Alternative NADPH-regenerating pathways to support methylene blue therapy in G6PD deficiency: the role of stress-induced reticulocyte metabolism

Abstract

Methylene blue (MB) is a potent chemical agent used to treat methemoglobinemia. However, its clinical use is dependent on NADPH. In a healthy body, the pentose phosphate pathway (PPP) provides NADPH via glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD). G6PD deficiency is the most common enzymopathy of erythrocytes, but 6PGD deficiency is among the rarest enzymopathies. G6PD deficiency creates a dangerous situation, depending on the severity of the enzyme deficiency. G6PD-deficient erythrocytes cannot regenerate enough NADPH and reduced glutathione. In NADPH-deficient cells, MB cannot be reduced to leucomethylene blue (LMB), leading to increased ROS and oxidative hemolysis. Because native transporters like Band 3 may fail to transport polar exogenous substrates efficiently, this study proposes G6PD-independent delivery strategies using 6-phosphogluconate, malate, and isocitrate. First, encapsulate these substrates within glycophorin A-containing liposomal vesicles to deliver them via membrane fusion. Second, modify them with pivaloyloxymethyl or isopropyloxycarbonyloxymethyl esters to make them lipophilic. After cellular entry, erythrocyte-specific esterases cleave these esters, releasing substrates to react with 6PGD, isocitrate dehydrogenase, and malic enzyme for NADPH reduction. Third, utilize Cell-Penetrating Peptides (CPPs) for direct transport. Fourth, utilize a hybrid system combining glycophorin A-containing nano-liposomes with arginine-rich CPPs. These approaches are theoretically expected to make MB therapy a safer treatment option for at-risk G6PD-deficient individuals. Mitochondrial pathways are limited in mature erythrocytes, as circulating reticulocytes constitute 1–2% of blood; however, stress erythropoiesis can boost this pool to 15%, transiently supporting clinical efficacy.