Synthesis, Biological Evaluation, and Molecular Modeling Studies of Chiral Chloroquine Analogues as Antimalarial Agents.

In a focused exploration, we designed, synthesized, and biologically evaluated chiral conjugated new chloroquine (CQ) analogues with substituted piperazines as antimalarial agents. In vitro as well as in vivo studies revealed that compound 7c showed potent activity (in vitro 50% inhibitory concentration, 56.98 nM for strain 3D7 and 97.76 nM for strain K1; selectivity index in vivo [up to at a dose of 12.5 mg/kg of body weight], 3,510) as a new lead antimalarial agent. Other compounds (compounds 6b, 6d, 7d, 7h, 8c, 8d, 9a, and 9c) also showed moderate activity against a CQ-sensitive strain (3D7) and superior activity against a CQ-resistant strain (K1) of Plasmodium falciparum Furthermore, we carried out docking and three-dimensional quantitative structure-activity relationship (3D-QSAR) studies of all in-house data sets (168 molecules) of chiral CQ analogues to explain the structure-activity relationships (SAR). Our new findings specify the significance of the H-bond interaction with the side chain of heme for biological activity. In addition, the 3D-QSAR study against the 3D7 strain indicated the favorable and unfavorable sites of CQ analogues for incorporating steric, hydrophobic, and electropositive groups to improve the antimalarial activity.

Design, synthesis and antiplasmodial activity of novel imidazole derivatives based on 7-chloro-4-aminoquinoline.

A series of short chain 4-aminoquinoline-imidazole derivatives have been synthesized in one pot two step multicomponent reaction using van leusen standard protocol. The diethylamine function of chloroquine is replaced by substituted imidazole derivatives containing tertiary terminal nitrogen. All the synthesized compounds were screened against the chloroquine sensitive (3D7) and chloroquine resistant (K1) strains of Plasmodium falciparum. Some of the compounds (6, 8, 9 and 17) in the series exhibited comparable activity to CQ against K1 strain of P. falciparum. All the compounds displayed resistance factor between 0.09 and 4.57 as against 51 for CQ. Further, these analogues were found to form a strong complex with hematin and inhibit the ß-hematin formation, therefore these compounds act via heme polymerization target.

Investigating Pharmacological Targeting of G-Quadruplexes in the Human Malaria Parasite.

The unique occurrence of G-quadruplexes in the AT-rich genome of human malaria parasite Plasmodium falciparum provides hints about their critical roles in parasite survival, pathogenesis, and host immune evasion. An intriguing question is whether these noncanonical structures can serve as molecular targets for small molecule-based interventions against malaria. In this study, we have investigated the pharmacological targeting of G-quadruplexes for parasite inhibition. We observed that bisquinolinium derivatives of 1,8-naphthyridine and pyridine affected the stability and molecular recognition properties of G-quadruplexes in telomeric and subtelomeric regions in P. falciparum. Parasite inhibition and cytotoxicity assays revealed that these ligands effectively inhibit parasite growth with minimal toxic effects in human cells. G-quadruplex interacting ligands caused degeneration and shortening of parasite telomeres. Ligand-induced perturbations in telomere homeostasis also affected transcriptional state of the subtelomeric region harboring antigenic variation genes. Taken together, our results suggest that quadruplex-ligand interaction disturbs telomeric/subtelomeric chromatin organization and induces DNA damage that consequently leads to parasite death. Our findings also draw attention to the striking differences in telomere dynamics in the protozoan parasite and human host that can be exploited for selective targeting of the telomeric quadruplex of the parasite as a potential antimalarial strategy.

Synthesis and Evaluation of Chirally Defined Side Chain Variants of 7-Chloro-4-Aminoquinoline To Overcome Drug Resistance in Malaria Chemotherapy.

A novel 4-aminoquinoline derivative [(S)-7-chloro-N-(4-methyl-1-(4-methylpiperazin-1-yl)pentan-2-yl)-quinolin-4-amine triphosphate] exhibiting curative activity against chloroquine-resistant malaria parasites has been identified for preclinical development as a blood schizonticidal agent. The lead molecule selected after detailed structure-activity relationship (SAR) studies has good solid-state properties and promising activity against in vitro and in vivo experimental malaria models. The in vitro absorption, distribution, metabolism, and excretion (ADME) parameters indicate a favorable drug-like profile.

