A New Set of Chemical Starting Points with Plasmodium falciparum Transmission-Blocking Potential for Antimalarial Drug Discovery

Abstract

The discovery of new antimalarials with transmission blocking activity remains a key issue in efforts to control malaria and eventually eradicate the disease. Recently, high-throughput screening (HTS) assays have been successfully applied to Plasmodium falciparum asexual stages to screen millions of compounds, with the identification of thousands of new active molecules, some of which are already in clinical phases. The same approach has now been applied to identify compounds that are active against P. falciparum gametocytes, the parasite stage responsible for transmission. This study reports screening results for the Tres Cantos Antimalarial Set (TCAMS), of approximately 13,533 molecules, against P. falciparum stage V gametocytes. Secondary confirmation and cytotoxicity assays led to the identification of 98 selective molecules with dual activity against gametocytes and asexual stages. Hit compounds were chemically clustered and analyzed for appropriate physicochemical properties. The TCAMS chemical space around the prioritized hits was also studied. A selection of hit compounds was assessed ex vivo in the standard membrane feeding assay and demonstrated complete block in transmission. As a result of this effort, new chemical structures not connected to previously described antimalarials have been identified. This new set of compounds may serve as starting points for future drug discovery programs as well as tool compounds for identifying new modes of action involved in malaria transmission.

Fig 1. Description of the critical path followed to generate and prioritize hits from TCAMS.

Fig 2. Relative distribution of TCAMS primary hits based on P. falciparum IC₅₀ in gametocytes.

Fig 3. Biological profile of the 56 prioritized compounds.

Bars and color intensity represent compound potency against P. falciparum gametocytes asexual stages, and cytotoxicity.

Fig 4. Three-dimensional plot displaying the structures, properties and parasitological activity of some of the 56 prioritized compounds and their analogs.

Selected compounds are represented as spheres, while the corresponding analogs (displayed at the same clogP and molecular weight as their proposed compound, but with their own inhibition value) are represented as cubes.

Fig 5a. Chemical analysis of the 56 prioritized hits by CSA and MCSI methods.

"TCAMS ID" represents the reference of the hit compound at TCAMS. "CSA Cluster" refers to the chemical scaffold obtained using the similarity-based computational search of analogs approach. "N Cl CSA" is the number of cluster using CSA approach. "MCS Cluster" depicts the chemical scaffold obtained using the similarity-based medicinal chemistry search of analogs approach and "N Cl MCS" indicates the number of cluster using MCS approach.

Fig 5b. Chemical analysis of the 56 prioritized hits by CSA and MCSI methods.

"TCAMS ID" represents the reference of the hit compound at TCAMS. "CSA Cluster" refers to the chemical scaffold obtained using the similarity-based computational search of analogs approach. "N Cl CSA" is the number of cluster using CSA approach. "MCS Cluster" depicts the chemical scaffold obtained using the similarity-based medicinal chemistry search of analogs approach and "N Cl MCS" indicates the number of cluster using MCS approach.

Fig 6. Effect of selected TCAMS on P. falciparum oocyst intensity after treatment of mature gametocytes for 48 h.

In the figure, each red dot represents the total number of oocysts in a single mosquito midgut, and the green line depicts the mean oocyst intensity of infection. The table below the graph depicts the total number of full-fed mosquitoes dissected per treatment, the percentage infection prevalence, transmission blocking potential, mean oocyst intensity reduction, and the percentage inhibition of exflagellation at 48 h post-treatment. The percentage transmission blocking potential, mean oocyst intensity reduction and exflagellation inhibition were calculated by normalizing with values from the control group. Compounds showed a statistically significant decrease in oocyst distribution when compared to the control (p<0.0001). The data above are representative from one of the two independent experiments.

Fig 7. Summary of biological data for the 6 compounds evaluated in the SMFA.