Conference Agenda

The treatment of malaria, a deadly infectious disease caused by Plasmodium parasites, relies on an arsenal of chemotherapies that are hampered by drug resistance. New antimalarial drugs with mechanisms of action that are different to currently used drugs are needed to combat Plasmodium drug resistance. Robenidine is a known antiparasitic drug with modest activity against P. falciparum (50% growth inhibitory concentration; IC₅₀ 0.76 µM), the most lethal human malaria parasite species. To aid in the identification of new antiplasmodial drug candidates, a library of robenidine analogues were tested for antiplasmodial activity and selectivity for parasites versus human cells (selectivity index; SI). Multiple compounds with hit and lead activities were identified including NCL146 and NCL123 which demonstrated IC₅₀ values of <100 nM against P. falciparum 3D7 parasites and a SI of 100. Further studies with NCL146 and NCL123 have demonstrated that these compounds are active against multiple multidrug resistant P. falciparum, zoonotic P. knowlesi (IC₅₀ <200 nM) and are well tolerated in vivo. Furthermore, parasites treated with NCL146 display a unique metabolic signature, suggesting a mode of action different to currently used drugs.

There is a need for compounds that can target the transmission-capable gametocytes of P. falciparum. Using a molecular and chemical biology approach we have developed a novel antimalarials with a unique mechanism of action against blood stage Plasmodium. Based on the structure of the human protein PF4, an engineered peptide termed PDIP, has potent activity against trophozoites. PDIP selectively enters parasitised erythrocytes and causes death by lysis of the digestive vacuole. We sought to determine whether PDIP was similarly active against gametocytes and other transmission stages. We found no direct gametocytocidal activity, however PDIP-fluorochrome conjugates rapidly enter gametocyte infected erythrocytes, and persist within for several days, highlighting their specificity. Novel peptide drug conjugates (PDCs) of PDIP covalently linked to known antimalarials, which use the PDIP scaffold to selectively deliver the compound to the parasite, have direct killing and inhibit further activation during gametogenesis. We are working with collaborators to determine the effects of PDIP in the full mosquito transmission infection model, and against sporozoite viability. This project reinforces the selectivity of membrane active proteins, and supports PDIP as a novel antiplasmodial compound. We also validate the PDC approach as a strategy for selective delivery of potent antimalarials.

The human malaria parasite Plasmodium falciparum scavenges key nutrients for its survival. Among these is riboflavin, the metabolites of which are used in crucial intracellular processes such as redox homeostasis. Flavin adenine dinucleotide synthetase (FADS) is an integral part of riboflavin metabolism, and consequently, a potential antimalarial target. The riboflavin analogue roseoflavin (RoF) binds competitively to FADS, disrupting riboflavin metabolism and generating toxic metabolites. Previous work identified an additional riboflavin analogue with antiplasmodial activity, namely 8-aminoriboflavin (8AF). We will employ a two-fold approach to investigating riboflavin metabolism; the characterisation of FADS and the testing of riboflavin analogue activity against transgenic parasites.

We will attempt to generate a conditional PfFADS-knockout to ascertain the enzyme’s necessity in parasite proliferation. Additionally, the apicomplexan parasite Leishmania tarentolae will be used to express PfFADS. This will enable us to characterise the enzyme in isolation. Literature has shown that the over-expression of an enzyme which metabolises an antiplasmodial compound can increase the sensitivity of the parasite to that compound. We will explore this effect in the context of RoF and 8AF by using transgenic parasites which are over-expressing FADS. This work will provide novel insights into the suitability of riboflavin metabolism as an antimalarial target.