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Topical antifungal drugs are used to treat fungal infections on the skin, scalp, nails, vagina or inside the mouth. These medications come as creams, gels, lotions, ointments, powders, shampoos, tinctures and sprays.

Most antifungal drugs induce fungal cell death by destroying the cell wall of the fungus. These drugs inhibit the production of ergosterol, which is a fundamental component of the fungal cell membrane and wall. Antifungal drugs are generally classified according to their chemical structures and their corresponding mechanism of actions. The four classes of topical antifungal drugs are azole antifungals, polyene antifungals, allylamine antifungals, and other antifungals. Azole antifungals inhibit the enzyme that converts lanosterol into ergosterol. Common examples of azole antifungals include clotrimazole, econazole, ketoconazole, miconazole, and tioconazole. The only polyene antifungal available topically is nystatin, which works by binding to ergosterol thus disrupting the integrity of the fungal cell membrane. Similar to azoles, allylamines disrupt the fungal cell wall synthesis through inhibition of the squalene epoxidase enzyme that converts squalene into ergosterol. Examples of allylamines antifungals comprise amorolfin, naftifine and terbinafine. The last group consists of antifungal drugs with a different mechanism of action than the other three classes. These drugs include benzoxaborole antifungals, ciclopirox olamine antifungals, thiocarbamate antifungals and undecylenic alkanolamide antifungals.

Common fungal infections
Fungal infections are commonly caused by dermatophytes, yeasts or molds. There are about 40 different species of dermatophytes. They obtain nutrients from keratinized material which are responsible for making up the skin, scalp or nails. Yeasts are normal inhabitants of our skin but sometimes they grow unheeded which can result in symptomatic infections. Molds are an uncommon cause of fungal infections but they can cause tinea nigra (painless brown or black patches on the skin) or hard-to-treat nail infections.

Common examples of fungal infections include Pityriasis capitis (Dandruff), oral candidiasis (oral thrush), onychomycosis (nail infection), tinea pedis (athlete’s foot), Pityriasis versicolor, tinea capitis, tinea corporis (ringworm), tinea cruris (jock itch) and tinea manuum. Most antifungal agents treat both dermatophyte and yeast infections. However, some, such as nystatin, are not suitable for dermatophyte fungal infections.

General mechanisms of antifungals
The unique components found in fungal cell membranes are usually the drug targets of antifungal drugs, in particular, ergosterol is a sterol exclusively found in the cell membranes to replace cholesterol found in mammalian cells. A medicine that selectively removes fungal infections from a host with minimum toxicity to the host is known as an antifungal agent.

Azole antifungals
Azole antifungals are generally divided into two groups, imidazoles and triazoles. Common examples of imidazoles include clotrimazole, econazole, miconazole, ketoconazole, while fluconazole, itraconazole, posaconazole and voriconazole fall under triazoles group. Both groups cause substantial damage in fungal membrane integrity by lowering ergosterol levels, along with the loss of cytoplasmic components, thus bringing similar effects to the polyene antifungals.

Mechanism of action
Azole antifungal drugs exert their effects by inhibiting the synthesis of the sterol components of the fungal membrane. Azoles are predominantly fungistatic which inhibit the growth of the fungus instead of killing the fungus. These drugs target the ergosterol biosynthetic pathway by inhibiting the C-14 α-demethylase, a cytochrome P450 enzyme, thus preventing the demethylation of lanosterol to ergosterol which is an essential component of the fungal cell membrane. The impaired synthesis of ergosterol leads to a cascade of membrane defects. Hence, fungal cell growth is inhibited due to the disrupted structure and function of the membrane.

Clinical use
Most topically used azole antifungals belong to the imidazole group, listed below are some of the most common azole antifungals according to the British National Formulary.

Cautions
Avoid contact with eyes and mucous membranes using topically. Avoid intravaginal preparations (particularly those that require the use of an applicator) in young girls who are not sexually active, unless there is no alternative (in children).

Polyene antifungals
Polyenes antifungals do not work well orally so they are mostly seen as a solution or given topically or intravenously as a systemic antifungal. They are not suitable for dermatophyte fungal infections.

Mechanism of action
Polyene antifungals have high affinity for ergosterol in fungal cell membranes. Upon binding to the fungal cell membrane and forming pores, membrane permeability and transport in fungus are altered. As a result, fungal cell death occurs. Nystatin is the only polyene antifungal that is available for topical administration. The drug is derived from Streptomyces noursei and has activity against Candida but not dermatophytes.

