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Antimicrobial Agents and Chemotherapy, April 2008, p. 1396-1400, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01512-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

In Vitro Antifungal Activities of Isavuconazole (BAL4815), Voriconazole, and Fluconazole against 1,007 Isolates of Zygomycete, Candida, Aspergillus, Fusarium, and Scedosporium Species{triangledown}

Jesús Guinea,1,2* Teresa Peláez,1,2 Sandra Recio,1 Marta Torres-Narbona,1 and Emilio Bouza1,2

Clinical Microbiology and Infectious Diseases Department, Hospital General Universitario Gregorio Marañón, Universidad Complutense de Madrid, Madrid, Spain,1 CIBER de Enfermedades Respiratorias (CIBER RES CD06/06/0058)2

Received 22 November 2007/ Returned for modification 29 December 2007/ Accepted 12 January 2008


   ABSTRACT
Top
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Isavuconazole (BAL4815) is a promising novel broad-spectrum triazole in late-stage clinical development that has proven active in vitro against Aspergillus and Candida species. We compared the in vitro activities of this agent with those of voriconazole and fluconazole by the CLSI (formerly NCCLS) M38-A and M27-A2 procedures against a large collection of 1,007 relevant opportunistic fungi collected from 1986 to 2007: Aspergillus spp. (n = 702), Candida spp. (n = 218), Zygomycetes (n = 45), Scedosporium spp. (n = 22), and Fusarium spp. (n = 20). All Candida isolates were from patients with candidemia. For isavuconazole, these techniques were also compared with the Etest. Isavuconazole and voriconazole had MICs at which 50% and 90% of isolates were inhibited (MIC50 and MIC90), respectively, of 1 and 1 µg/ml and 0.5 and 1 µg/ml against Aspergillus spp. and of 0.015 and 0.03 µg/ml and 0.25 and 0.125 µg/ml against Candida spp. (including fluconazole-resistant strains). The MIC50 partial and complete inhibition end points of isavuconazole and voriconazole against the non-Aspergillus molds were as follows: 1 and 2 µg/ml and 16 and >16 µg/ml against Zygomycetes; 1 and 4 µg/ml and 0.25 and 0.5 µg/ml against Scedosporium apiospermum; 4 to 16 and >16 µg/ml and 4 to 8 and 16 to >16 µg/ml (ranges) against Scedosporium prolificans; and 16 and 16 µg/ml and 4 and 4 µg/ml against Fusarium spp. Isavuconazole showed minimal fungicidal concentrations for 50% and 90% of the isolates of 1 and 1 µg/ml against Aspergillus, 16 and >16 µg/ml against Candida, and 4 and >16 µg/ml against Zygomycetes, respectively, and >16 µg/ml against the remaining molds. The Etest proved to be a suitable alternative method for determining the antifungal activities of isavuconazole against Aspergillus and Candida; the Etest results showed 96% and 93% agreement with the results of the CLSI M38-A and M27-A2 methods, respectively.


   INTRODUCTION
Top
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Invasive fungal infections (IFIs) are an important and increasing cause of morbidity and mortality in immunocompromised populations (6, 7). Despite correct antifungal treatment with available agents, the mortality rate of patients with IFIs remains extremely high.

Isavuconazole (formerly known as BAL4815) is a novel and promising broad-spectrum triazole in late-stage clinical development for the treatment of invasive aspergillosis. It is the active antifungal component of the water-soluble prodrug BAL8557, which can be administered intravenously and orally, and in preclinical studies it has demonstrated good pharmacokinetic parameters and low toxicity (11, 12).

Isavuconazole has proven active in vitro against Aspergillus and Candida spp. (13, 17). In an experimental neutropenic murine model of disseminated Aspergillus flavus infection, it showed high survival rates (19). However, in vitro studies with large numbers of Aspergillus and other mold isolates are necessary.

We compared the in vitro activities of isavuconazole with those of voriconazole and fluconazole against a large collection of clinically relevant opportunistic fungi. In addition, the results obtained by the Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS) M38-A procedure, the CLSI M27-A2 procedure, and the Etest for isavuconazole were compared.

