Onychomycosis TX: Be alert for drug interactions

December 1, 2005

National report — It's important that physicians be aware of potential drug interactions with commonly prescribed therapies used to treat onychomycosis, a leading clinician tells Dermatology Times.

Itraconazole, a synthetic antifungal agent, now carries a new "black box" warning indicating that the drug should not be used to treat onychomycosis in patients with evidence of ventricular dysfunction such as congestive heart failure (CHF) or a history of CHF. A study published in the Lancet in 2001 detailed 58 cases, mostly in women, that suggested CHF occurred with the use of itraconazole.

The drug is contraindicated when a patient is taking H1 antagonists, cisapride, lipid-lowering agents such as simvastatin or lovastatin, benzodiazepine, quinidine and pimozide.

Fluconazole, a standard treatment for onychomycosis, is given once weekly for Candida onychomycosis or paronychia. At a dose of 200 mg to 400 mg, the duration of therapy is anywhere from four to eight weeks.

"Apart from making a correct diagnosis, physicians have to stay abreast of drug interactions that can occur with oral therapy," says Boni E. Elewski, M.D., a professor of dermatology at the University of Alabama School of Medicine, director of clinical research and of the fungal reference laboratory.

"Physicians should not assume that if patients are younger that they are not at risk."

She notes that physicians need to look for early signs of CHF and pointsout that in the 2001 study, patients who developed CHF were not very old. The mean age of such patients was 57.

Diagnosing onychomycosis

One of the clues to making a diagnosis of onychomycosis is the presence of tinea pedis - affecting the web space or plantar surface.

When dermatologists see abnormal nails with normal plantar/web surface, it is unlikely that they are witnessing onychomycosis, according to Dr. Elewski, who acts as a consultant for Novartis Pharmaceuticals, Pfizer and Medicis.

About 20 percent of the U.S. population aged 40 to 60 suffers from nail fungus disease, with most cases being caused by dermatophytes. A minority of cases can occur because of nondermatophyte fungi such as molds or yeasts. The incidence of the condition is on the rise, particularly in specific populations such as individuals older than 60, patients infected with HIV and patients with diabetes mellitus.

A nationwide survey of 670 patients seen by primary care doctors and podiatrists demonstrated that the results from dermatophyte test medium and fungal cultures were consistent in 68 percent of cases. The survey, published in the Archives of Internal Medicine, found that the high level of agreement between the two tests supported the use of DTM as a confirmation of dermatophyte infection.

Despite those survey results, there has been a change in labeling for some of the oral antifungal agents, specifically oral terbinafine and itraconazole. The labels for these agents now indicate that dermatologists get nail specimens before they prescribe these medications to treat onychomycosis.

"The initial step to diagnosis is to do a potassium hydrochloride examination," says Dr. Elewski, a past president of the American Academy of Dermatology. "The next step is to determine the fungal pathogen through performing a culture."

By doing a potassium hydrochloride examination, clinicians can rule out non-fungal sources of nail disease, such as psoriasis. It is critical that physicians use an appropriate size specimen when they culture the nails, Dr. Elewski notes.

A number of pathogens are isolated in onychomycotic nails including Trichophyton rubrum, Trichophyton mentagrophytes, Candida albicans and aspergillus spp.

Abnormal fingernails with normal toenails is unlikely to be onychomycosis. However, an exception to that is Candida onycholysis, which is more common in women than it is in men.