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Antimicrobial Agents and Chemotherapy, April 2008, p. 1529-1532, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01097-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

In Vitro Interactions of Micafungin with Amphotericin B against Clinical Isolates of Candida spp.{triangledown}

Carolina Serena,1 Marçal Mariné,1 Guillermo Quindós,2 Alfonso J. Carrillo,3 J. F. Cano,1 F. Javier Pastor,1 and Josep Guarro1*

Unitat de Microbiologia, Facultat de Medicina i Ciències de la Salut, Universitat Rovira i Virgili, Reus, Spain,1 Departamento de Inmunología, Microbiología y Parasitología, Facultad de Medicina y Odontología, Universidad del País Vasco, Bilbao, Spain,2 Departamento de Microbiología, Asesoría Científica y de Investigación Aplicada, Barcelona, Spain3

Received 21 August 2007/ Returned for modification 10 November 2007/ Accepted 20 January 2008


   ABSTRACT
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The in vitro activity of amphotericin B in combination with micafungin was evaluated against 115 isolates representing seven species of Candida. Overall, the percentages of synergistic interactions were 50% and 20% when the MIC-2 (lowest drug concentration to cause a prominent reduction in growth) and MIC-0 (lowest drug concentration to cause 100% growth inhibition) end point criteria, respectively, were used. Antagonism was not observed. Some of the interactions were confirmed by time-kill assays.


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Candida spp. are an important cause of nosocomial infections with high morbidity and mortality (13). The spectrum of invasive candidiasis is changing, and the incidence of infections due to non-C. albicans species is rising (13). Amphotericin B (AMB) is one of the antifungal agents most commonly used to treat most invasive mycoses, but its toxicity limits its use. Fluconazole is commonly used for the treatment of candidiasis, but its activity against non-C. albicans species, especially C. glabrata, is variable, and it has no activity against C. krusei (18, 19). Echinocandins constitute a new and unique class, in that they inhibit synthesis of a distinct major cell wall component, β-(1,3)-D-glucan, resulting in morphological changes to the cell wall. Micafungin (MFG) is a promising echinocandin that was recently approved by the FDA and has demonstrated activity against Candida species, although it has shown high MICs (>8 µg/ml) against isolates of some species, such as C. parapsilosis (1, 6). A combination of MFG with AMB, two drugs with different targets, could be of interest for improving clinical results, shortening the duration of treatment, and reducing toxic drug doses, which is especially important for AMB. This combination has been synergistic in vitro against Cryptococcus spp., Rhodotorula glutinis, Trichosporon asahii, and Scedosporium spp. (15, 16, 21) and showed efficacy for the treatment of murine disseminated infection with Trichosporon asahii, C. glabrata, and Aspergillus spp. (3, 8, 12, 17). Moreover, the combination of other echinocandins, e.g., caspofungin, with AMB has shown positive in vitro and in vivo interactions against C. parapsilosis and C. glabrata (2, 12). We considered it interesting to evaluate whether this combination could also be beneficial for the treatment of infections by Candida spp., and we tested its activity against isolates representing seven of the most common species.

We tested a total of 145 clinical isolates (C. albicans [n = 35], C. dubliniensis [n = 20], C. glabrata [n = 15], C. krusei [n = 35], C. lusitaniae [n = 10], C. parapsilosis [n = 15], and C. tropicalis [n = 15]). In order to find out which species all the strains received as C. parapsilosis complex belonged to, the internal transcribed spacer 1 (ITS1) adjacent to the 5.8S rRNA gene was amplified and sequenced using primers ITS5 and ITS2 (20). All the sequences obtained showed 100% similarity with the sequence of the type strain of C. parapsilosis (CBS 604; GenBank accession number AJ635316). Drug interactions were assessed by a checkerboard microdilution method after 48 h of incubation at 35°C (4, 11). The drugs were obtained as pure powders. AMB (USP, Rockville, MD) was diluted in dimethyl sulfoxide and MFG (Astellas Pharma Inc., Tokyo, Japan) in sterile distilled water. The final dimethyl sulfoxide concentration was 1%. Antifungal agents were placed in rows or in columns in the trays, with concentrations ranging from 4 to 0.06 µg/ml for AMB and from 16 to 0.03 µg/ml for MFG. One of the limitations of the checkerboard method for some antifungal combinations is deciding which end point has to be used to evaluate interactions between the drugs (10). The recommended end points for AMB and MFG are different, i.e., the lowest drug concentration to show 100% growth inhibition (MIC-0) for the former (11) and a prominent reduction in growth (MIC-2) for the latter (14); however, the end points used in the checkerboard procedure should be the same for the two drugs tested. Since it is difficult to decide which of the two end points should be used, we used both for the two drugs when tested alone and in combination. We used the fractional inhibitory concentration index to quantify and classify drug interaction (4). Interaction was considered synergistic if the fractional inhibitory concentration index was ≤0.5, indifferent if it was >0.5 and ≤4, and antagonistic if it was >4. The procedure, conservation of the strains, and quality controls have all been detailed previously (15). Approximately 80% of the tests were repeated, and the results showed the same tendencies (data not shown). However, when the results did not coincide, the test was repeated and the mode of the three MICs was considered. The MICs were compared using the Mann-Whitney U test. Calculations were made using Graph Pad 4.0 and SPSS version 14.0 for Windows.