(This study was presented in part at the 3rd TIMM [Trends in Medical Mycology] Conference in Turin, Italy, in 2007 [poster no. P-026].)


   MATERIALS AND METHODS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Organisms and source of samples. We analyzed a collection of 1,007 environmental and clinically opportunistic fungi collected from 1986 to 2007. The collection comprised 315 environmental mold isolates from outdoor air in the province of Madrid (n = 257), hospital air (n = 47), and other sources (n = 11), as well as 692 clinical isolates from 534 different patients in our hospital. The clinical sources of the isolates were abscesses/normally sterile fluids (n = 23), biopsy specimens (n = 32), wounds (n = 40), respiratory samples (n = 339), blood samples (n = 222), skin and soft tissue samples (n = 19), and other samples (n = 17).

The isolates studied belonged to Aspergillus spp. (n = 702), Candida spp. (n = 218), Zygomycetes (n = 45), Scedosporium spp. (n = 22), and Fusarium spp. (n = 20). All Candida isolates were from patients with candidemia. Mold isolates were from the environment or from patients who had IFIs or were colonized.

All strains were cultured in Sabouraud dextrose agar, sheep blood agar, or Bactec medium and were identified by conventional morphological, chromogenic, and/or biochemical procedures. They were stored as spore or yeast suspensions in a solution of sterile distilled water at –70°C. To ensure viability and purity, each isolate was subcultured on potato dextrose agar or Sabouraud dextrose agar before being tested.

Antifungal agents. The antifungal drugs used in the study and obtained as reagent-grade powders from their respective manufacturers were isavuconazole (BAL4815) (Basilea Pharmaceutica, Basel, Switzerland), voriconazole (Pfizer Pharmaceutical Group, New York, NY), and fluconazole (Pfizer Pharmaceutical Group). The activities of isavuconazole and voriconazole were determined against all isolates and that of fluconazole only against Candida spp. Antifungal activities were determined using the CLSI M38-A and M27-A2 broth microdilution methods for molds and Candida spp., respectively. We also studied the antifungal activity of isavuconazole using the Etest.

CLSI M38-A and M27-A2 microdilution procedures. Stock solutions of isavuconazole, voriconazole, and fluconazole were prepared in dimethyl sulfoxide (Sigma, Madrid, Spain). Trays containing a 0.1-ml aliquot of the appropriate drug solution (2 x final concentration) in each well were first subjected to quality control and then sealed and stored at –70°C until use. The final concentrations of drug in the wells ranged from 0.015 µg/ml to 16 µg/ml for isavuconazole and voriconazole and from 0.125 µg/ml to 128 µg/ml for fluconazole.

All the inoculated microdilution trays were incubated at 35°C and read macroscopically at 24 h (for Zygomycetes) and 48 h (for the remaining molds and Candida spp.). According to the CLSI procedures, the MIC end point for isavuconazole and voriconazole with Aspergillus spp. was defined as the lowest concentration that produced complete inhibition of growth (MIC-0). For Fusarium spp., Scedosporium spp., and Zygomycetes, we also calculated MIC-1, defined as the lowest concentration that produced slight growth or approximately 25% that of the growth control (1). The MIC end point for azoles and Candida spp. was defined as the lowest concentration at which a prominent decrease in turbidity, corresponding to approximately 50% inhibition of growth, was observed (MIC-2) after 48 h of incubation (2).

We also calculated the minimum fungicidal concentration (MFC) for isavuconazole. For each isolate, 100 µl was removed from all wells with no visible fungal growth. Each aliquot was spot inoculated onto Sabouraud dextrose agar plates; the liquid was allowed to soak into the agar; and the plate was streaked. The plates were incubated at 35°C and read after 24 h (with confirmation after 48 h). The MFC was defined as the lowest concentration that killed 99% of spores or yeast cells present in the original inoculum in each well (50 or fewer colonies of molds and 2 colonies of yeasts), according to previous studies (17-19).

