1
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Kabeche S, Braukmann T, Doenier J, Meister T, Yeh E. A picomolar inhibitor of the Plasmodium falciparum IPP pathway. Antimicrob Agents Chemother 2024; 68:e0123823. [PMID: 39037239 DOI: 10.1128/aac.01238-23] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2023] [Accepted: 06/30/2024] [Indexed: 07/23/2024] Open
Abstract
We identified MMV026468 as a picomolar inhibitor of blood-stage Plasmodium falciparum. Phenotyping assays, including isopentenyl diphosphate rescue of parasite growth inhibition, demonstrated that it targets MEP isoprenoid precursor biosynthesis. MMV026468-treated parasites showed an overall decrease in MEP pathway intermediates, which could result from inhibition of the first MEP enzyme DXS or steps prior to DXS such as regulation of the MEP pathway. Selection of MMV026468-resistant parasites lacking DXS mutations suggested that other targets are possible. The identification of MMV026468 could lead to a new class of antimalarial isoprenoid inhibitors.
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Affiliation(s)
- Stephanie Kabeche
- Department of Biochemistry, Stanford University School of Medicine, Stanford, California, USA
| | - Thomas Braukmann
- Department of Pathology, Stanford University School of Medicine, Stanford, California, USA
| | - Jon Doenier
- Department of Biochemistry, Stanford University School of Medicine, Stanford, California, USA
| | - Thomas Meister
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California, USA
| | - Ellen Yeh
- Department of Pathology, Stanford University School of Medicine, Stanford, California, USA
- Department of Microbiology & Immunology, Stanford University School of Medicine, Stanford, California, USA
- Chan Zuckerberg Biohub, San Francisco, California, USA
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2
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Bague D, Wang R, Hodge D, Mikati MO, Roma JS, Boshoff HI, Dailey AL, Girma M, Couch RD, Odom John AR, Dowd CS. Inhibition of DXR in the MEP pathway with lipophilic N-alkoxyaryl FR900098 analogs. RSC Med Chem 2024; 15:2422-2439. [PMID: 39026652 PMCID: PMC11253873 DOI: 10.1039/d3md00642e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2023] [Accepted: 05/22/2024] [Indexed: 07/20/2024] Open
Abstract
In Mycobacterium tuberculosis (Mtb) and Plasmodium falciparum (Pf), the methylerythritol phosphate (MEP) pathway is responsible for isoprene synthesis. This pathway and its products are vital to bacterial/parasitic metabolism and survival, and represent an attractive set of drug targets due to their essentiality in these pathogens but absence in humans. The second step in the MEP pathway is the conversion of 1-deoxy-d-xylulose-5-phosphate (DXP) to MEP and is catalyzed by 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR). Natural products fosmidomycin and FR900098 inhibit DXR, but are too polar to reach the desired target inside some cells, such as Mtb. Synthesized FR900098 analogs with lipophilic substitution in the position α to the phosphorous atom showed promise, resulting in increased activity against Mtb and Pf. Here, an α substitution, consisting of a 3,4-dichlorophenyl substituent, in combination with various O-linked alkylaryl substituents on the hydroxamate moiety is utilized in the synthesis of a novel series of FR900098 analogs. The purpose of the O-linked alkylaryl substituents is to further enhance DXR inhibition by extending the structure into the adjacent NADPH binding pocket, blocking the binding of both DXP and NADPH. Of the initial O-linked alkylaryl substituted analogs, compound 6e showed most potent activity against Pf parasites at 3.60 μM. Additional compounds varying the phenyl ring of 6e were synthesized. The most potent phosphonic acids, 6l and 6n, display nM activity against PfDXR and low μM activity against Pf parasites. Prodrugs of these compounds were less effective against Pf parasites but showed modest activity against Mtb cells. Data from this series of compounds suggests that this combination of substituents can be advantageous in designing a new generation of antimicrobials.
