Β-haematin Inhibition Research Articles

Modular synthesis of antimalarial quinoline-based PGM metallarectangles.

A new ditopic, quinoline-based ligand L (7-chloro-4-(pyridin-4-yl)quinoline) was synthesized via a Suzuki cross-coupling reaction. The ligand was utilized to synthesize the corresponding half-sandwich iridium(III) and ruthenium(II) binuclear complexes (1c and 1d) and the subsequent metallarectangles (2c, 2d, 3c, and 3d), via [2 + 2] coordination-driven self-assembly. Single-crystal X-ray diffraction confirmed the proposed molecular structure of the binuclear complex [{IrCl2(Cp*)}2(μ-L)] (1c) and DFT calculations were used to predict the optimized geometry of the rectangular nature of [{Ir(μ-Cl)(Cp*)}4(μ-L)2](CF3SO3)4 (2c). All of the metallarectangles were isolated as their triflate salts and characterized using various spectroscopic (1H, 13C{1H}, DOSY NMR, and IR spectroscopy) and analytical techniques (ESI-MS). The synthesized compounds were screened against the NF54 chloroquine-sensitive (CQS) and K1 chloroquine-resistant (CQR) strains of Plasmodium falciparum. Incorporation of the ubiquitous quinoline core and metal complexation significantly enhanced the in vitro biological activity, with an increase in the nuclearity correlating with an increase in the resultant antiplasmodial activity. This was observed across both parasitic strains, alluding to the potential of supramolecular metallarectangles to act as antiplasmodial agents. Inhibition of haemozoin formation was considered a potential mechanism of action and selected metallarectangles exhibit β-haematin inhibition activity with near comparable activity to chloroquine.

Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites.

With the continued loss of antimalarials to resistance, drug repositioning may have a role in maximising efficiency and accelerating the discovery of new antimalarial drugs. Bayesian statistics was previously used as a tool to virtually screen USFDA approved drugs for predicted β-haematin (synthetic haemozoin) inhibition and in vitro antimalarial activity. Here, we report the experimental evaluation of nine of the highest ranked drugs, confirming the accuracy of the model by showing an overall 93% hit rate. Lapatinib, nilotinib, and lomitapide showed the best activity for inhibition of β-haematin formation and parasite growth and were found to inhibit haemozoin formation in the parasite, providing mechanistic insights into their mode of antimalarial action. We then screened the USFDA approved drugs for binding to the β-haematin crystal, applying a docking method in order to evaluate its performance. The docking method correctly identified imatinib, lapatinib, nilotinib, and lomitapide. Experimental evaluation of 22 of the highest ranked purchasable drugs showed a 24% hit rate. Lapatinib and nilotinib were chosen as templates for shape and electrostatic similarity screening for lead hopping using the in-stock ChemDiv compound catalogue. The actives were novel structures worthy of future investigation. This study presents a comparison of different in silico methods to identify new haemozoin-inhibiting chemotherapeutic alternatives for malaria that proved to be useful in different ways when taking into consideration their strengths and limitations.

Docking, synthesis and antimalarial activity of novel 4-anilinoquinoline derivatives

A series of 4-anilinoquinoline triazine derivatives were designed, synthesized and screened for in vivo antimalarial activity against a chloroquine-sensitive strain of Plasmodium berghei. The compounds were further subjected to in vitro antimalarial activity against chloroquine-resistant W2 strain of Plasmodium falciparum and β-haematin inhibition studies. All the compounds exhibited in vivo antimalarial activity better than that shown by the standard drug, chloroquine. Twelve out of fifteen compounds showed better inhibition than that of chloroquine against chloroquine-resistant W2 strain of Plasmodium falciparum. Ten compounds showed β-haematin inhibition, better than that of chloroquine, with IC50 values in the range of 18–25µM. One compound, 3k, was found to be better than artemisinin against W2 strain of Plasmodium falciparum and also displayed the best β-haematin inhibitory activity, thereby becoming eligible to be explored as a potential lead for antimalarial chemotherapy.

