- Open Access
Diversity-oriented synthesis and activity evaluation of substituted bicyclic lactams as anti-malarial against Plasmodium falciparum
- Vijeta Sharma†1,
- Shalini Agarwal†2,
- Sanjay M Madurkar†3,
- Gaurav Datta2,
- Poonam Dangi1,
- Ramu Dandugudumula1,
- Subhabrata Sen1Email author and
- Shailja Singh1Email author
© Sharma et al.; licensee BioMed Central Ltd. 2014
- Received: 29 September 2014
- Accepted: 19 November 2014
- Published: 28 November 2014
Malaria remains the world’s most important devastating parasitic disease. Of the five species of Plasmodium known to infect and cause human malaria, Plasmodium falciparum is the most virulent and responsible for majority of the deaths caused by this disease. Mainstream drug therapy targets the asexual blood stage of the malaria parasite, as the disease symptoms are mainly associated with this stage. The prevalence of malaria parasite strains resistance to existing anti-malarial drugs has made the control of malaria even more challenging and hence the development of a new class of drugs is inevitable.
Screening against different drug resistant and sensitive strains of P. falciparum was performed for few bicyclic lactam-based motifs, exhibiting a broad spectrum of activity with low toxicity generated via a focussed library obtained from diversity oriented synthesis (DOS). The synthesis and screening was followed by an in vitro assessment of the possible cytotoxic effect of this class of compounds on malaria parasite.
The central scaffold a chiral bicyclic lactam (A) and (A’) which were synthesized from (R)-phenylalaninol, levulinic acid and 3-(2-nitrophenyl) levulinic acid respectively. The DOS library was generated from A and from A’, by either direct substitution with o-nitrobenzylbromide at the carbon α- to the amide functionality or by conversion to fused pyrroloquinolines. Upon screening this diverse library for their anti-malarial activity, a dinitro/diamine substituted bicyclic lactam was found to demonstrate exceptional activity of >85% inhibition at 50 μM concentration across different strains of P. falciparum with no toxicity against mammalian cells. Also, loss of mitochondrial membrane potential, mitochondrial functionality and apoptosis was observed in parasite treated with diamine-substituted bicyclic lactams.
This study unveils a DOS-mediated exploration of small molecules with novel structural motifs that culminates in identifying a potential lead molecule against malaria. In vitro investigations further reveal their cytocidal effect on malaria parasite growth. It is not the first time that DOS has been used as a strategy to identify therapeutic leads against malaria, but this study establishes the direct implications of DOS in scouting novel motifs with anti-malarial activity.
- Bicyclic lactams
- Anti-malarial activity
- Plasmodium falciparum
Malaria, a leading cause of morbidity and mortality, is still the world’s most important parasitic disease . Globally, an estimated 3.4 billion people are at risk of malaria. Out of all known species of malaria parasite, Plasmodium falciparum causes the most virulent form of malaria and is a leading infectious cause of morbidity and mortality . Of the nearly 1,400 drugs registered worldwide in the last quarter of the 20th Century, only four were anti-malarials . No promising and efficient anti-malarial vaccine is available despite various vaccines being produced and tested [4, 5]. Adding to the woes are the prevalent P. falciparum strains resistant to the anti-malarial drugs, such as chloroquine and artemisinin [6, 7]. Despite this resistance, combination-based therapy can continue to cure patients provided novel drugs are introduced and included in the combination . Therefore, prompt endeavours are prerequisite to develop a new class of drugs as chemotherapeutic anti-malarials.
Diversity oriented synthesis (DOS) is one of the leading approaches in present day drug discovery to identify new class of compounds. It is considered one of the most useful techniques, to provide distinct molecular scaffolds that occupy different regions of chemical space . It utilizes a combination of building block-, appendage-, stereochemical-, and skeletal-diversity to access structurally disparate compounds . Among these, skeletal diversity, that deals with the scaffold architecture of the small molecules, is the most important to attain.
Herein, chiral bicyclic lactams (A and A’) (the central scaffolds of DOS-based strategy) were synthesized from (R)-phenylalaninol/levulinic acid and 3-(2-nitro)levulinic acid. The diversification strategies involved: (a) enolate-mediated substitution with o-nitrobenzylbromide at the carbon α –to the amide functionality (from A); and, (b) lewis acid-based ring opening of the oxazoline ring of the bicyclic lactams, followed by a 6-endo-trig ring closure to generate fused pyrroloquinolines (from A and A’). The nitroaryl lactams were further reduced to generate more lipophilic amines. Upon testing this focused library for their anti-malarial activity, dinitro/diamine substituted bicyclic lactams were found most effective growth inhibitory compound against both drug-resistant clone Dd2 and drug-sensitive clones HB3 and 3D7 of P. falciparum.
