The antifungal effectiveness of PDT has been assessed, in vitro, in different types of fungi using several different photosensitizers at different concentrations and light sources at various wavelengths. Table 1 summarizes the most important studies.

In 1978, Propst and Lubin demonstrated that dermatophyte fungi might be photosensitive in vitro to heterocyclic dyes when they observed that proflavine and blue light (455nm) were effective in killing both Trichophyton mentagrophytes and Microsporum gypseum.24

In 2 in vitro studies, Smijs et al.25,26 demonstrated the fungicidal effect of PDT on T. rubrum using porphyrin photosensitizers (Sylsens B and deuteroporfirín monomethylester) activated by broadband red or white light. They observed an effect that persisted several weeks after PDT. Moreover, it was seen at the different stages of fungal growth, although with interesting differences in sensitivity to the PDT: spores in suspension were more susceptible than fungal colonies in liquid culture.26 The fact that PDT destroys the hyphae and inactivates the fungal spores is important in establishing its potential use as a treatment for superficial mycoses. In a clinical situation spores are responsible for the initiation of the infection and often persist and survive in the skin after treatment, favoring reinfection.

The adhesion of dermatophytic fungi to keratinized tissue is essential in the pathogenesis of dermatophytosis. To investigate the outcomes of PDT in a situation similar to a clinical setting, Smijs et al. cultured T. rubrum in an ex vivo model using human stratum corneum.27 Using this model they investigated the susceptibility of the fungus to PDT at different growth phases, demonstrating the importance in fungal virulence of adherence to a keratinized structure. They observed that, compared with the results of in vitro studies, the susceptibility of mature mycelia to PDT decreased while that of conidia did not. PDT using Sylsens B 160μM and red light (108J/cm2) had a fungicidal effect in only 65% of cases, but this figure increased to 90% when a keratinase enzyme inhibitor was added to the incubation mixture. The same authors found that, when photodynamic inhibition was unsuccessful, the photosensitizer had not penetrated the fungal cell wall.

Kamp et al.28 observed a reduction of almost 50% in the growth of T. rubrum in vitro using 5-aminolevulinic acid (ALA) as a photosensitizer. The fungistatic effect could be due to the fact that ALA is a hydrophilic molecule. Consequently, its absorption and metabolism by T. rubrum was extremely slow: the first formation of protoporphyrin IX (PpIX) was observed only after 10 to 14 days of incubation. Increasing the lipophilicity of ALA by esterification is likely to improve its diffusion, uptake, and conversion by T. rubrum.28

Other authors have demonstrated the antifungal action of PDT in vitro using different photosensitizing agents, such as phenothiazines (Amorim et al.),29 hypericin (Peace-Cristobal et al.),30 and Rose Bengal (Morton et al.).31

There is little clinical experience with the use of PDT in the treatment of onychomycosis and no standardized protocol exists. Table 2 lists the cases reported and clinical trials published to date. Most of these studies include patients in whom previous antifungal treatments had failed and patients who had underlying diseases that contraindicated oral treatment. All used a light source that emitted a wavelength in the red spectrum, which is not absorbed by hemoglobin and can penetrate more deeply into living tissue, a property that is particularly important in the treatment of nail infections. The lamp most often used in these studies was light emitting diode (LED) with a wavelength of 630±10nm (Aktilite).22,32–35 LEDs are compact, require less energy to emit light at the desired wavelengths, do not cause thermal damage to biological tissues, and are made to produce multiple wavelengths.26

Although its effect in vitro is fungistatic, the photosensitizing agent most used in the literature was 20% ALA22,32 or its derivative 16% methyl-aminolevulinate (MAL).33–36 Both have been shown to be effective when applied topically, and have completely disappeared from the treated tissue within 24 to 48hours of application.23 Other photosensitizers used were 2% methylene blue37,38 and a hematoporphyrin derivative (Photogem).39

Most authors report a clinical and microbiological cure rate of 90% to 100% following treatment; however, this percentage decreases on follow-up. The efficacy of PDT appears to depend on the pretreatment of the nail with urea and/or mechanical abrasion to increase its permeability to the photosensitizing agent22,32 and active removal of hyperkeratosis.22,36

The first reported cases of onychomycosis treated with PDT were in 2008, when Watanabe et al.32 treated 2 patients with distal and lateral subungual dermatophyte onychomycosis affecting the first toenail. In both of these patients, other antifungal treatments had been applied with no success. In both cases, the affected nails were pretreated with a 20% urea ointment for 10hours to facilitate penetration of the photosensitizing agent ALA (incubated for 5h). The diseased nails were then irradiated with pulsed laser light at a wavelength of 630nm at 100J/cm2. Treatment was repeated once a week until clinical improvement was observed and no dermatophytes were detected by potassium hydroxide microscopy or by culture. The patients experienced mild pain during PDT, but this disappeared within a day. No recurrence (clinical or microbiological) was observed after 3 months in 1 of the 2 patients or on follow-up at 6 months in the other.

