Inter-individual variation in the metabolic activation of the antimalarial biguanides

The aryl-biguanides proguanil and chlorproguanil were developed as part of a collaborative programme between ICI and the Liverpool School of Tropical Medicine during the 1940s. The compounds were characterized by their absence of host toxicity. However, the rapid development of parasite resistance to the actions of these drugs and the development of the 4-aminoquinoline, chloroquine, severely limited their use. The subsequent widespread development of parasite resistance to chloroquine, together with the observations that the magnitude of dihydrofolate reductase inhibitor resistance (the site of action of the biguanides) developed to pyrimethamine is not directly correlated with biguanide resistance(1,2). has resulted in renewed interest in these drugs. In particular, proguanil is now the drug of choice for malaria prophylaxis, in combination with chloroquine; used in combination with a suitable sulphonamide, it may be of value in malaria therapy.

Chlorproguanil-dapsone for treatment of drug-resistant falciparum malaria in Tanzania

BACKGROUND

Resistance to the affordable malaria treatments chloroquine and pyrimethamine-sulfadoxine is seriously impeding malaria control through treatment in east Africa. We did an open, alternate drug allocation study to assess the efficacy of chlorproguanil-dapsone in the treatment of falciparum malaria clinically resistant to pyrimethamine-sulfadoxine.

METHODS

Children younger than 5 years with non-severe falciparum malaria, attending Muheza district hospital in Tanzania, were treated with the standard regimen of pyrimethamine-sulfadoxine. Patients whose clinical symptoms resolved but who remained parasitaemic 7 days after pyrimethamine-sulfadoxine were followed up for 1 month. Clinical malaria episodes were retreated with either single dose pyrimethamine-sulfadoxine or a 3-day regimen of chlorproguanil-dapsone. Those with parasitaemia after 7 days were treated with chlorproguanil-dapsone. Parasite DNA was collected on day 7 after first treatment with pyrimethamine-sulfadoxine and we looked for point mutations in the genes encoding dihydrofolate reductase (dhfr) and dyhydropteroate synthetase (dhps).

FINDINGS

360 children were enrolled and treated with pyrimethamine-sulfadoxine. On day 7, 192 (55%) of 348 had cleared parasitaemia. Of the remaining 156 parasitaemic children, 140 (90%) were followed up to day 28, and 92 (66%) of 140 developed clinical malaria. These 92 patients were alternately retreated with either pyrimethamine-sulfadoxine (46) or chlorproguanil-dapsone (46). 28 (61%) of 46 children retreated with pyrimethamine-sulfadoxine were still parasitaemic at day 7, compared with three (7%) of 44 [corrected] children retreated with chlorproguanil-dapsone. Resistance to pyrimethamine-sulfadoxine increased from 45% (156/348) at the first treatment to 61% (28/46) after retreatment. 83 of 85 parasite isolates collected after the first pyrimethamine-sulfadoxine treatment, and before and after the second treatments with pyrimethamine-sulfadoxine and chlorproguanil-dapsone showed triple-mutant dhfr alleles, associated with a variety of dhps mutations.

INTERPRETATION

Most patients treated with pyrimethamine-sulfadoxine, who remain parasitaemic at day 7, develop new malaria symptoms within 1 month. Chlorproguanil-dapsone was a practicable therapy under these circumstances. Analysis of parasite dhfr and dhps before and after treatment supports the view that pyrimethamine-sulfadoxine resistance in this part of Africa is primarily due to parasites with three mutations in the dhfr domain.

The disposition of intramuscular artemether in children with cerebral malaria; a preliminary study

The disposition of intramuscular artemether (AM) was studied in 26 Kenyan children with cerebral malaria. Antimalarial activity determined by bioassay was compared with total plasma AM plus dihydroartemisinin (DHA) determined by high power liquid chromatography (HPLC). Therapeutic levels were achieved in most subjects (21/26) within 1 h of receiving intramuscular AM (3.2 mg/kg), with close correlation between bioassay and HPLC measurements (r = 0.706). However, there was marked inter-individual variation, antimalarial activity was undetectable in 5 subjects ('non-absorbers'), and plasma concentrations were lower in subject with respiratory distress. The 50% parasite clearance time was significantly longer in non-absorbers (mean = 13.1 h, SD = 10.8 vs. mean = 7.8 h, SD = 5.5; P = 0.013). We conclude that the bioavailability of intramuscular AM in children with severe malaria may be highly variable, particularly in the presence of respiratory distress, and may be associated with an inadequate therapeutic response.

A study of the comparative bioavailability of pyrimethamine-sulfadoxine obtained from two oral preparations

The bioavailability of two oral preparations of pyrimethamine-sulfadoxine are compared in a randomised cross-over study. Pyrimethamine(PM)-sulfadoxine(SD) was given orally in a single dose to healthy male adults (SD 12 mgkg-1; PM 0.64 mgkg-1). For Fansidar(Hoffman-LaRoche) the mean AUC for PM and SD were; 0.47 mghl-1 and 45.4 mghl-1, and for Falcidin(Cosmos), the mean AUC for PM and SD were; 0.49 mg hl-1 and 42.9 mghl-1 respectively. There was no significant difference in bioavailability of PM or SD between the two preparations.

