Utility of alkylaminoquinolinyl methanols as new antimalarial drugs

The human malaria-Aotus monkey model: a historical perspective in antimalarial chemotherapy research at the Gorgas Memorial Laboratory-Panama

The human malaria-Aotus monkey model has served the malaria research community since its inception in 1966 at the Gorgas Memorial Laboratory (GML) in Panama. Spanning over five decades, this model has been instrumental in evaluating the in vivo efficacy and pharmacokinetics of a wide array of candidate antimalarial drugs, whether used singly or in combination. The animal model could be infected with drug-resistant and susceptible Plasmodium falciparum and Plasmodium vivax strains that follow a characteristic and reproducible course of infection, remarkably like human untreated and treated infections. Over the years, the model has enabled the evaluation of several synthetic and semisynthetic endoperoxides, for instance, artelinic acid, artesunate, artemether, arteether, and artemisone. These compounds have been evaluated alone and in combination with long-acting partner drugs, commonly referred to as artemisinin-based combination therapies, which are recommended as first-line treatment against uncomplicated malaria. Further, the model has also supported the evaluation of the primaquine analog tafenoquine against blood stages of P. vivax, contributing to its progression to clinical trials and eventual approval. Besides, the P. falciparum/Aotus model at GML has also played a pivotal role in exploring the biology, immunology, and pathogenesis of malaria and in the characterization of drug-resistant P. falciparum and P. vivax strains. This minireview offers a historical overview of the most significant contributions made by the Panamanian owl monkey (Aotus lemurinus lemurinus) to malaria chemotherapy research.

Discovery and Structural Optimization of Acridones as Broad-Spectrum Antimalarials

Malaria remains one of the deadliest diseases in the world today. Novel chemoprophylactic and chemotherapeutic antimalarials are needed to support the renewed eradication agenda. We have discovered a novel antimalarial acridone chemotype with dual-stage activity against both liver-stage and blood-stage malaria. Several lead compounds generated from structural optimization of a large library of novel acridones exhibit efficacy in the following systems: (1) picomolar inhibition of in vitro Plasmodium falciparum blood-stage growth against multidrug-resistant parasites; (2) curative efficacy after oral administration in an erythrocytic Plasmodium yoelii murine malaria model; (3) prevention of in vitro Plasmodium berghei sporozoite-induced development in human hepatocytes; and (4) protection of in vivo P. berghei sporozoite-induced infection in mice. This study offers the first account of liver-stage antimalarial activity in an acridone chemotype. Details of the design, chemistry, structure-activity relationships, safety, metabolic/pharmacokinetic studies, and mechanistic investigation are presented herein.

Molecular Insights into the Interaction of RONS and Thieno[3,2-c]pyran Analogs with SIRT6/COX-2: A Molecular Dynamics Study

SIRT6 and COX-2 are oncogenes target that promote the expression of proinflammatory and pro-survival proteins through a signaling pathway, which leads to increased survival and proliferation of tumor cells. However, COX-2 also suppresses skin tumorigenesis and their relationship with SIRT6, making it an interesting target for the discovery of drugs with anti-inflammatory and anti-cancer properties. Herein, we studied the interaction of thieno[3,2-c]pyran analogs and RONS species with SIRT6 and COX-2 through the use of molecular docking and molecular dynamic simulations. Molecular docking studies revealed the importance of hydrophobic and hydrophilic amino acid residues for the stability. The molecular dynamics study examined conformational changes in the enzymes caused by the binding of the substrates and how those changes affected the stability of the protein-drug complex. The average RMSD values of the backbone atoms in compounds 6 and 10 were calculated from 1000 ps to 10000 ps and were found to be 0.13 nm for both compounds. Similarly, the radius of gyration values for compounds 6 and 10 were found to be 1.87 ± 0.03 nm and 1.86 ± 0.02 nm, respectively. The work presented here, will be of great help in lead identification and optimization for early drug discovery.

