Pembrolizumab (P) in patients (pts) with metastatic breast cancer (MBC) with high tumor mutational burden (HTMB): Results from the Targeted Agent and Profiling Utilization Registry (TAPUR) Study.

1014 Background: TAPUR is a phase II basket study evaluating the anti-tumor activity of commercially available targeted agents in pts with advanced cancers with specific genomic alterations. P is an immune checkpoint inhibitor and HTMB is an emerging predictive biomarker for checkpoint inhibitor therapy. Results in a cohort of MBC pts with HTMB treated with P are reported. Methods: Eligible pts had advanced cancer, no standard treatment options, ECOG PS 0-1, measurable disease and acceptable organ function. Genomic testing was performed using commercially available tests selected by sites. Pts matched to P had HTMB defined as ≥9 mutations/megabase (Muts/Mb) by a FoundationOne test (n=20) or approved by the TAPUR Molecular Tumor Board for other tests (n=8). A Simon two-stage design was used to test a null rate of 15% vs. 35% (power = 0.85; α = 0.10). If ≥2 of 10 pts in stage 1 have disease control (DC) (objective response (OR) or stable disease at 16 weeks (wks) (SD16+)), an additional 18 pts are enrolled. If ≥7 of 28 pts have DC, the drug is considered worthy of further study. Secondary endpoints are progression-free survival (PFS), overall survival (OS) and safety. Results: Twenty-eight female MBC pts were enrolled from October 2016 to July 2018. Pts received P at 2 mg/kg (n=8) or 200 mg (n=20) IV over 30 minutes, every 3 wks. HTMB ranged from 9 to 37 Muts/Mb. Demographics and outcomes are summarized in Table (N=28). No relationship was observed between #Muts/Mb and PFS or OS. Two grade 3 AEs (weight loss and hypoalbuminemia) and 1 grade 2 SAE (urinary tract infection) were reported as at least possibly related to P. Conclusions: P demonstrated anti-tumor activity in heavily pre-treated MBC pts with HTMB . Additional study of P is warranted in MBC pts with HTMB. Clinical trial information: NCT02693535. [Table: see text]

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Author Susan Halabi Biostatistics & Bioinformatics, Division of Biostatistics