LAUSR

Background: PARP inhibitors (PARPi) are superior to chemotherapy in pts with BRCA1/2m MBC, with median PFS (mPFS) of 7 months with single-agent ola. Sapa is an oral nucleoside analog; the active metabolite, CNDAC, generates single-strand DNA breaks that are converted to double-strand breaks (DSB) during subsequent replication, resulting in cell death. CNDAC-induced DSB repair is dependent on homologous recombination. Sapa combined with ola may augment DNA damage compared to ola alone in BRCA1/2m tumors. Methods: We conducted a phase Ib study of sapa/ola in pts with BRCA1/2m HER2-negative MBC who were not previously PARPi-exposed. Primary objective was to determine the RP2D of sapa with ola. Secondary objectives included safety, tolerability and mPFS. RAD51 staining was performed on archival tissue. Serial blood samples were collected for circulating tumor DNA (ctDNA) analysis using low-pass WGS with targeted sequencing via the DDR Discovery Panel (Resolution Bioscience). Results: 10 women (median age 46) were enrolled before the study was terminated for hematologic toxicity. Germline BRCA1 and BRCA2 mutations were present in 3 and 7 pts, respectively. 4 pts had received prior platinum. All pts initially received standard dose ola (300 mg twice daily) with 100 mg (n = 3), 150 mg (n = 4), or 200 mg (n = 3) sapa once daily on days 1-5 and 8-12 of an every 28-day cycle. The RP2D was not determined. Hematologic AEs were common, including anemia (80%), neutropenia (80%), and thrombocytopenia (40%), with 1 DLT of G4 neutropenia lasting more than 7 days. The ORR was 50% (95% CI: 18.7% - 81.3%) with mPFS of 9.7 months (95% CI: 8.02 - NA). 3 pts had clinical benefit of 15+ months, exceeding expectations with olaparib alone, including 2 with PR, and 2 who remained on trial for 36+ months. 3 patients had confirmed PRs of short duration. Of the remaining patients, one withdrew consent early and the other 3 had non-CR/non-PD or SD of 4, 7 and 9 months. At progression, 2 pts had evidence of BRCA reversion mutation associated with an MMEJ signature; ctDNA also demonstrated MRE11 and PTEN amplifications in 1 of these pts, suggesting multiple resistance mechanisms. 3 pts had no evidence of reversion but had acquired putative non-reversion mechanisms of resistance, including changes suggesting enhanced MRN complex function, FANCD2 and ATR amplifications and PIK3CA mutation. The majority of tumors (6/10) were RAD51-negative. Other tumors with RAD51 foci demonstrated replication stress evidenced by high levels of pKAP1 and pRPA, suggesting benefit associated with sapa. Conclusions: Sapa/ola produces high rates of hematologic toxicity. However, the ORR of 50%, mPFS of 9.7 months, and durability of 3+ years in 2 pts suggest possible combinatorial benefit. Further exploration with different sapa schedules or with substitution of a PARP1-selective inhibitor to decrease hematologic toxicity is warranted. Citation Format: Filipa Lynce, Noah Graham, Bose S. Kochupurakkal, Huy Nguyen, Philip D. Poorvu, Victoria Attaya, Raechel Davis, Kavita Garg, Lee P. Lim, Mark Li, Cloud P. Paweletz, Nabihah Tayob, Geoffrey I. Shapiro, Sara M. Tolaney. A phase Ib study of sapacitabine (sapa) and olaparib (ola) in patients (pts) with BRCA1/2-mutated (BRCA1/2m) metastatic breast cancer (MBC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT188.