Studies on phosphonic acid antibiotics. IV. Synthesis and antibacterial activity of analogs of 3-(N-acetyl-N-hydroxyamino)-propylphosphonic acid (FR-900098)

Over 40 Years of Fosmidomycin Drug Research: A Comprehensive Review and Future Opportunities

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.

Fosmidomycin biosynthesis diverges from related phosphonate natural products

Fosmidomycin and related molecules comprise a family of phosphonate natural products with potent antibacterial, antimalarial and herbicidal activities. To understand the biosynthesis of these compounds, we characterized the fosmidomycin producer, Streptomyces lavendulae, using biochemical and genetic approaches. Surprisingly, we were unable to elicit production of fosmidomycin, instead observing the unsaturated derivative dehydrofosmidomycin, which we showed potently inhibits 1-deoxy-D-xylulose 5-phosphate reductoisomerase and has bioactivity against a number of bacteria. The genes required for dehydrofosmidomycin biosynthesis were established by heterologous expression experiments. Bioinformatics analyses, characterization of intermediates, and in vitro biochemistry show that the biosynthetic pathway involves conversion of a two-carbon phosphonate precursor into the unsaturated three-carbon product via a highly unusual rearrangement reaction, catalyzed by the 2-oxoglutarate dependent dioxygenase DfmD. The required genes and biosynthetic pathway for dehydrofosmidomycin differ substantially from that of the related natural product FR-900098, suggesting that the ability to produce these bioactive molecules arose via convergent evolution.

MEPicides: α,β-Unsaturated Fosmidomycin Analogues as DXR Inhibitors against Malaria

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.

Stereocontrolled C(sp3)–P bond formation with non-activated alkyl halides and tosylates

The current work provides a general approach for making C–P bonds with stereocontrol from chiral secondary alcohols.

The effect of chain length and unsaturation on Mtb Dxr inhibition and antitubercular killing activity of FR900098 analogs

Inhibition of the nonmevalonate pathway (NMP) of isoprene biosynthesis has been examined as a source of new antibiotics with novel mechanisms of action. Dxr is the best studied of the NMP enzymes and several reports have described potent Dxr inhibitors. Many of these compounds are structurally related to natural products fosmidomycin and FR900098, each bearing retrohydroxamate and phosphonate groups. We synthesized a series of compounds with two to five methylene units separating these groups to examine what linker length was optimal and tested for inhibition against Mtb Dxr. We synthesized ethyl and pivaloyl esters of these compounds to increase lipophilicity and improve inhibition of Mtb growth. Our results show that propyl or propenyl linker chains are optimal. Propenyl analog 22 has an IC50 of 1.07 μM against Mtb Dxr. The pivaloyl ester of 22, compound 26, has an MIC of 9.4 μg/mL, representing a significant improvement in antitubercular potency in this class of compounds.

Alpha-heteroatom derivatized analogues of 3-(acetylhydroxyamino)propyl phosphonic acid (FR900098) as antimalarials

To explore the hitherto successful derivatization of the α-carbon of fosmidomycin, a series of new α-substituted analogues was prepared. This was done by introduction of a heteroatom (N or O) in α-position to the phosphonate and using the resultant OH and NH₂ groups as a handle for appending a variety of substituents by means of several functional groups such as ether, amide, urea, and 1,4-triazole. The synthesized molecules, as a racemic mixture, were assayed for their EcDXR inhibitory potency. Both the α-azido-analogue and the α-hydroxylated analogue proved most promising, and docking experiments were performed. Although several compounds showed high potency when assayed against Plasmodium falciparum K1 in human erythrocytes, a clear correlation between the enzyme inhibition constants and P. falciparum inhibition concentrations could not be found.

Deciphering the late biosynthetic steps of antimalarial compound FR-900098

FR-900098 is a potent chemotherapeutic agent for the treatment of malaria. Here we report the heterologous production of this compound in Escherichia coli by reconstructing the entire biosynthetic pathway using a three-plasmid system. Based on this system, whole-cell feeding assays in combination with in vitro enzymatic activity assays reveal an unusual functional role of nucleotide conjugation and lead to the complete elucidation of the previously unassigned late biosynthetic steps. These studies also suggest a biosynthetic route to a second phosphonate antibiotic, FR-33289. A thorough understanding of the FR-900098 biosynthetic pathway now opens possibilities for metabolic engineering in E. coli to increase production of the antimalarial antibiotic and combinatorial biosynthesis to generate novel derivatives of FR-900098.

Cloning, expression, and biochemical characterization of Streptomyces rubellomurinus genes required for biosynthesis of antimalarial compound FR900098

The antibiotics fosmidomycin and FR900098 are members of a unique class of phosphonic acid natural products that inhibit the nonmevalonate pathway for isoprenoid biosynthesis. Both are potent antibacterial and antimalarial compounds, but despite their efficacy, little is known regarding their biosynthesis. Here we report the identification of the Streptomyces rubellomurinus genes required for the biosynthesis of FR900098. Expression of these genes in Streptomyces lividans results in production of FR900098, demonstrating their role in synthesis of the antibiotic. Analysis of the putative gene products suggests that FR900098 is synthesized by metabolic reactions analogous to portions of the tricarboxylic acid cycle. These data greatly expand our knowledge of phosphonate biosynthesis and enable efforts to overproduce this highly useful therapeutic agent.

