ABSTRACT
A high-throughput screen (HTS) was undertaken against the respiratory chain dehydrogenase component, NADH:menaquinone oxidoreductase (Ndh) of Mycobacterium tuberculosis (Mtb). The 11000 compounds were selected for the HTS based on the known phenothiazine Ndh inhibitors, trifluoperazine and thioridazine. Combined HTS (11000 compounds) and in-house screening of a limited number of quinolones (50 compounds) identified â¼100 hits and four distinct chemotypes, the most promising of which contained the quinolone core. Subsequent Mtb screening of the complete in-house quinolone library (350 compounds) identified a further â¼90 hits across three quinolone subtemplates. Quinolones containing the amine-based side chain were selected as the pharmacophore for further modification, resulting in metabolically stable quinolones effective against multi drug resistant (MDR) Mtb. The lead compound, 42a (MTC420), displays acceptable antituberculosis activity (Mtb IC50 = 525 nM, Mtb Wayne IC50 = 76 nM, and MDR Mtb patient isolates IC50 = 140 nM) and favorable pharmacokinetic and toxicological profiles.
Subject(s)
Mycobacterium tuberculosis/drug effects , Quinolones/chemical synthesis , Quinolones/pharmacology , Animals , Caco-2 Cells , Carbon-13 Magnetic Resonance Spectroscopy , Drug Design , Electron Transport/drug effects , Hep G2 Cells , High-Throughput Screening Assays , Humans , Microbial Sensitivity Tests , Mycobacterium tuberculosis/metabolism , Proton Magnetic Resonance Spectroscopy , Quinolones/chemistry , Quinolones/pharmacokinetics , Rats , Spectrometry, Mass, Electrospray Ionization , Structure-Activity Relationship , Toxicity TestsABSTRACT
Despite intense efforts, there has not been a truly new antimalarial, possessing a novel mechanism of action, registered for over 10 years. By virtue of a novel mode of action, it is hoped that the global challenge of multidrug-resistant parasites can be overcome, as well as developing drugs that possess prophylaxis and/or transmission-blocking properties, towards an elimination agenda. Many target-based and whole-cell screening drug development programs have been undertaken in recent years and here an overview of specific projects that have focused on targeting the parasite's mitochondrial electron transport chain is presented. Medicinal chemistry activity has largely focused on inhibitors of the parasite cytochrome bc1 Complex (Complex III) including acridinediones, pyridones and quinolone aryl esters, as well as inhibitors of dihydroorotate dehydrogenase that includes triazolopyrimidines and benzimidazoles. Common barriers to progress and opportunities for novel chemistry and potential additional electron transport chain targets are discussed in the context of the target candidate profiles for uncomplicated malaria.
Subject(s)
Antimalarials/chemistry , Antimalarials/pharmacology , Electron Transport Complex III/antagonists & inhibitors , Malaria, Falciparum/drug therapy , Plasmodium falciparum/enzymology , Animals , Antimalarials/therapeutic use , Drug Discovery , Electron Transport , Electron Transport Complex III/metabolism , Humans , Malaria, Falciparum/parasitology , Mitochondria/drug effects , Mitochondria/enzymology , Models, Molecular , Molecular Targeted Therapy , Plasmodium falciparum/drug effectsABSTRACT
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.
Subject(s)
Antimalarials/therapeutic use , Atovaquone/therapeutic use , Malaria/drug therapy , Antimalarials/pharmacology , Atovaquone/pharmacology , Chemoprevention/methods , Drug Combinations , Humans , Malaria/prevention & control , Proguanil/pharmacology , Proguanil/therapeutic use , United StatesABSTRACT
There is an urgent need for new antimalarial drugs with novel mechanisms of action to deliver effective control and eradication programs. Parasite resistance to all existing antimalarial classes, including the artemisinins, has been reported during their clinical use. A failure to generate new antimalarials with novel mechanisms of action that circumvent the current resistance challenges will contribute to a resurgence in the disease which would represent a global health emergency. Here we present a unique generation of quinolone lead antimalarials with a dual mechanism of action against two respiratory enzymes, NADH:ubiquinone oxidoreductase (Plasmodium falciparum NDH2) and cytochrome bc(1). Inhibitor specificity for the two enzymes can be controlled subtly by manipulation of the privileged quinolone core at the 2 or 3 position. Inhibitors display potent (nanomolar) activity against both parasite enzymes and against multidrug-resistant P. falciparum parasites as evidenced by rapid and selective depolarization of the parasite mitochondrial membrane potential, leading to a disruption of pyrimidine metabolism and parasite death. Several analogs also display activity against liver-stage parasites (Plasmodium cynomolgi) as well as transmission-blocking properties. Lead optimized molecules also display potent oral antimalarial activity in the Plasmodium berghei mouse malaria model associated with favorable pharmacokinetic features that are aligned with a single-dose treatment. The ease and low cost of synthesis of these inhibitors fulfill the target product profile for the generation of a potent, safe, and inexpensive drug with the potential for eventual clinical deployment in the control and eradication of falciparum malaria.
