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Beta-Lactam Antibiotics - Complete Group Discussion Notes

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Objective 1: Classification of Beta-Lactam Antibiotics

A. Penicillins

B. Cephalosporins (5 generations)

C. Carbapenems

D. Monobactams

E. Beta-Lactamase Inhibitors (combined with beta-lactams)

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Objective 2: Mechanism of Action of Beta-Lactams

1. Target structure: Bacterial cell walls are made of peptidoglycan - long polymer chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptides.
2. Normal synthesis: The enzyme transpeptidase (also called Penicillin-Binding Protein, or PBP) cross-links the peptide side chains of adjacent peptidoglycan strands - a process called transpeptidation. This cross-linking gives the cell wall its structural strength.
3. Beta-lactam action: Beta-lactam drugs are structural analogues of the D-Ala-D-Ala terminus of the peptide substrate. They bind covalently and irreversibly to the active site of PBPs (transpeptidases), inhibiting transpeptidation and halting peptidoglycan cross-linking.
4. Consequence: Incomplete cell wall formation leads to:
   • Accumulation of cell wall precursors
   • Activation of autolytic enzymes
   • Osmotic lysis and cell death

Key point: Different beta-lactams have different binding affinities for different PBP subtypes, explaining differences in spectrum. Cephalosporins, for example, do NOT bind essential PBPs in Enterococcus spp., which is why the entire class lacks useful activity against enterococci. - Goodman & Gilman, p.1169

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Objective 3: Antimicrobial Spectrum of Different Penicillins

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Objective 4: Mechanisms of Development of Resistance to Penicillins

1. Beta-Lactamase Production (Most Common)

• Bacteria produce beta-lactamase enzymes that hydrolyze the beta-lactam ring, inactivating the drug before it reaches its target PBP.
• Hundreds of different beta-lactamases are known:
  • Narrow-spectrum: TEM-1, SHV-1 (plasmid-encoded, common in E. coli, Klebsiella)
  • Extended-Spectrum Beta-Lactamases (ESBLs): Resistant to all penicillins and most cephalosporins
  • AmpC beta-lactamases: Chromosomally encoded in Enterobacter, Citrobacter, Pseudomonas; resistant to cephamycins
  • Carbapenemases (KPC, NDM, OXA): Hydrolyze even carbapenems

2. Modification of Target PBPs

• Altered PBPs with reduced affinity for beta-lactams; drug can no longer bind effectively.
• Classic example: MRSA - the mecA gene encodes a modified PBP (PBP2a) with very low affinity for all beta-lactams. This is why MRSA is resistant to ALL penicillins and most other beta-lactams.

3. Impaired Penetration to Target PBPs

• Gram-negative bacteria have an outer membrane that acts as a permeability barrier.
• Loss or mutation of porin proteins (e.g., OmpF, OmpC in E. coli) reduces drug entry into the periplasmic space, preventing access to PBPs.

4. Antibiotic Efflux

• Active efflux pumps expel beta-lactam molecules out of the bacterial cell before they can bind PBPs.
• Common in Pseudomonas aeruginosa (MexAB-OprM efflux system).

Katzung's Basic & Clinical Pharmacology 16th Ed., p.1240

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Objective 5: Pharmacokinetics of Different Penicillins

Absorption

• Penicillin G: Destroyed by gastric acid - must be given parenterally (IV/IM). IV preferred as large IM doses cause irritation.
• Penicillin V: Acid-stable - can be given orally.
• Nafcillin: Erratic GI absorption - not suitable orally. Given IV.
• Dicloxacillin, Ampicillin, Amoxicillin: Acid-stable; well absorbed orally. Serum levels ~4-8 mcg/mL after 500 mg oral dose.
• Amoxicillin: Uniquely, absorption is NOT impaired by food; others should be given 1-2 hours before or after meals.
• Depot forms: Benzathine penicillin G (IM) - maintains levels >0.02 mcg/mL for 10 days (used for streptococcal infections, syphilis). Procaine penicillin - useful concentrations for 12-24 hours.

Distribution

• Penicillins are polar molecules - good extracellular distribution, poor intracellular penetration.
• Protein binding varies: Nafcillin (highly bound) vs. Penicillin G/Ampicillin (less bound). Higher binding = lower free drug concentration.
• CNS penetration: Poor in normal conditions. With inflamed meninges (meningitis), penicillin G at high doses (18-24 million units/day IV) achieves CSF levels of 1-5 mcg/mL - sufficient against pneumococci and meningococci.
• Also excreted into sputum and breast milk (3-15% of serum levels).
• Poor penetration: eye, prostate.

