Rilopirox--a new hydroxypyridone antifungal with fungicidal properties

In vitro investigations on the mode of action of the hydroxypyridone antimycotics rilopirox and piroctone on Candida albicans

Rilopirox and piroctone belong to the class of hydroxypyridone antimycotics. This class is not related to other antimycotics. In contrast to azole antimycotics and polyene antimycotics the mode of action of hydroxypyridone antimycotics is not fully understood. Inhibition of cellular uptake of essential compounds as well as loss of other compounds seems to be only a secondary effect of a primary not known action of these drugs. The antifungal effect in vitro depends on the medium used. The hyphal induction of Candida albicans is inhibited by hydroxypyridone antimycotics, but this effect is compensated by iron ions. A damage of the cell membrane and a direct influence on adenosine triphosphate synthesis, respectively, do not seem to be part of the mode of action. But there are clear hints that reactive oxygen species (ROS) and available metabolic activity are important parts of the mode of action of the hydroxypyridone antimycotics rilopirox and piroctone.

Six novel antimycotics

We have reviewed six new antimycotic agents which have potential applications for human cutaneous and mucosal diseases. Information on these six drugs was obtained via an English language search of PubMed through the US National Library of Medicine. The antimycotic agents reviewed include rilopirox, lanoconazole, NND-502, butenafine, eberconazole and voriconazole. Rilopirox is a synthetic pyridone derivative, related to ciclopirox, with a fungicidal action. Rilopirox is a hydrophobic, topical agent with potential application in mucosal candida infections, tinea versicolor and seborrheic dermatitis. Lanoconazole, an imidazole, is a topical agent with potential application in tinea infections and cutaneous candidiasis. The drug has been available for clinical use in Japan since 1994 and once-daily application to affected areas is recommended. In addition to its antifungal effect, animal data suggest that application of lanoconazole 0.5 or 1% cream is associated with accelerated wound healing. NND-502, a stereoselective analog of lanoconazole, is a topical agent with potential application in tinea pedis infection. NND-502 appears to be more effective in inhibiting ergosterol biosynthesis than lanoconazole or bifonazole and clinical trials comparing these agents are awaited. Butenafine is the first member of a new class of antifungals, the benzylamine derivatives, and has been approved for topical use in Japan (since 1992) and the US. Butenafine has a potent fungicidal action and the drug has been shown to be effective in multiple clinical trials in patients with tinea pedis, tinea corporis and tinea cruris. Butenafine has also been reported to exert an anti-inflammatory action after topical application and this may offer potential benefit over other topical antifungal agents. Eberconazole, an imidazole derivative, is a topical antifungal agent that has been shown to be effective in clinical trials in patients with tinea infections. Preliminary data indicate that the eberconazole is effective against some triazole-resistant yeasts such as Candida krusei and Candida glabrata. Voriconazole is an azole antifungal derivative of fluconazole. The drug is available in both oral and parenteral formulations. Oral voriconazole 200mg twice daily has been effective in treating oropharyngeal candidiasis and apergillosis in immunocompromised patients. After 12 weeks' treatment, a similar dosage of the drug elicited a positive response in 69% of nonimmunocompromised patients with invasive aspergillosis.

In vitro activity of rilopirox against fluconazole-susceptible and fluconazole-resistant Candida isolates from patients with HIV infection

