Influence of Hemoglobin Level on Clinical Findings In Infants with Sickle Cell Anemia: Data From BABY HUG

Infants with sickle cell anemia (SCA) are at risk for organ damage and clinical events. Risk may vary depending on steady state hemoglobin level. BABY HUG (ClinicalTrials.org, NCT00006400), a NHLBI-NICHD supported phase III randomized placebo-controlled trial, examined the ability of hydroxyurea (HU) to reduce end organ damage to the kidneys and spleen and attenuate other complications of SCA in infants. In this secondary study, we investigated whether placebo-treated subgroups defined by extremes of baseline hemoglobin level differed from one another in frequency of sickle cell-related complications and other laboratory findings, and then compared these data to the entire group of infants treated with HU.

BABY HUG subjects randomized at ages 9 – 18 mo were treated with hydroxyurea (20 mg/kg/d) (N=96) or with placebo (N=97) for 2 years. Those randomized to placebo were classified according to their age-adjusted baseline hemoglobin level and subgroups in the lowest (n=24) and highest (n=24) quartiles compared. (Demarcating Hb values were: age 9 to <12 mo, <8.0 vs. >10.2gm/dL; age12 – 18 mo, <8.1 vs. >9.9gm/dL.)

BABY HUG primary endpoints of spleen (splenic uptake of ^99mTc sulfur colloid on liver-spleen scan) and kidney (GFR by DTPA clearance) function did not differ in placebo group subjects who were in the lowest and highest quartile hemoglobin levels. However, those in the lowest hemoglobin quartile had a higher incidence of acute chest syndrome (ACS) than those in the highest quartile (0.31 vs. 0.02 events/person-year, RR 14.4, p=0.01) and this difference was significantly attenuated by HU (0.05 events/person-year). The relative risk for developing a pain crisis did not differ between the two placebo subgroups (2.2 vs. 2.1 events/person-year), but HU significantly reduced pain frequency compared to either subgroup (0.94 events/person-year, p<0.001). Subjects in the lowest hemoglobin quartile had higher baseline mean TCD velocities than those in the highest quartile (126.2 vs. 112.4 cm/sec, p=0.008). These differences persisted over the two-year period of study, with exit values of 164.9 and 139.6 cm/sec, respectively (p=0.003). By comparison mean TCD velocity in the HU-treated group was 124.5 cm/sec at baseline and 145.6 at exit. Results of neurocognitive testing were not statistically different between groups; however, a trend toward a lower performance developmental index (PDI, p=0.07), but not a lower mental development index (MDI, p=0.15), was observed in subjects with the lowest hemoglobin levels. Subjects in the lowest quartile also had a higher mean WBC (18.0 vs. 11.4 × 109/L, p<0.001), absolute neutrophil count (5.3 vs. 3.6 × 109/L, p=0.001), and platelet count (416 vs. 315 × 109/L, p=0.0001) compared to those with the highest hemoglobin. The laboratory findings of patients receiving HU also were significantly lower than those of subjects in the lowest hemoglobin group and similar to those in the highest hemoglobin group.

Severe anemia in very young patients with SCA was associated with elevated WBC and platelet counts and higher TCD velocities. Interestingly, severe anemia also was associated with increased frequency of ACS, in contrast to the association of ACS with higher hemoglobin levels in older patients. Future studies will need to confirm this relationship and clarify to what extent associated findings (e.g., hyposplenia, elevated WBC or platelet count) contribute to this susceptibility. Severe anemia in young SCA patients is a negative prognostic factor that is significantly impacted by early hydroxyurea therapy.

Off Label Use: Hydroxyurea is not approved by FDA for infants with sickle cell disease. Miller:NIH/NHLBI; Emmaus Med Inc, Novartis Pharmaceutical; St Jude Childrens Hospital: Research Funding.