Synthesis and activity of some antimalarial bisquinolines

Design, synthesis, and characterization of novel aminoalcohol quinolines with strong in vitro antimalarial activity

Malaria is the fifth most lethal parasitic infections in the world. Herein, five new series of aminoalcohol quinolines including fifty-two compounds were designed, synthesized and evaluated in vitro against Pf3D7 and PfW2 strains. Among them, fourteen displayed IC₅₀ values below or near of 50.0 nM whatever the strain with selectivity index often superior to 100.17b was found as a promising antimalarial candidate with IC₅₀ values of 14.9 nM and 11.0 nM against respectively Pf3D7 and PfW2 and a selectivity index higher than 770 whatever the cell line is. Further experiments were achieved to confirm the safety and to establish the preliminary ADMET profile of compound 17b before the in vivo study performed on a mouse model of P. berghei ANKA infection. The overall data of this study allowed to establish new structure-activity relationships and the development of novel agents with improved pharmacokinetic properties.

Dimeric Drugs

This review intends to summarize the structures of an extensive number of symmetrical-dimeric drugs, having two monomers linked via a bridging entity while emphasizing the large versatility of biologically active substances reported to possess dimeric structures. The largest number of classes of these compounds consist of anticancer agents, antibiotics/antimicrobials, and anti-AIDS drugs. Other symmetrical-dimeric drugs include antidiabetics, antidepressants, analgesics, anti-inflammatories, drugs for the treatment of Alzheimer's disease, anticholesterolemics, estrogenics, antioxidants, enzyme inhibitors, anti-Parkisonians, laxatives, antiallergy compounds, cannabinoids, etc. Most of the articles reviewed do not compare the activity/potency of the dimers to that of their corresponding monomers. Only in limited cases, various suggestions have been made to justify unexpected higher activity of the dimers vs. the corresponding monomers. These suggestions include statistical effects, the presence of dimeric receptors, binding of a dimer to two receptors simultaneously, and others. It is virtually impossible to predict which dimers will be preferable to their respective monomers, or which linking bridges will lead to the most active compounds. It is expected that the extensive number of articles summarized, and the large variety of substances mentioned, which display various biological activities, should be of interest to many academic and industrial medicinal chemists.

Potent Antimalarial Activity of Two Arenes Linked with Triamine Designed To Have Multiple Interactions with Heme

Based on the idea that compounds designed to exhibit high affinity for heme would block hemozoin formation, a critical heme-detoxification process for malarial parasites, we synthesized a series of compounds with two π-conjugated moieties at terminal amino groups of triamine. These compounds exhibited moderate to high antimalarial activities in vitro toward both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. In a P. berghei-infected mouse model, 3a and 12a showed potent antimalarial activities compared to artesunate, as well as a prolonged duration of antimalarial effect. We found a good correlation between protective activity against hemin degradation and antimalarial activity. Compounds 8b and 3a strongly inhibited hemozoin formation catalyzed by heme detoxification protein.

Crystal structure and Hirshfeld surface analysis of N,N'-bis-(2-nitro-phen-yl)glutaramide

The asymmetric unit of the title compound, C17H16N4O6, contains two independent mol-ecules (A and B). The two benzene rings are twisted by an angle of 79.14 (7)° in mol-ecule A, whereas, in mol-ecule B, they are inclined by 19.02 (14)°. The conformations of the mol-ecules are stabilized by intra-molecular N-H⋯O hydrogen bonds between the amide nitro-gen atom and the O atom of the ortho-nitro substituent on the phenyl ring, enclosing an S(6) ring motif. In the amide and aliphatic segments, all the N-H, C=O and C-H bonds are anti to each other. In the crystal, the A and B mol-ecules are linked by inter-molecular amide-to-amide N-H⋯O hydrogen bonds, resulting in chains running along the b-axis direction. The inter-molecular inter-actions were analysed using Hirshfeld surface analysis. The two-dimensional fingerprint plots of the inter-molecular contacts indicate that the major contributions are from H⋯H and O⋯H inter-actions.

