Antibacterials: Cell Wall Inhibitors Flashcards
Why do gram positive bacteria typically have to produce larger quantities of beta-lactamases to confer beta-lactam resistance?
Gram + don’t have an outer membrane = beta lactamases are secreted vs. Gram - beta lactamases are confined to the periplasmic space.
What do drugs need to be able to do to enter Gram - bacteria?
Pass through porins in the outer membrane
What is the difference in Gram + vs. Gram - peptidoglycan?
Gram - –> Meso-diaminopimelic acid (DAP)
Gram + –> L-lysine (COOH of DAP replaced by H)
What are the general chemical constituents of pepditoglycan?
Polymer of N-acetylglucosamine (G) and N-Acetylmuramic acid (M) with L-ala, D-Glu, L-Lys (G+) or DAP (G-) attached to TWO D-Ala.
What forms the cross bridge in peptidoglycan synthesis?
transpeptidases
Between what do transpeptidases form the peptidoglycan cross bridge?
G- = 4th residue - D-Ala and DAP
G + = 4th residue - D-Ala and L-Lys
Briefly describe the mechanism of transpeptidation.
2 Addition Elimination Reactions:
Serine of Transpeptidase attacks + attaches to one peptidoglycan molecule
Second peptidoglycan molecule displaces the Transpeptidase
Form Peptide bond
How do beta lactams inhibit peptidoglycan synthesis? How does this lead to bacterial death?
Acylate the transpeptidase serine residue - blocks peptidoglycan cross linking.
Weak peptidoglycan = osmotic stress, cell lysis
What 2 things make penicillin highly reactive?
1) Ring strain of 4 membered ring (90 degrees)
2) Penicillin ring folding (to reduce ring strain) prevents N –> carbonyl resonance stabilization
Why do we see heterogeneity of responses of different bacteria to penicillin?
Many different transpeptidases (penicillin-binding proteins) in different bacteria.
What prevents bacterial transpeptidases from catalyzing reactions with host alanine residues?
Bacterial peptidoglycan substrate contains unnatural D-Ala residues.
What are 4 mechanisms of resistance to beta lactam antibiotics?
1) Reduced cellular uptake
2) Efflux pumps
3) Transpeptidase mutations - decreased affinity
4) Elaboration or induction of Transpeptidases
How do beta-lactamases inactivate beta lactam antibiotics?
1) serine of beta lactamase acylates beta lactam (addition elimination)
2) Water added to regenerate enzyme (addn elimin)
FAST REACTION - many drug molecules quickly inactivated
How do beta-lactamases cause allergic reactions? How do you test for allergenicity?
Drug is hapten - acylates host proteins.
Test w/ prick test - wheel and flare.
What reduces the bioavailability of penicillins? What can facilitate this reduced bioavailability?
IRREVERSIBLE Degradation/hyrdolysis to penicillenic acids reduces bioavailability:
Acidic (i.e. in stomach) - Enchomerid - side chain participates
Basic - Nucleophilic attack of carbonyl in beta lactam ring
Heavy metal ions catalyze penicillin degradation
Why is Penicillin V able to be taken orally but not PenG?
Electronegative O side group stabilizes the carbonyl group so it can’t participate in the acidic hydrolysis reaction in the stomach.
How does protein binding affect penicillins (what causes binding, bioavailability, degradation, half life)?
Liphophilic side chains = more protein bound
Protein binding = decreased bioavailability (free drug), degradation protection
No affect on half life - fast dissociation rates
Two ways for penicillin excretion. What is most common?
Renal, Biliary
Renal tubular secretion predominates
How can you increase the half life of penicillins?
Add a competitor for tubular secretion:
anion route - add an anion (e.g. PROBENECID)
cation - add a cation
What two actions contribute to antibiotic resistance?
Widespread livestock use
Lack of judicious use by healthcare workers
Why is methicillin not beta lactamase sensitive?
Two methoxy substituents attached ortho to the amide - steric hinderance prevents beta-lactamase nucleophilic attack of the beta lactam carbonyl.
How has Staph aureus developed Methicillin resistance?
Mutation in the transpeptidase - mecA gene coding for PBP2
What differentiates cephalosporins structurally from penicillins?
6 membered ring attached to the beta lactam ring vs the 5 membered ring in penicillins
How is cefepime inactivated?
Host esterases –> inactive lactone.
How does Aztreonam’s structural difference affect it’s reactivity?
Sulfur group electronegativity activates beta lactam ring for hydrolysis and PBP interaction.
How do penicillins work?
acylate transpeptidases - prevent peptidoglycan/cell wall cross linking/synthesis
Name 5 drugs/drug classes that have broad spectrum activity against G+ organisms. What are the unique characteristics of each?
