PHRM3101 Week 2 Antimicrobial Pharmacology - Inhibition of Cell Wall

Beta-lactams:

- Penicillins
- Cephalosporins
- Monobactams
- Carbapenems

Glycopeptides

The peptidoglycan (aka murein)-containing bacterial cell wall is not present in eukaryotic cells, allowing it to provide a selective target for bacteria.

The cell wall structure also varies between bacteria betweren gram-positive and gram-negative variants which affects the ability for different antibiotics to reach the site of action and thus, spectrum of antimicrobial activity.

Peptidoglycans incorporate a polysaccharide component that consists of i) alternating units of N-acetylglycosamine (NAG) and N-acetylmurate (NAM) carbohydrates. There is crosslinking between adjacent NAG-NAM chains in the peptidoglycan layer via ii) pentapeptides attached to NAM that terminate in D-alanine-D-alanine as shown. This bridging is made up of glycine:

The pentapeptide crosslinking is facilitated by a transpeptidase enzyme called penicillin-binding protein (PBP). This is the site to which penicillins irreversibly bind. 

PBP cleaves the terminal D-alanine amino acid of the pentapeptide and conjugates the remaining tetrapeptide sequence that extends from the pentapeptide L-lysine residue sidechain. 

- Structural rigidity
- Prevention of cell lysis under conditions of high osmotic pressure
- Plays a role in the regulation of cell division

Expressed by bacteria. Important for degrading old peptidoglycan to enable cell wall turnover 

- penicillins
- cephalosporins
- carbapenems

Core structures are as shown:



These are named after the beta-lactam ring (4-membered cyclic amide)

The beta-lactam ring functions as a structural analog of that related to the D-alanine-D-alanine sequence at the peptidoglycan pentapeptide terminus. The unique similarity alongside other beta-lactam antibiotic substituents allows beta-lactam antibiotics to bind to penicillin-binding proteins (PBPs). 

After this binding, the beta-lactam ring reacts with the PBP that leads to ring opening and covalent modification to the active site serine. Consequently, PBPs become inactivated, weakening the cell wall by inhibiting peptidoglycan crosslinking and leads to the activation of an autolytic process which initiates cell death.

lacks activity

Beta-lactamases are one of the primary reasons for resistance to beta-lactam antibiotics.

Beta-lactamases that target penicillin antibiotics and render them inactive are called penicillinaases while those that hydrolyse cephalosporin antibiotics are called cephalosporinases. Both these enzymes break the beta-lactam ring on the antibiotics. 

Co-administration of beta-lactam antibiotics with inhibitors of beta-lactamases can safeguard the antibiotics from inactivation. This protection mechanism offers a way to expand the activity of beta-lactam antibiotics that would normally be susceptible to beta-lactamases, towards bacteria that express beta-lactamase enzymes. 

Clavulanic acid

Tazobactam

Avibactam

*note avibactam is a broader spectrum than the other inhibitors and can cover some chromosomal AmpC beta-lactamases,. narrow and extended-spectrum beta-lactamases (ESBL), and Klebsiella pneumoniae carbapenemase (KPC) resistance mechanisms

Avibactam

nah

A. Natural Penicillins 

- Phenoxymethylpenicillin (Penicillin V)
- Benzylpenicillin (Penicillin G)

They are available in the following forms:

- Long lasting IM depots: procaine benzylpenicillin, benzathine benzylpenicillin
- Oral: Penicillin V
- Parenteral: Penicillin G

Natural penicillins are inactive towards S. aureus which is an important disease-associated bacteria, structural modifications have been made to provide improved resistance towards beta-lactamases and activity towards staphylococci

- Dicloxacillin
- Flucloxacillin

Both are available orally where Flucloxacillin is also available in IV formulation

No, as antibiotic resistance in MRSA is not associated with beta-lactamases

They are moderate-to-broad spectrum antibiotics that have been developed to have improved coverage towards gram-negative bacteria. They are also more water soluble than natural penicillins, which allow them to pass through porin channels in the cell wall of  gram-negative bacteria to reach the PBPs.

They are drugs of choice for enterococcal infections and sometimes combined with aminoglycosides (e.g. gentamicin to take advantance of synergistic improvements in bactericidal acivity). They are also used for upper respiratory tract infections (URTI)

- Amoxacillin
- Ampicillin

The natural penicillins are narrow spectrum antibiotics focussed on gram-positive bacteria and not active towards S. aureus due to susceptibility towards Staphylococcal beta-lactamases

Yes, and therefore not effective against staphylococci

Amoxacillin is available in both oral or parenteral (IV/IM)

Ampicillin is available in (IV/IM). Technically is orally available but amoxacillin has better bioavailability.

Amoxacillin-clavulanic acid (aka coamoxiclav) - available for oral and parenteral administration

Piperacillin-tazobactam (aka piptaz) - available for parenteral administration

These combinations are useful for empiric antibiotic treatment. If the beta-lactam antibiotic has intrinsic activity against a specific bacteria when it is not producing beta-lactamase, then coadministration with a beta-lactamase inhibitor may extend coverage to beta-lactamase producing forms of the bacteria.