Design, synthesis and in vitro antiplasmodial activity of some bisquinolines against chloroquine-resistant strain

A series of novel bisquinoline compounds comprising N¹ -(7-chloroquinolin-4-yl) ethane-1,2-diamine and 7-chloro-N-(2-(piperazin-1-yl)ethyl)quinolin-4-amine connected with 7-chloro-4-aminoquinoline containing various amino acids is described. We have bio-evaluated the compounds against both chloroquine-sensitive (3D7) and chloroquine-resistant (K1) strains of Plasmodium falciparum in vitro. Among the series, compounds 4 and 7 exhibited 1.8- and 10.6-fold superior activity as compared to chloroquine (CQ; IC₅₀  = 0.255 ± 0.049 μm) against the K1 strain with IC₅₀ values 0.137 ± 0.014 and 0.026 ± 0.007 μm, respectively. Furthermore, compound 7 also displayed promising activity against the 3D7 strain (IC₅₀  = 0.024 ± 0.003 μm) of P. falciparum when compared to CQ. All the compounds in the series displayed resistance factor between 0.57 and 4.71 as against 51 for CQ. These results suggest that bisquinolines can be explored for further development as new antimalarial agents active against chloroquine-resistant P. falciparum.

Identification of β-hematin inhibitors in a high-throughput screening effort reveals scaffolds with in vitro antimalarial activity

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Hemozoin formation is a prime drug target pathway to probe for new lead compounds.

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We examined the VICB library of compounds for in vitro β-hematin inhibition.

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β-Hematin inhibitors were tested for in vitro antimalarial activity in two P. falciparum strains.

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Chemical scaffolds with target-specific and in vitro antimalarial activity were identified.

The emergence of drug resistant strains of Plasmodium spp. creates a critical need for the development of novel antimalarials. Formation of hemozoin, a crystalline heme detoxification process vital to parasite survival serves as an important drug target. The quinoline antimalarials including chloroquine and amodiaquine owe their antimalarial activity to inhibition of hemozoin formation. Though in vivo formation of hemozoin occurs within the presence of neutral lipids, the lipophilic detergent NP-40 was previously shown to serve as a surrogate in the β-hematin (synthetic hemozoin) formation process. Consequently, an NP-40 mediated β-hematin formation assay was developed for use in high-throughput screening. Here, the assay was utilized to screen 144,330 compounds for the identification of inhibitors of crystallization, resulting in 530 hits. To establish the effectiveness of these target-based β-hematin inhibitors against Plasmodiumfalciparum, each hit was further tested in cultures of parasitized red blood cells. This effort revealed that 171 of the β-hematin inhibitors are also active against the parasite. Dose–response data identified 73 of these β-hematin inhibitors have IC₅₀ values ⩽5 μM, including 25 compounds with nanomolar activity against P. falciparum. A scaffold-based analysis of this data identified 14 primary scaffolds that represent 46% of the 530 total hits. Representative compounds from each of the classes were further assessed for hemozoin inhibitory activity in P. falciparum infected human erythrocytes. Each of the hit compounds tested were found to be positive inhibitors, while a negative control did not perturb this biological pathway in culture.

Resonance Raman spectroscopy in malaria research

In recent years, the field of Raman spectroscopy has witnessed a surge in technological development, with the incorporation of ultrasensitive, charge-coupled devices, improved laser sources and precision Rayleigh-filter systems. This has led to the development of sensitive confocal micro-Raman spectrometers and imaging spectrometers that are capable of obtaining high spatial-resolution spectra and images of subcellular components within single living cells. This review reports on the application of resonance micro-Raman spectroscopy to the study of malaria pigment (hemozoin), a by-product of hemoglobin catabolization by the malaria parasite, which is an important target site for antimalarial drugs. The review aims to briefly describe recent studies on the application of this technology, elucidate molecular and electronic properties of the malaria pigment and its synthetic analog beta-hematin, provide insight into the mechanism of hemozoin formation within the food vacuole of the parasite, and comment on developing strategies for using this technology in drug-screening protocols.

Synthesis and in vitro antimalarial activity of a series of bisquinoline and bispyrrolo[1,2a]quinoxaline compounds

Series of bisquinolines 4-15 and bispyrrolo[1,2a]quinoxalines 16-20 containing various polyamine linkers were synthesized. The aqueous solubility and distribution coefficient were experimentally determined. The compounds were screened for antimalarial activity alongside chloroquine against D10 and Dd2 strains of Plasmodium falciparum. The growth inhibitory effects of biscompounds 4-9 were assessed against various cancer cell lines. The aqueous solubility was found to increase with an increase in potential protonation sites. Bisquinolines 8 and 9 featuring triethylenetetramine and N,N'-bis(3-aminopropyl)ethylene-diamine linkers, respectively, were the most active of all synthesized compounds. They were found as potent as chloroquine against D10 but significantly more potent against the Dd2 strain, with good selectivity towards parasitic cells. Compound 4 containing a diethylenetriamine bridge displayed the most important anticancer activity of the series, and was a more effective antiproliferative inhibitor than etoposide against all three TK10, UACC62 and MCF7 cancer cell lines.