Chiral bicyclic lactams A and A’ were synthesized from (R)-phenylalaninol and levulinic acid and 3-(2-nitrophenyl) levulinic acid. A was transformed to B 1 , B 2 , C and F by substituting with various equivalents of o-nitrobenzylbromide at the carbon α to the amide functionality or by treating A with TiCl4via a known procedure in the literature . G was synthesized in a similar manner as F.
The bicyclic lactam A was synthesized by condensation of R-phenylalaninol with levulinic acid in presence of para-toluenesulfonic acid (PTSA) under toluene reflux with excellent yield (84%) and high diastreoselectivity (98%). Lactam A was alkylated with o-nitrobenzyl bromide via enolate-mediated alkylation. A typical procedure involved enolizing a tetrahydrofuran (THF) solution of A at −78°C with lithium hexamethyl disilyl amide (LHMDS) (2 eq.), followed by addition of o-nitrobenzylbromide (1 eq.). The reaction was warmed to room temperature (RT) and was monitored via liquid chromatography mass spectroscopy (LCMS). Once it indicated the completion of mono-alkylation (which is after ~12 hour) half of the reaction mixture was syringed to a different flask, then quenched and purified to generate two monobenzylated diastereomers B1 and B2. The remaining reaction mixture was further cooled to −78°C and then treated with LHMDS (2 eq.), and stirred for about 30 minutes, followed by the addition of an equivalent of o-nitrobenzylbromide. LCMS monitoring indicated completion of the reaction by next 16 hours, after which it was quenched and purified to obtain the dibenzylated bicyclic lactam C. Facile hydrogenation with 10% w/w Pd-C and H2 (at atm pressure), of the nitrobicyclic lactams B1/B2 and C generated the desired amines E1/E2 and D.
In an effort to synthesize compound A’, 3-(2-nitrophenyl) levulinic acid (12 g, 50.6 mmol), (R)-phenyl alaninol (7.3 g, 52 mmol) and PTSA (12 mg, cat) were dissolved in toluene (120 mL and the flask (equipped with a Dean-Stark trap) was heated to reflux. After 16 hours the solution was cooled, washed with saturated aqueous NaHCO3, dried, and concentrated. The final compound was purified by flash column chromatography on silica gel as stationary phase and Hexane/EtOAc, 1:1 as the eluent to afford the desired compound, which was then triturated with diethyl ether to afford the bicyclic lactam intermediate A’ (7.4 g, 41.5%) as white solid.
The intermediate A’ (500 mg, 1.42 mmol) was treated with titanium chloride (TiCl4) (0.4 mL, 3.55 mmol) in dichloromethane (DCM) (15 mL) and the resulting reaction mixture was stirred at 20°C. Once thin layer chromatography (TLC) indicated complete consumption of the starting material, the reaction was quenched with saturated aqueous solution of NaHCO3 (20 mL) and was filtered. The organic layer was separated and the aqueous layer was extracted further with dichloromethane (DCM). The combined organic layers were dried over sodium sulphate (Na2SO4) and evaporated to generate compound G in 88:12 diastereomeric ratio. The major product was purified by flash column chromatography. Compounds B1, B2, C, D, E1, E2, F and G were dissolved in dimethyl sulfoxide (DMSO; 10 mg/mL) (Sigma-Aldrich Chemie GmbH, Buchs, Switzerland) and stored at 4°C for use as anti-malarials.
Plasmodium falciparum clones used in this study 3D7, Dd2 and HB3 [12–14], were cultured in O+ human erythrocytes supplemented with RPMI 1640 (Invitrogen, USA) 24 mM sodium bicarbonate (Sigma, USA), 0.1 mM hypoxanthine (Invitrogen, USA), 25 mg/ml gentamicin (Invitrogen, USA) and 0.5% AlbuMax I (Invitrogen, USA), according to methods described earlier . Parasite culture was maintained in mixed gas environment (5% O2, 5% CO2 and 90% N2). Parasites were synchronized by sorbitol treatment at ring stage.
