Abstract PO1-12-10: Zoledronic acid to prevent rebound fractures after stopping adjuvant denosumab: a randomized controlled sub-study of the ABCSG-18 trial

Abstract Introduction ABCSG-18 trial showed that 60mg denosumab subcutaneously every 6 months halves fracture risk in postmenopausal patients with early breast cancer, who are treated with adjuvant aromatase inhibitors. However, it has been reported that stopping denosumab may lead to overactivation of osteoclasts resulting in so called rebound vertebral fractures. As shown previously, zoledronic acid (ZA), when given to patients of the ABCSG-18 trial after stopping denosumab prevents a high bone turnover state that can occur in some patients. Here we report the impact of a single ZA dose on bone mineral density (BMD) and fractures after stopping denosumab. Methods ABCSG-18 trial patients, who had received adjuvant denosumab were re-randomized to receive a single dose of ZA 5mg i.v. or not, 8 months +- 4 weeks after their last denosumab dose. DXA scans, lateral spine X-rays and bone turnover markers (CTX and osteocalcin) were assessed at baseline and at 6, 12 and 18 months thereafter. New clinical fractures were assessed until 18 months after randomization. Results 50 patients were randomized to either ZA (24pts) or control (26pts). All patients of the ZA arm and 25 out of 26 patients of the control arm had received 7 doses of denosumab within the ABCSG 18 trial before re-randomization. Baseline bone health factors were well balanced between the 2 arms: Baseline BMD did not differ between the two arms with regard to femoral neck (ZA: 0.817 g/cm2; SOC: 0.816g/cm2), total hip (ZA: 0.918g/cm2; SOC: 0.921g/cm2) and lumbar spine (ZA: 1.128 g/cm2; SOC 1.142 g/cm2), respectively. Relative changes in BMD differed significantly between both arms for femoral neck after 12 months (ZA: 0.5%; control: -4.98%, p 0.004) and for total hip after 6 months (ZA: -0.73%; control: -4.61%, p 0.001) and after 12 months (ZA: -0.13%; control: -5.62%; p 0.001). There was a trend towards a greater decline in BMD at lumbar spine after 6 months in the control arm versus the ZA arm (-3.35% vs 0.67%, p=0.052). After randomization, 3 asymptomatic fractures (lumbar vertebral and thoracic vertebral in one patient and thoracic vertebral in a second patient) occurred in the ZA arm, whereas 2 clinical fractures (one radius fracture and one lumbar vertebral fracture) occurred in the SOC arm. Conclusion A single dose of Zoledronic acid prevents high bone turnover state resulting in loss of BMD in some patients after stopping denosumab. ZA may therefore be offered to breast cancer patients with aromatase inhibitor after stopping denosumab. Citation Format: Georg Pfeiler, Christian F. Singer, Dominik Hlauschek, Manfreda Diether, Ferdinand Haslbauer, Paul Sevelda, Kristin Köck, Karl Tamussino, Arno C. Reichenauer, Florian Fitzal, Dietmar Heck, Richard Greil, Claudia Pasterk, Sonja Heibl, Walter Herz, Marija Balic, Daniel Egle, Lidija Sölkner, Michael Gnant. Zoledronic acid to prevent rebound fractures after stopping adjuvant denosumab: a randomized controlled sub-study of the ABCSG-18 trial abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-12-10.