Synthesis and Evaluation of Potent KCNQ2/3-Specific Channel Activators

KQT-like subfamily (KCNQ) channels are voltage-gated, noninactivating potassium ion channels, and their down-regulation has been implicated in several hyperexcitability-related disorders, including epilepsy, neuropathic pain, and tinnitus. Activators of these channels reduce the excitability of central and peripheral neurons, and, as such, have therapeutic utility. Here, we synthetically modified several moieties of the KCNQ2-5 channel activator retigabine, an anticonvulsant approved by the U.S. Food and Drug Administration. By introducing a CF3-group at the 4-position of the benzylamine moiety, combined with a fluorine atom at the 3-position of the aniline ring, we generated Ethyl (2-amino-3-fluoro-4-((4-(trifluoromethyl)benzyl)amino)phenyl)carbamate (RL648_81), a new KCNQ2/3-specific activator that is >15 times more potent and also more selective than retigabine. We suggest that RL648_81 is a promising clinical candidate for treating or preventing neurologic disorders associated with neuronal hyperexcitability.

The use of a prodrug approach to minimize potential CNS exposure of next generation quinoline methanols while maintaining efficacy in in vivo animal models

The use of mefloquine (MQ) for antimalarial treatment and prophylaxis has diminished largely in response to concerns about its neurologic side effects. An analog campaign designed to maintain the efficacy of MQ while minimizing blood-brain barrier (BBB) penetration has resulted in the synthesis of a prodrug with comparable-to-superior in vivo efficacy versus mefloquine in a P. berghei mouse model while exhibiting a sixfold reduction in CNS drug levels. The prodrug, WR319670, performed poorly compared to MQ in in vitro efficacy assays, but had promising in vitro permeability in an MDCK-MDR1 cell line BBB permeability screen. Its metabolite, WR308245, exhibited high predicted BBB penetration with excellent in vitro efficacy. Both WR319670 and WR308245 cured 5/5 animals in separate in vivo efficacy studies. The in vivo efficacy of WR319670 was thought to be due to the formation of a more active metabolite, specifically WR308245. This was supported by pharmacokinetics studies in non-infected mice, which showed that both IV and oral administration of WR319670 produced essentially identical levels of WR319670 and WR308245 in both plasma and brain samples at all time points. In these studies, the levels of WR308245 in the brain were 1/4 and 1/6 that of MQ in similar IV and oral studies, respectively. These data show that the use of WR319670 as an antimalarial prodrug was able to maintain efficacy in in vivo efficacy screens, while significantly lowering overall penetration of drug and metabolites across the BBB.

Synthesis of novel quinoline-2-one based chalcones of potential anti-tumor activity

Novel quinoline-2-one based chalcones were synthesized from a Claisen-Schmidt condensation by using the couple KOH/1,4-dioxane as reaction medium. A relatively stable aldol was isolated and identified as the intermediate species in the formation of the target chalcones. Nine of the obtained compounds were in vitro screened by the US National Cancer Institute (NCI) for their ability to inhibit 60 different human tumor cell lines. Products 16c, 16d, 16h and 27 exhibited the highest activity, being compound 27 the most active, displaying remarkable activity against 50 human tumor cell lines, thirteen of them with GI(50) values ≤1.0 μM, being the HCT-116 (Colon, GI(50) = 0.131 μM) and LOX IMVI (Melanoma, GI(50) = 0.134 μM) the most sensitive strains. Compound 27 was referred to in vivo acute toxicity and hollow fiber assay by the Biological Evaluation Committee of the NCI. The acute toxicity study indicated that compound 27 was well tolerated intraperitoneally (150 mg/kg/dose) by athymic nude mice. This compound may possibly be used as lead compound for developing new anticancer agents.

