Resistance of ten Thai isolates of Plasmodium falciparum to chloroquine and pyrimethamine by in vitro tests

In vitro drug resistance tests of ten isolates of Plasmodium falciparum from three different collection points in Central Thailand have been carried out, and the results compared with those of similar tests with a drug-sensitive West African isolate. Judged by concentration of drug tolerated, the Thai isolates appeared to be about 10 times as resistant to chloroquine, and usually about 10⁵ times as resistant to pyrimethamine, as the African isolate. A little variation amonst the Thai isolates was detected.     

In vitro assessment of sensitivity of Plasmodium falciparum to chloroquine and mefloquine in Ghana

An in vitro study of sensitivity of Plasmodium falciparum to chloroquine and mefloquine in Ghana is described. Results of 60 short-term cultures from 36 patients are evaluated. No sign of chloroquine resistance was found as all microtests showed complete inhibition of maturation at a level of 0.8 X 10(-6) M. For mefloquine schizont maturation was seen at higher levels of the drug. However, the estimated EC99, with 2.2830 X 10(-6) M is probably within the range of sensitivity.     

Evidence of increased chloroquine sensitivity in Thai isolates of Plasmodium falciparum

Sensitivity of Thai isolates of Plasmodium falciparum to chloroquine collected over the years 1978-1986 was measured by two methods: (i) by growth of previously cultured isolates for 72 h in presence of drug, and (ii) by the WHO standard in vitro microtest. During this period there were signs of a gradual increase in drug sensitivity, coinciding with the withdrawal of chloroquine for treatment of falciparum malaria in Thailand.     

In vitro sensitivity of Plasmodium falciparum to artesunate in Thailand.

Reported are the in vitro susceptibilities of Plasmodium falciparum to artesunate, mefloquine, quinine and chloroquine of 86 isolates and to dihydroartemisinin of 45 isolates collected from areas of high resistance to mefloquine within Thailand near the borders with Myanmar and Cambodia, and from southern Thailand where P. falciparum is generally still sensitive to mefloquine. All the isolates were highly sensitive to artesunate, but the geometric mean IC50S were higher in isolates from the Thai-Myanmar and Thai-Cambodian borders than in those from southern Thailand. The IC50S for mefloquine and artesunate were strongly correlated (Pearson r = 0.605; n = 86; P < 0.00001). As expected, the in vitro sensitivities to dihydroartemisinin and artesunate were similar and strongly correlated (at IC50, Pearson r = 0.695; n = 45; P < 0.00002). The correlation between the activity of mefloquine and artesunate requires further investigation in order to determine the potential for development of cross-resistance in nature. Our results suggest that combination with mefloquine is not the ideal way of protecting the usefulness of artemisinin and its derivatives. A search for more suitable partner drugs to these compounds and careful regulation of their use are necessary in the interest of ensuring their long therapeutic life span.     

Susceptibility of Plasmodium falciparum in The Gambia to pyrimethamine, Maloprim and chloroquine

A five-year malaria chemoprophylaxis study has begun with Maloprim in children aged three months to five years and pregnant women in a population of 13,000 in the area of Farafenni, The Gambia. Sensitivity of Plasmodium falciparum to pyrimethamine, Maloprim and chloroquine was assessed in vivo and in vitro in rural Gambian villages before drug intervention. 569 children aged one to seven years inclusive were sampled at the end of the wet season of 1982; 46% had positive blood films. All afebrile children were treated with a single dose of one of the antimalarials under study. Febrile children were treated with chloroquine. 109 infected children were retested 7 to 10 days after treatment and none showed asexual parasitaemia. 83 micro in vitro tests were successfully performed from fingerprick blood samples and the results confirmed the in vivo study. Pyrimethamine in combination with dapsone, in the proportion present in Maloprim, i.e., 1:8, showed a synergistic effect, the mean effective dose of pyrimethamine being reduced 13 times at the 50% inhibitory level.     

Sensitivity of Plasmodium falciparum to chloroquine and sulfadoxine/pyrimethamine in Nigerian children

The in vivo sensitivity of Plasmodium falciparum to chloroquine and sulfadoxine/pyrimethamine was evaluated in children under 5 years of age in two areas of southern Nigeria in 1987. A modification of the WHO Standard Field and Extended Tests (in vivo) was used, with follow-up on days, 2, 3, 7, and 14 after treatment with 25 mg chloroquine per kg body weight given over 3 days, or with standard doses of sulfadoxine/pyrimethamine. Clinical and parasitological evaluations were performed. At Igbo Ora, in Oyo State, where by day 7 chloroquine was clinically successful in 94.4% of 36 children and sulfadoxine/pyrimethamine in 91.7% of 36 children, there were no parasitological failures in either treatment group. Fever regressed significantly more rapidly with chloroquine than with sulfadoxine/pyrimethamine. At Oban, in Cross River State, initial parasite densities decreased markedly with the chloroquine regimen but 63.6% of 44 children were parasitological failures on days 3, 7, or 14; and all of the 26 children who failed parasitologically and completed follow-up were successfully treated with sulfadoxine/pyrimethamine. By day 7, clinical success was demonstrated for 77.3% of the children treated with chloroquine. The in vitro sensitivity to chloroquine, quinine, and mefloquine at Igbo Ora indicated that isolates of P. falciparum were sensitive to chloroquine and quinine, but had reduced sensitivity to mefloquine. Because of its continued clinical efficacy, chloroquine remains the recommended treatment for children with uncomplicated malaria in Nigeria. Health providers are, however, encouraged to maintain supplies of sulfadoxine/pyrimethamine as an alternative and to refer patients promptly if necessary.     

