Atovaquone resistance in malaria parasites

Drug-resistant malaria

In the past 21 years, a modest increase in the range of antimalarial drugs approved for clinical use has been complemented by a more impressive expansion in the analysis and understanding of the molecular mechanisms underlying resistance to these agents. Such resistance is a major factor in the increasing difficulty in controlling malaria, and important developments during this period are recounted here.

In vitro recrudescence of Plasmodium falciparum parasites suppressed to dormant state by atovaquone alone and in combination with proguanil

We studied the viability of Plasmodium falciparum parasites reappearing in long-term cultures after repetitive exposure to atovaquone and proguanil. Parasites (F32 and FCR3) exposed to 100-5000 nM atovaquone for 96 hours were reduced to <5% of initial parasitaemia but recrudesced after 9-15 days. Also, parasites exposed to 1000 nM atovaquone for 48, 72, 96 and 144 hours recrudesced after 9, 14, 21 and 23 days respectively. Immediately after removal of the drug, only 1-3 schizonts per 10000 red blood cells were found consistently, apparently unable to produce trophozoites and thus, possibly, adopting a "dormant state". Parasites (F32 and FCR3) exposed to 500 nM atovaquone for 72 hours reappeared after 14 days. These recrudescing parasites were then re-exposed and suppressed by atovaquone in three consecutive follow-up experiments. They reappeared after 12, 11 and 9 days respectively. No known point mutations in cytochrome b gene (cytb), associated with atovaquone resistance, were detected in any recrudescing parasites. Finally, parasites (F32) exposed to various concentrations of atovaquone and proguanil in combination for 72 hours reappeared after 9-17 days. The baseline susceptibilities of the parasites to individual drugs were similar before and after recrudescence in all experiments.

Isoquine and Related Amodiaquine Analogues: A New Generation of Improved 4-Aminoquinoline Antimalarials

Amodiaquine (AQ) (2) is a 4-aminoquinoline antimalarial that can cause adverse side effects including agranulocytosis and liver damage. The observed drug toxicity is believed to involve the formation of an electrophilic metabolite, amodiaquine quinoneimine (AQQI), which can bind to cellular macromolecules and initiate hypersensitivity reactions. We proposed that interchange of the 3' hydroxyl and the 4' Mannich side-chain function of amodiaquine would provide a new series of analogues that cannot form toxic quinoneimine metabolites via cytochrome P450-mediated metabolism. By a simple two-step procedure, 10 isomeric amodiaquine analogues were prepared and subsequently examined against the chloroquine resistant K1 and sensitive HB3 strains of Plasmodium falciparum in vitro. Several analogues displayed potent antimalarial activity against both strains. On the basis of the results of in vitro testing, isoquine (ISQ1 (3a)) (IC(50) = 6.01 nM +/- 8.0 versus K1 strain), the direct isomer of amodiaquine, was selected for in vivo antimalarial assessment. The potent in vitro antimalarial activity of isoquine was translated into excellent oral in vivo ED(50) activity of 1.6 and 3.7 mg/kg against the P. yoelii NS strain compared to 7.9 and 7.4 mg/kg for amodiaquine. Subsequent metabolism studies in the rat model demonstrated that isoquine does not undergo in vivo bioactivation, as evidenced by the complete lack of glutathione metabolites in bile. In sharp contrast to amodiaquine, isoquine (and Phase I metabolites) undergoes clearance by Phase II glucuronidation. On the basis of these promising initial studies, isoquine (ISQ1 (3a)) represents a new second generation lead worthy of further investigation as a cost-effective and potentially safer alternative to amodiaquine.

Detection of atovaquone and Malarone resistance conferring mutations in Plasmodium falciparum cytochrome b gene (cytb)

Clinical treatment failures of the hydroxynaphthoquinone atovaquone or its combination with proguanil (Malarone) in Plasmodium falciparum malaria has been recently documented. These events have been associated to single nucleotide polymorphisms (SNPs) in the parasite cytochrome b gene (cytb). In this report we describe a set of nest PCR-RFLP methods developed for the fast detection of all known cytb mutations associated to resistance to these drugs. The methods were successfully applied for the analysis of phenol-chloroform extracted DNA samples from patients not cured by Malarone, and from an established parasite clone. Further, the protocol for the detection of the A803C mutation was applied to 164 DNA field samples extracted through crude methanol-based protocols, originated from several malaria settings. The PCR-RFLP methods here presented can be used as a valuable for the clinical detection and study of Malarone and atovaquone P. falciparum resistance.

Malarone treatment failure and in vitro confirmation of resistance of Plasmodium falciparum isolate from Lagos, Nigeria

We report the first in vitro and genetic confirmation of Malarone (GlaxoSmithKline; atovaquone and proguanil hydrochloride) resistance in Plasmodium falciparum acquired in Africa. On presenting with malaria two weeks after returning from a 4-week visit to Lagos, Nigeria without prophylaxis, a male patient was given a standard 3-day treatment course of Malarone. Twenty-eight days later the parasitaemia recrudesced. Parasites were cultured from the blood and the isolate (NGATV01) was shown to be resistant to atovaquone and the antifolate pyrimethamine. The cytochrome b gene of isolate NGATV01 showed a single mutation, Tyr268Asn which has not been seen previously.

Are cytochrome b gene mutations the only cause of atovaquone resistance in Pneumocystis?

There is evidence that exposure of the opportunistic pathogen Pneumocystis to atovaquone enhances the development of resistance to the drug. Atovaquone is a structural analog of ubiquinone, which binds to the mitochondrial cytochrome bc(1) complex and inhibits electron transport. Like the parasites Plasmodium and Toxoplasma, atovaquone resistance can result from mutations in the cytochrome b gene of Pneumocystis. However, atovaquone resistance cannot be explained by cytochrome b gene mutations in all cases. The discovery that atovaquone also inhibits biosynthesis of ubiquinone in P. carinii may unfold other mechanisms by which drug resistance develops.