Polyene and Echinocandin Antifungals

The polyenes and the echinocandins are two antifungal classes defined by where they strike the fungal cell. Polyenes such as amphotericin B attack the fungal membrane by binding its sterol, while echinocandins such as caspofungin attack the fungal cell wall by blocking glucan synthesis. Both are broadly fungicidal against important pathogens, and together they complement the membrane-synthesis–inhibiting azoles.

Definition

Polyene antifungals are amphipathic macrolide compounds, exemplified by amphotericin B, that bind ergosterol in the fungal membrane and form pores or otherwise disrupt the membrane; echinocandin antifungals are lipopeptides that non-competitively inhibit beta-(1,3)-glucan synthase, blocking synthesis of an essential fungal cell-wall polymer.

Scope

This entry covers the two classes side by side: the polyene mechanism of ergosterol binding and membrane disruption, the echinocandin mechanism of beta-(1,3)-glucan synthase inhibition, the contrasting toxicity profiles, and the comparatively limited but distinct resistance each class faces. It is a reference description and not prescribing guidance.

Core questions

• How does binding membrane sterol kill the fungal cell?
• Why does targeting the cell wall give echinocandins a favourable toxicity profile?
• What explains amphotericin B's broad spectrum but notable toxicity?
• Why is acquired resistance to these classes comparatively uncommon, and how does it arise when it does?

Key concepts

• Ergosterol binding by polyenes
• Membrane pore formation and leakage
• Beta-(1,3)-glucan synthase (FKS) inhibition
• Cell-wall versus cell-membrane targeting
• Fungicidal activity
• Amphotericin B nephrotoxicity and lipid formulations
• FKS-mutation–mediated echinocandin resistance

Mechanisms

Polyenes are amphipathic molecules that intercalate into the fungal membrane by binding ergosterol; the classical model holds that they assemble into pores that cause leakage of ions and small molecules, killing the cell, and additional oxidative mechanisms have been proposed. Their relative affinity for ergosterol over the human sterol cholesterol underlies their selectivity, but the overlap contributes to host toxicity, notably nephrotoxicity, which lipid formulations were developed to mitigate (Lewis, 2011). Echinocandins act on the cell wall instead: they non-competitively inhibit beta-(1,3)-glucan synthase, depleting a load-bearing wall polymer and causing osmotic instability and cell death, an action confined to the fungus because mammalian cells lack this enzyme (Denning, 2003). Resistance to both classes is comparatively limited; echinocandin resistance, when it occurs, is driven mainly by mutations in the FKS genes that encode the glucan-synthase target.

Clinical relevance

Amphotericin B and the echinocandins are reference agents for serious invasive fungal infections, and both figure prominently in how candidiasis and other deep mycoses are studied and managed (Pappas et al., 2009). The contrast between amphotericin B's broad spectrum but higher toxicity and the echinocandins' favourable safety but narrower spectrum is a recurring teaching point. This entry describes the classes and their mechanisms and is not a basis for individual treatment or dosing decisions.

Epidemiology

Acquired resistance to polyenes is rare, which has kept amphotericin B a durable agent despite decades of use; echinocandin resistance remains uncommon overall but has been reported, particularly in some Candida species under selective pressure, and is monitored as part of resistance surveillance. The relative scarcity of resistance is one reason these classes are valued reserve agents.

History

Amphotericin B, introduced in the late 1950s, was for decades the mainstay of systemic antifungal therapy and remains a benchmark for broad-spectrum activity, with lipid formulations later developed to reduce its toxicity. The echinocandins arrived around the turn of the twenty-first century as the first new antifungal class in years to target the cell wall rather than the membrane, a development reviewed by Denning (2003).

Key figures

• David Denning
• Mahmoud Ghannoum
• Russell Lewis
• Peter Pappas

Seminal works

• denning-2003
• ghannoum-rice-1999

Frequently asked questions

How do polyenes and echinocandins differ in what they target?

Polyenes such as amphotericin B target the fungal cell membrane by binding its sterol, ergosterol, and disrupting the membrane. Echinocandins such as caspofungin target the fungal cell wall by inhibiting beta-(1,3)-glucan synthase. One acts on the membrane, the other on the wall.

Why are echinocandins considered relatively well tolerated?

Their target enzyme, beta-(1,3)-glucan synthase, builds a polymer found in the fungal cell wall but not in human cells, so the drug has little to act on in the host. This cell-wall specificity gives the class a generally favourable toxicity profile compared with amphotericin B.