Study of GVV858 as a Single Agent or in Combination With Endocrine Therapy in Patients With HR+/HER2- Breast Cancer and Other Advanced Solid Tumors

Inclusion Criteria: * Age ≥ 18 years old. * Patients with one of the following histologically or cytologically confirmed advanced cancers: Phase I (patients with one of the following cancers, from whom no standard therapy is available or appropriate in the judgment of the investigator): * HR+/HER2- advanced breast cancer (aBC) with disease progression on or following at least one line of hormone-based therapy in combination with a CDK4/6i and at least one additional line of systemic therapy for metastatic disease. * Locally advanced or metastatic cancer with a CCNE1 amplification. For dose expansion only: no more than 3 prior lines of therapy for advanced or metastatic disease. * Metastatic castration-resistant prostate adenocarcinoma, with no documented neuroendocrine component, castrate level of testosterone, and no more than 3 prior lines of systemic therapy for metastatic disease. Phase II: * HR+/HER2- aBC with disease progression on or after an endocrine therapy in combination, with a CDK4/6 inhibitor for advanced disease with no more than 2 lines of endocrine therapy and no prior cytotoxic chemotherapy or antibody-drug-conjugate for advanced disease. \- Measurable disease as determined by RECIST v1.1. * BC only: If no measurable disease is present, then at least one predominantly lytic bone lesion must be present that can be accurately assessed at baseline and is suitable for repeated assessment. * metastatic Castration-Resistant Prostate Cancer (mCRPC) only: If no measurable disease is present per PCWG3 modified RECIST, then at least 1 metastatic lesion must be present on bone scan imaging. Exclusion Criteria: * Patients with inadequate bone marrow and/or organ functions with out-of-range laboratory values. * Clinically significant, uncontrolled heart disease and/or cardiac repolarization abnormality including myocardial infarction (MI), coronary artery bypass graft (CABG), long QT syndrome, or risk factors for Torsades de Pointes (TdP). * Presence of symptomatic central nervous system (CNS) metastases or CNS metastases that require local therapy or increasing doses of corticosteroids within 2 weeks prior to study entry. * Patients with symptomatic visceral disease, including visceral crisis. * For patients with BC: Patient is concurrently using hormone replacement therapy. * Women of childbearing potential who are unwilling to use highly effective contraception methods, pregnant or nursing women. Other protocol-defined inclusion/exclusion criteria may apply.