Kidney fibroblast-like cell lines (COS-7 cells) were cultured in Dulbecco’s Modified Eagle’s Medium (Invitrogen, USA) supplemented with 10% foetal bovine serum (Gibco, USA) and penicillin (100 units/ml) and streptomycin (100 mg/ml). All cultures were incubated under standard culture conditions (37°C, 5% CO2).
Growth inhibition assays
Compounds were tested at different concentrations for inhibition of growth by P. falciparum strains 3D7, HB3 and Dd2. Briefly, the parasites were first synchronized by the purification of schizont-stage parasites on a Percoll gradient, followed by two rounds of treatment of the ring-stage parasites with sorbitol. Schizont-stage parasites at an initial parasitaemia of 1% and at 2% haematocrit were incubated with compounds at concentrations ranging from 0.5-50 μM, untreated as control, for one cycle of parasite growth (40 hours post invasion). Following a 40-hour incubation, the whole sample was collected and washed twice with PBS and stained with ethidium bromide (10 μM) for 15 minutes at room temperature in dark. The cells were washed with PBS, and analysed by flow cytometry on FACSCalibur (Becton Dickinson) using CellQuest software . Fluorescence signal (FL-2) was detected with the 590 nm band pass filter using an excitation laser of 488 nm collecting 100,000 cells per sample. Following acquisition, parasitaemia was estimated by detecting newly infected parasites, by determining the proportion of FL-2-positive cells using Cell Quest.
The data for the IC50 values and % parasite growth are expressed as the mean ± standard deviation (SD) of three independent experiments done in duplicates. IC50 values were calculated using Graph Pad Prism software.
The effect of compounds showing most potent growth inhibition (compounds C and D), was tested on progression of P. falciparum 3D7 strain at different stages (rings (R), trophozoites (T), schizonts (S)) and release of merozoites from schizonts. Ring-stage parasite culture was diluted to 2% parasitaemia and 2% haematocrit in a complete RPMI medium and treated with compound (50 μM) or solvent as control and incubated further for 12, 26, 48, and 54 hours blood-stage asexual cycle to monitor progression at each stage. Morphological analysis and counting (~1,000 cells/Giemsa-stained slides in duplicate) were done at each of these stages to monitor progression. Giemsa-stained thin blood smears of P. falciparum were made at every 12, 26, 48, and 54 hours post invasion and around 3,000 red blood cells (RBCs) were counted by light microscopy. To determine the relative per cent frequency of different stages of parasite (R, T, S), a pie diagram was made using Graph Pad Prism software.
Cytotoxicity assay with COS-7 cell line
COS-7 cells seeded in 96-well plates at a seeding density of 30,000 cells per well were allowed to adhere overnight at 37°C. Adhered cells were treated with compounds C and D at 50 μM concentrations for 24 hours. Cytotoxic effect was assessed using in vitro toxicology assay kit (Sigma-Aldrich, St Louis, MO, USA), based on cleavage of MTT ((3-[4,5- dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide)) by viable cells into formazan crystals.
Terminal deoxynucleotidyl transferase (TdT) mediated dUTP nick end labelling (TUNEL) assay
Parasites at late ring/early trophozoite stages were treated with either compound D or solvent alone and incubated for 12 hours. The DNA fragmentation in treated and untreated samples was assessed by TUNEL using In Situ Cell Death Detection Kit, TMR Red (Roche Applied Science), as described previously . Briefly, samples were fixed with 4% paraformaldehyde (Sigma-Aldrich, Fluka Chemicals) and 0.0075% glutaraldehyde in PBS for 30 minutes at room temperature, washed with PBS and permeabilized by 0.01% Triton-X 100. Following this, RBCs were incubated with a mix of TdT enzyme and TMR Red labelled dUTP for one hour at 37°C and washed thrice with 1X PBS. The labelled parasites were then observed in Nikon A1 confocal microscope and the percentage of TUNEL positive cells were calculated.
