E-test method for testing susceptibilities of Aspergillus spp. to the new triazoles voriconazole and posaconazole and to established antifungal agents: comparison with NCCLS broth microdilution method

Evaluation of disk diffusion method for determining posaconazole susceptibility of filamentous fungi: comparison with CLSI broth microdilution method

The disk diffusion method was evaluated for determining posaconazole susceptibility against 78 strains of molds using two culture media in comparison with the CLSI (Clinical Laboratory Standards Institute) broth microdilution method (M38-A). A significant correlation between disk diffusion and microdilution methods was observed with both culture media.

Lovastatin has significant activity against zygomycetes and interacts synergistically with voriconazole

Zygomycetes are emerging opportunistic molds resistant to most conventional antifungals. We evaluated the in vitro activity of lovastatin (LOV), a 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, against seven clinical isolates of Zygomycetes by using standard microdilution methods in three different media, disk diffusion testing, and viability dye staining. To further study the in vivo efficacy of LOV against zygomycetes, we developed a Drosophila melanogaster model of zygomycosis. In different experiments, groups of Toll-deficient (Tl) flies fed LOV-containing food were subsequently injected with two representative Zygomycetes isolates (Mucor and Rhizopus spp.). Finally, we examined the effects of LOV on voriconazole (VRC) activity against zygomycetes in vitro by checkerboard dilution, Epsilometer test-based methods, and bis-(1,3-dibutylbarbituric acid) trimethine oxonol staining and in vivo in Tl flies fed food containing LOV plus VRC and infected with zygomycetes. LOV exhibited significant, medium, and strain-independent fungicidal activity against all Zygomycetes isolates in vitro by all testing methods (MIC50, 48.0 microg/ml; 50% minimal fungicidal concentration, 56.0 microg/ml; 50% effective concentration, 29.4 microg/ml [6.6 to 38.9 microg/ml]). Tl flies fed LOV-containing food and infected with Mucor had a significantly better 6-day survival rate than did infected Tl flies fed regular food (P = 0.0005). LOV displayed in vitro synergy with VRC against all Zygomycetes isolates (fractional inhibitory concentration index, 0.104 to 0.290) by all methods used. LOV also displayed synergy with VRC in the Drosophila model of zygomycosis (P < 0.01). LOV is significantly active against zygomycetes and synergizes with triazoles inherently resistant to them, such as VRC. The clinical significance of these findings needs to be further explored.

Voriconazole-resistant disseminated Paecilomyces variotii infection in a neutropenic patient with leukaemia on voriconazole prophylaxis

Paecilomyces variotii, an emerging hyalohyphomycetes, has been reported to be susceptible in vitro to voriconazole. We describe a case of disseminated P. variotii infection in a neutropenic child with relapsed leukaemia who was on voriconazole prophylaxis. The P. variotii isolate was resistant to voriconazole in vitro. The patient responded to liposomal amphotericin B.

Increased frequency of non-fumigatus Aspergillus species in amphotericin B– or triazole–pre-exposed cancer patients with positive cultures for aspergilli

Invasive aspergillosis (IA) can occur despite prior prophylactic or empiric use of triazoles or amphotericin B (AMB). Although profound immunosuppression may account for breakthrough IA, resistance of Aspergillus to antifungals may also play a role. To examine this question, we measured the minimal inhibitory concentration of 105 Aspergillus isolates recovered from 105 cancer patients (64 with IA, 41 with Aspergillus colonization) to AMB, itraconazole (ITC), and voriconazole (VRC) using the National Committee for Clinical Laboratory Standards (NCCLS) M38-A microdilution and E-test methods. We also determined the minimal fungicidal concentration (MFC) of these agents and the minimal effective concentration (MEC) of caspofungin (CAS) using standardized methods. We then collected information regarding pre-exposure to AMB or triazoles (fluconazole, ITC, VRC) within 3 months before Aspergillus isolation. Pre-exposure of cancer patients to AMB or triazoles was associated with increased frequency of non-fumigatus Aspergillus species. Aspergillus isolates recovered from patients who previously received AMB exhibited higher E-test AMB MICs compared with isolates from patients without prior AMB exposure (P = 0.01). In addition, the AMB MICs by E-test were higher in triazole-pre-exposed patients compared with those not exposed to triazoles (P = 0.001). The ITC and VRC MICs by E-test were not affected by prior AMB or triazole exposure. Finally, neither the AMB, ITC, and VRC MICs and MFCs by NCCLS method nor CAS MECs showed such changes. In conclusion, cancer patients with positive Aspergillus cultures who are pre-exposed to AMB or triazoles have high frequency of non-fumigatus Aspergillus species. These Aspergillus isolates were found to be AMB-resistant by the more sensitive E-test method.

