Synthesis and biological evaluation of acridine derivatives as antimalarial agents

New N‐alkylaminoacridine derivatives attached to nitrogen heterocycles were synthesized, and their antimalarial potency was examined. They were tested in vitro against the growth of Plasmodium falciparum, including chloroquine (CQ)‐susceptible and CQ‐resistant strains. This biological evaluation has shown that the presence of a heterocyclic ring significantly increases the activity against P. falciparum. The best compound shows a nanomolar IC₅₀ value toward parasite proliferation on both CQ‐susceptible and CQ‐resistant strains. The antimalarial activity of these new acridine derivatives can be explained by the two mechanisms studied in this work. First, we showed the capacity of these compounds to inhibit heme biocrystallization, a detoxification process specific to the parasite and essential for its survival. Second, in our search for alternative targets, we evaluated the in vitro inhibitory activity of these compounds toward Sulfolobus shibatae topoisomerase VI‐mediated DNA relaxation. The preliminary results obtained reveal that all tested compounds are potent DNA intercalators, and significantly inhibit the activity of S. shibatae topoisomerase VI at concentrations ranging between 2.0 and 2.5 μM .     

N‐Cinnamoylation of Antimalarial Classics: Quinacrine Analogues with Decreased Toxicity and Dual‐Stage Activity

Plasmodium falciparum, the causative agent of the most lethal form of malaria, is becoming increasingly resistant to most available drugs. A convenient approach to combat parasite resistance is the development of analogues of classical antimalarial agents, appropriately modified in order to restore their relevance in antimalarial chemotherapy. Following this line of thought, the design, synthesis and in vitro evaluation of N‐cinnamoylated quinacrine surrogates, 9‐(N‐cinnamoylaminobutyl)‐amino‐6‐chloro‐2‐methoxyacridines, is reported. The compounds were found to be highly potent against both blood‐stage P. falciparum, chloroquine‐sensitive 3D7 (IC₅₀=17.0–39.0 nM ) and chloroquine‐resistant W2 and Dd2 strains (IC₅₀=3.2–41.2 and 27.1–131.0 nM , respectively), and liver‐stage P. berghei (IC₅₀=1.6–4.9 μM ) parasites. These findings bring new hope for the possible future “rise of a fallen angel” in antimalarial chemotherapy, with a potential resurgence of quinacrine‐related compounds as dual‐stage antimalarial leads.     

Preliminary Evaluation of Artemisinin–Cholesterol Conjugates as Potential Drugs for the Treatment of Intractable Forms of Malaria and Tuberculosis

To evaluate the feasibility of developing drugs that may be active against both malaria and tuberculosis (TB) by using in part putative cholesterol transporters in the causative pathogens and through enhancement of passive diffusion in granulomatous TB, artemisinin–cholesterol conjugates were synthesized by connecting the component molecules through various linkers. The compounds were screened in vitro against Plasmodium falciparum (Pf) and Mycobacterium tuberculosis (Mtb). Antimalarial activities (IC₅₀) against Pf drug‐sensitive NF54, and drug‐resistant K1 and W2 strains ranged from 0.03–2.6, 0.03–1.9, and 0.02–1.7 μm . Although the compounds are less active than the precursor artemisinin derivatives, the cholesterol moiety renders the compounds relatively insoluble in the culture medium, and variation in solubilities among the different compounds may reflect in the range of efficacies observed. Activities against Mtb H37Rv were assessed using a standardized colony‐forming unit (CFU) assay after 24 h pretreatment of cultures with each of the compounds. Percentage inhibition ranged from 3–38 % and 18–52 % at 10 and 80 μm , respectively. Thus, in contrast to the comparator drug artemether, the conjugates display enhanced activities. The immediate aims include the preparation of conjugates with enhanced aqueous solubilities, assays against malaria and TB in vivo, and for TB, assays using an infected macrophage model and assessment of granuloma influx.     