Cross-reactivity is low, allowing use in some cases for patients who are allergic to penicillins

moderate spectrum with activity against gram-positive bacteria.

Yes (except for MRSA) and therefore are good alternatives to the anti-staphylococcal penicillins (e.g. dicloxacillin), particularly in people who are allergic to penicillins.

Yes, but they do not cross the BBB, so not useful for CNS infections

Increases the risk of bleeding, particularly in patients who are taking warfarin or are malnourished due to its effects to reduce vitamin K production

Oral: Cefaclor
IV: Cefuroxime, cefoxitin

Moderate-spectrum antibiotics which when compared to first generation cephalosporins, have improved gram-negative cover with weakened gram-positive cover. In addition, they have reduced staphylococcal cover and do not cover S. aureus but gain anti-Haemophilus activity

Improved stability to gram-negative beta-lactamase producing bacteria

Cefuroxime provides better coverage of streptococcus pneumoniae than cefaclor. The orally administered agents are mainly used for bacterial URTIs (where H. influenzae is an important causative organism)

They do not cross the BBB so they are not useful for CNS infections

Cefotaxime, ceftazidime, ceftriaxone - all parenteral

They are broad-spectrum antibiotics, with greater gram-negative coverage, and less gram positive cover than earlier generations. This includes reduced staphylococcal cover and no cover of S. aureus like second generation cephalosporins.

- Improved stability to gram-negative beta-lactamase producing bacteria, but are highly associated with induction of antibiotic resistance in gram-negative bacteria (e.g. ESCHAPPM pathogens)

- Ceftazidime is available in combination with avibactam (beta-lactamase inhibitor), which extends its activity towards organisms producing extended-spectrum beta-lactamases (ESBLs), Klebsiella pneumoniae, carbapenemases (KPC) and AmpC beta-lactamases enzymes.

- Ceftazidime has anti-pseudomonal activity (like fourth-generation cephalosporin cefepime)

- Can cross BBB, and so are used for the treatment of CNS infections such as meningitis

- Strongly associated with Clostridioides difficle-related diarrhoea
- Ceftriaxone binds calcium to form crystals which has resulted in deaths. It is also associated with increased risk of bilirubin encephalopathy in neonates, and increases the risk of bleeding (particularly in patients taking warfarain or are malnourished due to its effects due to reduced vitamin K production)

The only available one is cefepime and it is administered parenterally

It is the broadest cephalosporin. It has similar gram-positive coverage to the first generation cephalosporins, including staphylococcus aureus (except MRSA) coverage. It covers many gram-negative bacteria including Pseudomonas (like ceftaizidime).

Induces less resistance in gram-negative organisms than the third generation cephalosporins.

The use of cefepime is reserved for the treatment of multiresistant organisms, or for treating sepsis in neutropenic or immunosuppressed persons

Ceftaroline fosamil, ceftolozane. All parenteral only

Reduced gram-negative coverage compared to fourth generation cephalosporins

Ceftaroline fosamil has a broad gram-positive coverage, including staphylococci. It is the only cephalosporin marketed in Australia that has MRSA coverage. It does not cover Pseudomonas aeruginosa.

Ceftolozane-tazobactam has reduced gram-positive coverage. In particular, it doesn't cover Staphylococci. It does however, cover Pseudomonas aeruginosa.

The beta-lactam is not fused to another ring structure 

Aztreonam - only parenterally administrered

Aztreonam and Ceftazidime. This is because they share the same side chain structure:

It has anti-pseudomonal activity. It is resistant to inducible AmpC beta-lactamases (e.g. produced by ESCHAPPM pathogens) and metallocarbapenemases compared to other beta-lactams. 

Imipenem
Ertapenem
Meropenem

(note: the suffix 'penem')

They are the most broad-spectrum antibiotics available

Activity towards gram-positive, gram-negative, and anaerobic pathogens. They are reserved for hospital aquired infections

Active towards S. aureus but not effective for MRSA or VRE

Ertapenem is given once daily and has poor activity against hospital aquired infections (e.g. Pseudomonas aeruginosa, Enterobacter faecalis, Acinetobacter species)

Imipenem is administered with cilastatin, preventings its inactivation in the kidney and generation of nephrotoxicity

low

vancomycin (oral), and parenterally via vancomycin and teicoplanin

Similar to beta-lactams. It is associated with preventing cross-linking of peptidoglycan. However, instead of inhibiting PBP transpeptidase activity, glycopeptides non-covalently bind with high affinity to the D-alanine-D-alanine terminus of peptidoglycan. Which inhibits PBP-mediated cross-linking of peptidoglycan

Vancomycin is mainly active towards gram-positive bacteria. Their large molecular weight prevents effective permeation of the gram-negative outer membrane.

Glycopeptides are useful for treaeting staphylococci, including MRSA, streptococci, and clostridioides difficle. They posesses some activity towards enterococci, but many have become resistant (vancomycin-resistant enterococci - VRE), especially Enterococcus faecium.