- Failed to achieve at least an absolute 5% increase in hemoglobin F while taking Hydroxyurea (HU) or unable to tolerate HU as described by the treating physician and may include but is not limited to lack of efficacy (such as people who have continued to have pain episodes more than 2 times a year or who have had acute chest or multiorgan failure syndromes or an episode of priapism), or other severe side effects while on HU (severe side effects include significant myelosuppression; skin cancer; or cytotoxicity evidenced by gastrointestinal symptoms, dermatological reactions, hepatic enzyme elevations, pulmonary fibrosis or neurological disturbances), or refusal of hydroxyurea therapy by the informed patient