JC-1 staining for estimation of mitochondrial membrane potential
The cell permeable lipophilic cation probe JC-1 (Molecular Probes, Eugene, OR, USA) exists as monomers in cytoplasm and emits green fluorescence (525 nm). Within the mitochondria, JC-1 forms aggregates emitting red fluorescence (590 nm) . In healthy cells with high mitochondrial transmembrane potential the aggregates show an intense red fluorescence. Cells in which the potential has been lost show a weak red fluorescence and mostly green fluorescence. A decrease in the red to green fluorescence ratio therefore signifies a loss in the mitochondrial transmembrane potential. To estimate any changes in mitochondrial membrane potential, parasites treated with compound D or solvent alone were incubated with 5 μM JC-1 for 30 minutes at 37°C, washed twice with PBS and imaged immediately using a Nikon A1 confocal microscope. To avoid bias, images for both the sets were captured using the same laser intensity, voltage and offset values in all the three channels. For each set, the ratio of red to green fluorescence in 100 parasites from randomly selected fields was estimated. For the estimation of fluorescence intensity in each channel, an infected and stained RBC was selected using the Region of Interest (ROI) tool provided in the Nikon NIS Elements AR Analysis V4.13.04 software. From this ROI area, the fluorescence intensity in each channel was noted as given by the software in ROI statistics. These values were used to calculate the ratio of red to green fluorescence in 100 RBCs in those fields for each set of parasite culture.
Synthesis of substituted chiral bicyclic lactams and pyrroloisoquinolines as potential anti-malarials
The diastereoselective alkylation of A afforded epimeric o-nitrobenzylbicyclic lactams B 1 and B 2 . Their subsequent hydrogenation generated the corresponding amine E 1 and E 2 . In a different reaction, excess of base and prolonged reaction time generated the dialkylated analog C that was also hydrogenated to provide the amine D. In a separate effort A was treated with TiCl4 in dichloromethane to undergo the oxazoline ring opening, followed by a 6-endo-trig cyclization to afford F. Finally, a novel bicyclic lactam A’ was synthesized by condensation of L-phenylalaninol with 2-(2-nitrophenyl)-levulinic acid. In a similar fashion as in A’, it was converted to G.
In vitro growth inhibitory activity of substituted bicyclic lactams against Plasmodium falciparum
Initially the screening of the biological activity of substituted bicyclic lactams was done at different concentrations for inhibition of intraerythrocytic growth by P. falciparum strain 3D7. The compounds C, D, B1, B2, E1, F, and G (as mentioned in Methods) were tested at different concentrations over one cycle of parasite growth. The untreated parasites served as the control and percentage growth inhibition was measured by flow cytometry. Compound C exhibited 39.9 and 85.27% of growth inhibition at 10 μM and 50 μM of concentration, respectively. Compound D exhibited 25.24 and 77.84% of growth inhibition at 10 μM and 50 μM of concentration, respectively (Figure 1B, Additional file 1). Concurrently, other compounds exhibited negligible effect (Figure 1B). This indicated that the dinitro-substituted compound C and the more lipophilic diamine-substituted compound D have more significant effect on growth inhibition compared to the other compounds in the same series (compound B1 ,B2 ,E 1 , F, and G) and are thus the most potent of all the substituted bicyclic lactams tested.
Half maximal inhibitory concentration (IC50) of compound C and compound D
Stage-specific effect of compounds on Plasmodium falciparum progression in vitro
Morphological analysis and counting (~3,000 cells/Giemsa-stained slides in duplicate) over the 48-hour blood stage cycle revealed healthy trophozoites 26 hours post-invasion (hpi) either in presence or absence of compounds (Figure 2B). At ~48 hpi healthy schizonts and new rings were observed in solvent control but were >70% reduced in the presence of either compound C or, compound D (Figure 2B). Parasites remained stalled at early to late trophozoites on treatment with compound C, D even at ~54 hpi.
Growth inhibition activity of compounds across three different strains of Plasmodium falciparum
Effect of compounds on mammalian cell viability
Compounds C and D, having significant effect on growth of malaria parasite, were further assessed for cytotoxic effects on COS-7 mammalian cell line. The degree of cytotoxicity caused was measured spectrophotometrically, based on the protocol described previously by Mossmann . No significant cytotoxicity was observed at concentration as high as 50 μM for both compounds C and D treated cells.
Substituted bicyclic lactam compound D reduces parasite-mitochondrial membrane potential
Compound D-induced apoptosis as detected via TUNEL assay
Next, DNA fragmentation was assessed in treated and untreated samples by TUNEL using In Situ Cell Death Detection Kit, TMR Red (Roche Applied Science). The labelled parasites were observed in Nikon A1 confocal microscope and the percentage of TUNEL-positive cells were calculated. The percentage of TUNEL-positive parasites after treatment with compound D increased by 60% compared to solvent control (Figure 4B (i)). Also the fluorescent image of parasites treated with compound D, stained with TMR Red (TUNEL staining) showed DNA fragmentation (Figure 4B (ii)).
