Macrolides and Ketolides

Macrolides: Intro & MOA - Ribosome Raiders

• Bacteriostatic agents; can be bactericidal at high concentrations.
• Structure: Large macrocyclic lactone ring with attached deoxy sugars.
• Mechanism of Action (MOA):
  • Bind reversibly to the 50S ribosomal subunit.
    • Specifically to the 23S rRNA component, near the peptidyltransferase center.
  • Inhibit bacterial protein synthesis by blocking the translocation step.
    • Prevents movement of peptidyl-tRNA from the A-site (acceptor) to the P-site (peptidyl).
  • 📌 Mnemonic (Ribosomal targets): "Buy AT 30, CCELL at 50" (E for Erythromycin/Macrolides).

⭐ Macrolides primarily inhibit protein synthesis by binding to the 23S rRNA of the 50S ribosomal subunit, thereby blocking polypeptide chain elongation and translocation.

Macrolides: PK & Spectrum - Germ Targets

Pharmacokinetics (PK) Comparison:

• Gram (+): Strep (pneumo, pyogenes), MSSA.
• Atypicals: Mycoplasma, Chlamydia, Legionella.
• Gram (-): H. influenzae, M. catarrhalis, N. gonorrhoeae, Campylobacter, H. pylori (Clarithro).
• Others: B. pertussis, MAC (Azithro/Clarithro).

⭐ Azithromycin's unique PK (long t½, high tissue conc.) allows short-course therapy (e.g., 3-5 days) for many infections.

Macrolides: Clinical Uses - Healing Hits

• Atypical pneumonias: Mycoplasma, Legionella, Chlamydia pneumoniae.
• Community-Acquired Pneumonia (CAP): Often first-line or in combination.
• Upper Respiratory Tract Infections (URTIs): Pharyngitis, sinusitis (if penicillin allergy).
• Skin and Soft Tissue Infections (SSTIs): Mild to moderate, especially if penicillin-allergic.
• Sexually Transmitted Infections (STIs): Chlamydia trachomatis (azithromycin single dose), chancroid.
• Pertussis (Whooping Cough): Drug of choice for treatment and prophylaxis.
• H. pylori eradication regimens (clarithromycin).
• Diphtheria: Carrier state and active infection (alternative to penicillin).
• Prophylaxis: Rheumatic fever (if penicillin allergy), MAC in HIV (azithromycin).

⭐ Azithromycin is highly effective as a single 1g oral dose for uncomplicated Chlamydia trachomatis genital infections.

📌 Mnemonic: "MACRO" for Mycoplasma, Atypical pneumonia, Chlamydia, Respiratory infections, Other (Pertussis, Diphtheria).

Macrolides: ADRs & Interactions - Drug Duels

• ADRs:
  • GI Upset (most common, esp. Erythromycin)
  • QT Prolongation & Torsades de Pointes (⚠️ Erythromycin > Clarithromycin > Azithromycin)
  • Cholestatic Hepatitis (Erythromycin estolate)
  • Ototoxicity (high doses, reversible)
  • 📌 MACRO (Erythromycin): Motility, Arrhythmia, Cholestatic hepatitis, Rash, eOsinophilia.
  • Ketolides (Telithromycin): Severe hepatotoxicity, Myasthenia Gravis exacerbation (⚠️ Contraindicated).
• Drug Interactions (CYP3A4 Inhibition - Erythromycin, Clarithromycin):
  • ↑ Warfarin, Statins (Simvastatin, Atorvastatin), Theophylline, Carbamazepine, Cyclosporine.
  • Avoid with other QT-prolonging drugs.
  • Azithromycin: Minimal CYP interaction.

⭐ Clarithromycin & Erythromycin are potent CYP3A4 inhibitors; Azithromycin is the safest regarding drug interactions.

Macrolides: Resistance & Ketolides - Bugs Fight Back

• Macrolide Resistance:
  • Methylation of 23S rRNA (erm genes): Main, high-level.
  • Efflux pumps (mef genes).
  • Esterase inactivation.
• Ketolides (e.g., Telithromycin):
  • Engineered to combat resistance.
  • Stronger, dual 23S rRNA binding; evade efflux.
  • Effective against macrolide-resistant S. pneumoniae.

⭐ Ketolides overcome erm-mediated resistance (if inducible) due to dual rRNA binding and stability against efflux pumps.

High‑Yield Points - ⚡ Biggest Takeaways

• Mechanism: Bind 50S ribosome, blocking protein synthesis (translocation).
• Spectrum: Atypical pathogens (Mycoplasma, Chlamydia, Legionella), some Gram-positives, H. pylori.
• Pharmacokinetics: CYP450 inhibition (Erythromycin, Clarithromycin); Azithromycin has long half-life, fewer interactions.
• Adverse Effects: GI upset (motilin agonism), QT prolongation, cholestatic hepatitis, ototoxicity.
• Resistance: Methylation of 23S rRNA (erm genes), efflux pumps.
• Ketolides (Telithromycin): Overcome some macrolide resistance; higher affinity for 50S ribosome.