The chemotherapy of rodent malaria. XLII. Halofantrine and halofantrine resistance.

As a blood schizontocide, halofantrine is about three times as active against a drug-sensitive line of Plasmodium berghei (N strain) as chloroquine, but it lacks any causal prophylactic effect. This activity is retained against parasites highly resistant to primaquine, cycloguanil, pyrimethamine, sulphaphenazole and menoctone. A marked resistance to halofantrine is shown by parasites resistant to mefloquine, quinine, chloroquine and amodiaquine, although the moderately chloroquine-resistant 'NS line' is sensitive. The artemisinin-resistant line shows a moderately reduced response. A high level of resistance was rapidly developed in the 'NS line' to halofantrine (NS/HAL line) but not in the N strain (N/HAL). The NS/HAL line was cross-resistant to mefloquine, amodiaquine and, to a lesser degree, artemisinin, but remained sensitive to chloroquine and quinine. The implications to a lesser degree, artemisinin, but remained sensitive to chloroquine and quinine. The implications of these observations for the possible future clinical deployment of halofantrine are discussed, and emphasis is laid on the need to find means of impeding the resistance to it of Plasmodium falciparum.