JAC Advance Access originally published online on June 6, 2006
Journal of Antimicrobial Chemotherapy 2006 58(2):458-461; doi:10.1093/jac/dkl237
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Caspofungin activity against clinical isolates of azole cross-resistant Candida glabrata overexpressing efflux pump genes
1 Istituto di Microbiologia, Università Cattolica del Sacro Cuore Rome, Italy 2 Institut de Microbiologie, Centre Hospitalier Universitaire Vaudois (CHUV) Lausanne, Switzerland
*Corresponding author. Tel: +39-06-30154964; Fax: +39-06-3051152; E-mail: msanguinetti{at}rm.unicatt.it
Received 16 March 2006; returned 14 April 2006; revised 10 May 2006; accepted 12 May 2006
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Abstract |
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Objectives: Several studies have documented the potent in vitro activity of caspofungin against Candida spp. This is of special concern for Candida glabrata infections that are often resistant to many azole antifungal agents and, consequently, difficult to treat. The aim of the present study was to expand the data on the in vitro activity of caspofungin against azole-resistant isolates of C. glabrata.
Methods: A total of 50 clinical isolates of C. glabrata were tested for susceptibility to caspofungin. The isolates were cross-resistant to multiple azoles, including fluconazole, itraconazole, ketoconazole and voriconazole. Expression of the resistance-related CgCDR1 and CgCDR2 genes was evaluated by quantitative RT–PCR analysis. The MICs of caspofungin were determined by using the National Committee for Clinical Laboratory Standards M27-A2 reference method.
Results: C. glabrata isolates exhibited increased expression of the CDR efflux pump(s), and this was in accordance with their high-level azole resistance. In contrast, all the isolates were highly susceptible to caspofungin (100% of isolates were inhibited at 
1 mg/L).
Conclusions: Our results represent further evidence for the excellent antifungal potency of caspofungin, particularly against C. glabrata isolates expressing cross-resistance to azoles.
Keywords: antifungals , susceptibility testing , CgCDR genes
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Introduction |
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Candida glabrata infections have markedly increased in the past decades and, when invasive, are associated with a high mortality rate, especially in immunocompromised patients such as intensive-care, post-surgical and neutropenic patients.1 This, in part, has been attributed to the reduced susceptibility of this pathogen to antifungal agents, especially the azoles.2 Recent studies revealed that the MICs of extended-spectrum triazoles (fluconazole, itraconazole and voriconazole) for C. glabrata were higher than those seen for most Candida albicans isolates.3–6 Fluconazole-resistant isolates have been found not only in AIDS patients with oropharyngeal candidiasis, but also in fungaemic patients and among vaginal isolates as a consequence of widespread and often indiscriminate use of the drug.7 C. glabrata is able to rapidly develop azole resistance, which is probably related to the haploid state of the microorganism.2 In this context, studying the resistance mechanisms of this pathogenic yeast may provide answers in developing new antifungal agents.6,8
Caspofungin is an echinocandin with potent fungicidal activity against many Candida species3,9 and could be particularly useful to treat C. glabrata infections, which often develop resistance to azoles. To further demonstrate the in vitro efficacy of this promising new antifungal agent in C. glabrata, we report here caspofungin susceptibility of a large collection of clinical isolates which were cross-resistant to multiple azoles. Molecular studies showed that azole resistance in these isolates is associated with the up-regulation of the ABC transporter genes CgCDR1 and CgCDR2.