Acylated mono-, bis- and tris- cinchona-based amines containing ferrocene or organic residues: synthesis, structure and in vitro antitumor activity on selected human cancer cell lines

A series of novel functionalized mono-, bis- and tris-(S)-{[(2S,4R,8R)-8-ethyl-quinuclidin-2-yl](6-methoxyquinolin-4-yl)}methanamines including ferrocene-containing derivatives was obtained by the reaction of the precursor amine with a variety of acylation agents. Their in vitro antitumor activity was investigated against human leukemia (HL-60), human neuroblastoma (SH-SY5Y), human hepatoma (HepG2) and human breast cancer (MCF-7) cells by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)-assay and the 50% inhibitory concentration (IC₅₀) values were determined. Our data indicate that the precursor amine has no antitumor activity in vitro, but the bis-methanamines with ureido-, thioureido and amide-type linkers display attractive in vitro cytotoxicity and cytostatic effects on HL-60, HepG2, MCF-7 and SH-SY5Y cells. Besides ¹H- and ¹³C-NMR methods the structures of the new model compounds were also studied by DFT calculations.

Synthesis, antiprotozoal, antimicrobial, β-hematin inhibition, cytotoxicity and methemoglobin (MetHb) formation activities of bis(8-aminoquinolines)

In continuing our search of potent antimalarials based on 8-aminoquinoline structural framework, three series of novel bis(8-aminoquinolines) using convenient one to four steps synthetic procedures were synthesized. The bisquinolines were evaluated for in vitro antimalarial (Plasmodiumfalciparum), antileishmanial (Leishmaniadonovani), antimicrobial (a panel of pathogenic bacteria and fungi), cytotoxicity, β-hematin inhibitory and methemoglobin (MetHb) formation activities. Several compounds exhibited superior antimalarial activities compared to parent drug primaquine. Selected compounds (44, 61 and 79) when tested for in vivo blood-schizontocidal antimalarial activity (Plasmodiumberghei) displayed potent blood-schizontocial activities. The bisquinolines showed negligible MetHb formation (0.2-1.2%) underlining their potential in the treatment of glucose-6-phosphate dehydrogenase deficient patients. The bisquinoline analogues (36, 73 and 79) also exhibited promising in vitro antileishmanial activity, and antimicrobial activities (43, 44 and 76) against a panel of pathogenic bacteria and fungi. The results of this study provide evidence that bis(8-aminoquinolines), like their bis(4-aminoquinolines) and artemisinin dimers counterparts, are a promising class of antimalarial agents.

Dual labeling with a far red probe permits analysis of growth and oxidative stress in P. falciparum-infected erythrocytes

The malaria parasite, Plasmodium falciparum, develops within human erythrocytes, consuming host hemoglobin to support its own growth. Reactive oxygen species (superoxide and hydrogen peroxide) are by-products of hemoglobin digestion and are believed to exert significant oxidative stress on the parasite. We have characterized a cell permeant, far red fluorescent nucleic acid-binding dye, SYTO 61, that can be used to distinguish between uninfected and infected erythrocytes in a flow cytometric format. The spectral properties of SYTO 61 make it suitable for use in combination with the fluorescent reactive oxygen species reporter 5-(and-6)-chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate acetyl ester. We have used this probe combination to measure oxidative stress in different stages of live P. falciparum. Low levels of the oxidized, fluorescent form of the reporter (2',7'-dichlorofluorescein, DCF) are detected in ring stage parasites; the DCF signal increases as the intraerythrocytic parasite matures into the trophozoite stage where active hemoglobin digestion occurs. Treatment of infected erythrocytes with the cysteine protease inhibitor, E-64, which inhibits hemoglobin digestion, decreases the DCF signal. We show that E-64 prevents schizont rupture but also causes delayed lethal effects when ring stage cultures are exposed to the drug. We also examined cultures of parasites in erythrocytes harboring 98% catalase inactivation and found no effect on growth and only a modest increase in DCF oxidation.

