Background Artemisinin resistance in Plasmodium falciparum threatens to reduce the efficacy of artemisinin combination therapies (ACTs), thus compromising global efforts to eliminate malaria. Recent treatment failures with dihydroartemisinin-piperaquine, the current first-line ACT in Cambodia, suggest that piperaquine resistance may be emerging in this country. We explored the relation between artemisinin resistance and dihydroartemisinin–piperaquine failures, and sought to confirm the presence of piperaquine-resistant P falciparum infections in Cambodia. Methods In this prospective cohort study, we enrolled patients aged 2–65 years with uncomplicated P falciparum malaria in three Cambodian provinces: Pursat, Preah Vihear, and Ratanakiri. Participants were given standard 3-day courses of dihydroartemisinin–piperaquine. Peripheral blood parasite densities were measured until parasites cleared and then weekly to 63 days. The primary outcome was recrudescent P falciparum parasitaemia within 63 days. We measured piperaquine plasma concentrations at baseline, 7 days, and day of recrudescence. We assessed phenotypic and genotypic markers of drug resistance in parasite isolates. The study is registered with ClinicalTrials.gov, number NCT01736319. Findings Between Sept 4, 2012, and Dec 31, 2013, we enrolled 241 participants. In Pursat, where artemisinin resistance is entrenched, 37 (46%) of 81 patients had parasite recrudescence. In Preah Vihear, where artemisinin resistance is emerging, ten (16%) of 63 patients had recrudescence and in Ratanakiri, where artemisinin resistance is rare, one (2%) of 60 patients did. Patients with recrudescent P falciparum infections were more likely to have detectable piperaquine plasma concentrations at baseline compared with non-recrudescent patients, but did not differ significantly in age, initial parasite density, or piperaquine plasma concentrations at 7 days. Recrudescent parasites had a higher prevalence of kelch13 mutations, higher piperaquine 50% inhibitory concentration (IC50) values, and lower mefloquine IC50 values; none had multiple pfmdr1 copies, a genetic marker of mefloquine resistance. Interpretation Dihydroartemisinin–piperaquine failures are caused by both artemisinin and piperaquine resistance, and commonly occur in places where dihydroartemisinin–piperaquine has been used in the private sector. In Cambodia, artesunate plus mefloquine may be a viable option to treat dihydroartemisinin–piperaquine failures, and a more effective first-line ACT in areas where dihydroartemisinin–piperaquine failures are common. The use of single low-dose primaquine to eliminate circulating gametocytes is needed in areas where artemisinin and ACT resistance is prevalent. Funding National Institute of Allergy and Infectious Diseases.

iIn 2008, dihydroartemisinin (DHA)-piperaquine (PPQ) became the first-line treatment for uncomplicated Plasmodium falciparum malaria in western Cambodia. Recent reports of increased treatment failure rates after DHA-PPQ therapy in this region suggest that parasite resistance to DHA, PPQ, or both is now adversely affecting treatment. While artemisinin (ART) resistance is established in western Cambodia, there is no evidence of PPQ resistance. To monitor for resistance to PPQ and other antimalarials, we measured drug susceptibilities for parasites collected in 2011 and 2012 from Pursat, Preah Vihear, and Ratanakiri, in western, northern, and eastern Cambodia, respectively. Using a SYBR green I fluorescence assay, we calculated the ex vivo 50% inhibitory concentrations (IC 50 s) of 310 parasites to six antimalarials: chloroquine (CQ), mefloquine (MQ), quinine (QN), PPQ, artesunate (ATS), and DHA. Geometric mean IC 50 s (GMIC 50 s) for all drugs (except PPQ) were significantly higher in Pursat and Preah Vihear than in Ratanakiri (P < 0.001). An increased copy number of P. falciparum mdr1 (pfmdr1), an MQ resistance marker, was more prevalent in Pursat and Preah Vihear than in Ratanakiri and was associated with higher GMIC 50 s for MQ, QN, ATS, and DHA. An increased copy number of a chromosome 5 region (X5r), a candidate PPQ resistance marker, was detected in Pursat but was not associated with reduced susceptibility to PPQ. The ex vivo IC 50 and pfmdr1 copy number are important tools in the surveillance of multidrug-resistant (MDR) parasites in Cambodia. While MDR P. falciparum is prevalent in western and northern Cambodia, there is no evidence for PPQ resistance, suggesting that DHA-PPQ treatment failures result mainly from ART resistance.

