LEE011 hydrochloride (ribociclib hydrochloride) is a targeted therapy primarily used in oncology for hormone receptor-positive (HR+), HER2-negative advanced or metastatic breast cancer in adults. It acts as a selective cyclin-dependent kinase (CDK) 4/6 inhibitor, crucial for regulating cell cycle progression. By inhibiting CDK4/6, it blocks the proliferation of cancer cells that rely on these pathways. It is typically administered orally in combination with endocrine therapies, such as aromatase inhibitors or fulvestrant, to enhance efficacy, delay disease progression, and improve survival outcomes. Ongoing clinical research investigates its role in other CDK4/6-dependent malignancies. Common side effects include neutropenia (low white blood cell count), fatigue, nausea, diarrhea, and hair loss; regular hematological monitoring is essential for safety. This drug advances precision medicine in cancer care.