Sickle cell disease in the new era: advances in drug treatment

Sickle cell disease is an inherited blood disorder afflicting an estimated 100,000 individuals in the United States and over 20 million people worldwide. The disease is heralded as the first molecular disease. However, despite its genetic simplicity, the pathophysiologic processes leading to its clinical sequelae are complex, heterogeneous and interrelated, making drug development to treat the disease challenging. For over two decades only one drug, hydroxyurea, had been used as disease-modifying therapy. New pharmacologic agents are rapidly evolving with three new drugs, with different mechanisms of action, approved by the United States Food and Drug Administration in recent years (L-glutamine, crizanlizumab and voxelotor). Several therapeutic approaches targeting different pathways in the disease pathophysiology are being investigated. These include inhibition of hemoglobin S polymerization such as by fetal hemoglobin induction or by increasing hemoglobin oxygen affinity, as well as intervention of downstream pathways including inhibiting cellular adhesion, reducing inflammation and oxidant stress, modulating platelet activation and coagulation abnormalities, and targeting nitric oxide signaling. This review will provide an overview of these therapeutic strategies, discuss the four currently approved drugs in detail, and summarize ongoing clinical trials of new drugs or drug indications for the treatment of sickle cell disease in different phases of development excluding those related to cellular therapies.