Pharmacokinetics and pharmacodynamics of oral 1 artesunate monotherapy in patients with uncomplicated 2 P . falciparum malaria in western Cambodia

29 Artemisinin-resistant malaria along the Thailand-Cambodian border is an important 30 public health concern, yet mechanisms of drug action and their contributions to the 31 development of resistance are poorly understood. The pharmacokinetics and 32 pharmacodynamics of oral artesunate monotherapy were explored in a dose-ranging 33 trial in an area of emerging artesunate resistance in western Cambodia. We enrolled 34 143 evaluable subjects with uncomplicated P. falciparum malaria in an open label 35 study of directly observed artesunate monotherapy at 3 dose levels (2, 4 and 6 36 mg/kg/day) for 7 days at Tasanh Health Center. Clinical outcomes were similar 37 among the 3 groups. Wide variability in plasma artesunate and dihydroartemisinin 38 concentrations were seen. No significant dose-effect or concentration-effect 39 relationships between pharmacokinetic and parasite clearance parameters were 40 observed, though baseline parasitemia was modestly correlated with increased 41 parasite clearance times. Overall parasite clearance times were prolonged compared 42 with a previous study at this site in 2006-7, but this did not persist when limiting the 43 evaluation to subjects with comparable artesunate dose (4mg/kg/d) and baseline 44 parasitemia from the two studies. Reduced plasma drug levels with higher 45 presentation parasitemias, previously hypothesized to result from partitioning into 46 infected red cells, was not observed in this population with uncomplicated malaria. 47 Neither in vitro parasite susceptibility, nor plasma drug concentrations appeared to 48 have a direct relationship with the pharmacodynamic effects of oral artesunate on 49 malaria parasites. While direct concentration-effect relationships were not found, it 50 remains possible that a population PK modelling approach which allows modelling of 51 greater dose separation might discern more subtle relationships. 52 on July 8, 2017 by gest httpaac.asm . rg/ D ow nladed fom INTRODUCTION 53 Oral artesunate is an important drug used in the control of multi-drug resistant 54 P. falciparum malaria. However, the mechanism of action for this class of drugs, 55 known as the Artemsisinins, and thus potential mechanisms for development of drug 56 resistance remain poorly understood. Semi-mechanistic models of parasite dynamics 57 have been proposed to explain the potential effects of drug on parasites sequestration 58 in unmeasurable compartments (16, 18). Selective effects of drug on early parasite 59 ring stages were found to explain differences in parasite clearance observed between 60 patients treated in Thailand and Cambodia using intrahost mathematical modeling of 61 PK-PD relationships (33). Autoinduction of metabolism has been suggested for drugs 62 in this class (15, 17), representing a possible confounder to parasite based 63 mechanisms of resistance. The current models provide only limited explanations for 64 artesunate resistance, creating an ongoing need for additional empiric data. 65 66 We recently reported evidence of artesunate resistance in malaria parasites from 67 patients treated in a clinical trial conducted in western Cambodia in 2006-2007 (30, 68 31). This study showed delayed parasite clearance times and reduced artemisinin 69 drug susceptibility in vitro in spite of what appeared to be therapeutic drug levels. 70 However, pharmacologic collection times in that study were limited to 2 time points 71 post dosing, prompting an effort to conduct more detailed analysis. A dose-ranging 72 clinical trial of artesunate monotherapy at 3 doses (2, 4 and 6 mg/kg/day once daily 73 for 7 days) was conducted to better understand pharmacokinetic-pharmacodynamic 74 relationships of oral artesunate monotherapy in 2008-9 (7). Rates of adequate clinical 75 and parasitological responses (ACPR) in all treatment arms to artesunate at 28 and 42 76 days were comparable to those observed in previous oral artesunate monotherapy 77 on July 8, 2017 by gest httpaac.asm . rg/ D ow nladed fom studies in the region (8, 9, 17, 26, 34). There were prolonged parasite clearance times 78 observed compared to the study carried out in 2006, with more than 50% of patients 79 remaining parasitemic at 72 hours after commencing artesunate. The 80 pharmacokinetics and pharmacodynamics of oral artesunate (AS) and 81 dihydroartemisinin (DHA), its active metabolite, were explored to test whether 82 alterations in pharmacokinetics could explain the apparent changes in parasite 83 pharmacodynamics. We investigated dose-effect and concentration-effect 84 relationships with key pharmacodynamic variables including parasite clearance and in 85 vitro parasite drug susceptibility to evaluate whether increasing the dose of oral 86 artesunate might improve pharmacodynamic responses by reducing rates of clinical 87 failure or decreasing parasite clearance times. 88