PD-L1 targeted radiotherapy can reduce tumor burden and delay tumor growth in a mouse model of triple-negative breast cancer

Introduction: Due to lack of targeted therapies, triple-negative breast cancer (TNBC) accounts for 40% of breast cancer mortalities despite only comprising 10-15% of diagnoses. Combination of PD-1/PD-L1 inhibitors with radiation therapy have been shown to have synergistic effects in TNBC. Targeted radiotherapy (TRT), in which a targeting probe is attached to a cytotoxic radioisotope, has emerged as a promising technique to deliver a therapeutic radiation payload to cells expressing the protein of interest. This is advantageous when targeting proteins such as PD-L1, which is highly expressed in TNBC and strongly upregulated after radiation-induced cytotoxicity. The purpose of this study was to assess the efficacy and safety of a novel PD-L1 targeted radiotherapy to improve immunotherapy outcomes in TNBC. Methods: The PD-L1-targeted antibody atezolizumab was radiolabeled with lutetium-177, a cytotoxic radioisotope, to selectively deplete PD-L1 + cells. Athymic nude mice bearing high PD-L1-expressing MDA-MB-231 tumors were treated with saline (n=6), 250µCi (n=3) or 500µCi (n=5) of [ 177 Lu]Lu-atezolizumab. Mice were imaged with Single Photon Emission Computed Tomography (SPECT) on days 1, 3, and 7 post-treatment, informing on distribution of the tracer in the tissue. Mice were monitored for 60 days for changes in tumor volume and body weight. Upon reaching endpoints, mice were euthanized and relevant organs (tumor, spleen, liver, kidney) were collected. Clearance organs were stained with hematoxylin and eosin to assess gross pathological changes. One- and two-way ANOVAs were utilized to assess differences between tumor volumes and SPECT imaging metrics across time, and log-rank tests were used to assess differences in survival between treatment groups. Results: Mice treated with 500µCi or 250µCi of [ 177 Lu]Lu-atezolizumab demonstrated a significant reduction in tumor volume compared to control by day 20 (p < 0.05) or day 24 (p < 0.05) post-treatment, respectively. Increased median survival was observed in both treated groups (35 days in control versus 52 and 60 days in 250µCi and 500µCi treated mice, respectively, p < 0.01). No weight loss was observed in any treated groups, and histology revealed minimal splenic and renal toxicity, with mild to moderate hepatotoxicity consistent with reversible changes. SPECT imaging revealed high uptake of [ 177 Lu]Lu-atezolizumab in the tumor by day 1 post treatment and high tumor to background ratios across all three imaging timepoints. Conclusions: Utilizing TRT to selectively deplete PD-L1 + cells resulted in decreased tumor volumes and increased survival in a mouse model of TNBC. Further, SPECT imaging validated high uptake of the PD-L1-targeted TRT within tumor tissue. These results validate [ 177 Lu]Lu-atezolizumab as a novel PD-L1-targeted radiotherapy which has the potential to improve outcomes of immunotherapy in TNBC patients.