Etest method. Etest strips (AB Biodisk, Solna, Sweden) of isavuconazole were supplied by Basilea Pharmaceutica. The inocula of spores and yeast cells were the same as those used for the microdilution procedure before the 50-fold dilution. A swab was dipped into the cell suspensions and streaked across the surfaces of RPMI agar plates with 2% glucose. The plates were incubated at 35°C and read at 24 h (for Zygomycetes) and 48 h (for the remaining molds and Candida spp.).

The MIC was determined by the Etest in accordance with the manufacturer's instructions. It was defined as the lowest drug concentration at which the border of the elliptical inhibition zone intercepted the scale on the antifungal strip.

Quality control. Quality control was ensured by testing the following strains: A. flavus ATCC 204304, Aspergillus fumigatus ATCC 204305, Candida krusei ATCC 6258, and Candida parapsilosis ATCC 22019. All results were within the recommended CLSI limits.

Data analysis. The activities of isavuconazole, voriconazole, and fluconazole were expressed as geometric means, MICs at which 90% and 50% of isolates were inhibited (MIC90 and MIC50, respectively), MFCs at which 90% and 50% of spores or yeast cells were killed (MFC90 and MFC50, respectively), and ranges of MICs and MFCs.

For Aspergillus spp. and the remaining molds, no breakpoints for the new triazoles have been established. For Candida spp., the classification of the strains for voriconazole was as follows: susceptible, breakpoint of ≤1 µg/ml; susceptible-dose dependent, 2 µg/ml; resistant, ≥4 µg/ml (9). The classification for fluconazole was as follows: susceptible, breakpoint of ≤8 µg/ml; susceptible-dose dependent, 16 to 32 µg/ml; resistant, ≥64 µg/ml.

Because the Etest strips contain a continuous gradient of isavuconazole instead of the established twofold drug dilutions, the MIC end point obtained by the Etest was raised to the next twofold dilution concentration matching the drug dilution on the scale used for the CLSI procedure. The CLSI and corrected Etest MIC distributions for isavuconazole against each clinical isolate were converted to log MICs. We compared the results of the Etest at 48 h of incubation with the CLSI results. MIC end point discrepancies of no more than ±2-fold dilutions were used to calculate the percentage of agreement between the two methods according to previous studies (4, 8).


   RESULTS
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Antifungal activity. For all 1,007 strains, the in vitro activity of each antifungal drug, expressed as the geometric mean, MIC90 and MIC50, range, and MFC90 and MFC50 (in micrograms per milliliter) is shown in Tables 1 to 4.


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  		TABLE 1. In vitro activities of voriconazole and isavuconazole against Aspergillus spp. by the CLSI M38-A procedurea

 

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  		TABLE 4. In vitro activities of isavuconazole (BAL4815) against all fungi, determined by the Etesta

 
Table 1 shows the activities of isavuconazole and voriconazole against the Aspergillus sp. isolates. Both drugs showed MIC90s of 1 µg/ml against Aspergillus spp. Although there were no significant differences in the activity of isavuconazole between different species of Aspergillus, isavuconazole tended to show slightly higher MICs (onefold dilution) against A. niger. Isavuconazole and voriconazole showed antifungal activity against A. terreus, a well-known amphotericin B-resistant species. Only for one A. nidulans strain was the voriconazole MIC 8 µg/ml, but the isavuconazole MIC for that strain was 1 µg/ml. For the remaining strains of the most clinically relevant Aspergillus species, voriconazole and isavuconazole MICs were ≤4 µg/ml. Isavuconazole also presented an equivalent MFC90 and MIC90 against Aspergillus spp.

Table 2 shows the activities of voriconazole and isavuconazole against the non-Aspergillus molds. Zygomycetes showed poor susceptibility to voriconazole, with a MIC90 of ≥16 µg/ml, independently of the MIC end point chosen (MIC-0 or MIC-1). In contrast, isavuconazole presented a limited antifungal effect against Zygomycetes, especially when the end point used was MIC-1 (MIC90 and MIC50, 8 µg/ml and 1 µg/ml, respectively). The two species of Scedosporium tested showed marked differences in susceptibility: whereas voriconazole and isavuconazole showed poor activity against Scedosporium prolificans, Scedosporium apiospermum showed more susceptibility to both agents, with a MIC90 very similar to those for Aspergillus spp. when MIC-1 was the end point. The MFC90 of isavuconazole against S. apiospermum was dramatically higher than that observed against Aspergillus spp. For Fusarium spp., which are also resistant to several antifungals, the geometric mean MIC of voriconazole (7.72 µg/ml) was lower than that of isavuconazole (16.56 µg/ml).