Synthesis, characterization and pharmacological evaluation of ferrocenyl azines and their rhodium(I) complexes

Ferrocenyl azines containing salicylaldimine motifs were prepared by Schiff-base condensation of salicylaldehyde hydrazones and (dimethylamino)methyl ferrocenecarboxaldehyde. Their corresponding Rh(I) complexes were prepared by reaction of the various ferrocenyl azines with [RhCl(COD)]2 (where COD = 1,5-cyclooctadiene) to yield heterobimetallic complexes. The compounds were characterized using standard spectroscopic and analytical techniques. The characterization data suggests that the ferrocenyl azine acts as a bidentate donor. The rhodium(I) centre binds to the imine nitrogen and phenolic oxygen of the salicylaldimine, forming a neutral complex. The compounds were screened against the NF54 chloroquine-sensitive (CQS) and K1 chloroquine-resistant (CQR) strains of Plasmodium falciparum. The ferrocene-containing salicylaldimines exhibited weak to moderate activity across both parasite strains. The heterometallic complexes exhibited enhanced activity compared to the ferrocenyl azines in both strains. Most of the compounds exhibited enhanced activity in the resistant strain compared to the sensitive strain. Inhibition of haemozoin formation was considered as a possible mechanism of action of these compounds and indeed they exhibited β-haematin inhibition activity, albeit weaker than chloroquine. All compounds were also screened against the G3 strain of Trichomonas vaginalis. The compounds inhibited no more than 50% parasite growth at the tested concentration. One complex exhibited moderate cytotoxicity against WHCO1 oesophageal cancer cells.

Improved antiparasitic activity by incorporation of organosilane entities into half-sandwich ruthenium(II) and rhodium(III) thiosemicarbazone complexes.

A series of ferrocenyl- and aryl-functionalised organosilane thiosemicarbazone compounds was obtained via a nucleophilic substitution reaction with an amine-terminated organosilane. The thiosemicarbazone (TSC) ligands were further reacted with either a ruthenium dimer [(η(6-i)PrC6H4Me)Ru(μ-Cl)Cl]2 or a rhodium dimer [(Cp*)Rh(μ-Cl)Cl]2 to yield a series of cationic mono- and binuclear complexes. The thiosemicarbazone ligands, as well as their metal complexes, were characterised using NMR and IR spectroscopy, and mass spectrometry. The molecular structure of the binuclear ruthenium(ii) complex was determined by single-crystal X-ray diffraction analysis. The thiosemicarbazones and their complexes were evaluated for their in vitro antiplasmodial activities against the chloroquine-sensitive (NF54) and chloroquine-resistant (Dd2) Plasmodium falciparum strains, displaying activities in the low micromolar range. Selected compounds were screened for potential β-haematin inhibition activity, and it was found that two Rh(iii) complexes exhibited moderate to good inhibition. Furthermore, the compounds were screened for their antitrichomonal activities against the G3 Trichomonas vaginalis strain, revealing a higher percentage of growth inhibition for the ruthenium and rhodium complexes over their corresponding ligand.

Bayesian models trained with HTS data for predicting β-haematin inhibition and in vitro antimalarial activity

A large quantity of high throughput screening (HTS) data for antimalarial activity has become available in recent years. This includes both phenotypic and target-based activity. Realising the maximum value of these data remains a challenge. In this respect, methods that allow such data to be used for virtual screening maximise efficiency and reduce costs. In this study both in vitro antimalarial activity and inhibitory data for β-haematin formation, largely obtained from publically available sources, has been used to develop Bayesian models for inhibitors of β-haematin formation and in vitro antimalarial activity. These models were used to screen two in silico compound libraries. In the first, the 1510 U.S. Food and Drug Administration approved drugs available on PubChem were ranked from highest to lowest Bayesian score based on a training set of β-haematin inhibiting compounds active against Plasmodium falciparum that did not include any of the clinical antimalarials or close analogues. The six known clinical antimalarials that inhibit β-haematin formation were ranked in the top 2.1% of compounds. Furthermore, the in vitro antimalarial hit-rate for this prioritised set of compounds was found to be 81% in the case of the subset where activity data are available in PubChem. In the second, a library of about 5000 commercially available compounds (AldrichCPR) was virtually screened for ability to inhibit β-haematin formation and then for in vitro antimalarial activity. A selection of 34 compounds was purchased and tested, of which 24 were predicted to be β-haematin inhibitors. The hit rate for inhibition of β-haematin formation was found to be 25% and a third of these were active against P. falciparum, corresponding to enrichments estimated at about 25- and 140-fold relative to random screening, respectively.

