Dynamic duo takes down fungal villains.

F ungal pathogens are notoriously difficult to treat because of antifungal drug toxicity and drug resistance, and the limited armamentarium (1). Some drug classes inhibit fungal growth but do not efficiently eliminate pathogens, and there is a pressing need for advances in antifungal therapy. The problem is especially acute in the immunocompromised. In a recent issue of PNAS, Cowen et al. (2) describe how harnessing heat shock protein 90 (Hsp90) dramatically enhances the efficacy of available antifungal drugs and provides a powerful new avenue for eradicating fungal infections. Hsp90 is an essential chaperone responsible for folding and maturation of client proteins. These include signal transducers with unstable conformations that only acquire their active structure in response to specific signals. Moreover, Hsp90 acts as a capacitor for evolution and buffers the effects of mutations, allowing their gradual accumulation (3, 4). When stress conditions overwhelm Hsp90, otherwise silent polymorphisms are expressed and can be selected if they provide a survival advantage. For example, Hsp90 potentiates the emergence of antifungal resistance and its inhibition reduces Candida albicans resistance to azoles and enhances action of caspofungin (CS) against Aspergilli (5, 6). These observations first implicated Hsp90 as a novel antifungal target and set the stage for Cowen et al. (2) to test synergistic effects of Hsp90 inhibitors and conventional antifungal drugs. In their study, Cowen et al. (2) tested several Hsp90 inhibitors combined with antifungal drugs of different mode of actions against evolutionarily divergent fungi in 2 complementary pathogenicity models. The Hsp90 inhibitors are geldanamycin (GdA) and analogs already in clinical trials as anticancer agents. 17-(Allylamino)17-demethoxygeldanamycin (17-AAG) and 17-(dimethylaminoethylamino)-17demethoxygeldanamycin (17-DMAG) were tested in association with 2 commonly used antifungal drugs, f luconazole (FL; an ergosterol biosynthesis inhibitor) and CS (inhibits 1,3-glucan synthase to block cell wall biosynthesis). The efficacy of combinatorial therapy against C. albicans and Aspergillus fumigatus, fungal pathogens associated with high mortality rates, was evaluated in Galleria mellonella and murine models. G. mellonella larvae serve as a facile, inexpensive, and ethically convenient model for fungal pathogenicity and allow evaluation of host immune response to infection (7). Studies using different fungal pathogens in the G. mellonella system show a significant correlation between virulence in larvae and mice, and the efficacy of antifungal therapies in this invertebrate corresponds well to results in humans (7). In all scenarios tested by Cowen et al. (2), inhibition of Hsp90 improved the response to antifungal drugs. Hsp90 inhibitors converted FL fungistatic activity against C. albicans to fungicidal, and this synergistic interaction rescued G. mellonella larvae from lethal infection. Similar effects were observed when A. fumigatus was treated with the GdA–CS combination in vitro and in G. mellonella. In a murine model of disseminated candidiasis, genetic abrogation of C. albicans HSP90 resulted in clearance of the fungal infection upon FL treatment. These findings support inhibition of Hsp90 in combination with fungistatic agents as a novel, efficacious broad-spectrum therapy against fungal pathogens (Fig. 1). Because Hsp90 is important for development of antifungal resistance (5), inhibiting Hsp90 early in infection has the potential to restrain phenotypic variation that might be selected and result in emergence of resistant isolates (Fig. 1). Blocking the evolvability of antifungal resistance is an entirely new mode of action, which has not been applied therapeutically and merits further exploration. According to the Tufts Center for the Study of Drug Development in Boston, up to 1.8 billion dollars and 18 years can be required to develop a new drug. Therefore, using a combination of marketed drugs as a novel treatment can accelerate therapeutic advance. Combinatorial therapies to enhance efficacy of individual drugs have been widely and successfully used to treat cancer and bacteria. The principle is simple: inhibiting multiple pathways simultaneously results in combinatorial blockage of cell viability and limits exposure to toxic side effects. Hence, simultaneous administration of available drugs is likely to provide several advantages to conventional antifungal therapy: broader spectrum and potency, decreased emergence of resistance, and reduction in the dosage of toxic agents such as amphotericin B (AmB). However, this therapeutic modality has not been implemented in clinical practice with the exception of cryptococcal meningitis, often treated with a flucytosine–AmB combination (8). The results of Cowen et al. (2) should accelerate further clinical applications of antifungal combination therapy.