Myelostat 500 mg Capsules Myelostat 500 contains Hydroxyurea, an antimetabolite that acts as an inhibitor of ribonucleotide reductase, an enzyme required for converting ribonucleotides to deoxyribonucleotides. By blocking DNA synthesis without interfering with RNA or protein synthesis, it causes cell cycle arrest in the S-phase. It is indicated for the treatment of chronic myelogenous leukemia (CML), essential thrombocythemia, polycythemia vera, head and neck cervical carcinomas, and for reducing the frequency of painful crises in sickle cell anemia.