Terbinafine

Mechanism of Action - Squalene's Demise

• Primary Target: Fungal enzyme squalene epoxidase.
• Action: Reversibly inhibits this enzyme, preventing the conversion of squalene to lanosterol, a key precursor for ergosterol.
• Dual Consequence:
  • ↓ Ergosterol synthesis → disrupts fungal cell membrane integrity.
  • ↑ Toxic intracellular accumulation of squalene.

⭐ Terbinafine is fungicidal due to the accumulation of toxic squalene, whereas azoles are typically fungistatic.

📌 TERbinafine TERminates squalene conversion.

Clinical Spectrum - Nail & Skin Savior

• Primary Use: Dermatophytoses (Tinea infections), especially effective against nail and skin fungi.
  • Onychomycosis (Tinea unguium): Highly effective, considered first-line therapy.
  • Tinea pedis (athlete's foot)
  • Tinea cruris (jock itch)
  • Tinea corporis (ringworm)
• Limited Activity: Less effective against Candida species (especially mucosal) and molds.

⭐ A classic board question involves treatment for onychomycosis (tinea unguium), which typically requires a long course of oral terbinafine (6 weeks for fingernails, 12 weeks for toenails).

Adverse Effects - The Bitter Pill

• Hepatotoxicity: ⚠️ Ranges from transient ↑ in liver enzymes to idiosyncratic, severe liver failure.

  ⭐ The risk of hepatotoxicity necessitates baseline liver function tests (LFTs) before starting oral terbinafine and periodic monitoring during treatment.

• GI Upset: Common, includes diarrhea, dyspepsia, and nausea.
• Sensory Disturbances:
  • Taste disturbance (dysgeusia), may be prolonged. 📌 Mnemonic: Terbinafine tastes terrible.
  • Visual disturbances.
• Dermatologic Reactions:
  • Rash, urticaria.
  • Rarely, severe cutaneous adverse reactions (SCARs) like SJS/TEN.
• Hematologic (Rare): Neutropenia, pancytopenia, thrombocytopenia.

Pharmacokinetics - A Drug's Journey

• Absorption:
  • Good oral absorption (~70-80%), but bioavailability is ↓ to ~40% due to significant first-pass metabolism.
  • Food enhances absorption.
• Distribution:
  • Highly lipophilic and protein-bound (>99%).
  • ⭐ > Its lipophilic nature and high affinity for keratin are why it concentrates in the skin, nails, and fat, making it so effective for dermatophyte infections.
• Metabolism:
  • Extensively metabolized in the liver by multiple CYP450 enzymes.
  • ⚠️ It is a potent inhibitor of CYP2D6.
• Excretion:
  • ~80% of the dose is excreted as inactive metabolites in the urine.
  • Requires dose adjustment in significant renal or hepatic impairment.

High-Yield Points - ⚡ Biggest Takeaways

• Inhibits squalene epoxidase, a key enzyme in the fungal ergosterol synthesis pathway, leading to the accumulation of toxic squalene.
• Highly effective for dermatophytoses, especially onychomycosis (nail fungus) and tinea capitis.
• It is fungicidal against dermatophytes.
• Significant adverse effect is hepatotoxicity; liver function tests (LFTs) must be monitored.
• Other side effects include GI distress, headache, and taste disturbance.
• Accumulates in keratin-rich structures like skin, nails, and hair, allowing for shorter treatment courses.