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Materials and methods |
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Yeast isolates and antifungal susceptibility testing
A total of 50 clinical isolates of azole-resistant C. glabrata were tested: 30 isolates were recovered from blood (4 isolates), respiratory tract (10), vagina (3), mouth (1), urine (10), skin (1) and drainage fluid (1) of patients hospitalized between January 2004 and June 2005, and 20 other isolates, cultured from various patient specimens (6 blood, 5 urine, 3 vagina, 2 respiratory tract, 2 mouth, 1 skin and 1 drainage fluid) before 2004, had been explored for molecular mechanisms of antifungal resistance in a previous study.6 Each isolate represented an individual infectious episode. The isolates showed high MICs of multiple azoles, as determined by the broth microdilution method (Table 1). A panel of 30 fluconazole-susceptible or fluconazole-susceptible dose-dependent (S-DD) isolates was also included. All isolates were identified to the species level by standard methods10 and stored as 20% glycerol stocks until use. Prior to testing, each isolate was subcultured onto Sabouraud dextrose agar. C. glabrata ATCC 36909 reference strain and two well-characterized C. glabrata isolates, DSY562 (a susceptible isolate) and DSY565 (a resistant isolate),11 were included as controls. Antifungal susceptibility testing was performed by the broth microdilution method in RPMI with 2% glucose according to the M27-A2 document of the Clinical and Laboratory Standards Institute (CLSI, formerly the NCCLS).12 The inoculum suspensions were adjusted spectrophotometrically at 530 nm to match the concentration of 1.5 ± 1.0 x 103 cells/mL. Stock solutions of caspofungin (Merck & Co., Whitehouse Station, PA, USA), fluconazole (Pfizer, Inc., New York, NJ, USA), itraconazole (Janssen, Beerse, Belgium), ketoconazole (Sigma Aldrich, Milan, Italy) and voriconazole (Pfizer) were prepared in water (caspofungin and fluconazole) or dimethyl sulphoxide (itraconazole, ketoconazole and voriconazole). The final concentrations of the antifungal agents were 0.008–8 mg/L for caspofungin, itraconazole, ketoconazole and voriconazole and 0.125–128 mg/L for fluconazole. The trays were incubated at 35°C, and MIC end points were read at 48 h. All tests were carried out in duplicate. For caspofungin, the MIC end point was defined as the lowest concentration determining complete inhibition of growth. This end point was chosen based on the fungicidal activity of caspofungin as already described.9 For azoles, the MIC end point was defined as the lowest concentration that produced a prominent decrease in turbidity (80% inhibition) compared with that of the growth control. The interpretive criteria used for fluconazole, itraconazole and voriconazole were those published by the CLSI.12,13 Interpretive breakpoints have not yet been established for either ketoconazole or voriconazole, but we used susceptibility breakpoints of 0.125 and 1 mg/L, respectively.6 The CLSI quality control strains Candida krusei ATCC 6258 and Candida parapsilosis ATCC 22019 were tested and MICs were within the expected range.
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Rhodamine 6G accumulation assay
The rhodamine 6G accumulation assay was performed as described previously.11,14 Following growth overnight in YEPD (1% yeast extract, 2% peptone, 2% glucose) liquid medium at 30°C, yeast cells were diluted 100-fold in 5 mL of the same medium. The culture was grown to reach a cell density of 
107 cells/mL, then placed at 4°C and divided into 1 mL aliquots. Labelling of the cells (107) with 10 µM rhodamine 6G (Società Italiana Chimici, Rome, Italy) was performed under constant shaking for 30 min at 30°C, then stopped by placing reaction tubes on ice. The labelled cells were diluted 40-fold in ice-cold 0.1 M phosphate-buffered saline (PBS) at pH 7.0 and then subjected to flow cytometry using a Coulter EPICS XL instrument (Beckman Coulter, Brea, CA, USA). Experiments were repeated two times.