Elimination

• Primarily renal: ~90% via tubular secretion, ~10% glomerular filtration.
• Half-life: Penicillin G - ~30 minutes (normal); up to 10 hours in renal failure.
• Ampicillin and extended-spectrum penicillins - half-life ~1 hour.
• Dose adjustment required in renal impairment: Give ~1/4 to 1/3 of normal dose if creatinine clearance ≤10 mL/min.
• Exception: Nafcillin - excreted primarily by the liver (biliary); NO dose adjustment needed for renal failure.
• Probenecid blocks tubular secretion, prolonging penicillin half-life (used historically to maintain higher serum levels).

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Objective 6: Important Therapeutic Uses of Penicillins

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Objective 7: Adverse Effects of Penicillins

1. Hypersensitivity Reactions (Most Important)

• Most common cause of drug allergy.
• The beta-lactam ring opens and binds to proteins, forming haptens that trigger immune responses.
• Spectrum:
  • Immediate (Type I, IgE-mediated): Anaphylaxis (0.01-0.05%), urticaria, angioedema, bronchospasm - can be fatal. Occurs within minutes.
  • Accelerated: Urticaria within 1-72 hours.
  • Delayed (Type IV): Maculopapular rash (most common overall reaction), occurs days later. Ampicillin/amoxicillin cause rash in ~5-8% of patients; nearly 100% of patients with infectious mononucleosis develop rash.
  • Serum sickness (Type III): Fever, arthralgia, lymphadenopathy - less common.
• Cross-reactivity: ~1-2% cross-reactivity between penicillins and cephalosporins (share the beta-lactam ring). Historical estimates were higher but are now considered overestimates.
• Management: Skin testing; desensitization if drug absolutely needed.

2. Neurological Toxicity

• Seizures: High CSF concentrations (from excessive doses or failure to adjust for renal failure) can be epileptogenic. Risk is higher in patients with prior CNS disease.
• Intrathecal/intraventricular injection causes severe arachnoiditis or fatal encephalopathy - must be avoided.
• Procaine penicillin - Hoigne syndrome: Accidental IV injection causes dizziness, tinnitus, headache, hallucinations, seizures (from rapid liberation of toxic procaine concentrations).
• Benzathine penicillin G: Accidental IV injection associated with cardiorespiratory arrest and death - must always be given IM.

3. GI Effects

• Nausea, diarrhea (especially ampicillin - up to 5-10%).
• Clostridium difficile pseudomembranous colitis - disruption of normal GI flora. Can follow both oral and parenteral penicillins.

4. Hematological Effects

• Neutropenia and granulocytopenia - particularly with high-dose nafcillin (monitor CBC).
• Hemolytic anemia - high-dose IV penicillin G can cause positive Coombs test and hemolysis.
• Platelet dysfunction - especially piperacillin at high doses (inhibits ADP-mediated platelet aggregation).

5. Electrolyte Disturbances

• Large IV doses of sodium penicillin G or ticarcillin can cause hypernatremia, hypokalemia, and fluid overload.

6. Hepatotoxicity

• Nafcillin and oxacillin - cholestatic jaundice, elevated transaminases (monitor LFTs).

7. Nephrotoxicity (Interstitial Nephritis)

• Rare; most commonly associated with methicillin (now discontinued). Presents with hematuria, pyuria, eosinophilia, rising creatinine.

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Objective 8: Spectrum of Activity and Uses of Different Cephalosporins

First-Generation

• Excellent Gram-positive: MSSA, Streptococci
• Modest Gram-negative: E. coli, Klebsiella, Proteus mirabilis
• NOT: MRSA, Enterococcus, Pseudomonas, Anaerobes (Bacteroides)

• Surgical prophylaxis (drug of choice - cefazolin)
• Skin and soft tissue infections (MSSA)
• UTI
• NOT for empiric meningitis

Second-Generation

• Gram-positive coverage maintained (slightly less than 1st-gen)
• Extended Gram-negative: adds H. influenzae, Moraxella catarrhalis, Neisseria gonorrhoeae
• Cephamycins (Cefoxitin, Cefotetan): Good anaerobic coverage (Bacteroides fragilis) due to methoxy group

• Community-acquired respiratory infections, otitis media, sinusitis
• Cefoxitin/cefotetan: intra-abdominal and pelvic infections, surgical prophylaxis for colorectal surgery

Third-Generation

• Dramatically enhanced Gram-negative coverage including Enterobacterales
• Penetrates CSF well
• Ceftazidime: anti-pseudomonal activity; loses Gram-positive coverage
• Do NOT use for Enterobacter infections even if in vitro susceptible - risk of AmpC beta-lactamase induction and resistance emergence

• IV infusion of 1 g produces serum levels 60-140 mcg/mL
• Ceftriaxone: Half-life 7-8 hours; once-daily dosing (1-2 g/day); biliary excretion - NO dose adjustment in renal failure
• Other 3rd-gen: renal excretion - require adjustment in renal insufficiency

• Meningitis (bacterial - pneumococcal, meningococcal, H. influenzae, Gram-negative rods) - ceftriaxone or cefotaxime
• Gonorrhea - IM ceftriaxone (drug of choice; cefixime no longer recommended due to resistance)
• Sepsis (empiric)
• Hospital-acquired infections

