An assessment of the utility of the yeast GreenScreen assay in pharmaceutical screening.

In this paper we describe an initial reproducibility study of 12 proprietary compounds followed by the assessment of 51 marketed pharmaceuticals and, lastly, a summary of the data so far from 2698 proprietary compounds from the Johnson & Johnson (J&J) compound library, in the yeast GreenScreen assay (GSA). In this assay, a reporter system in the yeast cells employs the DNA damage inducible promoter of the RAD54 gene, fused to the extremely stable green fluorescent protein (GFP). The assay proved to be very robust, the Excel templates provided by Gentronix with the assay interfaced well with in-house J&J systems with little adaptation, the assay was very rapid to perform and used very little compound. The results confirm previous work which suggests that the yeast GSA detects different classes of genotoxic compounds to the Ames assay and as a result can help screen out important genotoxic compounds at the pre-regulatory test phase that are missed by Ames-test-based screens alone. A combination of SAR evaluation of genotoxicity plus an Ames-test-based screen and the GSA provides a powerful pre-regulatory test battery to aid in the selection of successful drug candidates.

Antifungal susceptibility testing of yeasts: evaluation of technical variables for test automation.

The technical parameters for antifungal susceptibility testing with Candida species were reexamined to determine the optimal conditions for testing with semiautomated preparations of broth microdilution cultures, automated spectrophotometric readings of the cultures, and dose-response and endpoint determinations by means of a computer spreadsheet. Tests were based on proposed standard method M27P of the National Committee for Clinical Laboratory Standards for antifungal agents. RPMI 1640 broth with extra glucose to a final concentration of 2% gave higher and more reproducible drug-free control readings without affecting susceptibility endpoint readings. An inoculum of 8 x 10(4) yeasts per ml prepared from a carbon-limiting broth culture without further standardization was found to give optimal control readings after 48 h of incubation at 37 degrees C. For flucytosine, fluconazole, itraconazole, and ketoconazole, endpoints based on 50% growth inhibition (50% inhibitory concentration) gave the minimum variation with inoculum size and the fewest endpoint differences with RPMI 1640 medium obtained from two different suppliers. The 50% inhibitory concentration was also the optimal endpoint for fluconazole and ketoconazole susceptibilities in comparison with broth macrodilution MICs determined by the method of the National Committee for Clinical Laboratory Standards. Intralaboratory reproducibility was determined by retrospective analysis of replicate results for isolates retested at random over a 2-year period. This approach showed less favorable reproducibility than has been reported from purpose-designed, prospective antifungal susceptibility studies, but it may better reflect real-life test reproducibility. Susceptibility data for 616 clinical isolates of yeasts, representing 16 Candida and Saccharomyces spp., confirmed the tendency of Candida lusitaniae isolates to show relatively low susceptibilities to amphotericin B, the tendency of Candida krusei isolates to show low flucytosine and fluconazole susceptibilities, and the presence of some isolates in the species Candida albicans, Candida glabrata, and Candida tropicalis with low susceptibilities to azole derivative antifungal agents. The study demonstrates the value of automation and standardization in all stages of yeast susceptibility testing, from plate preparation to data analysis.

Oxygen as limiting nutrient for growth of Cryptococcus neoformans.

In microbroth cultures with RPMI 1640 medium, the growth yield of seven Cryptococcus neoformans isolates was unaffected by augmentation of the normal (0.2%) glucose concentration in the medium to 2%, and the addition of other potential carbon, nitrogen, and vitamin sources to the medium also failed to produce large changes in growth yield. However, macrobroth cultures of C. neoformans in RPMI 1640 that were agitated by rotation in air gave turbidities 6 to 37 times greater than those in identical cultures incubated statically, and similar levels of increase were seen whether the medium contained 0.2 or 2% glucose. Incubation of microplates under an oxygen atmosphere or with agitation by rotation led to an increase of up to twofold in growth turbidity of the yeast. The maximum increase was achieved by incubation with rotation and was dependent on the brand of microplate used. The findings implicate oxygen as a growth-limiting nutrient for C. neoformans. Incubation of microbroth cultures under conditions that enhance oxygen availability for antifungal susceptibility testing purposes may increase the speed of such tests and enhance the determination of MIC endpoints.

In vitro and in vivo activities of the novel azole antifungal agent r126638.

R126638 is a new triazole agent with potent antifungal activity in vitro against various dermatophytes, Candida spp., and Malassezia spp. Its activity against Malassezia spp. in vitro was superior to that of ketoconazole, the agent currently used for the treatment of Malassezia-related infections. R126638 showed activity comparable to or lower than that of itraconazole against dermatophytes in vitro; however, in guinea pig models of dermatophyte infections, R126638 given orally consistently showed antifungal activity superior to that of itraconazole, with 50% effective doses (ED(50)s) three- to more than eightfold lower than those of itraconazole, depending on the time of initiation and the duration of treatment. The ED(50) of R126638 in a mouse dermatophytosis model was more than fivefold lower than that of itraconazole. These data indicate that if the effects of R126638 seen when it is used to treat animals can be extrapolated to humans, the novel compound would be expected to show effects at doses lower than those of existing drugs and, hence, present a lower risk for side effects.

Susceptibility testing of pathogenic fungi with itraconazole: a process analysis of test variables.

A 2(10-5) fractional factorial model was used to investigate the influence of 10 process variables in broth microdilution susceptibility tests with itraconazole against eight isolates of Candida species and six isolates of filamentous fungi in two growth media. An analysis of variance (ANOVA) indicated that glucose concentration and incubation time both significantly influenced control turbidity optical density (OD) values for most of the Candida spp. isolates, while incubation in >10% CO(2) versus ambient air, incubation temperature and inoculum size significantly influenced these OD values for about half of the yeast isolates. Control OD values for the mould isolates were most influenced by incubation time and temperature, and by occlusion of the wells with an adhesive sticker. Three statistical approaches, ANOVA, rank transformation and Mann-Whitney U-test, were used to assess the influence of the variable combinations on MIC, determined with a 50% growth reduction end-point. Incubation temperature and time, glucose concentration and inoculum size were the variables that most often affected susceptibility results to the level of statistical significance; however, the supplier of RPMI 1640 medium, the use of adhesive stickers and the atmosphere of incubation significantly influenced the MIC for some isolates. The medium used to prepare the test inoculum, the solvent used to prepare the stock solution and the shape of the microdilution plate wells significantly affected outcome, but only sporadically. A principal component analysis of the data matrix confirmed this order of relative influence of the test variables on the MIC. Since each fungal isolate responded differently to combinations of process variables in the test, we conclude that any unified method for antifungal susceptibility determination represents a compromise, rather than an idealized system.