Clinical use
Nystatin is used topically for the treatment of Candida infections of the skin and mucous membranes.

Oral Candidiasis (Oral Thrush)

Nystatin is commonly used in treatment of lesions of the mouth caused by oral candidiasis. The drug can be formulated in pastilles or suspensions and is directly applied to the affected area. After application, patients should avoid taking food or drink for an hour to allow sufficient time for the drug to exert its effect locally. Oral irritation or sensitisation may occur in some patients after applying the drug to the oral mucous membrane.

Fungal skin infections

Nystatin in the forms of creams, ointments or powders can also be used to treat fungal infections of the skin. The medication is directly applied to the affected area at around the same times on a daily basis. Some patients may experience itchiness, pain or burning sensations in the area where the drug is applied, while some may develop rashes or hives. On rare occasions, patients may have difficulty breathing or swallowing after topical use of Nystatin.

Vaginal infections

Nystatin is also used to treat fungal infections in the vaginal area. It is commonly formulated into pessaries or vaginal cream for application. Ointments, gels, creams or dusting powder may be used when patients develop cutaneous lesions. Skin irritations may occur in some patients after applying the medication vaginally.

Patients who are concurrently using latex contraceptives should be reminded that some intravaginal preparations of nystatin may damage the contraceptive. Additional contraceptive measures may be needed during the treatment duration.

Mechanism of action
Allylamines are a new type of antifungal drug that is highly selective for the fungal enzyme but has a minimal effect on humans. They work in a similar way as azoles but have their effects earlier on in the ergosterol synthesis pathway. Allylamines allow the active ingredients in the medication to accumulate well within the stratum corneum of the skin and nails to exert their actions. They inhibit the enzyme squalene epoxidase which converts squalene into ergosterol. This inhibition results in decreased amounts of sterols, causing cell death and the disruption of the fungal cell wall synthesis. They are known to be quite effective against dermatophytes, yeast, and molds.

Clinical use
Common allylamine antifungals are naftifine and terbinafine. They are usually used topically for the treatment of skin infections.

Naftifine Naftifine is used topically for the treatment of dermatophytosis, including tinea cruris (jock itch), tinea corporis (ringworm of the body), tinea pedis (athlete's foot) and tinea manuum (ringworm of the hand). It is available in creams or gels. Occlusive dressings should be avoided after applying the topic preparations. Clinical improvement usually occurs within the first week of treatment with Naftifine. Topical preparations containing Naftifine are contraindicated in patients hypersensitive to the drug or any ingredient in the formulation. The major adverse effect of Naftifine is local irritation, skin dryness and rash.

Terbinafine Terbinafine is used to treat skin infections caused by a fungus (yeast), including tinea corporis (ringworm of the body), tinea cruris (jock itch), onychomycosis, Pityriasis Versicolor and tinea pedis (athlete's foot). It is available in gel, spray and cream. These topical preparations should not be administered intravaginally and applied in or near the mouth or eyes. In addition, they should not be used on nails or scalp directly. Topical preparations containing terbinafine are contraindicated in patients hypersensitive to the drug or any ingredient in the formulation. The major adverse effect of Terbinafine is transient burning and stinging, local irritation and rash.

Benzoxaborole antifungals
Mechanism of action

Benzoxaborole antifungals are a newer class of antifungal medication. They work by blocking the ability of the fungus to produce proteins in a highly specific way. Hence, disrupting the action of yeast cytoplasmic enzymes involved in the translation process. A common example of this class of antifungal is Tavaborole.

Clinical use

Onychomycosis Benzoxaborole antifungal is for the treatment of onychomycosis of toenails. It is applied to the affected toenail once daily for 48 weeks. The major adverse effects associated with its use is the ingrowing of toenails and application site reactions including exfoliation, erythema, and dermatitis.

Ciclopirox Olamine
Mechanism of action

Ciclopirox and ciclopirox olamine are synthetic antifungal agents. The exact mechanism of action of Ciclopirox olamine antifungals is not well understood. It appears that the medication works by causing depletion of important substrates such as amino acids and/or ions within fungal cells, resulting in the inhibition of transport of these substances across fungal cell membranes. Finally, it disrupts the synthesis of DNA, RNA and proteins in fungal cells and leads to cell death. Ciclopirox may also exert its antimycotic effects by altering fungal cell permeability. However, the leakage of cellular constituents resulting from the decreased cell permeability is only apparent at high drug concentrations.

