The most effective regimens were artemether-lumefantrine against P. falciparum and dihydroartemisinin-piperaquine against P. vivax. The relatively high rate of treatment failure with dihydroartemisinin-piperaquine against P. falciparum may reflect cross-resistance between chloroquine and piperaquine. (Australian New Zealand Clinical Trials Registry number, ACTRN12605000550606.)

Summaryobjective Recent clinical studies have shown high rates of malaria treatment failure in endemic areas of Papua New Guinea (PNG), necessitating a change of treatment from chloroquine (CQ) or amodiaquine (AQ) plus sulphadoxine-pyrimethamine to the artemisinin combination therapy (ACT) artemether plus lumefantrine (LM). To facilitate the monitoring of antimalarial drug resistance in this setting, we assessed the in vitro sensitivity of Plasmodium falciparum isolates from Madang Province.methods A validated colorimetric lactate dehydrogenase assay was used to assess growth inhibition of 64 P. falciparum isolates in the presence of nine conventional or novel antimalarial drugs [CQ, AQ, monodesethyl-amodiaquine (DAQ), piperaquine (PQ), naphthoquine (NQ), mefloquine (MQ), LM, dihydroartemisinin and azithromycin (AZ)].results The geometric mean (95% confidence interval) concentration required to inhibit parasite growth by 50% (IC 50 ) was 167 (141-197) nm for CQ, and 82% of strains were resistant (threshold 100 nm), consistent with near-fixation of the CQ resistance-associated pfcrt allele in PNG. Except for AZ [8.351 (5.418-12.871) nm], the geometric mean IC 50 for the other drugs was <20 nm. There were strong associations between the IC 50 s of 4-aminoquinoline (CQ, AQ, DAQ and NQ), bisquinoline (PQ) and aryl aminoalcohol (MQ) compounds suggesting cross-resistance, but LM IC 50 only correlated with that of MQ.conclusions Most PNG isolates are resistant to CQ in vitro but not to other ACT partner drugs. The non-isotopic semi-automated high-throughput nature of the Plasmodium lactate dehydrogenase assay facilitates the convenient serial assessment of local parasite sensitivity, so that emerging resistance can be identified with relative confidence at an early stage.