Antimalarial activity of novel 4-aminoquinolines active against drug resistant strains.

In the present study we have synthesized a new class of 4-aminoquinolines and evaluated against Plasmodium falciparum in vitro (3D7-sensitive strain & K1-resistant strain) and Plasmodium yoelii in vivo (N-67 strain). Among the series, eleven compounds (5, 6, 7, 8, 9, 11, 12, 13, 14, 15 and 21) showed superior antimalarial activity against K1 strain as compared to CQ. In addition, all these analogues showed 100% suppression of parasitemia on day 4 in the in vivo mouse model against N-67 strain when administered orally. Further, biophysical studies suggest that this series of compounds act on heme polymerization target.

In vitro susceptibility of Indian Plasmodium falciparum isolates to different antimalarial drugs & antibiotics.

Background & objectives: : The in vitro assays for susceptibility of Plasmodium falciparum to antimalarial drugs are important tools for monitoring drug resistance. During the present study, efforts were made to establish long-term continuous in vitro culture of Indian field isolates of P. falciparum and to determine their sensitivity to standard antimalarial drugs and antibiotics. Methods: Four (MZR-I, -II, -III and -IV) P. falciparum isolates were obtained from four patients who showed artemisinin-based combination therapy (ACT) from Mizoram, a north-eastern State of India, and characterized for their in vitro susceptibility to chloroquine diphosphate (CQ), quinine hydrochloride dehydrate, mefloquine, piperaquine, artemether, arteether, dihydro-artemisinin (DHA), lumefantrine and atovaquone and antibiotics, azithromycin and doxycycline. These patients showed ACT treatment failure. Two-fold serial dilutions of each drug were tested and the effect was evaluated using the malaria SYBR Green I fluorescence assay. K1 (chloroquine-resistant) and 3D7 (chloroquine-sensitive) reference strains were used as controls. Results: Growth profile of all field isolates was identical to that of reference parasites. The IC50 values of all the drugs were also similar against field isolates and reference parasite strains, except K1, exhibited high IC50 value (275±12.5 nM) of CQ for which it was resistant. All field isolates exhibited higher IC50 values of CQ, quinine hydrochloride dihydrate and DHA compared to reference strains. The resistance index of field isolates with respect to 3D7 ranged between 260.55 and 403.78 to CQ, 39.83 and 46.42 to quinine, and 2.98 and 4.16 to DHA, and with respect to K1 strain ranged between 6.51 and 10.08, 39.26 and 45.75, and 2.65 and 3.71. MZR-I isolate exhibited highest resistance index. Interpretation & conclusions: As the increase in IC50 and IC90 values of DHA against field isolates of P. falciparum was not significant, the tolerance to DHA-piperaquine (PPQ) combination might be because of PPQ only. Further study is required on more number of such isolates to generate data for a meaningful conclusion.

Correlation between in vitro and in vivo antimalarial activity of compounds using CQ-sensitive and CQ-resistant strains of Plasmodium falciparum and CQ-resistant strain of P. yoelii.

Present efforts have been made to establish a correlation between in vitro and in vivo antimalarial activity using MIC, IC50 and IC90 values against CQ-sensitive (3D7) and CQ-resistant (K1) strains of Plasmodium falciparum and in vivo activity against Plasmodium yoelii. The method of discriminant function analysis (DFA) was applied to analyze the data. It was observed that in vitro IC90 values against both 3D7 and K1 strains (p < 0.001) have strong correlation with in vivo curative activity. The respective IC50 and IC90 values of compounds, which cured mice (i.e., animals did not show recrudescence of parasitemia even after 60 days posttreatment), ranged between 3 and 14 nM and 14 and 186 nM against 3D7 and between 9 and 65 nM and 24 and 359 nM against the K1 strain of P. falciparum. Whereas the IC50 and IC90 values of compounds which exhibited in vivo suppressive activity in mice ranged between 10 and 307 nm and 61 and >965 nM, respectively, against 3D7 and 75 and >806 nm and 241 and >1232 nM against the K1 strain of P. falciparum. The findings suggest that IC90 values against both 3D7 and K1 strains (p < 0.02) are the main contributors for the prediction of in vivo curative activity of a new molecule. Apart from this, a reasonable correlation between MIC and IC50 values of compounds has also been established.

