Photodynamic inactivation of the dermatophyte Trichophyton rubrum^sup [para]^

ABSTRACT

Dermatophytes are fungi that can cause infections (known as tinea) of the skin, hair and nails because of their ability to use keratin. Superficial mycoses are probably the most prevalent of infectious diseases worldwide. One of the most distinct limitations of the current therapeutic options is the recurrence of the infection and duration of treatment. The present study shows that Trichophyton rubrum in suspension culture is susceptible to photodynamic treatment (PDT), a completely new application in this area. T. rubrum could be effectively killed with the use of the light-activated porphyrins deuteroporphyrin monomethylester (DP mme) and 5,10,15-tris(4-methylpyridinium)-20-phenyl-[21H,23H]-porphine trichloride (Sylsens B). The photodynamic efficacy was compared with that of some other photosensitizers that are well known in the field of PDT: the porphyrins deuteroporphyrin and hematoporphyrin, the drug Photofrin and several phthalocyanines. It was demonstrated that with the use of broadband white light, the phthalocyanines and Photofrin displayed a fungistatic effect for about 1 week, whereas all the porphyrins caused photodynamic killing of the dermatophyte. Sylsens B was the most effective sensitizer and showed no dark toxicity; therefore, in an appropriate formulation, it could be a promising candidate for the treatment of various forms of tinea. For Sylsens B and DP mme, which displayed the best results, a concentration-dependent uptake by T. rubrum was established.

Abbreviations: A1PcS^sub 4^, aluminum(III) phthalocyanine chloride tetrasulfonate; DP, deuteroporphyrin; DP mme, deuteroporphyrin monomethylester; HP, hematoporphyrin; MEA, malt extract agar; PcS^sub 4^, phthalocyanine tetrasulfonate; PDT, photodynamic treatment; Sylsens B, 5,10,15-tris(4-methylpyridinium)-20-phenyl-[21H,23H]-porphine trichloride; ZnPc, zinc phthalocyanine.

INTRODUCTION

Dermatophytes are fungi that can cause infections of the skin, hair and nails because of their ability to use keratin. They are unable to grow in living tissues but instead can multiply in structures containing keratin. The fungus colonizes the keratin tissues and causes inflammation as a result of the host's response to its metabolic by-products.

The distribution of the dermatophyte Trichophyton rubrum is worldwide (1). T. rubrum is seldom isolated from animals and is rarely found in soil.

Although there are a few geophylic dermatophytes, most of them are truly anthropophilic. The infections caused by dermatophytes, known as tinea, may have different manifestations depending on the site of the infection on the body. The inflammatory reactions caused by T. rubrum can give rise to severe damages to skin and particularly to nails. As soon as the infection becomes systemic, even death may follow if no treatment is performed. The drugs used today for the treatment of superficial fungal infections are certainly more effective than those available several years ago. In the 1960s griseofulvin was introduced (2) as a fungistatic with a treatment duration of 4 weeks up to 18 months (3), whereas in 1992 a topical treatment with the drug terbinafine (Lamisil) was introduced as a cream that promised to cure a dermatophyte infection after only a few applications (4,5). Recently, a sequential pulse therapy with itraconazole (a triazole) and terbinafine (an allylamine) was introduced to treat onychomycosis of fingernails (3,6,7). Allylamines are more effective against dermatophyte infections, but they are more expensive than azoles (3).

In general, limitations of the current therapeutic options include inadequate spectrum of activity, lack of efficacy, multiple drug interactions, inadequate pharmacokinetic profile, excessive costs, recurrence of the infection and duration of treatment (5,8). The aim of the present study is to determine whether the dermatophyte T. rubrum can be killed with the use of the light-activated porphyrins deuteroporphyrin monomethylester (DP mme) and 5,10,15-tris(4-methylpyridinium)-20-phenyl-[21H,23H]-porphine trichloride (Sylsens B) and thus develop a possible clinical application against infections caused by T. rubrum.

Sylsens B and DP mme are known to be able to kill certain bacteria, Chinese hamster ovary cells and the common fruit fly Drosophila melanogaster (G. M. T. Smijs, unpublished data). Therefore, we selected these compounds to investigate this relatively new application of photodynamic treatment (PDT). PDT refers to the use of light-activated agents called photosensitizers. Upon irradiation with light of the proper wavelength, photosensitizers can initiate a photochemical reaction resulting in the production of singlet oxygen, which can react with cellular components. The sequence of events is termed as photodynamic effect and can result in tissue destruction (9,10).

Little research has been reported concerning the treatment of fungal infections with the use of light-activated agents. In 1998 the photodynamic effect of different thiophenes on eight strains of dermatophytes with the use of UV-A radiation was studied (11). The growth of all tested strains was strongly inhibited by the thiophenes under investigation, but a complete inactivation was never detected. Moreover, UV-A is known to be carcinogenic.