Potential Microbiological Effects of Higher Dosing of Echinocandins

The antifungal "paradoxical effect” has been described as the reversal of growth inhibition at high doses of echinocandins, most usually caspofungin. This microbiological effect appears to be a cellular compensatory response to cell wall damage, resulting in alteration of cell wall content and structure as well as fungal morphology and growth. In vitro studies demonstrate this reproducible effect in a certain percentage of fungal isolates, but animal model and clinical studies are less consistent. The calcineurin and Hsp90 cell signaling pathways appear to play a major role in regulating these cellular and structural changes. Regardless of the clinical relevance of this paradoxical growth effect, understanding the specific actions of echinocandins is paramount to optimizing their use at either standard or higher dosing schemes, as well as developing future improvements in our antifungal arsena

Antifungal activity of compounds targeting the Hsp90-calcineurin pathway against various mould species

Objectives Invasive mould infections are associated with a high mortality rate and the emergence of MDR moulds is of particular concern. Calcineurin and its chaperone, the heat shock protein 90 (Hsp90), represent an important pathway for fungal virulence that can be targeted at different levels. We investigated the antifungal activity of compounds directly or indirectly targeting the Hsp90-calcineurin axis against different mould species. Methods The in vitro antifungal activity of the anticalcineurin drug FK506 (tacrolimus), the Hsp90 inhibitor geldanamycin, the lysine deacetylase inhibitor trichostatin A and the Hsp70 inhibitor pifithrin-μ was assessed by the standard broth dilution method against 62 clinical isolates of Aspergillus spp. and non-Aspergillus moulds (Mucoromycotina, Fusarium spp., Scedosporium spp., Purpureocillium/Paecilomyces spp. and Scopulariopsis spp.) Results FK506 had variable antifungal activity against different Aspergillus spp. and was particularly active against Mucor spp. Geldanamycin had moderate antifungal activity against Fusarium spp. and Paecilomyces variotii. Importantly, trichostatin A had good activity against the triazole-resistant Aspergillus ustus and the amphotericin B-resistant Aspergillus terreus as well as the MDR Scedosporium prolificans. Moreover, trichostatin A exhibited synergistic interactions with caspofungin against A. ustus and with geldanamycin against Rhizopus spp. for which none of the other agents showed activity. Pifithrin-μ exhibited little antifungal activity. Conclusions Targeting the Hsp90-calcineurin axis at different levels resulted in distinct patterns of susceptibility among different fungal species. Lysine deacetylase inhibition may represent a promising novel antifungal strategy against emerging resistant mould

Calcineurin Orchestrates Hyphal Growth, Septation, Drug Resistance and Pathogenesis of Aspergillus fumigatus: Where Do We Go from Here?

Studies on fungal pathogens belonging to the ascomycota phylum are critical given the ubiquity and frequency with which these fungi cause infections in humans. Among these species, Aspergillus fumigatus causes invasive aspergillosis, a leading cause of death in immunocompromised patients. Fundamental to A. fumigatus pathogenesis is hyphal growth. However, the precise mechanisms underlying hyphal growth and virulence are poorly understood. Over the past 10 years, our research towards the identification of molecular targets responsible for hyphal growth, drug resistance and virulence led to the elucidation of calcineurin as a key signaling molecule governing these processes. In this review, we summarize our salient findings on the significance of calcineurin for hyphal growth and septation in A. fumigatus and propose future perspectives on exploiting this pathway for designing new fungal-specific therapeutics

Heat Shock Protein 90 (Hsp90) in Fungal Growth and Pathogenesis

Invasive fungal infections, such as invasive candidiasis and invasive aspergillosis, have an important impact on morbidity and mortality in intensive care unit (ICU), cancer, and transplant patient populations. New therapies are required to overcome the limitations of the current antifungal armamentarium and the emergence of resistance. The heat shock protein 90 (Hsp90) is an essential molecular chaperone in eukaryotes that has engendered considerable interest as a potential target for novel cancer and antimicrobial therapies. Fungal Hsp90 was identified as a key regulator of antifungal resistance to both azole and echinocandin antifungals, with distinct features in the two major fungal pathogens, the yeast Candida albicans and the mold Aspergillus fumigatus. This review aims to provide a comprehensive summary on the role of Hsp90 in essential traits of fungal virulence, such as growth, development, stress adaptation and antifungal resistance, as well as the challenge of targeting this highly conserved protein to develop new antifungal strategies

Advances in Aspergillus fumigatus pathobiology

Aspergillus fumigatus is a human fungal pathogen that causes invasive aspergillosis (IA), a major infectious cause of death in the expanding population of immunocompromised individuals such as cancer patients and transplant recipients. The mortality of IA remains high (30-70%) and emerging resistance to triazoles, the first-line antifungal drug class, is of particular concern. Second-line therapies for IA are limited by their toxicity (polyenes) or their lack of fungicidal activity (echinocandins). Identification of novel antifungal targets is an urgent need for improving the outcome of IA. A. fumigatus is a filamentous fungus exhibiting a complex developmental cycle and elaborated mechanisms of adaptation to allow the initiation and progression of infection in the human host. The fungal cell wall, with its unique and dynamic structure, is crucial for protecting cell integrity and evading the host immune system, also contributing to biofilm formation and virulence, and thus representing an ideal antifungal target. The emergence of azole resistance implies various and complex mechanisms that need to be further elucidated. Other important processes, such as biosynthetic pathways and toxin/metabolite production are important for fungal survival and propagation in the host environment, ultimately leading to disease. Moreover, the host immune response is a determinant factor in influencing the course of infection. The objective of this topic issue is to provide an overview of the recent advances in our understanding of A. fumigatus pathobiology and of IA pathogenesis to outline future research

Calcineurin Inhibitor CN585 Exhibits Off-Target Effects in the Human Fungal Pathogen Aspergillus fumigatus

Calcineurin (CN) is an attractive antifungal target as it is critical for growth, stress response, drug resistance, and virulence in fungal pathogens. The immunosuppressive drugs, tacrolimus (FK506) and cyclosporin A (CsA), are fungistatic and specifically inhibit CN through binding to their respective immunophilins, FK506-binding protein (FKBP12), and cyclophilin (CypA). We are focused on CN structure-based approaches for the development of non-immunosuppressive FK506 analogs as antifungal therapeutics. Here, we examined the effect of the novel CN inhibitor, CN585, on the growth of the human pathogen Aspergillus fumigatus, the most common cause of invasive aspergillosis. Unexpectedly, in contrast to FK506, CN585 exhibited off-target effect on A. fumigatus wild-type and the azole- and echinocandin-resistant strains. Unlike with FK506 and CsA, the A. fumigatus CN, FKBP12, CypA mutants (ΔcnaA, Δfkbp12, ΔcypA) and various FK506-resistant mutants were all sensitive to CN585. Furthermore, in contrast to FK506 the cytosolic to nuclear translocation of the CN-dependent transcription factor (CrzA-GFP) was not inhibited by CN585. Molecular docking of CN585 onto human and A. fumigatus CN complexes revealed differential potential binding sites between human CN versus A. fumigatus CN. Our results indicate CN585 may be a non-specific inhibitor of CN with a yet undefined antifungal mechanism of activity