The in vitro antifungal activity of the new hydroxypyridone antimycotic rilopirox has been evaluated against 38 fluconazole-susceptible and -resistant clinical isolates of Candida albicans together with other Candida species isolated from patients with human immunodeficiency virus (HIV) infection and oropharyngeal candidosis. Minimum inhibitory concentrations (MICs) of both rilopirox and fluconazole were measured by a microdilution method using high-resolution medium supplemented with asparagine and glucose at pH 7.0. In comparison, an agar dilution technique was carried out for susceptibility testing of the antifungal agents. Rilopirox was found to be able to inhibit growth of all clinical yeast isolates. The rilopirox MICs at which 50% and 90% of strains were inhibited (MIC50 and MIC90 respectively), as determined by the microdilution method, were 4 and 8 micrograms ml-1 respectively. The highest MIC values for rilopirox using microdilution and the agar dilution method were 32 or 25 micrograms ml-1 respectively. On the other hand, for fluconazole, the MIC50 and MIC90 achieved were 0.5 and 128 micrograms ml-1, respectively, which means that the MIC90 value of fluconazole was 16-fold higher than that of rilopirox. Using the agar dilution technique, the MIC values of rilopirox were in the range 0.006-25 micrograms ml-1 with a median of 3.12 micrograms ml-1. For fluconazole, the MIC90 value was four-fold higher than that for rilopirox, indicating a considerable proportion of yeast strains with high MICs of 100 micrograms ml-1, suggesting in vitro resistance to this azole antifungal. All strains with diminished fluconazole susceptibility were susceptible to rilopirox. Even Candida krusei and Candida glabrata exhibited good in vitro susceptibility to rilopirox. Therefore, this new antifungal agent may be used as an alternative not only in the treatment of vaginal candidosis, but also in oropharyngeal Candida infections, e.g. in AIDS patients.

The hydroxypyridones: a class of antimycotics of its own

The hydroxypyridones chemically form a class of antimycotics not related to others. For a long time ciclopirox olamine has been the only compound to be used clinically. Recently, ciclopirox as a free acid and rilopirox have also been considered. The congeners are active against a broad spectrum of medically relevant fungi including dermatophytes, yeasts and moulds in vitro. According to investigations based on animal and in vivo models for fungal disease, this also applies in vivo. Low toxicity in the experimental animal has made topical applications in man possible. Clinical experience relates, in particular, to dermatophytosis and vaginal candidosis, but rarer conditions have also been investigated. High tolerability corresponds to efficacy in major dermatomycological conditions. Today, interest mainly focuses on ciclopirox lacquer for onychomycosis. Here as in general, the relative role of hydroxypyridones will have to be finally established in comparative trials.

Alterations in surface morphology of Candida albicans produced by rilopirox: a scanning electron microscopy study

The topological changes produced in Candida albicans cells by incubation in vitro with rilopirox have been investigated by scanning electron microscopy. Rilopirox is a new hydroxypyridone compound with fungicidal activity and the effects of 1x MIC (2.9 micrograms/ml) and 4 x MIC (11.6 micrograms/ml) after 1, 12, 24 hours of incubation were evaluated. The morphological alterations produced by rilopirox are round shapes, collapsed cells, surface folds, clusters, holes and thorn-like extrusion. The effects of rilopirox are already evident at 1 x MIC and after 1 h but their frequency and severity are correlated with the time of incubation and the MIC.

Ultrastructural investigations of Candida albicans after in vitro treatment with the new fungicidal agent rilopirox

Candida albicans was maintained in various culture media and incubated with different concentrations of the antifungal agent rilopirox. After fixation, dehydration and embedding in Spurr's medium, the cells were analysed at the ultrastructural level to investigate morphological aspects of the antifungal mode of action of this new hydroxpyridone compound. All untreated or sham-treated control cells exhibited a normal ultrastructural appearance. The cells were surrounded by a multilayered cell wall of typical structure, and the plasmalemma was in close contact with the cell wall. Also, the cell organelles of the protoplast corresponded well with the findings of other authors. After treatment with rilopirox, a variety of ultrastructural changes were seen, and the extent of damage was dependent on the specific culture condition, drug concentration and incubation time. After only 6 h and 1-10 micrograms ml-1 rilopirox, the plasmalemma exhibited elongated invaginations, the number and size of the lipid droplets had increased and greatly enlarged mitochondria containing electron-dense deposits became visible. The vacuolar system was strongly expanded and occupied nearly the whole cell. Exposure to higher concentrations of the antifungal agent and prolonged incubation times resulted in complete cytoplasmic autolysis and membrane breakdown, while the fungal cell wall remained unaffected. After treatment with 0.5% rilopirox suspension gel on agar cultures, the extent of cellular damage was clearly enhanced and included all cell types of a treated yeast colony, i.e. single blastospores and pseudohyphae.