Design,Synthesis, and Evaluation of Novel Ferroquine and Phenylequine Analogues as Potential Antiplasmodial Agents

7‐Chloroquinoline‐based antimalarial drugs are effective in the inhibition of hemozoin formation in the food vacuole of the Plasmodium parasite, the causative agent of malaria. We synthesized five series of ferroquine (FQ) and phenylequine (PQ) derivatives, which display good in vitro efficacy toward both the chloroquine‐sensitive (CQS) NF54 (IC₅₀: 4.2 nm ) and chloroquine‐resistant (CQR) Dd2 (IC₅₀: 33.7 nm ) strains of P. falciparum. Several compounds were found to have good inhibitory activity against β‐hematin formation in an NP‐40 detergent assay, with IC₅₀ values ranging between 10.4 and 19.2 μm .     

Expanding the SAR of Nontoxic Antiplasmodial Indolyl‐3‐ethanone Ethers and Thioethers

Despite major strides in reducing Plasmodium falciparum infections, this parasite still accounts for roughly half a million annual deaths. This problem is compounded by the decreased efficacy of artemisinin combination therapies. Therefore, the development and optimisation of novel antimalarial chemotypes is critical. In this study, we describe our strategic approach to optimise a class of previously reported antimalarials, resulting in the discovery of 1‐(5‐chloro‐1H‐indol‐3‐yl)‐2‐[(4‐cyanophenyl)thio]ethanone ( 13 ) and 1‐(5‐chloro‐1H‐indol‐3‐yl)‐2‐[(4‐nitrophenyl)thio]ethanone ( 14 ), whose activity was equipotent to that of chloroquine against the P. falciparum 3D7 strain. Furthermore, these compounds were found to be nontoxic to HeLa cells as well as being non‐haemolytic to uninfected red blood cells. Intriguingly, several of our most promising compounds were found to be less active against the isogenic NF54 strain, highlighting possible issues with long‐term dependability of malarial strains. Finally compound 14 displayed similar activity against both the NF54 and K1 strains, suggesting that it inhibits a pathway that is uncompromised by K1 resistance.     

Antiplasmodial and Antimalarial Activity of 3,5-Diarylidenetetrahydro-2H-pyran-4(3H)-ones via Inhibition of Plasmodium falciparum Pyridoxal Synthase

A series of 22 different 3,5-diarylidenetetrahydro-2H-pyran-4(3H)-ones (DATPs) were synthesized, characterized, and screened for their in vitro antiplasmodial activities against chloroquine (CQ)-sensitive Pf3D7, CQ-resistant PfINDO, and artemisinin-resistant PfMRA-1240 strains of Plasmodium falciparum. DATP 19 ( 3,5-bis(4-hydroxy-3,5-dimethoxybenzylidene)tetrahydro-2H-pyran-4(3H)-one) was found to be the most potent (IC₅₀ 1.07 μM) against PfMRA-1240, whereas 21 (3,5-bis(3,4,5-trimethoxybenzylidene)tetrahydro-2H-pyran-4(3H)-one) showed IC₅₀ values of 1.72 and 1.44 μM against Pf3D7 and PfINDO, respectively. Resistance indices (RI) as low as 0.2 to 0.5 for 10 (3,5-bis(4-nitrobenzylidene)tetrahydro-2H-pyran-4(3H)-one) and 20 (3,5-bis(3-nitrobenzylidene)tetrahydro-2H-pyran-4(3H)-one), and <1 for most other DATPs reveals their greater potency against resistant strains than the sensitive one. The single-crystal XRD data for DATP 21 are reported. In silico support was obtained through docking studies. Killing all three strains within 4–8 h, these DATPs showed rapid kill kinetics toward the trophozoite stage. Furthermore, DATP 18 (3,5-bis(quinolin-4-ylmethylene)tetrahydro-2H-pyran-4(3H)-one) inhibited PfPdx1 enzyme activity with IC₅₀ 20.34 μM, which is about twofold lower than that (IC₅₀ 43 μM) for an already known inhibitor 4PEHz. At an oral dose of 300 mg/kg body weight, DATPs 19 and 21 were found to be nontoxic to mice, and at 100 mg/kg body weight, DATP 19 was found to suppress parasitaemia, which led to an increase in median survival time by three days relative to untreated control mice in a malaria curative study.     

