Phase II (INSPIRE) trial of pembrolizumab (pembro) with serial immune and genomic profiling in patients (pts) with metastatic triple negative breast cancer (mTNBC).

1094Background: Pembro is a PD-1 immune checkpoint inhibitor with a low response rate in pretreated mTNBC. In this biomarker driven, investigator-initiated trial we evaluate changes in immune and genomic landscapes of mTNBC pts treated with pembro monotherapy. Methods: Pts with mTNBC, ECOG 0-1, progressing after or ineligible for standard therapy were treated with Pembro 200mg IV q3w. Imaging (RECIST v1.1) was performed every 9 weeks. Pre- and on-treatment (week 6-9) tumor biopsies (bx) were assessed by IHC (C223 antibody) for PD-L1 expression; single cell suspensions were pooled for exome/RNA-sequencing and immunophenotyping (IP) by flow cytometry. Serial blood samples were collected for IP, cytokines, and ctDNA. Results: Of 20 mTNBC pts enrolled, median age was 40 years, 35% had ≥3 lines of therapy, 35% had LDH ≥1.5xULN, 60% had visceral metastases (liver 25%, lung 44%) and 30% had PD-L1+ ( > 1%) tumors. Best response was 1 PR, 3 SD, and 16 PD. One pt discontinued Pembro after 3 cycles due to grade 3 pn...