Multiresistant falciparum malaria cured using atovaquone and proguanil

Cyclization-blocked proguanil as a strategy to improve the antimalarial activity of atovaquone

Atovaquone-proguanil (Malarone®) is used for malaria prophylaxis and treatment. While the cytochrome bc1-inhibitor atovaquone has potent activity, proguanil's action is attributed to its cyclization-metabolite, cycloguanil. Evidence suggests that proguanil has limited intrinsic activity, associated with mitochondrial-function. Here we demonstrate that proguanil, and cyclization-blocked analogue tBuPG, have potent, but slow-acting, in vitro anti-plasmodial activity. Activity is folate-metabolism and isoprenoid biosynthesis-independent. In yeast dihydroorotate dehydrogenase-expressing parasites, proguanil and tBuPG slow-action remains, while bc1-inhibitor activity switches from comparatively fast to slow-acting. Like proguanil, tBuPG has activity against P. berghei liver-stage parasites. Both analogues act synergistically with bc1-inhibitors against blood-stages in vitro, however cycloguanil antagonizes activity. Together, these data suggest that proguanil is a potent slow-acting anti-plasmodial agent, that bc1 is essential to parasite survival independent of dihydroorotate dehydrogenase-activity, that Malarone® is a triple-drug combination that includes antagonistic partners and that a cyclization-blocked proguanil may be a superior combination partner for bc1-inhibitors in vivo.

Artesunate, artemether or quinine in severe Plasmodium falciparum malaria?

Quinine and the artemisinin-derivative drugs artesunate and artemether are effective treatments for severe falciparum malaria. Trials comparing artemether with quinine have not demonstrated convincing evidence of a mortality advantage for artemether. The South East Asian Quinine Artesunate Malaria Trial (SEAQUAMAT), a multicenter, randomized, open-label trial in 1461 adults with severe malaria in Asia compared artesunate with quinine. Mortality was 15% in the artesunate group and 22% in the quinine group, a reduction of 34.7% (95% confidence interval: 18.5-47.6%) in the artesunate group, with almost all the benefit reported in those with high parasite counts. Artesunate should constitute first-line treatment for severe malaria in Asia. These results can probably be generalized to the treatment of severe malaria in adults from all areas, especially in those with hyperparasitemia. However, it is unclear whether these results can be generalized to children in Africa, who constitute the majority of those who die from severe malaria worldwide.

Cerebral malaria in spite of peripheral parasite clearance in a patient treated with atovaquone/proguanil

Cerebral malaria, the most frequent complication of falciparum malaria, is usually predicted by an increased count of asexual parasites in peripheral blood. We report a case of a female returnee from Ghana who developed cerebral malaria in spite of parasite clearance in peripheral blood after therapy with atovaquone/proguanil.

Atovaquone-proguanil for treating uncomplicated malaria

BACKGROUND

Many conventional treatments for uncomplicated malaria are failing because malaria parasites develop resistance to them. One way to combat this resistance is to treat people with a combination of drugs, such as atovaquone-proguanil.

OBJECTIVES

To compare atovaquone-proguanil with other antimalarial drugs (alone or in combination) for treating children and adults with uncomplicated Plasmodium falciparum malaria.

SEARCH STRATEGY

We searched the Cochrane Infectious Diseases Group Specialized Register (June 2005), CENTRAL (The Cochrane Library Issue 2, 2005), MEDLINE (1966 to June 2005), EMBASE (1980 to June 2005), LILACS (1982 to June 2005), reference lists, and conference abstracts. We also contacted relevant pharmaceutical manufacturers and researchers.

SELECTION CRITERIA

Randomized controlled trials comparing atovaquone-proguanil with other antimalarial drugs for treating children and adults confirmed to have uncomplicated P. falciparum malaria.

DATA COLLECTION AND ANALYSIS

Three authors independently assessed trial eligibility and methodological quality, and extracted data for an intention-to-treat analysis (where possible). We used relative risk (RR) and 95% confidence intervals (CI) for dichotomous data. We contacted trial authors for additional information where needed.

