<p>Breast cancer is the most common cancer among Canadian women with one in nine women expected to develop breast cancer in their lifetime. Until recently these breast tumors were thought to be a homogeneous cell population. Recent studies have shown that breast tumors contain a rare cell type termed breast tumor initiating cells (TICs) or cancer stem cells (CSCs) with the ability to elicit new tumor growth and metastases. These TICs exist apex of a tumor cell hierarchy and give rise to more TICs and non-tumorigenic cells. Traditionally, drugs were developed to target the highly proliferative cells population resulting in a decrease in tumor volume. However, these therapies spare the TICs, which results in tumor relapse demonstrating the need for new drugs that target the TICs. Because in cancer, mutated protein kinases are the controllers of cell proliferation, invasion and metastasis, they have become a target for drug development. Inhibition of these kinases could lead to the identification of compounds that selectively target breast TICs. Using mammary tumors from cancer prone mice propagated as non-adherent tumorspheres (TMS), which contain a high fraction of breast TICs and the same conditions to propagate the non-transformed mouse mammary epithelial stem and progenitor cells (MESC), as non-adherent mammospheres (MMS) a 240-kinase inhibitor library was screened using an AlamarBlue proliferation assay. Twenty percent of the compounds resulted in 75% decrease in proliferation of TMS derived cells and some of which were TMS-selective. Sunitinib, a multi-targeted kinase inhibitor, was one of the selective compounds identified and when administered to mice with subcutaneous mammary tumors resulted in tumor shrinkage. This was accompanied by an increase in apoptotic cells, decrease in proliferating cells and tumor vasculature, and a change in tumor morphology and composition. These findings show the efficacy of Sunitinib in shrinking mouse mammary tumors and suggest a potential use of Sunitinib for treatment of breast cancer.</p> / Master of Science (MSc)