In vitro susceptibility testing of Candida and Aspergillus spp. to voriconazole and other antifungal agents using Etest®: results of a French multicentre study

Minimum inhibitory concentrations (MICs) of the antifungal agent voriconazole were determined using the Etest and compared with those of amphotericin B, itraconazole and fluconazole using 1986 clinical isolates of Candida spp. Voriconazole MICs were also compared with those of amphotericin B and itraconazole using 391 clinical isolates of Aspergillus spp. Voriconazole was found to have more potent activity and lower MIC values than amphotericin B, itraconazole and fluconazole against C. albicans, C. tropicalis, C. parapsilosis and C. kefyr. Against C. glabrata and C. krusei, voriconazole was more active than either of the other two azole antifungals but had similar activity to amphotericin B. For species of Aspergillus, MIC values of voriconazole were lower than those of amphotericin B and itraconazole against A. fumigatus and A. flavus, and were similar to those of amphotericin B against A. niger. Against A. terreus, MIC values for voriconazole and itraconazole were similar. A. terreus is known to be resistant to amphotericin B, and this was reflected in higher MIC values compared with those of voriconazole and itraconazole. Voriconazole therefore compares very favourably with other antifungal agents against a large number of clinical isolates of Candida and Aspergillus spp.

Combination treatment of invasive fungal infections

The persistence of high morbidity and mortality from systemic fungal infections despite the availability of novel antifungals points to the need for effective treatment strategies. Treatment of invasive fungal infections is often hampered by drug toxicity, tolerability, and specificity issues, and added complications often arise due to the lack of diagnostic tests and to treatment complexities. Combination therapy has been suggested as a possible approach to improve treatment outcome. In this article, we undertake a historical review of studies of combination therapy and also focus on recent studies involving newly approved antifungal agents. The limitations surrounding antifungal combinations include nonuniform interpretation criteria, inability to predict the likelihood of clinical success, strain variability, and variations in pharmacodynamic/pharmacokinetic properties of antifungals used in combination. The issue of antagonism between polyenes and azoles is beginning to be addressed, but data regarding other drug combinations are not adequate for us to draw definite conclusions. However, recent data have identified potentially useful combinations. Standardization of assay methods and adoption of common interpretive criteria are essential to avoid discrepancies between different in vitro studies. Larger clinical trials are needed to assess whether combination therapy improves survival and treatment outcome in the most seriously debilitated patients afflicted with life-threatening fungal infections.

Comparison of the Sensititre YeastOne colorimetric antifungal panel with a modified NCCLS M38-A method to determine the activity of voriconazole against clinical isolates of Aspergillus spp

The susceptibilities of 63 isolates of Aspergillus spp. to voriconazole were evaluated by a modified NCCLS M38-A method and the Sensititre YeastOne method. The overall agreement was 82.5%, ranging from 100% for Aspergillus niger and Aspergillus terreus to 62.5% for Aspergillus flavus. Discrepancies between the methods were due to higher Sensititre MICs. The Sensititre YeastOne method could have potential value for susceptibility testing of Aspergillus spp. to voriconazole.

