Fosfomycin Permeation through the Outer Membrane Porin OmpF

Fosfomycin is a frequently prescribed drug in the treatment of acute urinary tract infections. It enters the bacterial cytoplasm and inhibits the biosynthesis of peptidoglycans by targeting the MurA enzyme. Despite extensive pharmacological studies and clinical use, the permeability of fosfomycin across the bacterial outer membrane is largely unexplored. Here, we investigate the fosfomycin permeability across the outer membrane of Gram-negative bacteria by electrophysiology experiments as well as by all-atom molecular dynamics simulations including free-energy and applied-field techniques. Notably, in an electrophysiological zero-current assay as well as in the molecular simulations, we found that fosfomycin can rapidly permeate the abundant Escherichia coli porin OmpF. Furthermore, two triple mutants in the constriction region of the porin have been investigated. The permeation rates through these mutants are slightly lower than that of the wild type but fosfomycin can still permeate. Altogether, this work unravels molecular details of fosfomycin permeation through the outer membrane porin OmpF of E. coli and moreover provides hints for understanding the translocation of phosphonic acid antibiotics through other outer membrane pores.

Emergence of Plasmid-Mediated Fosfomycin-Resistance Genes among Escherichia coli Isolates, France

FosA, a glutathione S-transferase that inactivates fosfomycin, has been reported as the cause of enzymatic resistance to fosfomycin. We show that multiple lineages of FosA-producing extended spectrum β-lactamase Escherichia coli have circulated in France since 2012, potentially reducing the efficacy of fosfomycin in treating infections with antimicrobial drug–resistant gram-negative bacilli.

[Proteomic analysis of Bacillus subtilis 168 transforming cis-propenylphosphonic acid to fosfomycin]

In this study, we investigated the mechanism of transformation by Bacillus subtilis strain 168 by proteomic analysis. B. subtilis strain 168 was able to stereoselectively transform cis-propenylphosphonic acid (cPPA) to fosfomycin. The maximal fosfomycin production was 816.6 microg/mL after two days cultivation, with a conversion rate of 36.05%. We separated the whole cellular proteins by two-dimensional gel electrophoresis (2-DE) method, and 562 protein spots were detected in the presence of cPPA in the medium, while 527 protein spots were detected in the absence of cPPA. Of them, 98 differentially expressed protein spots were found. Among them, 52 proteins were up-regulated whereas 20 were down-regulated in the presence of cPPA in the medium, and 26 induced at the presence of cPPA. The differentially expressed proteins were analyzed by combined MS and MS/MS methods. Eighty protein spots, including 45 up-regulated proteins, 17 down-regulated proteins, and 18 induced by cPPA were identified. Based on the results of proteomic analysis, we postulated two steps of transformation: in the first step, cPPA was hydrated to 2-hydroxypropylphosphonic acid; in the second step, 2-hydroxypropylphosphonic acid was transformed to fosfomycin via a dehydrogenation reaction.

Synthesis of tetrazole analogues of phosphonohydroxamic acids: an attempt to improve the inhibitory activity against the DXR

This work is focused on the design of new antimicrobial drugs and on the development of lipophilic inhibitors of the DXR, the second enzyme of the MEP pathway for the biosynthesis of isoprene units in most bacteria, by replacing the phosphonate group of fosmidomycin derivatives by a tetrazoyl moiety capable of multiple hydrogen bonding. The N- and C-substituted tetrazole analogues of phosphonohydroxamate inhibitors were synthesized and tested on the DXR of Escherichia coli. This work points out the hypothesis that the phosphonate/phosphate recognition site might be too rigid to accommodate other functional groups.

Metabolite profiling identified methylerythritol cyclodiphosphate efflux as a limiting step in microbial isoprenoid production

Isoprenoids are natural products that are all derived from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These precursors are synthesized either by the mevalonate (MVA) pathway or the 1-Deoxy-D-Xylulose 5-Phosphate (DXP) pathway. Metabolic engineering of microbes has enabled overproduction of various isoprenoid products from the DXP pathway including lycopene, artemisinic acid, taxadiene and levopimaradiene. To date, there is no method to accurately measure all the DXP metabolic intermediates simultaneously so as to enable the identification of potential flux limiting steps. In this study, a solid phase extraction coupled with ultra performance liquid chromatography mass spectrometry (SPE UPLC-MS) method was developed. This method was used to measure the DXP intermediates in genetically engineered E. coli. Unexpectedly, methylerythritol cyclodiphosphate (MEC) was found to efflux when certain enzymes of the pathway were over-expressed, demonstrating the existence of a novel competing pathway branch in the DXP metabolism. Guided by these findings, ispG was overexpressed and was found to effectively reduce the efflux of MEC inside the cells, resulting in a significant increase in downstream isoprenoid production. This study demonstrated the necessity to quantify metabolites enabling the identification of a hitherto unrecognized pathway and provided useful insights into rational design in metabolic engineering.

Structure of MurA (UDP-N-acetylglucosamine enolpyruvyl transferase) from Vibrio fischeri in complex with substrate UDP-N-acetylglucosamine and the drug fosfomycin

The development of new antibiotics is necessitated by the rapid development of resistance to current therapies. UDP-N-acetylglucosamine enolpyruvyl transferase (MurA), which catalyzes the first committed step of bacterial peptidoglycan biosynthesis, is a prime candidate for therapeutic intervention. MurA is the target of the antibiotic fosfomycin, a natural product produced by Streptomyces. Despite possessing a high degree of sequence conservation with MurA enzymes from fosfomycin-susceptible organisms, recent microbiological studies suggest that MurA from Vibrio fischeri (VfiMurA) may confer fosfomycin resistance via a mechanism that is not yet understood. The crystal structure of VfiMurA in a ternary complex with the substrate UDP-N-acetylglucosamine (UNAG) and fosfomycin has been solved to a resolution of 1.93 Å. Fosfomycin is known to inhibit MurA by covalently binding to a highly conserved cysteine in the active site of the enzyme. A comparison of the title structure with the structure of fosfomycin-susceptible Haemophilus influenzae MurA (PDB entry 2rl2) revealed strikingly similar conformations of the mobile substrate-binding loop and clear electron density for a fosfomycin-cysteine adduct. Based on these results, there are no distinguishing sequence/structural features in VfiMurA that would translate to a diminished sensitivity to fosfomycin. However, VfiMurA is a robust crystallizer and shares high sequence identity with many clinically relevant bacterial pathogens. Thus, it would serve as an ideal system for use in the structure-guided optimization of new antibacterial agents.

The Streptomyces-produced antibiotic fosfomycin is a promiscuous substrate for archaeal isopentenyl phosphate kinase

Isopentenyl phosphate kinase (IPK) catalyzes the phosphorylation of isopentenyl phosphate to form the isoprenoid precursor isopentenyl diphosphate in the archaeal mevalonate pathway. This enzyme is highly homologous to fosfomycin kinase (FomA), an antibiotic resistance enzyme found in a few strains of Streptomyces and Pseudomonas whose mode of action is inactivation by phosphorylation. Superposition of Thermoplasma acidophilum (THA) IPK and FomA structures aligns their respective substrates and catalytic residues, including H50 and K14 in THA IPK and H58 and K18 in Streptomyces wedmorensis FomA. These residues are conserved only in the IPK and FomA members of the phosphate subdivision of the amino acid kinase family. We measured the fosfomycin kinase activity of THA IPK [K(m) = 15.1 ± 1.0 mM, and k(cat) = (4.0 ± 0.1) × 10⁻² s⁻¹], resulting in a catalytic efficiency (k(cat)/K(m) = 2.6 M⁻¹ s⁻¹) that is 5 orders of magnitude lower than that of the native reaction. Fosfomycin is a competitive inhibitor of IPK (K(i) = 3.6 ± 0.2 mM). Molecular dynamics simulation of the IPK·fosfomycin·MgATP complex identified two binding poses for fosfomycin in the IP binding site, one of which results in a complex analogous to the native IPK·IP·ATP complex that engages H50 and the lysine triangle formed by K5, K14, and K205. The other binding pose leads to a dead-end complex that engages K204 near the IP binding site to bind fosfomycin. Our findings suggest a mechanism for acquisition of FomA-based antibiotic resistance in fosfomycin-producing organisms.

