Trypanosoma brucei brucei: expression of drug resistance in vitro.

The susceptibility of five Trypanosoma brucei brucei stocks to diminazene aceturate (Berenil) and isometamidium chloride (Samorin) was determined in vivo and in vitro. Two stocks were resistant in mice to both drugs. Differences in drug-induced growth inhibition occurred between drug-resistant and drug-sensitive stocks when trypanosome bloodstream forms were incubated for 24 hr at 37 degrees C in 4% CO2 in air in a feeder layer-free culture system. Diminazene concentrations which inhibited growth of trypanosomes by 50% (EC50) differed more than 50-fold between drug-resistant and drug-sensitive stocks, but there were only minor differences in EC50 values for isometamidium. However, differences in growth inhibition due to isometamidium occurred when trypanosomes were incubated with drug concentrations (0.001-0.05 micrograms/ml) below the EC50 values. Trypanosome stocks which were propagated in vitro in the presence of drugs for a period of 10 days greatly differed in their viability. In contrast to drug-sensitive trypanosomes, drug-resistant stocks could be maintained without loss of viability in the presence of 0.01 micrograms/ml isometamidium chloride.