Fueling Open Innovation for Malaria Transmission-Blocking Drugs: Hundreds of Molecules Targeting Early Parasite Mosquito Stages

Abstract

BackgroundDespite recent successes at controlling malaria, progress has stalled with an estimated 219 million cases and 435,000 deaths in 2017 alone. Combined with emerging resistance to front line antimalarial therapies in Southeast Asia, there is an urgent need for new treatment options and novel approaches to halt the spread of malaria. Plasmodium, the parasite responsible for malaria propagates through mosquito transmission. This imposes an acute bottleneck on the parasite population and transmission-blocking interventions exploiting this vulnerability are recognized as vital for malaria elimination.Methods13,533 small molecules with known activity against Plasmodium falciparum asexual parasites were screened for additional transmission-blocking activity in an ex vivo Plasmodium berghei ookinete development assay. Active molecules were then counterscreened in dose response against HepG2 cells to determine their activity/cytotoxicity window and selected non-toxic representative molecules were fully profiled in a range of transmission and mosquito infection assays. Furthermore, the entire dataset was compared to other published screens of the same molecules against P. falciparum gametocytes and female gametogenesis.Results437 molecules inhibited P. berghei ookinete formation with an IC50 < 10 μM. of which 273 showed >10-fold parasite selectivity compared to activity against HepG2 cells. Active molecules grouped into 49 chemical clusters of three or more molecules, with 25 doublets and 94 singletons. Six molecules representing six major chemical scaffolds confirmed their transmission-blocking activity against P. falciparum male and female gametocytes and inhibited P. berghei oocyst formation in the standard membrane feeding assay at 1 μM. When screening data in the P. berghei development ookinete assay was compared to published screens of the same library in assays against P. falciparum gametocytes and female gametogenesis, it was established that each assay identified distinct, but partially overlapping subsets of transmission-blocking molecules. However, selected molecules unique to each assay show transmission-blocking activity in mosquito transmission assays.ConclusionThe P. berghei ookinete development assay is an excellent high throughput assay for efficiently identifying antimalarial molecules targeting early mosquito stage parasite development. Currently no high throughput transmission-blocking assay is capable of identifying all transmission-blocking molecules.

Multistage and transmission-blocking targeted antimalarials discovered from the open-source MMV Pandemic Response Box

“…Importantly, our parallel screening approach on different life cycle stages yielded compounds and chemical scaffolds that not only have stage-specific asexual parasite activity but also selectively and specifically target the elusive gametocyte stages with activity in mosquito transmission assays. This unbiased approach, in place of the paradigm where compounds are only profiled for additional life cycle activity once asexual activity has been established, confirms the possibility of identifying gametocyte-specific compounds 7 , 8 , 10 . Indeed, we identified several active compounds that have no former documented antimalarial activity, simply because they were previously not screened against the correct life cycle stage of the parasite, where the relevant biology being targeted was essential.…”

“…in place of the paradigm where compounds are only profiled for additional life cycle activity once asexual activity has been established, confirms the possibility of identifying gametocytespecific compounds 7,8,10 . Indeed, we identified several active compounds that have no former documented antimalarial activity, simply because they were previously not screened against the correct life cycle stage of the parasite, where the relevant biology being targeted was essential.…”

“…Parallel screening against multiple life cycle stages would best identify such compounds and rely on selective and predictive assays for gametocytocidal activity 4 , transmissionblocking 5,6 , and hepatic development 7 . Recently, parallel screening of diversity sets has resulted in reports of such stage-specific compounds [7][8][9][10] , with the benefit that divergent biology associated with the different life cycle states can be targeted 5,6,11 .…”

A Hetero-Multimeric Chitinase-Containing Plasmodium falciparum and Plasmodium gallinaceum Ookinete-Secreted Protein Complex Involved in Mosquito Midgut Invasion

“…Plasmodium parasites cause human malaria, one of the deadliest vector-borne diseases, which has medical and economic impacts affecting nearly half of the world's populations (Delves et al, 2019). Human-infecting Plasmodium parasites complete a complex life cycle that alternates between definitive female Anopheline mosquito and the intermediate human hosts.…”

Natural Products: A Potential Source of Malaria Transmission Blocking Drugs?

“…Some natural compounds exhibit a TCP-5 activity profile while others have dual activity with additional potency against asexual parasites (defined with both TCP-1 and TCP-5 activity). The low hit rates of the synthetic compounds against sexual stage Plasmodium parasites [40][41][42][43], motivates expansion of the search for transmission-blocking drugs to natural products. This is justified particularly since their diverse chemical space and wide range of pharmacophores could lead to identification of novel lead compounds and associated targets in the parasite and as such avert existing drug resistance challenges.…”