Additional Study Information: This research study is designed to learn about the safety of an experimental (investigational) drug called RLY-2608, also called the study drug, when given to individuals who have been diagnosed with metastatic (spread to other areas of the body) or unresectable (cannot be removed surgically) cancers and have at least one mutation in a gene called PIK3CA. Investigational means that it is not yet known if this drug helps patients, has not been approved by the Food and Drug Administration (FDA) or any other regulatory agency, and is not available for sale as a marketed product. Patients with advanced breast cancer will also receive Fulvestrant in combination with RLY-2608.