Partial inhibition by mepacrine of the development of sulphonamide resistance in Plasmodium berghei

The chemotherapy of rodent malaria XXXV. Further studies on the retardation of drug resistance by the use of a triple combination of mefloquine, pyrimethamine and sulfadoxine in mice infected with P. berghei and 'P. berghei NS'

In the face of an increasing prevalence of Plasmodium falciparum resistant to chloroquine and to pyrimethamine-sulphonamide or -sulphone mixtures, the need for a new, effective blood schizontocide for treatment of acute malaria is urgent. The only such compound that is almost ready for release is mefloquine (M) but there is already a danger that parasites may become resistant to this compound if it is used extensively alone. Earlier studies using a rodent model indicated that mefloquine could be 'protected' by administering it with a pyrimethamine-sulfadoxine (PS) mixture, but the experimental technique used was open to criticism. The present experiments, using a relapse technique to develop drug resistance, more closely parallel the way antimalarials are likely to be deployed in human communities. They confirm that the development of resistance to the individual components in the P. berghei N line is delayed by the triple combination, and more so in the slightly chloroquine-resistant 'P. berghei NS' line. The combination did not prevent the development of resistance in a line initially resistant to PS, but the level reached was much less than those seen when P. berghei N or 'P. berghei NS' were exposed to MPS or to M alone. It was concluded that the use of a triple combination of M, P and S against P. falciparum would be of value in 'protecting' these compounds, and thus gain time while new agents against malaria are being developed.

The chemotherapy of rodent malaria, XXVI. The potential value of WR 122,455 (a 9-phenanthrenemethanol) against drug-resistant malaria parasites

The phenanthrenemethanol compound WR 122,455 is an effective blood schizontocide against lines of Plasmodium berghei that are highly resistant to primaquine, sulphonamides, pyrimethamine and cycloguanil. It is also active against the NS line that is moderately resistant to chloroquine. WR 122,455 is inactive against the RC line which is highly resistant to chloroquine. Resistance to WR 122,455 is fairly readily developed by the drug-sensitive N strain of P. berghei, using a relapse technique. Resistance develops very readily to the NS line of P. berghei. Both resistant lines exhibit cross-resistance to quinine, but a roughly normal response to chloroquine, primaquine, sulphonamides, dapsone, pyrimethamine and cycloguanil. Resistance to WR 122,455 is stable through cyclical transmission and through cryopreservation, as well as in the absence of drug selection pressure. The resistant parasites have an essentially normal morphology and virulence. A warning is given against the widescale use of WR 122,455 or similar new drugs for human malaria other than in a suitable combination, in order to minimize the danger of the development of resistance to them.