Fosfomycin

Drug Name

Fosfomycin is a bactericidal antibiotic produced by strains of Streptomyces (Jia et al., 2017), such as S. fradiae, S. viridochromogenes, and S. wedmorensis (Hendlin et al., 1969). It is a broad spectrum antibiotic typically used to treat uncomplicated urinary tract infections caused by Escherichia coli (E. coli) and Enterococcus faecalis (E. faecalis).

The antibacterial drug, fosfomycin tromethamine, is a phosphonic acid derivative of fosfomycin and is not chemically related to any other antibacterial agent (Michalopoulos et al., 2011); therefore, it does not belong to a specific drug class. The drug is available as a tromethamine or calcium salt to be taken orally; however, fosfomycin tromethamine is preferred over the calcium salt of fosfomycin, because of its superior oral bioavailability (Frimodt-Møller, 2010).

Table 1. Basic profile of fosfomycin.

Drug Information

Table 2. Chemical and physical properties (DrugBank).

Drug Target

Fosfomycin is an oral, broad-spectrum antibacterial drug that inhibits the enzyme UDP-N-acetylglucosamine enolpyruvyl transferase (MurA). MurA is responsible for the first cytoplasmic step of peptidoglycan biosynthesis and initiates production of the peptidoglycan precursor. The enzyme, as the name suggests, catalyzes transfer of enolpyruvate from phosphoenolpyruvate (PEP) to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc), which yields enolpyruvyl UDP-GlcNAc (Hrast et al., 2014). This reaction is required to initiate formation of the nucleotide muramyl peptides which will serve as precursors to the cell-wall building blocks (Hendlin et al., 1969). Click here to learn more about MurA.

In E. coli, fosfomycin reaches its intracellular target via two uptake systems: the hexose-6-phosphate transport system (because fosfomycin is a molecular mimic of glucose-6-phosphate, or G6P) and the L-alpha-glycerophosphate transport system (because fosfomycin is also a molecular mimic of glycerol-3-phosphate, or G3P), as illustrated in Figure 2 (Frimodt-Møller, 2010).

After entering the cell, fosfomycin is recognized by MurA and binds to the enzyme active site, where formation of a covalent adduct occurs in the drug-target complex. Irreversible inhibition of MurA by fosfomycin blocks bacterial cell wall biosynthesis causing cell lysis and death (Dijkmans et al., 2017).

Drug-Target Comples

The target of fosfomycin, MurA, is a 419 amino acid protein, consisting of two closely approximated globular domains (Skarzynski et al., 1996, Figure 3):

• Catalytic domain (residues 22-229, blue)
• C-terminal domain (residues 1-21 & 230-419, green)

The active site is located between the two domains (Skarzynski et al., 1996), where UDP-GlcNAc (gray stick figure) and fosfomycin (white ball-and-stick figure) bind (Figure 3).

In the absence of fosfomycin, MurA is thought to catalyze transfer of enolpyruvate from PEP to UDP-GlcNAc, yielding enolpyruvyl UDP-GlcNAc in a multi-step process. First, UDP-GlcNAc binds to MurA, inducing a significant conformational change in the structure of the protein. Thereafter, PEP binds to a second subsite within the enzyme active site, allowing the transfer reaction to proceed.

Fosfomycin, a PEP analogue, can also bind to the PEP-binding site of MurA (presumably after binding of UDP-GlcNAc) and functions by alkylating the catalytic cysteine residue (Cys115 in E.coli and E. cloacae) (Bensen et al., 2012). When the antibiotic binds, the enzyme catalyzes a single turnover reaction resulting in formation of a covalent adduct with Cys115. This covalent modification step irreversibly inactivates the enzyme (Hendlin et al., 1969), by preventing binding of PEP. The covalent interaction between fosfomycin and the catalytic residue is shown in Figure 3.

