Synthesis, Antimalarial Activity, and Structure–Activity Relationship of 7-(2-Phenoxyethoxy)-4(1H)-quinolones

3D-QSAR-based design, synthesis and biological evaluation of 2,4-disubstituted quinoline derivatives as antimalarial agents

2,4-Disubstituted quinoline derivatives were designed based on a 3D-QSAR study, synthesized and evaluated for antimalarial activity. A large dataset of 178 quinoline derivatives was used to perform a 3D-QSAR study using CoMFA and CoMSIA models. PLS analysis provided statistically validated results for CoMFA (r2ncv = 0.969, q2 = 0.677, r2cv = 0.682) and CoMSIA (r2ncv = 0.962, q2 = 0.741, r2cv = 0.683) models. Two series of a total of 40 2,4-disubstituted quinoline derivatives were designed with amide (quinoline-4-carboxamide) and secondary amine (4-aminoquinoline) linkers at the -C4 position of the quinoline ring. For the purpose of selecting better compounds for synthesis with good pEC50 values, activity prediction was carried out using CoMFA and CoMSIA models. Finally, a total of 10 2,4-disubstituted quinoline derivatives were synthesized, and screened for their antimalarial activity based on the reduction of parasitaemia. Compound #5 with amide linker and compound #19 with secondary amine linkers at the -C4 position of the quinoline ring showed maximum reductions of 64% and 57%, respectively, in the level of parasitaemia. In vivo screening assay confirmed and validated the findings of the 3D-QSAR study for the design of quinoline derivatives.

Molecular docking and antimalarial evaluation of hybrid para-aminobenzoic acid 1,3,5 triazine derivatives via inhibition of Pf-DHFR

In this study, a structurally guided pharmacophore hybridization strategy is used to combine the two key structural scaffolds, para-aminobenzoic acid (PABA), and 1,3,5 triazine in search of new series of antimalarial agents. A combinatorial library of 100 compounds was prepared in five different series as [4A (1-22), 4B (1-21), 4 C (1-20), 4D (1-19) and 4E (1-18)] using different primary and secondary amines, from where 10 compounds were finally screened out through molecular property filter analysis and molecular docking study as promising PABA substituted 1,3,5-triazine scaffold as an antimalarial agent. The docking results showed that compounds 4A12 and 4A20 exhibited good binding interaction with Phe58, IIe164, Ser111, Arg122, Asp54 (-424.19 to -360.34 kcal/mol) and Arg122, Phe116, Ser111, Phe58 (-506.29 to -431.75 kcal/mol) against wild (1J3I) and quadruple mutant (1J3K) type of Pf-DHFR. These compounds were synthesized by conventional as well as microwave-assisted synthesis and characterized by different spectroscopic methods. In-vitro antimalarial activity results indicated that two compounds 4A12 and 4A20 showed promising antimalarial activity against chloroquine-sensitive (3D7) and chloroquine-resistant (Dd2) strains of Plasmodium falciparum with IC50 (1.24-4.77 μg mL-1) and (2.11-3.60 μg mL-1). These hybrid PABA substituted 1,3,5-triazine derivatives might be used in the lead discovery towards a new class of Pf-DHFR inhibitors.Communicated by Ramaswamy H. Sarma.

Quinolone: a versatile therapeutic compound class

The discovery of nalidixic acid is one pinnacle in medicinal chemistry, which opened a new area of research that has led to the discovery of several life-saving antimicrobial agents (generally referred to as fluoroquinolones) for over decades. Although fluoroquinolones are frequently encountered in the literature, the utility of quinolone compounds extends far beyond the applications of fluoroquinolones. Quinolone-based compounds have been reported for activity against malaria, tuberculosis, fungal and helminth infections, etc. Hence, the quinolone scaffold is of great interest to several researchers in diverse disciplines. This article highlights the versatility of the quinolone pharmacophore as a therapeutic agent beyond the fluoroquinolone profile.

