Conference Agenda

Parasites, such as the malaria parasite P. falciparum, are critically dependent on host nutrients. Interference with nutrient uptake can lead to parasite death and, therefore, serve as a successful treatment strategy. P. falciparum parasites cannot synthesise cholesterol, and instead source this lipid from the host. Here, we tested whether cholesterol uptake pathways could be ‘hijacked’ for optimal drug delivery to the intracellular parasite. We found that fluorescent cholesterol analogues were delivered from the extracellular environment to the intracellular parasite. We investigated the uptake and inhibitory effects of conjugate compounds, where proven antimalarial drugs (primaquine and artesunate) were attached to steroids that mimic the structure of cholesterol. These conjugated antimalarial drugs improved the inhibitory effects against multiple parasite lifecycle stages, multiple parasite species, and drug-resistant parasites, whilst also lowering the toxicity to human host cells. Steroids with introduced peroxides also displayed antimalarial activity. Similar activity was observed against other apicomplexan and other protozoan parasites.

These results provide a proof-of-concept that cholesterol mimics can be developed as a drug delivery system against apicomplexan parasites with the potential to improve drug efficacy, increase therapeutic index, and defeat drug resistance.

Malaria parasites have developed resistance to all currently available antimalarials, and it is now critical novel targets against which new inhibitors can be developed are identified. To aid malaria elimination, these targets should play important roles in both malaria disease and transmission. We have identified Protein Disulfide Isomerases (PDIs), a family of enzymes responsible for the formation/cleavage of disulfide bonds ensuring correct protein folding, as novel targets for antimalarial development. Importantly, a wide repertoire of proteins involved in host cell invasion (associated with malaria disease) and transmission must be properly folded in order to function. Here we have interrogated whether PDIs can be inhibited by commercially available inhibitors, with a view to repurposing these compounds as novel antimalarials. We show that commercial PDI inhibitors prevent growth of key malaria-causing species (Plasmodium falciparum and P. knowlesi) whilst also inhibiting their ability to successfully invade host cells. Furthermore, through medicinal chemistry we have enhanced the activity of one of these inhibitors and our data suggests that it specifically targets PDI-Trans, a PDI found to be critical to malaria transmission. Collectively our data suggest that PDI inhibitors could provide dual stage activity and are attractive targets for antimalarial development.

Antibiotics that inhibit translation in the Plasmodium apicoplast are useful antimalarials, but these inhibitors have a “delayed death” effect. These compounds have no immediate impact on growth after treatment, and stop parasite growth only one cycle after drug exposure. A notable exception is the prophylactic drug doxycycline, which has a delayed death effect, but also kills parasites quickly at higher concentrations. We characterised the impact of delayed death drugs on Plasmodium protein translation using quantitative proteomics, simultaneously assaying translation in the apicoplast, mitochondrion and cytosol. We find that delayed death drugs lead to a specific block of apicoplast translation, but doxycycline additionally inhibits translation in the Plasmodium mitochondrion. Seahorse metabolic assays also show that doxycycline decreases parasite oxygen consumption rate, consistent with a perturbation to mitochondrial metabolism consequent to mitochondrial translation inhibition. This provides a rational for doxycycline’s faster inhibition of parasite growth at higher doses. Importantly, these data reveal the first known inhibitor of Plasmodium mitochondrial protein synthesis, and support mitochondrial translation as a potential target for future drugs. The discovery of an additional target for a widely-used malaria prophylactic may also inform future drug combinations for malaria treatment or prevention.