As illustrated in Figure 3, fosfomycin is tightly packed between UDP-GlcNAc and the enzyme (Skarzynski et al., 1996). Three active-site residues (Lys22, Arg120, and Arg397) surround the phosphonate group of fosfomycin. These positively-charged residues both neutralize and make a total of five hydrogen bonds with the phosphonate group (Figure 4a). Similar interactions enable recognition of the phosphate group of the substrate PEP, as shown in Figure 4b (Zhu et al., 2012).

A comparison of the co-crystal structures of MurA with fosfomycin (Figure 4a) and of MurA with PEP (Figure 4b) shows that fosfomycin acts by occluding the PEP binding site and preventing substrate binding (Skarzynski et al., 1996).

It was found that fosfomycin is able to carry out time-dependent inactivation of MurA more rapidly in the presence of UDP-GlcNAc (Zoeiby et al., 2003). This suggests that the conformational changes that UDP-GlcNAc binding induces in the active site are essential for inhibition of MurA by fosfomycin (Zoeiby et al., 2003).

Pharmacologic Properties and Safety

Drug Interactions and Side Effects

Table 4. Drug interactions and side effects of fosfomycin.

Regulatory Approvals/Commercial

The US FDA approved monurol (fosfomycin), as an oral antibacterial drug, in 1996. The drug is largely used for the treatment of uncomplicated urinary tract infections (acute cystitis) (Sastry et al., 2015).

Monurol is available as a single three-gram oral dose contained within a sachet. The contents of the sachet (white granules) must be dissolved in water before being consumed. Only one three-gram dose of monurol should be used to treat a single occurrence of acute cystitis. The cost of a single-dose sachet is about US $50.

In clinical trials, the most frequently reported side effects (>1%) in the study population included diarrhea, headache, vaginitis (vaginal inflammation), nausea, rhinitis (stuffy nose), and back pain. Other rarer side effects (<1%) included abnormal stools, anorexia, constipation, dry mouth, ear disorder, migraine, and vomiting. One patient developed unilateral optic neuritis, the occurrence of which is possibly related to treatment by monurol (FDA).

Off-Target Considerations

The US FDA approved antibacterial drug fosfomycin not only inhibits the bacterial enzyme MurA but also plays a role in the modulation of the host immune system (Falagas et al., 2016). Fosfomycin enhances the ability of neutrophils to phagocytically kill invading pathogens, and it was seen that neutrophils have an enhanced bactericidal ability in the presence of fosfomycin compared to other antimicrobial agents (Falagas et al., 2016).

Links

Table 5. Links

References

Baum, E. Z., Montenegro, D. A., Licata, L., Turchi, I., Webb, G. C., Foleno, B. D., and Bush, K. (2001) Identification and Characterization of New Inhibitors of the Escherichia coli MurA Enzyme. Antimicrobial Agents and Chemotherapy 45, 3182-3188. doi:10.1128/aac.45.11.3182-3188.2001

Bensen, D. C., Rodriguez, S., Nix, J., Cunningham, M. L., and Tari, L. W. (2012) Structure of MurA (UDP-N-acetylglucosamine enolpyruvyl transferase) from Vibrio fischeri in complex with substrate UDP-N-acetylglucosamine and the drug fosfomycin. Acta Crystallographica Section F: Structural Biology and Crystallization Communications 68, 382–385. doi:10.1107/S1744309112006720

Dijkmans, A. C., Zacarías, N. V., Burggraaf, J., Mouton, J. W., Wilms, E., Nieuwkoop, C. V., Touw, D. J., Stevens, J., and Kamerling, I. M. (2017) Fosfomycin: Pharmacological, Clinical and Future Perspectives. Antibiotics 6, 24. doi:10.3390/antibiotics6040024

Falagas, M. E., Vouloumanou, E. K., Samonis, G., and Vardakas, K. Z. (2016) Fosfomycin. Clinical Microbiology Reviews 29, 321-347. doi:10.1128/cmr.00068-15