Aspergillus fumigatus variant with decreased susceptibility to multiple antifungals

Isolates of Aspergillus fumigatus that demonstrate resistance to itraconazole (ITZ) have been described previously; however, the prevalence and clinical significance of ITZ resistance are not completely understood. In this study we assessed the ITZ susceptibilities of 128 A. fumigatus isolates that caused invasive infection in 82 stem cell transplant patients before and after the use of ITZ in our institution (study period, 1991 to 2000). The MICs for 10 isolates obtained from seven patients were high, > or 1 microg/ml. The average ITZ MIC increased after institutional use of the drug began in 1995. The majority of the isolates for which MICs were high (6 of 10) and one isolate for which the MIC was low (0.06 microg/ml) demonstrated an unusual phenotype, appearing as predominantly white colonies. For all seven atypical isolates, voriconazole MICs were high (> or = 2 microg/ml), and minimal effective concentrations of caspofungin were high (> or 4 microg/ml). For two of the seven atypical isolates, amphotericin B MICs were high (> or 2 microg/ml). The isolates appeared white due to slow sporulation; however, after prolonged incubations, the isolates sporulated with no difference in conidial color or conidiophore morphology compared with typical isolates. Randomly amplified polymorphic DNA-PCR patterns of these isolates were distinct compared with those of other A. fumigatus isolates. Sequencing of 18S rRNA genes confirmed that all were A. fumigatus; however, the mitochondrial cytochrome b gene sequences of all the atypical isolates were unique. These data suggest the potential presence of a genetically unique, poorly sporulating variant of A. fumigatus that demonstrates decreased susceptibilities to several antifungals.

Utility of mould susceptibility testing

PURPOSE OF REVIEW

As new antifungal agents were introduced for the treatment of fungal infections, reliable methods were developed or adapted for the in-vitro susceptibility testing of yeasts and moulds (filamentous fungi). This paper reviews the available methods for antifungal susceptibility testing of moulds as well as the scant data that have been published since 2002 regarding the clinical implications of in-vitro testing.

RECENT FINDINGS

Caspofungin and voriconazole have been recently licensed for treatment of certain mould infections that are usually refractory to treatment with established agents. The introduction of new agents and the frequent reports of mould resistance have underscored the role of the laboratory in patient management and initiated the study of mechanisms of resistance in moulds. In 2002, the US National Committee for Clinical Laboratory Standards M38-P document moved to the approved level of development (M38-A document). Although combination therapy and synergistic studies with the new agents have been documented, very little information is available on in-vitro/in-vivo correlations for moulds.

SUMMARY

The ultimate goal of in-vitro testing is the prediction of the clinical outcome of therapy. The use of National Committee for Clinical Laboratory Standards procedures has led to increased interlaboratory agreement of minimum inhibitory concentrations. Reproducibility of minimum inhibitory concentrations for yeasts has facilitated the establishment of interpretive breakpoints for fluconazole and itraconazole versus Candida spp., but breakpoints are not available for any antifungal against mould species. Although some insights have been documented regarding the potential utility of in-vitro testing for moulds, further documentation of in-vitro versus in-vivo data is needed.

Comparison of Etest with modified broth microdilution method for testing susceptibility of Aspergillus spp. to voriconazole

We compared the Etest with a broth microdilution method, performed according to a modified National Committee for Clinical Laboratory Standards guideline (M38-A), for determining the in vitro susceptibility of 77 isolates of Aspergillus spp. (26 A. fumigatus, 21 A. flavus, 10 A. terreus, 9 A. niger, 5 A. nidulellus, 4 A. glaucus, and 2 A. flavipes isolates). Overall, there was 92.2% agreement between both methods when Etest MICs were read at 24 h and 83.1% agreement when both methods were read at 48 h. When Etest MICs were read at 24 h, the agreement was >90% for all species tested except for A. fumigatus (84.6%). When Etest MICs were read at 48 h, the agreement ranged from 50 to 100%. The poorest agreement was seen with A. glaucus (50%) and A. fumigatus (65%). Where a discrepancy was observed between Etest and the reference method, the Etest MIC was generally higher. The Etest appears to be a suitable alternative procedure for testing the susceptibility of Aspergillus spp. to voriconazole.