Owning to immediate need of development of anti-malarials, a new class of substituted bicyclic lactams was developed and evaluated for their anti-malarial activity against the asexual erythrocytic stage of P. falciparum. Lactams are reported as a large class of antibiotics characterized by the presence of the azetidin-2-one/oxazol-2-one rings, which is the core of their biological activities. They are further differentiated by side chains, unsaturations, heteroatoms, and, in many cases, by the presence of another five- or six-membered rings [20, 21]. Biologically active lactams range from β-lactams, used in diverse therapeutic areas viz serine-dependent enzyme inhibitors, matrix-metalloprotease inhibitors, and even apoptosis inductors [22, 23], the inhibition of HIV-1 protease, antitumor activity, anti-malarial activity, and cholesterol absorption inhibition [24–30], peptides, bicyclic lactams (as potential anticancer and anti-malarials) etc. This study, reports the genesis (from DOS) of substituted bicyclic lactams and their cyotoxic effect on malaria parasite growth, with the objective to contribute new anti-malarial agents against P. falciparum. It is not the first time DOS has been used to generate efficacious small molecules against malaria. Schreiber et al. has relentlessly researched on various kinds of molecules ranging from small molecules to macrocycles evolved from DOS towards generating a therapeutic compound against malaria. NITD-609, Novartis’ recent molecule in phase 2 clinical trials for malaria is also the result of a DOS inspired effort. Additionally Brown et al. also demonstrated the prowess of DOS in generating antimalarial compounds [31–35]. A DOS library was generated from A and A’ by direct substitution with o-nitrobenzylbromide at the carbon α to the amide functionality and also converting A’ into fused pyrroloquinolines. A rational chemistry-based approach is undertaken to design molecules modifying the compounds for less toxicity, bioavailability, more stability, solubility, and other attractive features . Based on preliminary screening results, the dinitro-substituted bicyclic lactam has emerged as the most active compound in the series. The nitro group in compound C was consequently reduced to amine (compound D) as amines are more soluble due to their hydrophilic nature and hence are more bio-available compared to their nitro counterpart. The nitroaryl lactams (compound C) and the further reduced, more lipophilic amine forms (compound D) demonstrated exceptional activity, showing >85% inhibition at 50 μM concentration across different drug-sensitive and drug-resistant strains of P. falciparum (Figure 3A,B). It is noteworthy that the introduction of modification attributed not much change in antiparasitic activity as both compounds showed significant effect on the parasite growth and exerted no cytotoxic effect on mammalian cell line (Figure 3A,B,C). Amines, being more soluble owing to their hydrophilic nature, are more bio-available as compared to their nitro counterpart, which will benefit further in vivo antiplasmodial evaluation of the compound. Also the death of parasite following in vitro exposure to compound D was investigated using biochemical and morphological approaches. Diffused green fluorescence of JC-1 monomer observed in compound D-treated parasites suggested a loss of mitochondrial functionality and thus mitochondrial membrane potential. TUNNEL assay further revealed the presence of apoptotic cells in compound D-treated parasites. This study thus, establishes a strong demonstration of the superior efficacy and cytotoxicity of substituted bicyclic lactams against a broad range of P. falciparum strains. These findings advocate a new class of compounds, which could be considered as potential anti-malarials and inclusion of these compounds in combination-based therapy seems promising. The in vivo assessment of the antiparasitic potential of these bicyclic lactams could be beneficial to achieve success in development of novel anti-malarials and in eradication of this devastating disease. Last but not the least the initial lead compound obtained via our DOS strategy can be further elaborated and improved into a better drug candidate in the future. There are quite a few sites of diversity present in the molecules for example the aromatic ring of the benzylic moiety, the oxazoline side ring which can be changed to thiazoline side chain, the subsequent reduction of the nitro group followed by diverse array of acid-amine coupling or C-N bond formation reactions with aliphatic, aromatic and heteroaromatic halides and finally, further substituting the position α- to the amide carbonyl either by alkylation or by acylation will provide opportunities to improve the efficacy of this initial lead compound.