A phosphatidylcholine-BODIPY 581/591 conjugate allows mapping of oxidative stress in P. falciparum-infected erythrocytes

The chromophore, BODIPY 581/591, has an extended conjugated system that reacts with oxygen centered-radicals leading to changes in its spectral characteristics. Fatty acid-conjugated BODIPY 581/591 transfers readily between membrane bilayers and can be used as a sensor of oxidative stress in cell populations. We report here the use of a phosphatidylcholine (PC) derivative of BODIPY 581/591, which transfers much less rapidly between membranes. This allows the analysis of oxidative stress in individual cells and in different compartments within cells. Quantitative imaging and flow cytometry were used to measure the ratio of fully conjugated to oxidized probe in model systems and in Plasmodium falciparum-infected erythrocytes. We observed an increase in the oxidation of the parasite-associated BODIPY 581/591-PC as the intraerythrocytic parasite matures. By contrast, BODIPY 581/591-PC associated with the erythrocyte membrane experiences a low level of oxidation even in the later stages of parasite development. Treatment with a pro-oxidant compound caused increased oxidation of the probe in the parasite compartment, but less so in the host cell membrane. Conversely, treatment with ferricyanide increases oxidation of the probe in the erythrocyte cell membrane but does not inhibit parasite growth. Chromatographic analysis of the lipids in infected erythrocytes shows no evidence for loss of alpha-tocopherol or the accumulation of lipid hydroperoxides indicating that, despite the increased oxidative stress, the parasite membranes remain protected from substantial lipid oxidation. We have established BODIPY 581/591-PC as a useful probe of the spatial distribution of oxidative stress in P. falciparum-infected erythrocytes; however, the probe appears to be more sensitive to oxidative damage than endogenous lipids.

NAD(+)-dependent DNA ligase: a novel target waiting for the right inhibitor

DNA ligases (EC.6.5.1.1) are key enzymes that catalyze the formation of phosphodiester bonds at single stranded or double stranded breaks between adjacent 5' phosphoryl and 3' hydroxyl groups of DNA. These enzymes are important for survival because they are involved in major cellular processes like DNA replication/repair and recombination. DNA ligases can be classified into two groups on the basis of their cofactor specificities. NAD(+)-dependent DNA ligases are present in bacteria, some entomopox viruses and mimi virus while ATP-dependent DNA ligases are ubiquitous. The former have recently been drawing a lot of attention as novel targets for antibiotics to overcome current drug resistance issues. Currently a diverse range of inhibitors have been identified. There are several issues to be addressed in the quest for optimized inhibitors of the enzyme. In the first part of the review we summarize current structural work on these enzymes. Subsequently we describe the currently available classes of inhibitors. We also address modalities to improve the specificity and potencies of new inhibitors identified using protein structure based rational approaches. In conclusion, NAD(+)-dependent ligases show great promise and represent a novel drug target whose time has come.

New ferrocenic pyrrolo[1,2-a]quinoxaline derivatives: synthesis, and in vitro antimalarial activity

Following our search for antimalarial compounds, novel series of ferrocenic pyrrolo[1,2-a]quinoxaline derivatives 1-2 were synthesized from various substituted nitroanilines and tested for in vitro activity upon the erythrocytic development of Plasmodiumfalciparum strains with different chloroquine-resistance status. The pyrrolo[1,2-a]quinoxalines 1 were prepared in 6-8 steps through a regioselective palladium-catalyzed monoamination by coupling 4-chloropyrrolo[1,2-a]quinoxalines with 1,3-bis(aminopropyl)piperazine or -methylamine using Xantphos as the ligand. The ferrocenic bispyrrolo[1,2-a]quinoxalines 2 were prepared by reductive amination of previously described bispyrrolo[1,2-a]quinoxalines 9 with ferrocene-carboxaldehyde, by treatment with NaHB(OAc)(3). The best results were observed with ferrocenic pyrrolo[1,2-a]quinoxalines linked by a bis(3-aminopropyl)piperazine. Moreover, it was observed that a methoxy group on the pyrrolo[1,2-a]quinoxaline nucleus and no substitution on the terminal N-ferrocenylmethylamine function enhanced the pharmacological activity. Selected compounds 1b, 1f-h, 1l and 2a were tested for their ability to inhibit beta-haematin formation, the synthetic equivalent of hemozoin, by using the HPIA (heme polymerization inhibitory activity) assay. Of the tested compounds, only 2a showed a beta-haematin formation inhibition, but no inhibition of haem polymerization was observed with the other selected ferrocenic monopyrrolo[1,2-a]quinoxaline derivatives 1b, 1f-h and 1l, as the IC(50) values were superior to 10 equivalents.

