Susceptibility of Malaysian Plasmodium falciparum isolates to antimalarial drugs after cryopreservation and 12 months of continuous in vitro culture

Eleven Malaysian Plasmodium falciparum isolates were cultured in vitro and later subjected to antimalarial evaluations in 96-well microtiter plates. After cryopreservation, the IC50 (nM) for ST 195, ST 196, ST 197, ST 244 and ST 245 isolates were, respectively: 180.9, 198.7, 482.0, 580.0 and 690.1 for chloroquine; 3.4, 3.4, 9.2, 4.0 and 5.8 for mefloquine; 21.9, 10.5, 40.7, 40.1 and 48.7 for quinine; 136.7, 58.8, 116.4, 29.4 and 95.4 for cycloguanil, and 48.3, 57.5, 47.4, 61.5 and 37.8 for pyrimethamine. Before cryopreservation they were 172.5, 141.5, 453.2, 636.0 and 651.6 nM for chloroquine; 4.8, 2.6, 9.0, 6.9 and 5.8 nM for mefloquine; 21.3, 8.3, 41.9, 49.6 and 40.1 nM for quinine, 129.9, 47.3, 109.3, 30.6 and 95.4 nM for cycloguanil, and 45.4, 47.4, 40.2, 66.3 and 36.0 nM for pyrimethamine. IC50 (nM) for Gombak A, Gombak C, ST 9, ST 12, ST 85 and ST 148 isolates after 12 months of continuous in vitro culture were, respectively: 477.0, 492.3, 367.1, 809.4, 566.5 and 341.8 for chloroquine; 2.9, 11.1, 8.5, 16.9, 5.3 and 4.2 for mefloquine; 6.2, 58.3, 52.7, 36.7, 31.8 and 26.2 for quinine; 154.5, 57.2, 130.3, 94.2, 81.4 and 102.9 for cycloguanil, 26.9, 24.9, 43.8, 31.0, 14.1 and 56.7 for pyrimethamine. Before the 12-month culture they were 472.3, 452.9, 352.7, 773.7, 702.7 and 322.7 nM for chloroquine; 2.6, 13.2, 8.5, 17.2, 5.0 and 4.0 nM for mefloquine; 6.2, 85.4, 53.9, 38.5, 35.8 and 38.5 nM for quinine; 106.8, 74.3, 112.4, 89.8, 91.8 and 103.3 nM for cycloguanil, and 26.9, 31.4, 47.0, 28.1, 14.9 and 56.7 nM for pyrimethamine. Thus none of these isolates differed in their original susceptibilities after either of these procedures.