f Reduced Plasmodium falciparum sensitivity to short-course artemisinin (ART) monotherapy manifests as a long parasite clearance half-life. We recently defined three parasite founder populations with long half-lives in Pursat, western Cambodia, where reduced ART sensitivity is prevalent. Using the ring-stage survival assay, we show that these founder populations have reduced ART sensitivity in vitro at the early ring stage of parasite development and that a genetically admixed population contains subsets of parasites with normal or reduced ART sensitivity.

b Dihydroartemisinin-piperaquine is the current frontline artemisinin combination therapy (ACT) for Plasmodium falciparum malaria in Cambodia but is now failing in several western provinces. To investigate artesunate plus mefloquine (AS؉MQ) as a replacement ACT, we measured the prevalence of multiple pfmdr1 copies-a molecular marker for MQ resistance-in 844 P. falciparum clinical isolates collected in 2008 to 2013. The pfmdr1 copy number is decreasing in Western Cambodia, suggesting that P. falciparum is regaining in vitro susceptibility to MQ.A rtemisinin-based combination therapy (ACT) is used worldwide to treat uncomplicated Plasmodium falciparum malaria. Artesunate plus mefloquine (ASϩMQ) was adopted as Cambodia's first-line treatment in 2000. ASϩMQ treatment failures were first observed in 10 to 20% of patients in Pailin and Battambang Provinces, Western Cambodia, in 2003 to 2004 (1, 2). The presence of multiple (i.e., Ն2) pfmdr1 copies-a genetic marker of MQ resistance-was associated with ASϩMQ failures and reduced parasite in vitro susceptibility to MQ not only in Pailin and Battambang but in neighboring Pursat and Kampot Provinces as well (3)(4)(5). This development, along with emerging evidence of reduced in vitro susceptibility to MQ in other provinces (6), prompted Cambodia's National Malaria Control Program to adopt dihydroartemisinin-piperaquine (DHA-PPQ) as the firstline ACT in Western Cambodia in 2008 and in the entire country in 2010. Unfortunately, recent clinical studies suggest that the efficacy of DHA-PPQ is rapidly declining in five Western Cambodian provinces: Pursat, Battambang, Pailin, Oddar Meancheay, and Preah Vihear. For example, DHA-PPQ treatment failures were observed in 25% and 11% of patients in Pailin and Pursat in 2010 (7) and in 36% of patients in Oddar Meancheay in 2013 (8). The rapidly increasing prevalence of DHA-PPQ failures in these provinces, likely due to entrenched artemisinin resistance (9, 10) and suspected PPQ resistance (7, 8), demands additional evaluations of newer antimalarial drugs (11, 12), as well as reevaluation of previously used ACTs.Only a few ACTs, including ASϩMQ and artemether-lumefantrine (AL), are presently available for widespread use in Cambodia to replace DHA-PPQ as it fails. Although ASϩMQ and AL treatment failures have previously been reported from Battambang and Pursat (2, 13), the clinical efficacy of these regimens in the setting of DHA-PPQ resistance is not known. However, studies have suggested a lack of cross-resistance between PPQ and MQ in vitro; for example, pfmdr1 amplification is associated with decreased susceptibility to MQ but increased susceptibility to PPQ (14). Given these and related findings (15), and the fact that earlier use of DHA-PPQ was associated with decreasing prevalence of multiple pfmdr1 copies in Pailin (from 33% in 2005 to 5% in 2007) (16), we hypothesized that the recent substantially reduced use of ASϩMQ in Pursat and Preah Vihear would select for parasites that have regained sensitivity to MQ. To explore this hyp...