Recent advances in antimalarial drug development

Malaria caused by protozoa of the genus Plasmodium, because of its prevalence, virulence, and drug resistance, is the most serious and widespread parasitic disease encountered by mankind. The inadequate armory of drugs in widespread use for the treatment of malaria, development of strains resistant to commonly used drugs such as chloroquine, and the lack of affordable new drugs are the limiting factors in the fight against malaria. These factors underscore the continuing need of research for new classes of antimalarial agents, and a re-examination of the existing antimalarial drugs that may be effective against resistant strains. This review provides an in-depth look at the most significant progress made during the past 10 years in antimalarial drug development.

Plasmodium berghei: In vitro and in vivo activity of dequalinium

Bisquinoline compounds have exhibited remarkable activity in vitro and in vivo against Plasmodium parasites by inhibition of heme detoxification. We have tested the ability of dequalinium 1,1'-(1,10-decanediyl)bis(4-amino-2-methylquinoline), a known antimicrobial agent, to inhibit beta-hematin synthesis using a non-emzymatic colorimetric assay and globin proteolysis by electrophoretic analysis (SDS-PAGE-15%). Dequalinium was able to inhibit both processes in vitro with close correlation to a murine malaria model, reducing parasitemia levels, prolonging the survival time post-infection and curing 40% of infected mice using a combination therapy with a loading dose of chloroquine. These results confirm that dequalinium is a promising lead for antimalarial drug development.

Synergistic interaction of a chloroquine metabolite with chloroquine against drug-resistant malaria parasites

We have previously shown that structural modification of chlorpromazine to introduce a basic side chain converts this chloroquine (CQ) resistance-reversing agent into a compound that has activity against Plasmodium falciparum in vitro. In an effort to further dissect the structural features that determine quinoline antimalarial activity and drug resistance-reversing activity, we have studied a series of aminoquinolines that are structurally related to CQ. We have analysed their haematin-binding activities, their antimalarial activities and their abilities to synergise the effect of CQ against drug-resistant P. falciparum. We found that a number of the aminoquinolines were able to interact with haematin but showed no or very weak antiparasitic activity. Interestingly, 4-amino-7-chloroquinoline, which is the CQ nucleus without the basic side chain, was able to act as a resistance-reversing agent. These studies point to structural features that may determine the resistance-modulating potential of weakly basic amphipaths. Interestingly, 4-amino-7-chloroquinoline is a metabolic breakdown product of CQ and may contribute to CQ activity against resistant parasites in vivo.

Synthesis, Antimalarial Activity, and Molecular Modeling of New Pyrrolo[1,2-a]quinoxalines, Bispyrrolo[1,2-a]quinoxalines, Bispyrido[3,2-e]pyrrolo[1,2-a]pyrazines, and Bispyrrolo[1,2-a]thieno[3,2-e]pyrazines

Three pyrrolo[1,2-a]quinoxalines, 15 bispyrrolo[1,2-a]quinoxalines, bispyrido[3,2-e]pyrrolo[1,2-a]pyrazines, and bispyrrolo[1,2-a]thieno[3,2-e]pyrazines were synthesized from various substituted nitroanilines or nitropyridines and tested for their in vitro activity upon the erythrocytic development of Plasmodium falciparum strains with different chloroquine-resistance status. Bispyrrolo[1,2-a]quinoxalines showed superior antimalarial activity with respect to monopyrrolo[1,2-a]quinoxalines. The best activity was observed with bispyrrolo[1,2-a]quinoxalines linked by a bis(3-aminopropyl)piperazine. Moreover, it was observed that the presence of a methoxy group on the pyrrolo[1,2-a]quinoxaline nucleus increased the pharmacological activity. Drug effects upon beta-hematin formation were assayed and showed similar or higher inhibitory activities than CQ. A possible mechanism of interaction implicating binding of pyrroloquinoxalines to beta-hematin was supported by molecular modeling.

Novel Endoperoxide Antimalarials: Synthesis, Heme Binding, and Antimalarial Activity

We report the synthesis of a series of novel epoxy endoperoxide compounds that can be prepared in high yields in one to three steps from simple starting materials. Some of these compounds inhibit the growth of Plasmodium falciparum in vitro. Structure-activity studies indicate that an endoperoxide ring bisubstituted with saturated cyclic moieties is the pharmacophore. To study the molecular basis of the action of these novel antimalarial compounds, we examined their ability to interact with oxidized and reduced forms of heme. Some of the compounds interact with oxidized heme in a fashion similar to chloroquine and other 4-aminoquinolines, while some of the compounds interact with reduced heme. However, the level of antimalarial potency is not well correlated with these activities, suggesting that some of the endoperoxides may exert their antimalarial activities by a novel mechanism of action.

