Efflux-mediated antifungal drug resistance

Abstract

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

FIG. 1.

Domain arrangements of ABC and MFS transporters. The schematic representation of the ABC TMS in the plane of the membrane is based on the crystal structure of Sav1866 (67). H. sapiens, Homo sapiens.

FIG. 2.

Phylogeny of fungi. The numbers indicate the number of PDR ABC genes identified in each species. CTG indicates the reassignment of the CTG codon as serine in the majority of Candida spp. rather than encoding leucine as in other organisms. WGD indicates genomes that have undergone a whole-genome duplication (98). The tree is based on the work of Fitzpatrick et al. (98).

FIG. 3.

Phylogenetic tree for 45 Pdr proteins from S. cerevisiae (SACE), C. albicans (CAAL), C. lusitaniae (CALU), C. immitis (COIM), A. fumigatus (ASFU), and C. neoformans (CRNE). The amino acid sequences of the Pdrps were aligned using MUSCLE (86). The Phylip suite of programs (92) was used to calculate distances between amino acid sequences (PROTDIST) and to draw trees by a neighbor-joining method (144). The proteins could be differentiated into eight clusters (A to H). The amino acids in the GSGK/C core of the Walker A1 motif, which are common to all members of each cluster except E, are shown in square brackets. New clusters are those in which there are no members from Saccharomyces or Candida species, and therefore, they were not identified until now. Details for individual Pdrps are given in Table S1 in the supplemental material.

FIG. 4.

Structures of representative ABC pump substrates, ABC pump inhibitors, MFS pump substrates, and antifungal drugs that are not pump substrates. All the structures except those of milbemycin and fluconazole were obtained from the ChemSpider database and were visualized using MarvinView. The milbemycin and fluconazole structures were built using the MarvinSketch v.5.1.3_2 editor (ChemAxon).

FIG. 5.

Possible ways of overcoming efflux-mediated fungal drug resistance.