Structure-function relationships in aminoquinolines: effect of amino and chloro groups on quinoline-hematin complex formation, inhibition of beta-hematin formation, and antiplasmodial activity

Acetic Acid-Driven One-Pot Synthesis of 4,7-dihydro-[1,2,3]thiadiazolo[5,4-b]pyridine-6-carboxamides and Pharmacological Evaluations

A diverse set of 4,7-dihydro-[1,2,3]thiadiazolo[5,4-b]pyridine-6-carboxamides 4(a-o) was synthesized via a one-pot reaction of 5-amino-[1,2,3]thiadiazole, various aromatic aldehydes, and different acetoacetanilides, using glacial acetic acid. The resulting compounds were obtained in moderate to good yields. All the newly synthesized compounds were evaluated for their antimicrobial activity. Among them, compound 4 e demonstrated superior efficacy against the Salinivibrio proteolyticus strain of Gram-negative bacteria compared to ciprofloxacin. Compound 4 d exhibited the highest potency against the fungal strain Candida albicans, surpassing amphotericin B. The physicochemical characteristics of 4 d and 4 e were assessed. According to docking analysis, DHTDAPy 4 e shows a higher binding affinity of -7.2 kcal/mol in the binding cavity of the receptor. These findings illustrate the safety, tolerability, and potency of the newly synthesized DHTDAPy compounds against fungal and bacterial infections.

Exploring Quinoline Derivatives: Their Antimalarial Efficacy and Structural Features

OBJECTIVES

Malaria continues to be the primary cause of mortality worldwide, and timely recognition and prompt intervention are crucial in mitigating adverse consequences. This review article aims to examine the effectiveness and structural characteristics of quinoline-based compounds as antimalarial agents. It specifically focuses on their therapeutic effects as well as potential prospects for exploring structure-activity relationship (SAR). In addition, this study aims to identify lead compounds that can efficiently battle multidrug-resistant forms of Plasmodium falciparum and Plasmodium vivax.

METHODS

A comprehensive review was conducted to evaluate the effectiveness of quinoline-based antimalarial medications in eradicating P. falciparum and P. vivax. The mechanism of action and SAR of these compounds were analyzed.

RESULTS

Quinoline-based antimalarials demonstrated significant effectiveness in eliminating P. falciparum parasites, particularly in regions severely impacted by malaria, including Africa and Asia. These compounds were found to exhibit tolerance and immune-modulating properties, indicating their potential for more widespread utilization. The investigation identified various new quinoline compounds with improved antimalarial activity, including metal-chloroquine complexes, diaminealkyne chloroquines, and cinnamoylated chloroquine hybrids. This study explored different mechanisms by which these compounds interact with parasites, including their ability to accumulate in the parasite's acidic food vacuoles and disrupt heme detoxification. The derivatives demonstrated strong efficacy against chloroquine-resistant strains and yielded positive results.

CONCLUSION

Quinoline-based compounds represent a promising avenue for combating malaria due to their demonstrated efficacy against P. falciparum and P. vivax parasites. Further research on their mechanisms of action and SAR could lead to the development of more effective antimalarial medications.

Tetrahydropyridine appended 8-aminoquinoline derivatives: Design, synthesis, in silico, and in vitro antimalarial studies

Antimalarial drug resistance is a major obstacle in the ongoing quest against malaria. The disease affects half of the world's population. The majority of them are toddlers and pregnant women. Needed a potent compound to act on drug-resistant Pf at appropriate concentrations without endangering the host. Envisaged solving this issue through rational drug design by creating a novel hybrid drug possessing two pharmacophores that can act on two marvellous and independent aims within the cell. Synthesized a new series of substituted 4-phenyl-1,2,3,6-tetrahydropyridine (THP) 8-Aminoquinoline-based hybrid analogs which have been integrated with quinoline, chloroquine, pamaquine, and primaquine, which exhibited antimalarial activity against Pf. Out of thirteen 4-phenyl-1,2,3,6-THP appended 8-Aminoquinoline derivatives, the compounds 1j, 1e, 1b, and 1l have exhibited good antimalarial activity against chloroquine-sensitive (3D7) and chloroquine-resistant (RKL-9) strain with the minimum inhibitory concentration. Compound 1b was the most effective and showed consistently good potency against the drug-resistant (RKL-9) strain, although all other arrays showed good antimalarial efficacy. Additional docking and molecular dynamics studies were carried out at several targeting sites to quantify the structural parameters necessary for the activity.

A novel 4-aminoquinoline chemotype with multistage antimalarial activity and lack of cross-resistance with PfCRT and PfMDR1 mutants

Artemisinin-based combination therapy (ACT) is the mainstay of effective treatment of Plasmodium falciparum malaria. However, the long-term utility of ACTs is imperiled by widespread partial artemisinin resistance in Southeast Asia and its recent emergence in parts of East Africa. This underscores the need to identify chemotypes with new modes of action (MoAs) to circumvent resistance to ACTs. In this study, we characterized the asexual blood stage antiplasmodial activity and resistance mechanisms of LDT-623, a 4-aminoquinoline (4-AQ). We also detected LDT-623 activity against multiple stages (liver schizonts, stage IV-V gametocytes, and ookinetes) of Plasmodium's life cycle, a feature unlike other 4-AQs such as chloroquine (CQ) and piperaquine (PPQ). Using heme fractionation profiling and drug uptake studies in PfCRT-containing proteoliposomes, we observed inhibition of hemozoin formation and PfCRT-mediated transport, which constitute characteristic features of 4-AQs' MoA. We also found minimal cross-resistance to LDT-623 in a panel of mutant pfcrt or pfmdr1 lines, but not the PfCRT F145I mutant that is highly resistant to PPQ resistance yet is very unfit. No P. falciparum parasites were recovered in an in vitro resistance selection study, suggesting a high barrier for resistance to emerge. Finally, a competitive growth assay comprising >50 barcoded parasite lines with mutated resistance mediators or major drug targets found no evidence of cross-resistance. Our findings support further exploration of this promising 4-AQ.

