Primaquine is contraindicated in patients with P. vivax infections due to a risk of hemolysis. However, a novel humanized murine study identified specific conformational changes in glucose 6-phosphate dehydrogenase deficiency that confirm that activity of the enzyme decreases with RBC age and support dose escalation regimens, as currently suggested by clinical studies.
Approximately 500 million individuals globally have a genetic deficiency in glucose 6-phosphate dehydrogenase (G6PD). This deficiency shortens the antioxidant capacity of red blood cells (RBCs), rendering individuals susceptible to life-threatening hemolysis when exposed to certain drugs that induce oxidative stress.
Notably, the only approved drugs capable of curing liver-phase Plasmodium vivax (P. vivax) infections, namely primaquine, and tafenoquine, are contraindicated in G6PD-deficient (G6PDd) patients due to the associated risks of hemolysis. A new study shared at the 65th ASH Annual Meeting & Exposition suggests a new and safer approach to primaquine use in such patients.
Understanding Primaquine-Induced Hemolysis in Glucose-6 Phosphate Dehydrogenase-Deficient Patients
Effective treatment of G6PDd individuals infected with P. vivax requires a prolonged course of primaquine spanning 8 weeks. However, this protracted regimen often results in non-adherence, subsequently diminishing efficacy and elevating the risk of disease transmission. Despite extensive research efforts and large-scale drug screening programs, the identification of a non-hemolytic compound capable of eradicating P. vivax remains elusive.
The lack of progress in this direction is partly due to an incomplete mechanistic understanding of primaquine-induced hemolysis, compounded by the absence of an animal model that recapitulates G6PD instability. To address these challenges, researchers engineered a humanized mouse model in which murine genomic G6PD was replaced with either genomic human hG6PD(ND) or hG6PD(MED).
Recombinant forms of non-deficient human G6PD (hG6PD(ND)) and the deficient Mediterranean variant (hG6PD(MED)) were expressed and purified. These proteins were subjected to thermal proteome profiling and TMT10-assisted quantitative cross-linking proteomics to delineate specific conformational changes contributing to decreased specific activity and half-life in the hG6PD(MED) variant.
Results revealed that the hG6PD(MED) enzyme exhibited a 2.6-fold lower specific activity compared to that of the hG6PD(ND) enzyme. Proteomic analysis identified specific amino acid spacing associated with distinct conformational changes. The mouse model demonstrated that hG6PD(MED) RBCs had 5% of G6PD activity compared to hG6PD(ND) RBCs, with a decrease in G6PD activity correlating with RBC age. Exposure to primaquine metabolites induced various markers of hemolysis in hG6PD(MED) RBCs compared to hG6PD(ND) RBCs. However, younger cells with higher G6PD activity displayed resistance to hemolysis, both in vivo and in vitro.
The Bottom Line
These findings hold potential practical significance, justifying clinical studies proposing new dose escalation primaquine regimens to induce low-level early hemolysis, leading to reticulocytosis, and decreasing the mean age of circulating RBCs, thereby allowing increased doses of primaquine to be tolerated in G6PDd patients.
Zimring, J. (2023a, December 9). Mechanisms of primaquine induced hemolysis in a novel humanized murine model of Mediterranean G6PD deficiency. https://ash.confex.com/ash/2023/webprogram/Paper178739.html