Self-assembly of new cobalt complexes based on [Co (SCN)4], synthesis, empirical, antioxidant activity, and quantum theory investigations

The cobalt (II) complexes have been synthesized from the reaction of the cationic entities (3,4-dimethylaniline (1) and histamine (2)) with metallic salt CoCl₂⋅6H₂O and thiocyanate ion (SCN⁻) as a ligand in H₂O/ethanolic solution and processing by the evaporation crystal growth method at room temperature to get crystals. The synthesized complex has been fully characterized by single-crystal X-ray diffraction. UV–Visible, FTIR spectroscopy, TGA analysis, and DFT circulations were also performed. The crystal structural analysis reveals that the solid (1) {[Co(SCN)₄] (C₈H₁₂N)₃}·Cl crystallizes in the monoclinic system with the space group P2₁/n and the solid (2) {[Co(SCN)₄](C₅H₁₁N₃)₂}·2Cl crystallizes in the monoclinic space group P2₁/m. Metal cations are joined into corrugated chains parallel to the b-axis direction in (1) and (2) by four thiocyanate anions. The crystal structures of (1) and (2) were calculated using XRPD data, indicating that they are closely connected to the DRX mono-crystal results. Different interactions pack the system into a ring formed by N–H⋯Cl and N–H⋯S hydrogen bonds. C–H⋯π and the π⋯π stacking of anilinuim ring for (1) and N–H⋯S intermolecular interactions for (1) and (2) increase the crystals' robustness. Hirshfeld surface analysis cum 2D fingerprint plots visualize the main intermolecular interactions with their contributions in the solid-state phase. The molecular geometries of both complexes obtained from the crystal structure were used for quantum chemical calculation. Here, frontier orbital analysis and electrostatic potential illustrate the chemical reactivities of metal–organic complexes. QTAIM and NCI analysis reveal the strength of interactions at the electronic level.

Aminoalkoxycarbonyloxymethyl Ether Prodrugs with a pH-Triggered Release Mechanism: A Case Study Improving the Solubility, Bioavailability, and Efficacy of Antimalarial 4(1H)-Quinolones with Single Dose Cures

Preclinical and clinical development of numerous small molecules is prevented by their poor aqueous solubility, limited absorption, and oral bioavailability. Herein, we disclose a general prodrug approach that converts promising lead compounds into aminoalkoxycarbonyloxymethyl (amino AOCOM) ether-substituted analogues that display significantly improved aqueous solubility and enhanced oral bioavailability, restoring key requirements typical for drug candidate profiles. The prodrug is completely independent of biotransformations and animal-independent because it becomes an active compound via a pH-triggered intramolecular cyclization-elimination reaction. As a proof-of-concept, the utility of this novel amino AOCOM ether prodrug approach was demonstrated on an antimalarial compound series representing a variety of antimalarial 4(1H)-quinolones, which entered and failed preclinical development over the last decade. With the amino AOCOM ether prodrug moiety, the 3-aryl-4(1H)-quinolone preclinical candidate was shown to provide single-dose cures in a rodent malaria model at an oral dose of 3 mg/kg, without the use of an advanced formulation technique.

Exploring quinolone scaffold: Unravelling the chemistry of anticancer drug design

Globally, cancer is considered as the major leading cause in decreasing the patient health care system of human beings. The growing threat from drug-resistant cancers makes heterocyclic moieties as an urgent need to develop more successful candidates for anti-cancer therapy. In view of outstanding pharmacological activities Quinolone and its derivatives have attracted more attention towards drug designing and biological evaluation in the search of new drug molecules. The inspired researchers attempted efforts in order to discover quinolone based analogs due to its wide range of biological activities. Due to immense pharmacological importance, distinct synthetic methods have been executed to attain new drug entities from quinolones and all the reported molecules have shown constructive anticancer activity. Some of the synthetic protocol like, one pot synthesis, post-Ugi-transformation, catalysed based synthesis, enzyme-based synthesis and nano-catalyst based synthetic procedures are also discussed as recent advancement in production of quinolone derivatives. In this review, recent synthetic approaches in the medicinal chemistry of quinolones and potent quinolone derivatives on the basis of structural activity relationship are outlined. Moreover, their major methods and modifications are discussed.

Crystal structure of 1-(3-chlorophenyl)-4-(4-(((2,3-dihydro-1H-inden-5-yl)oxy)methyl)phenethyl)piperazine, C28H31ClN2O

C28H31ClN2O, monoclinic, P1̄ (no. 2), a = 21.9309(11) Å, b = 9.9648(5) Å, c = 11.0049(7) Å, β = 93.403(6)°, V = 2400.7(2) Å3, Z = 4, R gt(F) = 0.0566, wR ref(F 2) = 0.1355, T = 293 K.