Subject(s)
Antimalarials/pharmacology , Malaria, Falciparum/drug therapy , Malaria, Falciparum/prevention & control , Plasmodium falciparum/drug effects , Pyridines/pharmacology , Quinolones/pharmacology , Animals , Antimalarials/chemistry , Cells, Cultured , Electron Transport/drug effects , Electron Transport Complex I/antagonists & inhibitors , Electron Transport Complex III/antagonists & inhibitors , Hepatocytes/cytology , Hepatocytes/parasitology , Macaca mulatta , Malaria, Falciparum/parasitology , Male , Mice , Mice, Inbred Strains , Mitochondria/drug effects , Plasmodium berghei/drug effects , Plasmodium berghei/growth & development , Plasmodium cynomolgi/drug effects , Plasmodium cynomolgi/growth & development , Plasmodium falciparum/growth & development , Pyridines/chemistry , Quinolones/chemistryABSTRACT
Malaria is responsible for approximately 1 million deaths annually; thus, continued efforts to discover new antimalarials are required. A HTS screen was established to identify novel inhibitors of the parasite's mitochondrial enzyme NADH:quinone oxidoreductase (PfNDH2). On the basis of only one known inhibitor of this enzyme, the challenge was to discover novel inhibitors of PfNDH2 with diverse chemical scaffolds. To this end, using a range of ligand-based chemoinformatics methods, ~17000 compounds were selected from a commercial library of ~750000 compounds. Forty-eight compounds were identified with PfNDH2 enzyme inhibition IC(50) values ranging from 100 nM to 40 µM and also displayed exciting whole cell antimalarial activity. These novel inhibitors were identified through sampling 16% of the available chemical space, while only screening 2% of the library. This study confirms the added value of using multiple ligand-based chemoinformatic approaches and has successfully identified novel distinct chemotypes primed for development as new agents against malaria.
Subject(s)
Antimalarials/chemistry , Databases, Factual , Plasmodium falciparum/enzymology , Protozoan Proteins/antagonists & inhibitors , Quantitative Structure-Activity Relationship , Quinone Reductases/antagonists & inhibitors , Antimalarials/pharmacology , Bayes Theorem , High-Throughput Screening Assays , Informatics , Parasitic Sensitivity Tests , Plasmodium falciparum/drug effects , Principal Component Analysis , Protozoan Proteins/chemistry , Quinone Reductases/chemistryABSTRACT
A program was undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a dehydrogenase of the mitochondrial electron transport chain of the malaria parasite Plasmodium falciparum. PfNDH2 has only one known inhibitor, hydroxy-2-dodecyl-4-(1H)-quinolone (HDQ), and this was used along with a range of chemoinformatics methods in the rational selection of 17 000 compounds for high-throughput screening. Twelve distinct chemotypes were identified and briefly examined leading to the selection of the quinolone core as the key target for structure-activity relationship (SAR) development. Extensive structural exploration led to the selection of 2-bisaryl 3-methyl quinolones as a series for further biological evaluation. The lead compound within this series 7-chloro-3-methyl-2-(4-(4-(trifluoromethoxy)benzyl)phenyl)quinolin-4(1H)-one (CK-2-68) has antimalarial activity against the 3D7 strain of P. falciparum of 36 nM, is selective for PfNDH2 over other respiratory enzymes (inhibitory IC(50) against PfNDH2 of 16 nM), and demonstrates low cytotoxicity and high metabolic stability in the presence of human liver microsomes. This lead compound and its phosphate pro-drug have potent in vivo antimalarial activity after oral administration, consistent with the target product profile of a drug for the treatment of uncomplicated malaria. Other quinolones presented (e.g., 6d, 6f, 14e) have the capacity to inhibit both PfNDH2 and P. falciparum cytochrome bc(1), and studies to determine the potential advantage of this dual-targeting effect are in progress.
