Antifungal Classes and Mechanisms

Antifungal drugs are grouped into a small number of classes defined by their molecular target in the fungal cell. Because fungi are eukaryotes, the usable targets are few: the cell-membrane sterol ergosterol and its biosynthesis, the cell wall, and nucleic-acid synthesis. The major systemic classes—polyenes, azoles, echinocandins, and the pyrimidine analog flucytosine—each map to one of these targets.

Definition

Antifungal classes are categories of antifungal agents grouped by shared molecular mechanism: polyenes bind ergosterol, azoles inhibit ergosterol synthesis, echinocandins inhibit cell-wall glucan synthesis, and flucytosine disrupts fungal nucleic-acid synthesis.

Scope

The topic covers the principal antifungal classes used for systemic and serious infections and the mechanism by which each acts on the fungal cell. It explains why the target set is narrow and how mechanism relates to spectrum and toxicity. It does not provide dosing, regimen selection, or individualized therapy; those are clinical decisions outside the scope of this reference entry.

Core questions

• What molecular target defines each major antifungal class?
• Why is ergosterol such a central antifungal target?
• How does an agent's mechanism relate to whether it is fungistatic or fungicidal?

Key concepts

• Ergosterol as the fungal-membrane sterol
• Polyenes binding membrane ergosterol
• Azole inhibition of lanosterol 14-alpha-demethylase (Erg11/Cyp51)
• Echinocandin inhibition of beta-1,3-glucan synthase
• Flucytosine and fungal nucleic-acid synthesis
• Fungistatic versus fungicidal activity
• Spectrum differences across yeasts and molds

Mechanisms

Polyenes such as amphotericin B bind ergosterol in the fungal cell membrane, forming pores or otherwise disrupting membrane integrity and causing leakage of cell contents. Azoles inhibit the cytochrome P450 enzyme lanosterol 14-alpha-demethylase (Erg11/Cyp51), blocking ergosterol biosynthesis so that the membrane accumulates aberrant sterols and loses normal function. Echinocandins inhibit beta-1,3-glucan synthase, depleting a structural polysaccharide of the fungal cell wall and weakening it. Flucytosine is taken up and converted within the fungal cell to metabolites that interfere with DNA and RNA synthesis. Because each class targets a distinct structure, classes differ in spectrum, in whether they are predominantly static or cidal, and in their characteristic toxicities.

Clinical relevance

Knowing the target of each class explains observed differences in spectrum (for example, why echinocandins are inactive against certain molds) and in toxicity, and underpins critical reading of antifungal trials and stewardship discussions. This entry describes mechanisms and class properties for educational reference and is not a guide to selecting or dosing therapy for any individual.

Epidemiology

Invasive fungal infections, particularly invasive candidiasis and aspergillosis, are important causes of morbidity and mortality in immunocompromised and critically ill patients, and the limited number of antifungal classes constrains treatment options when resistance or intolerance arises.

History

Amphotericin B, introduced in the late 1950s, long anchored systemic antifungal therapy despite its toxicity. The azoles broadened oral and less-toxic options from the 1980s onward, and the echinocandins added a cell-wall-targeting class around the turn of the century, giving three mechanistically distinct pillars of systemic antifungal therapy.

Seminal works

• roemer-2014
• kullberg-2015
• cowen-2015

Frequently asked questions

Why do so many antifungals target ergosterol?

Ergosterol is the principal sterol of fungal membranes and has no exact counterpart in human cells (which use cholesterol), so binding it or blocking its synthesis offers a measure of selective toxicity that is hard to obtain elsewhere in a eukaryotic pathogen.

What do echinocandins target that the other classes do not?

Echinocandins inhibit synthesis of beta-1,3-glucan, a structural component of the fungal cell wall that is absent in human cells, making the cell wall the distinctive target of this class.