In vitro sensitivity of Plasmodium falciparum to chlorcycloguanil in The Gambia

Chlorcycloguanil (10732), the active metabolite of the antifolate chlorproguanil, has been tested in vitro against 17 isolates of Plasmodium falciparum in The Gambia. Minimum inhibitory concentrations were 10(-9) molar or less. 11 isolates simultaneously tested with pyrimethamine were sensitive to 10(-8) molar concentrations of that drug.     

Enzyme typing of southern African isolates of Plasmodium falciparum by polyacrylamide gel electrophoresis

Nineteen southern African isolates of Plasmodium falciparum were typed by polyacrylamide gel electrophoresis, using 5 enzymes (glucose phosphate isomerase, adenosine deaminase, lactate dehydrogenase, NADP-dependent glutamate dehydrogenase and 6-phosphogluconate dehydrogenase). Limited variation was found amongst the isolates and the frequencies of variants were similar to those of isolates from other parts of the world. Eight of the isolates contained 2 forms of glucose phosphate isomerase, indicating clonal heterogeneity. One of these 8 isolates also contained 2 forms of adenosine deaminase and another showed 2 forms of lactate dehydrogenase.     

In vitro response of Plasmodium falciparum to chloroquine in the Nandi district, Kenya

Thirty-six isolates of Plasmodium falciparum from Nandi district, Kenya, which were tested for their sensitivity to chloroquine using the WHO in vitro macrotechnique, yielded a total of 29 successful tests, one of which showed overt resistance with schizont maturation at chloroquine levels of > 1.5 × 10^-6 mol/l. The majority of isolates showed reduced sensitivity to chloroquine, and the EC₉₉ was 1.7218 × 10^-6 mol/l. These findings are indicative of widespread in vitro resistance of a low degree which may remain largely unnoticed in immune individuals. However, in nonimmune subjects one may expect also in vivo resistance because the parasites will not be completely cleared after a normally curative dose of chloroquine.     

Therapeutic efficacy of chloroquine and sulfadoxine/pyrimethamine against Plasmodium falciparum infection in Somalia

OBJECTIVE: To assess the efficacy of chloroquine and sulfadoxine/pyrimethamine in the treatment of uncomplicated Plasmodium falciparum infections in Somalia. METHODS: Patients with clinical malaria in Merca, an area of high transmission of the disease, were treated with the standard regimens of chloroquine (25 mg/kg) or sulfadoxine/pyrimethamine (25 mg sulfadoxine and 1.25 mg pyrimethamine per kg). Similar patients in Gabiley, an area of low transmission, received the standard regimen of chloroquine. The clinical and parasitological responses were monitored for 14 days. FINDINGS: Chloroquine treatment resulted in clinical failure in 33% (n = 60) and 51% (n = 49) of the patients in Merca and Gabiley respectively. There were corresponding parasitological failures of 77% RII/RIII and 35% RII/RIII. Patients who experienced clinical failure had significantly higher initial parasitaemia than those in whom there was an adequate clinical response, both in Merca (t = 2.2; P t = 2.8; P n = 50) of the patients achieved an adequate clinical response despite a parasitological failure rate of 76% RII/RIII. CONCLUSION: Chloroquine should no longer be considered adequate for treating clinical falciparum malaria in vulnerable groups in the areas studied. Doubts about the therapeutic life of sulfadoxine/pyrimethamine in relation to malaria are raised by the high levels of resistance in the Merca area and underline the need to identify suitable alternatives.     

In vivo and in vitro assessment of the sensitivity of Plasmodium falciparum to chloroquine in four districts of Tanga region, Tanzania

The sensitivity of Plasmodium falciparum to chloroquine was tested in Muheza, Pangani, Tanga and Korogwe districts in north-eastern Tanzania by applying both in vivo and in vitro tests in schoolchildren. A total dose of 25 mg chloroquine base/kg body-weight given over a period of three days (10 mg/kg on days 0 and one; and 5 mg/kg on day 2) failed to clear asexual parasites from the peripheral blood by day 7 in 12.5% of the children tested at Muheza, 5.9% at Pangani, 31.8% at Tanga, and 39.5% at Korogwe. In vitro micro tests were successfully carried out on 44 isolates at Muheza, 29 isolates at Pangani, 45 isolates at Tanga and 44 isolates at Korogwe. Schizont maturation at chloroquine concentrations of 1.14 mu mol/litre of blood and above, an indication of drug resistance, was observed in 20.5% of the isolates at Muheza, 41.4% at Pangani, 51.1% at Tanga and 45.5% at Korogwe. In vivo and in vitro results of the tests for resistance have been compared.