Artesunate Artemisinins

Artemisia annua L. A. apiacea Hance ex Walpers Qing Guo (Stinking artemisia) (aerial part) Dihydroartemisinin, artesunate, artemisinin, chloroquine, flavonoids, sesquiterpene.33-269-476 This herb is mildly toxic. A schizonticidal agent, antimalarial, treat infections of multidrug-resistant strains of Plasmodium falciparum, the cause of human malignant cerebral malaria. 

Artemisia annua L. China Dihydroartemisinin, artesunate, artemisinin, chloroquine.33 This herb is mildly toxic. A schizonticidal agent, antimalarial. 

Artemisinin and its derivatives, artesunate and arthemether, kill both asexual and sexual blood stages (Fig. 2). However, artemisinins are quickly eliminated from the body, resulting in parasite recrudescence, and are therefore combined with schizontocides that have a longer biological half-life, such as amodiaquine,... 

Antiprotozoal Drugs. Figure 5 Artemisinin combination therapy (ACT) Adding a 3-days artesunate course to mefloquine cleats the parasitaemia much more rapidly (A — A). The remaining parasites are exposed to higher mefloquine levels in ACT (B) compared to mefloquine monotherapy (B (with permission White, 1997 Antimicrob Agents Chemother 41 1413-1422). 

Deoxoartemisinin and carboxypropyldeoxoartimisinin have also been shown to have anti-tumour activity and, NMR studies on solution conformations have been reported <00BBR359>. One of the problems with artemisinin use is its poor water solubility characteristics. An attempt to rectify this, and to overcome stability problems associated with sodium artesunate in solution, has involved the introduction of amino group functionality as in 127 (eg. R = 0(CH2)3NR r2 where NR r2 = morpholine). The maleate salt of this compound has reasonable water solubility and aqueous solutions are stable at room temperature for an extended time. However activity against Plasmodium knowlesi in rhesus monkeys after oral administration was poorer compared with artesunic acid <00JMC1635>. 

Artemisinin-based regimens are often regarded as safe and effective drugs in the recent years however, clinically relevant artemisinin resistance has been reported both from laboratory and field studies. Some of these studies have shown that P. falciparum has reduced in vivo susceptibility to artesunate in western Cambodia as compared with northwestern Thailand. This resistance was characterized by a slow parasite clearance in vivo, without corresponding reductions on conventional in vitro susceptibility testing.Although this resistance to artemisinin is still very mild and limited, its emergence would be disastrous because of the lack of alternative treatments. 

Li GQ. (1989) Clinical studies on artemisinin suppository and on artesunate and artemether. In S. Jiaxiang (ed.), Antimalarial drug development in China. National Institute of Pharmaceutical Research and Development, Beijing, People s Republic of China, pp. 69-73. 

Efferth T, Romero MR, Wolf DG Stamminger T, Marin JJ, Marschall M. (2008) The antiviral activities of artemisinin and artesunate. Clin Infect Dis 47 804-811. 

Artemisinin (18) is a natural product for which many semisynthetic derivatives have been generated. The major rationale to produce these derivatives was to deal with the low aqueous solubility of artemisinin and its short half-life in plasma. The lipid-soluble arteether (22) and artemether (23), and the water-soluble sodium artesunate (24), were designed for... 

In addition to artemisinin, other synthetic trioxanes and endoperoxides (fenozan BO-7 4 and arteflene 5 " ) have enjoyed some success arteflene reached Phase II pre-clinical trials. More recently, Vennerstrom and coworkers have reported on the outstanding antimalarial properties of several 1,2,4-trioxolanes, one of which, OZ 277 (6), has entered clinical trials in man . These exciting, easily prepared drugs will be discussed in detail later in this chapter. In order to determine the parasiticidal action of this class of antimalarial, many research groups have focused their efforts on artemisinin and its semi-synthetic derivatives (artemether, arteether and artesunate Ic, Id and le), and this is the point where our discussion will begin. 

Woerdenbag and coworkers reported on the cytotoxicity of artemisinin endoperoxides to Ehrlich ascites tumour cells . Artemisinin had an IC50 of 29.8 p.M, whereas arteether, artemether, artelininc acid and sodium artesunate all had more potent activities, ranging from 12.2 to 19.9 p.M. It was found that opening of the lactone ring of artemisinin dramatically reduced the cytotoxicity. An ether dimer of dihydroartemisinin 106, prepared by... 

Artemisinin is very poorly soluble in water or oil and can thus only be administered orally. Active derivatives have been synthesized such as artemether, arteether and beta-arteether (Artemotil), artelinic acid and artesunate, which are used for oral, intramuscular, rectal and intravenous administration. Dihydroartemisinin is the active metabolite of all artemisinin compounds and is also available as a drug in itself (see Fig. 2). 

