Antifungal drugs explained: mechanisms of action
Fungi are eukaryotes like us, so antifungal drugs must hit the few things human cells lack. Classify the five classes by the fungal target each one inhibits.
Classify the antifungals (polyenes, azoles, echinocandins, terbinafine, flucytosine) by the fungal target each one inhibits, and use route and spectrum clues to name the class.
The short version
- Antifungal drugs exploit what fungi have and human cells lack: ergosterol in the membrane, a cell wall of chitin, mannans and glucans, and a fungal-only enzyme for flucytosine.
- Ergosterol is made in steps, squalene then lanosterol then ergosterol; polyenes, azoles and terbinafine act on ergosterol or its synthesis, while echinocandins hit the wall and flucytosine hits nucleic acid synthesis.
- Polyenes bind ergosterol, azoles block lanosterol 14-alpha-demethylase, and terbinafine blocks squalene epoxidase.
- Echinocandins block beta-1,3-D-glucan synthase in the wall, and flucytosine blocks nucleic acid synthesis.
- Match the drug to the map: membrane, pathway, wall or nucleic acid.
Why antifungal drugs are hard to design
A woman in her thirties finishes a course of antibiotics, and a week later her tongue is coated in thick white patches. Another antibacterial would not clear it, because this is Candida, a fungus.[1]
The difficulty is that fungi are eukaryotes, like human cells. Many targets are shared, so antifungal drugs have to hit something a human cell does not have.[1][2]
A fungal cell offers two features worth attacking:
The ergosterol pathway and the wall: a map of antifungal targets
Ergosterol is built in steps: squalene, then lanosterol, then ergosterol. The five classes in this lesson each hit one point on this map: the membrane sterol itself, its synthesis, the wall, or nucleic acid synthesis.[2][3]
Read the map in order. Squalene comes first, lanosterol follows, and ergosterol is the end product that the fungal membrane needs. A drug can therefore act on ergosterol itself, on one of the steps that build it, on the wall outside the membrane, or on nucleic acid synthesis inside the cell.[2][3]
| Class | Target | Part of the map |
|---|---|---|
| Polyenes (amphotericin B, nystatin) | Ergosterol | Membrane sterol itself |
| Azoles (fluconazole, itraconazole, voriconazole) | Lanosterol 14-alpha-demethylase | Ergosterol synthesis |
| Terbinafine | Squalene epoxidase | Ergosterol synthesis, one step earlier |
| Echinocandins (caspofungin, micafungin, anidulafungin) | Beta-1,3-D-glucan synthase | Cell wall |
| Flucytosine | Nucleic acid synthesis | RNA and DNA |
Polyenes: antifungal drugs that bind ergosterol
The polyenes are amphotericin B and nystatin. They bind ergosterol and form pores, so ions and cell contents leak out and the cell dies.[2][5]
They also bind cholesterol to a lesser degree, which helps explain the kidney injury. The infusion reactions come from immune activation.[3][4][5]
The key idea is that the target is the sterol itself, not an enzyme. Because polyenes bind ergosterol directly, they act on the membrane, and the pores they form are what kill the cell.[2][5]
Azoles and terbinafine: blocking ergosterol synthesis
The azoles include fluconazole, itraconazole and voriconazole. They inhibit lanosterol 14-alpha-demethylase, so ergosterol falls and the membrane fails. Against Candida they are mainly fungistatic, and drug interactions are common.[2][3][4]
Terbinafine works one step earlier. It inhibits squalene epoxidase, so squalene piles up inside the fungal cell and ergosterol falls. It collects in skin and nails, so it treats dermatophyte infections such as fungal nail disease.[2][3]
Echinocandins: attacking the fungal cell wall
The echinocandins are caspofungin, micafungin and anidulafungin. They inhibit beta-1,3-D-glucan synthase, so the wall weakens and the cell bursts. The wall is absent from mammalian cells.[2][3][4]
They are given intravenously, and they are fungicidal against Candida. Spectrum is part of the identity of the class[3][4]:
| Organism | Effect of echinocandins |
|---|---|
| Candida | Fungicidal |
| Aspergillus | Fungistatic only |
| Cryptococcus | Almost no effect |
A drug that leaves ergosterol alone but starves the wall of glucan is an echinocandin.[3][4]
Flucytosine: blocking fungal nucleic acid synthesis
A fungal enzyme, cytosine deaminase, converts flucytosine to 5-fluorouracil, which blocks RNA and DNA synthesis. Human cells lack that enzyme.[2][3][4]
Resistance appears fast, so flucytosine is paired with amphotericin B, for example in cryptococcal meningitis.[3][4]
Flucytosine is the odd one out because its target is neither the membrane nor the wall. The fungal-only enzyme cytosine deaminase makes the active drug, which is why human cells are spared.[3]
Matching antifungal drug names to the target
The names help among antifungals. A name ending in -azole points to the demethylase, and one ending in -fungin points to the wall.[2][4]
Try the map on three short cases. Each one is answered by locating the drug on the pathway, the wall or the membrane.
- Squalene piles upThe block comes before lanosterol: squalene epoxidase, so terbinafine.
- Ergosterol untouched, wall starved of glucanAn echinocandin.
- Pores punched in the membrane by binding ergosterolA polyene.