Fosmidomycin uptake into Plasmodium and Babesia-infected erythrocytes is facilitated by parasite-induced new permeability pathways

Background

Highly charged compounds typically suffer from low membrane permeability and thus are generally regarded as sub-optimal drug candidates. Nonetheless, the highly charged drug fosmidomycin and its more active methyl-derivative FR900098 have proven parasiticidal activity against erythrocytic stages of the malaria parasite Plasmodium falciparum. Both compounds target the isoprenoid biosynthesis pathway present in bacteria and plastid-bearing organisms, like apicomplexan parasites. Surprisingly, the compounds are inactive against a range of apicomplexans replicating in nucleated cells, including Toxoplasma gondii.

Methodology/Principal Findings

Since non-infected erythrocytes are impermeable for FR90098, we hypothesized that these drugs are taken up only by erythrocytes infected with Plasmodium. We provide evidence that radiolabeled FR900098 accumulates in theses cells as a consequence of parasite-induced new properties of the host cell, which coincide with an increased permeability of the erythrocyte membrane. Babesia divergens, a related parasite that also infects human erythrocytes and is also known to induce an increase in membrane permeability, displays a similar susceptibility and uptake behavior with regard to the drug. In contrast, Toxoplasma gondii-infected cells do apparently not take up the compounds, and the drugs are inactive against the liver stages of Plasmodium berghei, a mouse malaria parasite.

Conclusions/Significance

Our findings provide an explanation for the observed differences in activity of fosmidomycin and FR900098 against different Apicomplexa. These results have important implications for future screens aimed at finding new and safe molecular entities active against P. falciparum and related parasites. Our data provide further evidence that parasite-induced new permeability pathways may be exploited as routes for drug delivery.

Structure and mechanism of enzymes involved in biosynthesis and breakdown of the phosphonates fosfomycin, dehydrophos, and phosphinothricin

Recent years have seen a rapid increase in the mechanistic and structural information on enzymes that are involved in the biosynthesis and breakdown of naturally occurring phosphonates. This review focuses on these recent developments with an emphasis on those enzymes that have been characterized crystallographically in the past five years, including proteins involved in the biosynthesis of phosphinothricin, fosfomycin, and dehydrophos and proteins involved in resistance mechanisms.

Structure of 1-deoxy-D-xylulose 5-phosphate reductoisomerase in a quaternary complex with a magnesium ion, NADPH and the antimalarial drug fosmidomycin

The crystal structure of 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) from Escherichia coli complexed with Mg(2+), NADPH and fosmidomycin was solved at 2.2 A resolution. DXR is the key enzyme in the 2-C-methyl-D-erythritol 4-phosphate pathway and is an effective target of antimalarial drugs such as fosmidomycin. In the crystal structure, electron density for the flexible loop covering the active site was clearly observed, indicating the well ordered conformation of DXR upon substrate binding. On the other hand, no electron density was observed for the nicotinamide-ribose portion of NADPH and the position of Asp149 anchoring Mg(2+) was shifted by NADPH in the active site.

Fosfomycin resistance proteins: a nexus of glutathione transferases and epoxide hydrolases in a metalloenzyme superfamily

Three similar but mechanistically distinct fosfomycin resistance proteins that catalyze the opening of the oxirane ring of the antibiotic are known. FosA is a Mn(II) and K(+)-dependent glutathione transferase. FosB is a Mg(2+)-dependent L-cysteine thiol transferase. FosX is a Mn(II)-dependent fosfomycin-specific epoxide hydrolase. The expression, purification, kinetic, and physical characteristics of six fosfomycin resistance proteins including the FosA proteins from transposon TN2921 and Pseudomonas aeruginosa, the FosB proteins from Bacillus subtilis and Staphylococcus aureus, and the FosX proteins from Mesorhizobium loti and Listeria monocytogenes are reported.

Crystallization and preliminary X-ray crystallographic analysis of UDP-N-acetylglucosamine enolpyruvyl transferase from Haemophilus influenzae in complex with UDP-N-acetylglucosamine and fosfomycin

The bacterial enzyme UDP-N-acetylglucosamine enolpyruvyl transferase catalyzes the first committed step of peptidoglycan biosynthesis, i.e., transfer of enolpyruvate from phosphoenolpyruvate to UDP-N-acetyl-glucosamine. We have overexpressed the enzyme from Haemophilus influenzae in Escherichia coli and crystallized it in the apo-form, as well as in a complex with UDP-N-acetylglucosamine and fosfomycin using ammonium sulfate as the precipitant. X-ray diffraction data from a crystal of the apo-form were collected to 2.8 A resolution at 293 K. The crystal quality was improved by co-crystallization with UDP-N-acetylglucosamine and fosfomycin. X-ray data to 2.2 A have been collected at 100 K from a flash-frozen crystal of the complex. The complex crystals belong to the orthorhombic space group I222 (or I212121) with unit-cell parameters of a = 63.7, b = 124.5, and c = 126.3 A. Assuming a monomer of the recombinant enzyme in the crystallographic asymmetric unit, the calculated Matthews parameter (VM) is 2.71 A3 Da-1 and solvent content is 54.6%.

A novel inhibitor that suspends the induced fit mechanism of UDP-N-acetylglucosamine enolpyruvyl transferase (MurA)

MurA (UDP-N-acetylglucosamine enolpyruvyl transferase, EC 2.5.1.7) catalyzes the first committed step in the synthesis of the bacterial cell wall. It is the target of the naturally occurring, broad-spectrum antibiotic fosfomycin. Fosfomycin, an epoxide, is a relatively poor drug because an ever-increasing number of bacteria have developed resistance to fosfomycin. Thus, there is a critical need for the development of novel drugs that target MurA by a different molecular mode of action. We have identified a new scaffold of potent MurA inhibitors, derivatives of 5-sulfonoxy-anthranilic acid, using high-throughput screening. T6361 and T6362 are competitive inhibitors of MurA with respect to the first substrate, UDP-N-acetylglucosamine (UNAG), with a K(i) of 16 microM. The crystal structure of the MurA.T6361 complex at 2.6 angstrom resolution, together with fluorescence data, revealed that the inhibitor targets a loop, Pro112 to Pro121, that is crucial for the structural changes of the enzyme during catalysis. Thus, this new class of MurA inhibitors is not active site-directed but instead obstructs the transition from the open (unliganded) to the closed (UNAG-liganded) enzyme form. The results provide evidence for the existence of a MurA.UNAG collision complex that may be specifically targeted by small molecules different from ground-state analogs of the enzymatic reaction.