Malaria transmission-blocking drugs: implications and future perspectives

As the world gets closer to eliminating malaria, the scientific community worldwide has begun to realize the importance of malaria transmission-blocking interventions. The onus of breaking the life cycle of the human malaria parasite Plasmodium falciparum predominantly rests upon transmission-blocking drugs because of emerging resistance to commonly used schizonticides and insecticides. This third part of our review series on malaria transmission-blocking entails transmission-blocking potential of preclinical transmission-blocking antimalarials and other non-malaria drugs/experimental compounds that are not in clinical or preclinical development for malaria but possess transmission-blocking potential. Collective analysis of the structure and the activity of these experimental compounds might pave the way toward generation of novel prototypes of next-generation transmission-blocking drugs.

Crystal structure of 2-(3-(2-(4-phenylpiperazin-1-yl)ethyl)benzyl)isoindoline-1,3-dione, C27H27N3O2

C27H27N3O2, triclinic, P1̄ (no. 2), a = 6.9263(4) Å, b = 10.0832(7) Å, c = 17.3531(10) Å, α = 75.708(5)°, β = 87.142(5)°, γ = 74.935(6)°, V = 1133.92(13) Å3, Z = 2, Rgt(F) = 0.0605, wRref(F2) = 0.1661, T = 290(1) K.

Role of Trifluoromethyl Substitution in Design of Antimalarial Quinolones: a Comprehensive Review

Malaria represents a significant health issue, and novel effective drugs are needed to address parasite resistance that has emerged to the current drug arsenal. The most popular antimalarial drugs are focused on the 7-chloro-4-aminoquinoline [e.g., chloroquine (CQ), amodiaquine (AQ), isoquine (IQ), and tebuquine (TBQ)], artemisinin, and atovaquone systems. Recently, endochin has been used as a platform to design a variety of novel potent and safe antimalarial agents named endochin-like quinolones (ELQs). Also, antimalarial quinolones have been constructed from other quinolones drugs such as ICI-56780 and floxacrine. Trifluoromethyl substitution has provided a significant increase in the antimalarial response of many of the designed ELQs against Plasmodium-resistant strains and for in vivo models. In particular, attachment of a substituted trifluoromethoxy (or trifluoromethyl in some cases) biaryl side chain at 2-, 3-, 4-, or 6-position of the quinolone core has shown to be crucially important to generate selective and potent novel ELQs. Furthermore, 6-chloro and 7-methoxy moieties on the quinolone core have been identified as essential pharmacophores when the trifluoromethoxy biaryl side chain is placed at 2- or 3-position of the quinolone core. Methyl or ethyl ester attached at 3-position is essential when the trifluoromethoxy aryl side chain is attached at 6- or 7-position of the quinolone core. Some promising ELQs are currently under clinical trials, representing an excellent platform for the design of new potent, selective, effective, and safe antimalarial drugs against emergent resistance malarial models.

Antileishmanial activity, structure-activity relationship of series of 2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-ones

Trifluoromethyl-substituted quinolones and their analogues have emerged as an interesting platform in the last 6 years to design antiparasite agents. Many of their derivatives have been demonstrated to display excellent efficacy against flagellate parasites such as Plasmodium spp. In order to identify new analogues of trifluoromethyl-substituted quinolones to treat the American cutaneous leishmaniasis, we evaluated the antiproliferative activity of a series of 2-(trifluoromethyl)benzo[b]-[1,8]naphthyridin-4(1H)-ones on the Leishmania braziliensis and Leishmania mexicana parasites. The mentioned derivatives have never been evaluated against any parasite strain. In general, an in vitro evaluation on L.(L)mexicana and L.(V)braziliensis showed that L.(L)mexicana was more sensitive to the action of the compounds than L.(V)braziliensis, either in the promastigote or in the amastigote form. Five compounds exhibited moderate efficacy against L.(L)mexicana promastigotes, with IC50 values ranging from 9.65 to 14.76 µM. From the mentioned molecules, three compounds, 1e, 1f, and 1h, showed a discrete response against axenic and intracellular amastigotes, with LD50 values between 19 and 24 µM. Moreover, an in vitro evaluation was performed on an antimony-resistant amastigote strain and a human isolate amastigote strain. These three compounds showed discrete toxicity on peritoneal macrophages; however, their relatively good antiamastigote response compared to the drug glucantime promoted our trifluoromethyl-substituted benzo[b][1,8]naphthyridin-4(1H)-ones as a potential platform to design potent antileishmanial agents.

