A medicinal chemistry perspective on artemisinin and related endoperoxides.

ion and allylic carbon radical formation with subsequent triplet ground-state oxygen capture result ultimately in the formation of lipid hydroperoxides. The explicit mechanism depicted in Scheme 1 is supported by the work of Berman and Adams and others, and it was proposed that the damage caused to the parasite’s FV membrane leads to vacuolar rupture and parasite autodigestion.41 This mechanism is consistent with the observed oxygen dependence of antimalarial action of artemisinin and the morphological changes seen following artemisinin administration to parasites in vitro.44 The biological significance of hydroperoxides in relation to biological hydroxylation and autoxidation of, for example, lipids and membrane bilayers is well established. The generation of unsaturated lipid hydroperoxides provides a means of initiation of such processes. In contrast to these proposals, other workers in the field have suggested that membrane-bound heme may have a role to play in reducing the effectiveness of endoperoxides such as dihydroartemisinin. Further work is required to clarify the role of vacuolar membrane bound heme in the mechanism of action of endoperoxide antimalarials.45 An alternative nonlipid “artemisinin derived source of hydroperoxide” is discussed below. Although Scheme 1 would appear chemically plausible, several workers have proposed that the parasite death in the presence of artemisinin is probably not due to nonspecific or random cell damage caused by freely diffusing oxygen radical species but might involve specific radicals and targets, some of which are described later in this review. The following section details the specific “transitory” species that may be responsible for the antimalarial mechanism of action of artemisinin. Targets of these species are discussed and will conclude with the most recent studies by Eckstein-Ludwig who suggest that artemisinin derivatives target the PfATP6, the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) of the parasite.46 Transitory Species Mediating the Antimalarial Activity of Artemisinin: Carbon Radicals, Carbocations, Hydroperoxides, and High-Valent IronOxo Species. On the basis of the seminal work of Posner and co-workers in the early 1990s, the free radical chemistry of artemisinin is now very welldefined and has been shown to involve an initial chemical decomposition induced by heme Fe(II) (reduced hemin) or other sources of ferrous iron within the malaria parasite to produce initially an oxy radical that subsequently rearranges into one or both of two distinctive carbon-centered radical species.47 Scheme 2a summarizes the main radical pathways available for artemisinin following endoperoxide-mediated bioactivation. Since artemisinin is an unsymmetrical endoperoxide, the oxygen atoms of the peroxide linkage can associate with reducing ferrous ions in two ways. Association of Fe(II) with oxygen 1 provides an oxy radical that goes on to produce a primary carbon-centered radical (5a). A surrogate marker for the intermediacy of this radical species is the ring-contracted tetrahydrofuran (RCT) product 5b. Alternatively, association with oxygen 2 provides an oxy radical species that, via a 1,5-H shift, can produce a secondary carbon-centered radical (5c). Again, like the previous route, a stable end-product, hydroxydeoxoartemisinin (HDA) (5d), functions as a surrogate marker for this secondary carbon-centered radical species. It has been proposed that final alkylation by these reactive intermediates of biomacromolecules such as Scheme 1. Proposed Chemical Mechanism for the Observed Artemisinin Mediated Lipid Peroxidation of Cell Membranesa a Carbon radicals, generated from artemisinin, abstract allylic hydrogen atoms from unsaturated lipid bilayers to set in motion the downstream generation of a variety of reactive oxygen species by a classic lipid peroxidation mechanism. Heme or hematin/thiols associated with the lipid bilayer may be the catalysts responsible for initiation of these processes. Perspective Journal of Medicinal Chemistry, 2004, Vol. 47, No. 12 2947