Synthesis and Anti-Trypanosoma cruzi Activity of New Pyrazole-Thiadiazole Scaffolds

Chagas disease, a silent but widespread disease that mainly affects a socioeconomically vulnerable population, lacks innovative safe drug therapy. The available drugs, benznidazole and nifurtimox, are more than fifty years old, have limited efficacy, and carry harmful side effects, highlighting the need for new therapeutics. This study presents two new series of pyrazole-thiadiazole compounds evaluated for trypanocidal activity using cellular models predictive of efficacy. Derivatives 1c (2,4-diCl) and 2k (4-NO2) were the most active against intracellular amastigotes. Derivative 1c also showed activity against trypomastigotes, with the detachment of the flagellum from the parasite body being a predominant effect at the ultrastructural level. Analogs have favorable physicochemical parameters and are predicted to be orally available. Drug efficacy was also evaluated in 3D cardiac microtissue, an important target tissue of Trypanosoma cruzi, with derivative 2k showing potent antiparasitic activity and a significant reduction in parasite load. Although 2k potentially reduced parasite load in the washout assay, it did not prevent parasite recrudescence. Drug combination analysis revealed an additive profile, which may lead to favorable clinical outcomes. Our data demonstrate the antiparasitic activity of pyrazole-thiadiazole derivatives and support the development of these compounds using new optimization strategies.

Two new Nb-oxide indole alkaloids with β-hematin inhibitory activity from the stems and leaves of Rauvolfia dichotoma

Rauvolfia dichotoma, a shrub of Apocynaceae, was collected from the Islands of SAO Tome and Principe and cultivated locally for medicinal purpose. Phytochemical investigation of 95% ethanol extract from the stems and leaves of R. dichotoma led to the isolation of two new Nb-oxide indole alkaloids, namely Nb-oxide-mitoridine (1) and Nb-oxide-raucaffricine (2), together with two known alkaloids (3-4) and eleven known lignans (5-15). Their chemical structures were elucidated by extensive NMR and HR-ESI-MS data analysis. All compounds (except 13) were tested for their β-hematin inhibitory activity. Compounds 2, 4, 14, and 15 showed certain inhibitory activity, indicating that they may have an antimalarial effect.

Redox modulating small molecules having antimalarial efficacy

The search for effective antimalarial agents remains a critical priority because malaria is widely spread and drug-resistant strains are becoming more prevalent. In this review, a variety of small molecules capable of modulating redox processes were showcased for their potential as antimalarial agents. The compounds were designed to target the redox balance of Plasmodium parasites, which has a pivotal function in their ability to survive and multiply within the host organism. A thorough screening method was utilized to assess the effectiveness of these compounds against both drug-sensitive and drug-resistant strains of Plasmodium falciparum, the malaria-causing parasite. The results revealed that several of the tested compounds exhibited significant effectiveness against malaria, displaying IC50 values at a low micromolar range. Furthermore, these compounds displayed promising selectivity for the parasite, as they exhibited low cytotoxicity towards mammalian cells. Thorough mechanistic studies were undertaken to clarify how the active compounds exert their mode of action. The findings revealed that these compounds disrupted the parasites' redox balance, causing oxidative stress and interfering with essential cellular functions. Additionally, the compounds showed synergistic effects when combined with existing antimalarial drugs, suggesting their potential for combination therapies to combat drug resistance. Overall, this study highlights the potential of redox-modulating small molecules as effective antimalarial agents. The identified compounds demonstrate promising antimalarial activity, and their mechanism of action offers insights into targeting the redox balance of Plasmodium parasites. Further optimization and preclinical studies are warranted to determine their efficacy, safety, and potential for clinical development as novel antimalarial therapeutics.

Recent developments in the synthesis of hybrid antimalarial drug discovery

In this 21st century, Malaria remains a global burden and causes massive economic trouble to disease-endemic nations. The control and eradication of malaria is a major challenge that requires an urgent need to develop novel antimalarial drugs. To overcome the aforementioned situation, several researchers have given significant effort to develop hybrid antimalarial agents in the search for new antimalarial drugs. Hence, we have summarized those developments of hybrid antimalarial agents from 2017 to till date. This review illustrates the current progress in the recent synthesis of hybrid antimalarial agents along with focusing on their antimalarial evaluation to find the most potent hybrids. This present mini-review will also be useful for the scientific community for the development of new antimalarial drugs to eradicate malaria.

Is structural hybridization invoking new dimensions for antimalarial drug discovery research?