Synthesis and biological evaluation of estrone 3-O-ether derivatives containing the piperazine moiety

A series of new estrone derivatives were designed and synthesized, and their structures were confirmed by spectroscopic methods. All new estrone derivatives were investigated for their in vitro cytotoxic efficacies against a panel of three human prostate cancer cell lines (PC-3, LNCaP, and DU145). The derivatives 6, 7, 10, 15, 16, 20, 21, 22, 24 and 26 showed important cytotoxic actions against individual carcinoma cell line collections. Moreover, antagonistic activities of compounds (7, 15, 16 and 21) towards a₁-ARs (α_1A, α_1B, and α_1D) were further evaluated using dual-luciferase reporter assays, and the compounds 16 and 21 exhibited better a₁-ARs subtype selectivity. The structure-activity relationship (SAR) suggested that the substitute's type and position on the phenyl group leads to the interesting variations within pharmacological effects of resultant molecular systems.

Design and Synthesis of Orally Bioavailable Piperazine Substituted 4(1H)-Quinolones with Potent Antimalarial Activity: Structure-Activity and Structure-Property Relationship Studies

Malaria deaths have been decreasing over the last 10-15 years, with global mortality rates having fallen by 47% since 2000. While the World Health Organization (WHO) recommends the use of artemisinin-based combination therapies (ACTs) to combat malaria, the emergence of artemisinin resistant strains underscores the need to develop new antimalarial drugs. Recent in vivo efficacy improvements of the historical antimalarial ICI 56,780 have been reported, however, with the poor solubility and rapid development of resistance, this compound requires further optimization. A series of piperazine-containing 4(1H)-quinolones with greatly enhanced solubility were developed utilizing structure-activity relationship (SAR) and structure-property relationship (SPR) studies. Furthermore, promising compounds were chosen for an in vivo scouting assay to narrow selection for testing in an in vivo Thompson test. Finally, two piperazine-containing 4(1H)-quinolones were curative in the conventional Thompson test and also displayed in vivo activity against the liver stages of the parasite.

Crystal structure of 1-(4-((benzo[d][1,3]dioxol-5-yloxy)methyl)phenethyl)-4-(3-chlorophenyl) piperazin-1-ium chloride, C26H28Cl2N2O3

C26H28Cl2N2O3, monoclinic, P21/c (no. 14), a = 13.422(3) Å, b = 7.0011(14) Å, c = 26.249(5) Å, β = 101.06(3)°, V = 2420.8(9) Å3, Z = 4, R gt(F) = 0.0516, wR ref(F 2) = 0.1370, T = 296 K.

Antiplasmodial and antimalarial activities of quinolone derivatives: An overview

Malaria remains one of the most deadly infectious diseases globally. Considering the growing spread of resistance, development of new and effective antimalarials remains an urgent priority. Quinolones, which are emerged as one of the most important class of antibiotics in the treatment of various bacterial infections, showed potential in vitro antiplasmodial and in vivo antimalarial activities, making them promising candidates for the chemoprophylaxis and treatment of malaria. This review presents the current progresses and applications of quinolone-based derivatives as potential antimalarials to pave the way for the development of new antimalarials.

Antibiotics in malaria therapy: which antibiotics except tetracyclines and macrolides may be used against malaria?

