Date Published: April 24, 2014
Publisher: Public Library of Science
Author(s): Laura C. Pollitt, Silvie Huijben, Derek G. Sim, Rahel M. Salathé, Matthew J. Jones, Andrew F. Read, Tim J. C. Anderson.
The evolution of drug resistance, a key challenge for our ability to treat and control infections, depends on two processes: de-novo resistance mutations, and the selection for and spread of resistant mutants within a population. Understanding the factors influencing the rates of these two processes is essential for maximizing the useful lifespan of drugs and, therefore, effective disease control. For malaria parasites, artemisinin-based drugs are the frontline weapons in the fight against disease, but reports from the field of slower parasite clearance rates during drug treatment are generating concern that the useful lifespan of these drugs may be limited. Whether slower clearance rates represent true resistance, and how this provides a selective advantage for parasites is uncertain. Here, we show that Plasmodium chabaudi malaria parasites selected for resistance to artesunate (an artemisinin derivative) through a step-wise increase in drug dose evolved slower clearance rates extremely rapidly. In single infections, these slower clearance rates, similar to those seen in the field, provided fitness advantages to the parasite through increased overall density, recrudescence after treatment and increased transmission potential. In mixed infections, removal of susceptible parasites by drug treatment led to substantial increases in the densities and transmission potential of resistant parasites (competitive release). Our results demonstrate the double-edged sword for resistance management: in our initial selection experiments, no parasites survived aggressive chemotherapy, but after selection, the fitness advantage for resistant parasites was greatest at high drug doses. Aggressive treatment of mixed infections resulted in resistant parasites dominating the pool of gametocytes, without providing additional health benefits to hosts. Slower clearance rates can evolve rapidly and can provide a strong fitness advantage during drug treatment in both single and mixed strain infections.
The evolution of drug resistance in pathogens is a major public health concern, as it has the potential to undermine many of the health advances achieved in the last century. In addition to the individual health impacts of treatment failure, drug resistance has substantial economic costs, including research into alternative treatments and new drug development –. In the case of malaria, the introduction of new drugs has inevitably been followed by the evolution and spread of resistance –. Artemisinin-based drugs (hereafter, artemisinins), the current frontline drugs against malaria parasites, are highly valued for their ability to rapidly clear infections –. However, in the last few years, slower parasite clearance rates, possibly an early sign of resistance, have been reported in Western Cambodia –. Furthermore, studies showing a genetic basis for slower clearance rates – and the spread of this phenotype through South East Asia –, are causing concern that the useful lifespan of artemisinins could be limited . Of particular concern is the fact that, if the currently recommended treatments fail, there are no new antimalarial drugs ready for wide-scale use –.
We have demonstrated that susceptibility to artesunate was reduced within only three passages, with selected parasites breaking through a drug dose of 16 mg/kg that was able to fully clear the ancestral line. After ten passages, selected parasites were cleared more slowly and bounced back more rapidly from all drug doses tested (up to 64 mg/kg twice a day for 5 days: figure 2). The speed at which resistance evolves may be stochastic and likely varies depending on intrinsic differences between parasite strains or species, a question which will be important to address in future work. However our data provides proof of principle that susceptibility to artesunate can be rapidly lost under drug pressure. Consistent with recent reports from P. falciparum, increasing the drug dose did not increase clearance rates of selected parasites in our study, but, in single infections, gametocyte densities were reduced. The slower clearance rate and longer half-life of our selected lines were qualitatively similar to the rates observed from human malaria infections in South-East Asia .