Apatinib is a tyrosine kinase inhibitor targeting vascular endothelial growth factor receptor (VEGFR), and its anti-angiogenesis effect has been viewed in preclinical tests. The investigators' phase I study has shown that the drug's toxicity is manageable and the maximum tolerable daily dose is 850 mg. The hypothesis of this clinical research study is to discover if the study drug apatinib can shrink or slow the growth of triple-negative breast cancer. The safety of apatinib will also be studied. Patients physical state, symptoms, changes in the size of the tumor, and laboratory findings obtained while on-study will help the research team decide if apatinib is safe and effective in pretreated metastatic triple-negative breast cancer patients.