Antifungal drug resistance in Aspergillus

Effect of medium composition on static and cidal activity of amphotericin B, itraconazole, voriconazole, posaconazole and terbinafine against Aspergillus fumigatus: a multicenter study

The effect of the medium composition on the fungistatic (MIC) and fungicidal (MLC) activity of amphotericin B, itraconazole, voriconazole, posaconazole and terbinafine against four Aspergillus fumigatus strains has been investigated by four European laboratories. MICs were determined by broth microdilution, using RPMI 1640 and Antibiotic Medium 3 (AM3), three times in three independent determinations by the four laboratories. MLCs were determined for the three independent determinations by the four laboratories, subculturing 100 microl from each well showing no visible growth after 48 hours. Except for a 2-dilution difference observed in three cases, no differences were observed between MICs determined on the two media. In contrast, a 3- to 6-dilution discrepancy between the MLCs was observed for the azoles. Endpoints on RPMI were higher than those on AM3. A 1-2 dilution difference was noted between both the endpoints of amphotericin B and of terbinafine. The highest inter- and intra-laboratory agreements were reached on AM3. The azoles showed a medium-dependent fungicidal activity.

Sequencing the Aspergillus fumigatus genome

Aspergillus fumigatus is the most common mould pathogen of human beings and unusually causes both invasive disease in immunocompromised patients and allergic disease in patients with atopic immune systems. 4% of patients dying in modern European teaching hospitals have invasive aspergillosis and it is the leading infectious cause of death in leukaemia and bone marrow transplant patients. Until 2001, only two licensed antifungal drugs were available to treat aspergillosis-amphotericin B and itraconazole. Its 28-30Mb genome is being sequenced in an international collaboration, with the Wellcome Trust Sanger Institute (UK) and The Institute for Genomic Research (TIGR, USA) as the two main centres. A whole-genome shotgun approach was adopted and initiated in 2001 with an expected completion date in 2003. The complete sequence will permit identification of pathways specific to pathogenic Aspergillus species, help identify new targets for antifungal drugs, and enable investigations into the basic biology of fungi. Numerous secondary metabolic pathways with biotechnological applications and pharmacological properties are found in the Aspergilli and the genome sequence will facilitate research in this area.

Antifungal drug resistance of pathogenic fungi

Pathogenic fungi have many complex mechanisms of resistance to antifungal drugs. Information about the clinical, cellular, and molecular factors contributing to antifungal-drug resistance continues to accumulate. We critically review the diagnosis, epidemiology, and mechanisms of antifungal drug resistance of pathogenic fungi. Better understanding of this resistance should assist in developing better detection strategies for preventing and treating refractory mycoses in the future.

Comparison of fractional inhibitory concentration index with response surface modeling for characterization of in vitro interaction of antifungals against itraconazole-susceptible and -resistant Aspergillus fumigatus isolates

Although the fractional inhibitory concentration (FIC) index is most frequently used to define or to describe drug interactions, it has some important disadvantages when used for drugs against filamentous fungi. This includes observer bias in the determination of the MIC and no agreement on the endpoints (MIC-0, MIC-1, or MIC-2 [> or = 95, > or = 75, and > or = 50% growth inhibition, respectively]) when studying drug combinations. Furthermore, statistical analysis and comparisons are troublesome. The use of a spectrophotometric method to determine the effect of drug combinations yields quantitative data and permits the use of model fits to the whole response surface. We applied the response surface model described by Greco et al. (W. R. Greco, G. Bravo, and J. C. Parsons, Pharmacol. Rev. 47:331-385, 1995) to determine the interaction coefficient alpha (ICalpha) using a program developed for that purpose and compared the results with FIC indices. The susceptibilities of amphotericin B (AM), itraconazole (IT), and terbinafine (TB) were tested either alone or in combination against 10 IT-susceptible (IT-S) and 5 IT-resistant (IT-R) clinical strains of Aspergillus fumigatus using a modified checkerboard microdilution method that employs the dye MTT [3-(4,5-dimethyl-2-thiazyl)2,5-diphenyl-2H-tetrazolium bromide]. Growth in each well was determined by a spectrophotometer. FIC indices were determined and ICalpha values were estimated for each organism strain combination, and the latter included error estimates. Depending on the MIC endpoint used, the FIC index ranged from 1.016 to 2.077 for AM-IT, from 0.544 to 1.767 for AM-TB, and from 0.656 to 0.740 for IT-TB for the IT-S strains. For the IT-R strains the FIC index ranged from 0.308 to 1.767 for AM-IT, from 0.512 to 1.646 for AM-TB, and from 0.403 to 0.497 for IT-TB. The results indicate that the degree of interaction is not only determined by the agents themselves but also by the choice of the endpoint. Estimates of the ICalpha values showed more consistent results. Although the absolute FIC indices were difficult to interpret, there was a good correlation with the results obtained using the ICalpha values. The combination of AM with either IT or TB was antagonistic in vitro, whereas the combination of IT and TB was synergistic in vitro for both IT-S and IT-R strains. The use of response surface modeling to determine the interaction of drugs against filamentous fungi is promising, and more consistent results are obtained by this method than by using FIC indices.

