Silmitasertib (CX-4945) in Combination With Chemotherapy for Relapsed Refractory Solid Tumors

The purpose of this study is to evaluate the investigational drug, silmitasertib (a pill taken by mouth), in combination with FDA approved drugs for solid tumors. An investigational drug is one that has not been approved by the U.S. Food \& Drug Administration (FDA), or any other regulatory authorities around the world for use alone or in combination with any drug, for the condition or illness it is being used to treat. The goals of this part of the study are: * Establish a recommended dose of silmitasertib in combination with chemotherapy * Test the safety and tolerability of silmitasertib in combination with chemotherapy in subjects with cancer * To determine the activity of study treatments chosen based on: * How each subject responds to the study treatment * How long a subject lives without their disease returning/progressing