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Affiliation(s)
- Darean Bague
- Department of Chemistry, George Washington University Washington D.C. 20052 USA
| | - Ruiqin Wang
- Department of Chemistry, George Washington University Washington D.C. 20052 USA
| | - Dana Hodge
- Division of Infectious Diseases, Children's Hospital of Philadelphia Philadelphia PA 19104 USA
| | - Marwa O Mikati
- Department of Molecular Microbiology, Washington University School of Medicine St. Louis MO 63110 USA
| | - Jose S Roma
- Tuberculosis Research Section, LCIM, NIAID/NIH Bethesda MD 20892 USA
| | - Helena I Boshoff
- Tuberculosis Research Section, LCIM, NIAID/NIH Bethesda MD 20892 USA
| | - Allyson L Dailey
- Department of Chemistry and Biochemistry, George Mason University Fairfax VA 22030 USA
| | - Misgina Girma
- Department of Chemistry and Biochemistry, George Mason University Fairfax VA 22030 USA
| | - Robin D Couch
- Department of Chemistry and Biochemistry, George Mason University Fairfax VA 22030 USA
| | - Audrey R Odom John
- Division of Infectious Diseases, Children's Hospital of Philadelphia Philadelphia PA 19104 USA
- Department of Molecular Microbiology, Washington University School of Medicine St. Louis MO 63110 USA
| | - Cynthia S Dowd
- Department of Chemistry, George Washington University Washington D.C. 20052 USA
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3
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Wang X, Edwards RL, Ball HS, Heidel KM, Brothers RC, Johnson C, Haymond A, Girma M, Dailey A, Roma JS, Boshoff HI, Osbourn DM, Meyers MJ, Couch RD, Odom John AR, Dowd CS. MEPicides: α,β-unsaturated Fosmidomycin N-Acyl Analogs as Efficient Inhibitors of Plasmodium falciparum 1-Deoxy-d-xylulose-5-phosphate reductoisomerase. ACS Infect Dis 2023; 9:1387-1395. [PMID: 37310810 PMCID: PMC10880585 DOI: 10.1021/acsinfecdis.3c00132] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Malaria, a mosquito-borne disease caused by several parasites of the Plasmodium genus, remains a huge threat to global public health. There are an estimated 0.5 million malaria deaths each year, mostly among African children. Unlike humans, Plasmodium parasites and a number of important pathogenic bacteria employ the methyl erythritol phosphate (MEP) pathway for isoprenoid synthesis. Thus, the MEP pathway represents a promising set of drug targets for antimalarial and antibacterial compounds. Here, we present new unsaturated MEPicide inhibitors of 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR), the second enzyme of the MEP pathway. A number of these compounds have demonstrated robust inhibition of Plasmodium falciparum DXR, potent antiparasitic activity, and low cytotoxicity against HepG2 cells. Parasites treated with active compounds are rescued by isopentenyl pyrophosphate, the product of the MEP pathway. With higher levels of DXR substrate, parasites acquire resistance to active compounds. These results further confirm the on-target inhibition of DXR in parasites by the inhibitors. Stability in mouse liver microsomes is high for the phosphonate salts, but remains a challenge for the prodrugs. Taken together, the potent activity and on-target mechanism of action of this series further validate DXR as an antimalarial drug target and the α,β-unsaturation moiety as an important structural component.
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Affiliation(s)
- Xu Wang
- Department of Chemistry, George Washington University, Washington, District of Columbia 20052, United States
| | - Rachel L Edwards
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Haley S Ball
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Kenneth M Heidel
- Department of Chemistry, George Washington University, Washington, District of Columbia 20052, United States
| | - Robert C Brothers
- Department of Chemistry, George Washington University, Washington, District of Columbia 20052, United States
| | - Claire Johnson
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Amanda Haymond
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Misgina Girma
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Allyson Dailey
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Jose Santinni Roma
- Tuberculosis Research Section, LCIM, NIAID/NIH, Bethesda, Maryland 20892, United States
| | - Helena I Boshoff
- Tuberculosis Research Section, LCIM, NIAID/NIH, Bethesda, Maryland 20892, United States
| | - Damon M Osbourn
- Department of Chemistry, Saint Louis University, St. Louis, Missouri 63103, United States
| | - Marvin J Meyers
- Department of Chemistry, Saint Louis University, St. Louis, Missouri 63103, United States
| | - Robin D Couch
- Department of Chemistry and Biochemistry, George Mason University, Manassas, Virginia 20110, United States
| | - Audrey R Odom John
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Cynthia S Dowd
- Department of Chemistry, George Washington University, Washington, District of Columbia 20052, United States
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Elahi R, Prigge ST. New insights into apicoplast metabolism in blood-stage malaria parasites. Curr Opin Microbiol 2023; 71:102255. [PMID: 36563485 PMCID: PMC9852000 DOI: 10.1016/j.mib.2022.102255] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2022] [Revised: 11/21/2022] [Accepted: 11/23/2022] [Indexed: 12/24/2022]
Abstract
The apicoplast of Plasmodium falciparum is the only source of essential isoprenoid precursors and Coenzyme A (CoA) in the parasite. Isoprenoid precursor synthesis relies on the iron-sulfur cluster (FeS) cofactors produced within the apicoplast, rendering FeS synthesis an essential function of this organelle. Recent reports provide important insights into the roles of FeS cofactors and the use of isoprenoid precursors and CoA both inside and outside the apicoplast. Here, we review the recent insights into the roles of these metabolites in blood-stage malaria parasites and discuss new questions that have been raised in light of these discoveries.
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Affiliation(s)
- Rubayet Elahi
- Department of Molecular Microbiology and Immunology, Johns Hopkins University, Baltimore, MD, USA; The Johns Hopkins Malaria Research Institute, Baltimore, MD, USA
| | - Sean T Prigge
- Department of Molecular Microbiology and Immunology, Johns Hopkins University, Baltimore, MD, USA; The Johns Hopkins Malaria Research Institute, Baltimore, MD, USA.
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Over 40 Years of Fosmidomycin Drug Research: A Comprehensive Review and Future Opportunities. Pharmaceuticals (Basel) 2022; 15:ph15121553. [PMID: 36559004 PMCID: PMC9782300 DOI: 10.3390/ph15121553] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2022] [Revised: 12/01/2022] [Accepted: 12/05/2022] [Indexed: 12/15/2022] Open
Abstract
To address the continued rise of multi-drug-resistant microorganisms, the development of novel drugs with new modes of action is urgently required. While humans biosynthesize the essential isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) via the established mevalonate pathway, pathogenic protozoa and certain pathogenic eubacteria use the less well-known methylerythritol phosphate pathway for this purpose. Important pathogens using the MEP pathway are, for example, Plasmodium falciparum, Mycobacterium tuberculosis, Pseudomonas aeruginosa and Escherichia coli. The enzymes of that pathway are targets for antiinfective drugs that are exempt from target-related toxicity. 2C-Methyl-D-erythritol 4-phosphate (MEP), the second enzyme of the non-mevalonate pathway, has been established as the molecular target of fosmidomycin, an antibiotic that has so far failed to be approved as an anti-infective drug. This review describes the development and anti-infective properties of a wide range of fosmidomycin derivatives synthesized over the last four decades. Here we discuss the DXR inhibitor pharmacophore, which comprises a metal-binding group, a phosphate or phosphonate moiety and a connecting linker. Furthermore, non-fosmidomycin-based DXRi, bisubstrate inhibitors and several prodrug concepts are described. A comprehensive structure-activity relationship (SAR) of nearly all inhibitor types is presented and some novel opportunities for further drug development of DXR inhibitors are discussed.
