Mixed tetraoxanes 5a and 13 synthesized from cholic acid and 4-oxocyclohexanecarboxylic acid were as active as artemisinin against chloroquine-susceptible, chloroquine-resistant, and multidrug-resistant Plasmodium falciparum strains (IC50, IC90). Most active 13 is metabolically stable in in vitro metabolism studies. In vivo studies on tetraoxanes with a C(4'') methyl group afforded compound 15, which cured 4/5 mice at 600 and 200 mg, kg(-1), day(-1), and 2/5 mice at 50 mg, kg(-1), day(-1), showing no toxic effects. Tetraoxane 19 was an extremely active antiproliferative with LC50 of 17 nM and maximum tolerated dose of 400 mg/kg. In Fe(II)-induced scission of tetraoxane antimalarials only RO center dot radicals were detected by EPR experiments. This finding and the indication of Fe(IV)=O species led us to propose that RO center dot radicals are probably capable of inducing the parasite's death. Our results suggest that C radicals are possibly not the only lethal species derived from peroxide prodrug antimalarials, as currently believed.