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  		TABLE 2. In vitro activities of voriconazole and isavuconazole against Fusarium spp., Scedosporium spp., and Zygomycetes by the CLSI M38-A procedurea

 
Table 3 shows the activities of voriconazole, isavuconazole, and fluconazole against Candida spp. Isavuconazole MIC90s were low for fluconazole-susceptible isolates of C. albicans, C. parapsilosis, and C. tropicalis. For the fluconazole-resistant species C. krusei, isavuconazole MICs were low, although the number of strains included was limited. The MIC90s of the three azole derivatives for C. glabrata, regarded as a fluconazole-susceptible-dose-dependent species, were higher than those for the other Candida species evaluated.


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  		TABLE 3. In vitro activities of voriconazole, isavuconazole, and fluconazole against Candida spp. by the CLSI M27-A2 procedurea

 
Of all the Candida strains studied, 186 (85.32%) were susceptible to fluconazole, 22 (10.01%) were susceptible-dose dependent (including the 5 strains of the inherently fluconazole resistant species C. krusei), and 10 (4.59%) were resistant (6 C. albicans and 4 C. glabrata strains). No strains were resistant to voriconazole, and only for one strain of C. glabrata was the isavuconazole MIC 2 µg/ml. Two strains (1%) of C. glabrata were susceptible-dose dependent to voriconazole (with isavuconazole MICs of 1 µg/ml and 2 µg/ml, respectively). These two strains were fluconazole resistant (MIC, 128 µg/ml). Overall, the isavuconazole MIC for 99.5% of the Candida isolates was ≤1 µg/ml.

Table 4 shows the activities of isavuconazole determined by the Etest. For Candida and Aspergillus spp., the MIC90 obtained by the Etest tended to be slightly lower than that obtained by the CLSI procedure (approximately a onefold dilution). In contrast, the MICs of isavuconazole for Zygomycetes by the Etest were higher than those obtained by broth microdilution. S. prolificans and Fusarium spp. also proved to be less susceptible to isavuconazole by this method. However, wide discrepancies were found for S. apiospermum: the Etest value and CLSI MIC90 were >32 µg/ml and 4 µg/ml, respectively. For Candida spp., the two procedures yielded very similar activities for all the species evaluated.

Agreement between the CLSI M38-A procedure, the CLSI M27-A2 procedure, and the Etest for isavuconazole. To determine the levels of agreement between the "gold standard" procedures (CLSI M38-A and CLSI M27-A2) and the Etest, we calculated the percentage of strains for which the isavuconazole MICs by Etest differed by ± 1, ±2, and >±3 dilutions from those obtained by the gold standard. These results are summarized in Table 5. For Aspergillus spp., the correlation was very good, with an overall agreement above 96% (±2 dilutions) for all clinically relevant species. The correlation for Zygomycetes was moderate but variable between the different genera of the group. Isavuconazole showed poor antifungal activity against S. prolificans by both methods (agreement, 100% [±2 dilutions]), although there was a high percentage of disagreement for S. apiospermum. Isavuconazole showed poor activity against Fusarium spp. by both procedures. For Candida spp., the Etest correlated very well with the CLSI M27-A2 procedure (±2 dilutions), especially for C. albicans and C. parapsilosis, the most common etiological agents of candidemia.


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  		TABLE 5. Number of strains for which the MICs of isavuconazole (BAL4815) by Etest differed by ± 1, ±2, and >±3 log dilutions from those obtained by the CLSI M38-A and M27-A2 procedures

 
We detected only one strain of C. glabrata for which the isavuconazole MIC was 2 µg/ml by the CLSI M27-A2 procedure but 6 µg/ml by the Etest.