Subject(s)
Antimalarials/chemical synthesis , Plasmodium falciparum/enzymology , Quinolones/chemical synthesis , Quinone Reductases/antagonists & inhibitors , Administration, Oral , Animals , Antimalarials/chemistry , Antimalarials/pharmacology , Crystallography, X-Ray , Drug Design , Electron Transport Complex III/antagonists & inhibitors , Humans , In Vitro Techniques , Malaria/drug therapy , Male , Mice , Microsomes, Liver/metabolism , Models, Molecular , Parasitic Sensitivity Tests , Plasmodium berghei , Plasmodium falciparum/drug effects , Quinolones/chemistry , Quinolones/pharmacology , Structure-Activity RelationshipABSTRACT
Following a program undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a novel enzyme target within the malaria parasite Plasmodium falciparum, hit to lead optimization led to identification of CK-2-68, a molecule suitable for further development. In order to reduce ClogP and improve solubility of CK-2-68 incorporation of a variety of heterocycles, within the side chain of the quinolone core, was carried out, and this approach led to a lead compound SL-2-25 (8b). 8b has IC(50)s in the nanomolar range versus both the enzyme and whole cell P. falciparum (IC(50) = 15 nM PfNDH2; IC(50) = 54 nM (3D7 strain of P. falciparum) with notable oral activity of ED(50)/ED(90) of 1.87/4.72 mg/kg versus Plasmodium berghei (NS Strain) in a murine model of malaria when formulated as a phosphate salt. Analogues in this series also demonstrate nanomolar activity against the bc(1) complex of P. falciparum providing the potential added benefit of a dual mechanism of action. The potent oral activity of 2-pyridyl quinolones underlines the potential of this template for further lead optimization studies.
Subject(s)
Antimalarials/chemical synthesis , Plasmodium falciparum/enzymology , Pyridines/chemical synthesis , Quinolones/chemical synthesis , Quinone Reductases/antagonists & inhibitors , Administration, Oral , Animals , Antimalarials/chemistry , Antimalarials/pharmacology , Atovaquone/pharmacology , Crystallography, X-Ray , Cytochromes b/genetics , Drug Design , Drug Resistance , Humans , Malaria/drug therapy , Male , Mice , Microsomes, Liver/metabolism , Models, Molecular , Parasitic Sensitivity Tests , Plasmodium berghei , Plasmodium falciparum/drug effects , Plasmodium falciparum/genetics , Pyridines/chemistry , Pyridines/pharmacology , Quinolones/chemistry , Quinolones/pharmacology , Rats , Structure-Activity RelationshipABSTRACT
We extend our approach of combination chemotherapy through a single prodrug entity (O'Neill et al. Angew. Chem., Int. Ed. 2004, 43, 4193) by using a 1,2,4-trioxolane as a protease inhibitor carbonyl-masking group. These molecules are designed to target the malaria parasite through two independent mechanisms of action: iron(II) decomposition releases the carbonyl protease inhibitor and potentially cytotoxic C-radical species in tandem. Using a proposed target "heme", we also demonstrate heme alkylation/carbonyl inhibitor release and quantitatively measure endoperoxide turnover in parasitized red blood cells.
Subject(s)
Antimalarials/chemical synthesis , Chalcones/chemical synthesis , Cysteine Endopeptidases/metabolism , Cysteine Proteinase Inhibitors/chemical synthesis , Peroxides/chemical synthesis , Prodrugs/chemical synthesis , Antimalarials/chemistry , Antimalarials/pharmacology , Chalcones/chemistry , Chalcones/pharmacology , Cysteine Proteinase Inhibitors/chemistry , Cysteine Proteinase Inhibitors/pharmacology , Erythrocytes/drug effects , Erythrocytes/parasitology , Inhibitory Concentration 50 , Models, Molecular , Peroxides/chemistry , Peroxides/pharmacology , Plasmodium falciparum/drug effects , Prodrugs/chemistry , Prodrugs/pharmacology , Structure-Activity RelationshipSubject(s)
Antimalarials/chemistry , Spiro Compounds/chemistry , Tetraoxanes/chemistry , Alkylation , Antimalarials/chemical synthesis , Antimalarials/pharmacology , Artemisinins/chemical synthesis , Artemisinins/chemistry , Artemisinins/pharmacology , Drug Resistance, Multiple , Plasmodium falciparum/drug effects , Plasmodium falciparum/isolation & purification , Small Molecule Libraries/chemistry , Spiro Compounds/chemical synthesis , Tetraoxanes/chemical synthesis , Tetraoxanes/pharmacologyABSTRACT
Malaria is a global health problem that causes significant mortality and morbidity, with more than 1 million deaths per year caused by Plasmodium falciparum. Most antimalarial drugs face decreased efficacy due to the emergence of resistant parasites, which necessitates the discovery of new drugs. To identify new antimalarials, we developed an automated 384-well plate screening assay using P. falciparum parasites that stably express cytoplasmic firefly luciferase. After initial optimization, we tested two different types of compound libraries: known bioactive collections (Library of Pharmacologically Active Compounds [LOPAC] and the library from the National Institute of Neurological Disorders and Stroke [NINDS]) and a library of uncharacterized compounds (ChemBridge). A total of 12,320 compounds were screened at 5.5 microM. Selecting only compounds that reduced parasite growth by 85% resulted in 33 hits from the combined bioactive collection and 130 hits from the ChemBridge library. Fifteen novel drug-like compounds from the bioactive collection were found to be active against P. falciparum. Twelve new chemical scaffolds were found from the ChemBridge hits, the most potent of which was a series based on the 1,4-naphthoquinone scaffold, which is structurally similar to the FDA-approved antimalarial atovaquone. However, in contrast to atovaquone, which acts to inhibit the bc(1) complex and block the electron transport chain in parasite mitochondria, we have determined that our new 1,4-napthoquinones act in a novel, non-bc(1)-dependent mechanism and remain potent against atovaquone- and chloroquine-resistant parasites. Ultimately, this study may provide new probes to understand the molecular details of the malaria life cycle and to identify new antimalarials.
