative body radiotherapy followed by atezolizumab in advanced triple-negative breast cancer: a randomized phase II trial. Nature Communications.