The crystal structure of E.coli 1-deoxy-D-xylulose-5-phosphate reductoisomerase in a ternary complex with the antimalarial compound fosmidomycin and NADPH reveals a tight-binding closed enzyme conformation

The key enzyme in the non-mevalonate pathway of isoprenoid biosynthesis, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) has been shown to be the target enzyme of fosmidomycin, an antimalarial, antibacterial and herbicidal compound. Here we report the crystal structure of selenomethionine-labelled Escherichia coli DXR in a ternary complex with NADPH and fosmidomycin at 2.2 A resolution. The structure reveals a considerable conformational rearrangement upon fosmidomycin binding and provides insights into the slow, tight binding inhibition mode of the inhibitor. Although the inhibitor displays an unusual non-metal mediated mode of inhibition, which is an artefact most likely due to the low metal affinity of DXR at the pH used for crystallization, the structural data add valuable information for the rational design of novel DXR inhibitors. Using this structure together with the published structural data and the 1.9 A crystal structure of DXR in a ternary complex with NADPH and the substrate 1-deoxy-D-xylulose 5-phosphate, a model for the physiologically relevant tight-binding mode of inhibition is proposed. The structure of the substrate complex must be interpreted with caution due to the presence of a second diastereomer in the active site.

Evidence that the fosfomycin target Cys115 in UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) is essential for product release

MurA (UDP-N-acetylglucosamine enolpyruvyl transferase, EC 2.5.1.7) is an essential enzyme in the biosynthesis of the peptidoglycan layer of the bacterial cell. It provides an attractive template for the design of novel antibiotic drugs and is the target of the naturally occurring antibiotic fosfomycin, which covalently attaches to Cys115 in the active site of the enzyme. Mutations of Cys115 to Asp exist in pathogens such as Mycobacteria or Chlamydia rendering these organisms resistant to fosfomycin. Thus, there is a need for the elucidation of the role of this cysteine in the MurA reaction. We determined the x-ray structure of the C115S mutant of Enterobacter cloacae MurA, which was crystallized in the presence of the substrates of MurA. The structure depicts the product state of the enzyme with enolpyruvyl-UDP-N-acetylglucosamine and inorganic phosphate trapped in the active site. Kinetic analysis revealed that the Cys-to-Ser mutation results in an enzyme that appears to perform a single turnover of the reaction. Opposing the common view of Cys115 as a key residue in the chemical reaction of enolpyruvyl transfer, we now conclude that the wild-type cysteine is essential for product release only. On the basis of a detailed comparison of the product state with the intermediate state and an unliganded state of MurA, we propose that dissociation of the products is an ordered event with inorganic phosphate leaving first. Phosphate departure appears to trigger a suite of conformational changes, which finally leads to opening of the two-domain structure of MurA and the release of the second product enolpyruvyl-UDP-N-acetylglucosamine.

Role of K22 and R120 in the covalent binding of the antibiotic fosfomycin and the substrate-induced conformational change in UDP-N-acetylglucosamine enolpyruvyl transferase

UDP-N-acetylglucosamine enolpyruvyl transferase (MurA), catalyzes the first step in the biosynthesis of peptidoglycan, involving the transfer of the intact enolpyruvyl moiety from phosphoenolpyruvate to the 3'-hydroxyl group of UDP-N-acetylglucosamine (UDPNAG). The enzyme is irreversibly inhibited by the antibiotic fosfomycin. The inactivation is caused by alkylation of a highly conserved cysteine residue (C115) that participates in the binding of phosphoenolpyruvate. The three-dimensional structure of the enzyme suggests that two residues may play a decisive role in fosfomycin binding: K22 and R120. To investigate the role of these residues, we have generated the K22V, K22E, K22R and R120K single mutant proteins as well as the K22V/R120K and K22V/R120V double mutant proteins. We demonstrated that the K22R mutant protein behaves similarly to wild-type enzyme, whereas the K22E mutant protein failed to form the covalent adduct. On the other hand, the K22V mutant protein requires the presence of UDPNAG for the formation of the adduct indicating that UDPNAG plays a crucial role in the organization of productive interactions in the active site. This model receives strong support from heat capacity changes observed for the K22V/R120K and R120K mutant proteins: in both mutant proteins, the heat capacity changes are markedly reduced indicating that their ability to form a closed protein conformation is impeded due to the R120K exchange.

Structure of fosfomycin resistance protein FosA from transposon Tn2921

The crystal structure of fosfomycin resistance protein FosA from transposon Tn2921 has been established at a resolution of 2.5 A. The protein crystallized without bound Mn(II) and K+, ions crucial for efficient catalysis, providing a structure of the apo enzyme. The protein maintains the three-dimensional domain-swapped arrangement of the paired betaalphabetabetabeta-motifs observed in the genomically encoded homologous enzyme from Pseudomonas aeruginosa (PA1129). The basic architecture of the active site is also maintained, despite the absence of the catalytically essential Mn(II). However, the absence of K+, which has been shown to enhance enzymatic activity, appears to contribute to conformational heterogeneity in the K(+)-binding loops.

Impact of ozone on monoterpene emissions and evidence for an isoprene-like antioxidant action of monoterpenes emitted by Quercus ilex leaves

Quercus ilex (L.) leaves emit monoterpenes, particularly alpha-pinene, beta-pinene and sabinene. Apart from the monoterpene pools that are stored in specialized structures and have a clear defensive or attractive role, the function of monoterpenes in Q. ilex leaves is unknown. We tested whether monoterpenes have an antioxidant role, as has previously been found for isoprene in isoprene-emitting leaves. We exposed Q. ilex leaves to either mild and repeated ozone exposure (Experiment I) or to a single acute ozone exposure (Experiment II) at temperatures ranging between 20 and 32 degrees C. Both ozone treatments rapidly stimulated monoterpene synthesis, but had no effect on photosynthesis and caused no visible damage to leaves maintained at 25, 30 or 32 degrees C. Ozone inhibited both photosynthesis and monoterpene synthesis in leaves maintained at 20 degrees C. To characterize the relationship between monoterpenes and ozone-induced damage, we fed detached leaves fosmidomycin, a selective inhibitor of isoprene synthesis. Fosmidomycin caused rapid and complete inhibition of monoterpene emissions in leaves maintained at 30 degrees C, confirming that monoterpenes are synthesized by the same biochemical pathway as isoprene. However, over the experimental period, fosmidomycin did not affect concentrations of compounds that are formed from chloroplastic isoprenoids and that might have conferred antioxidant protection, either directly (carotenoids) or indirectly (chlorophylls, xanthophylls). In leaves whose monoterpene synthesis had been inhibited by fosmidomycin, ozone rapidly and significantly inhibited photosynthesis and increased the production of hydrogen peroxide and malonyldialdehyde. We conclude that monoterpenes produced by Q. ilex leaves share the same biosynthetic pathway and function as isoprene. Furthermore, all volatile isoprenoids may have similar antioxidant properties and may be stimulated by the same stress-inducing conditions.

