Biological resistance of hydroxychloroquine for Plasmodium vivax malaria in the Republic of Korea.

The Republic of Korea (ROK) Army instituted a vivax malaria chemoprophylaxis program (hydroxychloroquine [HCQ] 400 mg per week) in 1997 that was expanded to nearly 200,000 soldiers by 2007, raising concerns for the emergence of drug-resistant vivax malaria. Therefore, a study of whole blood HCQ concentrations for all malaria patients admitted to four ROK Army hospitals was conducted from June through September 2007. For all 142 vivax malaria patients enrolled, fevers returned to normal by Day 3 post-treatment and all thin blood films were negative for parasites by Day 7. Pre-treatment whole blood concentrations of HCQ for 14 patients were > 100 ng/mL. Eight of the patients were enrolled in the ROK Army chemoprophylaxis program that reported taking HCQ as directed, with the last pill taken > or = 4 days before diagnosis. Although there was no evidence of clinical resistance, chemoprophylaxis data indicates the biological resistance or tolerance to HCQ in ROK.