Background: A previous phase 2 study in metastatic breast cancer demonstrated a statistically significant improvement in overall survival (OS) in patients treated with pelareorep (pela), an intravenously delivered immuno-oncolytic reovirus, given in combination with paclitaxel (PTX) versus PTX alone [1]. We hypothesized that the OS benefit from pela + PTX may be attributed to an adaptive T cell response triggered by pela. To examine if pela can mediate the priming of an anti-tumor immune response, and the impact of checkpoint blockade therapy on this response, we and SOLTI research group are conducting the AWARE-1 study (NCT04102618) in patients with early breast cancer. The initial translational research results from this study are presented here. Methods: AWARE-1 is a window-of-opportunity study to evaluate the safety and effect of pela ± atezolizumab on the tumor microenvironment (TME) in 38 women with early breast cancer. Patients are treated with pela on days 1, 2, 8, and 9, and atezolizumab is administered on day 3. Tumor biopsies are collected at diagnosis, day 3, and day ~21. Five patient cohorts are being examined: Cohort 1: HR+/HER2-neg (10 patients) receiving pelareorep + letrozole (without atezolizumab); Cohort 2: HR+/HER2-neg (10 patients) receiving pelareorep + letrozole + atezolizumab; Cohort 3: TNBC (6 patients) receiving pelareorep + atezolizumab; Cohort 4: HR+/HER2+ (6 patients) receiving pelareorep + trastuzumab + atezolizumab; Cohort 5: HR-neg/HER+ (6 patients) receiving pelareorep + trastuzumab + atezolizumab. The primary endpoint is CelTIL score [2], a metric for quantifying changes in tumor cellularity and the number of tumor infiltrating lymphocytes (TILs), where an increase in CelTIL score has been associated with a favorable response to treatment. Tumor tissue is being examined for pela replication, changes to the TME by immunohistochemistry (IHC), PD-L1 expression by the Ventana SP142 assay used as the atezolizumab companion diagnostic, and T cell clonality by T cell receptor sequencing (TCR-seq). Peripheral blood is also being examined by TCR-seq. Results: Changes in the TME by IHC demonstrate that treatment with pela in the presence of atezolizumab increases the CD8/Treg ratio, a predictor of greater therapeutic efficacy, similar to preclinical breast cancer mouse models [3, 4]. Detailed TCR-seq, Ventana PD-L1 assay results, and IHC analysis will be presented, focusing on differences between patients receiving pela in the absence or presence of atezolizumab (Cohorts 1 and 2, respectively), and between CelTIL scores of responders and non-responders. Overall, these data demonstrate that pela can promote an inflamed tumor phenotype that allows for synergy with checkpoint blockade therapy in breast cancer.

References: [1] Bernstein, V., et al. Breast Cancer Res Treat, 2018. 167(2): p. 485-493. [2] Nuciforo, P., et al. Ann Oncol, 2018. 29(1): p. 170-177. [3] Mostafa, A.A., et al. Cancers (Basel), 2018. 10(6). [4] Lee, J., et al. Cancer Research, 2020. 80(16 Supplement): p. 2206-2206.

Citation Format: Luis Manso, Fernando Salvador, Patricia Villagrasa, Nuria Chic, Begoña Bermejo, Juan M. Cejalvo, Yann Izarzugaza, Blanca Cantos, Salvador Blanch, Mireia Margeli, Jose L. Alonso, Alejandro Martínez, Rafael Villanueva, Juan A. Guerra, Raquel Andrés, Pilar Zamora, Esteban Nogales, Manel Juan, Blanca Gonzalez-Farre, Grey A. Wilkinson, Thomas C. Heineman, Gerard Nuovo, Houra Loghmani, Matt Coffey, Azucena Gonzalez, Débora Martínez, Laia Paré, Tomás Pascual, Xavier Gonzalez, Aleix Prat, Joaquín Gavilá. A window-of-opportunity study with atezolizumab and the oncolytic virus pelareorep in early breast cancer (AWARE-1) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT191.