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6
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Synthesis, Structure−Activity Relationships, and Parasitological Profiling of Brussonol Derivatives as New Plasmodium falciparum Inhibitors. Pharmaceuticals (Basel) 2022; 15:ph15070814. [PMID: 35890113 PMCID: PMC9321043 DOI: 10.3390/ph15070814] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2022] [Revised: 06/24/2022] [Accepted: 06/26/2022] [Indexed: 11/17/2022] Open
Abstract
Malaria is a parasitic disease caused by protozoan parasites from the genus Plasmodium. Plasmodium falciparum is the most prevalent species worldwide and the causative agent of severe malaria. The spread of resistance to the currently available antimalarial therapy is a major concern. Therefore, it is imperative to discover and develop new antimalarial drugs, which not only treat the disease but also control the emerging resistance. Brussonol is an icetexane derivative and a member of a family of diterpenoids that have been isolated from several terrestrial plants. Here, the synthesis and antiplasmodial profiling of a series of brussonol derivatives are reported. The compounds showed inhibitory activities in the low micromolar range against a panel of sensitive and resistant P. falciparum strains (IC50s = 5-16 μM). Moreover, brussonol showed fast-acting in vitro inhibition and an additive inhibitory behavior when combined with the antimalarial artesunate (FICindex~1). The mode of action investigation indicated that brussonol increased the cytosolic calcium levels within the parasite. Hence, the discovery of brussonol as a new scaffold endowed with antiplasmodial activity will enable us to design derivatives with improved properties to deliver new lead candidates for malaria.
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7
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Bernard MM, Mohanty A, Rajendran V. Title: A Comprehensive Review on Classifying Fast-acting and Slow-acting Antimalarial Agents Based on Time of Action and Target Organelle of Plasmodium sp. Pathog Dis 2022; 80:6589403. [PMID: 35588061 DOI: 10.1093/femspd/ftac015] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2021] [Revised: 03/20/2022] [Accepted: 05/17/2022] [Indexed: 11/13/2022] Open
Abstract
The clinical resistance towards malarial parasites has rendered many antimalarials ineffective, likely due to a lack of understanding of time of action and stage specificity of all life stages. Therefore, to tackle this problem a more incisive comprehensive analysis of the fast and slow-acting profile of antimalarial agents relating to parasite time-kill kinetics and the target organelle on the progression of blood-stage parasites was carried out. It is evident from numerous findings that drugs targeting food vacuole, nuclear components, and endoplasmic reticulum mainly exhibit a fast-killing phenotype within 24h affecting first-cycle activity. Whereas drugs targeting mitochondria, apicoplast, microtubules, parasite invasion and egress exhibit a largely slow-killing phenotype within 96-120h, affecting second-cycle activity with few exemptions as moderately fast-killing. It is essential to understand the susceptibility of drugs on rings, trophozoites, schizonts, merozoites, and the appearance of organelle at each stage of 48h intraerythrocytic parasite cycle. Therefore, these parameters may facilitate the paradigm for understanding the timing of antimalarials action in deciphering its precise mechanism linked with time. Thus, classifying drugs based on the time of killing may promote designing new combination regimens against varied strains of P. falciparum and evaluating potential clinical resistance.
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Affiliation(s)
- Monika Marie Bernard
- Department of Microbiology, School of Life Sciences, Pondicherry University, Puducherry 605014, India
| | - Abhinab Mohanty
- Department of Microbiology, School of Life Sciences, Pondicherry University, Puducherry 605014, India
| | - Vinoth Rajendran
- Department of Microbiology, School of Life Sciences, Pondicherry University, Puducherry 605014, India
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Frasse PM, Miller JJ, Polino AJ, Soleimani E, Zhu JS, Jakeman DL, Jez JM, Goldberg DE, Odom John AR. Enzymatic and structural characterization of HAD5, an essential phosphomannomutase of malaria-causing parasites. J Biol Chem 2022; 298:101550. [PMID: 34973333 PMCID: PMC8808168 DOI: 10.1016/j.jbc.2021.101550] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2021] [Revised: 12/22/2021] [Accepted: 12/23/2021] [Indexed: 11/05/2022] Open
Abstract
The malaria-causing parasite Plasmodium falciparum is responsible for over 200 million infections and 400,000 deaths per year. At multiple stages during its complex life cycle, P. falciparum expresses several essential proteins tethered to its surface by glycosylphosphatidylinositol (GPI) anchors, which are critical for biological processes such as parasite egress and reinvasion of host red blood cells. Targeting this pathway therapeutically has the potential to broadly impact parasite development across several life stages. Here, we characterize an upstream component of parasite GPI anchor biosynthesis, the putative phosphomannomutase (PMM) (EC 5.4.2.8), HAD5 (PF3D7_1017400). We confirmed the PMM and phosphoglucomutase activities of purified recombinant HAD5 by developing novel linked enzyme biochemical assays. By regulating the expression of HAD5 in transgenic parasites with a TetR-DOZI-inducible knockdown system, we demonstrated that HAD5 is required for malaria parasite egress and erythrocyte reinvasion, and we assessed the role of HAD5 in GPI anchor synthesis by autoradiography of radiolabeled glucosamine and thin layer chromatography. Finally, we determined the three-dimensional X-ray crystal structure of HAD5 and identified a substrate analog that specifically inhibits HAD5 compared to orthologous human PMMs in a time-dependent manner. These findings demonstrate that the GPI anchor biosynthesis pathway is exceptionally sensitive to inhibition in parasites and that HAD5 has potential as a specific, multistage antimalarial target.
