Glycopeptide Antibiotics

Discovery

The rise of antibiotic-resistant Staphylococcus aureus in the 1950s led to the hospitalization of many patients (Griffith, 1984). In 1953, Dr. E.C Kornfield, a scientist from Eli Lilly, isolated vancomycin from soil samples obtained from the jungles of Borneo. Derived from the bacteria Streptomyces orientalis, this molecule had the ability to kill Staphylococci (Griffith, 1984). Since this molecule was relatively safe in patients with staphylococcal infections and "vanquished" the infections, it was named vancomycin and received a "fast track approval" by the US FDA in 1958! (Rubinstein and Keynan 2014.) The vancomycin initially obtained by fermentation had many impurities. It was brown in color and was called by the nickname “Mississippi Mud” (Rubinstein and Keynan 2014.) The impurities also had several toxic effects such as fevers, low blood pressure, various types of inflammation (including a rash in the face, neck, and upper torso, referred to as the "red person syndrome"), and kidney toxicity (Elting et al., 1998). Over time, the impurities were reduced/removed and several other related molecules (called glycopeptide antibiotics) were developed.

Another naturally occurring glycopeptide antibiotic, teicoplanin, was identified from Actinoplanes teichomyceticus in 1978 (Butler et al., 2014). It is a mixture of several related compounds that share the same glycopeptide core but have slightly different chemical groups attached to the core (DrugBank). Although Teicoplanin was approved for clinical use in Europe in 1998, it has never been approved for use in the United States (Butler et al., 2014). Other members of the glycopeptides antibiotic class are discussed below. Vancomycin still remains an effective agent against gram-positive bacteria and as a preventative antibiotic in cancer patients (Elting et al., 1998).

Overview of Chemistry

Glycopeptide antibiotics have a heptapeptide core. Amino acids in the heptapeptide are labeled 1-7 in magenta (Figure 1). The side chains of the seven amino acids in the peptide core are extensively crosslinked to form a tricyclic (or tetracyclic) structure that is glycosylated. Sometimes the molecule may have additional lipophilic fatty acid groups (e.g., in teicoplanin). The peptide cores of glycoproteins are synthesized non-ribosomally, by sets of gene clusters - e.g., bal, cep, dbv, sta and tcp gene clusters (Donadio et al., 2005). The heptapeptide backbone of the glycopeptide antibiotics binds to the C-terminal L-Lys-D-Ala-D-Ala subunit of the peptidoglycan precursor Lipid II

Figure 1. The chemical structure of Vancomycin (PRD_000204) is shown with its components marked. The peptide part is composed of 7 amino acids, and forms 3 cyclic structures. Amino acids 1 and 3 are circled with dashed lines. The glycans are attached to the peptide core. The figure was made using ChemAxon.

Types

Glycopeptide antibiotics are classified depending on the chemical characteristics of the amino acids at positions 1 and 3, as well as the linkages between amino-acid side chains (Butler et al., 2014). There are 4 distinct subclasses of glycopeptide antibiotics (Lancini 1989):
* Type I - the side chains of the amino acids at positions 1 and 3 of the heptapeptide (see Figure 1) are not aromatic. They may be valine at position 1 and either asparagine or glutamine at position 3. e.g., vancomycin contains an asparagine at position 3
* Type II - the residues at positions 1 and 3 of the heptapeptide are aromatic but not linked together - e.g., β-avoparcin
* Type III - the residues at positions 1 and 3 of the heptapeptide are aromatic and they are linked via an ether linkage - e.g., ristocetin A
* Type IV - the residues at positions 1 and 3 of the heptapeptide are aromatic and they are linked. In addition to the glycan in the molecule, a fatty acid component is attached to an amino sugar (e.g., teicoplanin A2)

Beyond the naturally produced glycopeptides, a second generation of semisynthetic lipoglycopeptide derivatives such as telavancin, dalbavancin, and oritavancin have been created to possess broader spectra of activity and improved pharmacokinetic properties. These molecules were engineered by adding additional sugars and lipophilic groups to the vancomycin core structure (Blaskovich et al., 2018).

Resistance

Glycopeptide antibiotics such as vancomycin, do not directly target an enzyme in the cell wall biosynthesis. Instead, they bind to the C-terminal L-Lys-D-Ala-D-Ala of the peptidoglycan precursor, preventing the trans-peptidation steps that strengthen the cell wall (Kahne et al., 2005).

