Used to treat systemic yeast infections; enzymes within yeast cells convert the drug to 5-fluorouracil, which inhibits an enzyme required for nucleic acid synthesis; not effective against most molds; resistant mutants are common.

Most of these interfere with the synthesis or function of a fungal membrane component. The few drugs that have been available to treat internal infections are quite toxic. Fortunately, however, a number of less toxic alternatives have recently been developed. The targets of antifungal drugs are illustrated in figure 21.17.

In general, there are no routine methods to determine the sensitivity of fungi to various drugs; instead, the drug that previous experience indicates will most likely eliminate the pathogen is generally prescribed. Table 21.4 summarizes the characteristics of the most common antifungal drugs.

Plasma Membrane Synthesis and Function

The target of most antifungal drugs is the chemical compound ergosterol, which is found in the plasma membrane of fungal but not human cells. Unfortunately, these drugs sometimes have a low therapeutic index because they interfere with similar compounds found in eukaryotic cells, limiting their internal use to the treatment of life-threatening infections. Some, however, can be applied topically to treat skin, hair, and nail infections.


The polyenes, a group of antibiotics produced by Streptomyces, bind to ergosterol. This disrupts the fungal membrane, causing leakage of the cytoplasmic contents and leading to cell death. Unfortunately, the polyenes are quite toxic to humans, which limits their systemic use to life-threatening infections. Amphotericin B causes severe side effects, but it is the most effective drug for treating many systemic infections. Newer lipid-based emulsions are less toxic but are more expensive. Nystatin is too toxic to be given systemically, but it is used topically.


The azoles are a large family of chemically synthesized drugs, some of which have antifungal activity. They include two classes, the im-idazoles and the newer triazoles; the latter are generally less toxic. Both classes inhibit the synthesis of ergosterol, resulting in defective fungal membranes that leak cytoplasmic contents. Fluconazole and itraconazole, which are triazoles, are increasingly being used to treat systemic fungal infections. Ketoconazole, an imidazole, is also used systemically, but it is associated with more severe side effects. Other imidazoles, including miconazole and clotrima-zole, are commonly used in nonprescription creams, ointments,

Plasma membrane synthesis/function

Polyenes Azoles Allyamines


Figure 21.17 Targets of Antifungal Drugs


Figure 21.17 Targets of Antifungal Drugs

Cell division


528 Chapter 21 Antimicrobial Medications and suppositories to treat vaginal yeast infections. They are also used topically to treat dermatophyte infections. ■ dermatophyte, p. 557


The allylamines inhibit an enzyme in the pathway of ergosterol synthesis. Naftifine and terbinafine can be administered topically to treat dermatophyte infections. Terbinafine can also be taken orally.

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