This work has been funded by Innovative Young Biotechnologist Award (IYBA) from the Department of Biotechnology, Ministry of Science and Technology, Government of India (DBT). Shailja Singh is a recipient of the IYBA Award from DBT. VS and PD are supported by Shiv Nadar Foundation scholarship. SA is supported by senior research fellowship (SRF) from DBT. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
- Prapansilp P, Medana I, Mai NT, Day NP, Phu NH, Yeo TW, Hien TT, White NJ, Anstey NM, Turner GD: A clinicopathological correlation of the expression of the angiopoietin-Tie-2 receptor pathway in the brain of adults with Plasmodium falciparum malaria. Malar J. 2013, 12: 50-10.1186/1475-2875-12-50.PubMed CentralView ArticlePubMedGoogle Scholar
- WHO: World Malaria Report. 2013, Geneva: World Health OrganizationGoogle Scholar
- Olliaro PL, Taylor WR: Antimalarial compounds: from bench to bedside. J Exp Biol. 2003, 206: 3753-3759. 10.1242/jeb.00653.View ArticlePubMedGoogle Scholar
- Ballou WR, Kester KE, Stoute JA, Heppner DG: Malaria vaccines: triumphs or tribulations?. Parassitologia. 1999, 41: 403-408.PubMedGoogle Scholar
- Broccolo F, Cainelli G, Caltabiano G, Cocuzza CE, Fortuna CG, Galletti P, Giacomini D, Musumarra G, Musumeci R, Quintavalla A: Design, synthesis, and biological evaluation of 4-Alkyliden-beta Lactams: new products with promising antibiotic activity against resistant bacteria. J Med Chem. 2006, 49: 2804-2811. 10.1021/jm0580510.View ArticlePubMedGoogle Scholar
- Ashley EA, Dhorda M, Fairhurst RM, Amaratunga C, Lim P, Suon S, Sreng S, Anderson JM, Mao S, Sam B, Sopha C, Chuor CM, Nguon C, Sovannaroth S, Pukrittayakamee S, Jittamala P, Chotivanich K, Chutasmit K, Suchatsoonthorn C, Runcharoen R, Hien TT, Thuy-Nhien NT, Thanh NV, Phu NH, Htut Y, Han KT, Aye KH, Mokuolu OA, Olaosebikan RR, Folaranmi OO: Spread of artemisinin resistance in Plasmodium falciparum malaria. N Engl J Med. 2014, 371: 411-423. 10.1056/NEJMoa1314981.PubMed CentralView ArticlePubMedGoogle Scholar
- Venkatesan M, Gadalla NB, Stepniewska K, Dahal P, Nsanzabana C, Moriera C, Price RN, Mårtensson A, Rosenthal PJ, Dorsey G, Sutherland CJ, Guérin P, Davis TM, Ménard D, Adam I, Ademowo G, Arze C, Baliraine FN, Berens-Riha N, Björkman A, Borrmann S, Checchi F, Desai M, Dhorda M, Djimdé AA, El-Sayed BB, Eshetu T, Eyase F, Falade C, Faucher JF: Polymorphisms in Plasmodium falciparum chloroquine resistance transporter and multidrug resistance 1 genes: parasite risk factors that affect treatment outcomes for P. falciparum malaria after artemether-lumefantrine and artesunate-amodiaquine. Am J Trop Med Hyg. 2014, 14: 833-843.View ArticleGoogle Scholar
- Visser BJ, van Vugt M, Grobusch MP: Malaria: an update on current chemotherapy. Expert Opin Pharmacother. 2014, 15: 2219-2254. 10.1517/14656566.2014.944499.View ArticlePubMedGoogle Scholar
- Schreiber SL: Target-oriented and diversity-oriented organic synthesis in drug discovery. Science. 2000, 287: 964-1969.View ArticleGoogle Scholar
- Tan DS: Diversity-oriented synthesis: exploring the intersections between chemistry and biology. Nat Chem Biol. 2005, 1: 74-84. 10.1038/nchembio0705-74.View ArticlePubMedGoogle Scholar
- Allina SM, Jamesa SL, Martinb WP, Smitha TAD: Stereoselective synthesis of the pyrroloisoquinoline ring system. Tetrahedron Lett. 2001, 42: 3943-3946. 10.1016/S0040-4039(01)00613-X.View ArticleGoogle Scholar
