Onvansertib + Paclitaxel In TNBC

This is a phase 1b/2 study of onvansertib in combination with paclitaxel in patients with triple negative invasive breast cancer with unresectable locally advanced or metastatic disease. In the phase 1b, different doses of onvansertib will be studied with a fixed dose of paclitaxel to determine the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D) of onvansertib. In the phase 2, the selected onvansertib RP2D in combination with paclitaxel will be studied following a Simon two-stage design. The study is divided into three time periods: a screening period; a treatment period; and a post-treatment follow-up period. The names of the study interventions involved in this study are: * Onvansertib * Paclitaxel Participants will receive study treatment for as long there are no serious side effects and the disease does not get worse. It is expected that about 50 people will take part in this research study. This is a Phase 1/2 clinical trial. A Phase 1 clinical trial tests the safety of an investigational drug and also tries to define the appropriate dose of the investigational drug to use for further studies. "Investigational" means that the drug is being studied. The U.S. Food and Drug Administration (FDA) has approved Paclitaxel as a treatment option for this disease. The use of Onvansertib is experimental which means that it is not approved by any regulatory authority, including the FDA, for treatment of metastatic breast cancer.