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Affiliation(s)
- Philip M Frasse
- Division of Infectious Diseases, Departments of Medicine and Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA
| | - Justin J Miller
- Department of Biology, Washington University in St. Louis, St. Louis, Missouri, USA
| | - Alexander J Polino
- Division of Infectious Diseases, Departments of Medicine and Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA
| | - Ebrahim Soleimani
- College of Pharmacy, Dalhousie University, Halifax, Nova Scotia, Canada; Department of Chemistry, Razi University, Kermanshah, Iran
| | - Jian-She Zhu
- College of Pharmacy, Dalhousie University, Halifax, Nova Scotia, Canada
| | - David L Jakeman
- College of Pharmacy, Dalhousie University, Halifax, Nova Scotia, Canada; Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada
| | - Joseph M Jez
- Department of Biology, Washington University in St. Louis, St. Louis, Missouri, USA
| | - Daniel E Goldberg
- Division of Infectious Diseases, Departments of Medicine and Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA
| | - Audrey R Odom John
- Division of Infectious Diseases, Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
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9
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Miller JJ, Shah IT, Hatten J, Barekatain Y, Mueller EA, Moustafa AM, Edwards RL, Dowd CS, Planet PJ, Muller FL, Jez JM, Odom John AR. Structure-guided microbial targeting of antistaphylococcal prodrugs. eLife 2021; 10:66657. [PMID: 34279224 PMCID: PMC8318587 DOI: 10.7554/elife.66657] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2021] [Accepted: 07/16/2021] [Indexed: 01/07/2023] Open
Abstract
Carboxy ester prodrugs are widely employed to increase oral absorption and potency of phosphonate antibiotics. Prodrugging can mask problematic chemical features that prevent cellular uptake and may enable tissue-specific compound delivery. However, many carboxy ester promoieties are rapidly hydrolyzed by serum esterases, limiting their therapeutic potential. While carboxy ester-based prodrug targeting is feasible, it has seen limited use in microbes as microbial esterase-specific promoieties have not been described. Here we identify the bacterial esterases, GloB and FrmB, that activate carboxy ester prodrugs in Staphylococcus aureus. Additionally, we determine the substrate specificities for FrmB and GloB and demonstrate the structural basis of these preferences. Finally, we establish the carboxy ester substrate specificities of human and mouse sera, ultimately identifying several promoieties likely to be serum esterase-resistant and microbially labile. These studies will enable structure-guided design of antistaphylococcal promoieties and expand the range of molecules to target staphylococcal pathogens.
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Affiliation(s)
- Justin J Miller
- Department of Pediatrics, Washington University School of Medicine, St. Louis, United States.,Department of Biology, Washington University in St. Louis, St. Louis, United States
| | - Ishaan T Shah
- Department of Pediatrics, Washington University School of Medicine, St. Louis, United States
| | - Jayda Hatten
- Department of Pediatrics, Washington University School of Medicine, St. Louis, United States
| | - Yasaman Barekatain
- Department of Cancer Systems Imaging, The University of Texas MD Anderson Cancer Center, Houston, United States
| | - Elizabeth A Mueller
- Department of Biology, Washington University in St. Louis, St. Louis, United States
| | - Ahmed M Moustafa
- Department of Pediatrics, Perelman School of Medicine, University of Pennsylvania, Children's Hospital of Philadelphia, Philadelphia, United States
| | - Rachel L Edwards
- Department of Pediatrics, Washington University School of Medicine, St. Louis, United States
| | - Cynthia S Dowd
- Department of Chemistry, The George Washington University, Washington, United States
| | - Paul J Planet
- Department of Pediatrics, Perelman School of Medicine, University of Pennsylvania, Children's Hospital of Philadelphia, Philadelphia, United States
| | - Florian L Muller
- Department of Cancer Systems Imaging, The University of Texas MD Anderson Cancer Center, Houston, United States
| | - Joseph M Jez
- Department of Biology, Washington University in St. Louis, St. Louis, United States
| | - Audrey R Odom John
- Department of Pediatrics, Washington University School of Medicine, St. Louis, United States.,Department of Pediatrics, Perelman School of Medicine, University of Pennsylvania, Children's Hospital of Philadelphia, Philadelphia, United States.,Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, United States
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10
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Non-hydroxamate inhibitors of 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR): A critical review and future perspective. Eur J Med Chem 2020; 213:113055. [PMID: 33303239 DOI: 10.1016/j.ejmech.2020.113055] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2020] [Revised: 11/18/2020] [Accepted: 11/21/2020] [Indexed: 12/22/2022]
Abstract
1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) catalyzes the second step of the non-mevalonate (or MEP) pathway that functions in several organisms and plants for the synthesis of isoprenoids. DXR is essential for the survival of multiple pathogenic bacteria/parasites, including those that cause tuberculosis and malaria in humans. DXR function is inhibited by fosmidomycin (1), a natural product, which forms a chelate with the active site divalent metal (Mg2+/Mn2+) through its hydroxamate metal-binding group (MBG). Most of the potent DXR inhibitors are structurally similar to 1 and retain hydroxamate despite the unfavourable pharmacokinetic and toxicity profile of the latter. We provide our perspective on the lack of non-hydroxamate DXR inhibitors. We also highlight the fundamental flaws in the design of MBG in these molecules, primarily responsible for their failure to inhibit DXR. We also suggest that for designing next-generation non-hydroxamate DXR inhibitors, approaches followed for other metalloenzymes targets may be exploited.