   DISCUSSION
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our study showed that isavuconazole was active against Aspergillus and Candida spp. and that there were high levels of agreement between both CLSI procedures and the Etest.

Previous series have shown good in vitro activities of voriconazole against Aspergillus spp, but the data for isavuconazole are still limited (3, 5, 10, 15, 17). Isavuconazole and voriconazole proved to be active against Aspergillus spp. in the present study. In addition, the MFC of isavuconazole was the same as or only onefold higher than the MIC, as reported in recent series (17). Isavuconazole also demonstrated substantial activity against A. terreus, a well-known amphotericin B-resistant species (14, 16). No Aspergillus strains were resistant to voriconazole, but other reports have shown that isavuconazole had in vitro activity against itraconazole-resistant strains (17).

Zygomycetes are known to be resistant to voriconazole in vitro and in vivo. Due to the limited number of drugs active against Zygomycetes and other rare but multiresistant molds, we decided to show the activity of isavuconazole using MIC-0 and MIC-1 as end points. Isavuconazole presented a limited antifungal effect against Zygomycetes.

The differences in the in vitro activities of isavuconazole against S. prolificans and S. apiospermum are similar to those reported for posaconazole (10). S. apiospermum is susceptible to the new triazoles. The activities of isavuconazole and voriconazole were also limited for Fusarium spp. However, due to the limited number of non-Aspergillus mold isolates included, other confirmatory studies are necessary.

Isavuconazole presented good antifungal activity against Candida strains isolated from patients with candidemia, including species inherently resistant to fluconazole (C. krusei) and those with the ability to develop such resistance (C. glabrata). In a recent study, Seifert et al. reported similar in vitro activities of isavuconazole against Candida spp. (13), indicating that antifungal resistance to the new triazoles is infrequent.

The Etest has the potential to become a good alternative to the CLSI M38-A and M27-A2 procedures for determining the antifungal activities of isavuconazole against Aspergillus and Candida spp., with levels of agreement above 96% and 93%, respectively. We found only one strain of C. glabrata with a MIC of 2 µg/ml by the CLSI procedure and 6 µg/ml by the Etest. In the absence of specific breakpoints, the Etest showed the ability to detect the strain of Candida with the highest isavuconazole microdilution MIC included in the study. One shortcoming of our study, however, is the limited number of Aspergillus and Candida strains with reduced susceptibility to isavuconazole. Susceptibility testing for the limited number of molds other than Aspergillus species makes it difficult to reach other conclusions.

In summary, isavuconazole and voriconazole showed good antifungal activities against Candida spp., including fluconazole-resistant strains, and Aspergillus spp. The activity of isavuconazole against the non-Aspergillus mold isolates should be investigated in a series with a larger number of isolates.


   ACKNOWLEDGMENTS
 
We thank Thomas O'Boyle for editing and proofreading this article.

This study does not present any conflicts of interest for its authors.

This study was partially financed by grants from Basilea Pharmaceutica and from the Spanish Social Security Health Investigation Fund (Fondo de Investigación Sanitaria) FIS PI070198 (Instituto de Salud Carlos III). Jesús Guinea is contracted by the Fondo de Investigación Sanitaria (FIS) CM05/00171.


   FOOTNOTES
 
* Corresponding author. Mailing address: Servicio de Microbiología Clínica y Enfermedades Infecciosas, Hospital General Universitario Gregorio Marañón, C/Dr. Esquerdo, 46, 28007 Madrid, Spain. Phone: 34 914265104. Fax: 34 915044906. E-mail: jguineaortega{at}yahoo.es Back

{triangledown} Published ahead of print on 22 January 2008. Back


   REFERENCES
Top
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Clinical and Laboratory Standards Institute. 2002. Reference method for broth dilution antifungal susceptibility testing of filamentous fungi. Approved standard. CLSI document M38-A. Clinical and Laboratory Standards Institute, Wayne, PA.
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Antimicrobial Agents and Chemotherapy, April 2008, p. 1396-1400, Vol. 52, No. 4
0066-4804/08/$08.00+0     doi:10.1128/AAC.01512-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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