Molecular imaging reveals a high degree of cross-seeding of spontaneous metastases in a novel mouse model of synchronous bilateral breast cancer

Synchronous bilateral breast cancer (SBBC) patients present with cancer in both breasts at the time of diagnosis or within a short time interval. They show higher rates of metastasis and lower overall survival compared to women with unilateral breast cancer. However, the lack of a preclinical model has led to a dearth in knowledge regarding the patterns of SBBC metastasis. Here we established an SBBC model and used molecular imaging to visualize the development of spontaneous lung metastases arising from each primary tumor. We engineered human breast cancer cells to express either Akaluc or Antares2 for bioluminescence imaging (BLI), and tdTomato or zsGreen for ex vivo fluorescence microscopy. Both cell populations were implanted into contralateral mammary fat pads of mice (n=10) and BLI was performed weekly for up to day 29 (n=3), 38 (n=4), or 42 (n=3). Signal from both Antares2 and Akaluc was first detected in the lungs on day 28 and was present in 9 of 10 mice at endpoint. Ex vivo fluorescence microscopy of the lungs revealed that for mice sacrificed on day 38, a significant percentage of micrometastases were composed of cancer cells from both primary tumors (mean 37%; range 27% to 45%), while two mice sacrificed on day 42 showed percentages of 51% and 70%. These results reveal a high degree of metastatic cross-seeding which may contribute to faster metastatic growth and intratumoral heterogeneity. We posit our work will help understand treatment resistance and optimal planning of treatment for SBBC patients.