Differences in anti-malarial activity of 4-aminoalcohol quinoline enantiomers and investigation of the presumed underlying mechanism of action

Background

A better anti-malarial efficiency and lower neurotoxicity have been reported for mefloquine (MQ) (+)- enantiomer. However, the importance of stereoselectivity remains poorly understood as the anti-malarial activity of pure enantiomer MQ analogues has never been described. Building on these observations, a series of enantiopure 4-aminoalcohol quinoline derivatives has previously been synthesized to optimize the efficiency and reduce possible adverse effects. Their in vitro activity on Plasmodium falciparum W2 and 3D7 strains is reported here along with their inhibition of β-haematin formation and peroxidative degradation of haemin, two possible mechanisms of action of anti-malarial drugs.

Results

The (S)-enantiomers of this series of 4-aminoalcohol quinoline derivatives were found to be at least as effective as both chloroquine (CQ) and MQ. The derivative with a 5-carbon side-chain length was the more efficient on both P. falciparum strains. (R )-enantiomers displayed an activity decreased by 2 to 15-fold as compared to their (S) counterparts. The inhibition of β-haematin formation was significantly stronger with all tested compounds than with MQ, irrespective of the stereochemistry. Similarly, the inhibition of haemin peroxidation was significantly higher for both (S) and (R)-enantiomers of derivatives with a side-chain length of five or six carbons than for MQ and CQ.

Conclusions

The prominence of stereochemistry in the anti-malarial activity of 4-aminoalcohol quinoline derivatives is confirmed. The inhibition of β-haematin formation and haemin peroxidation can be put forward as presumed mechanisms of action but do not account for the stereoselectivity of action witnessed in vitro.

Long-term effect of a simple nest-box on the reproductive efficiency and other life traits of an Aotus lemurinus lemurinus monkey colony: an animal model for malaria research

BACKGROUND

The long-term effect of a PVC pipe nest-box on the reproductive efficiency and other life traits of an Aotus monkey-breeding colony have not been characterized.

METHODS AND RESULTS

We analyzed laboratory records of the Gorgas Memorial Institute (GMI) Aotus monkey colony in Panama for the period 1999-2010 and found a 273% increase in the annual mean life births in the following 7 years after the introduction of a PVC pipe nest-box in 2002, as well as increases in the mean body mass and survival of laboratory-bred monkeys. Other life traits such as inter-birth interval, parity, birth sex distribution, mortality, and longevity were also determined.

CONCLUSIONS

The use of a PVC pipe nest-box significantly improved the reproductive efficiency and other life traits of the GMI Aotus breeding colony.

Anti-malarial activity of a non-piperidine library of next-generation quinoline methanols

BACKGROUND

The clinical utility for mefloquine has been eroded due to its association with adverse neurological effects. Better-tolerated alternatives are required. The objective of the present study was the identification of lead compounds that are as effective as mefloquine, but exhibit physiochemical properties likely to render them less susceptible to passage across the blood-brain barrier.

METHODS

A library of drug-like non-piperidine analogs of mefloquine was synthesized. These compounds are diverse in structure and physiochemical properties. They were screened in appropriate in vitro assays and evaluated in terms of their potential as lead compounds. The correlation of specific structural attributes and physiochemical properties with activity was assessed.

RESULTS

The most potent analogs were low molecular weight unconjugated secondary amines with no heteroatoms in their side-chains. However, these compounds were more metabolically labile and permeable than mefloquine. In terms of physiochemical properties, lower polar surface area, lower molecular weight, more freely rotatable bonds and fewer H-bond acceptors were associated with greater potency. There was no such relationship between activity and LogP, LogD or the number of hydrogen bond donors (HBDs). The addition of an H-bond donor to the side-chain yielded a series of active diamines, which were as metabolically stable as mefloquine but showed reduced permeability.

CONCLUSIONS

A drug-like library of non-piperidine analogs of mefloquine was synthesized. From amongst this library an active lead series of less permeable, but metabolically stable, diamines was identified.

