Gender differences in gastrointestinal disturbances and plasma concentrations of tafenoquine in healthy volunteers after tafenoquine administration for post-exposure vivax malaria prophylaxis.

In an open-label sequential cohort study, we compared gastrointestinal (GI) disturbances and plasma tafenoquine concentrations after administration of single-dose (400mg daily x 3 days; n=76 males, 11 females) and split-dose (200 mg twice daily x 3 days; n=73 males, 13 females) tafenoquine regimens in healthy Australian Defence Force volunteers for post-exposure malaria prophylaxis. The female and male volunteers had comparable demographic characteristics (age, weight, height) in the single- and split-dose treatment groups. GI disturbances were generally mild and self-limiting for both groups. The frequency of nausea and abdominal distress was over two-fold higher in females than in males for both treatment groups. Reporting of GI disturbances in the single-dose group differed significantly between males and females, but this gender difference was not seen for the split-dose group. In those volunteers who experienced GI disturbances, the mean plasma tafenoquine concentrations 12 h after the last dose of tafenoquine were approximately 1.3-fold higher in females than in males (means+/-SD: 737+/-118 ng/ml vs. 581+/-113 ng/ml). These preliminary findings suggest that further studies are required in a larger number of females to determine whether there is a need to reduce the dose of tafenoquine to minimise GI disturbances in females.

Therapeutic response of multidrug-resistant Plasmodium falciparum and P. vivax to chloroquine and sulfadoxine-pyrimethamine in southern Papua, Indonesia.

To determine the level of antimalarial drug resistance in southern Papua, Indonesia, we assessed the therapeutic efficacy of chloroquine plus sulfadoxine-pyrimethamine (CQ+SP) for Plasmodium falciparum infections as well as CQ monotherapy for P. vivax infections. Patients with P. falciparum failing therapy were re-treated with unsupervised quinine+/-doxycycline therapy and those with P. vivax with either unsupervised quinine+/-doxycycline or amodiaquine. In total, 143 patients were enrolled in the study (103 treated with CQ+SP and 40 with CQ). Early treatment failures occurred in four patients (4%) with P. falciparum and six patients (15%) with P. vivax. The failure rate by Day 28 for P. vivax was 65% (95% CI 49-81). After PCR correction for re-infections, the Day 42 recrudescence rate for P. falciparum infections was 48% (95% CI 31-65). Re-treatment with unsupervised quinine+/-doxycycline resulted in further recurrence of malaria in 48% (95% CI 31-65) of P. falciparum infections and 70% (95% CI 37-100) of P. vivax infections. Eleven patients with recurrent P. vivax were re-treated with amodiaquine; there were no early or late treatment failures. In southern Papua, a high prevalence of drug resistance of P. falciparum and P. vivax exists both to first- and second-line therapies. Preliminary data indicate that amodiaquine retains superior efficacy compared with CQ for CQ-resistant P. vivax.

The Plasmodium PI(4)K inhibitor KDU691 selectively inhibits dihydroartemisinin-pretreated Plasmodium falciparum ring-stage parasites.

Malaria control and elimination are threatened by the emergence and spread of resistance to artemisinin-based combination therapies (ACTs). Experimental evidence suggests that when an artemisinin (ART)-sensitive (K13 wild-type) Plasmodium falciparum strain is exposed to ART derivatives such as dihydroartemisinin (DHA), a small population of the early ring-stage parasites can survive drug treatment by entering cell cycle arrest or dormancy. After drug removal, these parasites can resume growth. Dormancy has been hypothesized to be an adaptive physiological mechanism that has been linked to recrudescence of parasites after monotherapy with ART and, possibly contributes to ART resistance. Here, we evaluate the in vitro drug sensitivity profile of normally-developing P. falciparum ring stages and DHA-pretreated dormant rings (DP-rings) using a panel of antimalarial drugs, including the Plasmodium phosphatidylinositol-4-OH kinase (PI4K)-specific inhibitor KDU691. We report that while KDU691 shows no activity against rings, it is highly inhibitory against DP-rings; a drug effect opposite to that of ART. Moreover, we provide evidence that KDU691 also kills DP-rings of P. falciparum ART-resistant strains expressing mutant K13.