While resistant strains rapidly arise following the use of most β-lactams and other antibiotics, vancomycin resistance didn’t appear until ~30 years after its clinical introduction (Blaskovich et al., 2018). The first vancomycin-resistant strain of Enterococcus (VRE) appeared in Europe in 1986, and 3 years later the first case of VRE reached the U.S. (NIAID, 2019). The slow emergence of resistance is traceable to the conserved nature of vancomycin’s target, along with the fact that this target is not encoded in the genome. Therefore resistance is unlikely to emerge as the result of random mutations in the genome. However, once resistance became established in enterococcal populations, it spread rapidly—in only four years, the percentage of VRE-positive enterococcal tests in the US rose from 0.3 to 7.9% (NIAID, 2019).

Enterococcal infections can be fatal, and the threat posed by VRE is significant. However, a potentially greater threat is the spread of resistance genes to other virulent microbes such as Staphylococcus aureus. In 2002, the first strain of Staphylococcus aureus which is highly resistant to vancomycin was identified in the US (Gardete and Tomasz, 2014). Critically, three isolates of methicillin-resistant Staphylococcus aureus (MRSA) were found to exhibit moderate to high levels of glycopeptide resistance after acquiring a plasmid-borne set of vancomycin resistance genes (Courvalin, 2006). Given that vancomycin is the drug of choice to treat MRSA infections, vancomycin-resistant MRSA infections leave healthcare providers with few resources for treatment. Fortunately, high levels of vancomycin resistance remain rare in MRSA; however, the incidence of intermediate levels of resistance is increasing and constitutes a serious public health concern.

The main source of vancomycin resistance comes from the horizontal gene transfer of van gene clusters (or operons). A large number of different van operons have been identified in Enterococci, including types A, B, C, D, E, G, L, M, and N (Guffey and Loll 2021). Of these, types A and B (corresponding to the vanA and vanB operons) are responsible for most VRE infections in humans.

Learn more about the van operons and other mechanisms of vancomycin resistance.

References

Blaskovich, M. A. T., Hansford, K. A., Butler, M. S., Jia, Z., Mark, A. E., Cooper, M. A. (2018) Developments in Glycopeptide Antibiotics. ACS Infect Dis. 2018 May 11;4(5):715-735. https://doi.org/10.1021/acsinfecdis.7b00258

Butler, M. S., Hansford, K. A., Blaskovich, M. A., Halai, R., Cooper, M. A. (2014) Glycopeptide antibiotics: back to the future. J Antibiot (Tokyo). 67(9):631-44. https://doi.org/10.1038/ja.2014.111

Courvalin, P. (2006) Vancomycin Resistance in Gram-Positive Cocci. Clinical Infectious Diseases 42, S25-S34. https://doi.org/10.1086/491711

Donadio, S., Sosio, M., Stegmann, E., Weber, T., Wohlleben, W. (2005) Comparative analysis and insights into the evolution of gene clusters for glycopeptide antibiotic biosynthesis. Mol Genet Genomics. 274(1):40-50. https://doi.org/10.1007/s00438-005-1156-3

Elting, L. S., Rubenstein, E. B., Kurtin, D., Rolston, K. V., Fangtang, J., Martin, C. G., Raad, I. I., Whimbey, E. E., Manzullo, E., Bodey, G. P. (1998) Mississippi mud in the 1990s: risks and outcomes of vancomycin-associated toxicity in general oncology practice. Cancer. 83(12):2597-607. https://doi.org/10.1002/(sici)1097-0142(19981215)83:12%3C2597::aid-cncr27%3E3.0.co;2-l

Gardete, S., and Tomasz, A. (2014) Mechanisms of vancomycin resistance in Staphylococcus aureus. Journal of Clinical Investigation 124, 2836-2840. https://doi.org/10.1172/JCI68834

Griffith, R. S. (1984) Vancomycin use--an historical review. J Antimicrob Chemother. 14 Suppl D:1-5. https://doi.org/10.1093/jac/14.suppl_d.1

Guffey, A. A., Loll, P. J. (2021) Regulation of Resistance in Vancomycin-Resistant Enterococci: The VanRS Two-Component System. Microorganisms. 9(10):2026. https://doi.org/10.3390/microorganisms9102026

Kahne, D., Leimkuhler, C., Lu, W., Walsh, C. (2005) Glycopeptide and lipoglycopeptide antibiotics. Chem Rev. 105(2):425-48. https://doi.org/10.1021/cr030103a

Lancini G. C. (1989) Fermentation and biosynthesis of glycopeptide antibiotics. in Progress in Industrial Microbiology Vol. 27 eds Bushell M. E., Graefe U., 283–296 Elsevier: New York, (1989).

NIAID. (2019) Vancomycin-Resistant Enterococci (VRE) Overview. Niaid.nih.gov.

Rubinstein, E., Keynan, Y. (2014) Vancomycin revisited - 60 years later. Front Public Health. 2:217. https://doi.org/10.3389/fpubh.2014.00217

Teicoplanin - DrugBank DB06149