Cleavage of DNA induced by 9-anilinoacridine inhibitors of topoisomerase II in the malaria parasite Plasmodium falciparum

Pyronaridine: a review of its clinical pharmacology in the treatment of malaria

Pyronaridine-artesunate was recently strongly recommended in the 2022 update of the WHO Guidelines for the Treatment of Malaria, becoming the newest artemisinin-based combination therapy (ACT) for both uncomplicated Plasmodium falciparum and Plasmodium vivax malaria. Pyronaridine-artesunate, available as a tablet and paediatric granule formulations, is being adopted in regions where malaria treatment outcome is challenged by increasing chloroquine resistance. Pyronaridine is an old antimalarial agent that has been used for more than 50 years as a blood schizonticide, which exerts its antimalarial activity by interfering with the synthesis of the haemozoin pigment within the Plasmodium digestive vacuole. Pyronaridine exhibits a high blood-to-plasma distribution ratio due to its tendency to accumulate in blood cells. This feature is believed to play a crucial role in its pharmacokinetic (PK) properties and pharmacological activity. The PK characteristics of pyronaridine include rapid oral absorption, large volumes of distribution and low total body clearance, resulting in a long terminal apparent half-life. Moreover, differences in PK profiles have been observed between healthy volunteers and malaria-infected patients, indicating a potential disease-related impact on PK properties. Despite a long history, there is only limited knowledge of the clinical PK and pharmacodynamics of pyronaridine, particularly in special populations such as children and pregnant women. We here provide a comprehensive overview of the clinical pharmacology of pyronaridine in the treatment of malaria.

A new look at 9-substituted acridines with various biological activities

Heterocycles have long been the focus of intensive study in attempts to develop novel therapeutic compounds, and acridine, a polynuclear nitrogen molecule containing a heterocycle, has attracted a considerable amount of scientific attention. Acridine derivatives have been studied in detail and have been found to possess multitarget properties, which inhibit topoisomerase enzymes that regulate topological changes in DNA and interfere with the essential biological function of DNA. This article describes some recent advancements in the field of new 9-substituted acridine heterocyclic agents and describes both the structure and the structure-activity relationship of the most promising molecules. The article will also present the IC₅₀ values of the novel derivatives against various human cancer cell lines. The mini review also investigates the topoisomerase inhibition and antibacterial and antimalarial activity of these polycyclic aromatic derivatives.

Pyronaridine: An update of its pharmacological activities and mechanisms of action

Pyronaridine (PYR) is an erythrocytic schizonticide with a potent antimalarial activity against multidrug-resistant Plasmodium. The drug is used in combination with artesunate for the treatment of uncomplicated P. falciparum malaria, in adults and children. The present review briefly retraces the discovery of PYR and recent antimalarial studies which has led to the approval of PYR/artesunate combination (Pyramax) by the European Medicines Agency to treat uncomplicated malaria worldwide. PYR also presents a marked antitumor activity and has revealed efficacy for the treatment of other parasitic diseases (notably Babesia and Trypanosoma infections) and to mitigate the Ebola virus propagation. On the one hand, PYR functions has an inhibitor of hemozoin (biomineral malaria pigment, by-product of hemoglobin digestion) formation, blocking the biopolymerization of β-hematin and thus facilitating the accumulation of toxic hematin into the digestive vacuole of the parasite. On the other hand, PYR is a bona fide DNA-intercalating agent and an inhibitor of DNA topoisomerase 2, leading to DNA damages and cell death. Inhibition of hematin polymerization represents the prime mechanism at the origin of the antimalarial activity, whereas anticancer effects relies essentially on the interference with DNA metabolism, as with structurally related anticancer drugs like amsacrine and quinacrine. In addition, recent studies point to an immune modulatory activity of PYR and the implication of a mitochondrial oxidative pathway. An analogy with the mechanism of action of artemisinin drugs is underlined. In brief, the biological actions of pyronaridine are recapitulated to shed light on the diverse health benefits of this unsung drug.