- Walliker D, Quakyi IA, Wellems TE, McCutchan TF, Szarfman A, London WT, Corcoran LM, Burkot TR, Carter R: Genetic analysis of the human malaria parasite Plasmodium falciparum. Science. 1987, 236: 1661-1666. 10.1126/science.3299700.View ArticlePubMedGoogle Scholar
- Wellems TE, Oduola AMJ, Phenton B, Des Jardins R, Panton LJ, DoRosario VE: Chromosome size variation occurs in cloned Plasmodium falciparum on in vitro cultivation. Braz J Genet. 1988, 11: 813-825.Google Scholar
- Bhasin VK, Trager W: Gametocyte-forming and non-gametocyteforming clones of Plasmodium falciparum. Am J Trop Med Hyg. 1984, 33: 534-537.PubMedGoogle Scholar
- Trager W, Jensen JB: Human malaria parasites in continuous culture. Science. 1976, 193: 673-675. 10.1126/science.781840.View ArticlePubMedGoogle Scholar
- Agarwal S, Singh MK, Garg S, Chitnis CE, Singh S: Ca (2+)-mediated exocytosis of subtilisin-like protease 1: a key step in egress of Plasmodium falciparum merozoites. Cell Microbiol. 2013, 15: 910-921. 10.1111/cmi.12086.View ArticlePubMedGoogle Scholar
- Rathore S, Jain S, Sinha D, Gupta M, Asad M, Srivastava A, Narayanan MS, Ramasamy G, Chauhan VS, Gupta D, Mohmmed A: Disruption of a mitochondrial protease machinery in Plasmodium falciparum is an intrinsic signal for parasite cell death. Cell Death Dis. 2011, 2: e231-10.1038/cddis.2011.118.PubMed CentralView ArticlePubMedGoogle Scholar
- Yakes FM, Van Houten B: Mitochondrial DNA damage is more extensive and persists longer than nuclear DNA damage in human cells following oxidative stress. Proc Natl Acad Sci U S A. 1997, 94: 514-519. 10.1073/pnas.94.2.514.PubMed CentralView ArticlePubMedGoogle Scholar
- Mossman T: Rapid colorimetric assay for cellular growth and survival: application to proliferation and cytotoxicity assays. J Immunol Methods. 1983, 65: 55-63. 10.1016/0022-1759(83)90303-4.View ArticleGoogle Scholar
- Wright AJ: The penicillins. Mayo Clin Proc. 1999, 74: 290-307. 10.4065/74.3.290.View ArticlePubMedGoogle Scholar
- Galletti G, Garbisa P, Giacomini S, Sartor D, Quintavalla L: 4-Alkylidene-azetidin-2-ones: novel inhibitors of leukocyte elastase and gelatinase. Bioorg Med Chem. 2003, 11: 5391-5399. 10.1016/j.bmc.2003.09.035.View ArticlePubMedGoogle Scholar
- Kazi A, Hill R, Long TE, Kuhn DJ, Turos E, Dou QP: Novel N-thiolated-beta-lactam antibiotics selectively induce apoptosis in human tumor and transformed, but not normal or nontransformed, cells. Biochem Pharmacol. 2004, 67: 365-374. 10.1016/j.bcp.2003.09.017.View ArticlePubMedGoogle Scholar
- Veinberg G, Vorona M, Shestakova I, Kanepe I, Lukevics E: Design of β-lactams with mechanism based nonbacterial activities. Curr Med Chem. 2003, 10: 1741-1757. 10.2174/0929867033457089.View ArticlePubMedGoogle Scholar
- Mehta PD, Sengar NPS, Pathak AK: 2-Azetidinone, a new profile of various pharmacological activities. Eur J Med Chem. 2010, 45: 5541-5560. 10.1016/j.ejmech.2010.09.035.View ArticlePubMedGoogle Scholar
- Sperka T, Pitlik J, Bagossi P, Toszér J: β-Lactam compounds as apparently uncompetitive inhibitors of HIV-1 protease. Bioorg Med Chem Lett. 2005, 15: 3086-3090. 10.1016/j.bmcl.2005.04.020.View ArticlePubMedGoogle Scholar
- Veinberg G, Shestakova I, Vorona M, Kanepe I, Lukevics E: Synthesis of antitumor 6-alkylidenepenicillanate sulfones and related 3-alkylidene-2-azetidinones. Bioorg Med Chem Lett. 2004, 14: 147-150. 10.1016/j.bmcl.2003.09.078.View ArticlePubMedGoogle Scholar