Despite effective prevention methods, malaria is a devastating, persistent infection caused by protozoal parasites that result in nearly half a million fatalities annually. Any progress made thus far in the eradication of the disease is jeopardized by the expansion of malaria parasites that have evolved to become resistant to a wide range of drugs, including first-line therapy. To surmount this significant obstacle, it is necessary to develop newly synthesized drugs with multiple modes of action that may have a novel target in various stages of Plasmodium parasite development and this is made possible by the hybridization concept. Hybridization is the combination of at least two diverse pharmacophore units with some linkers bringing about a single molecule with a diverse mode of action. It intensifies a drug's physiological and chemical characteristics, such as absorption, cellular target contact, metabolism, excretion, distribution, and toxicity. This review article outlines the currently published most potent hybrid drugs against the Plasmodium species.

"Magic Chloro": Profound Effects of the Chlorine Atom in Drug Discovery

Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen, and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The seemingly simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in remarkable improvements in potency of up to 100,000-fold and can lead to profound effects on pharmacokinetic parameters including clearance, half-life, and drug exposure in vivo. Following the literature terminology of the "magic methyl effect" in drugs, the term "magic chloro effect" has been coined herein. Although reports of 500-fold or 1000-fold potency improvements are often serendipitous discoveries that can be considered "magical" rather than planned, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery.

Quinoline-based heterocyclic hydrazones: Design, synthesis, anti-plasmodial assessment, and mechanistic insights

A library of quinoline-based hydrazones bearing 1H-1,2,3-triazole core was designed, synthesized, and evaluated for their antiplasmodial activity against the drug-resistant Plasmodium falciparum W2 strain. The inclusion of pyrazine-2-carboxylic acid with a flexible propyl spacer afforded the most active scaffold with an IC₅₀ value of 0.26 μM. Mechanistically, the compound inhibited heme to hemozoin formation, as demonstrated by UV-vis and mass spectral studies.

Discovery of potent Plasmodium falciparum protein kinase 6 (PfPK6) inhibitors with a type II inhibitor pharmacophore

Malaria is a devastating disease that causes significant global morbidity and mortality. The rise of drug resistance against artemisinin-based combination therapy demonstrates the necessity to develop alternative antimalarials with novel mechanisms of action. We report the discovery of Ki8751 as an inhibitor of essential kinase PfPK6. 79 derivatives were designed, synthesized and evaluated for PfPK6 inhibition and antiplasmodial activity. Using group efficiency analyses, we established the importance of key groups on the scaffold consistent with a type II inhibitor pharmacophore. We highlight modifications on the tail group that contribute to antiplasmodial activity, cumulating in the discovery of compound 67, a PfPK6 inhibitor (IC50 = 13 nM) active against the P. falciparum blood stage (EC50 = 160 nM), and compound 79, a PfPK6 inhibitor (IC50 < 5 nM) with dual-stage antiplasmodial activity against P. falciparum blood stage (EC50 = 39 nM) and against P. berghei liver stage (EC50 = 220 nM).

New Iron Twist to Chloroquine─Upgrading Antimalarials with Immunomodulatory and Antimicrobial Features

Herein, upgraded chloroquine (CQ) derivatives capable of overcoming Plasmodium resistance and, at the same time, suppressing excessive immune response and risk of concurrent bacteremia were developed. Twelve new ferrocene-CQ hybrids tethered with a small azathia heterocycle (1,3-thiazolidin-4-one, 1,3-thiazinan-4-one, or 5-methyl-1,3-thiazolidin-4-one) were synthesized and fully characterized. All hybrids were evaluated for their in vitro antiplasmodial, antimicrobial, and immunomodulatory activities. Additional assays were performed on selected hybrids to gain insights into their mode of action. Although only hybrid 4a was more potent than the parent drug toward CQ-resistant Dd2 Plasmodium falciparum strain, several other hybrids (such as 6b, 6c, and 6d) manifested substantially improved antimicrobial and immunomodulatory properties. Interesting structure-activity relationship data were obtained, hinting at future research for the development of new multitarget chemotherapies for malaria and other infectious diseases complicated by drug resistance, bacterial co-infection, and immune-driven pathology issues.

Recent contributions of quinolines to antimalarial and anticancer drug discovery research

Quinoline, a privileged scaffold in medicinal chemistry, has always been associated with a multitude of biological activities. Especially in antimalarial and anticancer research, quinoline played (and still plays) a central role, giving rise to the development of an array of quinoline-containing pharmaceuticals in these therapeutic areas. However, both diseases still affect millions of people every year, pointing to the necessity of new therapies. Quinolines have a long-standing history as antimalarial agents, but established quinoline-containing antimalarial drugs are now facing widespread resistance of the Plasmodium parasite. Nevertheless, as evidenced by a massive number of recent literature contributions, they are still of great value for future developments in this field. On the other hand, the number of currently approved anticancer drugs containing a quinoline scaffold are limited, but a strong increase and interest in quinoline compounds as potential anticancer agents can be seen in the last few years. In this review, a literature overview of recent contributions made by quinoline-containing compounds as potent antimalarial or anticancer agents is provided, covering publications between 2018 and 2020.

