Toxicity related to chloroquine treatment of resistant vivax malaria

OBJECTIVE: To report a case of severe chloroquine toxicity in the presence of high-grade chloroquine-resistant Plasmodium vivax. CASE SUMMARY: A febrile 36-year-old seaman from Mumbai (Bombay) was prescribed >5 times the usual dose of chloroquine for malaria diagnosed empirically onboard ship. His fever resolved, but he developed symptoms consistent with those of chloroquine toxicity. Fever recurred 30 days after his initial presentation, and blood smear-positive vivax malaria was diagnosed. A serum chloroquine concentration at this time (91 micro g/L) was above that considered effective for chloroquine-sensitive P. vivax (>15 micro g/L). The patient responded to atovaquone plus proguanil followed by primaquine. DISCUSSION: The patient was given chloroquine by his captain in a dosage regimen appropriate for quinine (2 tablets 3 times daily for 7 d). Pharmacokinetic modeling suggested that the patient's initial over-treatment was as reported and that the predicted maximum serum concentration of chloroquine (902 micro g/L) was within the range seen in fatal chloroquine overdose. CONCLUSIONS: Chloroquine-resistant vivax malaria is increasingly widespread, and transmission can occur within large tropical population centers. For drugs with a narrow therapeutic index such as chloroquine, recommended dosing regimens should be respected, and adequate information sources must be available where such drugs are dispensed by untrained personnel.     

In vitro susceptibility to a new antimalarial organometallic analogue,ferroquine, of Plasmodium falciparum isolates from the Haut-Ogooué region of Gabon

OBJECTIVES: To assess the activity of a new organometallic chloroquine analogue, ferroquine, against numerous Plasmodium falciparum isolates from Gabon. METHODS: The in vitro susceptibility of 116 P. falciparum isolates to chloroquine and ferroquine was assessed using the isotopic microtest. All isolates were from outpatients in the Franceville and Bakoumba medical centres in the province of Haut-Ogooué, south-east Gabon. RESULTS: The in vitro resistance to chloroquine was 51.8% in Franceville and 96.7% in Bakoumba. The IC50 geometric mean (95% CI) of ferroquine against isolates in Franceville was 16.0 (14.4-17.8) nM, with individual values ranging from 1.0 to 47.0 nM; in Bakoumba it was 27.9 (23.4-33.2) nM, with individual values ranging from 1.0 to 62.0 nM. Compared with chloroquine, ferroquine was 5.3 times more active on isolates susceptible to chloroquine, and 13.3 times more active on isolates resistant to chloroquine. A weak positive correlation was observed between responses of these two drugs, but too low to demonstrate cross-resistance. CONCLUSIONS: Ferroquine may be useful as an alternative drug for treating chloroquine-resistant malaria.     

Antimalarial 4-aminoquinolines: mode of action and pharmacokinetics

Summary— In the last ten years, the widespread increase in Plasmodium falciparum resistance to chloroquine has prompted research into antimalarial 4-aminoquinolines, empirically used up to now. The mechanism of action of 4-aminoquinolines is characterized by the concentration of the drug in the digestive vacuole of the intraerythrocytic parasite. Various hypotheses have been advanced to explain the specificity of action on the parasite; the most recent one is the inhibition of the haem polymerase of the parasite, leading to the accumulation of soluble haem toxic for the parasite. Chloroquine-resistant parasites accumulate the drug to a lesser extent than do sensitive parasites. Recent findings have shown that chloroquine resistance can be reversed by various tricyclic drugs, which are able to restore the effective concentrations of chloroquine in the infected erythrocyte, but intrinsic mechanisms of action of these reversing agents are unknown. Four-aminoquinolines are extensively distributed in tissues and characterized by a long elimination half-life. Despite similarities in their chemical structures, these drugs show differences in their biotransformation and routes of elimination: chloroquine is partly metabolized into a monodesethylderivative and eliminated mainly by the kidney. In contrast, amodiaquine is a prodrug and amopyroquine is poorly metabolized; both drugs are excreted mainly in the bile. The understanding of the pharmacokinetics of 4-aminoquinolines has led to an improvement in empirically defined therapeutic regimens. Finally, the emergence of severe adverse-effects after prolonged prophylaxis with amodiaquine and the lack of cross resistance of Plasmodium falciparum between chloroquine and amopyroquine, have led to a proposal for the use of intramuscular amopyroquine as an alternative for the treatment of chloroquine-resistant malaria.     

