Influence of the Plasmodium falciparum P-glycoprotein homologue 1 (pfmdr1 gene product) on the antimalarial action of cyclosporin

Targeting Artemisinin-Resistant Malaria by Repurposing the Anti-Hepatitis C Virus Drug Alisporivir

The emergence of Plasmodium falciparum resistance raises an urgent need to find new antimalarial drugs. Here, we report the rational repurposing of the anti-hepatitis C virus drug, alisporivir, a nonimmunosuppressive analog of cyclosporin A, against artemisinin-resistant strains of P. falciparum. In silico docking studies and molecular dynamic simulation predicted strong interaction of alisporivir with PfCyclophilin 19B, confirmed through biophysical assays with a K_d value of 354.3 nM. Alisporivir showed potent antimalarial activity against chloroquine-resistant (PfRKL-9 with resistance index [Ri] 2.14 ± 0.23) and artemisinin-resistant (PfKelch13^R539T with Ri 1.15 ± 0.04) parasites. The Ri is defined as the ratio between the IC₅₀ values of the resistant line to that of the sensitive line. To further investigate the mechanism involved, we analyzed the expression level of PfCyclophilin 19B in artemisinin-resistant P. falciparum (PfKelch13^R539T). Semiquantitative real-time transcript, Western blot, and immunofluorescence analyses confirmed the overexpression of PfCyclophilin 19B in PfKelch13^R539T. A 50% inhibitory concentration in the nanomolar range, together with the targeting of PfCyclophilin 19B, suggests that alisporivir can be used in combination with artemisinin. Since artemisinin resistance slows the clearance of ring-stage parasites, we performed a ring survival assay on artemisinin-resistant strain PfKelch13^R539T and found significant decrease in parasite survival with alisporivir. Alisporivir was found to act synergistically with dihydroartemisinin and increase its efficacy. Furthermore, alisporivir exhibited antimalarial activity in vivo. Altogether, with the rational target-based Repurposing of alisporivir against malaria, our results support the hypothesis that targeting resistance mechanisms is a viable approach toward dealing with drug-resistant parasite.

New In Vitro Interaction-Parasite Reduction Ratio Assay for Early Derisk in Clinical Development of Antimalarial Combinations

The development and spread of drug-resistant phenotypes substantially threaten malaria control efforts. Combination therapies have the potential to minimize the risk of resistance development but require intensive preclinical studies to determine optimal combination and dosing regimens. To support the selection of new combinations, we developed a novel in vitro-in silico combination approach to help identify the pharmacodynamic interactions of the two antimalarial drugs in a combination which can be plugged into a pharmacokinetic/pharmacodynamic model built with human monotherapy parasitological data to predict the parasitological endpoints of the combination. This makes it possible to optimally select drug combinations and doses for the clinical development of antimalarials. With this assay, we successfully predicted the endpoints of two phase 2 clinical trials in patients with the artefenomel-piperaquine and artefenomel-ferroquine drug combinations. In addition, the predictive performance of our novel in vitro model was equivalent to that of the humanized mouse model outcome. Last, our more informative in vitro combination assay provided additional insights into the pharmacodynamic drug interactions compared to the in vivo systems, e.g., a concentration-dependent change in the maximum killing effect (E_max) and the concentration producing 50% of the killing maximum effect (EC₅₀) of piperaquine or artefenomel or a directional reduction of the EC₅₀ of ferroquine by artefenomel and a directional reduction of E_max of ferroquine by artefenomel. Overall, this novel in vitro-in silico-based technology will significantly improve and streamline the economic development of new drug combinations for malaria and potentially also in other therapeutic areas.

The antimalarial action of FK506 and rapamycin: evidence for a direct effect on FK506-binding protein PfFKBP35

FK506 and rapamycin (Rap) are immunosuppressive drugs that act principally on T-lymphocytes. The receptors for both drugs are FK506-binding proteins (FKBPs), but the molecular mechanisms of immunosuppression differ. An FK506-FKBP complex inhibits the protein phosphatase calcineurin, blocking a key step in T-cell activation, while the Rap -FKBP complex binds to the protein kinase target of rapamycin (TOR), which is involved in a subsequent signalling pathway. Both drugs, and certain non-immunosuppressive compounds related to FK506, have potent antimalarial activity. There is however conflicting evidence on the involvement of Plasmodium calcineurin in the action of FK506, and the parasite lacks an apparent TOR homologue. We therefore set out to establish whether inhibition of the Plasmodium falciparum FKBP PfFKBP35 itself might be responsible for the antimalarial effects of FK506 and Rap. Similarities in the antiparasitic actions of FK506 and Rap would constitute indirect evidence for this hypothesis. FK506 and Rap acted indistinguishably on: (i) specificity for different intra-erythrocytic stages in culture, (ii) kinetics of killing or irreversible growth arrest of parasites and (iii) interactions with other antimalarial agents. Furthermore, PfFKBP35's inhibitory effect on calcineurin was independent of FK506 under a range of conditions, suggesting that calcineurin is unlikely to be involved in the antimalarial action of FK506.

Antimicrobial peptides: a new class of antimalarial drugs?

A range of antimicrobial peptides (AMP) exhibit activity on malaria parasites, Plasmodium spp., in their blood or mosquito stages, or both. These peptides include a diverse array of both natural and synthetic molecules varying greatly in size, charge, hydrophobicity, and secondary structure features. Along with an overview of relevant literature reports regarding AMP that display antiplasmodial activity, this review makes a few considerations about those molecules as a potential new class of antimalarial drugs.

