Abstract P4-06-08: Characterization of an estrogen-dependent murine model of human estrogen receptor-positive breast cancer bone metastasis

Abstract

Abstract Introduction: Breast cancer patients with estrogen receptor-positive (ER+) tumors have the highest rate of developing clinically evident osteolytic bone metastasis. However, much of our knowledge of breast cancer bone metastasis has been derived from ER- murine xenograft models, as robust pre-clinical models of ER+ bone metastasis are currently lacking. Thus we sought to develop and characterize a murine xenograft model of breast cancer bone metastases using an estrogen-dependent ER+ MCF-7 cell line. Methods: 28-day-old female nude mice were implanted with the following 60-day extended-release 17β-estradiol (E2) pellets: 0.72, 0.36, 0.18, 0.10, or 0.05 mg; with or without 1x105 MCF-7 cells (ATCC) inoculated via the left cardiac ventricle. Non-estradiol-supplemented female nude mice were also inoculated with either MCF-7 cells or a well-characterized, bone-tropic human ER- MDA-MB-231 cell line (referred to as MDA-SA), used as a comparison model. Radiographs and dual-energy X-ray absorptiometry's (DXAs) of hind femurs and tibias were obtained weekly to assess osteolytic lesion formation and BMD, respectively. Statistical significance was analyzed using one-way ANOVA with Tukey's post-hoc testing. Results: Using E2 doses previously reported in the literature to promote MCF-7 orthotopic tumors and bone metastases (0.72 mg 60-day 17β- E2 pellets), bone metastases developed with 100% incidence (n = 13) by day 28 in the MCF-7 model, with osteolytic lesion area increasing to 10.3±0.8 mm2 by day 42 (as compared with 100% incidence on day 21 in non-estrogen-supplemented mice inoculated with MDA-SA (n=8) with osteolytic lesion area of 10.7±2.0 mm2). However, histologic assessment of hind limbs of MCF-7 tumor bearing mice suggested that this estrogen dose stimulated a significant increase in bone mass. Therefore, the estrogen-dependency of bone metastasis formation and changes in BMD were explored. Bone metastasis incidence was estrogen-dependent; falling to less than 100% in mice with E2 doses less than 0.72 mg (50-75%, n = 4-12). BMD of distal femurs of mice supplemented with estradiol was significantly greater than naïve controls on day 28 (average increase of 65.3%, p<0.05), and was not dose dependent over the range of doses tested (0.05-0.72 mg pellets). Conclusion: Despite a marked increase in bone density, ER+ MCF-7 cells formed radiographically detectable osteolytic lesions with 100% incidence by day 28 in 0.72 mg 60-day 17β- E2 supplemented nude mice. It is possible that estrogen-stimulated increases in BMD contributed to the longer time required to achieve ER+ osteolytic lesion sizes comparable to those in standard ER- models. However, lower doses did not mitigate effects of estrogen on BMD in these 1-month-old mice, while lowering the incidence of bone metastasis formation. While further optimization is planned, intracardiac inoculation of MCF-7 cells in female nude mice provides a robust model of estrogen-dependent ER+ breast cancer bone metastases. Source of Research Support: R01 CA174926-01 R03 CA181893-01. Citation Format: Cheng JN, Frye JB, Whitman SA, Kunihiro AG, Funk JL. Characterization of an estrogen-dependent murine model of human estrogen receptor-positive breast cancer bone metastasis [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-06-08.