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11
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Mikati MO, Miller JJ, Osbourn DM, Barekatain Y, Ghebremichael N, Shah IT, Burnham CAD, Heidel KM, Yan VC, Muller FL, Dowd CS, Edwards RL, Odom John AR. Antimicrobial Prodrug Activation by the Staphylococcal Glyoxalase GloB. ACS Infect Dis 2020; 6:3064-3075. [PMID: 33118347 PMCID: PMC8543975 DOI: 10.1021/acsinfecdis.0c00582] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
With the rising prevalence of multidrug resistance, there is an urgent need to develop novel antibiotics. Many putative antibiotics demonstrate promising in vitro potency but fail in vivo due to poor drug-like qualities (e.g., serum half-life, oral absorption, solubility, and toxicity). These drug-like properties can be modified through the addition of chemical protecting groups, creating "prodrugs" that are activated prior to target inhibition. Lipophilic prodrugging techniques, including the attachment of a pivaloyloxymethyl group, have garnered attention for their ability to increase cellular permeability by masking charged residues and the relative ease of the chemical prodrugging process. Unfortunately, pivaloyloxymethyl prodrugs are rapidly activated by human sera, rendering any membrane permeability qualities absent during clinical treatment. Identification of the bacterial prodrug activation pathway(s) will allow for the development of host-stable and microbe-targeted prodrug therapies. Here, we use two zoonotic staphylococcal species, Staphylococcus schleiferi and S. pseudintermedius, to establish the mechanism of carboxy ester prodrug activation. Using a forward genetic screen, we identify a conserved locus in both species encoding the enzyme hydroxyacylglutathione hydrolase (GloB), whose loss-of-function confers resistance to carboxy ester prodrugs. We enzymatically characterize GloB and demonstrate that it is a functional glyoxalase II enzyme, which has the capacity to activate carboxy ester prodrugs. As GloB homologues are both widespread and diverse in sequence, our findings suggest that GloB may be a useful mechanism for developing species- or genus-level prodrug targeting strategies.
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Affiliation(s)
- Marwa O Mikati
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Justin J Miller
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Damon M Osbourn
- Department of Molecular Microbiology and Immunology, Saint Louis University School of Medicine, St. Louis, Missouri 63104, United States
| | - Yasaman Barekatain
- Department of Cancer Systems Imaging, The University of Texas MD Anderson Cancer Center, Houston, Texas 77054, United States
| | - Naomi Ghebremichael
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Ishaan T Shah
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Carey-Ann D Burnham
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
- Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110, United States
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Kenneth M Heidel
- Department of Chemistry, The George Washington University, Washington, DC 20052, United States
| | - Victoria C Yan
- Department of Cancer Systems Imaging, The University of Texas MD Anderson Cancer Center, Houston, Texas 77054, United States
| | - Florian L Muller
- Department of Cancer Systems Imaging, The University of Texas MD Anderson Cancer Center, Houston, Texas 77054, United States
| | - Cynthia S Dowd
- Department of Chemistry, The George Washington University, Washington, DC 20052, United States
| | - Rachel L Edwards
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
| | - Audrey R Odom John
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, United States
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12
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Synthesis and Antiplasmodial Activity of Novel Fosmidomycin Derivatives and Conjugates with Artemisinin and Aminochloroquinoline. Molecules 2020; 25:molecules25204858. [PMID: 33096817 PMCID: PMC7587979 DOI: 10.3390/molecules25204858] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2020] [Revised: 10/15/2020] [Accepted: 10/16/2020] [Indexed: 11/17/2022] Open
Abstract
Malaria, despite many efforts, remains among the most problematic infectious diseases worldwide, mainly due to the development of drug resistance by Plasmodium falciparum. The antibiotic fosmidomycin (FSM) is also known for its antimalarial activity by targeting the non-mevalonate isoprenoid synthesis pathway, which is essential for the malaria parasites but is absent in mammalians. In this study, we synthesized and evaluated against the chloroquine-resistant P. falciparum FcB1/Colombia strain, a series of FSM analogs, derivatives, and conjugates with other antimalarial agents, such as artemisinin (ART) and aminochloroquinoline (ACQ). The biological evaluation revealed four new compounds with higher antimalarial activity than FSM: two FSM-ACQ derivatives and two FSM-ART conjugates, with 3.5-5.4 and 41.5-23.1 times more potent activities than FSM, respectively.
