Squalene epoxidase inhibitors

Mechanism of fungicidal action

The biochemical target of these drugs is the enzyme squalene epoxidase. Blockade of this enzyme leads to two key processes:

1. The conversion of squalene into lanosterol, which serves as a direct precursor of ergosterol, is disrupted.
2. Toxic squalene metabolites accumulate inside the fungal cell.

Unlike azole antifungals, allylamines halt ergosterol synthesis at earlier stages. They do not directly affect mitosis processes like griseofulvin does, but instead target the membrane structure directly.

Classification and clinical spectrum

Chemically, these compounds are synthetic agents divided into two main categories:

• Allylamines (derivatives of N-methylnaphthalene): terbinafine, naftifine.
• Benzylamines: butenafine.

Although the drugs exhibit a broad spectrum of antimycotic activity, their primary clinical significance focuses on dermatophyte pathogens. They demonstrate high efficacy in fungal infections of the skin and nails.

Pharmacokinetics and clinical use of terbinafine

Terbinafine is available in topical and systemic formulations (tablets). Upon oral administration, its bioavailability is approximately 40% due to pronounced presystemic metabolism (first-pass hepatic effect), and plasma protein binding reaches 99%.

The drug is actively distributed with a tendency to accumulate in adipose cells and keratin-containing tissues, such as skin and nails. Long-term circulation is ensured by a substantial half-life ($T_{1/2}$) of approximately 300 hours.

Indications for oral administration:

• Onychomycosis (nail infections);
• Mycoses of smooth skin and scalp;
• Pityriasis versicolor.

Safety and drug interactions

Systemic use requires caution, as the drug is contraindicated during pregnancy and in renal or hepatic impairment.

• Frequent adverse effects: dyspepsia (nausea, abdominal pain, diarrhea, loss of appetite) and skin rashes.
• Rare but severe reactions: hepatotoxicity (regular laboratory monitoring of liver enzymes is required), neutropenia, Stevens–Johnson syndrome, and exacerbation of autoimmune conditions (psoriasis, subacute cutaneous lupus erythematosus).

Plasma levels of terbinafine are affected by cytochrome P450 modulators: cimetidine increases drug concentration acting as an inhibitor, while rifampin decreases it as an inducer.

Specifics of naftifine

Another well-known representative of the group — naftifine — functions similarly by inhibiting squalene-2,3-epoxidase. The drug is used exclusively topically (as a cream or gel) twice daily.

In laboratory settings (in vitro), it exhibits fungicidal action against dermatophytes and fungistatic action against yeast-like fungi of the genus Candida (including C. albicans). Naftifine is prescribed for tinea pedis and tinea manuum, cutaneous candidiasis, onychomycosis, and pityriasis versicolor. Tolerability is generally good, with occasional local reactions such as burning, dryness, or redness.

Class and exam questions

What else to study on this topic

• Chemical structure of N-methylnaphthalene derivatives
• Comparison of fungicidal and fungistatic effects of naftifine against various fungal species
• The role of cytochrome P450 in the interaction of terbinafine with cimetidine and rifampin
• Monitoring of liver enzymes and hematological parameters during long-term therapy
• Pathogenesis of dermatophytoses and the rationale for using keratophilic drugs

Other topics in the section "Antifungal agents"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.