Antimalarial activity of rifampicin in vitro and in rodent models.

The antimalarial activity of rifampicin, a specific inhibitor of bacterial ribonucleic acid (RNA) polymerase, was confirmed with Plasmodium falciparum in vitro and with P. chabaudi in vivo. The viability of ring forms of P. falciparum, measured by [3H]hypoxanthine and [14C]isoleucine uptake, was significantly reduced within 5 h of exposure to 2.5 microM rifampicin, the 50% inhibitory concentration. Streptolydigin and tagetitoxin, other specific inhibitors of bacterial RNA polymerase, were much less effective as antimalarials. A rifampicin-tolerant sub-line of P. falciparum was selected in vitro. When released from drug pressure, the tolerant line showed appreciably greater rates of incorporation of precursors and growth than the parent line, but over a period of months these characteristics gradually reverted. Rifampicin was effective against a chloroquine-resistant line of P. falciparum and the rifampicin-tolerant line had increased chloroquine sensitivity. Treatment of patent parasitaemias of P. chabaudi in mice with more than 100 mg/kg rifampicin twice daily significantly reduced the parasitaemia within 24 h and parasites were barely detectable on blood films by the fourth day. Recrudescence occurred on release of drug pressure.