How can natural products serve as a viable source of lead compounds for the development of new/novel anti-malarials?

Malaria continues to be an enormous global health challenge, with millions of new infections and deaths reported annually. This is partly due to the development of resistance by the malaria parasite to the majority of established anti-malarial drugs, a situation that continues to hamper attempts at controlling the disease. This has spurred intensive drug discovery endeavours geared towards identifying novel, highly active anti-malarial drugs, and the identification of quality leads from natural sources would greatly augment these efforts. The current reality is that other than compounds that have their foundation in historic natural products, there are no other compounds in drug discovery as part of lead optimization projects and preclinical development or further that have originated from a natural product start-point in recent years. This paper briefly presents both classical as well as some more modern, but underutilized, approaches that have been applied outside the field of malaria, and which could be considered in enhancing the potential of natural products to provide or inspire the development of anti-malarial lead compounds

Stereo-Selectivity of Human Serum Albumin to Enantiomeric and Isoelectronic Pollutants Dissected by Spectroscopy, Calorimetry and Bioinformatics

1–naphthol (1N), 2–naphthol (2N) and 8–quinolinol (8H) are general water pollutants. 1N and 2N are the configurational enantiomers and 8H is isoelectronic to 1N and 2N. These pollutants when ingested are transported in the blood by proteins like human serum albumin (HSA). Binding of these pollutants to HSA has been explored to elucidate the specific selectivity of molecular recognition by this multiligand binding protein. The association constants (Kb) of these pollutants to HSA were moderate (104–105 M−1). The proximity of the ligands to HSA is also revealed by their average binding distance, r, which is estimated to be in the range of 4.39–5.37 nm. The binding free energy (ΔG) in each case remains effectively the same for each site because of enthalpy–entropy compensation (EEC). The difference observed between ΔCpexp and ΔCpcalc are suggested to be caused by binding–induced flexibility changes in the HSA. Efforts are also made to elaborate the differences observed in binding isotherms obtained through multiple approaches of calorimetry, spectroscopy and bioinformatics. We suggest that difference in dissociation constants of pollutants by calorimetry, spectroscopic and computational approaches could correspond to occurrence of different set of populations of pollutants having different molecular characteristics in ground state and excited state. Furthermore, our observation of enhanced binding of pollutants (2N and 8H) in the presence of hemin signifies that ligands like hemin may enhance the storage period of these pollutants in blood that may even facilitate the ill effects of these pollutants

The Plasmodium PI(4)K inhibitor KDU691 selectively inhibits dihydroartemisinin-pretreated Plasmodium falciparum ring-stage parasites

Malaria control and elimination are threatened by the emergence and spread of resistance to artemisinin-based combination therapies (ACTs). Experimental evidence suggests that when an artemisinin (ART)-sensitive (K13 wild-type) Plasmodium falciparum strain is exposed to ART derivatives such as dihydroartemisinin (DHA), a small population of the early ring-stage parasites can survive drug treatment by entering cell cycle arrest or dormancy. After drug removal, these parasites can resume growth. Dormancy has been hypothesized to be an adaptive physiological mechanism that has been linked to recrudescence of parasites after monotherapy with ART and, possibly contributes to ART resistance. Here, we evaluate the in vitro drug sensitivity profile of normally-developing P. falciparum ring stages and DHA-pretreated dormant rings (DP-rings) using a panel of antimalarial drugs, including the Plasmodium phosphatidylinositol-4-OH kinase (PI4K)-specific inhibitor KDU691. We report that while KDU691 shows no activity against rings, it is highly inhibitory against DP-rings; a drug effect opposite to that of ART. Moreover, we provide evidence that KDU691 also kills DP-rings of P. falciparum ART-resistant strains expressing mutant K13

Strict tropism for CD71+/ CD234+ human reticulocytes limits Plasmodium cynomolgi's zoonotic potential

