Glycopeptides, fluoroquinolones, aminoglycosides

1. PHAR 532
LECTURE 6
GLYCOPEPTIDES, FLUOROQUINOLONES,
AMINOGLYCOSIDES
Antimicrobial agents and
Immunization

2. Glycopeptides
Examples of glycopeptides are vancomycin and
teicoplanin).
They interact with peptidoglycan precursors to kill
bacterial cells.
Vancomycin is very similar in its action to penicillin
as it blocks transpeptidation by binding specifically
to the D-Alanine-D-Alanine terminal sequence of
peptidoglycan. However it does not contain a β-
lactam ring and its structure is quite different.

3. Vancomycin
Synthesized by Nonribosomal Peptide Synthetases
(NPS). Enzymes that join amino acids without the
need for a mRNA.
Each NPS can synthesize only one type of peptide.
Nonribosomal peptides often have cyclic and/or
branched structures, can contain non-
proteinogenic amino acids including D-amino acids,
carry modifications like N-methyl and N-formyl
groups, or are glycosylated, acylated, halogenated,
or hydroxylated.

4. Vancomycin
Produced by the soil bacterium Amycolatopsis
orientalis.
Synthesis is the result of 7 different modules.
A set of multienzymes (peptide synthase CepA,
CepB, and CepC) are responsible for assembling the
heptapeptide.
After assembly further modifications lead to the
active form.

5. Vancomycin modules

6. Vancomycin

7. Mechanism of action
Vancomycin acts by inhibiting proper cell wall synthesis
in Gram-positive bacteria. In Gram negative vancomycin
is only active against some nongonococcal species
of Neisseria).
Vancomycin forms 5 point hydrogen bonds interactions
with D-Ala-D-Ala in the peptide chain of peptidoglycan.
This binding of vancomycin to the D-Ala-D-Ala prevents
cell wall synthesis of the long polymers NAM and NAG
that form the backbone strands of the bacterial cell wall.
If polymers do form, there is no cross linking.

8. Teicoplanin
Teicoplanin is a semisinthetic glycopeptide.
It interferes with cell wall synthesis and is effective
against Gram positive bacterial infections including
methicillin-resistant Staphylococcus aureus and
Enterococcus faecalis
Spectrum of activity similar to vancomycin

9. Resistance
Altered drug target.
The resistance phenotype is accomplished using
multiple proteins that result in the production of a
modified peptidoglycan.
Resistance to glycopeptides requires a complex set of
gene clusters.

10. Transposable elements for antibiotic resistance
Alekshun and Levy. Cell, 128 2007

11. Resistance to glycopeptides
Either a racemase or dehydrogenase results in the
production of serine (VanC, E, or G) or lactate from
pyruvate (Van A, B, or D). This results into D-Ala-D Ser
or D-Ala-D-Lac.
Two additional (or one for VanC) complete the
phenotype by removing the regular D-Ala-D-Ala.
A two component regulatory system controls expression
of the biosynthetic machinery (vanR, vanS).
Recently (2003) the enterococcal vanA gene cluster has
entered methicillin resistant Staphylococcus aureus.

12. Resistance to vancomycin

13. Telavancin
Telavancin, a lipoglycopeptide, was derived from
vancomycin by the addition of a hydrophobic
decylaminoethyl side chain to its vancosamine sugar.
Telavancin has a dual mechanism of antibacterial
action, disrupting peptidoglycan synthesis and cell
membrane function
Telavancin, unlike vancomycin, binds to bacterial
cell membranes, including the membrane-embedded
lipid II, and causes membrane depolarization and
increased membrane permeability.

14. Structures of telavancin and vancomycin.
James A. Karlowsky et al. Clin Infect Dis. 2015;61:S58-S68
© The Author 2015. Published by Oxford University Press on behalf of the Infectious Diseases
Society of America. All rights reserved. For Permissions, please e-mail:
journals.permissions@oup.com.

15. Mechanism of action of telavancin.
James A. Karlowsky et al. Clin Infect Dis. 2015;61:S58-S68
© The Author 2015. Published by Oxford University Press on behalf of the Infectious Diseases
Society of America. All rights reserved. For Permissions, please e-mail:
journals.permissions@oup.com.