Expedient synthesis and anticancer evaluation of dual-action 9-anilinoacridine methyl triazene chimeras

The efficient synthesis of molecular hybrids including a DNA-intercalating 9-anilinoacridine (9-AnA) core and a methyl triazene DNA-methylating moiety is described. Nucleophilic aromatic substitution (S_N Ar) and electrophilic aromatic substitution (EAS) reactions using readily accessible starting materials provide a quick entry to novel bifunctional anticancer molecules. The chimeras were evaluated for their anticancer activity. Chimera 7b presented the highest antitumor activity at low micromolar IC₅₀ values in antiproliferative assays performed with various cancer cell lines. In comparison, compound 7b outperformed DNA-intercalating drugs like amsacrine and AHMA. Mechanistic studies of chimera 7b suggest a dual mechanism of action: methylation of the DNA-repairing protein MGMT associated with the triazene structural portion and Topo II inhibition by intercalation of the acridine core.

Topoisomerase II from Human Malaria Parasites: EXPRESSION, PURIFICATION, AND SELECTIVE INHIBITION

Historically, type II topoisomerases have yielded clinically useful drugs for the treatment of bacterial infections and cancer, but the corresponding enzymes from malaria parasites remain understudied. This is due to the general challenges of producing malaria proteins in functional forms in heterologous expression systems. Here, we express full-length Plasmodium falciparum topoisomerase II (PfTopoII) in a wheat germ cell-free transcription-translation system. Functional activity of soluble PfTopoII from the translation lysates was confirmed through both a plasmid relaxation and a DNA decatenation activity that was dependent on magnesium and ATP. To facilitate future drug discovery, a convenient and sensitive fluorescence assay was established to follow DNA decatenation, and a stable, truncated PfTopoII was engineered for high level enzyme production. PfTopoII was purified using a DNA affinity column. Existing TopoII inhibitors previously developed for other non-malaria indications inhibited PfTopoII, as well as malaria parasites in culture at submicromolar concentrations. Even before optimization, inhibitors of bacterial gyrase, GSK299423, ciprofloxacin, and etoposide exhibited 15-, 57-, and 3-fold selectivity for the malarial enzyme over human TopoII. Finally, it was possible to use the purified PfTopoII to dissect the different modes by which these varying classes of TopoII inhibitors could trap partially processed DNA. The present biochemical advancements will allow high throughput chemical screening of compound libraries and lead optimization to develop new lines of antimalarials.

Comparison of hematin-targeting properties of pynacrine, an acridine analog of the benzonaphthyridine antimalarial pyronaridine

The hematin-targeting properties of pynacrine, an acridine analog of the schizontocidal antimalarial drug, pyronaridine, were evaluated to probe the role of the latter's benzonaphthyridine moiety. Pynacrine was as active as pyronaridine in inhibiting glutathione-induced hematin degradation and in enhancing hematin-mediated membrane lysis. It formed a 1:2 complex with hematin but was 50-fold less effective in inhibiting β-hematin formation. However, pynacrine was as potent as pyronaridine in inhibiting intra-erythrocytic Plasmodium falciparum growth in culture, suggesting that it has other off-target(s) effects.

Gametocytocidal screen identifies novel chemical classes with Plasmodium falciparum transmission blocking activity

Discovery of transmission blocking compounds is an important intervention strategy necessary to eliminate and eradicate malaria. To date only a small number of drugs that inhibit gametocyte development and thereby transmission from the mosquito to the human host exist. This limitation is largely due to a lack of screening assays easily adaptable to high throughput because of multiple incubation steps or the requirement for high gametocytemia. Here we report the discovery of new compounds with gametocytocidal activity using a simple and robust SYBR Green I- based DNA assay. Our assay utilizes the exflagellation step in male gametocytes and a background suppressor, which masks the staining of dead cells to achieve healthy signal to noise ratio by increasing signal of viable parasites and subtracting signal from dead parasites. By determining the contribution of exflagellation to fluorescent signal and using appropriate cutoff values, we were able to screen for gametocytocidal compounds. After assay validation and optimization, we screened an FDA approved drug library of approximately 1500 compounds, as well as the 400 compound MMV malaria box and identified 44 gametocytocidal compounds with sub to low micromolar IC₅₀s. Major classes of compounds with gametocytocidal activity included quaternary ammonium compounds with structural similarity to choline, acridine-like compounds similar to quinacrine and pyronaridine, as well as antidepressant, antineoplastic, and anthelminthic compounds. Top drug candidates showed near complete transmission blocking in membrane feeding assays. This assay is simple, reproducible and demonstrated robust Z-factor values at low gametocytemia levels, making it amenable to HTS for identification of novel and potent gametocytocidal compounds.