In vitro activities of piperaquine and other 4-aminoquinolines against clinical isolates of Plasmodium falciparum in Cameroon

The spread of chloroquine-resistant Plasmodium falciparum calls for a constant search for new drugs. The in vitro activity of piperaquine, a new Chinese synthetic drug belonging to the bisquinolines, was evaluated in 103 fresh clinical isolates of P. falciparum in Cameroon, Central Africa, and compared with that of other 4-aminoquinoline and Mannich base derivatives and dihydroartemisinin. Piperaquine was highly active (geometric mean 50% inhibitory concentration, 38.9 nmol/liter; range, 7.76 to 78.3 nmol/liter) and equally active (P > 0.05) against the chloroquine-sensitive and the chloroquine-resistant isolates. There was a significant but low correlation of response between chloroquine and piperaquine (r = 0.257, P < 0.05). These results suggest that further development of piperaquine, in combination with dihydroartemisinin, holds promise for use in chloroquine-resistant regions of endemicity.

Synthesis and in vitro and in vivo antimalarial activity of N1-(7-chloro-4-quinolyl)-1,4-bis(3-aminopropyl)piperazine derivatives

Three series of monoquinolines consisting of a 1,4-bis(3-aminopropyl)piperazine linker and a large variety of terminal groups were synthesized. Our aim was to prove that in related bisquinoline, it is the second quinoline moiety that is responsible for cytotoxicity and that it is not an absolute requirement for overcoming resistance to chloroquine (CQ). Eleven compounds displayed a higher selectivity index (ratio CC50/IC50 activity) than CQ, and one of them cured mice infected by Plasmodium berghei.

Parallel synthesis and anti-malarial activity of a sulfonamide library

Solution-phase synthesis and evaluation of a library of 31 sulfonamides as inhibitors of a chloroquine-resistant strain of Plasmodium falciparum are described. The most potent compound displayed an activity 100-fold better than chloroquine. Experiments using a fluorescent sulfonamide derivative suggest that their site of action inside the parasite is different to that of chloroquine.

Synthesis, antimalarial activity and inhibition of haem detoxification of novel bisquinolines

The synthesis of novel bisquinoline compounds comprising 4-(4-diethylamino-1-methylbutyl)aminoquinoline units joined through the 2-position by a (CH(2))(n) linker is described. Their ability to inhibit the growth of both chloroquine-sensitive (D10) and chloroquine-resistant (K1) strains of Plasmodium falciparum, the hydrogen peroxide-mediated pathway for decomposition of haem, and the conversion of haem to beta-haematin have been measured. The activity was affected by the length of the linker and the most active (6c, n=12) showed effects similar to chloroquine in three of the assays. However, it was even more active against the resistant strain [IC(50), 17 nM (K1); 43 nM (D10)], much superior to chloroquine (IC(50), 540 nM) and slightly better than mefloquine (IC(50), 30 nM) in this regard.

Antimalarial in-vivo activity of bis(9-amino-6-chloro-2-methoxyacridines)

In the fight against malaria, chemotherapy using bisacridines may represent an alternative method to overcoming chloroquine-resistance. Eight bis(9-amino-6-chloro-2-methoxyacridines), in which acridine moieties were linked by polyamines substituted with a side chain, were tested for their in-vivo activity upon mice infected by Plasmodium berghei. Three of the compounds revealed antimalarial activity but no relationship could be deduced from a comparison of in-vitro and in-vivo activities. N-alkylation of the central amino group generated toxicity and, therefore, only compounds N-acylated in this position can be selected as leads.

Antiplasmodial activity and cytotoxicity of bis-, tris-, and tetraquinolines with linear or cyclic amino linkers

Bisquinoline heteroalkanediamines were structurally modified in order to study the effects of enhanced bulkiness and rigidity on both their activity on strains of Plasmodium falciparum expressing different degrees of chloroquine (CQ) resistance and their cytotoxicity toward mammalian cells. While cyclization yielded molecules of greater rigidity that were not more active than their linear counterparts, they were characterized by an absence of cytotoxicity. Alternatively, dimerization of these compounds led to tetraquinolines that are very potent for CQ-resistant strains and noncytotoxic.

