Leustat, known generically as cladribine, is a medication primarily used in the treatment of hairy cell leukemia and other types of cancer. It functions as an antimetabolite, specifically targeting cells in the S phase of the cell cycle, thereby inhibiting DNA synthesis and triggering cell death. Cladribine is designed to be selectively toxic to lymphocytes and monocytes, which are types of white blood cells, making it effective in treating conditions characterized by abnormal proliferation of these cells. It can be administered either intravenously or subcutaneously, depending on the specific condition being treated and the patient's overall treatment plan. Due to its potent effects and the risk of serious side effects, Leustat is typically prescribed and managed by healthcare professionals specializing in oncology or hematology.