Abstract P1-13-02: Early change in topoisomerase 1 (Top1) positive circulating tumor cells (CTCs) is associated with overall survival (OS) in patients with advanced breast cancer after treatment with etirinotecan pegol

Background: Etirinotecan pegol (EP) is a long-acting Top1 inhibitor providing sustained levels of active metabolite throughout the entire chemotherapy cycle. The phase 3 BEACON trial compared EP to treatment of physician's choice (TPC) in patients with advanced breast cancer, demonstrating a non-statistically significant 2.1 month difference in survival favoring EP in the intent to treat population. A novel aspect of the BEACON trial is to explore the utility of biomarkers measured in CTCs for predicting efficacy with EP. Pre- and post-treatment CTCs were isolated from blood of 77% of the 852 BEACON patients. Target-specific pharmacodynamic biomarkers for EP measured in CTCs were analyzed to identify patients most responsive to treatment with EP. Methods: Donation of blood samples for CTC analysis was voluntary. Participating BEACON patients had serial (baseline, Cycle 2 Day 1 [C2D1], Cycle 4 Day 1 [C4D1], End of Treatment) 7.5-mL whole blood samples drawn in EDTA tubes and shipped within 96 hours ambient to ApoCell (Houston, TX) for processing. PBMCs were separated by Ficoll® gradient, and CTCs were isolated using ApoStream® technology. Isolated cells were deposited on three slides and stained for DAPI, CD45, cytokeratin markers, as well as Top1, Top2, Ki67, γH2AX, Rad51, ABCG2, and TUNEL. Biomarkers were quantified by iCys® laser scanning cytometer equipped with image analysis software, and correlated with OS using Cox multiple regression and Kaplan-Meier analyses. Results: The CTC substudy yielded 611 pre-treatment, 519 C2D1, 268 C4D1, and 431 End of Treatment samples. Among the successfully processed blood samples, 98% had detectable CTCs, with a median of 63, 46, 51, and 57 CTCs/mL at baseline, C2D1, C4D1, and End of Treatment, respectively. Cox regression analyses of CTC number and percentage of Top1, Top2, Ki67, or TUNEL positive CTCs identified a correlation for post-treatment number of Top1-positive CTCs with OS in patients receiving EP. To assess the impact of Top1-positive CTCs, patients were classified as Top1-High (> median) or Top1-Low (≤ median) based on the percent of Top1-positive CTCs at baseline. Among the Top1-High patients at baseline, significantly improved OS (HR 0.54, p=0.007) was observed for those who converted to Top1-Low after their first treatment with EP (C2D1), but not TPC (HR 1.12, p=0.613). These results suggest that decreased number of Top1-positive CTCs may reflect EP target engagement with Top1, as these patients derived the most benefit from treatment. Conclusions: CTC collection and analysis was successfully incorporated into the phase 3 BEACON study, with 77% patient participation. CTC detection rate using ApoStream® was high, permitting evaluation of biomarkers at baseline and post-treatment. Significantly improved OS was observed in patients who had a decreased number of Top1-positive CTCs following cycle 1 of EP. Citation Format: Rugo HS, Cortes J, Awada A, O'Shaughnessy J, Twelves C, Im S-A, Hannah AL, Lu L, Sy S, Caygill K, Zajchowski D, Davis DW, Hoch U, Perez EA. Early change in topoisomerase 1 (Top1) positive circulating tumor cells (CTCs) is associated with overall survival (OS) in patients with advanced breast cancer after treatment with etirinotecan pegol. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-13-02.