Intrinsic fluorescence properties of antimalarial pyrido[1,2-a]benzimidazoles facilitate subcellular accumulation and mechanistic studies in the human malaria parasite Plasmodium falciparum

Promising antimalarial hits from phenotypic screens: a review of recently-described multi-stage actives and their modes of action

Over the last two decades, global malaria cases caused by Plasmodium falciparum have declined due to the implementation of effective treatments and the use of insecticides. However, the COVID-19 pandemic caused major disruption in the timely delivery of medical goods and diverted public health resources, impairing malaria control. The emergence of resistance to all existing frontline antimalarials underpins an urgent need for new antimalarials with novel mechanisms of action. Furthermore, the need to reduce malaria transmission and/or prevent malaria infection has shifted the focus of antimalarial research towards the discovery of compounds that act beyond the symptomatic blood stage and also impact other parasite life cycle stages. Phenotypic screening has been responsible for the majority of new antimalarial lead compounds discovered over the past 10 years. This review describes recently reported novel antimalarial hits that target multiple parasite stages and were discovered by phenotypic screening during the COVID-19 pandemic. Their modes of action and targets in blood stage parasites are also discussed.

Antimalarial Imidazopyridines Incorporating an Intramolecular Hydrogen Bonding Motif: Medicinal Chemistry and Mechanistic Studies

We previously identified a novel class of antimalarial benzimidazoles incorporating an intramolecular hydrogen bonding motif. The frontrunner of the series, analogue A, showed nanomolar activity against the chloroquine-sensitive NF54 and multi-drug-resistant K1 strains of Plasmodium falciparum (PfNF54 IC₅₀ = 0.079 μM; PfK1 IC₅₀ = 0.335 μM). Here, we describe a cell-based medicinal chemistry structure-activity relationship study using compound A as a basis. This effort led to the identification of novel antimalarial imidazopyridines with activities of <1 μM, favorable cytotoxicity profiles, and good physicochemical properties. Analogue 14 ( PfNF54 IC₅₀ = 0.08 μM; PfK1 IC₅₀ = 0.10 μM) was identified as the frontrunner of the series. Preliminary mode of action studies employing molecular docking, live-cell confocal microscopy, and a cellular heme fractionation assay revealed that 14 does not directly inhibit the conversion of heme to hemozoin, although it could be involved in other processes in the parasite's digestive vacuole.

Antimalarial Pyrido[1,2-a]benzimidazoles Exert Strong Parasiticidal Effects by Achieving High Cellular Uptake and Suppressing Heme Detoxification

Pyrido[1,2-a]benzimidazoles (PBIs) are synthetic antiplasmodium agents with potent activity and are structurally differentiated from benchmark antimalarials. To study the cellular uptake of PBIs and understand the underlying phenotype of their antiplasmodium activity, their antiparasitic activities were examined in chloroquine (CQ)-susceptible and CQ-resistant Plasmodium falciparum in vitro. Moreover, drug uptake and heme detoxification suppression were examined in Plasmodium berghei-infected mice. The in vitro potency of PBIs is comparable to most 4-aminoquinolines. They have a speed of action in vitro that is superior to that of atovaquone and an ability to kill rings and trophozoites. The antiparasitic effects observed for the PBIs in cell culture and in infected mice are similar in terms of potency and efficacy and are comparable to CQ but with the added advantage of demonstrating equipotency against both CQ susceptible and resistant parasite strains. PBIs have a high rate of uptake by parasite cells and, conversely, a limited rate of uptake by host cells. The mechanism of cellular uptake of the PBIs differs from the ion-trap mechanism typically observed for 4-aminoquinolines, although they share key structural features. The high cellular uptake, attractive parasiticidal profile, and susceptibility of resistant strains to PBIs are desirable characteristics for new antimalarial agents.

Simple inorganic base promoted polycyclic construction using mucohalic acid as a C3 synthon: synthesis and AIE probe application of benzo[4,5]imidazo[1,2-a]pyridines

Using mucohalic acid as C3 synthon via a transition metal-free multicomponent reaction, an eco-friendly protocol to synthesize C1-functionalized benzo[4,5]imidazo[1,2-a]pyridines which can be applied as fluorescence probe for picric acid is described.

Simple inorganic base promoted C-N and C-C formation: synthesis of benzo[4,5]imidazo[1,2-a]pyridines as functional AIEgens used for detecting picric acid

Metal-free catalyzed intermolecular tandem Michael addition/cyclization has been developed for the synthesis of benzo[4,5]imidazo[1,2-a]pyridines from α-bromocinnamaldehyde and 2-substituted benzimidazoles. The reaction promoted by a simple inorganic base displays moderate to good yields and good functional group tolerance. The optical properties of some typical products have been investigated. We found that, due to the presence of the benzene ring at the C1-position of benzo[4,5]imidazo[1,2-a]pyridines which restricts intramolecular motion, as a new type of aggregation-induced emission (AIE) luminogen (AIEgen), they show very good solid-state fluorescence with quantum yields up to 88.80%. Importantly, the AIE performance of compound 3b can be useful to detect the nitroaromatic explosive picric acid (PA) with a detection limit and quenching constant of 42.5 nM and 7.27 × 10⁴ M^-M, respectively.

Benzimidazole Derivatives Are Potent against Multiple Life Cycle Stages of Plasmodium falciparum Malaria Parasites

The continued emergence of resistance to front-line antimalarial treatments is of great concern. Therefore, new compounds that potentially have a novel target in various developmental stages of Plasmodium parasites are needed to treat patients and halt the spread of malaria. Here, several benzimidazole derivatives were screened for activity against the symptom-causing intraerythrocytic asexual blood stages and the transmissible gametocyte stages of P. falciparum. Submicromolar activity was obtained for 54 compounds against asexual blood stage parasites with 6 potent at IC₅₀ < 100 nM while not displaying any marked toxicity against mammalian cells. Nanomolar potency was also observed against gametocytes with two compounds active against early stage gametocytes and two compounds active against late-stage gametocytes. The transmission-blocking potential of the latter was confirmed as they could prevent male gamete exflagellation and the lead compound reduced transmission by 72% in an in vivo mosquito feeding model. These compounds therefore have activity against multiple stages of Plasmodium parasites with potential for differential targets.

Chemical Biology Tools To Investigate Malaria Parasites

Parasitic diseases like malaria tropica have been shaping human evolution and history since the beginning of mankind. After infection, the response of the human host ranges from asymptomatic to severe and may culminate in death. Therefore, proper examination of the parasite's biology is pivotal to deciphering unique molecular, biochemical and cell biological processes, which in turn ensure the identification of treatment strategies, such as potent drug targets and vaccine candidates. However, implementing molecular biology methods for genetic manipulation proves to be difficult for many parasite model organisms. The development of fast and straightforward applicable alternatives, for instance small-molecule probes from the field of chemical biology, is essential. In this review, we will recapitulate the highlights of previous molecular and chemical biology approaches that have already created insight and understanding of the malaria parasite Plasmodium falciparum. We discuss current developments from the field of chemical biology and explore how their application could advance research into this parasite in the future. We anticipate that the described approaches will help to close knowledge gaps in the biology of P. falciparum and we hope that researchers will be inspired to use these methods to gain knowledge - with the aim of ending this devastating disease.