Abstract CT050: Expansion cohort of Phase I study of oral sapacitabine and oral seliciclib in patients with metastatic breast cancer andBRCA1/2mutations

Introduction: Sapacitabine, a nucleoside analog, and seliciclib, a cyclin-dependent kinase 2/9 inhibitor, constitute a novel oral regimen aimed at augmenting DNA damage and impairing cell cycle checkpoints. The initial phase I cohort investigating this combination demonstrated a 25% response rate in BRCA carriers. Hence, we developed an expansion cohort to assess the safety and efficacy of this regimen in patients with metastatic breast cancer and BRCA1/2 mutations. Methods: We enrolled 20 patients with HER2-negative metastatic breast cancer and germline or somatic BRCA1/2 mutations, who were treated with sapacitabine 50 mg twice daily for days 1-7 followed by seliciclib 800 mg twice daily for days 8-10 of a 21-day cycle. Baseline or archival biopsies underwent RAD51 immunohistochemistry to assess for functional homologous recombination proficiency. Available tissue was sent for whole exome and transcriptome sequencing, and pre- and post-treatment blood was submitted for cell-free DNA sequencing to assess for genomic correlates of response. Results: Participants received a median of 2 prior lines of chemotherapy for metastatic disease. Of the 9 patients who received a prior platinum agent, 6 progressed on this therapy. In addition to chemotherapy, 7 patients received and progressed on a prior PARP inhibitor. The overall response rate for sapacitabine and seliciclib in this cohort was 10%, consisting of 2 patients with partial responses lasting 4.7 and 9.0 months, respectively. The clinical benefit rate (CR + PR + SD ≥ 6 months) was 30%, and durations of stable disease ≥ 6 months ranged from 7.4 to 11.7 months. For all patients, median PFS was 3.7 months. The most frequent grade 3/4 adverse events were neutropenia (25% of patients), transaminitis (20%), and rash (10%). No patients who progressed on prior PARP inhibitor therapy and 6 of 13 patients (46%) with no history of PARP inhibitor resistance experienced clinical benefit (p = 0.052 by Fisher’s exact test). In contrast, 1 of 6 patients (17%) who progressed on prior platinum chemotherapy and 5 of 14 patients (36%) with no history of platinum resistance experienced clinical benefit (p = 0.61 by Fisher’s exact test). Notably, the tumors of some resistant patients harbored BRCA reversion mutations. Additional genomic analyses and RAD51 immunohistochemistry will be presented. Conclusions: The combination of sapacitabine and seliciclib was safe and led to durable clinical benefit in some patients with metastatic breast cancer and BRCA1/2 mutations. Prior progression on PARP inhibitors predicted resistance to this combination, associated in some cases with BRCA reversion mutations. Based on these results, the combination of sapacitabine and the PARP inhibitor olaparib is now being investigated in patients with PARP-naive metastatic HER2-negative breast cancer and germline BRCA1/2 mutations. Citation Format: Tanya Keenan, David Liu, Haitham Elmarakeby, Daniel Stover, Bose Kochupurakkal, Adam Tracy, Elaine Danielczyk, Leilani Anderson, Chelsea Andrews, Brendan Reardon, Beth Overmoyer, Eric Winer, Daniella Zheleva, Judy Chiao, David Blake, Eliezer Van Allen, Geoffrey I. Shapiro, Sara Tolaney. Expansion cohort of Phase I study of oral sapacitabine and oral seliciclib in patients with metastatic breast cancer and BRCA1/2 mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT050.