Chloroquine bioassay of plasma specimens obtained from soldiers on chloroquine plus doxycycline for malaria prophylaxis.

In this study we describe the application of a bioassay for measuring chloroquine equivalent concentrations in plasma samples obtained from soldiers on chloroquine (300 mg weekly) and doxycycline (50 or 100 mg daily) for malaria prophylaxis. Chloroquine, its principal metabolite monodesethylchloroquine and doxycycline were also measured by high performance liquid chromatography (HPLC). Physiological concentrations of doxycycline did not interfere with the measurement of chloroquine equivalent concentrations. The correlation between bioassay and HPLC was rs = 0.88, with a median bioassay/HPLC chloroquine concentration ration of 1.1 (range 0.6-2.4, n = 26). The bioassay is a valuable method, particularly under field conditions, for measuring chloroquine concentrations and can be very helpful in distinguishing drug failures from either poor compliance or inadequate drug absorption.

Disposition of proguanil in Thai patients with uncomplicated falciparum malaria.

The objective of this study was to examine the disposition of proguanil in malaria-infected Thai patients with acute uncomplicated falciparum malaria. Eleven patients were administered 500 mg of proguanil twice a day for three days (total dose = 3,000 mg). Four patients were tentatively classified as extensive metabolizers (EMs) and seven as poor metabolizers (PMs). The mean plasma clearances of proguanil for EMs and PMs were 1.31 and 1.10 L/hr/kg, respectively. The mean elimination half-life of proguanil was statistically longer in PMs than EMs (19.6 hr versus 16.1 hr; P = 0.01). Plasma clearance and elimination half-life of proguanil in the malaria patients were comparable with those reported in the literature for healthy Thai volunteers. In contrast to other ethnic groups. Thai EM patients had relatively low plasma concentrations of cycloguanil, the active metabolite of proguanil. None of the 11 patients treated with proguanil were cured of malaria and their phenotype status did not affect the treatment outcome. Although high levels of parasite resistance to cycloguanil were probably responsible for the poor response to proguanil treatment, the inability of Thai EM and PM patients to produce cycloguanil may have also contributed to the treatment outcome.

Malaria chemoprophylaxis using proguanil/dapsone combinations on the Thai-Cambodian border.

The Thai-Cambodian border is a difficult area in which to provide adequate malaria chemoprophylaxis because of multiple drug-resistant Plasmodium falciparum. In 1990-1991, Thai soldiers were randomly selected to receive proguanil (200 mg/day) combined with dapsone (4 mg or 12.5 mg/day) (n = 184) or pyrimethamine/dapsone (12.5 mg and 100 mg/week) (n = 177). Doxycycline (100 mg/day) was given to men with glucose-6-phosphate dehydrogenase deficiency (n = 77). Falciparum malaria attack rates were the same whether proguanil/dapsone (10.3%) or pyrimethamine/dapsone (11.3%) was used. However, proguanil/dapsone was more effective than pyrimethamine/dapsone in preventing vivax malaria (1.6% versus 12.4%). Men receiving doxycycline had falciparum malaria (3.9%) and vivax malaria (1.3%) at low rates. Adjusting the dapsone component from 4 mg to 12.5 mg did not improve the prophylactic effectiveness. Hematologic toxicity was not observed with the proguanil/dapsone combination. We conclude that proguanil/dapsone is not a useful alternative for malaria chemoprophylaxis on the Thai-Cambodian border.

Causal prophylactic and radical curative activity of WR182393 (a guanylhydrazone) against Plasmodium cynomolgi in Macaca mulatta.

