Mouse studies on inhibitors of Plasmodium falciparum Hsp90: progress and challenges

Synergistic Action between PfHsp90 Inhibitor and PfRad51 Inhibitor Induces Elevated DNA Damage Sensitivity in the Malaria Parasite

The DNA recombinase Rad51 from the human malaria parasite Plasmodium falciparum has emerged as a potential drug target due to its central role in the homologous recombination (HR)-mediated double-strand break (DSB) repair pathway. Inhibition of the ATPase and strand exchange activity of P. falciparum Rad51 (PfRad51) by a small-molecule inhibitor, B02 [3-(phenylmethyl)-2-[(1E)-2-(3-pyridinyl)ethenyl]-4(3H)-quinazolinone], renders the parasite more sensitive to genotoxic agents. Here, we investigated whether the inhibition of the molecular chaperone PfHsp90 potentiates the antimalarial action of B02. We found that the PfHsp90 inhibitor 17-AAG [17-(allylamino)-17-demethoxygeldanamycin] exhibits strong synergism with B02 in both drug-sensitive (strain 3D7) and multidrug-resistant (strain Dd2) P. falciparum parasites. 17-AAG causes a greater than 200-fold decrease in the half-maximal inhibitory concentration (IC50) of B02 in 3D7 parasites. Our results provide mechanistic insights into such profound synergism between 17-AAG and B02. We report that PfHsp90 physically interacts with PfRad51 and promotes the UV irradiation-induced DNA repair activity of PfRad51 by controlling its stability. We find that 17-AAG reduces PfRad51 protein levels by accelerating proteasomal degradation. Consequently, PfHsp90 inhibition renders the parasites more susceptible to the potent DNA-damaging agent methyl methanesulfonate (MMS) in a dose-dependent manner. Thus, our study provides a rationale for targeting PfHsp90 along with the recombinase PfRad51 for controlling malaria propagation.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

Structure-activity relationship of new antimalarial 1-aryl-3-susbtituted propanol derivatives: Synthesis, preliminary toxicity profiling, parasite life cycle stage studies, target exploration, and targeted delivery

Design, synthesis, structure-activity relationship, cytotoxicity studies, in silico drug-likeness, genotoxicity screening, and in vivo studies of new 1-aryl-3-substituted propanol derivatives led to the identification of nine compounds with promising in vitro (55, 56, 61, 64, 66, and 70-73) and in vivo (66 and 72) antimalarial profiles against Plasmodium falciparum and Plasmodium berghei. Compounds 55, 56, 61, 64, 66 and 70-73 exhibited potent antiplasmodial activity against chloroquine-resistant strain FCR-3 (IC₅₀s < 0.28 μM), and compounds 55, 56, 64, 70, 71, and 72 showed potent biological activity in chloroquine-sensitive and multidrug-resistant strains (IC₅₀s < 0.7 μM for 3D7, D6, FCR-3 and C235). All of these compounds share appropriate drug-likeness profiles and adequate selectivity indexes (77 < SI < 184) as well as lack genotoxicity. In vivo efficacy tests in a mouse model showed compounds 66 and 72 to be promising candidates as they exhibited significant parasitemia reductions of 96.4% and 80.4%, respectively. Additional studies such as liver stage and sporogony inhibition, target exploration of heat shock protein 90 of P. falciparum, targeted delivery by immunoliposomes, and enantiomer characterization were performed and strongly reinforce the hypothesis of 1-aryl-3-substituted propanol derivatives as promising antimalarial compounds.

Identification and characterization of the antiplasmodial activity of Hsp90 inhibitors

Background

The recent reduction in mortality due to malaria is being threatened by the appearance of Plasmodium falciparum parasites that are resistant to artemisinin in Southeast Asia. To limit the impact of resistant parasites and their spread across the world, there is a need to validate anti-malarial drug targets and identify new leads that will serve as foundations for future drug development programmes targeting malaria. Towards that end, the antiplasmodial potential of several Hsp90 inhibitors was characterized. Because, the Hsp90 chaperone has been suggested as a good drug target against multiple parasitic infections including malaria.

Results

Chemically diverse sets of Hsp90 inhibitors, evaluated in clinical trials as anti-cancer agents, were tested against the malaria parasite. Most of the compounds showed strong antiplasmodial activity in growth inhibition assays against chloroquine sensitive and resistant strains. There was a good agreement between the compound in vitro anti-parasitic activity and their affinity against the Plasmodium chaperone. The two most potent Hsp90 inhibitors also showed cytocidal activity against two P. falciparum strains. Their antiplasmodial activity affected all parasite forms during the malaria blood cycle. However, the compounds activity against the parasite showed no synergy when combined with anti-malarial drugs, like chloroquine or DHA.

