Penicillins Flashcards
How does drug penetration of cell membrane differ between gram-positive and gram-negative bacteria?
- Drugs can penetrate the outer layers of the gram-positive cell wall effectively
- The outer membrane of gram-negative bacteria excludes some drugs, but some drugs can still get through the porins in the outer membrane
How does the distribution of B-lactamases differ between gram-positive and gram-negative bacteria?
- In gram-positive bacteria, B-lactamases are excreted through the cell wall to the external environment so more B-lactamases have to be produced to have an effect
- In gram-negative bacteria, the B-lactamases are confined to the periplasmic space
What are the differences in cell membrane between gram-positive and gram-negative bacteria?
- gram-positive have one membrane
- gram-negative have two membranes
Gram-negative bacteria have a more complex cell wall that is more lipoidal
What are the differences in peptidoglycan between gram-positive and gram-negative bacteria?
thickness, content, bridging
- The gram-positive peptidoglycan layer is thicker compared to gram-negative
- The gram-negative peptidoglycan contains DAP but gram-positive peptidoglycan contains an L-lysine residue
- Gram-negative peptidoglycan is bridged by DAP and D-ala whereas gram-positive is bridged by L-lys and D-ala.
DAP = diaminopimelic acid
How does transpeptidase cross-link peptidoglycan stands?
- Transpeptidase binds to D-Ala-D-Ala sequence on peptidoglycan
- Transpeptidase serine residue performs a nucleophilic attack on the carbonyl carbon of the D-ala-D-ala bond which then releases one D-ala as a free molecule and forms an intermediate
- The transpeptidase then transfers the intermediate to a nearby peptidoglycan which cross-links via a covalent peptide bond
- Transpeptidase enzyme is regenerated
Forming the covalent bond strengthens the cell wall.
What is the mechanism of action of B-lactam antibiotics (penicillins)?
- The structure of the B-lactams resemble the D-ala-D-ala fragment that transpeptidase typically acts on.
- The B-lactam antibiotics acylate the transpeptidase serine residue which then forms a stable, inactive product.
In general, B-lactams inhibit transpeptidase so it can’t cross-link peptidoglycan strands together –> this makes the cell wall defective. The defective cell wall is subject to osmotic stress which then results in cell lysis and cell death.
What does the reactivity of B-lactams depend on?
The reactivity is due to the highly-strained four-membered ring.
What are mechanisms that lead to resistance to B-lactam abx?
- decreased cellular uptake of the drug
- mutation of PBPs that decrease their affinity for penicillins
- presence of efflux pump that pumps abx out of cell
- induction of bacterial B-lactamases
- B-lactamases catalyze the hydrolysis of the B-lactam moiety which inactivates the B-lactam. Beta-lactamases can hydrolyze many drug molecules rapidly.
- Hydrolysis of the B-lactam is irreversible
What is the cause of allergenicity of B-lactams?
- results from the drug acting as a hapten
- it acylates host cell proteins, which then raise antibodies that result in an allergic reaction
What structural features of penicillins can confer resistance to degradation under acidic conditions?
- Electronegative substituents on the side chain carbonyl reduce the nucleophilicity of the side chain amide carbonyl oxygen atom.
- This stabilizes the penicillin against hydrolysis under acidic conditions since the first step in the hydrolysis reaction is decelerated.
Which penicillin is more stable to hydrolysis in the stomach?
Penicillin V is more stable to hydrolysis in the stomach compared to Pen G because the electronegativity of the ether oxygen decreases the nucleophilicity of the amide carbonyl.
How should parenteral solutions of PCN be stored and used?
- store in the refrigerator between pH 6.0 and 6.8
- use promptly
What can promote penicillin degradation reactions?
Heavy metal ions catalyze penicillin degradation reactions.
Should be kept away from penicillin solutions
- How does the lipophilicity of penicillins impact their serum protein binding?
- How does serum protein binding impact bioavailability?
- penicillins with more lipophilic side chains are more highly protein bound
- protein binding reduces bioavailability by reducing the effective concentration of the free drug
These are additions that increase lipophilicity:
- CH3
- C(CH3)3
- F, -Cl, -I
- phenyl
- ester (COOR)
How are penicillins excreted?
- 90% by tubular secretion
- 10% by glomerular filtration
How does renal disease impact the half-life of pcns?
- renal disease increases the half-life of pcns
- pcns are renally excreted
- diseased kidneys –> slower excretion –> longer half-life
How does probenecid affect pcn half-life?
- Probenecid increases the half-life
- pcns and probenecid are both anionic
- probenecid will compete with the pcn for the anionic tubular secretions which then increases the half-life of the pcn since it is competing for excretion
What is this?
penam
What is this?
Penem
What is this?
carbapenem
What is this?
Cephem
What is this?
monobactam
What is this?
penicillin G
benzylpenicillin
Describe the antimicrobial spectrum, B-lactamase sensitivity, administration, toxicity, and precautions for penicillin G
- Antimicrobial spectrum: Gram-positive cocci
- Sensitive to B-lactamase
- parenteral administration
- acute allergic reactions
- Pen G should be used with caution in individuals with histories of significant allergies and/or asthma
What is this?
Penicillin V
phenoxymethyl penicillin
What are the differences between penicillin g and v?
- Pen V is more stable in acid
- Pen V is more stable in the stomach and produces higher and more sustained blood levels than penicillin G after oral admin.
The increase in stability in acid is attributed to the electronegative ether oxygen, which decreases the nucleophilicity of the side chain amide carbonyl and therefore decreases its participation in the B-lactam hydrolysis reaction
Why is methicillin resistant to hydrolysis by beta-lactamases?
- Methicillin is resistant to Beta-lactamases due to the steric hindrance of nucleophilic attack by the enzyme on the B-lactam carbonyl
- The two o-methoxy groups are ortho and in the way
Why is methicillin unstable in the stomach?
susceptible to hydrolysis in acid conditions
- Instability is due to electron donation toward the amide carbonyl oxygen by the o-methoxy groups
- this makes the amide carbonyl oxygen more nucleophilic
Describe methicillin’s antibiotic spectrum:
- effective against some gram-positive
- discontinued because many bacteria are resistant to it
MRSA is resistant to methicillin due to a mutation in mecA (methicillin resistance gene)
What is the B-lactamase sensitivity and acid stability of nafcillin?
- resistant to B-lactamase
- slightly more stable than methicillin in acid
What is this?
- Nafcillin
- B-lactamase resistant parenteral pcn
What are these?
- oxacillin
- cloxacillin
- dicloxacillin
- B-lactamase resistant oral pcns
What drug is this?
- ampicillin
- gram-positive and some gram-negative are sensitive to ampicillin
- B-lactamase sensitive
The charged amino group of ampicillin at physiological pH allows ampicillin to be transported into gram-negative bacteria through the porins
Why is ampicillin stable in acid?
- The amino group is protonated in the stomach, so the positively charged nitrogen is more electron-attracting
- This decreases the nucleophilicity of the amide carbonyl oxygen so that it does not participate in ring-opening of the lactam
What drug is this?
- amoxicillin
- broad spectrum
- B-lactamase sensitive
What drug is this?
- piperacillin
- B-lactamase-sensitive, broad spectrum, parenteral pcn
Why does piperacillin have a broadened spectrum of antibacterial activity and enhanced potency?
- the urea moiety make it hydrophilic which allows it to pass through the porins of gram-negative bacteria
- the added side chain fragment resembles a longer section of the peptidoglycan chain which increases its binding affinity to PBPs