Malaria, a parasite vector-borne disease, is one of the most significant health threats in tropical regions, despite the availability of individual chemoprophylaxis. Malaria chemoprophylaxis and chemotherapy remain a major area of research, and new drug molecules are constantly being developed before drug-resistant parasites strains emerge. The use of anti-malarial drugs is challenged by contra-indications, the level of resistance of Plasmodium falciparum in endemic areas, clinical tolerance and financial cost. New therapeutic approaches are currently needed to fight against this disease. Some antibiotics that have shown potential effects on malaria parasite have been recently studied in vitro or in vivo intensively. Two families, tetracyclines and macrolides and their derivatives have been particularly studied in recent years. However, other less well-known have been tested or are being used for malaria treatment. Some of these belong to older families, such as quinolones, co-trimoxazole or fusidic acid, while others are new drug molecules such as tigecycline. These emerging antibiotics could be used to prevent malaria in the future. In this review, the authors overview the use of antibiotics for malaria treatment.

2-Pyridylquinolone antimalarials with improved antimalarial activity and physicochemical properties

2-Pyridylquinolones with improved solubility, an improved metabolic stability profile, reduced off-target toxicity and 12 nM Plasmodium falciparum antimalarial activity are described.

Computational Study of Quinolone Derivatives to Improve their Therapeutic Index as Anti-malaria Agents: QSAR and QSTR

Malaria is a parasitic disease caused by five different species of Plasmodium. More than 40% of the world's population is at risk and malaria annual incidence is estimated to be more than two hundred million, malaria is one of the most important public health problems especially in children of the poorest parts of the world, annual mortality is about 1 million. The epidemiological status of the disease justifies to search for control measures, new therapeutic options and development of an effective vaccine. Chemotherapy options in malaria are limited, moreover, drug resistant rate is high. In spite of global efforts to develop an effective vaccine yet there is no vaccine available. In the current study, a series of quinolone derivatives were subjected to quantitative structure activity relationship (QSAR) and quantitative structure toxicity relationship (QSTR) analyses to identify the ideal physicochemical characteristics of potential anti-malaria activity and less cytotoxicity. Quinolone with desirable properties was built using HyperChem program, and conformational studies were performed through the semi-empirical method followed by the PM3 force field. Multi linear regression (MLR) was used as a chemo metric tool for quantitative structure activity relationship modeling and the developed models were shown to be statistically significant according to the validation parameters. The obtained QSAR model reveals that the descriptors PJI2, Mv, PCR, nBM, and VAR mainly affect the anti-malaria activity and descriptors MSD, MAXDP, and X1sol affect the cytotoxicity of the series of ligands.

Orally bioavailable 6-chloro-7-methoxy-4(1H)-quinolones efficacious against multiple stages of Plasmodium

The continued proliferation of malaria throughout temperate and tropical regions of the world has promoted a push for more efficacious treatments to combat the disease. Unfortunately, more recent remedies such as artemisinin combination therapies have been rendered less effective due to developing parasite resistance, and new drugs are required that target the parasite in the liver to support the disease elimination efforts. Research was initiated to revisit antimalarials developed in the 1940s and 1960s that were deemed unsuitable for use as therapeutic agents as a result of poor understanding of both physicochemical properties and parasitology. Structure–activity and structure–property relationship studies were conducted to generate a set of compounds with the general 6-chloro-7-methoxy-2-methyl-4(1H)-quinolone scaffold which were substituted at the 3-position with a variety of phenyl moieties possessing various properties. Extensive physicochemical evaluation of the quinolone series was carried out to downselect the most promising 4(1H)-quinolones, 7, 62, 66, and 67, which possessed low-nanomolar EC₅₀ values against W2 and TM90-C2B as well as improved microsomal stability. Additionally, in vivo Thompson test results using Plasmodium berghei in mice showed that these 4(1H)-quinolones were efficacious for the reduction of parasitemia at >99% after 6 days.

Synthesis and cytotoxic activity evaluation of novel arylpiperazine derivatives on human prostate cancer cell lines

A series of novel arylpiperazine derivatives was synthesized. The in vitro cytotoxic activities of all synthesized compounds against three human prostate cancer cell lines (PC-3, LNCaP, and DU145) were evaluated by a CCK-8 assay. Compounds 9 and 15 exhibited strong cytotoxic activities against LNCaP cells (IC₅₀ < 5 μM), and compound 8 (IC₅₀ = 8.25 μM) possessed the most potent activity against DU145 cells. However, these compounds also exhibited cytotoxicity towards human epithelial prostate normal cells RWPE-1. The structure–activity relationship (SAR) of these arylpiperazine derivatives was also discussed based on the obtained experimental data.