Mechanistic diversity of fosfomycin resistance in pathogenic microorganisms

Microbial resistance to the antibiotic fosfomycin [(1R,2S)-epoxypropylphosphonic acid, 1] is known to be mediated by thiol transferase enzymes FosA and FosB, which catalyze the addition of glutathione and l-cysteine to C1 of the oxirane, respectively. A probe of the microbial genome database reveals a related group of enzymes (FosX). The genes mlr3345 from Mesorhizobium loti and lmo1702 from Listeria monocytogenes were cloned and the proteins expressed. This heretofore unrecognized group of enzymes is shown to catalyze the Mn(II)-dependent addition of water to C1 of the oxirane. The ability of each enzyme to confer resistance in Escherichia coli is correlated with their catalytic efficiency, such that the M. loti protein confers low resistance while the Listeria enzyme confers very robust resistance. The crystal structure of the FosX from M. loti was solved at a resolution of 1.83 A. The structure reveals an active-site carboxylate (E44) located about 5 A from the expected position of the substrate that appears to be poised to participate in catalysis. Single turnover experiments in H218O and kinetic analysis of the E44G mutant of the FosX enzymes indicate that the carboxylate of E44 acts as a general base in the direct addition of water to 1. The FosX from M. loti also catalyzes the addition of glutathione to the antibiotic. The catalytic promiscuity and low efficiency of the M. loti protein suggest that it may be an intermediate in the evolution of clinically relevant fosfomycin resistance proteins such as the FosX from Listeria monocytogenese.

Fosmidomycin resistance in adenylate cyclase deficient (cya) mutants of Escherichia coli

Adenylate cyclase deficient (cya) mutants of E. coli K-12 were found to be resistant to fosmidomycin, a specific inhibitor of the non-mevalonate pathway, just like to fosfomycin. E. coli glpT mutants were resistant to fosfomycin and also to fosmidomycin. This fact shows that fosmidomycin was transported inside via the glycerol-3-phosphate transporter, GlpT. DNA micro-array analysis showed that the transcription of glpT and other genes concerning glycerol utilization were highly dependent on the presence of cAMP.

Mevalonate and nonmevalonate pathways for the biosynthesis of isoprene units

Isoprenoids are synthesized by consecutive condensations of their five-carbon precursor, isopentenyl diphosphate, to its isomer, dimethylallyl diphosphate. Two pathways for these precursors are known. One is the mevalonate pathway, which operates in eucaryotes, archaebacteria, and cytosols of higher plants. The other is a recently discovered pathway, the nonmevalonate pathway, which is used by many eubacteria, green algae, and chloroplasts of higher plants. To date, five reaction steps in this new pathway and their corresponding enzymes have been identified. EC numbers of these enzymes have been assigned by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB) and are available at http://www.chem.qmw.ac.uk/iubmb/enzyme/reaction/terp/nonMVA.html.

EPR study of substrate binding to the Mn(II) active site of the bacterial antibiotic resistance enzyme FosA: a better way to examine Mn(II)

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.

Identification and characterization of new inhibitors of the Escherichia coli MurA enzyme

The bacterial enzyme MurA catalyzes the transfer of enolpyruvate from phosphoenolpyruvate (PEP) to uridine diphospho-N-acetylglucosamine (UNAG), which is the first committed step of bacterial cell wall biosynthesis. From high-throughput screening of a chemical library, three novel inhibitors of the Escherichia coli MurA enzyme were identified: the cyclic disulfide RWJ-3981, the purine analog RWJ-140998, and the pyrazolopyrimidine RWJ-110192. When MurA was preincubated with inhibitor, followed by addition of UNAG and PEP, the 50% inhibitory concentrations (IC(50)s) were 0.2 to 0.9 microM, compared to 8.8 microM for the known MurA inhibitor, fosfomycin. The three compounds exhibited MICs of 4 to 32 microg/ml against Staphylococcus aureus; however, the inhibition of DNA, RNA, and protein synthesis in addition to peptidoglycan synthesis by all three inhibitors indicated that antibacterial activity was not due specifically to MurA inhibition. The presence of UNAG during the MurA and inhibitor preincubation lowered the IC(50) at least fivefold, suggesting that, like fosfomycin, the three compounds may interact with the enzyme in a specific fashion that is enhanced by UNAG. Ultrafiltration and mass spectrometry experiments suggested that the compounds were tightly, but not covalently, associated with MurA. Molecular modeling studies demonstrated that the compounds could fit into the site occupied by fosfomycin; exposure of MurA to each compound reduced the labeling of MurA by tritiated fosfomycin. Taken together, the evidence indicates that these inhibitors may bind noncovalently to the MurA enzyme, at or near the site where fosfomycin binds.

Elementary steps in the acquisition of Mn2+ by the fosfomycin resistance protein (FosA)

The fosfomycin resistance protein, FosA, catalyzes the Mn(2+)-dependent addition of glutathione to the antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid, rendering the antibiotic inactive. The enzyme is a homodimer of 16 kDa subunits, each of which contains a single mononuclear metal site. Stopped-flow absorbance/fluorescence spectrometry provides evidence suggesting a complex kinetic mechanism for the acquisition of Mn(2+) by apoFosA. The binding of Mn(H(2)O)(6)(2+) to apoFosA alters the UV absorption and intrinsic fluorescence characteristics of the protein sufficiently to provide sensitive spectroscopic probes of metal binding. The acquisition of metal is shown to be a multistep process involving rapid preequilibrium formation of an initial complex with release of approximately two protons (k(obsd) > or = 800 s(-1)). The initial complex either rapidly dissociates or forms an intermediate coordination complex (k > 300 s(-1)) with rapid isomerization (k > or = 20 s(-1)) to a set of tight protein-metal complexes. The observed bimolecular rate constant for formation of the intermediate coordination complex is 3 x 10(5) M(-1) s(-1). The release of Mn(2+) from the protein is slow (k approximately 10(-2) s(-1)). The kinetic results suggest a more complex chelate effect than is typically observed for metal binding to simple multidentate ligands. Although the addition of the substrate, fosfomycin, has no appreciable effect on the association kinetics of enzyme and metal, it significantly decreases the dissociation rate, suggesting that the substrate interacts directly with the metal center.

Asparagine 23 and aspartate 305 are essential residues in the active site of UDP-N-acetylglucosamine enolpyruvyl transferase from Enterobacter cloacae

UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) catalyzes the transfer of the intact enolpyruvyl moiety of phosphoenolpyruvate (PEP) to the 3'-hydroxyl group of UDP-N-acetylglucosamine (UDPNAG). This reaction constitutes the first committed step in the biosynthesis of the bacterial cell wall component peptidoglycan (murein). The transfer reaction involves the nucleophilic attack of the 3'-hydroxyl group of UDPNAG at the C-2 of PEP. The three-dimensional structure of MurA complexed with UDPNAG revealed an aspartate residue (D305 in the En. cloacae sequence) close to the 3'-hydroxyl group of UDPNAG, suggesting that it may act as an acid-base catalyst in the active center of the enzyme. In addition to aspartate 305, asparagine 23 also interacts with the 3'-hydroxyl group; however, its role in catalysis or binding of the UDPNAG substrate is unclear. To gain information on the role of these two amino acids in the MurA-catalyzed reaction we have exchanged D305 for alanine, cysteine, histidine, and glutamate, and N23 for alanine and serine using site-directed mutagenesis. While the D305 alanine, cysteine, and histidine mutant proteins do not have detectable enzymatic activity, the D305E mutant protein exhibits a low residual activity (ca. 0.1% of the wild-type enzyme). Unlike with wild-type MurA, no exothermic signal was obtained when the D305A and -E mutant proteins were titrated with UDPNAG, demonstrating that the affinity of the sugar nucleotide substrate is reduced as a result of the amino acid exchange. The reduced affinity to UDPNAG leads to a lower propensity of C115 to form either the O-phosphothioketal with PEP or the thioether with the antibiotic fosfomycin. These findings emphasize the dual role of D305 as a general base and an essential binding partner to UDPNAG in the active site of MurA. Similarly, the two N23 mutant proteins showed a much lower catalytic activity although binding of UDPNAG was not as much affected as in the case of the D305 mutant proteins. This result indicates that this amino acid residue is mainly involved in stabilization of transition states.