Clinical use

Ciclopirox olamine comes in different formulations, which can be used topically in a wide variety of fungal infections.

Fungal skin infections

Ciclopirox olamine cream or lotion is used to treat certain dermatophytosis such as tinea pedis, tinea cruris, and tinea corporis. They can also be used for the treatment of cutaneous candidiasis and pityriasis versicolor. The medication is sometimes formulated into gels when treating tinea corporis and interdigital tinea pedis.

Scalp infections

Ciclopirox olamine gels and shampoos are used topically to treat seborrheic dermatitis of the scalp.

Nail infections

Nail lacquers are formulated from ciclopirox olamine solutions to treat mild to moderate onychomycosis of fingernails and toenails without the involvement of the lunula (the white area at the base of a nail).

Adverse effects

In general, ciclopirox and ciclopirox olamine preparations are well tolerated by patients when used topically for treatment of fungal infections. Pain, itchiness and burning sensations may occur following topical application of the medication. For patients using ciclopirox nail lacquers, redness of the skin around the nail and the proximal nail fold may occur, while a few of them develop nail disorders such as ingrown nails, changes in shape, and discoloration.

The above adverse reactions are usually mild and self-terminating. They are rarely severe enough to require discontinuation of the drug.

Thiocarbamate antifungals
 Mechanism of action

Thiocarbamate antifungals have a similar mechanism of action as allylamine antifungals. They both inhibit the enzyme squalene-epoxidase, which converts squalene to lanosterol, the raw material for producing ergosterol in fungal cells. By blocking the sterol synthesis in fungal cells and cell extracts, the integrity of fungal cell membranes will be interrupted, thus leading to impeded fungal growth.

Clinical use

Tolnaftate is one example of thiocarbamate antifungals.

Tolnaftate

Tolnaftate is a topically used antifungal for treating or preventing superficial dermatophyte infections, also known as skin infections, ringworm or tinea, and pityriasis versicolor. It is available in gel, solution, powder, ointment, or cream. Repeated treatment may be required if conditions do not improve within two to six weeks. It is important to note that tolnaftate is effective in inhibiting the growth of dermatophytes such as Epidermphyton and Microsporum, but the drug has no use against candida or bacteria infections. Contraindications include nail or scalp infections, and sulfa allergy or hypersensitivity to tolnaftate itself. Tolnaftate is usually useful for patients desiring self-medication of mild tinea infections.

Adverse effects

Side effects are rare with tolnaftate, these side reactions may include irritation, burning, pruritus (itchiness), and contact dermatitis.

Undecylenic alkanolamide antifungals
Mechanism of action Undecyclenic alkanolamide antifungals, also referred to as undecylenic acid and undecenoic acid, is a type of unsaturated fatty acid that inhibits fungal growth on the skin.

Clinical use

Although undecylenic acids possess antifungal properties, the drug itself is seldom used on its own for antifungal purposes. Salts of undecylenic acid such as zinc undecenoate and calcium undecenoate are commonly formulated into antifungal creams, ointments, solutions or powders. Undecenoic acid and its salt forms are applied topically to treat superficial dermatophytosis, in particular tinea corporis (ringworm), tinea pedis (athlete’s foot), and tinea cruris (jock itch). Undecenoic acid is also formulated into several other salts and derivatives such as methyl, phenyl and propyl undecenoate, which are also used to treat fungal infections on the skin. There has been limited evidence showing the effectiveness of undecylenic acid in treating nail and scalp fungal infections.

Adverse effects

Irritation may occur in some patients after topical application of undecylenic acid or its salts and derivatives.

General Precautions
Topical antifungals are generally safe to use in adults, seniors and children. Different dosages may be required for patients of different age groups. Discuss your conditions with your healthcare provider before using any topical antifungal medications.

Prior to using topical antifungals, wash the affected area with soap and water and dry it completely. Wash your hands thoroughly after applying topical preparations. Avoid using occlusive dressings or wrappings unless otherwise directed by a clinician. Topical antifungal medications usually come with side effects. Some patients may develop itching or local irritations after the application of these products. Consult a pharmacist or clinician if the treated area shows signs of increased irritation or possible sensitization such as erythema, pruritus, burning, blistering, swelling, oozing. These drugs can also interact with food and various medications. Inform your pharmacist or clinician of existing or contemplated concomitant therapy, including prescription and OTC drugs, dietary or herbal supplements and any concomitant illnesses. Therefore, topical antifungals should be used carefully after seeking advice from medical professionals.