ABSTRACT
Most malaria drug development focuses on parasite stages detected in red blood cells, even though, to achieve eradication, next-generation drugs active against both erythrocytic and exo-erythrocytic forms would be preferable. We applied a multifactorial approach to a set of >4000 commercially available compounds with previously demonstrated blood-stage activity (median inhibitory concentration < 1 micromolar) and identified chemical scaffolds with potent activity against both forms. From this screen, we identified an imidazolopiperazine scaffold series that was highly enriched among compounds active against Plasmodium liver stages. The orally bioavailable lead imidazolopiperazine confers complete causal prophylactic protection (15 milligrams/kilogram) in rodent models of malaria and shows potent in vivo blood-stage therapeutic activity. The open-source chemical tools resulting from our effort provide starting points for future drug discovery programs, as well as opportunities for researchers to investigate the biology of exo-erythrocytic forms.
Subject(s)
Antimalarials/pharmacology , Drug Discovery , Imidazoles/pharmacology , Liver/parasitology , Malaria/drug therapy , Piperazines/pharmacology , Plasmodium/drug effects , Animals , Antimalarials/chemistry , Antimalarials/pharmacokinetics , Antimalarials/therapeutic use , Cell Line, Tumor , Drug Evaluation, Preclinical , Drug Resistance , Erythrocytes/parasitology , Humans , Imidazoles/chemistry , Imidazoles/pharmacokinetics , Imidazoles/therapeutic use , Malaria/parasitology , Malaria/prevention & control , Mice , Mice, Inbred BALB C , Molecular Structure , Piperazines/chemistry , Piperazines/pharmacokinetics , Piperazines/therapeutic use , Plasmodium/cytology , Plasmodium/growth & development , Plasmodium/physiology , Plasmodium berghei/cytology , Plasmodium berghei/drug effects , Plasmodium berghei/growth & development , Plasmodium berghei/physiology , Plasmodium falciparum/cytology , Plasmodium falciparum/drug effects , Plasmodium falciparum/growth & development , Plasmodium falciparum/physiology , Plasmodium yoelii/cytology , Plasmodium yoelii/drug effects , Plasmodium yoelii/growth & development , Plasmodium yoelii/physiology , Polymorphism, Single Nucleotide , Protozoan Proteins/chemistry , Protozoan Proteins/genetics , Protozoan Proteins/metabolism , Random Allocation , Small Molecule Libraries , Sporozoites/drug effects , Sporozoites/growth & developmentABSTRACT
BACKGROUND: Most medicinal plants contain a mixture of bioactive compounds, including chemicals that interact with intracellular targets and others that can act as adjuvants to facilitate absorption of polar agents across cellular membranes. However, little is known about synergistic effects between such potential drug candidates and adjuvants. To probe for such effects, we tested the green tea compound epigallocatechin gallate (EGCG) and the membrane permeabilising digitonin on Plasmodium sporozoite motility and viability. METHODOLOGY/PRINCIPAL FINDINGS: Green fluorescent P. berghei sporozoites were imaged using a recently developed visual screening methodology. Motility and viability parameters were automatically analyzed and IC50 values were calculated, and the synergism of drug and adjuvant was assessed by the fractional inhibitory concentration index. Validating our visual screening procedure, we showed that sporozoite motility and liver cell infection is inhibited by EGCG at nontoxic concentrations. Digitonin synergistically increases the cytotoxicity of EGCG on sporozoite survival, but shows an additive effect on sporozoite motility. CONCLUSIONS/SIGNIFICANCE: We proved the feasibility of performing highly reliable visual screens for compounds against Plasmodium sporozoites. We thereby could show an advantage of administering mixtures of plant metabolites on inhibition of cell motility and survival. Although the effective concentration of both drugs is too high for use in malaria prophylaxis, the demonstration of a synergistic effect between two plant compounds could lead to new avenues in drug discovery.