the risk is either absent or very small. Nevertheless, it has been a barrier for its wider use in patients with sickle-cell disease. Mechanism of action Hydroxycarbamide decreases the production of deoxyribonucleotides via inhibition of the enzyme ribonucleotide reductase by scavenging tyrosyl free radicals as they are involved in the reduction of nucleoside diphosphates (NDPs). In the treatment of sickle-cell disease, hydroxycarbamide increases the concentration of fetal hemoglobin. The precise mechanism of action is not yet clear, but it appears that hydroxycarbamide increases nitric oxide levels, causing soluble guanylyl cyclase activation with a resultant rise in cyclic GMP, and the activation of gamma globin gene expression and subsequent gamma chain synthesis necessary for fetal hemoglobin (HbF) production (which does not polymerize and deform red blood cells like the mutated HbS, responsible for sickle cell disease). Adult red cells containing more than 1% HbF are termed F cells. These cells are progeny of a small pool of immature committed erythroid precursors (BFU-e) that retain the ability to produce HbF. Hydroxyurea also suppresses the production of granulocytes in the bone marrow which has a mild immunosuppressive effect particularly at vascular sites where sickle cells have occluded blood flow. Natural occurrence Hydroxyurea has been reported as endogenous in human blood plasma at concentrations of approximately 30 to 200 ng/ml. Chemistry Hydroxyurea has been prepared in many different ways since its initial synthesis in 1869. Medical chemists, Dresler and Stein created Hydroxyurea from hydroxylamine, hydrochloric acid and potassium cyanide as a technical exercise in organic chemistry, as part of a series of experiments generating derivatives of urea. Hydroxyurea lay dormant for more than fifty years until it was studied as part of an investigation into the toxicity of protein metabolites. Due to its chemical properties hydroxyurea was explored as an antisickling agent in the treatment of hematological conditions. One common mechanism for synthesizing hydroxyurea is by the reaction of calcium cyanate with hydroxylamine nitrate in absolute ethanol and by the reaction of salt (i.e. sodium or potassium) cyanates and hydroxylamine hydrochloride in aqueous solution. Hydroxyurea has also been prepared by converting a quaternary ammonium anion exchange resin from the chloride form to the cyanate form with sodium cyanate and reacting the resin in the cyanate form with hydroxylamine hydrochloride. This method of hydroxyurea synthesis was patented by Hussain et al. (2015). Pharmacology Hydroxyurea is a monohydroxyl-substituted urea (hydroxycarbamate) antimetabolite. Similar to other antimetabolite anti-cancer