Change in tumor microenvironment and PD-L1 expression with rucaparib treatment in triple-negative breast cancer.

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e12569 Background: Triple-Negative Breast Cancers (TNBCs) represent a diverse subset of tumors characterized by varied responses to therapies including immunotherapy (IO). While IO is approved in early-stage and advanced TNBC, responses are modest. Modulation of the tumor microenvironment to improve responses to IO is an active area of research. In-vitro and in-vivo studies have shown Poly-ADP Ribose Polymerase inhibitors (PARPi) to increase PD-L1 expression in sporadic, BRCA1 and BRCA2 mutated TNBC cell lines improving response to IO. In addition, TNBCs have diverse mechanisms leading to Homologous Recombination Deficiency and the role of PARPi in sporadic TNBCs is still unclear. Methods: To further investigate the role of PARPi in tumor immuno-modulation we conducted a single-arm, open-label, window of opportunity (WOO) trial in patients with early-stage TNBC (NCT03911453). This study was approved by our institutional review board. Patients with histologically documented stage I-III TNBC were eligible for definitive surgery were enrolled. Patients were treated with 600mg BID rucaparib for 21 days. Pre- and post-treatment tumor biopsies were collected. Post-treatment, patients went to surgery or proceeded with neoadjuvant chemotherapy (NAC) followed by definitive surgery. Our primary endpoint was to assess change in PD-L1 expression. In addition, change in tumor microenvironment was by evaluating change in percentage of B cells, T cells and macrophages was calculated. Results: Twenty patients were enrolled. Nineteen patients completed 21 days and 1 patient went on extended duration PARPi during the COVID19 pandemic. Almost 50% of patients went to surgery while the rest went onto NAC after PARPi therapy. Pre-treatment tumor samples were obtained from the diagnostic core biopsies while post-treatment, fresh frozen and Formalin Fixed Paraffin Embedded were obtained either at time of surgery or a research biopsy prior to NAC initiation. Primary biomarkers data was successfully done in 14 paired samples. Treatment with rucaparib has increased expression of PD-L1 from median of 15% in pre-treatment tumor samples to a median of 26% in post-treatment tumor samples. Table below summarizes our results. Conclusions: Our study revealed that treatment with PARPi, rucaparib, changed the tumor microenvironment as noted by changes in expression of PD-L1, and immune milieu in and around the tumor. This could lead to change in responsiveness to therapies including CKI. This strategy should be further investigated to evaluate improving responses to immunotherapies. Clinical trial information: NCT03911453 . [Table: see text]