Parasite Viability as a Superior Measure of Antimalarial Drug Activity in Humans

Abstract

Background Artemisinin derivatives are the leading class of antimalarial drugs due to their rapid onset of action and rapid clearance of circulating parasites. The parasite clearance (PC) half-life measures the rate of loss of parasites from blood after treatment, and this is currently used to assess antimalarial activity of novel agents and to monitor resistance. However, a number of recent studies have challenged the use of PC to measure drug activity, arguing that many circulating parasites may be non-viable. Methods Plasmodium falciparum infected subjects (n=10) in a malaria volunteer infection study were administered a single dose of artesunate (2 mg/kg). Circulating parasite concentration was assessed by quantitative PCR (qPCR). Parasite viability after artesunate administration was estimated by mathematical modelling of the ex vivo growth of parasites collected from subjects. Results We showed that in artemisinin-sensitive infection, viable parasites declined to <0.1% of baseline within 8 h after artesunate administration, while the total number of circulating parasites measured by qPCR remained unchanged. In artemisinin-resistant infections over the same interval, viable parasites declined to 51.4% (SEM: 4.6%) of baseline. Conclusions These results demonstrate that in vivo drug activity of artesunate is faster than is indicated by the PC half-life.

A new in vitro checkerboard-parasite reduction ratio interaction assay for early de-risk of clinical development of antimalarial combinations

“…Apart from ethical concerns regarding the use of animals, and 4 the considerable amount of time and cost required by these in vivo studies, only a small number of doses can be tested, hence preventing the exploration of the full combined response surface of the antimalarial effects. Another important limitation of these studies is the uncertainty of parasite killing following drug treatment since conventional methods cannot reliably distinguish viable from dying or dead parasites (9,10). Indeed, the parasite clearance rate (viable versus dead parasites), measured immediately after treatment, may differ between in vivo models owing different clearance mechanisms of dead parasites from the bloodstream.…”

Questioning the Claimed Superiority of Malaria Parasite Ex Vivo Viability Reduction Over Observed Parasite Clearance Rate?

“…To the Editor —In a study of 10 Plasmodium falciparum –infected volunteers with submicroscopic parasitemias given a single 200-mg dose of artesunate, Rebelo et al [ 1 ] reported a substantial difference in the ex vivo growth of sequentially sampled circulating ring-stage [ 2 ] parasites comparing infections with artemisinin-sensitive (Pfkelch wild-type) and artemisinin-resistant (Pfkelch R539T) parasites. In the 5 artemisinin-sensitive infections, they derived an estimated ex vivo mean parasite “viability” reduction half-life of 0.75 hour, considerably shorter than the corresponding 3.2-hour in vivo mean parasite clearance half-life estimate.…”

“…The serial quantitative polymerase chain reaction derived parasitemia profiles shown by Rebelo et al [ 1 ], fig 3 strongly suggest continued input into the circulation from ongoing schizogony [ 2 , 8 ]. This explains why parasite densities in blood do not fall for approximately 8 hours.…”

“…The title of the article, “Parasite viability as a superior measure of antimalarial drug activity in humans” [ 1 ], suggests a significant advance, but it is not clear why or how it would be used to assess antimalarial drugs. It is stated that “the use of parasite clearance to measure drug activity and to inform decisions about drug development should be reconsidered in view of these new insights.” It is unclear what these insights are and whether these difficult and laborious serial in vivo studies would offer any advantage over the currently used, simple ring-stage in vitro tests [ 4 , 6 ], which identify the loss of ring-stage activity in artemisinin-resistant parasites very well.…”

“…As for dose finding, the results presented in [ 1 ] fig 3 suggest that the fits to the serial viability log-linear declines are poor and, thus, the derived viability half-lives are imprecise in comparison with the parasite clearance profiles. Indeed, it is unclear whether declines are exponential and, therefore, whether the model is appropriate.…”