Clinical, cellular, and molecular factors that contribute to antifungal drug resistance

Abstract

In the past decade, the frequency of diagnosed fungal infections has risen sharply due to several factors, including the increase in the number of immunosuppressed patients resulting from the AIDS epidemic and treatments during and after organ and bone marrow transplants. Linked with the increase in fungal infections is a recent increase in the frequency with which these infections are recalcitrant to standard antifungal therapy. This review summarizes the factors that contribute to antifungal drug resistance on three levels: (i) clinical factors that result in the inability to successfully treat refractory disease; (ii) cellular factors associated with a resistant fungal strain; and (iii) molecular factors that are ultimately responsible for the resistance phenotype in the cell. Many of the clinical factors that contribute to resistance are associated with the immune status of the patient, with the pharmacology of the drugs, or with the degree or type of fungal infection present. At a cellular level, antifungal drug resistance can be the result of replacement of a susceptible strain with a more resistant strain or species or the alteration of an endogenous strain (by mutation or gene expression) to a resistant phenotype. The molecular mechanisms of resistance that have been identified to date in Candida albicans include overexpression of two types of efflux pumps, overexpression or mutation of the target enzyme, and alteration of other enzymes in the same biosynthetic pathway as the target enzyme. Since the study of antifungal drug resistance is relatively new, other factors that may also contribute to resistance are discussed.

FIG. 1

Molecular mechanisms of azole resistance. In a susceptible cell, azole drugs enter the cell through an unknown mechanism, perhaps by passive diffusion. The azoles then inhibit Erg11 (pink circle), blocking the formation of ergosterol. Two types of efflux pumps are expressed at low levels. The CDR proteins are ABCT with both a membrane pore (green tubes) and two ABC domains (green circles). The MDR protein is an MF with a membrane pore (red tubes). In a “model” resistant cell, the azoles also enter the cell through an unknown mechanism. The azole drugs are less effective against Erg11 for two reasons; the enzyme has been modified by specific point mutations (dark slices in pink circles) and the enzyme is overexpressed. Modifications in other enzymes in the ergosterol biosynthetic pathway contribute to azole resistance (dark slices in blue spheres). The sterol components of the plasma membrane are modified (darker orange of membrane). Finally, the azoles are removed from the cell by overexpression of the CDR genes (ABCT) and MDR (MF). The CDR genes are effective against many azole drugs, while MDR appears to be specific for fluconazole. Reprinted from reference with permission of the publisher.

FIG. 2

Time course of the development of fluconazole resistance in a patient with AIDS. Isolates are shown on the x axis in the order in which they were obtained from the patient. The dose that was administered to the patient (○) at the time each isolate was obtained is graphed at the bottom. Doses are given on a linear scale on the right axis in milligrams per day. MICs of fluconazole were determined by macrobroth dilution (□) and microbroth dilution (■). Only two differences were observed between the two methods. MICs of itraconazole (▴), ketoconazole (▵), and amphotericin B (⧫) were determined by E-tests and confirmed by the NCCLS macrobroth dilution method. MICs (in micrograms per milliliter) are given on a logarithmic scale on the left axis. Genetic changes that were identified are summarized above the graph and are described in detail in the text. Reprinted from reference with permission of the publisher.