Targeting the Erythrocytic and Liver Stages of Malaria Parasites with s‐Triazine‐Based Hybrids

A diversity‐oriented library of s‐triazine‐based hybrids was screened for activity against the chloroquine‐resistant Plasmodium falciparum W2 strain. The most striking result was sub‐micromolar activity against cultured erythrocytic‐stage parasites of hybrid molecules containing one or two 8‐aminoquinoline moieties. These compounds were not clearly toxic to human cells. The most effective blood‐schizontocidal s‐triazine derivatives were then screened for activity against the liver stage of malaria parasites. The s‐triazine hybrid containing two 8‐aminoquinoline moieties and one chlorine atom emerged as the most potent against P. berghei liver‐stage infection, active in the low nanomolar region, combined with good metabolic stability in rat liver microsomes. These results indicate that s‐triazine‐8‐aminoquinoline‐based hybrids are excellent starting points for lead optimization as dual‐stage antimalarials.     

A partial convergence in action of methylene blue and artemisinins: antagonism with chloroquine, a reversal with verapamil, and an insight into the antimalarial activity of chloroquine

Artemisinins rapidly oxidize leucomethylene blue (LMB) to methylene blue (MB); they also oxidize dihydroflavins such as the reduced conjugates RFH₂ of riboflavin (RF), and FADH₂ of the cofactor flavin adenine dinucleotide (FAD), to the corresponding flavins. Like the artemisinins, MB oxidizes FADH₂, but unlike artemisinins, it also oxidizes NAD(P)H. Like MB, artemisinins are implicated in the perturbation of redox balance in the malaria parasite by interfering with parasite flavoenzyme disulfide reductases. The oxidation of LMB by artemisinin is inhibited by chloroquine (CQ), an inhibition that is abruptly reversed by verapamil (VP). CQ also inhibits artemisinin‐mediated oxidation of RFH₂ generated from N‐benzyl‐1,4‐dihydronicotinamide (BNAH)–RF, or FADH₂ generated from NADPH or NADPH–Fre, an effect that is also modulated by verapamil. The inhibition likely proceeds by the association of LMB or dihydroflavin with CQ, possibly involving donor–acceptor or π complexes that hinder oxidation by artemisinin. VP competitively associates with CQ, liberating LMB or dihydroflavin from their respective CQ complexes. The observations explain the antagonism between CQ–MB and CQ–artemisinins in vitro, and are reconcilable with CQ perturbing intraparasitic redox homeostasis. They further suggest that a VP–CQ complex is a means by which VP reverses CQ resistance, wherein such a complex is not accessible to the putative CQ‐resistance transporter (PfCRT).     

An Endoperoxide‐Based Hybrid Approach to Deliver Falcipain Inhibitors Inside Malaria Parasites

The emergence of artemisinin‐resistant Plasmodium falciparum malaria in Southeast Asia has reinforced the urgent need to discover novel chemotherapeutic strategies to treat and control malaria. To address this problem, we prepared a set of dual‐acting tetraoxane‐based hybrid molecules designed to deliver a falcipain‐2 (FP‐2) inhibitor upon activation by iron(II) in the parasite digestive vacuole. These hybrids are active in the low nanomolar range against chloroquine‐sensitive and chloroquine‐resistant P. falciparum strains. We also demonstrate that in the presence of FeBr₂ or within infected red blood cells, these molecules fragment to release falcipain inhibitors with nanomolar protease inhibitory activity. Molecular docking studies were performed to better understand the molecular interactions established between the tetraoxane‐based hybrids and the cysteine protease binding pocket residues. Our results further indicate that the intrinsic activity of the tetraoxane partner compound can be masked, suggesting that a tetraoxane‐based delivery system offers the potential to attenuate the off‐target effects of known drugs.     