Design, Synthesis, and Biological Evaluation of Novel 1,2,4-Trioxanes as Potential Antimalarial Agents.

A series of substituted 1,2,4-trioxanes were synthesized and evaluated for their antimalarial potential, in silico ADME properties and cytotoxicity on neuronal cell lines. Among the 15 synthesized substituted 1,2,4-trioxanes, two compounds (compound 15, IC50 = 25.71 nM; compound 21, IC50 = 19.6 nM) exhibited promising in vitro antimalarial potential comparable to those of the existing drugs chloroquine and artemisinin. Both of these compounds were found to be nontoxic up to 20 µM concentration in neuronal PC-12 cells. Compound 21 may serve as an optimized lead compound because of its less in vitro toxicity and lower probability to cross the blood brain barrier.

Synthesis of S-(-)-5,6-Dihydrocanthin-4-ones via a Triple Cooperative Catalysis-Mediated Domino Reaction.

An enantioselective synthesis of S-(-)-5,6-dihydrocanthin-4-ones via a triple cooperative catalysis-mediated domino reaction having a broad substrate scope is reported. The reaction between substituted 1-formyl-9H-ß-carbolines and terminal alkynes in the presence of catalytic amounts of Jorgensen-Hayashi catalyst, copper iodide, and Hunig base proceeded via a multicascade route, affording the title compounds in good yields and excellent ees with interesting mechanistic features. These compounds were assessed for in vitro antiplasmodial activity against P. falciparum strains. Additionally, 5,6-dihydrocanthin-4-ones are demonstrated to be a versatile precursor to different fused ß-carboline derivatives via simple synthetic transformations.

Pyrrolidine-Acridine hybrid in Artemisinin-based combination: a pharmacodynamic study.

Aiming to develop new artemisinin-based combination therapy (ACT) for malaria, antimalarial effect of a new series of pyrrolidine-acridine hybrid in combination with artemisinin derivatives was investigated. Synthesis, antimalarial and cytotoxic evaluation of a series of hybrid of 2-(3-(substitutedbenzyl)pyrrolidin-1-yl)alkanamines and acridine were performed and mode of action of the lead compound was investigated. In vivo pharmacodynamic properties (parasite clearance time, parasite reduction ratio, dose and regimen determination) against multidrug resistant (MDR) rodent malaria parasite and toxicological parameters (median lethal dose, liver function test, kidney function test) were also investigated. 6-Chloro-N-(4-(3-(3,4-dimethoxybenzyl)pyrrolidin-1-yl)butyl)-2-methoxyacridin-9-amine (15c) has shown a dose dependent haem bio-mineralization inhibition and was found to be the most effective and safe compound against MDR malaria parasite in Swiss mice model. It displayed best antimalarial potential with artemether (AM) in vitro as well as in vivo. The combination also showed favourable pharmacodynamic properties and therapeutic response in mice with established MDR malaria infection and all mice were cured at the determined doses. The combination did not show toxicity at the doses administered to the Swiss mice. Taken together, our findings suggest that compound 15c is a potential partner with AM for the ACT and could be explored for further development.

Synthesis, in vitro and in silico antimalarial activity of 7-chloroquinoline and 4H-chromene conjugates.

A new series of chloroquinoline-4H-chromene conjugates incorporating piperizine or azipane tethers were synthesized and their anti-malarial activity were evaluated against two Plasmodium falciparum strains namely 3D7 chloroquine sensitive (CQS) and K1 chloroquine resistant (CQR). Chloroquine was used as the standard and also reference for comparison. The conjugates exhibit intense UV absorption with λmax located at 342 nm (log ε=4.0), 254 nm (log ε=4.2), 223 nm (log ε=4.4) which can be used to spectrometrically track the molecules even in trace amounts. Among all the synthetic compounds, two molecules namely 6-nitro and N-piperazine groups incorporated 7d and 6-chloro and N-azapane incorporated 15b chloroquinoline-4H-chromene conjugates showed significant anti-malarial activity against two strains (3D7 and K1) of P. falciparum. These values are lesser than the values of standard antimalarial compound. Molecular docking results suggested that these two compounds showing strong binding affinity with P. falciparum lactate dehydrogenase (PfLDH) and also they occupy the co-factor position which indicated that they could be the potent inhibitors for dreadful disease malaria and specifically attack the glycolytic pathway in parasite for energy production.