Investigating Structural Property Relationships to Enable Repurposing of Pharmaceuticals as Zinc Ionophores

The importance of zinc in biology has gained greater recognition in recent years due to its essential contributions to the function of many endogenous enzymes. Disruption of zinc homeostasis may be useful in treating pathological conditions, such as Alzheimer's, and for antiviral purposes. Despite the growth of knowledge and increased interest in zinc, little is known about the structure and function of zinc ionophores. In this study we analyse the Cambridge Structural Database and solution complexation studies found in the literature to identify key functional groups which may confer zinc ionophorism. Pharmaceuticals, nutraceuticals and amino acids with these functionalities were selected to enable us to explore the translatability of ionophoric activity from in vitro assays to cellular systems. We find that although certain species may complex to zinc in the solid and solution states, and may carry ions across simple membrane systems, this does not necessarily translate into ionophoric activity. We propose that the CSD can help refine key functionalities but that ionophoric activity must be confirmed in cellular systems.

Heme Detoxification in the Malaria Parasite: A Target for Antimalarial Drug Development

Over the last century, malaria deaths have decreased by more than 85%. Nonetheless, there were 405 000 deaths in 2018, mostly resulting from Plasmodium falciparum infection. In the 21st century, much of the advance has arisen from the deployment of insecticide-treated bed nets and artemisinin combination therapy. However, over the past few decades parasites with a delayed artemisinin clearance phenotype have appeared in Southeast Asia, threatening further gains. The effort to find new drugs is thus urgent. A prominent process in blood stage malaria parasites, which we contend remains a viable drug target, is hemozoin formation. This crystalline material consisting of heme can be readily seen when parasites are viewed microscopically. The process of its formation in the parasite, however, is still not fully understood.In early work, we recognized hemozoin formation as a biomineralization process. We have subsequently investigated the kinetics of synthetic hemozoin (β-hematin) crystallization catalyzed at lipid-aqueous interfaces under biomimetic conditions. This led us to the use of neutral detergent-based high-throughput screening (HTS) for inhibitors of β-hematin formation. A good hit rate against malaria parasites was obtained. Simultaneously, we developed a pyridine-based assay which proved successful in measuring the concentrations of hematin not converted to β-hematin.The pyridine assay was adapted to determine the effects of chloroquine and other clinical antimalarials on hemozoin formation in the cell. This permitted the determination of the dose-dependent amounts of exchangeable heme and hemozoin in P. falciparum for the first time. These studies have shown that hemozoin inhibitors cause a dose-dependent increase in exchangeable heme, correlated with decreased parasite survival. Electron spectroscopic imaging (ESI) showed a relocation of heme iron into the parasite cytoplasm, while electron microscopy provided evidence of the disruption of hemozoin crystals. This cellular assay was subsequently extended to top-ranked hits from a wide range of scaffolds found by HTS. Intriguingly, the amounts of exchangeable heme at the parasite growth IC₅₀ values of these scaffolds showed substantial variation. The amount of exchangeable heme was found to be correlated with the amount of inhibitor accumulated in the parasitized red blood cell. This suggests that heme-inhibitor complexes, rather than free heme, lead to parasite death. This was supported by ESI using a Br-containing compound which showed the colocalization of Fe and Br as well as by confocal Raman microscopy which confirmed the presence of a complex in the parasite. Current evidence indicates that inhibitors block hemozoin formation by surface adsorption. Indeed, we have successfully introduced molecular docking with hemozoin to find new inhibitors. It follows that the resulting increase in free heme leads to the formation of the parasiticidal heme-inhibitor complex. We have reported crystal structures of heme-drug complexes for several aryl methanol antimalarials in nonaqueous media. These form coordination complexes but most other inhibitors interact noncovalently, and the determination of their structures remains a major challenge.It is our view that key future developments will include improved assays to measure cellular heme levels, better in silico approaches for predicting β-hematin inhibition, and a concerted effort to determine the structure and properties of heme-inhibitor complexes.

Synthesis and Antimalarial Activity of 4-Methylaminoquinoline Compounds against Drug-Resistant Parasite

A series of novel 4-aminoquinoline analogues bearing a methyl group at 4-aminoquinoline moiety were synthesized via a new and robust synthetic route comprising in situ tert-butoxycarbonyl (Boc) deprotection-methylation cascade resulting in the corresponding N-methylated secondary amine using Red-Al and an efficient microwave-assisted strategy for the fusion of N-methylated secondary amine with 4-chloroquinoline nucleus to access the series of novel 4-N-methylaminoquinoline analogues. The new series of compounds were evaluated for their antimalarial activity in in vitro and in vivo models. Among 21 tested compounds, 9a-i have shown a half-maximal inhibitory concentration (IC₅₀) value less than 0.5 μM (i.e., <500 nM) against both chloroquine-sensitive strain 3D7 and chloroquine-resistant strain K1 of Plasmodium falciparum with acceptable cytotoxicity. Based on the in vitro antimalarial activity, selected compounds were screened for their in vivo antimalarial activity against Plasmodium yoelii nigeriensis (a multidrug-resistant) parasite in Swiss mice. Most of the compounds have shown significant inhibition on day 4 post infection at the oral dose of 100 mg/kg. Compound 9a has shown 100% parasite inhibition on day 4, and out of five treated mice, two were cured till the end of the experiment. The present study suggests that 4-methylamino substitution is well tolerated for the antiplasmodial activity with reduced toxicity and therefore will be highly useful for the discovery of a new antimalarial agent against drug-resistant malaria.