Discussion

The Hsp90 inhibitors anti-parasitic activity correlates with their affinity to their predicted target the P. falciparum chaperone Hsp90. However, the most effective compounds also showed high affinity for a close homologue, Grp94. This association points to a mode of action for Hsp90 inhibitors that correlate compound efficacy with multi-target engagement. Besides their ability to limit parasite replication, two compounds also significantly impacted P. falciparum viability in vitro. Finally, a structural analysis suggests that the best hit represents a promising scaffold to develop parasite specific leads according.

Conclusion

The results shown that Hsp90 inhibitors are lethal against the malaria parasite. The correlation between biochemical and in vitro data strongly supports Hsp90 as a drug target against the malaria parasite. Furthermore, at least one Hsp90 inhibitor developed as anticancer therapeutics could serve as starting point to generate P. falciparum-specific lead compounds.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1940-7) contains supplementary material, which is available to authorized users.

In vitro and in vivo anti-malarial activity of novel harmine-analog heat shock protein 90 inhibitors: a possible partner for artemisinin

Background

The emergence of artemisinin-resistant Plasmodium falciparum strains poses a serious challenge to the control of malaria. This necessitates the development of new anti-malarial drugs. Previous studies have shown that the natural beta-carboline alkaloid harmine is a promising anti-malarial agent targeting the P. falciparum heat-shock protein 90 (PfHsp90). The aim of this study was to test the anti-malarial activity of harmine analogues.

Methods

Forty-two harmine analogues were synthesized and the binding of these analogues to P. falciparum heat shock protein 90 was investigated. The in vitro anti-malarial activity of two of the analogues, 17A and 21A, was evaluated using a 72-h growth inhibition assay. The in vivo anti-malarial activity was tested in Plasmodium berghei infection of BALB/c mice. The potential of 21A for a combination treatment with artemisinin was evaluated using in vivo combination study with dihydro-artemisinin in BALB/c mice. Cytotoxicity of the harmine analogues was tested in vitro using HepG2 and HeLa cell lines.

Results

17A and 21A bound to PfHsp90 with average IC₅₀ values of 12.2 ± 2.3 and 23.1 ± 8.8 µM, respectively. They also inhibited the P. falciparum W2 strain with average IC₅₀ values of 4.2 ± 1.3 and 5.7 ± 1.7 µM, respectively. In vivo, three daily injections of P. berghei-infected BALB/c mice with 100 mg/kg of either 17A or 21A showed significant reduction in parasitaemia with a 51.5 and 56.1% reduction, respectively. Mice treated with 17A and 21A showed a median survival time of 11 and 14 days, respectively, while the vehicle control mice survived a median of only 8.5 days. A dose-ranging experiment with 21A showed that the compound has a dose-dependent anti-malarial effect. Furthermore, treatment of infected mice with a combination of 21A and dihydroartemisinin (DHA) showed a dramatic reduction in parasitaemia compared to treatment with DHA alone.

Conclusion

A novel and non-toxic harmine analogue has been synthesized which binds to PfHsp90 protein, inhibits P. falciparum in vitro at micromolar concentration, reduces parasitaemia and prolongs survival of P. berghei-infected mice with an additive anti-malarial effect when combined with DHA.

Macrolactam analogues of macrolide natural products

The chemical modification of macrolide natural products into aza- or lactam analogues is a strategy employed to improve their metabolic stability and biological activity.

Potential Interaction of Plasmodium falciparum Hsp60 and Calpain

After invasion of red blood cells, malaria matures within the cell by degrading hemoglobin avidly. For enormous protein breakdown in trophozoite stage, many efficient and ordered proteolysis networks have been postulated and exploited. In this study, a potential interaction of a 60-kDa Plasmodium falciparum (Pf)-heat shock protein (Hsp60) and Pf-calpain, a cysteine protease, was explored. Pf-infected RBC was isolated and the endogenous Pf-Hsp60 and Pf-calpain were determined by western blot analysis and similar antigenicity of GroEL and Pf-Hsp60 was determined with anti-Pf-Hsp60. Potential interaction of Pf-calpain and Pf-Hsp60 was determined by immunoprecipitation and immunofluorescence assay. Mizoribine, a well-known inhibitor of Hsp60, attenuated both Pf-calpain enzyme activity as well as P. falciparum growth. The presented data suggest that the Pf-Hsp60 may function on Pf-calpain in a part of networks during malaria growth.