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Edwards RL, Heueck I, Lee SG, Shah IT, Miller JJ, Jezewski AJ, Mikati MO, Wang X, Brothers RC, Heidel KM, Osbourn DM, Burnham CAD, Alvarez S, Fritz SA, Dowd CS, Jez JM, Odom John AR. Potent, specific MEPicides for treatment of zoonotic staphylococci. PLoS Pathog 2020; 16:e1007806. [PMID: 32497104 PMCID: PMC7297381 DOI: 10.1371/journal.ppat.1007806] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2019] [Revised: 06/16/2020] [Accepted: 04/11/2020] [Indexed: 12/20/2022] Open
Abstract
Coagulase-positive staphylococci, which frequently colonize the mucosal surfaces of animals, also cause a spectrum of opportunistic infections including skin and soft tissue infections, urinary tract infections, pneumonia, and bacteremia. However, recent advances in bacterial identification have revealed that these common veterinary pathogens are in fact zoonoses that cause serious infections in human patients. The global spread of multidrug-resistant zoonotic staphylococci, in particular the emergence of methicillin-resistant organisms, is now a serious threat to both animal and human welfare. Accordingly, new therapeutic targets that can be exploited to combat staphylococcal infections are urgently needed. Enzymes of the methylerythritol phosphate pathway (MEP) of isoprenoid biosynthesis represent potential targets for treating zoonotic staphylococci. Here we demonstrate that fosmidomycin (FSM) inhibits the first step of the isoprenoid biosynthetic pathway catalyzed by deoxyxylulose phosphate reductoisomerase (DXR) in staphylococci. In addition, we have both enzymatically and structurally determined the mechanism by which FSM elicits its effect. Using a forward genetic screen, the glycerol-3-phosphate transporter GlpT that facilitates FSM uptake was identified in two zoonotic staphylococci, Staphylococcus schleiferi and Staphylococcus pseudintermedius. A series of lipophilic ester prodrugs (termed MEPicides) structurally related to FSM were synthesized, and data indicate that the presence of the prodrug moiety not only substantially increased potency of the inhibitors against staphylococci but also bypassed the need for GlpT-mediated cellular transport. Collectively, our data indicate that the prodrug MEPicides selectively and robustly inhibit DXR in zoonotic staphylococci, and further, that DXR represents a promising, druggable target for future development.
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Affiliation(s)
- Rachel L. Edwards
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Isabel Heueck
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Soon Goo Lee
- University of North Carolina-Wilmington, Wilmington, North Carolina, United States of America
| | - Ishaan T. Shah
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Justin J. Miller
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Andrew J. Jezewski
- Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Marwa O. Mikati
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Xu Wang
- Department of Chemistry, George Washington University, Washington, DC, United States of America
| | - Robert C. Brothers
- Department of Chemistry, George Washington University, Washington, DC, United States of America
| | - Kenneth M. Heidel
- Department of Chemistry, George Washington University, Washington, DC, United States of America
| | - Damon M. Osbourn
- Department of Chemistry, Saint Louis University, St. Louis, Missouri, United States of America
| | - Carey-Ann D. Burnham
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Sophie Alvarez
- Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, Nebraska, United States of America
| | - Stephanie A. Fritz
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Cynthia S. Dowd
- Department of Chemistry, George Washington University, Washington, DC, United States of America
| | - Joseph M. Jez
- Department of Biology, Washington University, St. Louis, Missouri, United States of America
| | - Audrey R. Odom John
- Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri, United States of America
- Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, United States of America
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Liang YF, Liu H, Li H, Gao WY. Determination of the Activity of 1-Deoxy-D-Xylulose 5-Phosphate Synthase by Pre-column Derivatization-HPLC Using 1,2-Diamino-4,5-Methylenedioxybenzene as a Derivatizing Reagent. Protein J 2019; 38:160-166. [PMID: 30707333 DOI: 10.1007/s10930-019-09816-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
α-Ketoacids can be determined by HPLC through pre-column derivatization with 1,2-diamino-4,5-methylenedioxybenzene (DMB) as a derivatizing reagent. Using this method, the specific activity and the steady-state kinetic of 1-deoxy-D-xylulose-5-phosphate synthase (DXS) were measured. Firstly, DXS substrate pyruvate was derivatized with DMB in acidic solution; then the corresponding quinoxalinone was elucidated by LC-ESI-MS and quantified by HPLC-UV. The optimum derivatization conditions were as follows: aqueous medium at pH 1.0, reaction temperature 80 °C, reaction time 60 min, molar ratio of DMB to pyruvate 10:1. The HPLC was run with isocratic elution using the mixture of methanol and water (60:40, v/v) as a mobile phase. The detective limit and the linear correlation range of the method were 0.05 µM and 0.002-1.0 mM (R = 0.994), respectively. The relative standard deviation (RSD) of six determinations was 2.48%. The steady-state kinetic parameters of DXS for pyruvate determined with the method were identical to the reported data. The established method is a practical route for evaluation of DXS activity, especially in the research and development of DXS inhibitors.