Comparative X-ray analysis of the un-liganded fosfomycin-target murA

MurA, an essential enzyme for the synthesis of the bacterial cell wall, follows an induced-fit mechanism. Upon substrate binding, the active site forms in the interdomain cleft, involving movements of the two domains of the protein and a reorientation of the loop Pro112-Pro121. We compare two structures of un-liganded MurA from Enterobacter cloacae: a new orthorhombic form, solved to 1.80 A resolution, and a monoclinic form, redetermined to 1.55 A resolution. In the monoclinic form, the loop Pro112-Pro121 stretches into solvent, while in the new form it adopts a winded conformation, thereby reducing solvent accessibility of the critical residue Cys115. In the interdomain cleft a network of 27 common water molecules has been identified, which partially shields negative charges in the cleft and stabilizes the orientation of catalytically crucial residues. This could support substrate binding and ease domain movements. Near the hinge region an isoaspartyl residue has been recognized, which is the product of post-translational modification of the genetically encoded Asn67-Gly68. The homogeneous population with L-isoaspartate in both structures suggests that the modification in Enterobacter cloacae MurA is not a mere aging defect but rather the result of a specific in vivo process.

Lysine 22 in UDP-N-acetylglucosamine enolpyruvyl transferase from Enterobacter cloacae is crucial for enzymatic activity and the formation of covalent adducts with the substrate phosphoenolpyruvate and the antibiotic fosfomycin

UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) catalyzes the first committed step in the biosynthesis of the bacterial cell wall component peptidoglycan. The enzyme is the target of the antibiotic fosfomycin. A lysine residue (K22), strictly conserved in MurAs and the structurally and mechanistically related 5-enolpyruvylshikimate 3-phosphate synthases (EPSPS), is located near the active center of the enzyme. This residue is thought to be involved directly in the binding of the substrate phosphoenolpyruvate (PEP) and also to participate in the conformational change leading to the formation of the catalytically competent enzyme complex. Using site-directed mutagenesis, we have replaced this lysine with arginine (K22R), valine (K22V), and glutamate (K22E). These mutant proteins were expressed, purified, and characterized in comparison to wild-type MurA and a previously described inactive C115S mutant protein. It was found that all three K22 mutant proteins had less than 0.5% of the wild-type activity. Using isothermal titration calorimetry, it could be shown that the binding parameters for the UDP-sugar nucleotide substrate are not affected by the mutations, except for the K22E mutant protein. Similarly, binding of PEP was found to be unaffected in the K22 mutant proteins as demonstrated by tryptophan fluorescence quench titrations. On the other hand, the level of formation of a covalent adduct with either PEP or fosfomycin with the thiol group of cysteine 115 was diminished. The propensity to form an adduct with PEP decreased in the following order: wild type >> K22R > K22V > K22E. A comparable effect was found on the formation of the inhibitory covalent adduct of MurA and the antibiotic fosfomycin. These results are discussed in terms of an involvement of lysine 22 in a conformational change of MurA.

Elucidation of a monovalent cation dependence and characterization of the divalent cation binding site of the fosfomycin resistance protein (FosA)

The fosfomycin resistance protein FosA is a member of a distinct superfamily of metalloenzymes containing glyoxalase I, extradiol dioxygenases, and methylmalonyl-CoA epimerase. The dimeric enzyme, with the aid of a single mononuclear Mn2+ site in each subunit, catalyzes the addition of glutathione (GSH) to the oxirane ring of the antibiotic, rendering it inactive. Sequence alignments suggest that the metal binding site of FosA is composed of three residues: H7, H67, and E113. The single mutants H7A, H67A, and E113A as well as the more conservative mutants H7Q, H67Q, and E113Q exhibit marked decreases in the ability to bind Mn2+ and, in most instances, decreases in catalytic efficiency and the ability to confer resistance to the antibiotic. The enzyme also requires the monovalent cation K+ for optimal activity. The K+ ion activates the enzyme 100-fold with an activation constant of 6 mM, well below the physiologic concentration of K+ in E. coli. K+ can be replaced by other monovalent cations of similar ionic radii. Several lines of evidence suggest that the K+ ion interacts directly with the active site. Interaction of the enzyme with K+ is found to be dependent on the presence of the substrate fosfomycin. Moreover, the E113Q mutant exhibits a kcat which is 40% that of wild-type in the absence of K+. This mutant is not activated by monovalent cations. The behavior of the E113Q mutant is consistent with the proposition that the K+ ion helps balance the charge at the metal center, further lowering the activation barrier for addition of the anionic nucleophile. The fully activated, native enzyme provides a rate acceleration of >10(15) with respect to the spontaneous addition of GSH to the oxirane.

Emergence of fosfomycin-resistant isolates of Shiga-like toxin-producing Escherichia coli O26

We evaluated the susceptibilities of 129 Shiga-like toxin-producing Escherichia coli (STEC) isolates to various antibiotics. The numbers of isolates for which MICs were high (> or = 128 micrograms/ml) were as follows: 5 for fosfomycin, 14 for ampicillin, 1 for cefaclor, 6 for kanamycin, 22 for tetracycline, and 2 for doxycycline. For two isolates of STEC O26 MICs of fosfomycin were high (1,024 and 512 micrograms/ml, respectively). Conjugation experiments and glutathione S-transferase assays suggested that the fosfomycin resistance in these isolates was determined not by a plasmid but chromosomally. The amount of active intracellular fosfomycin in these STEC isolates was 100- to 200-fold less than that in E. coli C600 harboring pREFTT47B408 in the presence of either L-alpha-glycerophosphate or glucose-6-phosphate. Cloning, sequencing, and Northern blot analysis demonstrated that the transcriptional level of the murA gene encoding UDP-N-acetylglucosamine enolpyruvoyl transferase in these isolates was greater than that in E. coli C600. Our results suggest that the fosfomycin resistance in these STEC isolates is due to concurrent effects of alteration of the glpT and/or uhp transport systems and of the enhanced transcription of the murA gene.

A phosphonate-induced gene which promotes Penicillium-mediated bioconversion of cis-propenylphosphonic acid to fosfomycin

Penicillium decumbens is able to epoxidize cis-propenylphosphonic acid (cPA) to produce the antibiotic fosfomycin [FOM; also referred to as phosphonomycin and (-)-cis-1,2-epoxypropylphosphonic acid], a bioconversion of considerable commercial significance. We sought to improve the efficiency of the process by overexpression of the genes involved. A conventional approach of isolating the presumed epoxidase and its corresponding gene was not possible since cPA epoxidation could not be achieved with protein extracts. As an alternative approach, proteins induced by cPA were detected by two-dimensional gel electrophoresis. The observation that a 31-kDa protein (EpoA) was both cPA induced and overaccumulated in a strain which more efficiently converted cPA suggested that it might take part in the bioconversion. EpoA was purified, its amino acid sequence was partially determined, and the corresponding gene was isolated from cosmid and cDNA libraries with oligonucleotide probes. The DNA sequence for this gene (epoA) contained two introns and an open reading frame encoding a peptide of 277 amino acids having some similarity to oxygenases. When the gene was subcloned into P. decumbens, a fourfold increase in epoxidation activity was achieved. epoA-disruption mutants which were obtained by homologous recombination could not convert cPA to FOM. To investigate the regulation of the epoA promoter, the bialaphos resistance gene (bar, encoding phosphinothricin acetyltransferase) was used to replace the epoA-coding region. In P. decumbens, expression of the bar reporter gene was induced by cPA, FOM, and phosphorous acid but not by phosphoric acid.