Syntheses and biological activities of novel diheterocyclic compounds containing 1,2,4‐triazolo[1,5‐a]pyrimidine and 1,3,4‐oxadiazole

In a search for novel agrochemicals with high activity and low toxicity, a series of diheterocyclic compounds containing 1,2,4‐triazolo[1,5‐a]pyrimidine and 1,3,4‐oxadiazole rings were designed and synthesized by a four‐step synthetic route starting from 2‐mercapto‐5,7‐dimethyl‐1,2,4‐triazolo[1,5‐a]pyrimidine. The structures of all the compounds synthesized were confirmed by ¹H NMR, mass spectroscopy and elemental analysis. The preliminary bioassay against Brassica campestris L and Echinochloa crusgalli Beavu indicated that the title compounds displayed herbicidal activity at the concentration of 100 ppm and that compounds 5a (R = CH₃), 5d (R = C₂H₅) and 5f (R = i‐Bu) were found to have particularly high activities. In addition, the results of an in vivo test at a concentration of 50 ppm showed that all the compounds prepared were highly active against Rhizoctonia slain, but not active against Fusarium oxysporum, Gibberella zeave and Phoma sparagi. A further in vivo test showed that compound 5j possessed better fungicidal activity against Rhizoctonia solani at a concentration of 200 ppm than Carbendazim and Validamycin A, which are well known for their fungicidal activity against Rhizoctonia solani. To our knowledge, this is the first report that 1,2,4‐triazolo[1,5‐a]pyrimidine derivatives display fungicidal activity against Rhizoctonia solani. © 2001 Society of Chemical Industry     

Challenges Based on Antiplasmodial and Antiviral Activities of 7-Chloro-4-aminoquinoline Derivatives

We report the structural functionalization of the terminal amino group of N¹-(7-chloroquinolin-4-yl) butane-1,4-diamine, leading to a series of 7-chloro-4-aminoquinoline derivatives, and their evaluation as potent anti-malarial and anti-viral agents. Some compounds exhibited promising anti-malarial effects against the Plasmodium falciparum 3D7 (chloroquine-sensitive) and Dd2 (chloroquine-resistant) strains. In addition, these compounds were assayed in vitro against influenza A virus (IAV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Compound 5 h , bearing an N-mesityl thiourea group, displayed pronounced anti-infectious effects against malaria, IAV, and SARS-CoV-2. These results provide new insights into drug discovery for the prevention or treatment of malaria and virus co-infection.     

Replacement of the 4′‐Hydroxy Group of Amodiaquine and Amopyroquine by Aromatic and Aliphatic Substituents: Synthesis and Antimalarial Activity

4′‐Substituted analogues of amodiaquine and amopyroquine were synthesized using Csp²–Csp² and Csp²–Csp³ Suzuki–Miyaura cross‐coupling reactions as the key step. The new derivatives were found to be active against both chloroquine (CQ)‐sensitive and CQ‐resistant strains of P. falciparum, with IC₅₀ values in the range of 7–200 nM ; one compound showed in vivo activity.

     

Probing the Azaaurone Scaffold against the Hepatic and Erythrocytic Stages of Malaria Parasites

The potential of azaaurones as dual‐stage antimalarial agents was investigated by assessing the effect of a small library of azaaurones on the inhibition of liver and intraerythrocytic lifecycle stages of the malaria parasite. The whole series was screened against the blood stage of a chloroquine‐resistant Plasmodium falciparum strain and the liver stage of P. berghei, yielding compounds with dual‐stage activity and sub‐micromolar potency against erythrocytic parasites. Studies with genetically modified parasites, using a phenotypic assay based on the P. falciparum Dd2‐ScDHODH line, which expresses yeast dihydroorotate dehydrogenase (DHODH), showed that one of the azaaurone derivatives has the potential to inhibit the parasite mitochondrial electron‐transport chain. The global urgency in finding new therapies for malaria, especially against the underexplored liver stage, associated with chemical tractability of azaaurones, warrants further development of this chemotype. Overall, these results emphasize the azaaurone chemotype as a promising scaffold for dual‐stage antimalarials.     