Expression levels of ABC transporter genes
Quantitative expression of the CgCDR1 and CgCDR2 genes was performed by real-time RT–PCR analysis with an i-Cycler iQ system (Bio-Rad Laboratories, Hercules, CA, USA), following a previously described protocol.6 Relative quantification of the target gene was carried out by using a primer pair and a Taqman probe in combination with primers and a probe specific for the URA3 reference gene. Each reaction was performed in triplicate. For each isolate, fold increases in gene expression were calculated from the mean normalized expression relative to the mean normalized expression of the DSY562 isolate, used as a susceptible control.15 Genes with a 
CT value, calculated as CT [test gene]–CT [URA3], that fell over the 3-SD range measured in the azole-susceptible isolates were considered overexpressed, as reported previously.16
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Results and discussion |
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Table 1 summarizes the in vitro susceptibility of 80 C. glabrata clinical isolates to caspofungin, itraconazole, ketoconazole and voriconazole stratified by fluconazole susceptibility category. With respect to fluconazole MIC, the isolates fell into the following groups: susceptible (15 isolates), S-DD (15 isolates) and resistant (50 isolates). All fluconazole-resistant isolates were also cross-resistant to the other azoles tested (Table 1), and this phenotype was found to be associated with the overexpression of drug efflux pump-encoding CgCDR genes. The expression of CgCDR1 and CgCDR2 was strongly increased in the majority of the 50 azole-resistant isolates compared with that of the susceptible control isolate DSY562, but not in the 15 azole-susceptible isolates studied (Figure 1). Of the resistant isolates 46 exhibited 12.4- to 483.0-fold up-regulation of the ABC transporter gene CgCDR1. Notably, 27 of these 46 also exhibited 3.9- to 70.6-fold up-regulation of CgCDR2, with the remaining 4 isolates showing 26.1- to 61.2-fold up-regulation of this second ABC transporter gene. The 15 fluconazole-S-DD isolates up-regulated CgCDR1 or CgCDR2 (Figure 1), but the levels of up-regulation were significantly less than those by the resistant isolates studied (109.9- to 5.5-fold for CgCDR1 and 17.4- to 2.6-fold for CgCDR2). Furthermore, the increased expression of efflux pumps in the fluconazole-resistant and fluconazole-S-DD isolates was also accompanied by decreased accumulation of rhodamine 6G, as shown by their mean fluorescence values, which differed from those of the susceptible isolates by 96.1- and 37.0-fold, respectively (Figure 1). In contrast, caspofungin was very active in vitro against all the fluconazole-resistant and fluconazole-S-DD isolates and, as expected, against the fluconazole-susceptible isolates. MIC90 (MIC at which 90% of the isolates were inhibited) of fluconazole-resistant isolates was 1 mg/L. MIC90s of fluconazole-S-DD isolates and fluconazole-susceptible isolates were 0.25 and 0.125 mg/L, respectively. For all the isolates, the MIC of caspofungin was
1 mg/L. Low MICs (0.06 mg/L) of caspofungin were also obtained for the C. glabrata reference strain, ATCC 36909, and two well-characterized C. glabrata isolates, DSY562 and DSY565.
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Cross-resistance within a class of antimicrobial agents is an important issue, especially if therapeutic options are limited by a poor number of available compounds as for the antifungal agents. Although the new systemic antifungal triazoles (ravuconazole and voriconazole) are more active in vitro than fluconazole and itraconazole against Candida spp.,17 cross-resistance between fluconazole and ravuconazole has been observed, so that fluconazole was recently proposed as a surrogate marker to predict the susceptibility of Candida spp. to ravuconazole.18 The cross-resistance involving ravuconazole,18 like that involving voriconazole,4,6 applies most directly to fluconazole-resistant C. glabrata. As stated by Sobel,7 C. glabrata can be considered the Achilles heel of all available azoles, thus C. glabrata infections treated with these drugs are difficult to eradicate.
Our results showed that caspofungin had excellent activity against fluconazole (multiazole)-resistant isolates, as well as against isolates susceptible and dose-dependent susceptible to fluconazole. This is not surprising considering that the echinocandin caspofungin belongs to a class of antifungal agents that act by inhibiting the cell wall glucan synthesis. Interestingly, fluconazole-resistant isolates showed a higher MIC90 value compared with fluconazole-S-DD or fluconazole-susceptible isolates. This difference indicates that azole resistance can contribute to an increase in caspofungin MIC. The basis for such a difference is still unknown.
In conclusion, the present study confirms the efficacy of caspofungin against clinical isolates of C. glabrata, but, for the first time, the potent activity of this drug was assessed with a large number of C. glabrata isolates whose cross-azole resistance was well documented by molecular methods. Furthermore, these findings support the use of caspofungin to treat those C. glabrata infections for which amphotericin B represents the sole therapeutic choice.
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None to declare.
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Acknowledgements |
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This work was supported by grants from the Istituto Superiore di Sanità (National Program of Research on AIDS, No. 50 F.31) and from the Università Cattolica del S. Cuore (Fondi Ateneo Linea D1-2005).
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References |
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