Studies of the allosteric properties of maize leaf phosphoenolpyruvate carboxylase with the phosphoenolpyruvate analog phosphomycin as activator

The antibiotic phosphomycin (1,2-epoxypropylphosphonic acid), an analog of phosphoenolpyruvate (PEP), behaved not as an inhibitor, but as an activator, of the enzyme phosphoenolpyruvate carboxylase (PEPC) from maize leaves. Multiple activation studies indicated that the analog binds to the Glc6P-allosteric site producing a more activated enzyme than Glc6P itself. Because of this, we used phosphomycin as a tool to further extend our understanding of the mechanisms of allosteric regulation of C4-PEPC. Initial velocity data from detailed kinetic studies, in which the concentrations of free and Mg-complexed PEP and phosphomycin were controlled, are consistent with: (1) the true activator is free phosphomycin, which competes with free PEP for the Glc6P-allosteric site; and (2) the Mg-phosphomycin complex caused inhibition by binding to the active site in competition with MgPEP. Therefore, although the Glc6P-allosteric site and the active site are able to bind the same ligands, they differ in the form of substrate and activator they bind. This important difference allows the full expression of the potential of activation and prevents inhibition by the activators, including the physiological ones, which are mostly uncomplexed at physiological free Mg2+ concentrations. At fixed low substrate concentrations, the saturation kinetics of the enzyme by phosphomycin showed positive cooperativity at pH 7.3 and 8.3, although at the latter pH, the kinetics of saturation by the substrate was hyperbolic. The cosolute glycerol greatly increased the affinity of the enzyme for phosphomycin and abolished the cooperativity in its binding, but did not eliminate the heterotropic effects of the activator. Therefore, the heterotropic and homotropic effects of the activator are not always coupled to the homotropic effects of the substrate, which argues against the two-state model previously proposed to explain the allosteric properties of maize-leaf PEPC.

Biodegradation of phosphonomycin by Rhizobium huakuii PMY1

The biodegradation by Rhizobium huakuii PMY1 of up to 10 mM phosphonomycin as a carbon, energy, and phosphorus source with accompanying P(i) release is described. This biodegradation represents a further mechanism of resistance to this antibiotic and a novel, phosphate-deregulated route for organophosphonate metabolism by Rhizobium spp.

Structure of UDP-N-acetylglucosamine enolpyruvyl transferase, an enzyme essential for the synthesis of bacterial peptidoglycan, complexed with substrate UDP-N-acetylglucosamine and the drug fosfomycin

BACKGROUND

UDP-N-acetylglucosamine enolpyruvyl transferase (MurA), catalyses the first committed step of bacterial cell wall biosynthesis and is a target for the antibiotic fosfomycin. The only other known enolpyruvyl transferase is 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, an enzyme involved in the shikimic acid pathway and the target for the herbicide glyphosate. Inhibitors of enolpyruvyl transferases are of biotechnological interest as MurA and EPSP synthase are found exclusively in plants and microbes.

RESULTS

The crystal structure of Escherichia coli MurA complexed with UDP-N-acetylglucosamine (UDP-GlcNAc) and fosfomycin has been determined at 1.8 A resolution. The structure consists of two domains with the active site located between them. The domains have a very similar secondary structure, and the overall protein architecture is similar to that of EPSP synthase. The fosfomycin molecule is covalently bound to the cysteine residue Cys115, whereas UDP-GlcNAc makes several hydrogen-bonding interactions with residues from both domains.

CONCLUSIONS

The present structure reveals the mode of binding of the natural substrate UDP-GlcNAc and of the drug fosfomycin, and provides information on the residues involved in catalysis. These results should aid the design of inhibitors which would interfere with enzyme-catalyzed reactions in the early stage of the bacterial cell wall biosynthesis. Furthermore, the crystal structure of MurA provides a model for predicting active-site residues in EPSP synthase that may be involved in catalysis and substrate binding.

Crystal structure of UDP-N-acetylglucosamine enolpyruvyltransferase, the target of the antibiotic fosfomycin

BACKGROUND

The ever increasing number of antibiotic resistant bacteria has fuelled interest in the development of new antibiotics and other antibacterial agents. The major structural element of the bacterial cell wall is the heteropolymer peptidoglycan and the enzymes of peptidoglycan biosynthesis are potential targets for antibacterial agents. One such enzyme is UDP-N-acetylglucosamine enolpyruvyltransferase (EPT) which catalyzes the first committed step in peptidoglycan biosynthesis: the transfer of the enolpyruvyl moiety of phosphoenolpyruvate (PEP) to the 3-hydroxyl of UDP-N-acetylglucosamine (UDPGlcNAc). EPT is of potential pharmaceutical interest because it is inhibited by the broad spectrum antibiotic fosfomycin.

RESULTS

The crystal structure of substrate-free EPT has been determined at 2.0 A resolution. The structure reveals a two-domain protein with an unusual fold (inside out alpha/beta barrel) which is built up from the sixfold repetition of one folding unit. The only repetitive element in the amino acid sequence is a short motif, Leu-X3-Gly(Ala), which is responsible for the formation of hydrogen-bond interactions between the folding units. An enzyme which catalyzes a similar reaction to EPT, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), has a very similar structure despite an amino acid sequence identity of only 25%. To date, only these two enzymes appear to display this characteristic fold.

CONCLUSIONS

The present structure reflects the open conformation of the enzyme which is probably stabilized through two residues, a lysine and an arginine, located in the cleft between the domains. Binding of the negatively charged UDPGlcNAc to these residues could neutralize the repulsive force between the two domains, thereby allowing the movement of a catalytically active cysteine residue towards the cleft.

Evidence that the reaction of the UDP-N-acetylglucosamine 1-carboxyvinyltransferase proceeds through the O-phosphothioketal of pyruvic acid bound to Cys115 of the enzyme

The enzyme UDP-N-acetylglucosamine 1-carboxyvinyltransferase (enolpyruvyltransferase, EC 2.5.1.7) catalyses the transfer of the intact 1-carboxyvinyl moiety of phosphoenolpyruvate to the 3'-hydroxyl group of the glucosamine moiety of UDP-(2')-N-acetylglucosamine with the concomitant release of inorganic phosphate, the first committed step in the biosynthesis of the bacterial cell wall peptidoglycan. Overexpression of the enzyme from Enterobacter cloacae in Escherichia coli allowed the isolation of large amounts of purified enzyme (approx. 900 mg/20 g fresh mass bacteria). By incubating the enzyme with 14C-labelled phosphoenolpyruvate, 32P-labelled orthophosphate and unlabelled UDP-(2')-N-acetyl-(3')-1-carboxyvinylglucosamine, we were able to isolate and characterise a reaction intermediate, covalently bound to the protein. It contains stoichiometric quantities of the C3 moiety (0.98 mol/mol) as well as of the phosphate moiety (0.95 mol/mol) of phosphoenolpyruvate relative to protein. The rapid turnover of this protein-bound intermediate in the presence of UDP-(2')-N-acetylglucosamine towards the product UDP-(2')-N-acetyl-(3')-1-carboxyvinylglucosamine suggests that the intermediate is kinetically competent. We also present evidence that the intermediate is bound as the O-phosphothioketal of pyruvic acid to Cys115 of the enzyme. This is the same Cys residue to which phosphomycin, an irreversible inhibitor of the UDP-GlcNAc carboxyvinyltransferase, binds covalently. Exchange of Cys115 for a Ser residue resulted in an inactive enzyme, demonstrating the essential role of Cys115 for the reaction. The only other enzyme known to catalyse the transfer of the intact 1-carboxyvinyl moiety of phosphoenolpyruvate to a substrate is the 3-phosphoshikimate 1-carboxyvinyltransferase (EC 2.5.1.19), the sixth enzyme of the shikimate pathway. The reaction of this synthase is known to proceed through a single, tightly but not covalently bound, tetrahedral intermediate. Even though the two enzymes share similarities in their primary amino acid sequences, their reaction mechanisms appear to be substantially different.