Absence of effect of chloroquine in vivo on neutrophil oxidative metabolism in human subjects

High levels of antimalarial drugs have been shown to be inhibitory to the oxidative metabolism of human neutrophils in vitro. In this study, chemiluminescence of, and chloroquine concentrations in, neutrophils from volunteers treated with prophylactic or therapeutic malaria regimens were assessed. Although detectable chloroquine levels were found in neutrophils, their chemiluminescence was not reduced at prophylactic or at therapeutic drug levels, in comparison with controls without chloroquine treatment. At these clinically achievable concentrations, chloroquine was not found to interfere with the oxidative metabolism of neutrophils.     

Chloroquine-resistant Plasmodium falciparum local strain in Taiz Governorate, Republic of Yemen

BACKGROUND: The emergence of Plasmodium falciparum resistance to widely used antimalarial drugs such as chloroquine has made malaria control and treatment much more difficult. In Yemen, 60% of the total population live in malarious areas. The problem of chloroquine resistance in Yemen is gradually worsening since the detection of first indigenous cases of P. falciparum resistance to chloroquine in 1989. METHODS: In a cross-sectional malariometric parasitic survey, 447 Yemeni children were enrolled from two selected districts (Hethran and Al-mafatch) representing Taiz Governorate. Duplicate thin and thick blood smears were prepared, stained with Giemsa stain and examined microscopically. Fifty-six students satisfied all criteria of the WHO for the assessment of P. falciparum response to chloroquine using a 7-day in vivo test. RESULTS: Out of 447 examined slides, 83 cases (100%) were found with falciparum malaria. The overall malaria parasite rate in Taiz Governorate was 18.6%, a prevalently mesoendemic condition. The obtained results of the 7-day in vivo study revealed that out of 83 P. falciparum cases who completed the study period, 56 cases did not respond to the standard dose of chloroquine, i.e. the overall resistance rate was 16.1%. The prevalence of chloroquine resistance was higher in the Hethran district (19.4%) compared with 10.0% in the Al-mafatch district. The majority had an RI resistance level. CONCLUSION: Chloroquine resistance of the local strain of P. falciparum was recorded in all studied districts in Taiz Governorate. This calls for an urgent revision of the current malaria treatment policy which still considers chloroquine as the first-line drug for treatment of uncomplicated P. falciparum malaria. To assess the magnitude of the problem, these districts could be the basis of future sentinel posts for continuous monitoring of chloroquine resistance in the whole country.     

Chloroquine Resistance in Malaria: Accessibility of Drug Receptors to Mefloquine

The process of mefloquine accumulation was studied in mouse erythrocytes infected with either Plasmodium berghei CS (chloroquine susceptible) or P. berghei CR (chloroquine resistant). In both cases, mefloquine was accumulated by a saturable process with an apparent dissociation constant of 2.5 x 10(-6) M and an apparent maximal capacity of 700 mumol per kg of erythrocyte pellet; uninfected mouse erythrocytes accumulated more than half as much mefloquine as infected erythrocytes. The process of accumulation was not stimulated by providing glucose as a substrate, and it was not inhibited in infected erythrocytes by azide, iodoacetate, or incubation at 2 degrees C. Although mefloquine was accumulated more effectively than chloroquine by uninfected erythrocytes and by erythrocytes infected with P. berghei CR, competition between chloroquine and mefloquine was observed, raising the possibility that the same process of accumulation serves both drugs. Chloroquine competitively inhibits mefloquine accumulation, with an apparent inhibitor constant of 1.7 x 10(-3) M, and mefloquine competitively inhibits chloroquine accumulation, with an apparent inhibitor constant of 2 x 10(-6) M. The same process of accumulation and the same group of receptors could serve both drugs if mefloquine has greater access than chloroquine to the receptors. Regardless of whether the same process serves both drugs, undiminished accumulation by erythrocytes infected with P. berghei CR provides an explanation for the superiority of mefloquine in treating chloroquine-resistant malaria.     