Review of pyronaridine anti-malarial properties and product characteristics

Pyronaridine was synthesized in 1970 at the Institute of Chinese Parasitic Disease and has been used in China for over 30 years for the treatment of malaria. Pyronaridine has high potency against Plasmodium falciparum, including chloroquine-resistant strains. Studies in various animal models have shown pyronaridine to be effective against strains resistant to other anti-malarials, including chloroquine. Resistance to pyronaridine appears to emerge slowly and is further retarded when pyronaridine is used in combination with other anti-malarials, in particular, artesunate. Pyronaridine toxicity is generally less than that of chloroquine, though evidence of embryotoxicity in rodents suggests use with caution in pregnancy. Clinical pharmacokinetic data for pyronaridine indicates an elimination T_1/2 of 13.2 and 9.6 days, respectively, in adults and children with acute uncomplicated falciparum and vivax malaria in artemisinin-combination therapy. Clinical data for mono or combined pyronaridine therapy show excellent anti-malarial effects against P. falciparum and studies of combination therapy also show promise against Plasmodium vivax. Pyronaridine has been developed as a fixed dose combination therapy, in a 3:1 ratio, with artesunate for the treatment of acute uncomplicated P. falciparum malaria and blood stage P. vivax malaria with the name of Pyramax® and has received Positive Opinion by European Medicines Agency under the Article 58 procedure.

Global gene expression profiling of Plasmodium falciparum in response to the anti-malarial drug pyronaridine

BACKGROUND

Pyronaridine (PN) and chloroquine (CQ) are structurally related anti-malarial drugs with primarily the same mode of action. However, PN is effective against several multidrug-resistant lines of Plasmodium falciparum, including CQ resistant lines, suggestive of important operational differences between the two drugs.

METHODS

Synchronized trophozoite stage cultures of P. falciparum strain K1 (CQ resistant) were exposed to 50% inhibitory concentrations (IC50) of PN and CQ, and parasites were harvested from culture after 4 and 24 hours exposure. Global transcriptional changes effected by drug treatment were investigated using DNA microarrays.

RESULTS

After a 4 h drug exposure, PN induced a greater degree of transcriptional perturbation (61 differentially expressed features) than CQ (10 features). More genes were found to respond to 24 h treatments with both drugs, and 461 features were found to be significantly responsive to one or both drugs across all treatment conditions. Filtering was employed to remove features unrelated to primary drug action, specifically features representing genes developmentally regulated, secondary stress/death related processes and sexual stage development. The only significant gene ontologies represented among the 46 remaining features after filtering relate to host exported proteins from multi-gene families.

CONCLUSIONS

The malaria parasite's molecular responses to PN and CQ treatment are similar in terms of the genes and pathways affected. However, PN appears to exert a more rapid response than CQ. The faster action of PN may explain why PN is more efficacious than CQ, particularly against CQ resistant isolates. In agreement with several other microarray studies of drug action on the parasite, it is not possible, however, to discern mechanism of drug action from the drug-responsive genes.

Acridine and acridinones: old and new structures with antimalarial activity

Since emergence of chloroquine-resistant Plasmodium falciparum and reports of parasite resistance to alternative drugs, there has been renewed interest in the antimalarial activity of acridines and their congeners, the acridinones. This article presents literature compilation of natural acridinone alkaloids and synthetic 9-substituted acridines, acridinediones, haloalcoxyacridinones and 10-N-substituted acridinones with antimalarial activity. The review also provides an outlook to antimalarial modes of action of some described compounds.