In Silico Exploration for New Antimalarials: Arylsulfonyloxy Acetimidamides as Prospective Agents.

A strategy is described to identify new antimalarial agents to overcome the drug resistance and/or failure issues through in silico screening of multiple biological targets. As a part of this, three enzymes namely CTPS, CK, and GST were selected, from among 56 drug targets of P. falciparum, and used them in virtual screening of ZINC database entries which led to the design and synthesis of arylsulfonyloxy acetimidamides as their consensus inhibitors. From these, two compounds showed good activity against sensitive (3D7; IC50, 1.10 and 1.45 µM) and resistant (K1; IC50, 2.10 and 2.13 µM) strains of the parasite, and they were further investigated through docking and molecular dynamics simulations. The findings of this study collectively paved the way for arylsulfonyloxy acetimidamides as a new class of antimalarial agents.

Sulfur mobilization for Fe-S cluster assembly by the essential SUF pathway in the Plasmodium falciparum apicoplast and its inhibition.

The plastid of the malaria parasite, the apicoplast, is essential for parasite survival. It houses several pathways of bacterial origin that are considered attractive sites for drug intervention. Among these is the sulfur mobilization (SUF) pathway of Fe-S cluster biogenesis. Although the SUF pathway is essential for apicoplast maintenance and parasite survival, there has been limited biochemical investigation of its components and inhibitors of Plasmodium SUFs have not been identified. We report the characterization of two proteins, Plasmodium falciparum SufS (PfSufS) and PfSufE, that mobilize sulfur in the first step of Fe-S cluster assembly and confirm their exclusive localization to the apicoplast. The cysteine desulfurase activity of PfSufS is greatly enhanced by PfSufE, and the PfSufS-PfSufE complex is detected in vivo. Structural modeling of the complex reveals proximal positioning of conserved cysteine residues of the two proteins that would allow sulfide transfer from the PLP (pyridoxal phosphate) cofactor-bound active site of PfSufS. Sulfide release from the l-cysteine substrate catalyzed by PfSufS is inhibited by the PLP inhibitor d-cycloserine, which forms an adduct with PfSufS-bound PLP. d-Cycloserine is also inimical to parasite growth, with a 50% inhibitory concentration close to that reported for Mycobacterium tuberculosis, against which the drug is in clinical use. Our results establish the function of two proteins that mediate sulfur mobilization, the first step in the apicoplast SUF pathway, and provide a rationale for drug design based on inactivation of the PLP cofactor of PfSufS.

Synthesis and evaluation of new diaryl ether and quinoline hybrids as potential antiplasmodial and antimicrobial agents.

Synthesis and bioevaluation of new diaryl ether hybridized quinoline derivatives as antiplasmodial, antibacterial and antifungal agents is reported. It was encouraging to discover that several compounds displayed 2-3 folds better efficacy than chloroquine in chloroquine-resistant K1 strain of Plasmodium falciparum. Further, a few members of the library displayed good antibacterial efficacy against gram positive strains of bacteria but none of the compounds displayed any significant antifungal activity.

Antiplasmodial activity of new 4-aminoquinoline derivatives against chloroquine resistant strain.

Emergence and spread of multidrug resistant strains of Plasmodium falciparum has severely limited the antimalarial chemotherapeutic options. In order to overcome the obstacle, a set of new side-chain modified 4-aminoquinolines were synthesized and screened against chloroquine-sensitive (3D7) and chloroquine-resistant (K1) strains of P. falciparum. The key feature of the designed molecules is the use of methylpiperazine linked α, ß(3)- and γ-amino acids to generate novel side chain modified 4-aminoquinoline analogues. Among the evaluated compounds, 20c and 30 were found more potent than CQ against K1 and displayed a four-fold and a three-fold higher activity respectively, with a good selectivity index (SI=5846 and 11,350). All synthesized compounds had resistance index between 1.06 and >14.13 as against 47.2 for chloroquine. Biophysical studies suggested that this series of compounds act on heme polymerization target.