Mefloquine exposure induces cell cycle delay and reveals stage-specific expression of the pfmdr1 gene

Drug-resistant Plasmodium falciparum malaria is a major public health problem. An elevated pfmdr1 gene copy number (CN) is known to decrease parasite sensitivity to the commonly used antimalarial mefloquine (MFQ). To understand the relationship between pfmdr1 CN and mefloquine resistance, we evaluated pfmdr1 transcript levels in three P. falciparum strains with different CNs in the presence and absence of MFQ. Parasite strains with multiple pfmdr1 gene copies exhibited higher relative transcript levels than single-copy parasites, and MFQ induced pfmdr1 expression above the levels without treatment in all three strains evaluated. Concomitant morphology analyses of the sampled cultures revealed that MFQ treatment of synchronized ring-stage parasites induced a delay in parasite maturation through the intraerythrocytic cycle. pfmdr1 expression peaks in the ring stage, and MFQ could be causing increased transcription by delaying parasite maturation. However, pretreatment with mefloquine did not affect the artemisinin in vitro half-maximal inhibitory concentration (IC(50)). These results suggest that MFQ-induced increases in pfmdr1 expression are the direct result of the maturation delay at the ring stage but that this change in expression does not affect the antimalarial activity of artemisinin.

A family of cyclophilin-like molecular chaperones in Plasmodium falciparum

The cyclophilins are a large family of proteins implicated in folding, transport and regulation of other proteins and are potential drug targets in cancer and in some viral and parasitic infections. The functionality of cyclophilins appears to depend on peptidyl-prolyl cis-trans isomerase (foldase) and/or molecular chaperone activities. In this study we assessed the peptidyl-prolyl isomerase and chaperone activities of 8 members of the Plasmodium falciparum cyclophilin family, all produced recombinantly using a common host/vector system. While only two of these proteins had isomerase activity, all of them displayed chaperone function as judged by the ability to prevent the thermal aggregation of model substrates. We suggest that the cyclophilins constitute a family of molecular chaperones in malarial parasites that complement the functions of other chaperones such as the heat-shock proteins.

In silico analysis of the cyclophilin repertoire of apicomplexan parasites

Background

Cyclophilins (Cyps) are peptidyl cis/trans isomerases implicated in diverse processes such as protein folding, signal transduction, and RNA processing. They are also candidate drug targets, in particular for the immunosuppressant cyclosporine A. In addition, cyclosporine is known to exhibit anti-parasitic effects on a wide range of organisms including several apicomplexa. In order to obtain new non-immunosuppressive drugs targeting apicomplexan cyclophilins, a profound knowledge of the cyclophilin repertoire of this phylum would be necessary.

Results

BLAST and maximum likelihood analyses identified 16 different cyclophilin subfamilies within the genomes of Cryptosporidium hominis, Toxoplasma gondii, Plasmodium falciparum, Theileria annulata, Theileria parva, and Babesia bovis. In addition to good statistical support from the phylogenetic analysis, these subfamilies are also confirmed by comparison of cyclophilin domain architecture. Within an individual genome, the number of different Cyp genes that could be deduced varies between 7–9 for Cryptosporidia and 14 for T. gondii. Many of the putative apicomplexan cyclophilins are predicted to be nuclear proteins, most of them presumably involved in RNA processing.

Conclusion

The genomes of apicomplexa harbor a cyclophilin repertoire that is at least as complex as that of most fungi. The identification of Cyp subfamilies that are specific for lower eukaryotes, apicomplexa, or even the genus Plasmodium is of particular interest since these subfamilies are not present in host cells and might therefore represent attractive drug targets.

Increased uptake of quinine into the brain by inhibition of P-glycoprotein

The impact of P-glycoprotein (P-gp) on the distribution of quinine between brain and plasma was studied experimentally in mice. Administration of quinine (20mg/kg, i.v.) to mdrla knockout mice resulted in enhanced brain concentrations of quinine as compared to the wild-type mice (7.9+/-1.4 microg/g versus 1.6+/-0.8 microg/g, respectively). Quinine concentrations and quinine-to-3-hydroxyquinine ratio in plasma were similar in normal and P-gp-deficient mice. The effect of intravenously administered drugs before quinine (20mg/kg, i.v.) was evaluated on brain uptake and biotransformation of quinine in mice. Cyclosporine A (50 mg/kg), erythromycine (40 mg/kg), verapamil (5mg/kg) or mefloquine (20 mg/kg) increased the brain-to-plasma quinine concentration ratio (by factors of 3.8-, 1.8-, 1.9- and 2.5-fold, respectively) and the quinine-to-3-hydroxyquinine ratio in plasma (by factors 2.1-, 3.7-, 1.8- and 2.0-fold, respectively). After cinchonine (40 mg/kg) and halofantrine (40 mg/kg) pre-treatment, the brain-to-plasma ratio for quinine increased by factor of 2.3 and 1.8, respectively without changes of quinine or metabolite concentrations in plasma. Doxycycline (20 mg/kg), artesunate (50 mg/kg) or artemether (50 mg/kg) did not alter quinine disposition. These results confirm in vivo that quinine is a substrate for mdr1a P-gp. Drug associations led not only to metabolic interactions but also increased quinine uptake by tissues protected by P-gp. Such interactions may have implications for the improvement of chemotherapy but should be also taken into account for potential enhancement of adverse effects.