Towards optimal regimens of parenteral quinine for young African children with cerebral malaria: the importance of unbound quinine concentration

Young African children with severe malaria are given quinine using a regimen designed for Thai adults. We measured quinine in the blood, plasma and plasma water of young children in Kenya after rapid intravenous and intramuscular dosing, and calculated the therapeutic range of unbound quinine. The peak plasma quinine concentration after rapid intravenous dosing was 12.3 +/- 3.7 mg/L (mean +/- SD), 43% higher than in adults given the same regimen previously; this was due to a smaller apparent volume of distribution in the children. The therapeutic range of unbound quinine was calculated as 0.2-2.0 mg/L. Simulations of unbound quinine were made for the standard quinine regimen: unbound drug concentrations rose above the therapeutic range after each dose. The possible risks of quinine-induced visual impairment are discussed. Alternative, lower dose regimens for young African children with severe malaria are described.

Prophylactic phenobarbitone in young children with severe falciparum malaria: pharmacokinetics and clinical effects

1. A method is described for the measurement of phenobarbitone (PB) by reversed phase high performance liquid chromatography (h.p.l.c.) from small samples of whole blood dried onto filter paper strips. 2. The disposition of PB given prophylactically to young children with severe malaria on parenteral quinine is contrasted with that in aparasitaemic Kenyan children on no antimalarial drugs. There were no differences in the disposition of PB between the two groups. 3. Peak blood PB concentrations were equal to or greater than 15 mg l-1 in 27% of the patients on quinine and 23% of those not on quinine; a concentration of 10 mg l-1 was achieved or exceeded by 100% and 92% of each group, respectively, and was maintained for 39 +/- 24 h (mean +/- s.d.), and 33 +/- 21 h, respectively. 4. In an open, dose-finding study, the progress of young children with cerebral malaria given prophylactic PB (10 mg kg-1), was contrasted with that of controls given no seizure prophylaxis. 5. The drug had no apparent effect on depth or duration of coma, but neither was the incidence of seizures reduced. 6. A controlled trial of prophylactic PB in young children with cerebral malaria is needed, but a larger dose than 10 mg kg-1 should be studied.

The disposition of oral and intramuscular pyrimethamine/sulphadoxine in Kenyan children with high parasitaemia but clinically non-severe falciparum malaria

1. H.p.l.c. methods are described for the measurement of pyrimethamine and sulphadoxine in small volumes of plasma dried on filter paper strips. 2. Pyrimethamine/sulphadoxine (Fansidar, Hoffman LaRoche) was given by mouth and by intramuscular injection to children with uncomplicated falciparum malaria but with high parasitaemia (n = 8 for both routes; pyrimethamine 1.25 mg kg-1, sulphadoxine 25 mg kg-1). 3. Plasma concentrations of pyrimethamine and sulphadoxine associated with synergistic effects against pyrimethamine-resistant strains of Plasmodium falciparum in vitro were achieved within 1 h of administration and were maintained beyond the end of sampling. 4. After both oral and parenteral administration the plasma concentrations of both compounds were lower than those predicted by data from healthy subjects. 5. Areas under the plasma concentration-time curves of sulphadoxine after oral and i.m. administration did not differ significantly, although maximum plasma drug concentrations were higher after the i.m. route (P = 0.03). 6. The AUC values of pyrimethamine did not differ significantly between the two routes of administration. However, after i.m. administration AUC(0,24 h) values were smaller (P = 0.03), and the time to maximum plasma drug concentration (tmax) was longer (P = 0.004) than when the drug was given orally.

Inter-subject variability in the metabolism of proguanil to the active metabolite cycloguanil in man

1. The metabolism of proguanil to the active metabolite cycloguanil has been evaluated in 135 British Troops and 26 Kenyan schoolchildren. 2. Large inter-subject variability was observed in both plasma and urinary concentrations of proguanil and cycloguanil after standard doses of drug. 3. Based on the ratio of proguanil to cycloguanil (P/C) in urine the British troops formed a non-normal distribution. 90% of the population formed a discrete distribution with P/C ranging from 0.5 to 9.0 while the remaining 10% were scattered throughout the distribution to an extreme value of 39. A similar pattern of variability was observed using P/C from a 6 h plasma sample. 4. This variability was due to differences in the ability of individuals to metabolise proguanil to cycloguanil. 5. Thirteen schoolchildren who had experienced malaria during prophylaxis with proguanil and thirteen matched controls each received proguanil (100 mg). We could not discriminate between the two groups based on P/C ratio in either a 6 h plasma or 0-6 h urine sample.