The chemotherapy of rodent malaria, VII

The chemotherapy of rodent malaria. LXII. Drug combinations to impede the selection of drug resistance, part 5: rates of development of resistance to some inhibitors of folate metabolism and to artesunate

In recent years infection with chloroquine-resistant Plasmodium falciparum has been combatted with two long-acting antimalarials, pyrimethamine and sulfadoxine, in the combination known as Fansidar that exerts a strong, synergistic action on the asexual stages of the parasite. This second-line regimen, however, is failing increasingly because of the selection of resistant clones in endemic areas, and effective, safe, alternative drugs or drug combinations that are also affordable are urgently needed. Antimalarial drugs with shorter half-lives than those of pyrimethamine or sulfadoxine are likely to be slower to select resistant parasites. In the experiments reported here, the baseline in-vivo responses of rodent malarial parasites to chlorproguanil and proguanil and their active metabolites, chlorcycloguanil and cycloguanil, as well as those to dapsone and artesunate, were explored. In general, the most sensitive parasite to all of these compounds was P. chabaudi. When the drugs were used, individually, to select resistance via the '2%-relapse technique', relatively stable resistance to each was obtained in P. chabaudi as well as in P. berghei and P. yoelii ssp. NS, the last of these being also highly resistant to chloroquine. Of most concern was the rapidity and high level of resistance developed by P. chabaudi to artesunate. The experiments also validated the use of chlorcycloguanil or cycloguanil as surrogates for chlorproguanil or proguanil. Further studies to investigate the possible value of administering chlorproguanil-dapsone, with or without artesunate, are under way and will be reported separately.

The chemotherapy of rodent malaria. LXIII. Drug combinations to impede the selection of drug resistance, part 6: the potential value of chlorproguanil and dapsone in combination, and with the addition of artesunate

Resistance is readily produced in rodent malaria using the single-dose, '2%-relapse technique' (2%RT) against the individual compounds chlorproguanil (CPG), chlorcycloguanil (CCG), cycloguanil, dapsone (DDS) and artesunate (ASN). Using the '4-day test', a low level of synergism or a simple additional action between CPG and DDS was observed with multiple dosing of these two compounds in a combination. Resistance to a 1 : 3 combination of CPG-DDS was selected in each of three parasite lines: Plasmodium berghei NK65, P. yoelii ssp. NS and P. chabaudi AS. Of these lines, P. chabaudi AS was found to be the most sensitive to the 1 : 3 combination in the 2%RT (and was also previously found to be the most sensitive when the compounds were used individually). Plasmodium chabaudi AS was also the line found most sensitive to a 7 : 21 : 300 combination of CPG-DDS-ASN (CDA). In mice infected with P. chabaudi AS, compared with the use of the individual components, the CPG-DDS combination only a gave a modest level of protection (as indicated by the increase in the time required to select resistance in the 2%RT) but the triple CDA combination was totally effective over the duration of the experiment. New pharmacokinetic data to be reported elsewhere indicate, however, that the antimalarial action of CPG in mice is exerted by a mechanism that is not associated with the drug's conversion to the antifolate triazine, CCG. The question thus arises as to how, in the present model, the protective action of CDA was effected. The present results nevertheless reinforce the hypothesis that a CDA combination, appropriately proportioned for human use, should be of practical value, in protecting the individual components, when used for the treatment of multidrug-resistant P. falciparum, and possibly other Plasmodium species, in endemic areas. Clinical trials, both with a CPG-DDS combination (Lapdap) and CDA, are currently under way in tropical Africa. Further studies are now required to determine whether DDS, CPG or an as-yet unidentified metabolite of CPG interact with ASN, and whether a simple double combination of ASN with one or other of these would be as protective, against the selection of resistance, as CDA.

The in vivo sensitivity of Plasmodium falciparum to chloroquine and to sulphadoxine-pyrimethamine combination in Ibadan, Nigeria

Fifty-eight Nigerian children with Plasmodium falciparum malaria were allocated randomly into two groups and treated with either chloroquine (25 mg/kg over three days) or Fansidar (35 mg sulphadoxin (+ 1/20 pyrimethamine) per kg single dose)). They were observed for 28 days during which blood films were examined periodically for malaria parasites. Asexual forms of P. falciparum, which were present in the blood films of all the patients in both groups before commencing treatment, disappeared rapidly from the blood so that by the fourth day after starting treatment no parasites were seen in the blood films. The blood films thereafter remained negative in both groups throughout the rest of the 28-day observation period. The rate of fever clearance was also similar in both groups. The study did not show resistance to Fansidar or to chloroquine. There is therefore, at present, no case for the indiscriminate use of Fansidar on the basis of suspected chloroquine resistance.