Effect of chloroquine on the urinary excretion of ciprofloxacin

Ciprofloxacin is an inexpensive antibacterial, whereas chloroquine is an inexpensive antimalarial. The coadministration of chloroquine and ciprofloxacin is easily encountered because both drugs are commonly prescribed to patients in the tropics. Five healthy male volunteers aged 19 to 31 years who were not taking any of the prescribed medications and who had no sensitivity to either ciprofloxacin or chloroquine each received 500 mg ciprofloxacin orally with 250 mL of water, and after a 2-week washout period, 500 mg ciprofloxacin plus 600 mg chloroquine was administered orally with 250 mL of water after providing informed consent. A urine sample (7 mL) was collected just before taking the drug at 8:00 AM representing 0 hour and continued afterward at 1, 2, 4, 8, 12, and 24 hours the next day. The samples were stored at -20 degrees C until analyzed. The minimum inhibitory concentrations by diffusion through agar technique were used for the assay of urine ciprofloxacin. The rate of ciprofloxacin excretion and cumulative urine ciprofloxacin were significantly increased. The coadministration of chloroquine increased the cumulative urinary concentration and excretion rate of ciprofloxacin.     

Chemotherapeutic Activity of Combination Doses of Chloroquine, Pyrimethamine, and Sulfamethoxy-diazine, a Long-Acting Sulfanilamide, Against Plasmodium berghei Infections in Mice

During the course of antimalarial screening, it was discovered that sulfamethoxydiazine, a long-acting sulfanilamide extensively used in genitourinary tract infections, not only was effective against Plasmodium berghei infections in mice when administered alone but also was active when used in combination with chloroquine, in effect making it possible to use half as much of the latter drug as normally required to achieve the same results. The triple combination of chloroquine, sulfamethoxydiazine, and pyrimethamine, when administered in a ratio of 30:10:1, was found to be potentiating against both blood-induced and sporozoite-induced P. berghei NK(65) infections. Mean effective dose values were calculated for chloroquine, sulfamethoxydiazine, and pyrimethamine against blood-induced P. berghei infection, and when a combination of the three drugs was administered therapeutically in the ratio given above, only one-tenth as much chloroquine, one-thirtieth as much sulfamethoxydiazine, and one-sixtieth as much pyrimethamine were needed to cure 50% of murine infections as was needed for each drug alone. The triple combination also showed enhanced activity against lethal sporozoite-induced P. berghei NK(65) infection in A/J strain mice.     

Sequestration of the chloroquine receptor in cell-free preparations of erythrocytes infected with Plasmodium berghei.

When mouse erythrocytes infected with Plasmodium berghei were preincubated with [14C]chloroquine and then lysed by hypotonic shock, chloroquine remained bound to the resulting cell-free preparation. In an isotonic medium at pH 7.4 and 25 degrees C, chloroquine was bound to the cell-free preparation with an apparent dissociation constant of 1.8 x 10(-7) M. The bound [14C]chloroquine could be displaced by nonradioactive chloroquine, amodiaquine, quinacrine, and mefloquine, as would be predicted from knowledge of the specificity of ferriprotoporphyrin IX for antimalarial drugs. Also, as predicted, primaquine did not displace the [14C]chloroquine. The ability of these cell-free preparations to bind chloroquine with high affinity decreased rapidly with incubation at 37 degrees C and became undetectable within 1 h; at 4 degrees C the decrease occurred more slowly. This behavior of the endogenous receptor-chloroquine complex was duplicated by an exogenous ferriprotoporphyrin IX-chloroquine complex loaded into cell-free preparations of erythrocytes infected with P. berghei. These findings support the hypothesis that ferriprotoporphyrin IX is the endogenous chloroquine receptor of P. berghei and indicate that it can be sequestered rapidly in a form that is inaccessible to chloroquine.     

Malaria chemoprophylaxis: when should we use it and what are the options?

Malaria chemoprophylaxis concerns prescribing healthy individuals medication for an infection they have an unknown chance of getting. Sensible use of malaria chemoprophylaxis is a balance between the risk of infection and death, and the risk of side effects. The risk of infection can be broken down into the risk of being bitten by a malaria-infected mosquito and the risk of the malaria parasites being resistant to the drug used for prophylaxis. Our knowledge of these parameters is patchy. The risk of infection is not uniform at a given location and the standard of living will greatly influence risk. It is suggested that chemoprophylaxis should not be recommended in areas with less than ten reported cases of P. falciparum malaria per 1000 inhabitants per year. The resistance pattern is known to a certain extent but, for instance, diverging opinion of how much resistance to chloroquine there is in West Africa illustrates the lack of data. There is much debate on rare adverse events, which usually escape Phase III studies prior to registration and are only picked up by passive, postmarketing surveillance. The lessons over the past 20 years with the introduction of amodiaquine, pyrimethamine/dapsone (Maloprim, GlaxoSmithKline) and pyrimethamine/sulfadoxine (Fansidar, Roche), which were all withdrawn for prophylaxis after a few years, show how sensitive drugs for chemoprophylaxis are to side effects. Three levels of chemoprophylaxis are used: chloroquine in areas with sensitive P. falciparum, chloroquine plus proguanil in areas with low level chloroquine resistance, and atovaquone/proguanil (Malarone, GlaxoSmithKline), doxycycline or mefloquine (Lariam, Roche) in areas with extensive resistance against chloroquine and proguanil. Primaquine and the primaquone analog tafenoquine may be future alternatives but otherwise there are few new drugs for chemoprophylaxis on the horizon.     

