One tube, seven days — the pharmacokinetics of a single-dose antifungal

Tinea pedis — athlete’s foot — is caused by dermatophyte fungi that colonise the stratum corneum of the foot. It is enormously common, rarely serious, and genuinely difficult to clear, not because the drugs fail but because the treatment does. Conventional topical antifungals ask the patient to apply a cream once or twice daily for one to four weeks. Real-world adherence to that regimen is poor, and inadequate courses breed recurrence.

The compliance problem, restated as pharmacokinetics

Terbinafine is an allylamine that inhibits squalene epoxidase, an enzyme early in fungal sterol synthesis. It is fungicidal, and it is highly lipophilic — so it partitions readily into and stays within the lipid-rich stratum corneum. That property is the key. If a vehicle can deposit a reservoir of terbinafine into the skin in a single application, the drug’s own tissue kinetics can carry the treatment for the rest of the course.

How a film-forming vehicle works

A single-application film-forming gel is painted on once. The volatile carrier evaporates and the remaining polymer cures into a thin, transparent, water-resistant film across the affected skin. That film is a depot: it releases terbinafine steadily into the stratum corneum over the following days, where the drug’s lipophilicity keeps it at fungicidal concentrations. Published pharmacokinetic work on this format has shown effective drug levels sustained for around thirteen days after a single dose.

Why it matters clinically

The elegance is not chemical novelty — terbinafine is decades old — but the collapse of a two-week regimen into a single, supervised event. The patient cannot forget to apply tomorrow’s dose, because there is no tomorrow’s dose. For a condition where the failure mode is behavioural rather than pharmacological, moving the burden from the patient to the vehicle is exactly the right design.