In vitro susceptibility testing of filamentous fungi: comparison of Etest and reference M38-A microdilution methods for determining posaconazole MICs

The performance of the Etest for posaconazole susceptibility testing of 72 isolates of filamentous fungi was assessed in comparison with the National Committee for Clinical Laboratory Standards (NCCLS) approved standard broth microdilution method (M38-A). The NCCLS method employed RPMI 1640 broth medium, and MICs were read after incubation for 48h at 35 degrees C. Etest MICs were determined with RPMI agar containing 2% glucose and were read after incubation for 48h at 35 degrees C. The isolates included Aspergillus fumigatus, A. flavus, A. nidulans, A. niger, A. versicolor, A. oryzae, A. terreus, Cladosporium spp., Curvularia sp., Exophiala sp., Fusarium spp., Paecilomyces spp., Pithomyces sp., Penicillium spp. and Scedosporium apiospermum. Overall agreement between Etest and microdilution MICs was 84% for Aspergillus spp. and 100% for the less common opportunistic molds, with the exception of Penicillium spp. (67%). Where a discrepancy was observed between Etest and the reference method, the Etest tended to give lower values. The Etest method using RPMI agar appears to be a useful method for determining posaconazole susceptibilities of filamentous fungi.

In vitro susceptibility testing of Aspergillus spp.: comparison of Etest and reference microdilution methods for determining voriconazole and itraconazole MICs

The performance of the Etest for voriconazole and for itraconazole susceptibility testing of 376 isolates of Aspergillus spp. was assessed in comparison with the National Committee for Clinical Laboratory Standards (NCCLS) proposed standard microdilution broth method. The NCCLS method employed RPMI 1640 broth medium, and MICs were read after incubation for 48 h at 35 degrees C. Etest MICs were determined with RPMI agar containing 2% glucose and were read after incubation for 48 h at 35 degrees C. The isolates included A. fumigatus, A. flavus, A. niger, A. terreus, A. versicolor, A. glaucus, A. nidulans, A. ustus, and A. sydowii. Overall agreement percentages between the Etest and microdilution MICs were 97.6% for voriconazole and 95.8% for itraconazole. Where a discrepancy was observed between Etest and the reference method, the Etest tended to give lower values with voriconazole and higher values with itraconazole. The Etest method using RPMI agar appears to be a useful method for determining the voriconazole and itraconazole susceptibilities of Aspergillus spp.

Laboratory evaluation of new antifungal agents against rare and refractory mycoses

PURPOSE OF REVIEW

An increase in refractory invasive fungal infections in the setting of marrow/solid organ transplantation and other immune-compromising clinical entities has provided the impetus for the development of new, more efficacious/less toxic antifungal agents. This review (1) examines currently available laboratory methods for the in-vitro evaluation of these new agents against both yeasts and filamentous fungi; (2) provides a summary of the most attractive investigational agents currently undergoing clinical trials/development; and (3) outlines the major refractory mycoses in contemporary medicine.

RECENT FINDINGS

Fluconazole-resistant Candida spp., Trichosporon spp., zygomycetous genera, the endemic mycoses, Scedosporium, Aspergillus, and Fusarium spp., and an ever-expanding list of lesser-known hyaline and phaeoid genera inciting invasive fungal infections comprise the bulk of refractory mycoses in the immune-compromised host. In-vitro data generated from reference-based antifungal susceptibility testing methods indicate an increased armamentarium of potentially efficacious agents against most of these mycoses.

SUMMARY

The newly approved antifungal agents caspofungin and voriconazole, used either as monotherapy or in combination regimens, have a significantly improved spectrum of activity over previously available therapeutic options. Correlation of clinical outcomes with investigational agents demonstrating in-vivo/in-vitro activity will provide critical information needed for the development of clinically significant minimum inhibitory concentration interpretative breakpoints.