Primaquine is the only currently available drug effective against persistent tissue stages of relapsing malaria in humans. Causal prophylactic and radical curative properties of WR182393 (a guanylhydrazone) were investigated as part of an effort to evaluate alternatives to primaquine in the rhesus monkey (Macaca mulatta)/Plasmodium cynomolgi test model. The drug was suspended in dimethylsulfoxide for intramuscular (im) injection. A pilot study indicated causal prophylactic activity in a regimen of 40 mg base/kg/day im for three days beginning the day before intravenous challenge with 1 x 10(6) P. cynomolgi sporozoites. Regimens of 31, 10, 3.1, and 0 mg base/kg/day im for three days were then tested in groups of two monkeys given a similar challenge. The two animals given 31 mg base/kg/day remained parasite-free. Average time to parasitemia for the lower dosage groups was 38, 18, and 8 days respectively. Groups of two monkeys with sporozoite-induced P. cynomolgi infections were also treated for seven days with 31, 10, 3.1, and 0 mg base/kg/day im in combination with 10 mg base/kg/day of chloroquine orally. Both monkeys given 31 mg base/kg/day did not relapse. The average time to relapse following treatment was 48, 29, and 8 days, respectively, for the lower dosage groups. Compound WR182393 is the first non-8-aminoquinoline class of drug to exhibit both causal prophylactic and radical curative properties against a relapsing primate, vivax-like malaria.

Halofantrine for the treatment of mefloquine chemoprophylaxis failures in Plasmodium falciparum infections.

Thai soldiers who became slide-positive for malaria while receiving mefloquine chemoprophylaxis were treated with halofantrine to study its efficacy against mefloquine-resistant falciparum malaria. Thirty-two patients received three doses of 500 mg (1,500 mg total) of halofantrine at six-hr intervals, and were then observed for four weeks. Parasite recrudescence following treatment (median 21 days) occurred in seven of 23 patients (30%) who had mefloquine serum concentrations indicative of regular prophylaxis (greater than 500 ng/ml). Serum concentrations of mefloquine in all 32 patients averaged 950 ng/ml (range 26-2,515) prior to halofantrine treatment. The halofantrine serum concentrations were higher in patients cured by halofantrine than in patients with drug failure, but this was not statistically significant. Patients who were cured by halofantrine had parasites that were more sensitive in in vitro testing to mefloquine (mean [inhibitory concentration] IC50 = 12.5 ng/ml) than in patients whose parasitemias recrudesced (mean IC50 = 23.8 ng/ml) (P less than 0.01, by Wilcoxon rank sum test). These observations suggest that the current formulation and regimen of halofantrine are not optimal for the treatment of multiple drug-resistant falciparum malaria from Thailand.

Comparison of capillary whole blood, venous whole blood, and plasma concentrations of mefloquine, halofantrine, and desbutyl-halofantrine measured by high-performance liquid chromatography.

Whole blood mefloquine, halofantrine, and desbutyl-halofantrine concentrations were measured by high-performance liquid chromatography in capillary blood, venous blood, and venous plasma samples from patients along the Thai/Burmese border with falciparum malaria who were treated with either mefloquine (25 mg/kg) or halofantrine (24 mg/kg or 72 mg/kg). The limits of detection for mefloquine, halofantrine, and desbutyl-halofantrine were 50, 15, and 10 ng/ml, respectively, with 200 microliters whole blood samples. There was a good linear correlation (r > 0.9) between capillary and venous blood and between whole blood and plasma for all three compounds. Mefloquine concentrations in venous and capillary blood were very similar (mean ratio 1.02, 95% confidence intervals [CI] 0.95-1.09, n = 60), but were 1.15 times higher (95% CI 1.03-1.29) in whole blood than in plasma (n = 22). The halofantrine and desbutyl-halofantrine concentrations were 1.27 (1.12-1.45, n = 23) and 1.34 (1.16-1.55, n = 24) times higher in venous compared to capillary blood, while halofantrine but not desbutyl-halofantrine concentrations were lower in whole blood than in plasma (mean ratios: halofantrine: 0.83 [0.72, 0.94], n = 39 and desbutyl-halofantrine: 1.05 [0.96-1.15], n = 41). Measurement of mefloquine, halofantrine, or desbutyl-halofantrine in capillary blood is an accurate and practical alternative to venous blood sampling, and is particularly useful for sampling with children, and under field conditions when technical facilities are limited.

