Phase 1/2 Study of ETX-636 in Participants With Advanced Solid Tumors

Key Inclusion Criteria: * Metastatic or locally advanced and unresectable solid tumor that has progressed on or after at least one available therapy. * Tumor harboring an activating PIK3CA mutation detected in either tumor tissue or ctDNA. * At least 1 measurable lesion or evaluable disease per RECIST v1.1. * An ECOG performance status score of 0 or 1. * Adequate organ function. Additional key inclusion criterion for Parts B and C: \- Confirmed metastatic or locally advanced HR+/HER2- breast cancer not amenable to surgical resection with curative intent and must have received at least 1 prior CDK4/6 inhibitor and at least 1 prior anti-estrogen therapy. Key Exclusion Criteria: * Has history (within ≤2 years before screening) of a solid tumor or hematological malignancy that is histologically distinct from the cancers being studied. * Has symptomatic brain or spinal metastases or a known or suspected history of untreated or uncontrolled central nervous system (CNS) involvement. * Has an established diagnosis of diabetes mellitus type 1 or has uncontrolled diabetes mellitus type 2. * Has received treatment with any local or systemic anticancer therapy or investigational anticancer agent within 14 days prior to start of treatment. * Has toxicities from previous anticancer therapies that have not resolved to baseline levels with the exception of alopecia and peripheral neuropathy. * Has had radiotherapy outside the target tumor lesions within 14 days prior to start of treatment.