Effects of amodiaquine, chloroquine, and mefloquine on human polymorphonuclear neutrophil function in vitro.

This study concerns the in vitro interaction with human polymorphonuclear neutrophils (PMNs) of amodiaquine, chloroquine, and mefloquine, three antimalarial drugs currently in use for the treatment and prophylaxis of malaria. It was found that mefloquine (100 and 50 micrograms/ml) significantly altered PMN viability while the other two drugs did not. Neutrophil chemotaxis was impaired by chloroquine (100 micrograms/ml) and mefloquine (greater than 10 micrograms/ml) but not by amodiaquine. Phagocytosis was decreased by about 50% in the presence of chloroquine (100 micrograms/ml) or mefloquine (10 micrograms/ml). The three antimalarial drugs altered neutrophil oxidative metabolism as assessed by luminol-amplified chemiluminescence. The strongest effect was observed with mefloquine, which abolished almost completely the neutrophil burst at concentrations of greater than 10 micrograms/ml whatever the stimulus used. This effect was not reversed by washing. Chloroquine and amodiaquine also impaired this PMN response by approximately 80 and 50%, respectively, but only at the highest concentration used (100 micrograms/ml). In the case of amodiaquine, the neutrophil response was restored by washing, except for stimulation with opsonized particles. After washing, the depressive effect of chloroquine was reversed completely in the case of phorbol myristate acetate stimulation and partly in the case of opsonized particle stimulation, but the formylmethionyl-leucyl-phenylalanine-induced response was not restored. These data show that although they are structurally related, amodiaquine and chloroquine exhibit qualitatively and quantitatively different depressive effects on PMN function and probably interfere at different points of cell activation, although the precise mechanisms are as yet unresolved.     

Therapeutic responses to different antimalarial drugs in vivax malaria

The therapeutic responses to the eight most widely used antimalarial drugs were assessed in 207 adult patients with Plasmodium vivax malaria. This parasite does not cause marked sequestration, so parasite clearance can be used as a direct measure of antimalarial activity. The activities of these drugs in descending order were artesunate, artemether, chloroquine, mefloquine, quinine, halofantrine, primaquine, and pyrimethamine-sulfadoxine (PS). Therapeutic responses to PS were poor; parasitemias did not clear in 5 of the 12 PS-treated patients, whereas all the other patients made an initial recovery. Of 166 patients monitored for > or =28 days, 35% had reappearance of vivax malaria 11 to 65 days later and 7% developed falciparum malaria 5 to 21 days after the start of treatment. There were no significant differences in the times taken for vivax malaria reappearance among the different groups except for those given mefloquine and chloroquine, in which all vivax malaria reappearances developed >28 days after treatment, suggesting suppression of the first relapse by these slowly eliminated drugs. There was no evidence of chloroquine resistance. The antimalarial drugs vary considerably in their intrinsic activities and stage specificities of action.     

Enhancement of human interferon production by neutral red and chloroquine: analysis of inhibition of protein degradation and macromolecular synthesis

Two lysosomotrophic drugs, neutral red and chloroquine, enhance polyinosinic:polycytidylic acid-induced interferon production by a strain of diploid human fibroblasts (FS-4). Treatment of cells with neutral red or chloroquine between 2.5 and 3.5 h after induction increases interferon yields 16- to 64- and 4- to 16-fold, respectively, in the subsequent 20.5 h. The two drugs inhibit the rates of protein degradation and of RNA and protein synthesis. In addition, neutral red is a very potent inhibitor of uridine transport into cells. Normalized dose-effect curves show that interferon superinduction is correlated with the inhibition of macromolecular synthesis, but not with that of protein degradation. Treatment of cells with chloroquine at low concentration (25 mug/ml) for a prolonged period of time (24 h) caused approximately 40% reduction in the rate of protein degradation. The usual rapid shutoff of interferon production and the effectiveness of effectiveness of actinomycin D superinduction are not altered by this treatment. This strongly suggests that inhibition of intralysosomal protein degradation does not significantly contribute to interferon superinduction. Degradation of the rapidly and the slowly turning over proteins was unaffected by actinomycin D under conditions of treatment known to enhance interferon production. Treatment with cycloheximide (5 or 50 mug/ml for 5 h) inhibited the rate of degradation of the rapidly turning over component by 10% and the slow component by 30-40%, which suggests that the two components turn over by distinct cellular mechanisms.     

