Antifungal Agents

Antifungal agents are drugs that kill or inhibit fungi. Because fungi are eukaryotes, like human cells, fewer unique targets are available than in bacteria, and antifungal development has historically been more constrained; the major classes act chiefly on the fungal cell membrane or cell wall.

Definition

Antifungal agents are substances used to treat fungal infections by destroying fungi or suppressing their growth, principally by disrupting components that distinguish fungal cells from human cells—above all the membrane sterol ergosterol and the fungal cell wall.

Scope

This topic introduces what antifungal agents are, the fungal-specific structures they target, and why the class is comparatively limited. It is a conceptual reference and does not provide drug selection, dosing, or treatment regimens for any fungal infection.

Core questions

• Why are there fewer antifungal targets than antibacterial targets?
• Which fungal-specific structures do the main antifungal classes act on?
• What distinguishes the polyene, azole, and echinocandin classes?
• Why is antifungal resistance an emerging concern?

Key concepts

• Selective toxicity against a eukaryotic pathogen
• Ergosterol as a membrane target
• Polyenes (binding membrane ergosterol)
• Azoles (inhibiting ergosterol synthesis)
• Echinocandins (inhibiting cell-wall β-glucan synthesis)
• Antifungal resistance

Mechanisms

The principal antifungal classes exploit structures specific to fungi. Polyenes bind ergosterol, the characteristic sterol of the fungal membrane, disrupting the membrane directly. Azoles inhibit an enzyme in the ergosterol biosynthetic pathway, depleting the membrane of this sterol. Echinocandins inhibit synthesis of β-glucan, a component of the fungal cell wall that human cells lack. Because fungi are eukaryotes that share much of their cellular machinery with human cells, the set of selectively toxic targets is smaller than for bacteria, which is one reason the antifungal armamentarium is comparatively narrow and why antifungal resistance is a growing concern.

Clinical relevance

Antifungal agents are essential to the management of fungal infections, particularly in immunocompromised patients, and form a distinct branch of anti-infective pharmacology. This entry describes the classes conceptually; it is not a guide to drug choice, dosing, or regimens, which depend on the specific fungus, the clinical context, and current guidelines such as the IDSA candidiasis guideline.

Evidence & guidelines

Reviews of antifungal drug development describe the limited target space and the unmet clinical needs of the class, while infection-specific clinical guidelines—such as the IDSA guideline for the management of candidiasis—set out treatment recommendations, which are voluntary and periodically updated.

History

Systemic antifungal therapy long depended on the polyene amphotericin B, with the azoles and later the echinocandins broadening the options over subsequent decades. The relatively slow expansion of the class, against a backdrop of rising fungal disease in immunocompromised populations, has kept antifungal development an active and challenging area.

Seminal works

• roemer-krysan-2014
• pappas-2016

Frequently asked questions

Why are antifungal drugs harder to develop than antibiotics?

Fungi are eukaryotes that share much of their cellular biology with human cells, so there are fewer fungus-specific targets that can be attacked without harming the host, which narrows the available drug classes.

What is ergosterol and why does it matter for antifungals?

Ergosterol is the principal sterol of the fungal cell membrane, analogous to cholesterol in human cells; several antifungal classes work either by binding ergosterol or by blocking its synthesis.