A Study of LOXO-783 in Patients With Breast Cancer/Other Solid Tumors

The main purpose of this study is to learn more about the safety, side effects, and effectiveness of LOXO-783. LOXO-783 may be used to treat breast cancer and other solid tumors that have a change in a particular gene (known as the PIK3CA gene). Participation could last up to 36 months (3 years) and possibly longer if the disease does not get worse.

Inclusion Criteria: * Have advanced breast cancer or another solid tumor with the presence of a phosphatidylinositol 3-kinase catalytic subunit alpha (PIK3CA) H1047R mutation (or other Sponsor and safety review committee (SRC)-approved, activating PIK3CA mutations other than H1047R mutation) * Have adequate archival tumor tissue sample available or be approved by the Sponsor for enrollment if no tumor sample is available. * Have stopped all cancer treatment and have recovered from the major side effects * Have adequate organ function, as measured by blood tests * Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) scale * Patients must have * Measurable disease \--- Patients with non-breast tumor types must have at least 1 measurable lesion * Non-measurable bone disease (at least 1 bone lesion in breast cancer patients only) * For patients with an estrogen receptor (ER)+ breast cancer diagnosis: * If female, must be postmenopausal * If male, must agree to use hormone suppression * Phase 1a: \-- Dose escalation and backfill patients: * Advanced solid tumor * Patients may have had up to 5 prior regimens for advanced disease * Phase 1b: * Part A: * ER+/human epidermal growth factor receptor 2 (HER2)- advanced breast cancer * Patients may have had up to 5 prior regimens for advanced disease ---- Prior cyclin dependent kinase (CDK)4/6 inhibitor therapy required * Part B: * ER+/HER2- advanced breast cancer * Patients may have had up to 2 prior regimens for advanced disease. * Part C: * ER+/HER2- advanced breast cancer * Patients may have had up to 5 prior regimens for advanced disease. \---- Prior CDK4/6 inhibitor therapy required. * Have a diagnosis of diabetes mellitus Type 2 * Part D: * Advanced breast cancer * Patients may have had up to 5 prior regimens for advanced disease. * Part E: * Advanced solid tumor * Patients may have had up to 3 prior regimens for advanced disease advanced disease * Part F: * ER+/HER2- advanced breast cancer * Patients may have had up to 5 prior regimens for advanced disease * Prior cyclin dependent kinase (CDK)4/6 inhibitor therapy required Exclusion Criteria: * Medical Conditions * Colorectal cancer * Endometrial cancers with specific concurrent oncogenic alterations * A history of known active or suspected * Diabetes mellitus Type 1 or * Diabetes mellitus Type 2 requiring antidiabetic medication (Phase 1a and all parts of Phase 1b except Part C). * Serious concomitant systemic disorder * Known or suspected history of untreated or uncontrolled central nervous system (CNS) involvement. * Active uncontrolled systemic bacterial, viral, fungal, or parasitic infection, or other clinically significant active disease process * Prior exposure to phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) inhibitor(s), except in certain circumstances