Proguanil plus sulfamethoxazole is not causally prophylactic in the Macaca mulatta--Plasmodium cynomolgi model.

New drugs for causal prophylaxis of malaria are needed. A proguanil/sulfamethoxazole combination was investigated using a rhesus monkey model (Macaca mulatta infected with Plasmodium cynomolgi) to determine whether causal prophylaxis could be achieved. When a five-day regimen of proguanil (40 mg/kg/day) combined with sulfamethoxazole (100 mg/kg/day) was used, infection of all animals (6 of 6) was observed, with an extended prepatent period (median 40 days). Two control animals became infected on days 9 and 23 following sporozoite inoculation. Plasma concentrations indicated that proguanil and sulfamethoxazole were adequately absorbed and metabolized to cycloguanil and N4-acetylsulfamethoxazole, respectively. Analysis of liver biopsy specimens demonstrated that the drugs were present two days following sporozoite inoculation but were not detectable one week later. Proguanil plus sulfamethoxazole does not eliminate exoerythrocytic-stage parasites in the rhesus monkey--P. cynomolgi model.

Evaluation of WR250417 (a proguanil analog) for causal prophylactic activity in the Plasmodium cynomolgi-Macaca mulatta model.

The Plasmodium cynomolgi-Macaca mulatta model has been used to test the antimalarial activity of new drugs for both radical cure and casual prophylaxis. The proguanil analog WR250417 (also known as PS-15) was evaluated for causal prophylactic activity in rhesus monkeys infected with P. cynomolgi bastianelli. Four monkeys were orally dosed with 40 mg/kg/day of WR250417 over three days (-1, 0, and +1). Sporozoite-induced infection of P. cynomolgi was initiated on day 0 with 1 x 10(6) sporozoites to each monkey. Compound WR250417 extended the prepatent period from an average of 8.5 days for controls (n = 2) to a mean of 18.3 days (range 18-19 days, n = 4) for drug-treated monkeys. Analysis of plasma drug concentrations by high-performance liquid chromatography showed that the monkeys converted the WR250417 to its putative principal active metabolite WR99210 (a dihydrotriazine). These findings demonstrate that WR250417 and its principal metabolite do not prevent primary infection by P. cynomolgi.

Chlorproguanil and chlorcycloguanil concentrations in human plasma and urine after Lapudrine administration.

Chlorproguanil and chlorcycloguanil concentrations in human plasma and urine were measured using a high performance liquid chromatographic method. The detection limit in plasma and urine was 5 ng/ml for chlorproguanil and 10 ng/ml for chlorcycloguanil. The elimination half-life of chlorproguanil (Lapudrine) in 2 healthy volunteers, after a single oral dose of the drug, was 14.7 and 16.1 h. No chlorcycloguanil could be detected in plasma over 32 h after dosing.

Race-linked differences in serum concentrations of dapsone, monoacetyldapsone and pyrimethamine during malaria prophylaxis.

Serum concentrations of dapsone (DDS), monoacetyldapsone (MADDS), the principal acetylated metabolite of DDS, and pyrimethamine (PYR) were measured in 55 Caucasian adults (31 males, 24 females) and 159 Papua New Guinean adults (140 males, 19 females) following the oral administration of Folaprim (100mg DDS; 12.5mg PYR). Blood samples were collected at mean sampling times of eight hours after medication and 18 hours before the next weekly dose for malaria prophylaxis. Clearance of DDS and MADDS from serum were significantly faster (p less than 0.001) in Caucasians than in Papua New Guineans. Significantly lower (p less than greater 0.001) serum concentrations of PYR were found in Papua New Guineans than in Caucasians at both sampling times, an observation which may reflect differences in the bioavailability of PYR between the two racial groups. The theoretical implications of these results are that Caucasians may be more susceptible to PYR-resistant Plasmodium falciparum malaria than Papua New Guineans whilst Papua New Guineans may be more susceptible to P. vivax; malaria than Caucasians.