Fosfomycin - DrugBank. Drugbank.ca. https://www.drugbank.ca/drugs/DB00828

Fosfomycin. Drugs.com. https://www.drugs.com/mtm/fosfomycin.html

Fosfomycin. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/fosfomycin#section=Top

Frimodt-Møller, N. (2010) Fosfomycin. Kucers' The Use of Antibiotics Sixth Edition 935-944. doi:10.1201/b13787-167

Han, H., Yang, Y., Olesen, S. H., Becker, A., Betzi, S., and Schönbrunn, E. (2010) The Fungal Product Terreic Acid Is a Covalent Inhibitor of the Bacterial Cell Wall Biosynthetic Enzyme UDP-N-Acetylglucosamine 1-Carboxyvinyltransferase (MurA). Biochemistry 49, 4276-4282. doi:10.1021/bi100365b PDB ID: 3kr6

Hendlin, D., Stapley, E. O., Jackson, M., Wallick, H., Miller, A. K., Wolf, F. J., Miller, T. W., Chaiet, L., Kahan, F. M., Foltz, E. L., Woodruff, H. B., Mata, J. M., Hernandez, S., and Mochales, S. (1969) Phosphonomycin, a New Antibiotic Produced by Strains of Streptomyces. Science 166, 122-123. doi:10.1126/science.166.3901.122

Hrast, M., Sosič, I., Šink, R., and Gobec, S. (2014) Inhibitors of the peptidoglycan biosynthesis enzymes MurA-F. Bioorganic Chemistry 55, 2-15. doi:10.1016/j.bioorg.2014.03.008

Jia, B., Raphenya, A. R., Alcock, B., Waglechner, N., Guo, P., Tsang, K. K., Lago, B. A., Dave, B. M., Pereira, S., Sharma, A. N., Doshi, S., Courtot, M., Lo, R., Williams, L. E., Frye, J. G., Elsayegh, T., Sardar, D. Westman, E. L., Pawlowski, A. C., Johnson, T. A., Brinkman, F. S., Wright, G. D., and McArthur, A. G. (2017) CARD 2017: expansion and model-centric curation of the Comprehensive Antibiotic Resistance Database. Nucleic Acids Research 45, D566-573. doi: 10.1093/nar/gkw1004

LiverTox - Clinical and Research Information on Drug-Induced Liver Injury. National Institutes of Health. https://livertox.nlm.nih.gov/Fosfomycin.htm

Michalopoulos, A. S., Livaditis, I. G., and Gougoutas, V. (2011) The revival of fosfomycin. International Journal of Infectious Diseases 15, e732-e739. doi:10.1016/j.ijid.2011.07.007
Monurol. Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2008/050717s005lbl.pdf

Raz, R. (2012) Fosfomycin: An old—new antibiotic. Clinical Microbiology and Infection 18 , 4-7. doi:10.1111/j.1469-0691.2011.03636.x

Sastry, S., Clarke, L. G., Alrowais, H., Querry, A. M., Shutt, K. A., and Doi, Y. (2015) Clinical Appraisal of Fosfomycin in the Era of Antimicrobial Resistance. Antimicrobial Agents and Chemotherapy 59, 7355-7361. doi:10.1128/aac.01071-15

Silver, L. L. (2017) Fosfomycin: Mechanism and Resistance. Cold Spring Harbor Perspectives in Medicine 7. doi:10.1101/cshperspect.a025262

Skarzynski, T., Mistry, A., Wonacott, A., Hutchinson, S. E., Kelly, V. A., and Duncan, K. (1996) 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. Structure 4, 1465-1474. doi:10.1016/s0969-2126(96)00153-0

Zhu, J., Yang, Y., Han, H., Betzi, S., Olesen, S. H., Marsilio, F., and Schönbrunn, E. (2012) Functional Consequence of Covalent Reaction of Phosphoenolpyruvate with UDP-N-acetylglucosamine 1-Carboxyvinyltransferase (MurA). Journal of Biological Chemistry 287, 12657-12667. doi:10.1074/jbc.m112.342725 PDB ID: 3swd