4(1H)-pyridone and 4(1H)-quinolone derivatives as antimalarials with erythrocytic, exoerythrocytic, and transmission blocking activities

Infectious diseases are the second leading cause of deaths in the world with malaria being responsible for approximately the same amount of deaths as cancer in 2012. Despite the success in malaria prevention and control measures decreasing the disease mortality rate by 45% since 2000, the development of single-dose therapeutics with radical cure potential is required to completely eradicate this deadly condition. Targeting multiple stages of the malaria parasite is becoming a primary requirement for new candidates in antimalarial drug discovery and development. Recently, 4(1H)- pyridone, 4(1H)-quinolone, 1,2,3,4-tetrahydroacridone, and phenoxyethoxy-4(1H)-quinolone chemotypes have been shown to be antimalarials with blood stage activity, liver stage activity, and transmission blocking activity. Advancements in structure-activity relationship and structure-property relationship studies, biological evaluation in vitro and in vivo, as well as pharmacokinetics of the 4(1H)-pyridone and 4(1H)-quinolone chemotypes are discussed.

4-(1H)-Quinolones and 1,2,3,4-Tetrahydroacridin-9(10H)-ones prevent the transmission of Plasmodium falciparum to Anopheles freeborni

Malaria kills approximately 1 million people a year, mainly in sub-Saharan Africa. Essential steps in the life cycle of the parasite are the development of gametocytes, as well as the formation of oocysts and sporozoites, in the Anopheles mosquito vector. Preventing transmission of malaria through the mosquito is necessary for the control of the disease; nevertheless, the vast majority of drugs in use act primarily against the blood stages. The study described herein focuses on the assessment of the transmission-blocking activities of potent antierythrocytic stage agents derived from the 4(1H)-quinolone scaffold. In particular, three 3-alkyl- or 3-phenyl-4(1H)-quinolones (P4Qs), one 7-(2-phenoxyethoxy)-4(1H)-quinolone (PEQ), and one 1,2,3,4-tetrahydroacridin-9(10H)-one (THA) were assessed for their transmission-blocking activity against the mosquito stages of the human malaria parasite (Plasmodium falciparum) and the rodent parasite (P. berghei). Results showed that all of the experimental compounds reduced or prevented the exflagellation of male gametocytes and, more importantly, prevented parasite transmission to the mosquito vector. Additionally, treatment with ICI 56,780 reduced the number of sporozoites that reached the Anopheles salivary glands. These findings suggest that 4(1H)-quinolones, which have activity against the blood stages, can also prevent the transmission of Plasmodium to the mosquito and, hence, are potentially important drug candidates to eradicate malaria.

Cell-based medicinal chemistry optimization of high-throughput screening (HTS) hits for orally active antimalarials. Part 1: challenges in potency and absorption, distribution, metabolism, excretion/pharmacokinetics (ADME/PK)

Malaria represents a significant health issue, and novel and effective drugs are needed to address parasite resistance that has emerged to the current drug arsenal. Antimalarial drug discovery has historically benefited from a whole-cell (phenotypic) screening approach to identify lead molecules. This approach has been utilized by several groups to optimize weakly active antimalarial pharmacophores, such as the quinolone scaffold, to yield potent and highly efficacious compounds that are now poised to enter clinical trials. More recently, GNF/Novartis, GSK, and others have employed the same approach in high-throughput screening (HTS) of large compound libraries to find novel scaffolds that have also been optimized to clinical candidates by GNF/Novartis. This perspective outlines some of the inherent challenges in cell-based medicinal chemistry optimization, including optimization of oral exposure and hERG activity.

Recent advances in malaria drug discovery

This digest covers some of the most relevant progress in malaria drug discovery published between 2010 and 2012. There is an urgent need to develop new antimalarial drugs. Such drugs can target the blood stage of the disease to alleviate the symptoms, the liver stage to prevent relapses, and the transmission stage to protect other humans. The pipeline for the blood stage is becoming robust, but this should not be a source of complacency, as the current therapies set a high standard. Drug discovery efforts directed towards the liver and transmission stages are in their infancy but are receiving increasing attention as targeting these stages could be instrumental in eradicating malaria.

