Hydroxyurea Therapy Requires HbF Induction for Clinical Benefit in a Sickle Cell Mouse Model.

To evaluate whether there are clinical benefits from chronic hydroxyurea administration that are independent of HbF induction using a murine SCD model in which fetal hemoglobin (HbF) cannot be induced.

Cohorts of sex- and aged-matched SCD mice were generated by transplanting lethally irradiated C57/BL6 mice with bone marrow from BERK mice. Only mice fully engrafted with SCD hematopoiesis were used for study. Transplanted SCD mice were injected by intraperitoneal route five days per week. SCD mice with high levels of HbF were generated by stem cell gene transfer using a gamma-globin lentiviral vector followed by transplantation.

We identified a dose of hydroxyurea (50 mg/kg) that would lead to a stable, well-tolerated reduction in neutrophil count, much like what is done to titrate dosage in human patients with SCD. Hydroxyurea dosed at 25 mg/kg produced no difference in blood counts compared to control mice, while 75 mg/kg and 100 mg/kg both produced critical pancytopenia. As expected, cellulose acetate gel electrophoresis and HPLC analysis showed that HbF was undetectable in both hydroxyurea-treated and saline-treated mice. Based on this dose-finding data, we treated SCD mice with 50 mg/kg hydroxyurea (n=20) and saline (n=13) five days/week for 20 weeks in order to determine whether chronic hydroxyurea therapy could improve both the anemia and organ damage of SCD. Blood counts obtained after 10 weeks again demonstrated a reduction in white blood cells (26.1 vs. 31.2 ×10⁹/L, p<0.005), absolute neutrophil counts (2.9 vs. 4.6 ×10⁹/L, p<0.005), platelets (780 vs 870 × 10⁹/L, p<0.05), without improvement in the anemia (6.7 vs 6.6 g/dL). Consistent with this data, the serum LDH and total bilirubin values remained elevated, similar to control mice, suggesting no improvement in the rate of hemolysis. Necropsy and pathologic analyses of major organs were performed on six mice from each group after 18-20 weeks of hydroxyurea therapy. Hydroxyurea-treated mice showed no improvement in the severe, multi-organ damage, compared to saline-treated, control mice. In contrast, six SCD mice with high levels of HbF resulting from stem cell gene transfer but not treated with hydroxyurea had a significant correction of their anemia (10.8 g/dL) along with a reduction in both total white blood cell (11.7 ×10⁹/L) and absolute neutrophil counts (2.6 × 10⁹/L). The reduction in the neutrophil count secondary to the correction of the anemia by gene therapy was similar to the levels demonstrated with hydroxyurea administration (hydroxyurea ANC 2.9 × 10⁹/L vs. gene therapy ANC 2.6 × 10⁹/L). Importantly, the SCD mice with high HbF demonstrated no significant organ damage.

Despite causing a significant reduction in the leukocytosis and thrombocytosis, hydroxyurea treatment did not improve the severe anemia and multi-organ disease pathology in SCD mice. In contrast, SCD mice with high levels of HbF resulting from stem cell gene therapy showed resolution of both the anemia and organ pathology. These data suggest that induction of HbF is a necessary and major contributor to the beneficial effects of hydroxyurea in SCD.

Off Label Use: Hydroxyurea use in pediatric patients sickle cell disease. This abstract does not discuss the off label use of Hydroxyurea in pediatric patients with sickle cell disease. However, discussion of this abstract would likely result in referencing the off label use of hydroxyurea in pediatric patients with sickle cell disease.