Discovery and Pharmacophore Mapping of a Low-Nanomolar Inhibitor of P. falciparum Growth

The treatment of malaria, the most common parasitic disease worldwide and the third deadliest infection after HIV and tuberculosis, is currently compromised by the dramatic increase and diffusion of drug resistance among the various species of Plasmodium, especially P. falciparum (Pf). In this view, the development of new antiplasmodial agents that are able to act via innovative mechanisms of action, is crucial to ensure efficacious antimalarial treatments. In one of our previous communications, we described a novel class of compounds endowed with high antiplasmodial activity, characterized by a pharmacophore never described before as antiplasmodial and identified by their 4,4’-oxybisbenzoyl amide cores. Here, through a detailed structure-activity relationship (SAR) study, we thoroughly investigated the chemical features of the reported scaffolds and successfully built a novel antiplasmodial agent active on both chloroquine (CQ)-sensitive and CQ-resistant Pf strains in the low nanomolar range, without displaying cross-resistance. Moreover, we conducted an in silico pharmacophore mapping.     

Enantiopure Indolizinoindolones with in vitro Activity against Blood‐ and Liver‐Stage Malaria Parasites

Malaria continues to be a major cause of morbidity and mortality to this day, and resistance to drugs like chloroquine has led to an urgent need to discover novel chemical entities aimed at new targets. Here, we report the discovery of a novel class of potential antimalarial compounds containing an indolizinoindolone scaffold. These novel enantiopure indolizinoindolones were synthesized, in good‐to‐excellent yields and excellent diastereoselectivities, by cyclocondensation reaction of (S)‐ or (R)‐tryptophanol and 2‐acyl benzoic acids, followed by intramolecular α‐amidoalkylation. Interestingly, we were able to synthesize for the first time 7,13b‐cis indolizinoindolones in a two‐step route. The novel compounds showed promising activity against erythrocytic stages of the human malaria parasite, Plasmodium falciparum, and liver stages of the rodent parasite Plasmodium berghei. In particular, an (S)‐tryptophanol‐derived isoindolinone was identified as a promising starting scaffold to search for novel antimalarials, combining excellent activity against both stages of the parasite′s life cycle with low cytotoxicity and excellent metabolic and chemical stability in vitro.     

Structure–Activity and Structure–Toxicity Relationships of Peptoid-Based Histone Deacetylase Inhibitors with Dual-Stage Antiplasmodial Activity

Novel malaria intervention strategies are of great importance, given the development of drug resistance in malaria-endemic countries. In this regard, histone deacetylases (HDACs) have emerged as new and promising malaria drug targets. In this work, we present the design, synthesis, and biological evaluation of 20 novel HDAC inhibitors with antiplasmodial activity. Based on a previously discovered peptoid-based hit compound, we modified all regions of the peptoid scaffold by using a one-pot multicomponent pathway and submonomer routes to gain a deeper understanding of the structure–activity and structure–toxicity relationships. Most compounds displayed potent activity against asexual blood-stage P. falciparum parasites, with IC₅₀ values in the range of 0.0052–0.25 μm and promising selectivity over mammalian cells (SI^Pf3D7/HepG2: 170–1483). In addition, several compounds showed encouraging sub-micromolar activity against P. berghei exo-erythrocytic forms (PbEEF). Our study led to the discovery of the hit compound N-(2-(benzylamino)-2-oxoethyl)-N-(4-(hydroxycarbamoyl)benzyl)-4-isopropylbenzamide ( 2 h ) as a potent and parasite-specific dual-stage antiplasmodial HDAC inhibitor (IC₅₀ Pf3D7=0.0052 μm , IC₅₀ PbEEF=0.016 μm ).     