Exploration of 4(1H)-pyridones as a novel family of potent antimalarial inhibitors of the plasmodial cytochrome bc1

A novel family of antimalarials based on the 4(1H)-pyridone scaffold is described. The compounds display potent antimalarial activity against Plasmodium falciparum in vitro and in vivo. Like atovaquone, 4(1H)-pyridones exert their antimalarial action by inhibiting selectively the electron-transport chain in P. falciparum at the cytochrome bc1 level (complex III). However, despite the similar mechanism of action, no cross-resistance with atovaquone has been found, suggesting that the binding mode of 4(1H)-pyridones might be different from that of atovaquone. The medicinal chemistry program, focused on improving potency and physicochemical properties, ultimately led to the discovery of GSK932121, which was progressed efficiently into first time in human studies. However, progression of GSK932121 was terminated when new toxicology results were obtained in the rat with a soluble phosphate prodrug of the candidate, indicating a potentially narrow therapeutic index.

Quinolone-3-diarylethers: a new class of antimalarial drug

The goal for developing new antimalarial drugs is to find a molecule that can target multiple stages of the parasite's life cycle, thus impacting prevention, treatment, and transmission of the disease. The 4(1H)-quinolone-3-diarylethers are selective potent inhibitors of the parasite's mitochondrial cytochrome bc1 complex. These compounds are highly active against the human malaria parasites Plasmodium falciparum and Plasmodium vivax. They target both the liver and blood stages of the parasite as well as the forms that are crucial for disease transmission, that is, the gametocytes, the zygote, the ookinete, and the oocyst. Selected as a preclinical candidate, ELQ-300 has good oral bioavailability at efficacious doses in mice, is metabolically stable, and is highly active in blocking transmission in rodent models of malaria. Given its predicted low dose in patients and its predicted long half-life, ELQ-300 has potential as a new drug for the treatment, prevention, and, ultimately, eradication of human malaria.

Antimalarial pharmacology and therapeutics of atovaquone

Atovaquone is used as a fixed-dose combination with proguanil (Malarone) for treating children and adults with uncomplicated malaria or as chemoprophylaxis for preventing malaria in travellers. Indeed, in the USA, between 2009 and 2011, Malarone prescriptions accounted for 70% of all antimalarial pre-travel prescriptions. In 2013 the patent for Malarone will expire, potentially resulting in a wave of low-cost generics. Furthermore, the malaria scientific community has a number of antimalarial quinolones with a related pharmacophore to atovaquone at various stages of pre-clinical development. With this in mind, it is timely here to review the current knowledge of atovaquone, with the purpose of aiding the decision making of clinicians and drug developers involved in the future use of atovaquone generics or atovaquone derivatives.

4(1H)-Quinolones with liver stage activity against Plasmodium berghei

With the exception of primaquine, tafenoquine, and atovaquone, there are very few antimalarials that target liver stage parasites. In this study, a transgenic Plasmodium berghei parasite (1052Cl1; PbGFP-Luc(con)) that expresses luciferase was used to assess the anti-liver stage parasite activity of ICI 56,780, a 7-(2-phenoxyethoxy)-4(1H)-quinolone (PEQ), as well as two 3-phenyl-4(1H)-quinolones (P4Q), P4Q-146 and P4Q-158, by using bioluminescent imaging (BLI). Results showed that all of the compounds were active against liver stage parasites; however, ICI 56,780 and P4Q-158 were the most active, with low nanomolar activity in vitro and causal prophylactic activity in vivo. This potent activity makes these compounds ideal candidates for advancement as novel antimalarials.

Lead optimization of 3-carboxyl-4(1H)-quinolones to deliver orally bioavailable antimalarials

Malaria is a protozoal parasitic disease that is widespread in tropical and subtropical regions of Africa, Asia, and the Americas and causes more than 800,000 deaths per year. The continuing emergence of multidrug-resistant Plasmodium falciparum drives the ongoing need for the development of new and effective antimalarial drugs. Our previous work has explored the preliminary structural optimization of 4(1H)-quinolone ester derivatives, a new series of antimalarials related to the endochins. Herein, we report the lead optimization of 4(1H)-quinolones with a focus on improving both antimalarial potency and bioavailability. These studies led to the development of orally efficacious antimalarials including quinolone analogue 20g, a promising candidate for further optimization.