DNA topoisomerases in apicomplexan parasites: promising targets for drug discovery

The phylum Apicomplexa includes a large group of protozoan parasites responsible for a wide range of animal and human diseases. Destructive pathogens, such as Plasmodium falciparum and Plasmodium vivax, causative agents of human malaria, Cryptosporidium parvum, responsible of childhood diarrhoea, and Toxoplasma gondii, responsible for miscarriages and abortions in humans, are frequently associated with HIV immunosuppression in AIDS patients. The lack of effective vaccines, along with years of increasing pressure to eradicate outbreaks with the use of drugs, has favoured the formation of multi-drug resistant strains in endemic areas. Almost all apicomplexan of medical interest contain two endosymbiotic organelles that contain their own mitochondrial and apicoplast DNA. Apicoplast is an attractive target for drug testing because in addition to harbouring singular metabolic pathways absent in the host, it also has its own transcription and translation machinery of bacterial origin. Accordingly, apicomplexan protozoa contain an interesting mixture of enzymes to unwind DNA from eukaryotic and prokaryotic origins. On the one hand, the main mechanism of DNA unwinding includes the scission of one-type I-or both DNA strands-type II eukaryotic topoisomerases, establishing transient covalent bonds with the scissile end. These enzymes are targeted by camptothecin and etoposide, respectively, two natural drugs whose semisynthetic derivatives are currently used in cancer chemotherapy. On the other hand, DNA gyrase is a bacterial-borne type II DNA topoisomerase that operates within the apicoplast and is effectively targeted by bacterial antibiotics like fluoroquinolones and aminocoumarins. The present review is an update on the new findings concerning topoisomerases in apicomplexan parasites and the role of these enzymes as targets for therapeutic agents.

Acridinediones: selective and potent inhibitors of the malaria parasite mitochondrial bc1 complex

The development of drug resistance to affordable drugs has contributed to a global increase in the number of deaths from malaria. This unacceptable situation has stimulated research for new drugs active against multidrug-resistant Plasmodium falciparum parasites. In this regard, we show here that deshydroxy-1-imino derivatives of acridine (i.e., dihydroacridinediones) are selective antimalarial drugs acting as potent (nanomolar K(i)) inhibitors of parasite mitochondrial bc(1) complex. Inhibition of the bc(1) complex led to a collapse of the mitochondrial membrane potential, resulting in cell death (IC(50) approximately 15 nM). The selectivity of one of the dihydroacridinediones against the parasite enzyme was some 5000-fold higher than for the human bc(1) complex, significantly higher ( approximately 200 fold) than that observed with atovaquone, a licensed bc(1)-specific antimalarial drug. Experiments performed with yeast manifesting mutations in the bc(1) complex reveal that binding is directed to the quinol oxidation site (Q(o)) of the bc(1) complex. This is supported by favorable binding energies for in silico docking of dihydroacridinediones to P. falciparum bc(1) Q(o). Dihydroacridinediones represent an entirely new class of bc(1) inhibitors and the potential of these compounds as novel antimalarial drugs is discussed.

In vitro efficiency of new acridyl derivatives against Plasmodium falciparum

A series of new 9-substituted acridyl derivatives were synthesized and their in vitro antimalarial activity was evaluated against one chloroquine-sensitive strain (3D7) and three chloroquine-resistant strains [W2 (Indochina), Bre1 (Brazil) and FCR3 (Gambia)] of Plasmodium falciparum. Some compounds inhibit the growth of malarial parasite with IC50 <or= 0.20 microM.

Targeting of hematin by the antimalarial pyronaridine

Pyronaridine, 2-methoxy-7-chloro-10[3',5'-bis(pyrrolidinyl-1-methyl-)4'hydroxyphenyl]aminobenzyl-(b)-1,5-naphthyridine, a new Mannich base schizontocide originally developed in China and structurally related to the aminoacridine drug quinacrine, is currently undergoing clinical testing. We now show that pyronaridine targets hematin, as demonstrated by its ability to inhibit in vitro beta-hematin formation (at a concentration equal to that of chloroquine), to form a complex with hematin with a stoichiometry of 1:2, to enhance hematin-induced red blood cell lysis (but at 1/100 of the chloroquine concentration), and to inhibit glutathione-dependent degradation of hematin. Our observations that pyronaridine exerted this mechanism of action in situ, based on growth studies of Plasmodium falciparum K1 in culture showing antagonism of pyronaridine in combination with antimalarials (chloroquine, mefloquine, and quinine) that inhibit beta-hematin formation, were equivocal.

