The Fe²⁺‐Mediated Decomposition, PfATP6 Binding, and Antimalarial Activities of Artemisone and Other Artemisinins: The Unlikelihood of C‐Centered Radicals as Bioactive Intermediates

Abstract

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 aminoartemisinins tend to be inert to aqueous iron(II) sulfate and anhydrous iron(II) 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 % yield from the trioxolane, and 3 % from the artemisinin. Artemisone provides a trapped product in 10 % 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 (DFO) 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.

Experimental Therapy of Hepatoma with Artemisinin and Its Derivatives: In vitro and In vivo Activity, Chemosensitization, and Mechanisms of Action

“…The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and other chemicals used in this study were of analytic grade and purchased from Sigma-Aldrich, Inc. Tween 20 was purchased from Promega Corp., and the Annexin V-FITC Apoptosis Detection kit was purchased from BioVision, Inc. The primary antibodies against Bcl-2 (100), Bax (N-20), caspase-3 p20 (N-19), E2F1 (C-20), cyclin D1 (DCS-6), cyclin E (HE12), cyclin-dependent kinase (Cdk) 2 (M2), Cdk4 (H-22), Cip1/p21 (187), Kip1/p27 (C-19), poly(ADP-ribose) polymerase (PARP; H-250), Rb (C-15), MDM2 (SMP14), p53 (Pab1801), glyceraldehyde-3-phosphate dehydrogenase (0411), and h-actin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) were from Santa Cruz Biotechnology, Inc. The secondary antibodies, horseradish peroxidase -linked anti-mouse immunoglobulin G, antigoat immunoglobulin G, and anti-rabbit immunoglobulin G, were also purchased from Santa Cruz Biotechnology.…”

Relationship between Antimalarial Activity and Heme Alkylation for Spiro- and Dispiro-1,2,4-Trioxolane Antimalarials

“…Despite the known potency of artemisinin and the numerous semisynthetic and fully synthetic analogues, the exact mechanism of action remains unresolved (9,10). It is known that the peroxide bond is essential for activity, and it is generally accepted that the peroxide reacts with intraparasitic iron to form free radicals, carbocations, or other reactive species (15).…”

Synthesis, reactions and biological activity of 11-azaartemisinin and derivatives

“…10 While reaction with methyl propiolate and DMAP in acetonitrile yielded the product 14 in high yield after short reaction time, the additions of 6a to 3-butyn-2-one and N,N-dimethyl propiolamide were much more sluggish and incomplete. NMR analysis of the adduct suggests E-stereochemistry of the double bond, based on the magnitude of the coupling constant J 13-14 (14)(15)(16). However, when the EWG was a nitrile, both isomers were formed, with the major product (40% isolated yield) possessing the Z-geometry (J 13-14 = 11 Hz).…”

“…In the course of their study on the Fe 2+ -mediated decomposition of artemisinin derivatives, the group of Haynes isolated the desoxo enamines 35a and 35b as part of the reduction products of 36a and 36b respectively (Structure block 6). 16 Enamine 35a could be converted to the corresponding hydroxy lactam 37 by treating it with ruthenium trichloride/sodium periodate.…”