Fluorescence Response and Self-Assembly of a Tweezer-Type Synthetic Receptor Triggered by Complexation with Heme and Its Catabolites

There is increasing interest in the development and applications of synthetic receptors that recognize target biomolecules in aqueous media. We have developed a new tweezer-type synthetic receptor that gives a significant fluorescence response upon complexation with heme in aqueous solution at pH 7.4. The synthetic receptor consists of a tweezer-type heme recognition site and sulfo-Cy5 as a hydrophilic fluorophore. The receptor-heme complex exhibits a supramolecular amphiphilic character that facilitates the formation of self-assembled aggregates, and both the tweezer moiety and the sulfo-Cy5 moiety are important for this property. The synthetic receptor also exhibits significant fluorescence responses to biliverdin and bilirubin, but shows very weak fluorescence responses to flavin mononucleotide, folic acid, and nicotinamide adenine dinucleotide, which contain smaller π-scaffolds.

Physicochemical Profiling and Comparison of Research Antiplasmodials and Advanced Stage Antimalarials with Oral Drugs

To understand the property space of antimalarials, we collated a large dataset of research antiplasmodial (RAP) molecules with known in vitro potencies and advanced stage antimalarials (ASAMs) with established oral bioavailability. While RAP molecules are "non-druglike", ASAM molecules display properties closer to Lipinski's and Veber's thresholds. Comparison within the different potency groups of RAP molecules indicates that the in vitro potency is positively correlated to the molecular weight, the calculated octanol-water partition coefficient (clog P), aromatic ring counts (#Ar), and hydrogen bond acceptors. Despite both categories being bioavailable, the ASAM molecules are relatively larger and more lipophilic, have a lower polar surface area, and possess a higher count of heteroaromatic rings than oral drugs. Also, antimalarials are found to have a higher proportion of aromatic (#ArN) and basic nitrogen (#BaN) counts, features implicitly used in the design of antimalarial molecules but not well studied hitherto. We also propose using descriptors scaled by the sum of #ArN and #BaN (SBAN) to define an antimalarial property space. Together, these results may have important applications in the identification and optimization of future antimalarials.

Novel Antimalarial Tetrazoles and Amides Active against the Hemoglobin Degradation Pathway in Plasmodium falciparum

Malaria control programs continue to be threatened by drug resistance. To identify new antimalarials, we conducted a phenotypic screen and identified a novel tetrazole-based series that shows fast-kill kinetics and a relatively low propensity to develop high-level resistance. Preliminary structure-activity relationships were established including identification of a subseries of related amides with antiplasmodial activity. Assaying parasites with resistance to antimalarials led us to test whether the series had a similar mechanism of action to chloroquine (CQ). Treatment of synchronized Plasmodium falciparum parasites with active analogues revealed a pattern of intracellular inhibition of hemozoin (Hz) formation reminiscent of CQ's action. Drug selections yielded only modest resistance that was associated with amplification of the multidrug resistance gene 1 (pfmdr1). Thus, we have identified a novel chemical series that targets the historically druggable heme polymerization pathway and that can form the basis of future optimization efforts to develop a new malaria treatment.

Synthesis and antiplasmodial evaluation of 1H-1,2,3-triazole grafted 4-aminoquinoline-benzoxaborole hybrids and benzoxaborole analogues

A library of 1H-1,2,3-triazole-tethered 4-aminoquinoline-benzoxaborole hybrids as well as aryl substituted benzoxaborole analogues was synthesized and screened for their anti-plasmodial efficacy against both chloroquine-susceptibility 3D7 and chloroquine-resistant W2 strains of P. falciparum. The inclusion of quinoline core among the synthesized analogues resulted in substantial enhancement of anti-plasmodial activities. Further, the spacer of a flexible alkyl chain is marginally preferred over piperazyl-ethyl in inhibiting growth of P. falciparum. The most potent 4-aminoquinoline-benzoxaborole conjugate with ethyl as spacer exhibited IC₅₀ values of 4.15 and 3.78 μM against 3D7 CQ-susceptible and W2 CQ-resistant strains of P. falciparum with lower cross resistance with Chloroquine. There was no difference in anti-plasmodial activities between the CQ-susceptible 3D7 and CQ-resistant W2 strains of P. falciparum for the benzoxaborole derivatives lacking a quinoline core.

Amide Tethered 4-Aminoquinoline-naphthalimide Hybrids: A New Class of Possible Dual Function Antiplasmodials

A series of amide tethered 4-aminoquinoline-naphthalimide hybrids has been synthesized to assess their in vitro antiplasmodial potential against chloroquine-susceptible (3D7) and chloroquine-resistant (W2) strains of Plasmodium falciparum. The most active and noncytotoxic compound had an IC₅₀ value of 0.07 μM against W2 strain and was more active than standard antimalarial drugs, including chloroquine, desethylamodiaquine, and quinine, particularly for drug resistant malaria. The promising scaffold, when subjected to heme binding and molecular modeling studies, was identified as a possible potent inhibitor of hemozoin formation and P. falciparum chloroquine resistance transporter (PfCRT), respectively, and, therefore, could act as a dual function antiplasmodial.