In order to confirm the results obtained with the checkerboard method, time-kill studies were performed using two strains (one that showed indifference and one that showed synergism by the first methodology at the MIC-2 end point) belonging to the three most common species (C. albicans FMR 9600 and FMR 9542, C. glabrata FMR 8498 and FMR 8489, and C. parapsilosis FMR 9601 and FMR 9544). Both AMB and MFG were used at 1x MIC and 2x MIC. The numbers of CFU were determined at 0, 2, 4, 6, and 24 h. The limit of detection was 20 CFU/ml. Synergism and antagonism were defined, respectively, as an increase or decrease of ≥2 log10 CFU/ml in antifungal activity compared with the most active single agent, while a change of <2 log10 CFU/ml was considered indifferent (4).

Table 1 shows the in vitro results using the MIC-0 or MIC-2 as the end point for both drugs. The MIC-0 and MIC-2 of MFG were significantly different for all the species (P < 0.05) except C. albicans (P = 0.09), C. glabrata (P = 0.12), and C. lusitaniae (P = 0.19). The geometric mean MIC (MIC-2) of MFG was lower than 1 µg/ml against all species tested with the exception of C. parapsilosis, which agrees with results reported by other authors, who also obtained high MFG MICs for this species (1, 6). Differences between MIC-0 and MIC-2 were lower for AMB, being significant only for C. parapsilosis, C. tropicalis, and C. dubliniensis (P < 0.0001, P = 0.0078, and P = 0.009, respectively). For every species there was more synergism when MIC-2 values rather than MIC-0 values were used as end points. Using the MIC-2, synergistic interactions were obtained for 52% (75/145) of the isolates, while using the MIC-0, synergistic interactions were obtained for only 22% (32/145) of them. This combination could potentially be more useful in the case of those species with strains that are resistant to both antifungals, such as C. parapsilosis. Although this species mainly showed very high MFG MICs, these were reduced significantly when the two drugs were combined (P < 0.0001), showing 40 to 60% synergistic interactions. The efficacy of the combination of AMB with one echinocandin, caspofungin, has already been proved in a murine model of disseminated infection with C. parapsilosis (2).


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  		TABLE 1. In vitro activities of MFG and AMB, alone and in combination, against clinical isolates of Candida spp. as determined using the MIC-0 or MIC-2 end point

 
One hundred percent agreement was obtained between the checkerboard and time-kill (at both concentrations) procedures for the six strains tested. Figure 1 shows the time-kill curves for the three strains that showed synergism or indifference at 1x MIC.


Figure 1
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  		FIG. 1. Time-kill studies conducted at 1x MIC for C. albicans, C. parapsilosis, and C. glabrata. Synergistic interactions were observed for C. albicans FMR 9542 (A), C. parapsilosis FMR 9544 (C), and C. glabrata FMR 8489 (E), and indifferent interactions were observed for C. albicans FMR 9600 (B), C. parapsilosis FMR 9601 (D), and C. glabrata FMR 8498 (F).

 
In summary, we have demonstrated that the combination MFG and AMB has some synergistic effect against Candida species, although with important differences between the two reading criteria used. Further studies with animal models are warranted to determine the efficacy of this combination and to determine which reading criterion is more predictive for clinical outcome.


   ACKNOWLEDGMENTS
 
This work was supported by a grant from Fondo de Investigaciones Sanitarias from the Ministerio de Sanidad y Consumo of Spain (PI 050031).


   FOOTNOTES
 
* Corresponding author. Mailing address: Unitat de Microbiologia, Facultat de Medicina, Universitat Rovira i Virgili, Carrer Sant Llorenç, 21.43201 Reus, Spain. Phone: 977-759359. Fax: 977-759322. E-mail: josep.guarro{at}urv.cat Back

{triangledown} Published ahead of print on 28 January 2008. Back


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Antimicrobial Agents and Chemotherapy, April 2008, p. 1529-1532, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01097-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.




This Article
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