Formation of an adduct between fosfomycin and glutathione: a new mechanism of antibiotic resistance in bacteria

Plasmid-borne resistance to fosfomycin in bacteria is due to modification of the antibiotic molecule by a glutathione S-transferase that catalyzes the formation of a covalent bond between the sulfhydryl residue of the cysteine in glutathione and the C-1 of fosfomycin. This reaction results in opening of the epoxide ring of the antibiotic to form an inactive adduct, the structure of which was confirmed by nuclear magnetic resonance. Dialyzed extracts prepared from resistant Escherichia coli strains were unable to modify fosfomycin unless exogenous glutathione was added to the reaction mixtures. Similarly, mutants defective in glutathione biosynthesis were susceptible to fosfomycin, despite harboring a resistance plasmid. Extracts of resistant but not susceptible strains could join glutathione to 1-chloro-2,4-dinitrobenzene, confirming the nature of the enzymatic activity. Adduct formation appeared to be specific for glutathione: none of the other thiols tested (cysteine, N-acetylcysteine, and dithiothreitol) could modify fosfomycin.

Pharmacokinetic evaluation of fosmidomycin, a new phosphonic acid antibiotic

The pharmacokinetics of fosmidomycin in phase I with a total of 127 healthy male volunteers is described through single--and repeated--dose studies using oral and parenteral routes of administration. The study results indicate that fosmidomycin is well tolerated even when given in repeated doses of 8 g/day i.v. for 7 days, 4 g/day i.m. for 5 days, and 4 g/day p.o. for 7 days. The gastrointestinal absorption rate after oral dosing of 500 mg is in general about 20-40%, which can be calculated to be on average about 30% (in comparison with fosfomycin which is on average only about 11%). Absorption seems to be slow and moderate. In the single-dose studies, the mean peak serum concentrations were 2.45 micrograms/ml and 12.3 micrograms/ml after 500 mg p.o. and 7.5 mg/kg (ca. 500 mg) i.m. doses respectively. The mean concentration after 15 mg/kg (ca. 2.2 g) i.v. dose was 157 micrograms/ml at 0.25 h. The serum half-lives were 1.65 h, 1.58 h and 1.87 h after i.v., i.m. and p.o. doses respectively. The recovery rate in urine was 85.5%, 66.4% and 26% after i.v., i.m. and p.o. doses respectively. In repeated-dose studies, no serum accumulation could be observed after 1 g q6h for 5 days, 1 g q6h for 7 days or 2 g q6h for 7 days. Mean peak serum levels of 34.0-35.5 micrograms/ml were recorded at steady state. The serum protein binding found in man was less than 1%. Unmetabolized fosmidomycin was the only bioactive substance found in the urine.(ABSTRACT TRUNCATED AT 250 WORDS)

Trometamol-fosfomycin (Monuril) bioavailability and food-drug interaction

A new water-soluble monobasic salt of fosfomycin with trometamol has recently been developed for oral administration. The objective of this study was to evaluate the pharmacokinetics of trometamol-fosfomycin (Monuril) in serum and in urine in 10 healthy volunteers after oral administration of one single dose (50 mg/kg). In the same volunteers the concentrations of fosfomycin were measured before and after food absorption, in serum and urine samples taken at t = 0, 2, 4, 6, 8 and 24 h after the dose (plus 0.5 and 1 h for serum samples). The measurement of fosfomycin levels was carried out by means of a microbiological procedure using Proteus mirabilis ATCC 21100 as test organism. The results indicated at 2 or 4 h serum peak levels ranging from 9 to 28 micrograms/ml, with mean values of about 17-21 micrograms/ml. The urine concentrations reached 2,000-2,500 micrograms/ml at 2 h, with high levels maintained till the 8th hour (1,200-2,750 micrograms/ml) and persistence of noticeable concentrations at 24 h (100-700 micrograms/ml). The influence of food absorption, even variable, decreased significantly the rate of absorption, with lower serum and urine levels, as measured in the same volunteers. Taking into account this factor influencing the bioavailability and the distribution of the drug, the results of the study confirm that a high proportion of the oral dose of trometamine salt of fosfomycin is absorbed; the extremely high urinary recovery of the drug even after food administration could certainly allow short-course therapy or even single-dose therapy in the treatment of uncomplicated urinary tract infection.

Pharmacokinetic profile of fosfomycin trometamol (Monuril)

The kinetics of fosfomycin in serum and in urine was studied in 5 healthy volunteers after intravenous administration of disodium fosfomycin and after oral administration of trometamol fosfomycin at 50 mg/kg. The presence of secondary peaks in serum kinetics, more evident after oral administration, requires the use of a compartmental model with enterohepatic recirculation to which all the data of each subject were simultaneously fitted. The following values of the various pharmacokinetic parameters were calculated: central volume 10.6 +/- 0.92 liters; bioavailability 0.58 +/- 0.04; delay time in the recirculation 2.00 +/- 0.92 h; half-life 2.43 +/- 0.31 h; total clearance 8.3 +/- 1.6 l/h; urinary clearance 7.0 +/- 0.9 l/h; peak level 32.1 +/- 3.0 micrograms/ml; time of the peak 2.2 +/- 0.44 h. The fosfomycin concentration in urine remains above 1,000 micrograms/ml for 12 h and above 100 micrograms/ml for 48 h. The oral kinetics of fosfomycin is dose-dependent, as shown by serum and urine kinetics in other 4 volunteers after oral administration of 2, 3, 4 and 5 g; the fraction of the dose excreted by urine goes from 50% for 2 g to 22% for 5 g.

