In recent years, candidiasis has reemerged with higher prevalence and mortality rates that are nearly 45% among compromised population groups [12
]. Moreover, clinicians have encountered the new challenge of failure of treatment with existing antifungal agents. This may be due to either emergence of non-albicans species such as C. krusei
and C. glabrata
, which are more resistant to commonly used antifungals. Therefore, there is an urgent need for new antifungal agents for the efficient management of candidal infections. Research on the active constituents of natural or traditional medicines, which are a potential source for new drugs, is drawing more attention. The aim of our study was to examine the anticandidal effect of four constituents of TCMs, which have been used for antifungal treatment for thousands of years in China.
In the initial screening with the disc diffusion assay, pseudolaric acid B showed considerable zones of growth inhibition. The remaining three TCM extracts, gentiopicrin, rhein, and alion, did not exert a significant anticandidal effect, although the plants (Gentian
, Radix et Rhizoma Rhei,
) from which they were isolated have been traditionally used to treat fungal skin infections. Pseudolaric acid is extracted from “tujingpi” or Cortex pseudolaricis, a TCM. To date, these natural products and their derivatives have been reported to exhibit antifungal, antifertility, cytotoxic, and antiangiogenic activities [13
]. Although tujingpi has been used in China for the treatment of fungal skin infection since the 17th century, recent, studies on TCM extracts have led to the identification of the pseudolaric acids as the main antifungal constituents, in which pseudolaric acid B is one of the major antifungal components. Subsequent in vitro
studies have shown that pseudolaric acid B is active against C. albicans
, Torulopsis petrophilum
, Trichophyton mentagrophytes,
and Microsporum gypseum
]. However, the anticandidal effect of pseudolaric acid B against non-Candida
species has rarely been reported, and there have been no reports of its activity against orally isolated Candida
species. The purpose of the present study was therefore to investigate activity of pseudolaric acid B against oral Candida
, with an emphasis on non-albicans and Candida
spp. from different sources.
It is well known that fungal infection is much more common in immunocompromised individuals, such as HIV-positive and Sjögren's syndrome patients. Furthermore, there are increasing reports on C. albicans
that is resistant to antifungal medications especially in HIV-infected hosts and Sjögren's syndrome patients who have undergone repeated courses of antifungal therapy [4
]. Therefore, C. albicans
strains isolated from HIV-infected and Sjögren's syndrome patients were included as part of our study. We found that pseudolaric acid B was effective against C. albicans
isolated from HIV-positive, HIV-negative, and Sjögren's syndrome patients, including both fluconazole-sensitive and -resistant strains. This potent nonselective effect of pseudolaric acid B implies that it has potential as a novel antifungal agent, especially against clinically resistant infections.
More interestingly, pseudolaric acid B demonstrated approximately similar antifungal activity against C. albicans and non-albicans Candida. This phenomenon is of great importance, because susceptibility to antifungal medication differs significantly among Candida spp. For example, C. krusei has natural resistance to fluconazole, a standard antifungal agent commonly used in the clinic. Another example is C. glabrata, which possesses a low level of intrinsic resistance to the azole drugs and fluconazole and ketoconazole. The antifungal activity of pseudolaric acid B against different Candida spp. demonstrated in the present study may provide a possible answer to the frustrating drug resistance and warrants further research.
In this study, the phenomenon of the interactions between pseudolaric acid B and fluconazole was observed by agar diffusion assay and determined qualitatively by checkerboard microdilution method. The FICI model is the most commonly used approach to study the interaction between antifungal drugs and has been used to interpret the data. It was found that a combination of pseudolaric acid B and fluconazole exhibited good synergism against oral azole-resistant isolates of C. albicans
, which has not been reported previously. It is well established that the development of antifungal drug resistance in C. albicans
follows from prolonged exposure to azole antifungals [17
]. In patients with advanced AIDS, oral candidiasis continues to be a common presenting illness associated with significant morbidity. In recent years, oral fluconazole, given its low toxicity, has become the most common form of treatment for symptomatic oral candidiasis [19
]. The widespread use of fluconazole has, however, led to an increased incidence of clinically resistant oral candidiasis due to infection with fluconazole-resistant organisms. The finding of synergy in the present study was significant because azole resistance is an emerging issue and it is necessary to find an effective and novel therapeutic method to overcome the problem of drug resistance.