About Hydroxyurea Antimetabolite,a ribonucleotide reductase inhibitor, Antineoplastic. Mechanism of Action of Hydroxyurea Drug converted into free radical nitroxide invivo, and transported by diffusion into cells where it quenches the tyrosyl free radical at the active site of the M2 protein subunit of ribonucleotide reductase, is inactivated. This cause DNA synthesis inhibition selectively and producing cell death in S phase. Repair of DNA damaged by chemicals or irradiation is also inhibited by hydroxyurea.This produces synergistic effect with radiation or alkylating agent.Hydroxyurea also increase the level of fetal Hb, leading to a reduction in the incidence of vasooclussive crisis in sickle cell anaemia.The drug selectively reduces the level of episomal DNA and thus potentially reduces drug resistance associated with duplicated genes retained as episomes.