Oral formulations of artemisinin and its derivatives are absorbed rapidly but incompletely. Peak plasma concentrations are reached in 1-2 h. A relative bioavailability of 43% was found for oral artemether compared to intramuscular administration. The absolute bioavailability of artesunate, the only derivative for which an intravenous formulation exists, was about 15%. Artesunate is extensively hydrolyzed to dihydroartemisinin in the gastro-intestinal lumen before first-pass metabolism in the gut wall and liver takes place. Artesunate acts like a prodrug with fast transformation into... 

Artemisinin DihydroartemIsinIn Artemether Arteether Artelinic acid Artesunic acid... 

Three currently-used artemisinin based combination therapies (ACT) artesunate-mefloquine, artemether-lumefantrine and dihydroartemisinin-piperaquine, have been proven highly simple, safe and effective in the treatment of multidrug resistant P. falciparum malaria. 

Artemisinin is a natural endoperoxide-containing sesquiterpene, isolated from a plant used in traditional Chinese medicine. Acetalic artemisinin derivatives (arte-mether, artesunate) are very active against chemo-resistant forms of Plasmodium falciparum, and are clinically used for the treatment. However, they suffer from an unfavourable pharmacological profile. They are quickly metabolised by fast oxidative metabolism, hydrolytic cleavage and glucuronidation. 

Dihydroartemisinin (DHA) is the active metabolite of acetalic derivatives of artemisinin (artemether, artesunate). Oxidation by cytochrome P450 enzymes or/and hydrolysis provides DHA, which is itself poorly stable in vivo. Indeed, the corresponding oxonium ion, a precursor of inactive metabolites by ring opening or by glucuronidation, can easily be formed (Figure 4.15). 

An FIPLC method using electrochemical detection in the reductive mode for the determination of artemether 28a and its metabolite dihydroartemisinin 29a <1997JCFI(B)145> and for the simultaneous quantification of artesunate 31 and dihydroartemisinin 29a in plasma has been developed <1997JCFI(B)259, 1998JCFI(B)201>. An effective reversed-phase FIPLC method using electrochemical and UV detection has been developed for the simultaneous determination in plant extracts of artemisinin and its bioprecursors such as arteannuin B 32a, and artemisitene 27 <1995JNP798, 2001JIC489>. 

Since this chapter was prepared, the only chemical developments in compounds belonging to this class have involved artemisinin and its derivatives, and some of these will be summarized here. Using combustion calorimetry, Liu and co-workers have determined the standard molar enthalpies of formation of artemisinin 9a, artemether 28a and artesunate 31 as —1493, —2420 and —3320kJmoP respectively, and the thermal stabilities of these compounds were also investigated by thermal analysis in combination with FT-IR <2007MI1045>. 

Artemisinins (artesunate, artemether,1 dihydroartemisinin1) Sesquiterpene lactone endoperoxides Treatment of P falciparum infections oral combination therapies for uncomplicated disease intravenous artesunate for severe disease... 

Mefloquine is effective in treating most falciparum malaria. The drug is not appropriate for treating individuals with severe or complicated malaria, since quinine, quinidine, and artemisinins are more rapidly active, and since drug resistance is less likely with those agents. The combination of artesunate plus mefloquine showed excellent antimalarial efficacy in regions of Southeast Asia with some resistance to mefloquine, and this regimen is now one of the combination therapies recommended by the WHO for the treatment of uncomplicated falciparum malaria (Table 52-4). Artesunate-mefloquine is the first-line therapy for uncomplicated malaria in a number of countries in Asia and South America. 

Artemisinins are also proving to have outstanding efficacy for the treatment of complicated falciparum malaria. Large randomized trials and meta-analyses have shown that intramuscular artemether has an efficacy equivalent to that of quinine and that intravenous artesunate is superior to intravenous quinine in terms of parasite clearance time and—most important—patient survival. Intravenous artesunate also has a superior side-effect profile compared with that of intravenous quinine or quinidine. Thus, intravenous artesunate will likely replace quinine as the standard of care for the treatment of severe falciparum malaria, although it is not yet widely available in most areas. Artesunate and artemether have also been effective in the treatment of severe malaria when administered rectally, offering a valuable treatment modality when parenteral therapy is not available. 

The pharmaceutical properties of artemisinin are far from optimal it is insoluble in water and only marginally soluble in oil. It has poor oral bioavailability and has been administered for the treatment of Plasmodium falciparum malaria in humans at total doses of about 1 g (over 3 days). Early studies by Chinese scientists in 1979 led to the discovery of dihydroartemisinin 3, artemether 4 (Artenam), and sodium artesunate 5, oil and water soluble derivatives, respectively (Figure 9.1 ).6-7 These drugs are currently in clinical use in Asia in a number of preparations such as suppositories, i.v. injectables, oil depos, to name only a few.8 Capsules containing 0.5 g of artemisinin for oral administration are available in Vietnam. 

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