16. Telavancin
In S. aureus, telavancin causes a rapid, concentration-
dependent depolarization of the plasma membrane,
increased permeability, and leakage of cellular adenosine
triphosphate and potassium.
Telavancin is 10 times more active than vancomycin in
inhibiting both the transglycosylation and the synthesis
of peptidoglycan because of its ability to bind to bacterial
cell membranes.
Telavancin is the most potent agent tested
against Streptococcus pneumoniae and inhibits all
isolates at an MIC of ≤0.03 µg/mL

17. Telavancin resistance
Intrinsic resistance: Gram negative.
Plasmid-mediated resistance to vancomycin most
frequently arises from spread of the vanA operon in
E. faecium and less commonly by the vanB operon.
VanA and vanB operons are less commonly found in
E. faecalis. VanA and vanB operons mediate
resistance to glycopeptides by altering the C-
terminal D-Ala-D-Ala dipeptide of peptidoglycan to
D-Ala-D-Lac, for which vancomycin displays low
affinity

18. Telavancin resistance
Telavancin resistance arises in enterococci harboring the
vanA operon; expression of vanA and vanB is induced by
the presence of vancomycin.
The potential for in vitro selection of resistance to
telavancin with serial passage, for gram-positive bacteria,
is low.
The potential for resistance development with clinical
use of telavancin is not known. To date (2015), only 1
report of a patient isolate demonstrating increased MICs
while on telavancin therapy has been published (Clin.
Infect.Dis. 2015,61 Telavancin: Mechanisms of Action, In Vitro Activity,
and Mechanisms of Resistance Karlowsky et al)

19. Oritavancin
Oritavancin is a semisynthetic lipoglycopeptide analogue
of vancomycin that contains the heptapeptide core
common to all glycopeptides.
One mechanism of action of oritavancin is inhibition of
transglycosylation (important in peptidoglycan
synthesis) by binding to D-alanyl-D-alanine stem termini
in Gram-positive bacteria. Secondary binding of
oritavancin to the pentaglycyl (Asp/Asn) bridging
segment in peptidoglycan also occurs, which
distinguishes it from vancomycin and contributes to
oritavancin’s activity versus vancomycin-resistant
organisms.

20. Chemical structures of vancomycin, chloroeremomycin, and oritavancin.
George G. Zhanel et al. Clin Infect Dis. 2012;54:S214-S219
© The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases
Society of America. All rights reserved. For Permissions, please e-mail:
journals.permissions@oup.com.

22. Oritavancin
Oritavancin demonstrates activity against clinical isolates
of VanA, VanB, and VanC enterococci.However, it has
been demonstrated in vitro that moderate decreases in
susceptibility level resistance to oritavancin can occur in
enterococci isolates exhibiting the VanA or VanB
phenotype.
Limited data are available regarding the selection of
oritavancin-resistant mutants in the laboratory. This is
likely because, in agar assays, oritavancin diffuses slowly
and binds to agar, which makes interpretation of
experiments to investigate single-step resistance
selection difficult.

23. Fluoroqinolones
Synthetic, broad spectrum. Some can be isolated
from natural sources.
Prevent chromosomal DNA unwinding and
replication.
Essential structure of all quinolone antibiotics: R is
usually piperazine; if the connection contains
fluorine, it is a fluoroquinolone.

24. Ciprofloxacin Levofloxacin
Ciprofloxacin Levofloxacin

25. Mechanism of action
Usually inhibit gyrase and topoisomerase, which
leads to DNA fragmentation.
Fluoroquinolones can enter cells easily
via porins and, therefore, are often used to
treat intracellular pathogens such as Legionella
pneumophila and Mycoplasma pneumoniae. For
many gram-negative bacteria, DNA gyrase is the
target, whereas topoisomerase IV is the target for
many gram-positive bacteria. Some compounds in
this class have been shown to inhibit the synthesis
of mitochondrial DNA.

26. Resistance
Mutations within the drug target molecules.
These are chromosomal mutations and happen more
frequently at particular amino acids than others.
Multiple mutational events can be selected in a
stepwise manner to train resistance in a bacterium.

27. Aminoglycosides
Protein synthesis inhibitors. They all contain a
cyclohexane ring and amino sugars.
Bind to 30S ribosomal subunit, interfere with
protein synthesis by directly inhibiting the process
and by causing misreading of the messenger RNA.

28. Aminoglycosides

29. Aminoglycosides
Most effective against Gram negatives.
Can be quite toxic and cause deafness renal damage,
loss of balance, nausea, and allergic responses.

30. Resistance
Enzymes that modify the drug can be acquired
through transposable elements.
The modification can result in:
 N-acetylation
 Phosphorylation
 Adenylation