Antimalarial, antitrypanosomal, and antileishmanial activities and cytotoxicity of bis(9-amino-6-chloro-2-methoxyacridines): influence of the linker

Forty bis(9-amino-6-chloro-2-methoxyacridines), in which acridine moieties are joined by alkanediamines, polyamines, or polyamines substituted by a side chain, were synthesized and tested for their in vitro activity upon the erythrocytic stage of Plasmodium falciparum, trypomastigote stage of Trypanosoma brucei, and amastigote stage of Trypanosoma cruzi and Leishmania infantum as well as for their cytotoxic effects upon MRC-5 cells. Results clearly showed the importance of the nature of the linker and of its side chain for antiparasitic activity, cytotoxicity, and cellular localization. Among several compounds devoid of cytotoxic effects at 25 microM upon MRC-5 cells, one displayed IC(50) values ranging from 8 to 18 nM against different P. falciparum strains while three others totally inhibited T. brucei at 1.56 microM.

Specific inhibition of the eubacterial DNA ligase by arylamino compounds

All known DNA ligases catalyze the formation of a phosphodiester linkage between adjacent termini in double-stranded DNA via very similar mechanisms. The ligase family can, however, be divided into two classes: eubacterial ligases, which require NAD(+) as a cofactor, and other ligases, from viruses, archaea, and eukaryotes, which use ATP. Drugs that discriminate between DNA ligases from different sources may have antieubacterial activity. We now report that a group of arylamino compounds, including some commonly used antimalarial and anti-inflammatory drugs and a novel series of bisquinoline compounds, are specific inhibitors of eubacterial DNA ligases. Members of this group of inhibitors have different heterocyclic ring systems with a common amino side chain in which the two nitrogens are separated by four carbon atoms. The potency, but not the specificity of action, is influenced by the DNA-binding characteristics of the inhibitor, and the inhibition is noncompetitive with respect to NAD(+). The arylamino compounds appear to target eubacterial DNA ligase in vivo, since a Salmonella Lig(-) strain that has been rescued with the ATP-dependent T4 DNA ligase is less sensitive than the parental Salmonella strain.

Kinetics of inhibition of glutathione-mediated degradation of ferriprotoporphyrin IX by antimalarial drugs

We have shown previously that chloroquine and amodiaquine inhibit the glutathione-dependent degradation of ferriprotoporphyrin IX (FP). We have also demonstrated that treatment of human erythrocytes infected with Plasmodium falciparum with chloroquine or amodiaquine results in a dose- and time-dependent accumulation of FP in the membrane fraction of these cells in correlation with parasite killing. High levels of membrane FP are known to perturb the barrier properties of cellular membranes, and could thereby irreversibly disturb the ion homeostasis of the parasite and cause parasite death. We here report on the effect of various 4-aminoquinolines, as well as pyronaridine, halofantrine and some bis-quinolines, on glutathione-mediated destruction of FP in aqueous solution, when FP was bound non-specifically to a protein, and when it was dissolved in human erythrocyte ghost membranes. We showed that all drugs were capable of inhibiting FP degradation in solution. The inhibitory efficacy of some drugs declined when FP was bound non-specifically to protein. Quinine and mefloquine were unable to inhibit the degradation of membrane-associated FP, in line with their inability to increase membrane-associated FP levels in malaria-infected cells following drug treatment. The discrepancy between chloroquine and amodiaquine on the one hand, and quinine and mefloquine on the other, is discussed in terms of the particular location of drugs and FP in the phospholipid membrane, and may suggest differences in the mechanistic details of the antimalarial action of these drugs.

Synthesis of bisquinolines and their in vitro ability to produce methemoglobin in canine hemolysate

Synthesis of a number of derivatives of bisquinolines (3-9) have been reported here. Effect of these compounds on in vitro methemoglobin formation and methemoglobin reductase activity has resulted in the identification of two potential compounds (5 & 7), showing negligible methemoglobin toxicity.

Bisquinoline antimalarials: their role in malaria chemotherapy

Quinoline compounds, such as chloroquine, are used widely to treat malaria; however, the malarial parasite is rapidly becoming resistant to the drugs currently available. Presently, rational drug design is hindered considerably due to the mode of action of chloroquine being poorly understood. We rely on serendipity, rather than solid structural evidence, to generate new antimalarials. Hence any insight into the possible modes of action of quinoline antimalarials, including the bisquinolines, would greatly aid rational drug design. The quinoline antimalarial drugs, chloroquine, quinine and mefloquine, are thought to act by interfering with the digestion of haemoglobin in the blood stages of the malaria life-cycle. These quinoline antimalarials traverse down the pH gradient to accumulate to millimolar concentrations in the acidic vacuole of the parasite. It has been suggested that this high intravacuolar concentration prevents haem sequestration, causing a build up of the toxic haem moiety and the death of the parasite by its own toxic waste. The actual mechanism by which the parasite sequesters haem and the drug target(s) during this process, however, still remains elusive. As a consequence, haem polymerisation and the efficiency of quinoline antimalarials, including the bisquinolines, as inhibitors of this process has been investigated. In this paper, the potential role of the bisquinolines in the fight against chloroquine-resistant malaria is addressed.

