Optimizing the Correlation between Results of Testing In Vitro and Therapeutic Outcome In Vivo for Fluconazole by Testing Critical Isolates in a Murine Model of Invasive Candidiasis

Abstract

The trailing growth phenomenon seen when determining the susceptibilities of Candida isolates to the azole antifungal agents makes consistent endpoint determination difficult, and the M27-A method of the National Committee for Clinical Laboratory Standards addresses this problem by requiring an 80% reduction in growth after 48 h of incubation. For some isolates, however, minor variations of this endpoint criterion can produce up to 128-fold variations in the resulting MIC. To investigate the significance of this effect, isolates of Candida that exhibited various forms of trailing growth when tested against fluconazole were identified. The isolates were examined in a murine model of invasive candidiasis and were ranked by their relative response to fluconazole by using both improvement in survival and reduction in fungal burden in the kidney. The resulting rank order of in vivo response did not match the MICs obtained by using the M27-A criterion, and these MICs significantly overestimated the resistance of three of the six isolates tested. However, if the MIC was determined after 24 h of incubation and the endpoint required a less restrictive 50% reduction in growth, MICs which better matched the in vivo response pattern could be obtained. Minor variations in the M27-A endpoint criterion are thus required to optimize the in vitro-in vivo correlation for isolates that demonstrate significant trailing growth when tested against fluconazole.

Evaluation of Virulence Factors In vitro, Resistance to Osmotic Stress and Antifungal Susceptibility of Candida tropicalis Isolated from the Coastal Environment of Northeast Brazil

“…In vivo studies have demonstrated that cells are actually susceptible to FLU (Rex et al, 1998). We also verified a phenomenon similar to the paradoxical growth described for Candida cells treated with echinocandins in our environmental strains against FLU.…”

Discovery of a hapE Mutation That Causes Azole Resistance in Aspergillus fumigatus through Whole Genome Sequencing and Sexual Crossing

“…Trailing endpoints caused problems when testing Candida spp. with azoles as well, but animal models and clinical experience argue that trailing endpoints do not indicate true resistance [56], [60]–[61], suggesting that the progeny with trailing endpoints for itraconazole might not be truly resistant.…”

Current Epidemiology of Candida Infection

“…Following almost 20 years of the use of fluconazole for the primary treatment of C. albicans infections, overall resistance (defined now as isolates with an MIC value of 8 μg/ml) still remains between 0% and 3% (4,(14)(15)(16)(17). Occasionally, there are reports of higher resistance rates at single institutions, but these could be either outbreaks of single clonal resistant strains or misinterpretation of "trailing" MIC values that caused susceptible isolates to show false in vitro resistance (25). While development of resistance during long-term therapy is still a concern, overall resistance of naïve C. albicans isolates to fluconazole is rare.…”