Echinocandin resistance, susceptibility testing and prophylaxis: implications for patient management

This article addresses the emergence of echinocandin resistance among Candida species, mechanisms of resistance, factors that promote resistance and confounding issues surrounding standard susceptibility testing. Fungal infections remain a significant cause of global morbidity and mortality, especially among patients with underlying immunosupression. Antifungal therapy is a critical component of patient management for acute and chronic diseases. Yet, therapeutic choices are limited due to only a few drug classes available to treat systemic disease. Moreover, the problem is exacerbated by the emergence of antifungal resistance, which has resulted in difficult to manage multidrug resistant strains. Echinocandin drugs are now the preferred choice to treat a range of candidiasis. These drugs target and inhibit the fungal-specific enzyme glucan synthase, which is responsible for the biosynthesis of a key cell wall polymer. Therapeutic failures involving acquisition of resistance among susceptible organisms like Candida albicans is largely a rare event. However, in recent years, there is an alarming trend of increased resistance among strains of Candida glabrata, which in many cases are also resistant to azole drugs. Echinocandin resistance is always acquired during therapy and the mechanism of resistance is well established to involve amino acid changes in "hot-spot" regions of the Fks subunits carrying the catalytic portion of glucan synthase. These changes significantly decrease the sensitivity of the enzyme to drug resulting in higher MIC values. A range of drug responses, from complete to partial refractory response, is observed depending on the nature of the amino acid substitution, and clinical responses are recapitulated in pharmacodynamic models of infection. The cellular processes promoting the formation of resistant Fks strains involve complex stress response pathways, which yield a variety of adaptive compensatory genetic responses. Stress-adapted cells become drug tolerant and can form stable drug resistant FKS mutations with continued drug exposure. A major concern for resistance detection is that classical broth microdilution techniques show significant variability among clinical microbiology laboratories for certain echinocandin drugs and Candida species. The consequence is that susceptible strains are misclassified according to established clinical breakpoints, and this has led to confusion in the field. Clinical factors that appear to promote echinocandin resistance include the expanding use of antifungal agents for empiric therapy and prophylaxis. Furthermore, host reservoirs such as biofilms in the gastrointestinal tract or intra-abdominal infections can seed development of resistant organisms during therapy. A fundamental understanding of the primary molecular resistance mechanism, along with cellular and clinical factors that promote resistance emergence, is critical to develop better diagnostic tools and therapeutic strategies to overcome and prevent echinocandin resistance.