Kojic acid derived hydroxypyridinone–chloroquine hybrids: Synthesis, crystal structure, antiplasmodial activity and β-haematin inhibition

Aminochloroquinoline–kojic acid hybrids were synthesized and evaluated for β-haematin inhibition and antiplasmodial activity against drug resistant (K1) and sensitive (3D7) strains of Plasmodium falciparum. Compound 7j was the most potent compound in both strains (IC503D7=0.004μM; IC50K1=0.03μM) and had the best β-haematin inhibition activity (0.07 IC50 equiv vs 1.91 IC50 equiv for chloroquine). One compound 8c was found to be equipotent in both strains (IC50=0.04μM).

Synthesis, β-haematin inhibition, and in vitro antimalarial testing of isocryptolepine analogues: SAR study of indolo[3,2-c]quinolines with various substituents at C2, C6, and N11

A series of indolo[3,2-c]quinolines were synthesized by modifying the side chains of the ω-aminoalkylamines at the C6 position and introducing substituents at the C2 position, such as F, Cl, Br, Me, MeO and NO2, and a methyl group at the N11 position for an SAR study. The in vitro antiplasmodial activities of the derivative agents against two different strains (CQS: NF54 and CQR: K1) and the cytotoxic activity against normal L6 cells were evaluated. The test results showed that compounds 6k and 6l containing the branched methyl groups of 3-aminopropylamino at C6 with a Cl atom at C2 exhibited a very low cytotoxicity with IC50 values above 4000nM, high antimalarial activities with IC50 values of about 11nM for CQS (NF54), IC50 values of about 17nM for CQR (K1), and RI resistance indices of 1.6. Furthermore, the compounds were tested for β-haematic inhibition, and QSAR revealed an interesting linear correlation between the biological activity of CQS (NF54) and three contributing factors, namely solubility, hydrophilic surface area, and β-haematin inhibition for this series. In vivo testing of 6l showed a reduction in parasitaemia on day 4 with an activity of 38%.

In vitro antimalarial activity, β-haematin inhibition and structure–activity relationships in a series of quinoline triazoles

A novel series of quinoline triazole amide analogues (38–51) has been synthesized. Analogues 38–44 had a Cl substituent at the 7-position of the quinoline ring, while 45–51 had a CN substituent at this position. Compounds 40, 45 and 49 were found to be the most active in the series against the Plasmodium falciparum chloroquine-sensitive D10 strain, with IC50 values in the range of 349–1247 nM, with 40 and 45, but not 49 also exhibiting similar activity against the chloroquine-resistant K1 strain of parasite. Quinoline triazoles 40 and 44 were the most active β-haematin inhibitors, with 50% inhibitory concentrations of 14.7 and 8.9 μM respectively. In vitro antimalarial activity of the 7-Cl bearing analogues 38–44 exhibited a strong linear dependence of log(1/IC50) on log P. Thus, the more lipophilic, the more active it was found be. The 7-CN series 45–51 showed no such dependence. The resistance index (IC50 K1/IC50 D10) also exhibited a linear dependence on log P, with a substantially steeper slope in the case of the 7-Cl series. The findings demonstrate the feasibility of producing hydrophilic analogues with strong activity and low cross-resistance with chloroquine.

Synthesis and antimalarial testing of neocryptolepine analogues: Addition of ester function in SAR study of 2,11-disubstituted indolo[2,3-b]quinolines

This report describes the synthesis, and in vitro and in vivo antimalarial evaluations of certain ester-modified neocryptolepine (5-methyl-5H-indolo[2,3-b]quinoline) derivatives. The modifications were carried out by introducing ester groups at the C2 and/or C9 position on the neocryptolepine core and the terminal amino group of the 3-aminopropylamine substituents at the C11 position with a urea/thiourea unit. The antiplasmodial activities of our derivative agents against two different strains (CQS: NF54, and CQR: K1) and the cytotoxic activity against normal L6 cells were evaluated. The test results showed that the ester modified neocryptolepine derivatives have higher antiplasmodial activities against both strains and a low cytotoxic activity against normal cells. The best results were achieved by compounds 9c and 12b against the NF54 strain with the IC50/SI value as 2.27 nM/361 and 1.81 nM/321, respectively. While against K1 strain, all the tested compounds showed higher activity than the well-known antimalarial drug chloroquine. Furthermore, the compounds were tested for β-haematin inhibition and 12 were found to be more active than chloroquine (IC50 = 18 μM). Structure activity relationship studies exposed an interesting linear correlation between polar surface area of the molecule and β-haematin inhibition for this series. In vivo testing of compounds 7 and 8a against NF54 strain on Plasmodium berghei female mice showed that the introduction of the ester group increased the antiplasmodial activity of the neocryptolepine core substantially.

