Abstract 4207: Zoledronic acid reduces quiescent bone-disseminated human ER+ breast cancer tumor cell burden and estrogen-responsiveness in a pre-clinical model
Authors: Cheng Julia N. , Frye Jennifer B. , Whitman Susan A. , Funk Janet L. 
Published: March 2024

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 &lt; 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&lt; 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&lt; 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(6_Suppl):Abstract nr 4207.