Abstract 4207: Zoledronic acid reduces quiescent bone-disseminated human ER+ breast cancer tumor cell burden and estrogen-responsiveness in a pre-clinical model

Abstract Unique to early-stage ER+ breast cancer, ER+ tumor cells already bone disseminated at time of diagnosis can remain dormant at this site until an initiating event (e. g. , activating ESR1 mutation acquisition) allows for osteolytic bone metastases (BMET) progression even decades later. Adjuvant use of anti-resorptive bisphosphonates, but not denosumab, reduces BMET risk in post-menopausal women with early stage breast cancer, suggesting protective effects may be unrelated to changes in bone turnover. Pre-clinical modeling of dormant, bone-disseminated ER+ breast cancer cells may enhance our understanding and thus our ability to better target these inherently treatment-resistant cells. In studies reported here, a model of bone-disseminated ER+ dormancy was characterized and used to query effects of zoledronic acid (ZA) on dormant cells. When female nude mice were inoculated (intracardiac) with human ER+ MCF-7 cells labelled with DiD, a membrane dye that dissipates with each cell division, DiD-positive tumor cell number in harvested tibial bone marrow remained unchanged from 24 hours to 28 days post-tumor cell inoculation. When 17ß-estradiol (E2) treatment (0. 72 mg 60-day pellets) was initiated on D28 following tumor cell inoculation, detectable DiD+ tumor cell number in harvested tibial bone marrow increased 1. 8-fold within one week (D35 vs. D28, p 0. 0001), consistent with initiation of E2-stimulated proliferation, and osteolytic, histology-confirmed BMET appeared subsequently with 60% incidence by 6-weeks post-start of E2 treatment. When mice were instead zoledronic acid-treated during the 28d dormancy period (70 ug/kg subcutaneously on D7 and on D21 post-inoculation), DiD-positive cell number in marrow harvested on D28 was reduced by more than half vs. vehicle-treated controls (p 0. 05). Moreover, DiD-positive cell numbers on D35, one week after start of E2-treatment, remained unchanged in tibial marrow of ZA-treated mice and were thus substantially (3. 8-fold) lower than controls (p 0. 001). When followed for 6-weeks after initiation of E2 treatment, no osteolytic BMET were detected in ZA-treated mice. In toto, these results recapitulate clinical findings, thus validating the methodologic approach, and suggest that in addition to reducing dormant ER+ tumor cell burden in bone, bisphosphonates may also alter their estrogen responsiveness. Acknowledgements: Special thanks to undergraduates Alyssa Magee and Geethika Ameneni for assistance with data acquisition, and the METAvivor Foundation and NIH-NCI (NCI R03 CA181893, R01 CA174926, P30 CA023074, T32 CA009213) for financial support. Citation Format: Julia N. Cheng, Jennifer B. Frye, Susan A. Whitman, Janet L. Funk. Zoledronic acid reduces quiescent bone-disseminated human ER+ breast cancer tumor cell burden and estrogen-responsiveness in a pre-clinical model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 4207.