Activity of PS-15 and its metabolite, WR99210, against Plasmodium falciparum in an in vivo-in vitro model.

An in vivo-in vitro model was used to assess the antimalarial activity of PS-15 and its metabolite, WR99210, against Plasmodium falciparum. WR99210, an antifolate triazine compound, was given as a single oral dose of 30 mg/kg to 8 Saimiri sciureus monkeys and, 3 months later, the parent compound, PS-15, was given similarly to the same monkeys. Serum samples were collected at various times after drug administration, serially diluted with control serum, and their antimalarial activity in vitro was determined against the multidrug-resistant K1 isolate of P. falciparum. Serum concentrations of PS-15 and WR99210 were estimated by high performance liquid chromatography. The maximum dilutions of serum that inhibited parasite growth were 20- to 86-fold higher 3 and 6 h after administration of PS-15 than following WR99210 administration. Substantial serum antimalarial activity was observed even at 48 h after medication with PS-15. Serum drug concentrations provided further evidence that PS-15 was absorbed far better from the gastrointestinal tract than WR99210. The substantial and sustained activity of PS-15 suggests that a single dose, or several smaller doses given once a day, should be effective in curing drug-resistant infections of P. falciparum.

Proguanil polymorphism does not affect the antimalarial activity of proguanil combined with atovaquone in vitro.

Clinical studies have shown proguanil (PROG) combined with atovaquone (ATQ) to be an effective and safe antimalarial combination for the treatment of multidrug-resistant falciparum malaria. PROG is a prodrug, which undergoes hepatic metabolism to its pharmacologically active metabolite cycloguanil (CYC). Individuals exhibit genetic polymorphism with respect to PROG, and can be phenotyped as either extensive metabolizers (EMs) or poor metabolizers (PMs) by measuring their PROG/CYC concentration ratio in plasma following PROG/ATQ administration. PMs produce lower plasma concentrations of CYC than EMs and thus may be more susceptible to prophylaxis or treatment failure. Both PROG and CYC potentiate the activity of ATQ in vitro. The antimalarial activity ex vivo of Thai patients' plasma samples obtained from EMs and PMs given concurrent PROG and ATQ was studied using the K1 isolate of Plasmodium falciparum. This isolate is resistant to PROG and CYC, but sensitive to ATQ. Maximum inhibitory dilution profiles of the patients' plasma samples containing PROG and ATQ from EMs and PMs were similar. These findings indicate that differences in plasma drug concentrations between EMs and PMs did not alter the antimalarial activity in vitro against the K1 isolate. The phenotypic status of individuals is not an important issue in the treatment of patients with PROG/ATQ.

Antimalaria activity of the triple combination of proguanil, atovaquone and dapsone.

The combination of proguanil and atovaquone has been shown to be more effective in curing drug-resistant infections of falciparum malaria than atovaquone or proguanil alone. Our current study sought to determine whether the antimalaria activity could be increased by adding dapsone. Plasma samples, obtained from individuals 4-72 h after proguanil-atovaquone administration, were 2-3 times more active against Plasmodium falciparum in vitro when dapsone was added to them. The enhanced activity of the combination of proguanil, atovaquone and dapsone is probably due to the combined activity of two synergistic combinations: proguanil-atovaquone and cycloguanil (metabolite of proguanil)-dapsone. These findings suggest that further studies are needed to evaluate the clinical value of the triple drug combination of proguanil, atovaquone and dapsone in the treatment of multi-drug resistant malaria.

Ex vivo antimalarial activity of proguanil combined with dapsone against cycloguanil-resistant Plasmodium falciparum isolates.

