(573a) Low Doses of Paclitaxel Induce a Dormant State in Brain Metastatic Breast Cancer Spheroids

Surviving residual cells after primary breast cancer treatment are known to contribute to tumor recurrence at secondary organs. These recurrent (metastatic) tumors are highly lethal, as they contribute to 90% of breast cancer related mortalities. Residual cells upon disseminating into metastatic sites can exhibit a dormant state and evade cell death. The mechanisms involved in attaining a dormant state are able to protect cells from immune surveillance and even aid them in gaining resistance to chemotherapeutic drugs, thereby enabling them to survive in a hostile microenvironment. The treatment options available for metastatic breast cancer are very few when compared to the primary tumor. In addition, the mechanisms through which cancer cells survive existing therapies at the secondary organ by exhibiting dormancy is not well understood.

In this study, we demonstrate that brain metastatic breast cancer (BMBC) spheroids are able to survive low doses of chemotherapy drug paclitaxel by exhibiting a dormant state through upregulation of phosphorylated p38 (p-p38). 10,000 MDA-MB-231Br BMBC cells were utilized to prepare spheroids. Spheroids were cultured in different drug concentrations (ranging from 0 to 100 µM) for a period of 7 days. Our results demonstrated that the cross-sectional area of the spheroids cultured in 80 nM paclitaxel remained constant throughout the culture time. We observed a decrease in spheroid areas for drug concentrations > 80 nM. 80 nM paclitaxel treated spheroids contained lower levels of proliferating cells than the control spheroids as demonstrated via Ki67 and EdU staining. The percentage of apoptotic cells increased with increase in drug dosages. We investigated the ratio of % extracellular signal - regulated kinase 1/2 (p-ERK) to % p-p38 positive cells as low ratio of p-ERK to p-p38 has been reported to be a characteristic of the dormant state. The ratio of % p-ERK: % p-p38 positive cells were higher in untreated spheroids compared to 80 nM paclitaxel treated spheroids. In addition, 80 nM paclitaxel treated spheroids attained growth upon the withdrawal of drug demonstrating reversibility of the dormant state. Overall, these results providing insight into regulation of the dormant state mediated via low dose chemotherapy.