MAIN RESULTS

Ten trials, with a total of 2345 participants, met the inclusion criteria. The trials were conducted in four geographical regions and were often small, but they included comparisons across eight drugs. Nine trials were funded by a pharmaceutical company, only three carried out an intention-to-treat analysis, and allocation concealment was unclear in seven. Atovaquone-proguanil had fewer treatment failures by day 28 than chloroquine (RR 0.04, 95% CI 0.00 to 0.57; 27 participants, 1 trial), amodiaquine (RR 0.22, 95% CI 0.13 to 0.36; 342 participants, 2 trials), and mefloquine (RR 0.04, 95% CI 0.00 to 0.73; 158 participants, 1 trial). There were insufficient data to draw a conclusion for this outcome from comparisons with sulfadoxine-pyrimethamine (172 participants, 2 trials), halofantrine (205 participants, 1 trial), artesunate plus mefloquine (1063 participants, 1 trial), quinine plus tetracycline (154 participants, 1 trial), and dihydroartemisinin-piperaquine-trimethoprim-primaquine (161 participants, 1 trial). Adverse events were mainly common symptoms of malaria and did not differ in frequency between groups.

AUTHORS' CONCLUSIONS

Data are limited but appear to suggest that atovaquone-proguanil is more effective than chloroquine, amodiaquine, and mefloquine. There are insufficient data for comparisons against sulfadoxine-pyrimethamine, halofantrine, artesunate plus mefloquine, quinine plus tetracycline, and dihydroartemisinin-piperaquine-trimethoprim-primaquine in treating malaria. There are not enough data to assess safety, but a number of adverse events were identified with all drugs. Large trials comparing atovaquone-proguanil with other new combination therapies are needed.

Cycloguanil and its parent compound proguanil demonstrate distinct activities against Plasmodium falciparum malaria parasites transformed with human dihydrofolate reductase

The lack of suitable antimalarial agents to replace chloroquine and pyrimethamine/sulfadoxine threatens efforts to control the spread of drug-resistant strains of the malaria parasite Plasmodium falciparum. Here we describe a transformation system, involving WR99210 selection of parasites transformed with either wild-type or methotrexate-resistant human dihydrofolate reductase (DHFR), that has application for the screening of P. falciparum-specific DHFR inhibitors that are active against drug-resistant parasites. Using this system, we have found that the prophylactic drug cycloguanil has a mode of pharmacological action distinct from the activity of its parent compound proguanil. Complementation assays demonstrate that cycloguanil acts specifically on P. falciparum DHFR and has no other significant target. The target of proguanil itself is separate from DHFR. We propose a strategy of combination chemotherapy incorporating the use of multiple parasite-specific inhibitors that act at the same molecular target and thereby maintain, in combination, their effectiveness against alternative forms of resistance that arise from different sets of point mutations in the target. This approach could be combined with traditional forms of combination chemotherapy in which two or more compounds are used against separate targets.

The pregnant traveler

The care of the pregnant traveler is both challenging and rewarding. It requires clinical information and skills that are derived from many disciplines. This article reviews preparatory guidelines for safe travel by the pregnant mother and her most important travel companion, the developing fetus. Issues considered are pretravel risk assessment, immunizations, and prevention of travelers' diarrhea and hepatitis. The safety and efficacy of malaria chemoprophylaxis in the present context of widespread multidrug-resistant malaria is discussed, and guidelines are offered for both prevention and treatment. A safety profile of commonly used travel medications, antibiotics, and antiparasitic drugs is reviewed.

Population pharmacokinetics of atovaquone in patients with acute malaria caused by Plasmodium falciparum

The population pharmacokinetics of atovaquone were examined in 458 black, Oriental, and Malay patients with acute Plasmodium falciparum malaria receiving atovaquone alone or concomitantly with other drugs. Oral clearance (CL/F) showed a 0.674 power relationship with weight and is similar in Oriental and Malay subjects but 58.5% lower in black subjects. On the basis of mean body weight, the population estimate of CL/F is 3.28, 8.49, and 9.13 L/hr in black, Oriental, and Malay subjects, respectively. The relationship between apparent volume of distribution (V area/F) and weight was linear and similar in all three races at 7.98 L/kg. The population estimate of V area/F is 345, 383, and 428 L in black, Oriental, and Malay subjects, respectively. The bioavailability of the high and low doses of atovaquone was similar. Neither CL/F nor V area/F were significantly affected by age, gender, and the coadministration with chloroguanide (proguanil), pyrimethamine, and tetracycline. Half-life (t1/2) showed a 0.326 power relationship with weight; thus, the population estimate of t1/2 in black, Oriental, and Malay subjects is 72.9, 31.3, and 32.5 hours, respectively. The final magnitudes of interpatient variability in CL/F and V area/F were 68% and 49%, respectively.

