An Open-label, Multi-center, Phase I/Ib Study of IEV407 as a Single Agent and in Combination With Endocrine Therapy in Patients With Advanced Hormone Receptor Positive, HER2- Negative Breast Cancer

Inclusion Criteria: * Age ≥ 18 years old * Patients with one of the following indications: * Dose escalation (IEV407 single agent and in combination with fulvestrant or letrozole): HR+/HER2- aBC with disease progression on or following, or have been intolerant to, at least one line of endocrine-based therapy in combination with a CDK4/6 inhibitor and at least one additional line of systemic therapy in the unresectable/metastatic setting and not be a candidate for any available standard therapy, in the investigator's judgement. \- Dose expansion of IEV407 in combination with fulvestrant: HR+/HER2- aBC with disease progression on or following, or have been intolerant to, endocrine-based therapy in combination with a CDK4/6 inhibitor. They must not have received more than two prior lines of endocrine-based therapy in the unresectable/metastatic setting. Prior cytotoxic chemotherapy and/or antibody-drug conjugate therapies in the unresectable/metastatic setting are not allowed. Exclusion Criteria: * Patients with inadequate bone marrow and/or organ functions with out-of-range laboratory values. * Impaired cardiac function or clinically significant cardiac disease. * Concurrent use of hormone replacement therapy. * Women of childbearing potential who are unwilling to use highly effective contraception methods, pregnant or nursing women. * For the combination treatment of IEV407 with fulvestrant or letrozole: Patients with symptomatic visceral disease or any disease burden that makes the patient ineligible for endocrine-based therapy. Other protocol-defined inclusion/exclusion criteria may apply.