SABC Abstracts have been publishedPublication Number: PS12-08 A window-of-opportunity study with atezolizumab and the oncolytic virus pelareorep in early breast cancer (REO-027, AWARE-1) Luis Manso 1, Patricia Villagrasa 2, Nuria Chic 3, Begona Bermejo 4, Juan Miguel Cejalvo 4, Yann Izarzugaza 5, Blanca Cantos 6, Salvador Blanch 7, Mireia Margeli8, Jose Luis Alonso 9, Alejandro Martinez 10, Rafael Villanueva 11, Juan Antonio Guerra 12, Raquel Andres 13, Pilar Zamora 14, Esteban Nogales 15, Manel Juan 3, Blanca Gonzalez 3, Rita Laeufle 16, Gerard Nuovo 17, Grey Wilkinson 16, Matt Coffey 16, Azucena Gonzalez 3, Debora Martinez 18, Laia Pare 2, Fernando Salvador2, Xavier Gonzalez 19, Aleix Prat20 and Joaquin Gavila 7. 1Hospital Universitario 12 de Octubre, Madrid, Spain 2SOLTI Breast Cancer Research Group, Barcelona, Spain 3Hospital Clinic de Barcelona, Barcelona, Spain 4Hospital Clinico Universitario de Valencia, Valencia, Spain 5Hospital Universitario Fundacion Jimenez Diaz, Madrid, Spain 6Hospital Universitario Puerta de Hierro-Majadahonda, Madrid, Spain 7Instituto Valenciano de Oncologia, Valencia, Spain 8Institut Catala d'Oncologia, ICO Badalona, Barcelona, Spain 9Hospital Clinico Universitario Virgen de la Arrixaca-IMIB, Murcia, Spain 10Hospital Universitari Quiron Dexeus, Barcelona, Spain 11Hospital Duran i Reynals, Barcelona, Spain 12Hospital Universitario de Fuenlabrada, Madrid, Spain 13Hospital Clinico Universitario Lozano Blesa, Zaragoza, Spain 14Hospital Universitario La Paz, Madrid, Spain 15Hospital Universitario Virgen Macarena, Sevilla, Spain 16Oncolytics Biotech Inc, San Diego, CA;17Ohio State University Arthur G. James Cancer Hospital and Richard J. Solove Research Institute, Colombus, OH;18Hospital Clinic de Barcelona/August Pi i Sunyer Biomedical Research Institute (IDIBAPS), Barcelona, Spain 19Hospital Universitari General de Catalunya, San Cugat del Valles, Barcelona, Spain 20Hospital Clinic de Barcelona/SOLTI Breast Cancer Research Group/August Pi i Sunyer Biomedical Research Institute (IDIBAPS)/Medicine Department, University of Barcelona, Barcelona, Spain Background: A previous phase 2 study in metastatic breast cancer compared treatment with intravenously delivered oncolytic reovirus, pelareorep (pela), in combination with paclitaxel (PTX) versus PTX alone. This study demonstrated a statistically significant improvement in overall survival (OS), without differences in objective response or progression-free survival. We hypothesized that the OS benefit from pela + PTX may be attributed to an adaptive immune response triggered by pela. To test this hypothesis, and examine if pela can mediate the priming of an anti-tumor immune response, we designed a study called AWARE-1 (A window-of-opportunity study of pela in Early Breast Cancer), which is currently enrolling and for which initial translational research results are presented. Methods: AWARE-1 is evaluating 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, while atezolizumab is administered on day 3. Tumor biopsies are collected at diagnosis, day 3, and day ~21. Five cohorts will be examined: Cohort 1: Hormone Receptor-positive/HER2-negative (HR+/HER2-neg) (10 patients), pelareorep + letrozole. Cohort 2: HR+/HER2-neg (10 patients), pelareorep + letrozole + atezolizumab. Cohort 3: Triple Negative Breast Cancer (TNBC) (6 patients), pelareorep + atezolizumab. Cohort 4: Hormone Receptor-positive/HER2-positive (HR+/HER2+) (6 patients), pelareorep + trastuzumab + atezolizumab. Cohort 5: Hormone Receptor-negative/HER2-positive (HR-/HER2+) (6 patients), pelareorep + trastuzumab + atezolizumab. The primary endpoint of the study is CelTIL score, a metric for quantifying the changes in tumor cellularity and infiltration of TILs, where an increase in CelTIL is associated with a favorable response to treatment. Tumor tissue was examined for pela replication, and changes to the TME were assessed by imaging mass cytometry (IMC), immunohistochemistry, and T cell receptor sequencing (TCR-seq). Peripheral blood was also examined by TCR-seq. Results: Detailed translational research results will be presented from patients in cohort 1, who received just pelareorep and letrozole. CelTIL score increased in 5/10 patients at day 3 biopsies and 6/10 patients at day 21 biopsies. Preliminary results show high levels of viral replication (>50% of tumor cells) while immunohistochemistry and IMC analysis revealed changes to the TME, with increases in CD8+ T cells and upregulation of PD-L1 at both day 3 and day 21 biopsies. Overall, preliminary data from cohort 1 of AWARE-1 demonstrate pela-mediated priming of an adaptive immune response. (NCT04102618)