Discovery of fast-acting dual-stage antimalarial agents by profiling pyridylvinylquinoline chemical space via copper catalyzed azide-alkyne cycloadditions

To identity fast-acting, multistage antimalarial agents, a series of pyridylvinylquinoline-triazole analogues have been synthesized via CuAAC. Most of the compounds display significant inhibitory effect on the drug-resistant malarial Dd2 strain at low submicromolar concentrations. Among the tested analogues, compound 60 is the most potent molecule with an EC₅₀ value of 0.04 ± 0.01 μM. Our current study indicates that compound 60 is a fast-acting antimalarial compound and it demonstrates stage specific action at the trophozoite phase in the P. falciparum asexual life cycle. In addition, compound 60 is active against both early and late stage P. falciparum gametocytes. From a mechanistic perspective, compound 60 shows good activity as an inhibitor of β-hematin formation. Collectively, our findings suggest that fast-acting agent 60 targets dual life stages of the malarial parasites and warrant further investigation of pyridylvinylquinoline hybrids as new antimalarials.

Aminoquinolines afford resistance to cerebral malaria in susceptible mice

OBJECTIVES

Malaria treatment is impeded by increasing resistance to conventional antimalarial drugs. Here we explored the activity of ten novel benzothiophene, thiophene and benzene aminoquinolines.

METHODS

In vitro testing was performed by the lactate dehydrogenase assay in chloroquine (CQ)-sensitive Plasmodium falciparum strain 3D7 and CQ-resistant (CQ^R) P. falciparum strain Dd2. In vivo activity was evaluated by a modified Thompson test using C57BL/6 mice infected with Plasmodium berghei ANKA strain.

RESULTS

Nine of the ten compounds had a lower 50% inhibitory concentration (IC₅₀) than CQ against the CQ^R strain Dd2. Five of these compounds that were available for in vivo evaluation were shown to be non-toxic. All five compounds administered at a dose of 160mg/kg/day for 3 days prolonged the survival of treated compared with untreated mice. Untreated control mice died by Day 7 with a mean parasitaemia of 15%. Among treated mice, a dichotomous outcome was observed, with a two-third majority of treated mice dying by Day 17 with a low mean parasitaemia of 5%, whilst one-third survived longer with a mean hyperparasitaemia of 70%; specifically, five of these mice survived a mean of 25 days, whilst two even survived past Day 31.

CONCLUSIONS

The significant antimalarial potential of this aminoquinoline series is illustrated by its excellent in vitro activity against the CQ^RP. falciparum strain and significant in vivo activity. Interestingly, compounds ClAQ7, ClAQ9 and ClAQ11 were able to confer resistance to cerebral malaria and afford a switch to hyperparasitaemia to mice prone to the neurological syndrome.

Synthesis and biological evaluation of novel quinoline-piperidine scaffolds as antiplasmodium agents

The parasitic disease malaria places almost half of the world's population at risk of infection and is responsible for more than 400,000 deaths each year. The first-line treatment, artemisinin combination therapies (ACT) regimen, is under threat due to emerging resistance of Plasmodium falciparum strains in e.g. the Mekong delta. Therefore, the development of new antimalarial agents is crucial in order to circumvent the growing resistance. Chloroquine, the long-established antimalarial drug, still serves as model compound for the design of new quinoline analogues, resulting in numerous new active derivatives against chloroquine-resistant P. falciparum strains over the past twenty years. In this work, a set of functionalized quinoline analogues, decorated with a modified piperidine-containing side chain, was synthesized. Both amino- and (aminomethyl)quinolines were prepared, resulting in a total of 18 novel quinoline-piperidine conjugates representing four different chemical series. Evaluation of their in vitro antiplasmodium activity against a CQ-sensitive (NF54) and a CQ-resistant (K1) strain of P. falciparum unveiled highly potent activities in the nanomolar range against both strains for five 4-aminoquinoline derivatives. Moreover, no cytotoxicity was observed for all active compounds at the maximum concentration tested. These five new aminoquinoline hit structures are therefore of considerable value for antimalarial research and have the potency to be transformed into novel antimalarial agents upon further hit-to-lead optimization studies.

Unsymmetrical Bisquinolines with High Potency against P. falciparum Malaria

Quinoline-based scaffolds have been the mainstay of antimalarial drugs, including many artemisinin combination therapies (ACTs), over the history of modern drug development. Although much progress has been made in the search for novel antimalarial scaffolds, it may be that quinolines will remain useful, especially if very potent compounds from this class are discovered. We report here the results of a structure-activity relationship (SAR) study assessing potential unsymmetrical bisquinoline antiplasmodial drug candidates using in vitro activity against intact parasites in cell culture. Many unsymmetrical bisquinolines were found to be highly potent against both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum parasites. Further work to develop such compounds could focus on minimizing toxicities in order to find suitable candidates for clinical evaluation.

Functionalized Naphthalimide-4-aminoquinoline Conjugates as Promising Antiplasmodials, with Mechanistic Insights

A series of 25 conjugates has been synthesized to evaluate their antiplasmodial potency and cytotoxicity against the chloroquine resistant (CQR) W2 strain of P. falciparum and Vero kidney cell lines, respectively. Most of the compounds showed IC₅₀ values in the lower nM range and proved to be many fold more active than chloroquine (CQ). The studies were extended to decipher modes of action using techniques including UV-vis absorption, NMR titrations, and mass spectrometry, and conclusions were strengthened by docking and density functional theory (DFT) simulations. The most active compound, with IC₅₀ 15 nM and selectivity index >4000, proved to be an interesting template for antimalarial drug discovery. To the best of our knowledge this is the first report of a potent naphthalimide based antiplasmodial conjugate.

