Design, Synthesis, and Evaluation of Allosteric Effectors for Hemoglobin

Conspectus Sickle cell disease (SCD) is an inherited blood disorder caused by a point mutation in hemoglobin (Hb), the protein in the red blood cell (RBC) responsible for the transport of oxygen (O 2 ) throughout the body. The mutation leads to the expression of sickle cell hemoglobin (HbS). Both Hb and HbS exist in equilibrium between oxygenated and deoxygenated forms; however, deoxygenated HbS can polymerize to form long fibers which distort the shape of RBCs into the characteristic sickled shape. The misshapen RBCs can obstruct blood vessels and capillaries, resulting in a vaso-occlusive crisis. Vaso-occulsion deprives tissues and organs of O 2 and can cause intense pain which often results in hospitalization. Chronic organ damage is a major cause of reduced life expectancy for SCD patients. Allosteric effectors are molecules which regulate protein function. HbS allosteric effectors can be used to decrease polymerization by stabilizing the oxygenated form of HbS, which leads to an increase in O 2 uptake and a decrease in the sickling of RBCs. Allosteric effectors that have been evaluated for the treatment of SCD include vanillin, 5-hydroxymethyl furfural (5-HMF), and voxelotor, which was approved by the U.S. Food and Drug Administration (FDA) for the treatment of SCD in 2019. 5-HMF did not progress to phase III clinical trials since it suffered from rapid metabolic degradation. However, several derivatives of 5-HMF and vanillin have been synthesized and evaluated as potential candidates for SCD treatment. Derivatives of these compounds have shown promise, but their shortcomings, such as high levels of oxidative metabolism, have prevented them from progressing into marketable drugs. Our efforts have produced multiple 5-HMF derivatives which have been evaluated for their potential to treat SCD. Each derivative was evaluated for its ability to increase O 2 affinity (i.e., P 50, the partial pressure at which hemoglobin is 50% saturated with O 2 ). The synthesized aryl ether derivatives were evaluated, and results suggest that compounds with multiple aromatic aldehydes may have enhanced biological properties. One such derivative, compound 5, which features two furan aldehyde rings, exhibited increased O 2 affinity ( P 50 = 8.82 ± 1.87 mmHg) over that of unmodified Hb ( P 50 = 13.67 ± 0.22 mmHg). Future studies include obtaining crystal structures of the 5-HMF derivatives complexed with HbS to confirm the protein–allosteric effector interactions.