The Fe2+-mediated decomposotion, PfATP6 binding, and antimalarial activities of artemisone and other arteminisins:: The unlikehood of C-centered radicals as bioactive intermediates

The results of Fe2+-induced decomposition of the clinically used artemisinins, artemisone, other aminoartemisinins, 10-deoxoartemisinin, and the 4-fluorophenyl derivative have been compared with their antimalarial activities and their ability to inhibit the parasite SERCA PfATP6. The clinical artemisinins and artemisone decompose under aqueous conditions to give mixtures of C radical marker products, carbonyl compounds, and reduction products. The 4-fluorophenyl derivative and aminoortemisinins tend to be inert to aqueous iron(II) sulfate and anhydrous iron(11) acetate. Anhydrous iron(II) bromide enhances formation of the carbonyl compounds and provides a deoxyglycal from DHA and enamines from the aminoartemisinins. Ascorbic acid (AA) accelerates the aqueous Fe2+-mediated decompositions, but does not alter product distribution. 4-Oxo-TEMPO intercepts C radicals from a mixture of an antimalaria-active trioxolane, 10-deoxoartemisinin, and anhydrous iron(II) acetate to give trapped products in 73 96 yield from the trioxolane, and 3 % from the artemisinin. Artemisone provides a trapped product in 10 916 yield Thus, in line with its structural rigidity, only the trioxolane provides a C radical eminently suited for intermolecular trapping. In contrast the structural flexibility of the C radicals from the artemisinins allows facile extrusion of Fe2+ and collapse to benign isomerization products. The propensity towards the formation of radical marker products and intermolecular radical trapping have no relationship with the in vitro antimalarial activities of the artemisinins and trioxolane. Desferrioxamine (DFQ) attenuates inhibition of PfATP6 by, and antagonizes antimalarial activity of the aqueous Fe2+ -susceptible artemisinins, but has no overt effect on the aqueous Fe2+-inert artemisinins. It is concluded that the C radicals cannot be responsible for antimalarial activity and that the Fe2+ -susceptible artemisinins may be competitively decomposed in aqueous extra- and intracellular compartments by labile Fe2+, resulting in some attenuation of their antimalarial activities. Interpretations of the roles of DFO and AA in modulating antimalarial activities of the artemisinins, and a comparison with antimalarial properties of simple hydroperoxides and their behavior towards thapsigargin-sensitive SERCA ATPases are presented. The general basis for the exceptional antimalarial activities of artemisinins in relation to the intrinsic activity of the peroxide within the uniquely stressed environment of the malaria parasite is thereby adumbrated.