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Affiliation(s)
- Yan-Fei Liang
- National Engineering Research Center for Miniaturized Detection Systems and College of Life Sciences, Northwest University, 229 North Taibai Road, Xi'an, 710069, Shaanxi, China
| | - Hui Liu
- National Engineering Research Center for Miniaturized Detection Systems and College of Life Sciences, Northwest University, 229 North Taibai Road, Xi'an, 710069, Shaanxi, China
| | - Heng Li
- National Engineering Research Center for Miniaturized Detection Systems and College of Life Sciences, Northwest University, 229 North Taibai Road, Xi'an, 710069, Shaanxi, China
| | - Wen-Yun Gao
- National Engineering Research Center for Miniaturized Detection Systems and College of Life Sciences, Northwest University, 229 North Taibai Road, Xi'an, 710069, Shaanxi, China.
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Targeting the apicoplast in malaria. Biochem Soc Trans 2019; 47:973-983. [PMID: 31383817 DOI: 10.1042/bst20170563] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2018] [Revised: 05/22/2019] [Accepted: 06/10/2019] [Indexed: 12/25/2022]
Abstract
Malaria continues to be one of the leading causes of human mortality in the world, and the therapies available are insufficient for eradication. Severe malaria is caused by the apicomplexan parasite Plasmodium falciparum Apicomplexan parasites, including the Plasmodium spp., are descendants of photosynthetic algae, and therefore they possess an essential plastid organelle, named the apicoplast. Since humans and animals have no plastids, the apicoplast is an attractive target for drug development. Indeed, after its discovery, the apicoplast was found to host the target pathways of some known antimalarial drugs, which motivated efforts for further research into its biological functions and biogenesis. Initially, many apicoplast inhibitions were found to result in 'delayed death', whereby parasite killing is seen only at the end of one invasion-egress cycle. This slow action is not in line with the current standard for antimalarials, which seeded scepticism about the potential of compounds targeting apicoplast functions as good candidates for drug development. Intriguingly, recent evidence of apicoplast inhibitors causing rapid killing could put this organelle back in the spotlight. We provide an overview of drugs known to inhibit apicoplast pathways, alongside recent findings in apicoplast biology that may provide new avenues for drug development.
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Heidel KM, Dowd CS. Phosphonate prodrugs: an overview and recent advances. Future Med Chem 2019; 11:1625-1643. [PMID: 31469328 PMCID: PMC6722485 DOI: 10.4155/fmc-2018-0591] [Citation(s) in RCA: 52] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2018] [Accepted: 03/29/2019] [Indexed: 01/04/2023] Open
Abstract
Phosphonates, often used as isosteric replacements for phosphates, can provide important interactions with an enzyme. Due to their high charge at physiological pH, however, permeation into cells can be a challenge. Protecting phosphonates as prodrugs has shown promise in drug delivery. Thus, a variety of structures and cleavage/activation mechanisms exist, enabling release of the active compound. This review describes the structural diversity of these pro-moieties, relevant cleavage mechanisms and recent advances in the design of phosphonate prodrugs.
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Affiliation(s)
- Kenneth M Heidel
- Department of Chemistry, George Washington University, Washington, DC 20052, USA
| | - Cynthia S Dowd
- Department of Chemistry, George Washington University, Washington, DC 20052, USA
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Wang X, Edwards RL, Ball H, Johnson C, Haymond A, Girma M, Manikkam M, Brothers RC, McKay KT, Arnett SD, Osbourn DM, Alvarez S, Boshoff HI, Meyers MJ, Couch RD, Odom John AR, Dowd CS. MEPicides: α,β-Unsaturated Fosmidomycin Analogues as DXR Inhibitors against Malaria. J Med Chem 2018; 61:8847-8858. [PMID: 30192536 DOI: 10.1021/acs.jmedchem.8b01026] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Severe malaria due to Plasmodium falciparum remains a significant global health threat. DXR, the second enzyme in the MEP pathway, plays an important role to synthesize building blocks for isoprenoids. This enzyme is a promising drug target for malaria due to its essentiality as well as its absence in humans. In this study, we designed and synthesized a series of α,β-unsaturated analogues of fosmidomycin, a natural product that inhibits DXR in P. falciparum. All compounds were evaluated as inhibitors of P. falciparum. The most promising compound, 18a, displays on-target, potent inhibition against the growth of P. falciparum (IC50 = 13 nM) without significant inhibition of HepG2 cells (IC50 > 50 μM). 18a was also tested in a luciferase-based Plasmodium berghei mouse model of malaria and showed exceptional in vivo efficacy. Together, the data support MEPicide 18a as a novel, potent, and promising drug candidate for the treatment of malaria.