Resistance of Candida species to antifungal agents: molecular mechanisms and clinical consequences

Candida albicans and related species pathogenic for man become resistant to antifungal agents, in particular triazole compounds, by expression of efflux pumps that reduce drug accumulation, alteration of the structure or concentration of antifungal target proteins, and alteration of membrane sterol composition. The clinical consequences of antifungal resistance can be seen in treatment failures in patients and in changes in the prevalences of Candida species causing disease. These effects were seen unequivocally in HIV-infected patients with oropharyngeal candida infections, but their incidence has decreased dramatically with the introduction of highly active antiretroviral therapy. The evidence for similar emergence of antifungal-resistant yeast strains and species in other types of candida infections is confounded by non-standardised susceptibility testing methods and definitions of a resistant fungal isolate. Recent large-scale surveys of yeasts isolated from blood cultures, based on standardised methodology and resistance definitions, do not support the view that antifungal resistance in pathogenic yeasts constitutes a significant or growing therapeutic problem.

Azole cross-resistance in Aspergillus fumigatus

We susceptibility tested 17 clinical isolates of Aspergillus fumigatus, for most of which MICs of itraconazole were elevated (MIC at which 50% of the isolates tested are inhibited, 16 microg/ml), against itraconazole, posaconazole, ravuconazole, and voriconazole. Posaconazole was the most active against itraconazole-susceptible isolates. A complex pattern of cross-resistance and hypersusceptibility was seen with voriconazole and ravuconazole, suggesting marked differences in activity and mechanisms of resistance.

Antifungal susceptibility testing: practical aspects and current challenges

Development of standardized antifungal susceptibility testing methods has been the focus of intensive research for the last 15 years. Reference methods for yeasts (NCCLS M27-A) and molds (M38-P) are now available. The development of these methods provides researchers not only with standardized methods for testing but also with an understanding of the variables that affect interlaboratory reproducibility. With this knowledge, we have now moved into the phase of (i) demonstrating the clinical value (or lack thereof) of standardized methods, (ii) developing modifications to these reference methods that address specific problems, and (iii) developing reliable commercial test kits. Clinically relevant testing is now available for selected fungi and drugs: Candida spp. against fluconazole, itraconazole, flucytosine, and (perhaps) amphotericin B; Cryptococcus neoformans against (perhaps) fluconazole and amphotericin B; and Aspergillus spp. against (perhaps) itraconazole. Expanding the range of useful testing procedures is the current focus of research in this area.

Optimal susceptibility testing conditions for detection of azole resistance in Aspergillus spp.: NCCLS collaborative evaluation. National Committee for Clinical Laboratory Standards

The most important role of susceptibility testing is to identify potentially resistant isolates for the agent being evaluated. Standard testing guidelines recently have been proposed for antifungal susceptibility testing of filamentous fungi (molds). This collaborative (eight centers) study evaluated further newly proposed guidelines (NCCLS, proposed standard M38-P, 1998) and other testing conditions for antifungal susceptibility testing of Aspergillus spp. to itraconazole and three new triazoles, posaconazole (SCH56592), ravuconazole (BMS-207147), and voriconazole. MICs of itraconazole, posaconazole, ravuconazole, and voriconazole for 15 selected isolates of three species of Aspergillus (A. fumigatus, A. flavus, and A. terreus) with well documented in vitro, clinical, or animal data were determined in each center by using four medium formulations (standard RPMI-1640 [RPMI], RPMI with 2% dextrose, antibiotic medium 3 [M3], and M3 with 2% dextrose) and two criteria of MIC determination (complete [MIC-0s] and prominent [MIC-2s] growth inhibition) at 24, 48, and 72 h. The highest reproducibility (92 to 99%) was seen with the standard RPMI and M3 media. Moreover, the distinction between itraconazole-resistant (MICs of >8 microg/ml for clinically resistant strains) and -susceptible (MICs of 0.03 to 1 microg/ml) isolates, as well as between a voriconazole-resistant laboratory mutant and other isolates (voriconazole MICs of 2 to >8 versus 0.12 to 2 microg/ml), was more consistently evident with the standard RPMI medium and when MIC-0s were determined at 48 h. These results provide further refinement of the testing guidelines for susceptibility testing of Aspergillus spp. and warrant consideration for inclusion in the future NCCLS document M38-A.