Alkylation of H-phosphinate esters under basic conditions

An efficient and general procedure was developed for the direct alkylation of H-phosphinate esters with LHMDS at low temperature. The simplicity of the reaction allows the use of various H-phosphinate esters and takes place with a wide range of electrophiles. The approach can be employed to access some GABA analogues or precursors to GABA analogues. The isolated yields are moderate to good. This is the first report of an alkylation with a secondary iodide or a primary chloride.

Evaluation of fosmidomycin analogs as inhibitors of the Synechocystis sp. PCC6803 1-deoxy-d-xylulose 5-phosphate reductoisomerase

Analogs of the antibiotic fosmidomycin, an inhibitor of the methylerythritol phosphate pathway to isoprenoids, were synthesized and evaluated against the recombinant Synechocystis sp. PCC6803 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR). Fosfoxacin, the phosphate analog of fosmidomycin, and its acetyl congener were found to be more potent inhibitors of DXR than fosmidomycin.

Synthesis of 1-Hydroxypyrrolidin-2,5-dione Derivatives of the Phosphonic - Hydroxamic Acid Antibiotic SF-2312

Analogues of the antibiotic SF-2312 with a 3-substituted 1-hydroxypyrrolidine moiety have been prepared from 1-benzyloxy-3-bromopyrrolidin-2,5-dione (1) through Michaelis–Arbusov reaction with various trialkyl phosphites and subsequent alkylation of ring position 3. Cleavage of phosphonic esters 3 by TMSBr afforded the corresponding phosphonic acids 7, which were treated with ethanolamine to give the monosalts 8. Finally, hydrogenolysis furnished 1-hydroxypyrrolidin-2,5-diones 9.

Fosmidomycin analogues as inhibitors of monoterpenoid indole alkaloid production in Catharanthus roseus cells

Substituted 3-[2-(diethoxyphosphoryl)propyl]oxazolo[4,5-b]pyridine-2(3H)-ones were obtained by functionalization at 6-position with various substituents (aryl, vinyl, carbonyl chains) via reactions catalysed with palladium. We found that these new fosmidomycin analogues inhibited the accumulation of ajmalicine, a marker of monoterpenoid indole alkaloids production in plant cells. Some of them have greater inhibitory effect than fosmidomycin and fully inhibit alkaloid accumulation at the concentration of 100 microM.

Asymmetric hydrogenation of alpha,beta-unsaturated phosphonates with Rh-BisP* and Rh-MiniPHOS catalysts: scope and mechanism of the reaction

Optically active 1,2-bis(alkylmethylphosphino)ethanes and bis(alkylmethylphosphino)methanes are unique diphosphine ligands combining the simple molecular structure and P-stereogenic asymmetric environment. This work shows that these ligands exhibit excellent enantioselectivity in rhodium-catalyzed asymmetric hydrogenation of alpha,beta-unsaturated phosphonic acid derivatives. The enantioselective hydrogenation mechanism elucidated by NMR study is also described.

Escherichia coli produces phosphoantigens activating human gamma delta T cells

Human Vgamma9delta2 T lymphocytes are suggested to play an important role in the immune response to various microbial pathogens. In contrast to alphabeta T cells, gammadelta T lymphocytes recognize small, non-protein, phosphate-bearing antigens (phosphoantigens) in a major histocompatibility complex-independent manner. Four different phosphoantigens termed TUBag1 to TUBag4 with a common 3-formyl-1-butyl-pyrophosphate moiety and isopentenyl-pyrophosphate have been isolated and identified from mycobacteria. However, natural occurring gammadelta T cell ligands from other bacterial species were not characterized so far. Here, we describe the structural identification of the two compounds responsible for the gammadelta T cell-stimulating capacity of Escherichia coli as similar to the mycobacterial phosphoantigens 3-formyl-1-butyl-pyrophosphate and its M(r) 275 homologue TUBag2. In addition, E. coli phosphoantigens exert bioactivities on gammadelta T cells with similar potencies to the mycobacterial phosphoantigens at 5-15 nm concentration. Furthermore, our results clearly prove that the deoxyxylulose 5-phophate pathway (also referred to as Rohmer metabolic route of isoprenoid biosynthesis) is essential for the biosynthesis of the phosphoantigens in E. coli. Because this pathway is absent from human cells, it proves an ideal target for focusing efficiently the antimicrobial selectivity of human gammadelta T lymphocytes.

Structural determinants for ligand binding and catalysis of triosephosphate isomerase

The crystal structure of leishmania triosephosphate isomerase (TIM) complexed with 2-(N-formyl-N-hydroxy)-aminoethyl phosphonate (IPP) highlights the importance of Asn11 for binding and catalysis. IPP is an analogue of the substrate D-glyceraldehyde-3-phosphate, and it is observed to bind with its aldehyde oxygen in an oxyanion hole formed by ND2 of Asn11 and NE2 of His95. Comparison of the mode of binding of IPP and the transition state analogue phosphoglycolohydroxamate (PGH) suggests that the Glu167 side chain, as well as the triose part of the substrate, adopt different conformations as the catalysed reaction proceeds. Comparison of the TIM-IPP and the TIM-PGH structures with other liganded and unliganded structures also highlights the conformational flexibility of the ligand and the active site, as well as the conserved mode of ligand binding.