br Experimental section br Acknowledgements The authors than

Experimental section
Acknowledgements The authors thank Prof. Yongbing Cao from School of Pharmacy, the Second Military Medical University, for providing the fluconazole-resistant strains of Candida albicans(strain 100 and strain 103). This work was supported by Program for Innovative Research Team of the Ministry of Education and Program for Liaoning Innovative Research Team in University.
Introduction Invasive fungal infections represent a continuous and serious threat to human health and they are associated with at least 1.5 million deaths worldwide each year [1], [2]. Estimates in the literature support a 30–40% mortality for invasive candidiasis, 20–30% for disseminated cryptococcosis and a similar percentage for invasive aspergillosis [3], [4]. Such infections are very common in immunocompromised patients as a result of aggressive therapies (e.g. anticancer chemotherapy, long-term corticosteroids treatment, or organ transplant) and immunosuppressive infections such as HIV/AIDS. About 90% of these deaths are caused by species belonging to genera of fungi Candida, Aspergillus, Cryptococcus, Pneumocystis, Mucor and Rhizopus[5]. However, new and emerging fungal pathogen species belonging to Zygomycetes, Fusarium or Scedosporium have becoming increasingly important as etiological agents of invasive mycoses [6]. Fungi also produced superficial infections (involving the skin and mucosal surfaces) which have greater incidence than the invasive infections diminishing the quality of life of affected individuals. Superficial mycoses are caused by Malasseria globose and M. furfur. Cutaneous and subcutaneous mycoses that affect keratinized structures are caused by dermatophyte genera like Trichophyton, Epidermophyton and Microsporum[7]. Mucosal infections are mostly caused by opportunistic yeasts being those belonging to the Candida genus the most frequent by far. In comparison with the development of new antibacterial drugs, antifungal drug development is more challenging because fungi are eukaryotes and many potential targets for therapy are also found in humans with substantial host toxicity risk [8], [9]. Currently, four histone methyltransferase of antifungal agents (azoles, echinocandins, polyenes and pyrimidine analogs) are used orally, topically or intravenously for the treatment of fungal infections [6], [9]. A fifth class, allylamines, is also existing although compounds of this class are used only for treating superficial dermathophytic infections. However, these antifungals have various drawbacks in terms of toxicity, spectrum of activity, safety and pharmacokinetic properties [1], [10]. The emergence of strains resistant to the current antifungal agents has led great efforts to develop new drugs with different mechanisms of action that target the biosynthesis of fungal proteins, lipids and cell wall [10].
Classification of antifungal agents Based on their targets for antifungal therapy (Fig. 1) antifungal agents can be classified in the following groups:
Strategies to develop new antifungal compounds The number of antifungal agents is limited as compared to antibacterial drugs. Fungi are eukaryotic organisms that parasitize eukaryotic hosts and therefore the scarce physiologic differences between both make more difficult to develop safe and broad spectrum antifungal agents. A number of antifungal agents are available in the market but, excluding amphotericin B and a few more compounds, almost all them are fungistatic [85]. Trehalose production seems to be required for the virulence of several fungal species. Deletion of TPS2 encoding trehalose-6-phosphate phosphatase leads to accumulation of trehalose-6-phosphate and cell death. Recently, the crystal structure of C. albicans Tsp2 has been reported [86] allowing for the identification of specific inhibitors for this potential target. The signaling molecules Ras GTPases play major roles in fungal virulence as they are required for growth at high temperatures. Therefore, several inhibitors of Ras function, like farnesyltransferase inhibitors, have been proposed for the specific design of antifungal agents [87]. Calcium/calmodulin signaling is another important regulator of stress responses in fungi, including resistance to antifungal treatment. Some calcineurin inhibitors are known to have a potent antifungal activity although some of them are being used as immunosuppressant drugs (e.g. tacrolimus). Therefore, development of non-immunosuppressive calcineurin inhibitors could be another way to develop antifungal drugs. Another protein being explored as antifungal drug target is the Hsp90 heat shock protein since in fungi this protein is involved in azole- and echinocandins-resistance. To date, combination of Hsp90 inhibitors, like geldanamycin, with echinocandins or fluconazole improved the fungicidal effect [88].