Abstract P2-02-04: De-escalation of bone-targeted treatment: Does the number of 6-monthly adjuvant zoledronate infusions received affect treatment efficacy for early breast cancer? A sub-study of ABCSG-12

Abstract Background: The results of the Early Breast Cancer Trialist Group (EBCTCG) meta-analysis (Lancet 2015) led to the widespread adoption of bisphosphonates as adjuvant therapy for postmenopausal early-stage breast cancer (EBC). Despite evaluating multiple bisphosphonate agents and regimens, there was no signal of varying efficacy with different agents, routes of administration or dose/dose intensity. We evaluated the question of treatment de-escalation using long-term outcome data from the prospective randomized ABCSG-12 trial. Patients and methods: Between 1999 and 2006, ABCSG-12 accrued 1803 patients with hormone-receptor positive EBC on ovarian function suppression for three years that were randomized to receive 4 mg zoledronic acid 6-monthly or not (and tamoxifen or anastrozole, in a 2:2 factorial design). In the current retrospective study, we evaluated whether the number of zoledronate infusions actually received had an impact on breast cancer-specific and fragility fracture outcomes. Based on the results of the EBCTCG meta-analysis, we hypothesized that amongst patients who receive less infusions than the planned seven zoledronate infusion in this trial, the number of infusions has no differential effect on these outcomes. Time to event endpoints were analyzed with Cox models and Kaplan Meier curves starting from a 3-year landmark. BMD subset analyses were restricted to patients who participated in the BMD sub-study with available BMD data. Results: 725 patients who received at least one zoledronate infusion were included in the time-to-event-analysis. There was no statistically significant difference in disease-free survival (adjusted HR 0.83, 95% CI 0.48–1.44, p=0.51) or overall survival (HR 0.97, 95% 0.30-3.11, p=0.96) in patients who received ≤6 zoledronate infusions (n=170) compared to those who received ≥7 zoledronate infusions (n=555) (adjusted HR 0.83, 95% CI 0.48 – 1.44, p=0.51). Both subgroups show stable lumbar spine and total hip BMD measurements across the five years. Conclusions: Comparable to the metastatic bone disease and fragility fracture settings, there was no evidence observed to indicate that a reduced number of zoledronate infusions is associated with reduced adjuvant efficacy. Further studies to define optimal regimens of adjuvant bone-targeted therapies (prospective evaluation of “how-low-can-you-go”) are required. Table 1: Multivariate Cox Regression: DFS and OS Table 2: BMD and percent change from baseline at the total hip and lumbar spine (L1-L4) over 60 months Citation Format: Ana-Alicia Beltran-Bless, Mark Clemons, Christian Fesl, Dominik Hlauschek, Lidija Soelkner, Gregory R. Pond, Lisa Vandermeer, Richard Greil, Marija Balic, Vesna Bjelic-Radisic, Christian F. Singer, Guenther Steger, Ruth Helfgott, Daniel Egle, Simon P. Gampenrieder, Stephanie Kacerovsky-Strobl, Christoph Suppan, Magdalena Ritter, Gabriel Rinnerthaler, Georg Pfeiler, Hannes Fohler, John Hilton, Michael Gnant. De-escalation of bone-targeted treatment: Does the number of 6-monthly adjuvant zoledronate infusions received affect treatment efficacy for early breast cancer? A sub-study of ABCSG-12 [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-02-04.