Synthesis of chiral chloroquine and its analogues as antimalarial agents.

In this investigation, we describe a new approach to chiral synthesis of chloroquine and its analogues. All tested compounds displayed potent activity against chloroquine sensitive as well as chloroquine resistant strains of Plasmodium falciparum in vitro and Plasmodium yoelii in vivo. Compounds S-13 b, S-13c, S-13 d and S-13 i displayed excellent in vitro antimalarial activity with an IC50 value of 56.82, 60.41, 21.82 and 7.94 nM, respectively, in the case of resistant strain. Furthermore, compounds S-13a, S-13c and S-13 d showed in vivo suppression of 100% parasitaemia on day 4 in the mouse model against Plasmodium yoelii when administered orally. These results underscore the application of synthetic methodology and need for further lead optimization.

A natural product inspired hybrid approach towards the synthesis of novel pentamidine based scaffolds as potential anti-parasitic agents.

A natural product inspired molecular hybridization approach led us to a series of novel pentamidine based pyrimidine and chalcone scaffolds. All the hybrids were evaluated for their anti-leishmanial potential. Most of the screened compounds have showed significant in vitro anti-leishmanial activity with less cytotoxicity in comparison to the standard drugs (pentamidine, sodium stibogluconate, and miltefosine). Additionally, anti-malarial screening of these compounds was also done and four compounds have shown superior activity against chloroquine resistance strain (K1) of Plasmodium falciparum.

Design and synthesis of 3-[(7-chloro-1-oxidoquinolin-4-ylamino)alkyl]-1,3-thiazolidin-4-ones as antimalarial agents.

A new series of quinoline analogs have been synthesized and found active against P. falciparum in vitro and P. yoelli in vivo. Compounds 8, 10 and 11 exhibited superior in vitro activity compared to chloroquine. Selected compounds 8, 10 and 11 exhibited significant suppression of parasitaemia in vivo assay. These analogs form a complex with hematin and inhibit the ß-hematin formation, suggesting that this class of compounds act on a heme polymerization target. Further this study confirms that quinoline ring nitrogen is essential for both transportation of the molecule across the membrane as well as for tight binding to hematin.

Design, synthesis of 4-aminoquinoline-derived thiazolidines and their antimalarial activity and heme polymerization inhibition studies.

The present study describes the synthesis of a series of new 4-aminoquinoline-derived thiazolidines and evaluation of their antimalarial activity against a NF-54 strain of Plasmodium falciparum in vitro and N-67 strain of Plasmodium yoelii in vivo. Among the series, two compounds, 2-(4-chloro-phenyl)-thiazolidine-4-carboxylic acid [2-(7-chloro-quinolin-4-ylamino)-ethyl]-amide hydrochloride (14) and 2-(2,6-dichloro-phenyl)-thiazolidine-4-carboxylic acid [2-(7-chloro-quinolin-4-ylamino)-ethyl]-amide hydrochloride (22) exhibited significant suppression of parasitaemia in the in vivo assay. All the analogues were found to form strong complex with haematin and inhibited the ß-haematin formation in vitro. These results suggest that these compounds act on heme polymerization target.

Synthesis and in vitro evaluation of new chloroquine-chalcone hybrids against chloroquine-resistant strain of Plasmodium falciparum.

The control of malaria has been complicated with increasing resistance of malarial parasite against existing antimalarials. Herein, we report the synthesis of a new series of chloroquine-chalcone based hybrids (8-22) and their antimalarial efficacy against both chloroquine-susceptible (3D7) and chloroquine-resistant (K1) strains of Plasmodium falciparum. Most of the compounds showed enhanced antimalarial activity as compared to chloroquine in chloroquine-resistant (K1) strain of Plasmodium falciparum. Furthermore, to unfold the mechanism of action of these synthesized hybrid molecules, we carried out hemin dependent studies, in which three compounds were found to be active.