Penicillin G - IV
Penicillin V - ORAL
Aminopenicillins (Ampicillin, amoxicillin) - Used in combo w/ beta-lactamase inhibitor
Cephalosporins - Less susceptible to penicillinases
Carbapenems - Imipenem/cilastatin - penicillinase resistant; meropenem - stable to DHP1 & less toxicity
Name 5 drugs/drug classes that have broad spectrum activity against G- organisms.
Aminopenicillins (ampicillin, amoxicillin) Ticarcillin, piperacillin, carbenicillin 3rd generation cephalosporins Carbepenems Aztreonam
What drugs can be used to treat pseudomonas?
Ticarcillin, piperacillin, carbenicillin
3rd (Ceftazidime) & 4th Generation cephalosporins
Aztreonam
In addition to G+ organisms, what can PenG and PenV be used to treat?
N. gonorrhoeae, H. influenza, spirochetes
What are Methicillin, oxacillin/nafcillin/dicloxacillin primarily used to treat? How is methicillin given and why?
Staph aureus (NOT MRSA)
Methicillin - IV only - not acid stable.
Name 4 drugs resistant to beta-lactamases and one group that is less susceptible to penicillinases.
- Methicillin
- Oxacillin/Nafcillin/Dicloxacillin
- Imipenem/cilastatin
- Aztreonam
Cephalosporins - less susceptible
What is the difference in coverage of the different generations of cephalosporins?
1st. G+ mostly; PEcKS - Proteus, E. coli, Klebsiella, Serratia
2nd. G+ mostly; HEN PEcKS - H. influenzae, Enterobacter, Neisseria, PEcKS
3rd. G- mostly
4th. G+ and pseudomonas
What is used to treat infant vs. adult meningitis?
Infant - Cefotaxime
Adult - Ceftriaxone
How do carbapenems differ from penicillin? When should they be used?
“Carb” = carbon analogs of penicillin; S replaced with a C = more reactive
“magic bullets” - use judiciously - broadest spectrum available
How does aztreonam differ from penicillin? When should Aztreonam be used?
C2 replaced with sulfamic acid.
Use with G- infections; good for penicillin allergic patients and renal insufficient patients.
What drugs should be used with penicillinase inhibitors?
Aminopenicillins & Antipseudomonals (Ticarcillin, piperacillin, carbenicillin)
What are the three penicillinase inhibitors? What are the drug names when they are used in combination with aminopenicillins?
Unasyn - Sulbactam + Ampicillin
Zoxyn - Tazobactam + Piperacillin
Augmentin - Amoxicillin + Clavulanic Acid
What is Dehydropeptidase-1? What medication does this affect, and what needs to be done to mediate its affects?
DHP-1 = renal peptidase that hydrolyzes Imipenem. Must give Imipenem with cilastatin = a DHP-1 inhibitor.
What is the major concern with penicillin family use?
Hypersensitivity and cross-reactive hypersensitiviey.
What are some toxicities of cephalosporins?
Hypersensitivity, vitamin K deficiency, increased nephrotoxicity of aminoglycosides.
What is Vancomycin used for?
G+ - serious only - resistance development; MRSA, C. Diff
How does Vancomycin inhibit cell wall synthesis?
Inhibits transpeptidation: Binds the D-ala-D-ala terminus of peptidoglycan precursor.
How does Vanco mechanism relate to and differ from that of penicillin?
Both inhibit transpeptidation.
Vanco surrounds precursor
Penicillin binds transpeptidases/PBPs
What toxicities are associated with Vancomycin?
Nephrotoxicity, Ototoxicity, Thrombophlebitis, Red Man Syndrome (diffuse flushing - directly activates mast cells)
What is Red Man Syndrome? What causes this and why?
Diffuse facial flushing. Vancomycin directly activates mast cells.
Why is vanco use restricted to only serious G+ infections, resistant organisms, and C. diff?
Toxicity
Resistance develops quickly.
How does resistance to Vancomycin form?
D-ala D-ala changes to D-ala D-lac on bacteria. Change can undergo transpeptidation but does not bind drug.
How do beta lactamase inhibitors work?
Acylate the beta lactamase serine OH in the active site.
What is the mechanism of action of Fosphomycin?
Inhibits enzyme (MurA) responsible for catalyzing production of activated NAG for cell wall synthesis. MurA SH group opens Fosphomycin epoxide ring.
What are Repository Penicillins used for and how are they administered? Name three.
Used for longer duration of action in URI and susceptible Strep infections. Given deep IM injection only.
Benzylpenicillin, Benzathine, Benzylpenicillin Procaine
What toxicities are associated with Repository Penicillins?
Cardiac arrest and permanent neurological damage if administered into the blood stream or near nerves.
Why should methicillin use be limited to only when absolutelly necessary?
Induces beta lactamase - contributes to development of resistance.