[Transfer of fosfomycin into human burn blister fluid and its pharmacokinetic analysis]

Fosfomycin (FOM) (50 mg/kg) was administered to burned patients by intravenous bolus injection. Burn blister fluid and serum were taken during 8 hours after injection, and concentrations of FOM in burn blister fluid and serum were determined by bioassay using Proteus sp. (MB-838) as the test organism. The serum concentrations of FOM were 257 +/- 34.6 micrograms/ml at 15 minutes, 222 +/- 34.8 micrograms/ml at 30 minutes, 166 +/- 34.6 micrograms/ml at 1 hour, 114 +/- 43.9 micrograms/ml at 2 hours, 79.5 +/- 34.9 micrograms/ml at 3 hours, 63 +/- 36.4 micrograms/ml at 4 hours, 44.3 +/- 27.6 micrograms/ml at 5 hours, 29.6 +/- 20.9 micrograms/ml at 6 hours and 17.9 +/- 12.8 micrograms/ml at 8 hours after the injection. FOM concentrations in burn blister fluid were 64.4 +/- 18.1 micrograms/ml at 30 minutes, 77 +/- 26.0 micrograms/ml at 1 hour, 71.6 +/- 24.7 micrograms/ml at 2 hours, 64.8 +/- 23.6 micrograms/ml at 3 hours, 43.2 +/- 8.8 micrograms/ml at 4 hours, 24.8 +/- 7.9 micrograms/ml at 6 hours and 17.9 +/- 10.5 micrograms/ml at 8 hours after the injection. The obtained data were analysed pharmacokinetically. The serum levels were analysed by a two-compartment model, and the transfer of FOM into burn blister was analysed by a modified deconvolution method. In results, Tmax and Cmax of FOM levels in burn blister fluid were calculated as 1.3 hours and 80.9 micrograms/ml, respectively. The transfer rate constant of FOM from serum to burn blister fluid (K1) and that from burn blister fluid to serum (K2) were calculated as 0.612 hr-1 and 1.10 hr-1, respectively.

[The transport mechanism of antibiotics using microvillous membrane vesicles (placental transport of fosfomycin)]

Using the rapid filtration technique, the uptake of fosfomycin into microvillous membrane vesicles isolated from human term placental trophoblast was investigated. The microvillous membrane vesicles exhibited the uptake of fosfomycin into an osmotically reactive intravesicular space and it was indicated that the uptake of fosfomycin by microvillous membrane vesicles represented transport into membrane vesicles. The uptake of fosfomycin by microvillous membrane vesicles was not dependent on the Na+ electrochemical gradient or membrane potential. The initial uptake of fosfomycin by microvillous membrane vesicles did not exhibit saturation kinetics with respect to fosfomycin concentration, and increased linearly as the fosfomycin concentration increased. These results indicated that fosfomycin was transported across the microvillous membrane by simple diffusion. L-alanine, L-valine, L-lysine, inorganic phosphate or D-glucose did not inhibit the uptake of fosfomycin into microvillous membrane vesicles. On the other hand, fosfomycin did not inhibit the uptake of L-alanine, L-valine, L-lysine inorganic phosphate or D-glucose into microvillous membrane vesicles. These results revealed that fosfomycin did not affect the placental transport activity of other nutrients.

Linearity of the pharmacokinetics of phosphomycin in serum and interstitial tissue fluid in rabbits

A study was carried out on the access and residence of (-)(1R,2S)-1,2-epoxypropylphosphonic acid (phosphomycin) in interstitial tissue fluid produced experimentally by subcutaneous implantation of spiral steel cages after administration of doses of 20, 30 and 60 mg/kg of the antibiotic to rabbits. The levels reached by the drug in serum and interstitial tissue fluid (ITF) were determined by a microbiological plate diffusion method. The elimination half-lives of phosphomycin ranged between 1.16 and 1.57 h. These values are similar to those found for the disappearance half-lives from ITF. Phosphomycin reached maximum concentrations in ITF between 31.95 and 80.37 micrograms/ml. Linear relationships were established between the (AUC) 0 infinity in serum, the (AUC) 0 infinity in ITF and Cmax in ITF and the doses studied, revealing the non-dose-dependent kinetics of phosphomycin. The linearity of phosphomycin kinetics was checked in serum and ITF by applying the superposition principle.

[Studies of the penetrating ability of fosfomycin into the aqueous humor and vitreous body of the eye]

Fosfomycin is an antibiotic with a new simple chemical structure and a broad spectrum of antibacterial activity, especially against staphylococci. In the study reported here, the penetration of fosfomycin into the aqueous humor and vitreous body was investigated after intravenous infusion of 4 and 8 g. The highest average concentration approximately 2 hours after administration was 28.3 micrograms/ml after the 4 g dose and 52 micrograms/ml after the 8 g dose. Both the 4 g and 8 g doses thus produced aqueous humor levels exceeding the minimum inhibitory concentrations required for most gram-positive and gram-negative organisms commonly responsible for bacterial endophthalmitis, including strains of pseudomonas aeruginosa. Extremely high aqueous humor levels of fosfomycin were observed in two patients with inflamed eyes because of a change in the blood-aqueous barrier.

[Diffusion of fosfomycin into the human and rabbit eye (aqueous humor and vitreous body)]

The intraocular distribution of fosfomycin was studied in 32 patients undergoing cataract surgery and in 8 rabbits after experimental infection of one eye by Staphylococcus aureus. Concentrations found 1 to 6 hours after termination of a 4 g fosfomycin infusion ranged from 14 to 18.8 mg/l in the aqueous humor and 8 to 12.5 mg/l in the vitreous body. These levels are higher than the MICs for 80 to 90% of the bacteria responsible for endophthalmitis. In each rabbit, the fosfomycin concentration in the infected eye as compared to the healthy eye was increased 2.5 to 5--fold for the aqueous humor and 4.9 to 19.2--fold for the vitreous body. Fosfomycin, in association with a third generation cephalosporin (ceftriaxone) or one of the new quinolones (pefloxacin) can be recommended for the prevention or early treatment of endophthalmitis.

[Effect of fosfomycin on auditory organs and its transfer to cochlear lymph following application of its solution into a middle ear cavity]

Male guinea pigs were given 0.1 ml of 2, 3 or 5% fosfomycin (FOM) ototopical solution once a day for 5 days into a middle ear cavity through artificially perforated ear drum. Kanamycin A (KM) was used at 2% ototopical solution as control drug. Four animals of each group were sacrificed under pentobarbital anesthesia to isolate the cochlea 10 days after the final application. The cochlea was washed with 0.1 M phosphate buffer (pH 7.0), followed by fixing with 2.5% glutaraldehyde and 1% osmic acid. Cochlear specimens were prepared by standard method for scanning electron microscopic observation. The scanning electron microscopic observations revealed some damages in outer and inner hair cells, such as partial deformation or loss of auditory hair in hair cells, but these damages were not correlated to drug treatments. In order to determine the transfer of FOM and KM from middle ear cavity to cochlear lymph in male guinea pigs, the cochlear lymph was collected 0.5, 1, 2 and 4 hours after an application of 0.1 ml of 3 or 5% FOM and 2% KM ototopical solution into a middle ear cavity, followed by estimating content of these antibiotics in the lymph. The results showed the peak concentration in lymph at 1 to 2 hours after an application of 3% FOM was lower than that after 2% KM, but the AUC of 3% FOM was higher than that of 2% KM. The AUC value of FOM was dependent on the applied concentration of FOM. The value of half-life time was about 4.8 hours at FOM and about 2.3 hours at KM.(ABSTRACT TRUNCATED AT 250 WORDS)

Fosfomycin causes transient lysis in Escherichia coli strains carrying fosfomycin-resistance plasmids

Escherichia coli cells carrying fosfomycin-resistance plasmids show high levels of resistance towards this drug. However, the plasmid-carrying strains exhibited a transient lytic phase induced by fosfomycin when grown in rich liquid media. This lytic phase was not observed if the cells were grown in liquid minimal media. Fosfomycin-induced lysis depended on the accumulation of drug inside the bacteria, presumably as a result of the saturation of the fosfomycin modification system. Growth recovery after lysis was not due to drug inactivation in the culture medium and could be explained by selection of mutants showing impaired fosfomycin transport when high concentrations of fosfomycin were used. However, there was no selection of mutants with low drug concentrations.