Investigation of β-cyclodextrin–norfloxacin inclusion complexes. Part 1. Preparation,physicochemical and microbiological characterization

Malaria chemoprophylaxis concerns prescribing healthy individuals medication for an infection they have an unknown chance of getting. Sensible use of malaria chemoprophylaxis is a balance between the risk of infection and death, and the risk of side effects. The risk of infection can be broken down into the risk of being bitten by a malaria-infected mosquito and the risk of the malaria parasites being resistant to the drug used for prophylaxis. Our knowledge of these parameters is patchy. The risk of infection is not uniform at a given location and the standard of living will greatly influence risk. It is suggested that chemoprophylaxis should not be recommended in areas with less than ten reported cases of P. falciparum malaria per 1000 inhabitants per year. The resistance pattern is known to a certain extent but, for instance, diverging opinion of how much resistance to chloroquine there is in West Africa illustrates the lack of data. There is much debate on rare adverse events, which usually escape Phase III studies prior to registration and are only picked up by passive, postmarketing surveillance. The lessons over the past 20 years with the introduction of amodiaquine, pyrimethamine/dapsone (Maloprim^®, GlaxoSmithKline) and pyrimethamine/sulfadoxine (Fansidar^®, Roche), which were all withdrawn for prophylaxis after a few years, show how sensitive drugs for chemoprophylaxis are to side effects. Three levels of chemoprophylaxis are used: chloroquine in areas with sensitive P. falciparum, chloroquine plus proguanil in areas with low level chloroquine resistance, and atovaquone/proguanil (Malarone^®, GlaxoSmithKline), doxycycline or mefloquine (Lariam^®, Roche) in areas with extensive resistance against chloroquine and proguanil. Primaquine and the primaquone analog tafenoquine may be future alternatives but otherwise there are few new drugs for chemoprophylaxis on the horizon.     

Susceptibility of melanized and nonmelanized Cryptococcus neoformans to the melanin-binding compounds trifluoperazine and chloroquine.

Cryptococcus neoformans is an opportunistic fungal pathogen which becomes heavily melanized in the presence of phenolic substrates such as L-dopa. Various drugs are known to bind to melanin with high affinity, including the antipsychotic agent trifluoperazine and the antimalarial agent chloroquine. We hypothesized that drugs which bind melanin may have different toxicities for melanized and nonmelanized C. neoformans cells. The effects of trifluoperazine and chloroquine or C. neoformans were determined by measuring cell viability after exposure to these drugs. Cell viability was measured by CFU determination and flow cytometry with propidium iodide staining. Melanized cells were more susceptible than nonmelanized cells to the fungicidal effects of trifluoperazine. Chloroquine had no fungicidal effect on either melanized or nonmelanized C. neoformans under the conditions studied. Flow cytometry of trifluoperazine-treated C. neoformans cells stained with the mitochondrial stain dihydrorhodamine 123 revealed fluorescence changes consistent with mitochondrial damage. Our results indicate that melanized and nonmelanized C. neoformans cells can differ in susceptibility to certain drugs and suggest that strategies which target melanin may be productive for antifungal-drug discovery.     

Quantitative assessment of seminal vesicle and prostate drug concentrations by use of a noninvasive method

BACKGROUND: The male genital tract is a complex collection of anatomically and biochemically distinct compartments that contribute to the ejaculate. Understanding the pharmacokinetics in these compartments should inform rational therapeutics involving these glands. METHODS: Nineteen men were administered a single dose of 600 mg chloroquine (base) and 975 mg aspirin before providing a semen sample by masturbation with fractionation into a 5-compartment collection device. Fractions were assayed for fructose (unique seminal vesicle marker), prostate-specific antigen (unique prostate marker), salicylate, and chloroquine. Seminal vesicle and prostate concentrations of salicylate and chloroquine were estimated via a novel analytic method involving a multilevel latent-variable model implemented by use of Bayesian methods. RESULTS: The geometric mean chloroquine semen/blood ratio was 4.02 (95% confidence interval [CI], 2.36-6.86); for salicylate, the primary metabolite of aspirin, the semen/blood ratio was 0.10 (95% CI, 0.08-0.14). The estimated mean prostate/seminal vesicle ratio for salicylate, 0.38 (95% CI by Bayesian methods, 0.12-0.73), was consistent with our hypothesis that salicylate would achieve higher concentrations in the seminal vesicle than in the prostate. Chloroquine, however, did not demonstrate a statistically significant seminal vesicle/prostate difference (4.41; 95% CI by Bayesian methods, 0.14-30.52). CONCLUSIONS: We successfully demonstrated the quantitative, noninvasive estimation of drug concentrations in the prostate gland fluid distinct from the seminal vesicle fluid using our optimized method of split-ejaculate collection and a novel mixed-effects model with Bayesian estimation. Our methods can be applied to gland-specific quantitation of drugs and other substances of interest, thus enabling pharmacokinetic, pharmacodynamic, and pathophysiologic studies to inform rational therapeutics within different glands of the male genital tract.     

