Exploiting differential metabolism in parasite life cycles to limit the spread of drug resistance
School of BioSciences, University of Melbourne.
Drug resistance is a major factor limiting our ability to control parasitic diseases. Many medically and economically significant parasites undergo immense changes in metabolic activity as they move between their mammalian hosts and invertebrate vectors. Consequently, drug selection for resistance during the vertebrate stage can have dire consequences for parasite fitness during transmission. Using the malaria parasite, Plasmodium, we are investigating anti-malarial drugs whose targets are under markedly reduced selection pressure in the mammalian host. Resistance mutations in these targets tend to drive parasite failure during development in the mosquito vector, thereby inhibiting disease transmission. This approach is particularly effective with drugs targeting the parasite organelles, due to their increased metabolic activity during the mosquito stages and the non-Mendelian inheritance of their genomes. Clinically relevant mutations conferring resistance to the widely used anti-malarial MalaroneTM arise in the mitochondrion-encoded cytochrome b gene. These mutations completely arrest development of infectious sporozoites in the mosquito stages of both P. falciparum and P. berghei, thereby blocking transmission of these resistance genes. The macrolide antibiotic azithromycin kills malarial parasites by inhibiting apicoplast protein synthesis. We generated two azithromycin resistant lines in P. berghei (PbAZMR) harbouring mutations in the apicoplast-encoded rpl4 gene. Such mutations also confer azithromycin resistance in P.falciparum. PbAZMR parasites infect mosquitoes at a lower rate, produce fewer sporozoites, and fail to infect naive mice. Major defects in apicoplast development underly this transmission block. Our results demonstrate that resistance trapping due to differential selection between life stages is a common phenomenon. It can be exploited to develop drugs with greatly reduce levels of resistance transmission; not only in malaria but possibly in other parasites with invertebrate vectors.