Subsequently, Piraccini et al.36 reported the case of a patient with onychomycosis caused by T. rubrum affecting the first toenails of both feet; topical antifungal treatment had proved unsuccessful and oral therapy was contraindicated. On the days preceding treatment, 40% urea was applied and nail hyperkeratosis was removed. On the day of PDT treatment, 16% MAL was applied and incubated under an occlusive dressing for 3hours. The affected area was then irradiated with red light (Aktilite) (630nm, 37J/cm2). Three PDT sessions separated by an interval of 15 days achieved clinical and mycological cure. No recurrence of the infection was observed on follow-up at 24 months.

Aspiroz et al.33,34 and Gilaberte and colleagues,35 using a protocol based on that of Piraccini et al,36 treated several cases of onychomycosis caused by nondermatophyte molds (Acremonium sclerotigenum, Fusarium oxysporum, Aspergillus terreus) and yeasts (mixed infection with Candida albicans and Malassezia furfur). All these cases of fingernail infections treated successfully with PDT, provide further evidence to support the use of PDT in this type of onychomycosis when the response to oral and topical antifungal therapies is inadequate.40

Silva et al.39 described the effective use of PDT with a hematoporphyrin derivative (Photogem) in the treatment of longstanding onychomycosis. The results of a culture performed after completion of treatment was negative, but there was no subsequent follow-up.

To date, only 3 published clinical trials have been carried out. Sotiriou et al.22 used 20% ALA and red light (40J/cm2, Waldmann PDT 1200) to treat 30 patients with distal and lateral subungual onychomycosis caused by T. rubrum in whom topical antifungal therapy had proved unsuccessful and there were confirmed contraindications to oral antifungal agents. In that study, 22 (73.3%) of the patients had involvement of the first toenail. The number of treatment sessions and the intervals between them were similar to those used by Piraccini et al., with the difference that treatment was preceded by 10 nights with 20% urea under occlusive dressing. On follow-up at 12 months, clinical and microbiological cure was observed in 13 patients (43.3%), no clinical signs were apparent in 5 (16.6%), and in the other 8 (26.6%) patients there were residual changes affecting less than 10% of the nail plate and the results of culture were negative. On follow-up at 18 months, 11 patients (36.6%) were still disease free; the recurrence in 2 cases may have been due to poor penetration of ALA or because only 1 nail was treated when several were infected.

Figueiredo Souza and colleagues37,38 carried out 2 studies using methylene blue as the photosenstizing agent. One of them—the largest trial published to date—enrolled 80 patients with onychomycosis of different etiologies, including T. rubrum, T. mentagrophytes, E. floccosum, Aspergillus spp., Candida spp. and Fusarium spp.37 In this 24-week blind study, patients were randomized to receive either PDT with 2% methylene blue at 15-day intervals or oral fluconazole. In both groups, patients with hyperkeratosis greater than 2mm, longitudinal onychomycosis, or dermatophytomas were treated with mechanical abrasion of the nail to facilitate penetration of the photosensitizing agent. On completion of treatment, the cure rate was 90% in the group treated with PDT and methylene blue versus 45% in the group receiving oral fluconazole (P<.002). After 12 months of follow-up, the cure rate in the PDT group was 80%, irrespective of whether or not the patients had received prior abrasion. The results of that study show that PDT can be an effective treatment for onychomycosis, regardless of the causative pathogen involved. However, the use of oral fluconazole as a comparator may have introduced a bias because this drug is less effective than terbinafine or itraconazole.41,42 The clinical cure rate of 90% is the highest obtained with PDT and is probably due to the use of mechanical abrasion in hyperkeratotic lesions and those with longitudinal spikes or dermatophytoma, features often associated with treatment failure.43