Identification and characterization of thiamine analogs with antiplasmodial activity.

Thiamine is metabolized into thiamine pyrophosphate (TPP), an essential enzyme cofactor. Previous work has shown that oxythiamine, a thiamine analog, is metabolized by thiamine pyrophosphokinase (TPK) into oxythiamine pyrophosphate within the malaria parasite and then inhibits TPP-dependent enzymes, killing the parasite and . To identify a more potent antiplasmodial thiamine analog, 11 commercially available compounds were tested against and . Five active compounds were identified, but only N3-pyridyl thiamine (N3PT), a potent transketolase inhibitor and candidate anticancer lead compound, was found to suppress proliferation with an IC value 10-fold lower than that of oxythiamine. N3PT was active against and was >17 times less toxic to human fibroblasts, as compared to oxythiamine. Increasing the extracellular thiamine concentration reduced the antiplasmodial activity of N3PT, consistent with N3PT competing with thiamine/TPP. A transgenic line overexpressing TPK was found to be hypersensitized to N3PT. Docking studies showed an almost identical binding mode in TPK between thiamine and N3PT. Furthermore, we show that [H]thiamine accumulation, resulting from a combination of transport and metabolism, in isolated parasites is reduced by N3PT. Treatment of -infected mice with 200 mg/kg/day N3PT reduced their parasitemia, prolonged their time to malaria symptoms, and appeared to be non-toxic to mice. Collectively, our studies are consistent with N3PT competing with thiamine for TPK binding and inhibiting parasite proliferation by reducing TPP production, and/or being converted into a TPP antimetabolite that inhibits TPP-dependent enzymes.