The ex vivo antimalarial activity of plasma samples obtained from 20 healthy Caucasian volunteers following daily proguanil (200 mg) plus dapsone (8 mg) for malaria chemoprophylaxis inhibited five cycloguanil-resistant Thai isolates of Plasmodium falciparum. All volunteers were phenotyped as extensive metabolisers (EMs) of proguanil. Three of the five isolates were obtained from Thai soldiers who had failed malaria prophylaxis on daily proguanil (200 mg) plus dapsone (4.0 or 12.5 mg). The Thai soldiers were also classified as EMs, but had relatively lower plasma cycloguanil concentrations compared to values reported in the literature for Caucasians and black Kenyans. Although the high level of parasite resistance to cycloguanil was the most likely explanation for the Thai soldiers failing prophylaxis on proguanil plus dapsone, their low cycloguanil concentrations may have also contributed to their lack of protection. However, in areas where parasites are more susceptible to cycloguanil, such as in certain regions of Africa, proguanil plus dapsone may still be an effective chemoprophylactic drug combination.

Plasma chloroquine concentrations in young and older malaria patients treated with chloroquine.

Plasma chloroquine (CQ) concentrations were measured by bioassay in young (0-4 years, n = 9) and older (5-60 years, n = 21) patients from Vanuatu infected with malaria following treatment with 25 mg/kg CQ over 3 days. CQ concentrations in young children tended to be lower than in older patients at days 2, 3, 4 and 7 after onset of treatment, with no drug present in two young children on day 3 and in one child on day 7. The greater difficulty experienced by young children to ingest all of their prescribed medication could have contributed to the lower CQ concentrations observed in the younger age group. The possibility that sub-therapeutic CQ concentrations are responsible for treatment failures in young children should be considered in areas where a high degree of CQ resistance has not yet been established. In such areas, the presence or prevalence of CQ-resistant infections should not be based on treatment failures observed in young children unless it can be confirmed that adequate blood CQ concentrations were achieved after treatment.

Doxycycline for malaria prophylaxis in Australian soldiers deployed to United Nations missions in Somalia and Cambodia.

The operational effectiveness of daily doxycycline alone or combined with weekly chloroquine were assessed during deployments of Australian Defence Force personnel to malaria-endemic countries. Doxycycline was given as part of mandated disease prevention measures during United Nations missions to Somalia (900 men for 4 months) and Cambodia (600 men for 12 months over two annual rotations). In Somalia the soldiers were in an area of low endemicity and experienced only three malaria cases (one Plasmodium falciparum, two P. vivax), all occurring after returning to Australia. In Cambodia the level of malaria exposure varied greatly, resulting in eight malaria cases during the entire 2-year mission (two P. falciparum, six P. vivax). Doxycycline was generally well tolerated, with 1.7% (Somalia) and 0.6% (Cambodia) of the men requiring a change of medication to mefloquine due to adverse effects. Doxycycline is an effective chemoprophylactic agent during operational deployments when soldiers truly take it every day.

Pharmacokinetics of proguanil in malaria patients treated with proguanil plus atovaquone.

Clinical studies have shown atovaquone (ATQ), a new blood schizontocidal drug, in combination with proguanil (PROG) to be very effective in the treatment of acute multidrug-resistant falciparum malaria. The multiple dose pharmacokinetics of PROG were determined in Thai patients with acute falciparum malaria given PROG alone (200 mg PROG twice a day for 3 days, n = 4) and concurrently PROG and ATQ (200 mg PROG and 500 mg ATQ twice a day for 3 days, n = 12). There were no statistical differences (p > 0.05) in the area under the plasma drug concentration-time curve (AUC), apparent oral clearance (CL/F) and elimination half-life (t1/2) of PROG between patients given PROG alone and PROG/ ATQ. The median (range) kinetic values of PROG in patients given PROG alone and PROG/ATQ were respectively: CL/F = 1.25 l/h/kg (0.99-1.45) and 0.95 (0.73-1.32) l/h/kg, and t1/2 = 14.2 hours (9.3-16.8) and 13.6 hours (9.1-17.6). The CL/F and t1/2 of PROG in the Thai patients treated with the 2 treatment regimens were also comparable to values reported in healthy Thai volunteers given a standard prophylactic dose (200 mg PROG). The results of this preliminary study suggest that ATQ is unlikely to affect the pharmacokinetics of PROG to a clinically important extent at an ATQ dosage of 500 mg twice a day for 3 days in malaria infected patients.