Introduction
Terbinafine hydrochloride is a synthetic allylamine antifungal. It is highly lipophilic in nature and tends to accumulate in skin, nails, and fatty tissues. Like other allylamines, terbinafine inhibits ergosterol synthesis by inhibiting the fungal squalene monooxygenase (also called squalene epoxidase), an enzyme that is part of the fungal cell wall synthesis pathway.
Uses
Terbinafine is an allylamine antifungal used to treat dermatophyte infections of toenails and fingernails as well as other fungal skin infections.
Terbinafine hydrochloride is indicated to treat fungal skin and nail infections caused by Trichophyton species, Microsporum canis, Epidermophyton floccosum, and Tinea species. Terbinafine hydrochloride also treats yeast infections of the skin caused by Candida species and Malassezia furfur.
Associated Conditions
Pharmacodynamics
Terbinafine is an allylamine antifungal that inhibits squalene epoxidase (also known as squalene monooxygenase) to prevent the formation of ergosterol and cause an accumulation of squalene, weakening the cell wall of fungal cells. Terbinafine distributes into tissues and has a long terminal elimination half life, so the duration of action is long. Overdose with terbinafine is rare, even above the therapeutic dose, so the therapeutic index is wide. Patients taking oral terbinafine should have liver function tests performed prior to treatment to reduce the risk of liver injury.
Mechanism of Action
Terbinafine inhibits the enzyme squalene monooxygenase (also called squalene epoxidase), preventing the conversion of squalene to 2,3-oxydosqualene, a step in the synthesis of ergosterol. This inhibition leads to decreased ergosterol, which would normally be incorporated into the cell wall, and accumulation of squalene.
Generation of a large number of squalene containing vesicles in the cytoplasm may leach other lipids away from, and further weaken, the cell wall.
Absorption
Oral terbinafine is >70% absorbed but only 40% bioavailable after first pass metabolism, reaching a Cmax of 1µg/mL with a Tmax of 2 hours an an AUC of 4.56µg*h/mL. Over the course of a week, 1% topical terbinafine's Cmax increases from 949-1049ng/cm2
Volume of Distribution
A single 250mg oral dose of terbinafine has a volume of distribution at steady state of 947.5L or 16.6L/kg.
Protein Binding
Terbinafine is >99% bound to proteins in plasma, mostly to serum albumin, high and low density lipoproteins, and alpha-1-acid glycoprotein to a lesser extent.
Route of Elimination
Terbinafine is approximately 80% eliminated in urine, while the remainder is eliminated in feces. The unmetabolized parent drug is not present in urine.
Half Life
Oral terbinafine has an effective half life of approximately 36 hours. However, the terminal half life ranges from 200-400 hours as it distributes into skin and adipose tissue. 1% topical terbinafine's half life increases over the first seven days from approximately 10-40 hours.
Clearance
A single 250mg oral dose of terbinafine has a clearance of 76L/h or 1.11L/h/kg.
Toxicity
The subcutaneous LD50 in rats and mice is >2g/kg. The TDLO for women is 210mg/kg/6W.
Overdose data with terbinafine is rare, however symptoms are expected to be nausea, vomiting, abdominal pain, dizziness, rash, frequent urination, and headache. Treat overdose with activated charcoal as well as symptomatic and supportive therapy.
Food Interactions
- Limit caffeine intake. Terbinafine may reduce the metabolism of caffeine by approximately 19%, monitor for increased effects of caffeine.
- Take with or without food.
Dosage
- For the treatment of fingernail onychomycosis: Terbinafine 250 mg (one tablet), once daily for 6 weeks.
- For the treatment of toenail onychomycosis: Terbinafine 250 mg (one tablet), once daily for 12 weeks.
- The optimal clinical effect is seen some months after mycological cure and cessation of treatment. This is related to the period required for the outgrowth of healthy nail.
- Body Weight: <25 kg: 125 mg/day up to 6 weeks
- Body Weight: 25-35 kg: 187.5 mg/day up to 6 weeks
- Body Weight: >35 kg: 250 mg/day up to 6 weeks
- Tinea corporis, cruris: 1 to 2 weeks
- Tinea pedis: 1 week
- Cutaneous candidiasis: 2 weeks
- Pityriasis versicolor: 2 weeks
Terbinafine 1% Spray: This spray is applied once or twice daily, depending on the indication. The affected areas should be cleansed and dried thoroughly before application of this spray. A sufficient amount of solution should be applied to wet the treatment area(s) thoroughly.
- Tinea pedis: once a day,1 week
- Tinea corporis/cruris: once a day,1 week
- Pityriasis versicolor: twice a day, 1 week