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Affiliation(s)
- Xu Wang
- Department of Chemistry , George Washington University , Washington D.C. 20052 , United States
| | - Rachel L Edwards
- Department of Pediatrics , Washington University School of Medicine, Washington University , St. Louis , Missouri 63110 , United States
| | - Haley Ball
- Department of Chemistry and Biochemistry , George Mason University , Manassas , Virginia 20110 , United States
| | - Claire Johnson
- Department of Chemistry and Biochemistry , George Mason University , Manassas , Virginia 20110 , United States
| | - Amanda Haymond
- Department of Chemistry and Biochemistry , George Mason University , Manassas , Virginia 20110 , United States
| | - Misgina Girma
- Department of Chemistry and Biochemistry , George Mason University , Manassas , Virginia 20110 , United States
| | - Michelle Manikkam
- Tuberculosis Research Section, LCIM , NIAID/NIH , Bethesda , Maryland 20892 , United States
| | - Robert C Brothers
- Department of Chemistry , George Washington University , Washington D.C. 20052 , United States
| | - Kyle T McKay
- Department of Chemistry , George Washington University , Washington D.C. 20052 , United States
| | - Stacy D Arnett
- Department of Pharmacology and Physiology , Saint Louis University , St. Louis , Missouri 63104 , United States
| | - Damon M Osbourn
- Department of Molecular Microbiology and Immunology , Saint Louis University , St. Louis , Missouri 63104 , United States
| | - Sophie Alvarez
- Proteomics & Metabolomics Facility, Center for Biotechnology, Department of Agronomy and Horticulture , University of Nebraska-Lincoln , Lincoln , Nebraska 68588 , United States
| | - Helena I Boshoff
- Tuberculosis Research Section, LCIM , NIAID/NIH , Bethesda , Maryland 20892 , United States
| | - Marvin J Meyers
- Department of Pharmacology and Physiology , Saint Louis University , St. Louis , Missouri 63104 , United States.,Department of Chemistry , Saint Louis University , St. Louis , Missouri 63103 , United States
| | - Robin D Couch
- Department of Chemistry and Biochemistry , George Mason University , Manassas , Virginia 20110 , United States
| | - Audrey R Odom John
- Department of Pediatrics , Washington University School of Medicine, Washington University , St. Louis , Missouri 63110 , United States
| | - Cynthia S Dowd
- Department of Chemistry , George Washington University , Washington D.C. 20052 , United States
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18
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Bartee D, Wheadon MJ, Freel Meyers CL. Synthesis and Evaluation of Fluoroalkyl Phosphonyl Analogues of 2- C-Methylerythritol Phosphate as Substrates and Inhibitors of IspD from Human Pathogens. J Org Chem 2018; 83:9580-9591. [PMID: 29870251 DOI: 10.1021/acs.joc.8b00686] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Targeting essential bacterial processes beyond cell wall, protein, nucleotide, and folate syntheses holds promise to reveal new antimicrobial agents and expand the potential drugs available for combination therapies. The synthesis of isoprenoid precursors, isopentenyl diphosphate (IDP) and dimethylallyl diphosphate (DMADP), is vital for all organisms; however, humans use the mevalonate pathway for production of IDP/DMADP while many pathogens, including Plasmodium falciparum and Mycobacterium tuberculosis, use the orthogonal methylerythritol phosphate (MEP) pathway. Toward developing novel antimicrobial agents, we have designed and synthesized a series of phosphonyl analogues of MEP and evaluated their abilities to interact with IspD, both as inhibitors of the natural reaction and as antimetabolite alternative substrates that could be processed enzymatically to form stable phosphonyl analogues as potential inhibitors of downstream MEP pathway intermediates. In this compound series, the S-monofluoro MEP analogue displays the most potent inhibitory activity against Escherichia coli IspD and is the best substrate for both the E. coli and P. falciparum IspD orthologues with a Km approaching that of the natural substrate for the E. coli enzyme. This work represents a first step toward the development of phosphonyl MEP antimetabolites to modulate early isoprenoid biosynthesis in human pathogens.
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Affiliation(s)
- David Bartee
- Department of Pharmacology and Molecular Sciences , The Johns Hopkins University School of Medicine , Baltimore , Maryland 21205 , United States
| | - Michael J Wheadon
- Department of Pharmacology and Molecular Sciences , The Johns Hopkins University School of Medicine , Baltimore , Maryland 21205 , United States
| | - Caren L Freel Meyers
- Department of Pharmacology and Molecular Sciences , The Johns Hopkins University School of Medicine , Baltimore , Maryland 21205 , United States
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19
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Wang X, Dowd CS. The Methylerythritol Phosphate Pathway: Promising Drug Targets in the Fight against Tuberculosis. ACS Infect Dis 2018; 4:278-290. [PMID: 29390176 DOI: 10.1021/acsinfecdis.7b00176] [Citation(s) in RCA: 40] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is a severe infectious disease in need of new chemotherapies especially for drug-resistant cases. To meet the urgent requirement of new TB drugs with novel modes of action, the TB research community has been validating numerous targets from several biosynthetic pathways. The methylerythritol phosphate (MEP) pathway is utilized by Mtb for the biosynthesis of isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP), the universal five-carbon building blocks of isoprenoids. While being a common biosynthetic pathway in pathogens, the MEP pathway is completely absent in humans. Due to its unique presence in pathogens as well as the essentiality of the MEP pathway in Mtb, the enzymes in this pathway are promising targets for the development of new drugs against tuberculosis. In this Review, we discuss three enzymes in the MEP pathway: 1-deoxy-d-xylulose-5-phosphate synthase (DXS), 1-deoxy-d-xylulose-5-phosphate reductoisomerase (IspC/DXR), and 2 C-methyl-d-erythritol 2,4-cyclodiphosphate synthase (IspF), which appear to be the most promising antitubercular drug targets. Structural and mechanistic features of these enzymes are reviewed, as well as selected inhibitors that show promise as antitubercular agents.
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Affiliation(s)
- Xu Wang
- Department of Chemistry, George Washington University, 800 22nd Street NW, Washington, D.C. 20052, United States
| | - Cynthia S. Dowd
- Department of Chemistry, George Washington University, 800 22nd Street NW, Washington, D.C. 20052, United States
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