Two malaria parasites of Southeast Asian macaques, Plasmodium knowlesi and P. cynomolgi, can infect humans experimentally. In Malaysia, where both species are common, zoonotic knowlesi malaria has recently become dominant, and cases are recorded throughout the region. By contrast, to date only a single case of naturally acquired P. cynomolgi has been found in humans. In this study we show that whereas P. cynomolgi merozoites invade monkey red blood cells (RBCs) indiscriminately in vitro, for humans they are restricted to reticulocytes expressing both transferrin receptor 1 (Trf1 or CD71) and the Duffy antigen/chemokine receptor (DARC or CD234). This likely contributes to the paucity of detectable zoonotic cynomolgi malaria. We further describe post-invasion morphological and rheological alterations in P. cynomolgi-infected human reticulocytes that are strikingly similar to those observed for P. vivax These observations stress the value of P. cynomolgi as a model in the development of blood stage vaccines against vivax malaria

Strict tropism for CD71+/ CD234+ human reticulocytes limits Plasmodium cynomolgi's zoonotic potential

Two malaria parasites of Southeast Asian macaques, Plasmodium knowlesi and P. cynomolgi, can infect humans experimentally. In Malaysia, where both species are common, zoonotic knowlesi malaria has recently become dominant, and cases are recorded throughout the region. By contrast, to date only a single case of naturally acquired P. cynomolgi has been found in humans. In this study we show that whereas P. cynomolgi merozoites invade monkey red blood cells (RBCs) indiscriminately in vitro, for humans they are restricted to reticulocytes expressing both transferrin receptor 1 (Trf1 or CD71) and the Duffy antigen/chemokine receptor (DARC or CD234). This likely contributes to the paucity of detectable zoonotic cynomolgi malaria. We further describe post-invasion morphological and rheological alterations in P. cynomolgi-infected human reticulocytes that are strikingly similar to those observed for P. vivax These observations stress the value of P. cynomolgi as a model in the development of blood stage vaccines against vivax malaria

Hepatic spheroids used as an in vitro model to study malaria relapse

Hypnozoites are the liver stage non-dividing form of the malaria parasite that are responsible for relapse and acts as a natural reservoir for human malaria Plasmodium vivax and P. ovale as well as a phylogenetically related simian malaria P. cynomolgi. Our understanding of hypnozoite biology remains limited due to the technical challenge of requiring the use of primary hepatocytes and the lack of robust and predictive in vitro models. In this study, we developed a malaria liver stage model using 3D spheroid-cultured primary hepatocytes. The infection of primary hepatocytes in suspension led to increased infectivity of both P. cynomolgi and P. vivax infections. We demonstrated that this hepatic spheroid model was capable of maintaining long term viability, hepatocyte specific functions and cell polarity which enhanced permissiveness and thus, permitting for the complete development of both P. cynomolgi and P. vivax liver stage parasites in the infected spheroids. The model described here was able to capture the full liver stage cycle starting with sporozoites and ending in the release of hepatic merozoites capable of invading simian erythrocytes in vitro. Finally, we showed that this system can be used for compound screening to discriminate between causal prophylactic and cidal antimalarials activity in vitro for relapsing malaria

Robust continuous in vitro culture of the Plasmodium cynomolgi erythrocytic stages

The ability to culture pathogenic organisms substantially enhances the quest for fundamental knowledge and the development of vaccines and drugs. Thus, the elaboration of a protocol for the in vitro cultivation of the erythrocytic stages of Plasmodium falciparum revolutionized research on this important parasite. However, for P. vivax, the most widely distributed and difficult to treat malaria parasite, a strict preference for reticulocytes thwarts efforts to maintain it in vitro. Cultivation of P. cynomolgi, a macaque-infecting species phylogenetically close to P. vivax, was briefly reported in the early 1980s, but not pursued further. Here, we define the conditions under which P. cynomolgi can be adapted to long term in vitro culture to yield parasites that share many of the morphological and phenotypic features of P. vivax. We further validate the potential of this culture system for high-throughput screening to prime and accelerate anti-P. vivax drug discovery efforts