Enhanced anticryptococcal activity of chloroquine in phosphatidylserine-containing liposomes in a murine model

OBJECTIVES: The anticryptococcal activity of chloroquine was assessed after incorporation in phosphatidylserine (PS)-containing negatively charged liposomes in a murine model. METHODS: In the present study, we investigated the antifungal activity of chloroquine entrapped in PS liposomes against Cryptococcus neoformans in the macrophage cell line J 774 and in a murine model. Mice were treated with free as well as liposomal formulations of chloroquine before and after challenging with C. neoformans infection. The anticryptococcal activity of chloroquine was also evaluated in combination with fluconazole in the treatment of systemic murine cryptococcosis. The efficacy of chloroquine treatment was assessed by continued survival as well as by colony forming units (cfu) in liver and brain of treated mice. RESULTS: Chloroquine entrapped in PS liposomes shows increased activity against C. neoformans infection both in in vitro and in vivo studies. Moreover, the antifungal activity of fluconazole increases when used in combination with liposomal chloroquine. Chloroquine in PS liposomes was found to be more effective in comparison with the same dose of free chloroquine or chloroquine entrapped in neutral liposomes. CONCLUSIONS: The enhanced anticryptococcal activity of chloroquine in PS liposomes seems to be due to uptake of drug-containing PS liposomes by macrophages. It can be assumed that liposome-mediated delivery of chloroquine to macrophages results in an unfavourable (alkaline) environment for the growth of C. neoformans inside macrophages.     

Synergistic in vitro antimalarial activity of omeprazole and quinine

Previous studies have shown that the proton pump inhibitor omeprazole has antimalarial activity in vitro. The interactions of omeprazole with commonly used antimalarial drugs were assessed in vitro. Omeprazole and quinine combinations were synergistic; however, chloroquine and omeprazole combinations were antagonistic. Artemisinin drugs had additive antimalarial activities with omeprazole.     

Antimalarial efficacy and drug interactions of the novel semi-synthetic endoperoxide artemisone in vitro and in vivo

OBJECTIVES: The in vitro and in vivo efficacy and drug-drug interactions of the novel semi-synthetic endoperoxide artemisone with standard antimalarials were investigated in order to provide the basis for the selection of the best partner drug. METHODS: Antimalarial activity and drug interactions were evaluated in vitro against Plasmodium falciparum by the incorporation of [(3)H]hypoxanthine. In vivo efficacy and drug interactions were assessed using the standard 4-day Peters' test. RESULTS: Artemisone was 10 times more potent than artesunate in vitro against a panel of 12 P. falciparum strains, independent of their susceptibility profile to antimalarial drugs, and consistently 4 to 10 times more potent than artesunate in rodent models against drug-susceptible and primaquine- or sulfadoxine/pyrimethamine-resistant Plasmodium berghei lines and chloroquine- or artemisinin-resistant lines of Plasmodium yoelii. Slight antagonistic trends were found between artemisone and chloroquine, amodiaquine, tafenoquine, atovaquone or pyrimethamine and additive to slight synergistic trends with artemisone and mefloquine, lumefantrine or quinine. Various degrees of synergy were observed in vivo between artemisone and mefloquine, chloroquine or clindamycin. CONCLUSIONS: These results confirm the increased efficacy of artemisone over artesunate against multidrug-resistant P. falciparum and provide the basis for the selection of potential partner drugs for future deployment in areas of multidrug-resistant malaria. Artemisone represents an important addition to the repertoire of artemisinin combination therapies currently in use, as it has enhanced antimalarial activity, improved bioavailability and stability over current endoperoxides.