Chronic Hydroxyurea Therapy in Children with Sickle Cell Anemia: Mechanisms of Action, Systemic Effects, and Long-Term Safety.

Sickle cell disease (SCD) is the most common monogenic disorder worldwide and remains a major cause of morbidity and mortality. Sickle cell anemia (SCA), the homozygous HbSS genotype, represents the most severe and frequent form within the spectrum of SCD. Hydroxyurea (HU), a ribonucleotide reductase inhibitor, represents the first and most widely used disease-modifying therapy for SCA. This review summarizes current evidence on the mechanisms of action, clinical efficacy, systemic effects, and long-term safety of chronic HU therapy in patients with SCA. A comprehensive literature search was conducted in PubMed up to 2025 using the terms "sickle cell disease," "sickle cell anemia", "hydroxyurea," and "children" or "paediatric." Eligible studies included randomized controlled trials, cohort studies, and systematic reviews evaluating HU therapy in SCA. Literature analysis showed that HU exerts pleiotropic effects by inducing fetal hemoglobin (HbF) synthesis, improving red blood cell deformability, reducing leukocyte and platelet counts, and enhancing nitric oxide bioavailability. These mechanisms lead to decreased vaso-occlusive crises, acute chest syndrome, transfusion requirements, and overall mortality. Beyond hematologic improvement, HU confers neuroprotective benefits, modulates inflammatory and immune pathways, and supports normal growth and endocrine development in children. Adverse events, primarily mild bone marrow suppression, are dose-dependent and reversible with appropriate monitoring. No evidence supports an increased risk of malignancy with long-term use. In conclusion, chronic HU therapy is a safe, effective, and multifaceted treatment that substantially improves survival and quality of life in patients with SCA. Early initiation and individualized dosing maximize its therapeutic benefits and help prevent irreversible organ damage.