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SSO 2022 Press Release – Preliminary Results of a Phase ib/2 Window of Opportunity Clinical Trial (NCT04454528) Assessing the Effects of Preoperative (preop) Pembrolizumab +/- radiation in Early-stage Breast Cancer (eBC)

Julia Tchou, MD PhD (Presenter;Submitter) – University of Pennsylvannia;

INTRODUCTION: Immune checkpoint inhibition (ICI) is currently only used in combination with chemotherapy in breast cancer since ICI alone is ineffective. It is unclear if non-chemotherapy strategies, such as radiation therapy (RT) specifically targeting the tumor may improve ICI effectiveness through immune-stimulatory effects. To test this hypothesis, we initiated a phase 1b/2 trial to evaluate the feasibility and efficacy of preop pembrolizumab (ICI) +/- a tumor-directed RT fraction (7 Gy) in patients with eBC.

Arms 1 & 2 differ by the treatment order of preop RT and ICI. In arm 3, patients receive preop ICI alone. In Phase 1b, patients are assigned 1:1 into Arms 1 & 2 according to tumor subtype. In Phase 2, patients are assigned (1:1:1) into arms 1, 2, & 3 according to tumor subtype. Standard of care surgery occurs on day 21 after enrollment (day 0). Standard adjuvant systemic therapy +/- RT follows as recommended by treating oncologists. RESULTS: The study was initiated in January, 2021. Phase 1b was completed and feasibility established in June 2021. Thus far, 9 patients (7 TNBC, 1 HR+HER2- and 1 HER2+) have enrolled. Of the 7 patients with TNBC, tumor shrinkage was observed In 4 patients (57%). Exceptional pathologic response, defined as < 10% viable tumor, was noted in 2 patients with near complete pathologic response (pCR) reminiscent of the histologic changes (tumor necrosis/fibrosis in residual tumor bed) seen with pCR after neoadjuvant chemotherapy (NAC).

CONCLUSIONS: Preop ICI+RT may achieve near pCR in some patients with e-BC. Samples collected at various timepoints in this study will help elucidate mechanisms and biomarkers to identify strategies to improve pCR rate and to predict response a priori. Our results may lead to a larger study in which chemotherapy omission, i.e, treatment de-escalation, may be feasible in select patients with near pCR after preop ICI+RT.

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