The chemotherapy of rodent malaria, XXVII. Studies on mefloquine (WR 142,490)

Mefloquine (WR 142,490) is a potent blood schizontocide active against drug-sensitive and drug-resistant lines of Plasmodium berghei. The ED50 and ED90 against the P. berghei N strain in albino mice are 1.5 and 3.8 mg/kg respectively. The highly chloroquine-resistant RC line is less sensitive and mefloquine is not fully effective at the maximum tolerated dose in the '4-day test'. Mefloquine has a similar mode of action to quinine both in vitro and (as demonstrated by the morphological changes it induces in P. berghei) in vivo, but is some 100 times more potent. Unlike quinine and WR 122,455 it appears not to interact with DNA. It has no causal prophylactic effect. Mixtures of mefloquine with pyrimethamine, sulphaphenazole or primaquine have an additive effect.

Experimental studies of the potentiation of proguanil and pyrimethamine by dapsone using Plasmodium berghei in white mice

An accurate system with stringent criteria has been established to test antimalarial drugs and drug combinations against Plasmodium berghei in mice. Minimum effective doses for a number of antimalarial drugs have been determined when used in this system. Considerable potentiation of the activity of pyrimethamine and to a somewhat less extent of proguanil by dapsone has been demonstrated. The need to extend studies of these combinations to the parasites of human malaria is discussed in terms of establishing a safe chemoprophylactic regime in the presence of drug-resistant strains of parasites.

The chemotherapy of rodent malaria, XXV. Antimalarial activity of WR 122,455 (a 9-phenanthrenemethanol) in vivo and in vitro

WR 122,455, 3,6-bis-(trifluoromethyl)-alpha-(2-piperidinyl)-9-phenanthrenemethanol HCl, suppresses infection with drug-sensitive Plasmodium berghei N strain in mice. It acts rapidly and affects all the stages of the asexual intraerythrocytic parasites, the effective dose levels being about three times those of chloroquine and one-twelfth to one-fifteenth those of quinine. Under the influence of WR 122,455 haemozoin seems to disappear from the affected parasites following an initial coarsening of the fine pigment granules. These changes are similar to those exerted by quinine. Large doses of WR 122,455 have a residual affect due in part, at least, to deposition of insoluble material in the tissues. The drug appears to exert an antagonistic action on chloroquine when both drugs are administered simultaneously. It has no causal prophylactic effect. In vitro WR 122,455 is a competitive antagonist of chloroquine in a similar manner to quinine, and appears to have a dissociation constant (Ki) of 2-26 x 10(-8) M, making it about 18 times as active as quinine. WR 122,455 interacts strongly with calf thymus DNA, but the mechanism of interaction has yet to be defined. Mice tolerate single doses of a saline/Tween 80 suspensions up to about 400 mg/kg but sc administration induces necrotic changes at the injection site. Up to 30 mg/kg daily po for seven consecutive days is well tolerated systemically but local tissue reaction may occur if the drug is given by the sc or ip routes. However, systemically up to 60 mg/kg is tolerated sc or ip. The relation of WR 122,455 to drug resistant malaria will be reported later.

Pharmacokinetic rationale for a malarial suppressant administered once monthly

Serum was collected from six adults participating in a field trial of sulfadoxine and pyrimethamine in combination which was being administered once monthly for malaria suppression. Samples were drawn during each of two consecutive months three hours, and 7, 14 and 28 days following a dose of 1 500 mg sulfadoxine. Serum sulfadoxine concentration was measured using the method of Bratton and Marshall (1939). Initial serum concentrations averaged 19-9 plus or minus 2-4 (SD) mg/100 ml and decayed to 6-2 plus or minus 2-8 mg/100 ml at 14 days. Serum sulfadoxine concentrations were still detectable at 28 days following a dose (2-1 plus or minus 1-5 mg/100 ml). Elimination half-time averaged 195 plus or minus 44 hours. The presistent serum concentrations of sulfadoxine following monthly doses documented here during field-use of this drug are in agreement with the successful clinical results reported for such a regimen (Lewis and Ponnampalam, 1974; O'Holohan and Hugoe-Mathews, 1971; Wolfensberger, 1971).

Antimalarial drugs and their actions

New antimalarial drugs are required, partly because of the emergence of drug resistant strains of malaria parasites and partly because better compounds are needed to cure relapsing tertian malaria. In reviewing the diverse modes of action of currently used anti-malarials, against a background of the pathogenesis of malaria, attention is drawn to deficiencies in our knowledge. Even less do we understand how the malaria parasite becomes resistant to certain drugs, in particular chloroquine. New approaches to the problem include the application of combinations of existing antimalarials, and the search for new drugs on an unprecedentedly vast scale. Out of over a quarter million compounds that have recently been screened, a handful are now in clinical trial and are showing great promise for the treatment of multiple resistant falciparum malaria. The paper concludes by summarizing current recommendations for the prophylaxis and therapy of malaria due to drug resistant parasites.