Fourth-Generation

• Retains 3rd-gen Gram-negative coverage AND strong Gram-positive coverage
• Anti-pseudomonal
• More resistant to chromosomal AmpC beta-lactamases (vs. 3rd-gen)
• Half-life 2 hours; penetrates CSF; renal clearance

Fifth-Generation

• Active against MRSA (the key differentiator from all other cephalosporins)
• Good Gram-positive and Gram-negative coverage (except Pseudomonas)
• Used for: complicated skin/soft tissue infections, community-acquired pneumonia

Siderophore Cephalosporin

• Uses bacterial iron-transport systems (siderophore iron chelate) to penetrate outer membrane
• Active against carbapenem-resistant organisms including Pseudomonas, Acinetobacter, Klebsiella with KPC

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Objective 9: Adverse Effects of Cephalosporins

1. Hypersensitivity reactions - Similar to penicillins but less frequent (~1-3%). Cross-reactivity with penicillin is ~1-2% (lower than previously believed). Anaphylaxis is rare. Maculopapular rash more common. Patients with penicillin allergy can generally receive cephalosporins with caution.
2. Cefazolin exception: Despite being a first-generation cephalosporin, cefazolin has very low cross-reactivity with penicillin allergy and is generally safe even in patients with documented penicillin allergy.
3. Nephrotoxicity: Older cephalosporins (cephaloridine, no longer available) were nephrotoxic. Current cephalosporins rarely cause direct nephrotoxicity, but can potentiate nephrotoxicity of aminoglycosides.
4. Bleeding diathesis (Hypoprothrombinemia): Cephalosporins with N-methylthiotetrazole (NMTT) side chain at position 3 (cefoperazone, cefamandole) - interfere with vitamin K metabolism, causing hypoprothrombinemia and potential bleeding. Also cause disulfiram-like reaction with alcohol.
5. Positive Coombs Test: High-dose cephalosporins can coat RBCs; hemolytic anemia is rare but reported.
6. Biliary sludge/pseudolithiasis: Ceftriaxone precipitates in bile as a calcium salt and can form biliary sludge or pseudolithiasis (especially in children, with high doses). Usually reversible on stopping drug.
7. GI effects: Diarrhea, C. difficile colitis (especially oral broad-spectrum cephalosporins; ceftriaxone is a known risk factor due to biliary excretion altering GI flora).
8. Neurotoxicity: Less common than with penicillins. High doses, especially with renal failure, can cause encephalopathy, myoclonus, seizures.

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Objective 10: Carbapenems and Monobactams

Carbapenems

• Imipenem (given with cilastatin - a renal dehydropeptidase inhibitor that prevents renal tubular inactivation of imipenem)
• Meropenem - stable to renal dehydropeptidase; no need for cilastatin
• Ertapenem - once-daily dosing; NOT active against Pseudomonas or Acinetobacter
• Doripenem

• Gram-positive (including MSSA, Streptococci, Enterococcus - imipenem/meropenem)
• Gram-negative including Pseudomonas aeruginosa (imipenem, meropenem, doripenem - NOT ertapenem)
• Anaerobes (Bacteroides fragilis)
• Extended-spectrum beta-lactamase (ESBL)-producing organisms
• NOT active against: MRSA, VRE (generally), Stenotrophomonas maltophilia, fungi

• Severe polymicrobial infections (intra-abdominal, pelvic)
• Hospital-acquired and ventilator-associated pneumonia
• Neutropenic fever (empiric therapy)
• ESBL-producing organism infections
• "Last resort" for multi-drug-resistant Gram-negatives

• Imipenem: Seizures (lowers seizure threshold - particularly at high doses/renal failure). Less common with meropenem.
• Nausea, vomiting (especially imipenem)
• Hypersensitivity (cross-reactivity with penicillin - use with caution)

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Monobactams

• Gram-positive organisms
• Anaerobes

• No cross-reactivity with penicillin allergy (monocyclic structure; no shared antigenic determinants with penicillins or cephalosporins)
• Can be safely used in patients with severe penicillin or cephalosporin allergy when a Gram-negative coverage is needed

• Aerobic Gram-negative infections in penicillin-allergic patients
• Urinary tract infections
• Pseudomonal infections (alternative when other beta-lactams cannot be used)
• Often combined with agents covering Gram-positive organisms (e.g., vancomycin) for broad coverage in penicillin-allergic patients

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Objective 11: Beta-Lactamase Inhibitors

Older Generation (Inhibit Plasmid-Encoded Beta-Lactamases)

• AmpC chromosomal beta-lactamases (Enterobacter, Citrobacter, Pseudomonas)
• Carbapenemases (KPC, metallo-beta-lactamases)

Newer Generation (Broader Spectrum Inhibition)

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Quick Summary Table: Key "One-Liners" for Exam

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