Epileptogenic potential of mefloquine chemoprophylaxis: a pathogenic hypothesis

Background

Mefloquine has historically been considered safe and well-tolerated for long-term malaria chemoprophylaxis, but prescribing it requires careful attention in order to rule out contraindications to its use. Contraindications include a history of certain neurological conditions that might increase the risk of seizure and other adverse events. The precise pathophysiological mechanism by which mefloquine might predispose those with such a history to seizure remains unclear.

Presentation of the hypothesis

Studies have demonstrated that mefloquine at doses consistent with chemoprophylaxis accumulates at high levels in brain tissue, which results in altered neuronal calcium homeostasis, altered gap-junction functioning, and contributes to neuronal cell death. This paper reviews the scientific evidence associating mefloquine with alterations in neuronal function, and it suggests the novel hypothesis that among those with the prevalent EPM1 mutation, inherited and mefloquine-induced impairments in neuronal physiologic safeguards might increase risk of GABAergic seizure during mefloquine chemoprophylaxis.

Testing and implications of the hypothesis

Consistent with case reports of tonic-clonic seizures occurring during mefloquine chemoprophylaxis among those with family histories of epilepsy, it is proposed here that a new contraindication to mefloquine use be recognized for people with EPM1 mutation and for those with a personal history of myoclonus or ataxia, or a family history of degenerative neurologic disorder consistent with EPM1. Recommendations and directions for future research are presented.

(3RS,4SR)-Methyl 4-(2-chloro-5,8-di-methoxy-quinolin-3-yl)-1-phenyl-pyrrolidine-3-carboxyl-ate

The mol­ecule of the title compound, C₂₃H₂₃ClN₂O₄, contains a quinolyl unit linked to a functionalized pyrrolidine system with a 3,4-trans arrangement of the substituents. The unit cell contains two stereoisomers that have the absolute stereochemistry 3S,4R and 3R,4S. The pyrrolidine ring adopts a twist conformation with pseudo-rotation parameters P = 258.2 (3)° and τ(M) = 35.3 (1)°. The packing is stabilized by C—H⋯π inter­actions and offset π–π stacking (centroid-to-centroid distance = 3.849 Å, inter­planar distance = 3.293 Å and slippage = 1.994 Å) between phenyl rings, leading to a two-dimensional network.

Antimalarial activity of phenylthiazolyl-bearing hydroxamate-based histone deacetylase inhibitors

The antimalarial activity and pharmacology of a series of phenylthiazolyl-bearing hydroxamate-based histone deacetylase inhibitors (HDACIs) was evaluated. In in vitro growth inhibition assays approximately 50 analogs were evaluated against four drug resistant strains of Plasmodium falciparum. The range of 50% inhibitory concentrations (IC(50)s) was 0.0005 to >1 microM. Five analogs exhibited IC(50)s of <3 nM, and three of these exhibited selectivity indices of >600. The most potent compound, WR301801 (YC-2-88) was shown to cause hyperacetylation of P. falciparum histones, which is a marker for HDAC inhibition in eukaryotic cells. The compound also inhibited malarial and mammalian HDAC activity in functional assays at low nanomolar concentrations. WR301801 did not exhibit cures in P. berghei-infected mice at oral doses as high as 640 mg/kg/day for 3 days or in P. falciparum-infected Aotus lemurinus lemurinus monkeys at oral doses of 32 mg/kg/day for 3 days, despite high relative bioavailability. The failure of monotherapy in mice may be due to a short half-life, since the compound was rapidly hydrolyzed to an inactive acid metabolite by loss of its hydroxamate group in vitro (half-life of 11 min in mouse microsomes) and in vivo (half-life in mice of 3.5 h after a single oral dose of 50 mg/kg). However, WR301801 exhibited cures in P. berghei-infected mice when combined at doses of 52 mg/kg/day orally with subcurative doses of chloroquine. Next-generation HDACIs with greater metabolic stability than WR301801 may be useful as antimalarials if combined appropriately with conventional antimalarial drugs.