Synthesis and <i>in Vitro</i> Testing of Antimalarial Activity of Non-natural-Type Neocryptolepines: Structure–Activity Relationship Study of 2,11- and 9,11-Disubstituted 6-Methylindolo[2,3-<i>b</i>]quinolines

This report describes the synthesis and in vitro anti-malarial evaluations of certain C2 or C8 and C11-disubstituted 6-methyl-5H-indolo[2,3-b]quinoline (neocryptolepine congener) derivatives. To attain higher activities, the structure–activity relationship (SAR) studies were conducted by varying the kind of alkylamino or ω-aminoalkylamino stubstituents at C11 and with Cl at the C2 position, or CO2Me at the C9 position. The anti-malarial activities of the tested compounds were significantly increased compared to the 11-non(alkylamino) derivatives. The 3-aminopropylamino group at C11 was further modified to urea and thiourea, which improved the cytotoxicity against normal cells. The best results were achieved with compounds 8 and 9d against the NF54 strain with the IC(50)/SI values as of 86 nM/20 and 317 nM/370, respectively. Furthermore, the compounds were tested for β-haematin inhibition. Twelve were found to have IC(50) values below 100 µM and a linear correlation between the β-haematin inhibition and cell growth inhibition in the NF54 strain was found for those derivatives with basic amino side chains. A second correlation was identified between the NF54 activity and physico-chemical factors related to solvation and polarity.

Amino acid, dipeptide and pseudodipeptide conjugates of ring-substituted 8-aminoquinolines: Synthesis and evaluation of anti-infective, β-haematin inhibition and cytotoxic activities

Three new series of 8-aminoquinolines with modifications in the side-chain by conjugation with amino acids, dipeptides and pseudodipeptides have been synthesized. The synthesized compounds were tested for in vitro antimalarial activity against chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum strains, in vitro cytotoxicity in mammalian kidney cells (Vero), in vitro antileishmanial activity against Leishmania donovani, in vitro antimicrobial activity and in vitro inhibition of β-haematin formation. The promising compounds were also evaluated for in vivo blood-schizontocidal antimalarial activity against Plasmodium berghei infected mice. The analogues 55 and 101 produced highest antimalarial activities, in vitro. Analogues 52 and 59 exhibited promising antileishmanial and broad spectrum of antifungal activities, respectively.

Activity of piperaquine and other 4-aminoquinoline antiplasmodial drugs against chloroquine-sensitive and resistant blood-stages of Plasmodium falciparum: Role of β-haematin inhibition and drug concentration in vacuolar water- and lipid-phases

Chloroquine (CQ), a 4-aminoquinoline, accumulates in acidic digestive vacuoles of the malaria parasite, preventing conversion of toxic haematin to β-haematin. We examine how bis 4-aminoquinoline piperaquine (PQ) and its hydroxy-modification (OH-PQ) retain potency on chloroquine-resistant (CQ-R) Plasmodium falciparum. For CQ, PQ, OH-PQ and 4 and 5, representing halves of PQ, β-haematin inhibitory activity (BHIA) was assayed, while potency was determined in CQ-sensitive (CQ-S) and CQ-R P. falciparum. From measured p K as and the pH-modulated distribution of base between water and lipid (log D), the vacuolar accumulation ratio (VAR) of charged drug from plasma water (pH 7.4) into vacuolar water (pH 4.8) and lipid accumulation ratio (LAR) were calculated. All agents were active in BHIA. In CQ-S, PQ, OH-PQ and CQ were equally potent while 4 and 5 were 100 times less potent. CQ with two basic centres has a VAR of 143,482, while 4 and 5, with two basic centres of lower p K as have VARs of 1287 and 1966. In contrast PQ and OH-PQ have four basic centres and achieve VARs of 104,378 and 19,874. This confirms the importance of VAR for potency against CQ-S parasites. Contrasting results were seen in CQ-R. 5, PQ and OH-PQ with LARs of 693; 973,492 and 398,118 (compared with 8.25 for CQ) showed similar potency in CQ-S and CQ-R. Importance of LAR for potency against CQ-R parasites probably reflects ability to block efflux by hydrophobic interaction with PfCRT but may relate to β-haematin inhibition in vacuolar lipid.