Semustine (methyl-CCNU) is a synthetic nitrosourea compound used as an alkylating agent in chemotherapy. It is primarily indicated for the treatment of certain cancers, including brain tumors (such as glioblastoma multiforme and astrocytoma), Hodgkin's lymphoma, colorectal cancer, and gastric cancer in select cases. Semustine interferes with cancer cell DNA, preventing replication and growth, and is particularly effective due to its ability to cross the blood-brain barrier, making it valuable for central nervous system malignancies. It is typically administered orally as part of combination regimens. As a pharmaceutical intermediate or active ingredient, it requires careful handling. Common side effects include bone marrow suppression, leading to anemia, increased infection risk, and thrombocytopenia; use must be under strict medical supervision. Chemical formula: C₁₁H₁₉ClN₄O₂; CAS: 13909-09-6.