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

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