Activity of benzothiazoles and chemical derivatives on Plasmodium falciparum

Malaria is a major health concern particularly in Africa which has about 90% of the worldwide annual clinical cases. The increasing number of drug-resistant Plasmodium falciparum justifies the search for new drugs in this field. Antimalarial activity of 2-substituted 6-nitro- and 6-amino-benzothiazoles and their anthranilic acids has been tested. An in vitro study has been performed on W2 and 3D7 strains of P. falciparum and on clinical isolates from malaria-infected patients. Toxicity has been assessed on THP1 human monocytic cells. For the most active drug candidates, the in vitro study was followed by in vivo assays on P. berghei-infected mice and by in vitro assays in order to determine the stage-dependency and the mechanism of action. Of 39 derivatives tested in vitro, 2 had specific antimalarial properties. Each compound was active on all stages of the parasite, but one was markedly active on mature schizonts, while the other was more active on young schizont forms. Both drugs were also active on mitochondrial membrane potential. In vivo data confirmed efficiency with a sustained decrease of parasitaemia. Products A12 and C7 may be considered as potential antimalarial worthy of further chemical and biological research.

Arylsulfonyl acridinyl derivatives acting on Plasmodium falciparum

Several arylacridinyl sulfones have been synthesized and their antimalarial action was tested on Plasmodium falciparum. PABA (para-aminobenzoic acid) has no antagonistic effect with these compounds as opposed to the observed effect with dapsone and sulfonamides previously studied. A possible relationship between the ability of cleavage of the S-9C acridinic bond and activity is suggested.

Antimalarial 9-anilinoacridine compounds directed at hematin

Antimalarial 9-anilinoacridines are potent inhibitors of parasite DNA topoisomerase II both in vitro and in situ. 3,6-diamino substitution on the acridine ring greatly improves parasiticidal activity against Plasmodium falciparum by targeting DNA topoisomerase II. A series of 9-anilinoacridines were investigated for their abilities to inhibit beta-hematin formation, to form drug-hematin complexes, and to enhance hematin-induced lysis of red blood cells. Inhibition of beta-hematin formation was minimal with 3,6-diamino analogs of 9-anilinoacridine and greatest with analogs with a 3,6-diCl substitution together with an electron-donating group in the 1'-anilino position. On the other hand, the presence of a 1'-N(CH3)2 group in the anilino ring produced compounds that strongly inhibited beta-hematin formation but which did not appear to be sensitive to the nature of the substitutions in the acridine nucleus. The derivatives bound hematin, and Job's plots of UV-visible absorbance changes in drug-hematin complexes at various molar ratios indicated a stoichiometric ratio of 1:2. The drugs enhanced hematin-induced red blood cell lysis at low concentrations (<4 microM). These studies open up the novel possibility of development of 9-anilinoacridine antimalarials that target not only DNA topoisomerase II but also beta-hematin formation, which should help delay the rapid onset of resistance to drugs acting at only a single site.

Inhibition of Plasmodium falciparum proliferation in vitro by antisense oligodeoxynucleotides against malarial topoisomerase II

The development of new effective antimalarial agents is urgently needed due to the ineffectiveness of current drug regimes on the most virulent human malaria parasite Plasmodium falciparum. Antisense (AS) oligodeoxynucleotides (ODNs) have shown promise as chemotherapeutic agents. Phosphorothioate AS ODNs against different regions of P. falciparum topoisomerase II gene were investigated. Chloroquine- and pyrimethamine-resistant P. falciparum K1 strain was exposed to phosphorothioate AS ODNs for 48 h and growth was determined by flow cytometric assay or by microscopic assay. Exogenous delivery of phosphorothioate AS ODNs between 0.01 and 0.5 microM significantly inhibited parasite growth compared with sense sequence controls suggesting sequence specific inhibition. This inhibition was shown to occur during maturation stages, with optimal inhibition being detected after 36 h. These results should prove useful in future designs of novel antimalarial agents.

DNA topoisomerases as targets for anticancer drugs

DNA topoisomerases are essential enzymes that regulate the conformational changes in DNA topology by catalysing the concerted breakage and rejoining of DNA strands during normal cellular growth. Over the past few years there has been considerable pharmacological interest in these enzymes because inhibitors of DNA topoisomerases represent a major class of anticancer drugs. This review highlights topoisomerase-targeting drugs that have shown promising anticancer activities. The mechanisms by which those drugs interfere with the catalytic cycles of type I and type II DNA topoisomerases and the factors involved in the development of resistance to these drugs are discussed.