The primary efficacy endpoint was chosen as the effect of ETA receptor blockade on the reduction of microalbuminuria based upon findings in diabetic nephropathy, which has a similar pathogenesis to sickle nephropathy. A 30% reduction in proteinuria is rather conservative and realistic based upon the results of studies of ETA receptor blockade in diabetic nephropathy that consistently resulted in 40-45% reduction in proteinuria. Pre-clinical data has shown that changes can be detected in urinary nephrin excretion before overt proteinuria is observed in a model of chronic ET-1 elevation. Furthermore, tubular injury that can occur as a result of an intrarenal vaso-occlusive crisis (VOC) should be expected to increase the level of renal tubular injury markers, neutrophil gelantinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), and netrin. Treatment with ambrisentan would be expected to attenuate these changes. Dr. Jennifer Pollock, has extensive experience with all of the biomarker assays including ET-1. Dr. Pollock runs bio-analytical core labs for two PO1s and has published extensively on these and similar methods in human and animal samples.