Venous (up to 10 mL) blood samples will be obtained at time zero (pretreatment), 30 min, 1h, 1.5 2, 2.5, 3.0, 3.5 and 4h following the first oral dose of hydroxyurea, and pretreatment and at the same times following the sixth oral dose of hydroxyurea (i.e., 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5 and 24 h).