Potent,Metabolically Stable 2‐Alkyl‐8‐(2H‐1,2,3‐triazol‐2‐yl)‐9H‐adenines as Adenosine A2A Receptor Ligands

Inhibition of adenosine A_2A receptors has been shown to elicit a therapeutic response in preclinical animal models of Parkinson’s disease (PD). We previously identified the triazolo‐9H‐purine, ST1535, as a potent A_2AR antagonist. Studies revealed that ST1535 is extensively hydroxylated at the ω‐1 position of the butyl side chain. Here, we describe the synthesis and evaluation of derivatives in which the ω‐1 position has been substituted (F, Me, OH) in order to block metabolism. The stability of the compounds was evaluated in human liver microsomes (HLM), and the affinity for A_2AR was determined. Two compounds, (2‐(3,3‐dimethylbutyl)‐9‐methyl‐8‐(2H‐1,2,3‐triazol‐2‐yl)‐9H‐purin‐6‐amine ( 3 b ) and 4‐(6‐amino‐9‐methyl‐8‐(2H‐1,2,3‐triazol‐2‐yl)‐9H‐purin‐2‐yl)‐2‐methylbutan‐2‐ol ( 3 c ), exhibited good affinity against A_2AR (K_i=0.4 nM and 2 nM , respectively) and high in vitro metabolic stability (89.5 % and 95.3 % recovery, respectively, after incubation with HLM for two hours).     

Quinoline-Dihydropyrimidin-2(1H)-one Hybrids: Synthesis,Biological Activity,and Mechanistic Studies

A novel class of quinoline-dihydropyrimidin-2(1H)-one (DHPM) hybrids was synthesized and in vitro antiplasmodial activity was evaluated against chloroquine sensitive (D10) and chloroquine resistant (Dd2) strains of Plasmodium falciparum, the human malaria parasite. The antiplasmodial activity was compared to previously reported DHPM based molecular hybrids. Dual mode of antiplasmodial action of the most active member has been evaluated through heme binding study and in silico docking in the active site of dihydrofolate enzymes (wild-type as well as mutant). Favourable pharmacokinetic parameters were predicted in the ADMET evaluation. The new hybrids were also tested against a number of DNA and RNA viruses. No antiviral activity was found, except for one hybrid that showed mild inhibitory activity against two strains of cytomegalovirus (AD-169 and Davis), The most active hybrid was found to be a selective inhibitor of the growth of P. falciparum as well as a modest inhibitor of varicella zoster virus in HEL cells. Cytotoxicity of all hybrids was assessed in HEL, HeLa, Vero, MDCK, and CRFK cell cultures.     

Pyrotechnic Countermeasures. III: The Influence of Oxygen Balance of an Aromatic Fuel on the Color Ratio of Spectral Flare Compositions

The influence of the oxygen balance, Λ, of an anthracene‐type aromatic fuel in flare compositions on the spectral color ratio, that is the ratio of radiation emitted in band A (λ=1.8–2.5 μm) versus radiation emitted in band B (λ=3.5–4.8 μm), is investigated. Eight compositions based on potassium perchlorate, polychloroprene binder and either of the following fuels, anthracene, C₁₄H₁₀ ( 1 ), anthracene‐d¹⁰, C₁₄D₁₀ ( 2 ), phenanthrene C₁₄H₁₀ ( 3 ), anthrone, C₁₄H₁₀O ( 4 ), anthraquinone, C₁₄H₈O₂ ( 5 ), 1‐aminoanthraquinone, C₁₄H₉NO₂ ( 6 ), 1,5‐dihydroxyanthraquinone, C₁₄H₈O₄ ( 7 ) and 1,5‐dinitroanthraquinone, C₁₄H₆N₂O₆ ( 8 ) were prepared and radiometrically investigated.