Subject(s)
Antimalarials/pharmacology , Drug Resistance, Multiple/drug effects , Parasitic Sensitivity Tests/methods , Plasmodium falciparum/drug effects , Animals , Antimalarials/chemistry , Molecular StructureABSTRACT
On the basis of a mechanistic understanding of the toxicity of the 4-aminoquinoline amodiaquine (1b), three series of amodiaquine analogues have been prepared where the 4-aminophenol "metabolic alert" has been modified by replacement of the 4'-hydroxy group with a hydrogen, fluorine, or chlorine atom. Following antimalarial assessment and studies on mechanism of action, two candidates were selected for detailed ADME studies and in vitro and in vivo toxicological assessment. 4'-Fluoro-N-tert-butylamodiaquine (2k) was subsequently identified as a candidate for further development studies based on potent activity versus chloroquine-sensitive and resistant parasites, moderate to excellent oral bioavailability, low toxicity in in vitro studies, and an acceptable safety profile.
Subject(s)
Aminoquinolines/chemical synthesis , Amodiaquine/analogs & derivatives , Amodiaquine/chemical synthesis , Antimalarials/chemical synthesis , Aminoquinolines/pharmacokinetics , Aminoquinolines/pharmacology , Amodiaquine/chemistry , Amodiaquine/pharmacokinetics , Amodiaquine/pharmacology , Animals , Antimalarials/pharmacokinetics , Antimalarials/pharmacology , Cell Survival , Chloroquine/pharmacology , Dogs , Drug Resistance , Female , Haplorhini , Hepatocytes/cytology , Hepatocytes/drug effects , Humans , In Vitro Techniques , Malaria/drug therapy , Malaria/parasitology , Male , Mice , Parasitic Sensitivity Tests , Plasmodium berghei/drug effects , Plasmodium falciparum/drug effects , Plasmodium yoelii/drug effects , Rats , Rats, Wistar , Structure-Activity RelationshipABSTRACT
A novel series of semi-synthetic trioxaquines and synthetic trioxolaquines were prepared, in moderate to good yields. Antimalarial activity was evaluated against both the chloroquine-sensitive 3D7 and resistant K1 strain of Plasmodium falciparum and both series of compounds were shown to be active in the low nanomolar range. For comparison the corresponding 9-amino acridine analogues were also prepared and shown to have low nanomolar activity like their quinoline counterparts.
Subject(s)
Antimalarials/chemical synthesis , Peroxides/chemistry , Plasmodium falciparum/drug effects , Quinolines/chemical synthesis , Aminacrine/chemical synthesis , Aminacrine/chemistry , Animals , Antimalarials/chemistry , Antimalarials/pharmacology , Artemisinins/chemical synthesis , Artemisinins/chemistry , Peroxides/chemical synthesis , Quinolines/chemistry , Quinolines/pharmacology , Structure-Activity RelationshipABSTRACT
N-tert-Butyl isoquine (4) (GSK369796) is a 4-aminoquinoline drug candidate selected and developed as part of a public-private partnership between academics at Liverpool, MMV, and GSK pharmaceuticals. This molecule was rationally designed based on chemical, toxicological, pharmacokinetic, and pharmacodynamic considerations and was selected based on excellent activity against Plasmodium falciparum in vitro and rodent malaria parasites in vivo. The optimized chemistry delivered this novel synthetic quinoline in a two-step procedure from cheap and readily available starting materials. The molecule has a full industry standard preclinical development program allowing first into humans to proceed. Employing chloroquine (1) and amodiaquine (2) as comparator molecules in the preclinical plan, the first preclinical dossier of pharmacokinetic, toxicity, and safety pharmacology has also been established for the 4-aminoquinoline antimalarial class. These studies have revealed preclinical liabilities that have never translated into the human experience. This has resulted in the availability of critical information to other drug development teams interested in developing antimalarials within this class.