- Sun L, Vasilevich NI, Fuselier JA, Hocart SJ, Coy DH: Examination of the 1,4-disubstituted azetidinone ring system as a template for combretastatin A-4 conformationally restricted analogue design. Bioorg Med Chem Lett. 2004, 14: 2041-2046. 10.1016/j.bmcl.2004.02.050.View ArticlePubMedGoogle Scholar
- Nivsarkar M, Thavaselvam D, Prasanna S, Sharma M, Kaushik MP: Design, synthesis and biological evaluation of novel bicyclic β-lactams as potential antimalarials. Bioorg Med Chem Lett. 2005, 15: 1371-1373. 10.1016/j.bmcl.2005.01.011.View ArticlePubMedGoogle Scholar
- Kvaerno L, Werder M, Houser H, Carreira EM: Synthesis and in vitro evaluation of inhibitors of intestinal cholesterol absorption. J Med Chem. 2005, 48: 6035-6053. 10.1021/jm050422p.View ArticlePubMedGoogle Scholar
- Alcaide B, Almendros P, Aragoncillo C: β-Lactams: versatile building blocks for the stereoselective synthesis of non-β-lactam products. Chem Rev. 2007, 107: 4437-4492. 10.1021/cr0307300.View ArticlePubMedGoogle Scholar
- Thera MA, Doumbo OK, Coulibaly D, Laurens MB, Ouattara A, Kone AK, Guindo AB, Traore K, Traore I, Kouriba B, Diallo DA, Diarra I, Daou M, Dolo A, Tolo Y, Sissoko MS, Niangaly A, Sissoko M, Takala-Harrison S, Lyke KE, Wu Y, Blackwelder WC, Godeaux O, Vekemans J, Dubois MC, Ballou WR, Cohen J, Thompson D, Dube T, Soisson L: A field trial to assess a blood-stage malaria vaccine. N Engl J Med. 2013, 65: 1004-1013.Google Scholar
- Heidebrecht RW, Mulrooney C, Austin CP, Barker RH, Beaudoin JA, Cheng KC, Comer E, Dandapani S, Dick J, Duvall JR, Ekland EH, Fidock DA, Fitzgerald ME, Foley M, Guha R, Hinkson P, Kramer M, Lukens AK, Masi D, Marcaurelle LA, Su XZ, Thomas CJ, Weïwer M, Wiegand RC, Wirth D, Xia M, Yuan J, Zhao J, Palmer M, Munoz B: Diversity-oriented synthesis yields a novel lead for the treatment of malaria. ACS Med Chem Lett. 2012, 3: 112-117. 10.1021/ml200244k.PubMed CentralView ArticlePubMedGoogle Scholar
- Brown LE, Chih-Chien Cheng K, Wei WG, Yuan P, Dai P, Trilles R, Ni F, Yuan J, MacArthur R, Guha R, Johnson RL, Su XZ, Dominguez MM, Snyder JK, Beeler AB, Schaus SE, Inglese J, Porco JA: Discovery of new antimalarial chemotypes through chemical methodology and library development. Proc Natl Acad Sci U S A. 2011, 108: 6775-6780. 10.1073/pnas.1017666108.PubMed CentralView ArticlePubMedGoogle Scholar
- Rottmann M, McNamara C, Yeung BK, Lee MC, Zou B, Russell B, Seitz P, Plouffe DM, Dharia NV, Tan J, Cohen SB, Spencer KR, González-Páez GE, Lakshminarayana SB, Goh A, Suwanarusk R, Jegla T, Schmitt EK, Beck HP, Brun R, Nosten F, Renia L, Dartois V, Keller TH, Fidock DA, Winzeler EA, Diagana TT: Spiroindolones, a potent compound class for the treatment of malaria. Science. 2010, 329: 1175-1180. 10.1126/science.1193225.PubMed CentralView ArticlePubMedGoogle Scholar
- Yeung BK, Zou B, Rottmann M, Lakshminarayana SB, Ang SH, Leong SY, Tan J, Wong J, Keller-Maerki S, Fischli C, Goh A, Schmitt EK, Krastel P, Francotte E, Kuhen K, Plouffe D, Henson K, Wagner T, Winzeler EA, Petersen F, Brun R, Dartois V, Diagana TT, Keller TH: Spirotetrahydro beta-carbolines (spiroindolones): a new class of potent and orally efficacious compounds for the treatment of malaria. J Med Chem. 2010, 53: 5155-5564. 10.1021/jm100410f.View ArticlePubMedGoogle Scholar
- Flannery EL, Chatterjee AK, Winzeler EA: Antimalarial drug discovery - approaches and progress towards new medicines. Nat Rev Microbiol. 2013, 11: 849-862. 10.1038/nrmicro3138.PubMed CentralView ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.