[(7-chloroquinolin-4-yl)amino]acetophenones and their copper(II) derivatives: Synthesis, characterization, computational studies and antimalarial activity

The synthesis of the compounds [(7-chloroquinolin-4-yl)amino]acetophenones (4, 5) and their copper(II) complexes (4a, 5a) is reported. The compounds were characterized using a wide range of spectroscopic and spectrometric techniques, such as FTIR, UV-vis, NMR, EPR, ESI-CID-MS². The spectral results suggested that the ligand acted as chelating species coordinating the metal through the endocyclic nitrogen of the quinoline ring in both complexes, with general formulae expressed in two ways, according to the phase in which they are: [Cu(L)₂Cl₂] for solid phase and [Cu(L)₂][2Cl] for liquid phase. The EPR study of the Cu (II) complexes indicated a probable distorted tetrahedral coordination geometry. This result was confirmed by the calculated optimized structures at the DFT/B3LYP method with the 6-31G (d,p) basis set. The characterization of the fragmentation pattern of protonated free ligands was extended here to fragments as low as m/z 43, while for coordination complexes it extends to fragments at m/z 80 and m/z 111. The antimalarial activity of the compounds was determined through three different tests: inhibitory activity against in vitro growth of Plasmodium falciparum (W2), inhibition of hemozoin formation (β-hematin) and in vitro inhibitory activity against recombinant falcipain-2, where compound 5 showed considerable activity. However, the activity of free ligands against P. falciparum was increased by complexing with the Cu (II) metal ion. The values of the HOMO-LUMO energy gap of 3.847 eV (4a) and 3.932 eV (5a) were interpreted with high chemical activity and thus, could influence on biological activity. In both compounds, the total electron density surface mapped with electrostatic potential clearly revealed the presence of high negative charge on the Cu atom. Also, this study reported the molecular docking of free ligands (4, 5) using software package ArgusLab 4.0.1. The results revealed the importance of water molecules as interaction bridges through hydrogen bonds between free ligands and β-hematin; at the same time, the hypothesis that π-π interaction between quinoline derivatives and the electronic system of hematin governs the formation of adducts was confirmed.

TMSBr-Promoted Cascade Cyclization of ortho-Propynol Phenyl Azides for the Synthesis of 4-Bromo Quinolines and Its Applications

Difficult-to-access 4-bromo quinolines are constructed directly from easily prepared ortho-propynol phenyl azides using TMSBr as acid-promoter. The cascade transformation performs smoothly to generate desired products in moderate to excellent yields with good functional groups compatibility. Notably, TMSBr not only acted as an acid-promoter to initiate the reaction, and also as a nucleophile. In addition, 4-bromo quinolines as key intermediates could further undergo the coupling reactions or nucleophilic reactions to provide a variety of functionalized compounds with molecular diversity at C4 position of quinolines.

Antiplasmodial Activity and In Vivo Bio-Distribution of Chloroquine Molecules Released with a 4-(4-Ethynylphenyl)-Triazole Moiety from Organometallo-Cobalamins

We have explored the possibility of using organometallic derivatives of cobalamin as a scaffold for the delivery of the same antimalarial drug to both erythro- and hepatocytes. This hybrid molecule approach, intended as a possible tool for the development of multi-stage antimalarial agents, pivots on the preparation of azide-functionalized drugs which, after coupling to the vitamin, are released with a 4-(4-ethynylphenyl)-triazole functionality. Three chloroquine and one imidazolopiperazine derivative (based on the KAF156 structure) were selected as model drugs. One hybrid chloroquine conjugate was extensively studied via fluorescent labelling for in vitro and in vivo bio-distribution studies and gave proof-of-concept for the design. It showed no toxicity in vivo (zebrafish model) as well as no hepatotoxicity, no cardiotoxicity or developmental toxicity of the embryos. All 4-(4-ethynylphenyl)-triazole derivatives of chloroquine were equally active against chloroquine-resistant (CQR) and chloroquine-sensitive (CQS) Plasmodium falciparum strains.

Current scenario of artemisinin and its analogues for antimalarial activity

Human malaria, one of the most striking, reemerging infectious diseases, is caused by several types of Plasmodium parasites. Whilst advances have been made in lowering the numbers of cases and deaths, it is clear that a strategy based solely on disease control year on year, without reducing transmission and ultimately eradicating the parasite, is unsustainable. Natural products have served as a template for the design and development of antimalarial drugs currently in the clinic or in the development phase. Artemisinin combine potent, rapid antimalarial activity with a wide therapeutic index and an absence of clinically important resistance. The alkylating ability of artemisinin and its semi-synthetic analogues toward heme related to their antimalarial efficacy are underlined. Although impressive results have already been achieved in malaria research, more systematization and concentration of efforts are required if real breakthroughs are to be made. This review will concisely cover the clinical, preclinical antimalarial and current updates in artemisinin based antimalarial drugs. Diverse classes of semi-synthetic analogs of artemisinin reported in the last decade have also been extensively studied. The experience gained in this respect is discussed.

Chloroquine analogs as antimalarial candidates with potent in vitro and in vivo activity

In spite of recent efforts to eradicate malaria in the world, this parasitic disease is still considered a major public health problem, with a total of 216 million cases of malaria and 445,000 deaths in 2016. Artemisinin-based combination therapies remain effective in most parts of the world, but recent cases of resistance in Southeast Asia have urged for novel approaches to treat malaria caused by Plasmodium falciparum. In this work, we present chloroquine analogs that exhibited high activity against sensitive and chloroquine-resistant P. falciparum blood parasites and were also active against P. berghei infected mice. Among the compounds tested, DAQ, a chloroquine analog with a more linear side chain, was shown to be the most active in vitro and in vivo, with low cytotoxicity, and therefore may serve as the basis for the development of more effective chloroquine analogs to aid malaria eradication.

