SICKLE CELL DISEASE : CHALLENGES AND PROGRESS Beyond hydroxyurea : new and old drugs in the pipeline for sickle cell disease

The simplicity of the genetic mutation that causes sickle cell disease (SCD) belies the complexity of the disease’s pathophysiology.A single base-pair change (A→T), and the ensuing alteration of one amino acid (glutamic acid replaced by valine) in theb chain of hemoglobin (Hb), a protein only expressed in erythrocytes, nevertheless causes a multiorgan disease with many complex pathophysiologic mechanisms (Figure 1). Thus, therapeutic approaches may target the “root cause” (ie, by replacement of the abnormal hemoglobin), as do stem cell transplantation and gene therapy, or one ormore of themany damaging and interwoven pathways responsible for the disease’s cardinal manifestations—episodic severely painful vaso-occlusive episodes (VOC), hemolytic anemia, and progressive multiorgan damage. Red cells that contain primarily HbS or HbSwith one of the variants that interacts with it, such as HbC, are abnormal in many respects, including that as a result of hemolysis they are overall much younger than normal erythrocytes. The fundamental defect in sickle red blood cells (SS RBCs) is the insolubility of HbS when it becomes deoxygenated, leading to formation of polymers that aggregate into tubular fibers and, as they enlarge, deform red cells, causing the characteristic sickle shape. In addition, SS RBCs become dehydrated, have abnormally activated intracellular signaling pathways, have decreased nitric oxide and adenosine triphosphate content and antioxidant capacity, demonstrate oxidative damage to many cellular components, and reflect dysregulation of miRNAs and gene expression during erythropoiesis. Cellular dehydration contributes to deoxygenated hemoglobin polymer formationandultimatelycell sicklingandhemolysis. Signaling pathways downstream of the b2 adrenergic receptor and protein kinase A result in activation of MEK and ERK as well as several cell surface adhesion receptors.Oxidativedamageofmembraneproteins andaggregation of proteins along the inner surface of the plasma membrane led to further intracellular abnormalities. At their surfaces, SSRBCsdemonstrate altered lipid sidedness,with markedly increased phosphatidylserine exposure. Along with the formation of microparticles, phosphatidylserine exposure contributes to the procoagulant activity of SS RBCs. SS RBCs also evince abnormal adhesive properties, including activation of known adhesion receptors (including BCAM/Lu, ICAM-4, and CD44) and increased interactions with leukocytes, platelets, endothelial cells, and extracellular matrix proteins. Abnormal SS RBC cell-cell signaling can activate both leukocytes and endothelial cells, making both more easily involved in adhesive interactions and also driving endothelial cell expression of procoagulant proteins. SS RBCs are also stiffer than normal red cells in the circulation. Wide-field digital interferometry (WFDI) examination of normal red cells, normal-appearing SS RBCs, and sickled RBCs has shown that normal-appearingHbSS red cells are 2 to 3 times stiffer thanHbAA red cells, and sickled RBCs are about 2 times stiffer than normal-appearing SS RBCs. Thus,newdrugdevelopment aswell as trials of existing compounds have targeted one or more of these pathophysiologic factors (Figure 1) in an effort to improve the overall prognosis of SCDaswell as to reduce or treat its cardinal manifestation, vaso-occlusion. Given the diversity of therapeutic targets and pharmacokinetics of potential drugs, trials of new therapies have focused on a variety of different outcomes, including prevention of SCD events (such as the frequency of both VOC and acute chest syndrome (ACS), both reduced by hydroxyurea) and the ability to shorten the course of acute VOC. However, because resolution of VOC-related pain is a patient-reported outcome generally not accepted as a clinical outcome for SCD therapies, other end points—such as length of stay, time to discharge, or time to readiness for discharge—have been used. Unfortunately, these end points occur at highly variable time points among patients, so that achievement of statistically significant differences has beenquite challenging.Partly for that reason, phase 2 studies have often used more easily quantified and sometimes less variable surrogate end points, although not always with great success.