Effects of dual combinations of antifolates with atovaquone or dapsone on nucleotide levels in Plasmodium falciparum

The triazine antifolates, cycloguanil and 4,6-diamino-1,2-dihydro-2,2-dimethyl-1-[(2,4,5-trichlorophenoxy)propy loxy]-1,3,5-triazine hydrobromide (WR99210), and their parent biguanide compounds, proguanil and N-[3-(2,4,5-trichlorophenoxy)propyloxy]-n-(1-methylethyl)-imido dicarbonimidic-diamine hydrochloride (PS-15), were tested in combination with a series of antimalarial drugs for synergism against Plasmodium falciparum growing in erythrocytic culture. Four synergistic combinations were found: cycloguanil dapsone, WR99210-dapsone, proguanil-atovaquone, and PS-15-atovaquone. Cycloguanil-dapsone or WR99210-dapsone had a profound suppressive effect on the concentration of dTTP in parasites while that of dATP increased. Depletion of dTTP is consistent with cycloguanil or WR99210 inhibiting dihydrofolate reductase and dapsone inhibiting dihydropteroate synthase. For the combinations proguanil-atovaquone and PS-15-atovaquone, the levels of nucleoside triphosphates (NTPs) and dNTPs were generally suppressed, suggesting that inhibition is not through nucleotide pathways but probably through another metabolic mechanism(s). Combinations of two synergistic pairs of antimalarial drugs, (proguanil-atovaquone)-(cycloguanil-dapsone) and (PS-15-atovaquone)-(WR99210-dapsone), were tested, and it was found that NTPs and dNTPs decreased much more than for a single synergistic combination. Dual synergistic combinations could play an important role in the therapy of multidrug-resistant malaria, just as combination chemotherapy is used to treat cancer.

Atovaquone and proguanil for Plasmodium falciparum malaria

BACKGROUND

The increasing spread of multidrug-resistant Plasmodium falciparum malaria emphasises the urgent need for alternative treatment regimens. The objective of the study was to establish the efficacy of a novel drug combination. We compared a combination of atovaquone and proguanil with amodiaquine in the treatment of acute uncomplicated P falciparum malaria in Lambaréné, Gabon.

METHODS

142 adults were randomly allocated either a combination treatment of atovaquone 1000 mg daily and proguanil 400 mg daily for 3 days or treatment with amodiaquine 600 mg on admission, 600 mg 24 h later, and 300 mg after a further 24 h. Symptoms and clinical signs were recorded and giemsa-stained thick blood smears were done every 12 h until patients had been symptom-free and aparasitaemic for 24 h. 126 patients were followed up for 28 days or until recrudescence.

FINDINGS

In the atovaquone plus proguanil group 62 (87%) of 71 patients were cured and only one had recrudescent infection. By contrast, the cure rate was significantly lower (p=0.022) with amodiaquine (51 [72%] of 71; there were 12 recrudescences in the amodiaquine group). Eight patients in each group were lost to follow-up. Patients treated with atovaquone plus proguanil complained of nausea (33%) and vomiting (29%), and the most commonly reported adverse effects of amodiaquine were pruritus (43%) and insomnia (27%).

INTERPRETATION

Atovaquone and proguanil was a highly effective and safe drug combination in patients with acute uncomplicated P falciparum malaria in Gabon.

Frequency of drug resistance in Plasmodium falciparum: a nonsynergistic combination of 5-fluoroorotate and atovaquone suppresses in vitro resistance

A combination of 5-fluoroorotate and atovaquone eliminated Plasmodium falciparum in long-term cultures more efficiently than either compound alone. The improved potency came not through synergistic activity but through decreased frequency of drug resistance. In support of this finding, it was shown that 5-fluoroorotate and atovaquone do not act in a synergistic fashion, that 5-fluoroorotate-resistant and atovaquone-resistant P. falciparum organisms generated in vitro do not show cross-resistance, and that the frequency of simultaneous resistance to the two compounds approached the product of their individual resistance frequencies. To demonstrate the last finding, and establish proof of principle, an in vitro method was developed for measuring the frequency of drug resistance in P. falciparum. By this method, it was shown that the frequency of resistance to 10(-7) M 5-fluoroorotate was about 10(-6) and the frequency of resistance to 10(-8) M atovaquone was about 10(-5); the frequency of simultaneous resistance to a combination of 10(-7) M 5-fluoroorotate and 10(-8) M atovaquone was less than 5 x 10(-10). On the basis of additional measurements, it was estimated that the frequency of simultaneous resistance to higher, pharmacologically more relevant, concentrations of 10(-6) M 5-fluoroorotate and 10(-7) M atovaquone would be less than 10(-17). Control experiments demonstrated that these drug combinations did not cause increased toxicity to mammalian cells in culture. On this basis, it is predicted that a combination of 5-fluoroorotate and atovaquone will successfully eliminate typical malarial infections in animals and in human patients at doses that are readily tolerated.