Chemical Proteomics and Super-resolution Imaging Reveal That Chloroquine Interacts with Plasmodium falciparum Multidrug Resistance-Associated Protein and Lipids

It is well established that chloroquine, a quinoline antimalarial, inhibits hemozoin formation in the malaria parasite. Counterintuitively, this archetypal antimalarial is also used in the treatment of diseases in which hemozoin biocrystallization does not play a role. Hence, we decided to investigate whether chloroquine possesses binding targets other than Fe(III) protoporphyrin IX in blood stage Plasmodium falciparum parasites and whether these are related to sites of accumulation within the parasite other than the digestive vacuole. A 7-nitrobenz-2-oxa-1,3-diazole (NBD)-labeled fluorescent derivative of chloroquine, especially sensitive to regions outside the digestive vacuole and retaining the antiplasmodial pharmacophore, was synthesized to investigate subcellular localization in the parasite. Super-resolution microscopy revealed association with membranes including the parasite plasma membrane, the endoplasmic reticulum, and possibly also the mitochondrion. A drug-labeled affinity matrix was then prepared to capture protein binding targets of chloroquine. SDS-PAGE revealed a single prominent band between 200 and 250 kDa from the membrane-associated fraction. Subsequent proteomic analysis revealed that this band corresponded to P. falciparum multidrug resistance-associated protein (PfMRP1). Intrigued by this finding, we demonstrated pull-down of PfMRP1 by matrices labeled with Cinchona alkaloids quinine and quinidine. While PfMRP1 has been implicated in resistance to quinolines and other antimalarials, this is the first time that these drugs have been found to bind directly to this protein. Based on previous reports, PfMRP1, the only prominent protein found to bind to quinolines in this work, is likely to modulate the activity of these antimalarials in P. falciparum rather than act as a drug target.

Novel antimalarial chloroquine- and primaquine-quinoxaline 1,4-di-N-oxide hybrids: Design, synthesis, Plasmodium life cycle stage profile, and preliminary toxicity studies

Emergence of drug resistance and targeting all stages of the parasite life cycle are currently the major challenges in antimalarial chemotherapy. Molecular hybridization combining two scaffolds in a single molecule is an innovative strategy for achieving these goals. In this work, a series of novel quinoxaline 1,4-di-N-oxide hybrids containing either chloroquine or primaquine pharmacophores was designed, synthesized and tested against both chloroquine sensitive and multidrug resistant strains of Plasmodium falciparum. Only chloroquine-based compounds exhibited potent blood stage activity with compounds 4b and 4e being the most active and selective hybrids at this parasite stage. Based on their intraerythrocytic activity and selectivity or their chemical nature, seven hybrids were then evaluated against the liver stage of Plasmodium yoelii, Plasmodium berghei and Plasmodium falciparum infections. Compound 4b was the only chloroquine-quinoxaline 1,4-di-N-oxide hybrid with a moderate liver activity, whereas compound 6a and 6b were identified as the most active primaquine-based hybrids against exoerythrocytic stages, displaying enhanced liver activity against P. yoelii and P. berghei, respectively, and better SI values than primaquine. Although both primaquine-quinoxaline 1,4-di-N-oxide hybrids slightly reduced the infection of mosquitoes, they inhibited sporogony of P. berghei and compound 6a showed 92% blocking of transmission. In vivo liver efficacy assays revealed that compound 6a showed causal prophylactic activity affording parasitaemia reduction of up to 95% on day 4. Absence of genotoxicity and in vivo acute toxicity were also determined. These results suggest the approach of primaquine-quinoxaline 1,4-di-N-oxide hybrids as new potential dual-acting antimalarials for further investigation.

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 β-hematin formation, therefore these compounds act via heme polymerization target.

Prediction Model for Antimalarial Activities of Hemozoin Inhibitors by Using Physicochemical Properties

The rapid spread of strains of malaria parasites that are resistant to several drugs has threatened global malaria control. Hence, the aim of this study was to predict the antimalarial activity of chemical compounds that possess anti-hemozoin-formation activity as a new means of antimalarial drug discovery. After the initial in vitro anti-hemozoin-formation high-throughput screening (HTS) of 9,600 compounds, a total of 224 hit compounds were identified as hemozoin inhibitors. These 224 compounds were tested for in vitro erythrocytic antimalarial activity at 10 μM by using chloroquine-mefloquine-sensitive Plasmodium falciparum strain 3D7A. Two independent experiments were conducted. The physicochemical properties of the active compounds were extracted from the ChemSpider and SciFinder databases. We analyzed the extracted data by using Bayesian model averaging (BMA). Our findings revealed that lower numbers of S atoms; lower distribution coefficient (log D) values at pH 3, 4, and 5; and higher predicted distribution coefficient (ACD log D) values at pH 7.4 had significant associations with antimalarial activity among compounds that possess anti-hemozoin-formation activity. The BMA model revealed an accuracy of 91.23%. We report new prediction models containing physicochemical properties that shed light on effective chemical groups for synthetic antimalarial compounds and help with in silico screening for novel antimalarial drugs.