Bisquinolines. 2. Antimalarial N,N-bis(7-chloroquinolin-4-yl)heteroalkanediamines

N,N-Bis(7-chloroquinolin-4-yl)heteroalkanediamines 1-11 were synthesized and screened against Plasmodium falciparum in vitro and Plasmodium berghei in vivo. These bisquinolines had IC50 values from 1 to 100 nM against P. falciparum in vitro. Six of the 11 bisquinolines were significantly more potent against the chloroquine-resistant W2 clone compared to the chloroquine-sensitive D6 clone. For bisquinolines 1-11 there was no relationship between the length of the bisquinoline heteroalkane bridge and antimalarial activity and no correlation between in vitro and in vivo antimalarial activities. Bisquinolines with alkyl ether and piperazine bridges were substantially more effective than bisquinolines with alkylamine bridges against P. berghei in vivo. Bisquinolines 1-10 were potent inhibitors of hematin polymerization with IC50 values falling in the narrow range of 5-20 microM, and there was a correlation between potency of inhibition of hematin polymerization and inhibition of parasite growth. Compared to alkane-bridged bisquinolines (Vennerstrom et al., 1992), none of these heteroalkane-bridged bisquinolines had sufficient antimalarial activity to warrant further investigation of the series.

4-Aminoquinolines--past, present, and future: a chemical perspective

The 4-aminoquinoline chloroquine (1) can be considered to be one of the most important synthetic chemotherapeutic agents in history. Since its discovery, chloroquine has proved to be a highly effective, safe, and well-tolerated drug for the treatment and prophylaxis of malaria. However, the emergence of chloroquine-resistant strains of the malarial parasite has underlined the requirement for a synthetic alternative to chloroquine. This review describes structure-activity relationships for the 4-aminoquinolines, along with views on the mechanism of action and parasite resistance. A description of drug metabolism and toxicity also is included, with a brief description of potential approaches to the design of new synthetic derivatives.

Quinoline antimalarials: mechanisms of action and resistance

The quinoline-containing antimalarial drugs, chloroquine, quinine and mefloquine, are a vital part of our chemotherapeutic armoury against malaria. These drugs are thought to act by interfering with the digestion of haemoglobin in the blood stages of the malaria life cycle. Chloroquine is a dibasic drug which diffuses down the pH gradient to accumulate about a 1000-fold in the acidic vacuole of the parasite. The high intravacuolar concentration of chloroquine is proposed to inhibit the polymerisation of haem. As a result, the haem which is released during haemoglobin breakdown builds up to poisonous levels, thereby killing the parasite with its own toxic waste. The more lipophilic quinolinemethanol drugs, mefloquine and quinine, are not concentrated so extensively in the food vacuole and probably have alternative sites of action. The technique of photoaffinity labelling has been used to identify a series of proteins which interact specifically with mefloquine. These studies have led us to speculate that the quinolinemethanols bind to high density lipoproteins in the serum and are delivered to the erythrocytes where they interact with an erythrocyte membrane protein, known as stomatin, and are then transferred to the intracellular parasite via a pathway used for the uptake of exogenous phospholipid. The final target(s) of quinine and mefloquine action are not yet fully characterised, but may include parasite proteins with apparent molecular weights of 22 kDa and 36 kDa. As resistance to the quinoline antimalarials rises inexorably, there is an urgent need to understand the molecular basis for decreased drug sensitivity. A parasite-encoded homologue of P-glycoprotein has been implicated in the development of drug resistance, possibly by controlling the level of accumulation of the quinoline-containing drugs. As our molecular understanding of these processes increases, it should be possible to design novel antimalarial strategies which circumvent the problem of drug resistance.

Novel bisquinoline antimalarials. Synthesis, antimalarial activity, and inhibition of haem polymerisation

We report the synthesis of two series of novel bisquinoline compounds that inhibit the growth of both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. To study the molecular basis of the action of these novel antimalarial drugs, we examined their ability to inhibit haem polymerisation in the presence and absence of parasite extracts. The level of antimalarial potency was correlated with the level of inhibition of haem polymerisation, suggesting that these bisquinolines exert their antimalarial activity by antagonising the sequestration of toxic haem moieties.