Practice question
Original · USMLE styleA 52-year-old man with acute leukemia has had fever for 9 days despite broad-spectrum intravenous antibacterial therapy. He has been neutropenic since chemotherapy, with an absolute neutrophil count of 200/mm³. Blood cultures grow Candida species. He is started on an intravenous drug that is fungicidal against Candida species and has no activity against Cryptococcus species.
Common questions
How do antifungal drugs work without harming human cells?
They hit targets human cells lack or do not use in the same way: ergosterol instead of cholesterol, the cell wall, and a fungal enzyme (cytosine deaminase) that activates flucytosine.[2][3]
What is the mechanism of action of azole antifungals?
Azoles such as fluconazole, itraconazole and voriconazole inhibit lanosterol 14-alpha-demethylase, so ergosterol falls and the membrane fails. Against Candida they are mainly fungistatic.[2][3]
What does terbinafine inhibit?
Terbinafine inhibits squalene epoxidase, one step before lanosterol, so squalene piles up and ergosterol falls. It collects in skin and nails and treats dermatophyte infections such as fungal nail disease.[2][3]
Transcript
Read the full narration, by chapter644 words
0:00The puzzle: thrush after antibiotics, and a fungus that is like us
A woman in her thirties finishes a course of antibiotics, and a week later her tongue is coated in thick white patches. Another antibacterial would not clear it, because this is Candida, a fungus. But fungi are eukaryotes, like us. So what can a drug hit that our own cells do not have?
0:19Today's goal, the colour key and two features worth attacking
Today: classify the antifungals by the fungal target each one inhibits. A fungal cell has two features worth attacking. First, its membrane uses ergosterol, where human membranes use cholesterol. Second, outside the membrane sits a cell wall of chitin, mannans and glucans, which human cells do not have.
0:42The drug map: squalene, lanosterol, ergosterol
Ergosterol is built in steps: squalene, then lanosterol, then ergosterol. The five classes in this lesson each hit one point on this map: the membrane sterol itself, its synthesis, the wall, or nucleic acid synthesis.
0:58Polyenes: pores in the membrane (amphotericin B, nystatin)
Start with the membrane. The polyenes are amphotericin B and nystatin. They bind ergosterol and form pores, so ions and cell contents leak out and the cell dies. They also bind cholesterol to a lesser degree, which helps explain the kidney injury. The infusion reactions come from immune activation.
1:20Azoles: lanosterol 14-alpha-demethylase
Next, the pathway. The azoles include fluconazole, itraconazole and voriconazole. They inhibit lanosterol 14-alpha-demethylase, so ergosterol falls and the membrane fails. Against Candida they are mainly fungistatic, and drug interactions are common.
1:39Terbinafine: squalene epoxidase
Terbinafine works one step earlier. It inhibits squalene epoxidase, so squalene piles up inside the fungal cell and ergosterol falls. It collects in skin and nails, so it treats dermatophyte infections such as fungal nail disease.
1:57Echinocandins: beta-1,3-D-glucan synthase and the spectrum
Now the wall. The echinocandins are caspofungin, micafungin and anidulafungin. They inhibit beta-1,3-D-glucan synthase, so the wall weakens and the cell bursts. They are given intravenously, and they are fungicidal against Candida. They are only fungistatic against Aspergillus, and they have almost no effect on Cryptococcus.
2:22Flucytosine: cytosine deaminase and nucleic acid synthesis
Last, flucytosine. A fungal enzyme, cytosine deaminase, converts it to 5-fluorouracil, which blocks RNA and DNA synthesis. Human cells lack that enzyme. But resistance appears fast, so flucytosine is paired with amphotericin B, for example in cryptococcal meningitis.
2:44Names point to the target, then try the map
The names help among antifungals. A name ending in azole points to the demethylase, and one ending in fungin points to the wall. Try the map. A drug that makes squalene pile up must act before lanosterol, so the block is squalene epoxidase, and the drug is terbinafine. A drug that leaves ergosterol alone but starves the wall of glucan is an echinocandin. A drug that punches pores in the membrane by binding ergosterol is a polyene.
3:17Practice question
Your turn. A 52-year-old man with acute leukaemia has had fever for 9 days despite broad-spectrum antibacterial therapy. He has been neutropenic since chemotherapy, with an absolute neutrophil count of 200 per cubic millimetre. Blood cultures grow Candida, and he is started on an intravenous drug that is fungicidal against Candida and has no activity against Cryptococcus. Which of the following is the most likely target of this drug? A, ergosterol. B, lanosterol 14-alpha-demethylase. C, squalene epoxidase. D, beta-1,3-D-glucan synthase. E, fungal nucleic acid synthesis. Pause here and decide.
4:08Answer and explanation
A is out: ergosterol is the polyene target, and amphotericin B is fungicidal and intravenous, but it does work against Cryptococcus. B is out: azoles block the demethylase, but they are mainly fungistatic against Candida. C is out: squalene epoxidase is the terbinafine target, a drug for skin and nail infections, not for bloodstream Candida. E is out: flucytosine acts on nucleic acid synthesis, and because resistance emerges it is almost always paired with another antifungal. So the answer is D. Echinocandins inhibit glucan synthase, are fungicidal against Candida, and miss Cryptococcus.
4:54Recap and next lesson
Three things to keep. One: polyenes bind ergosterol, azoles block the demethylase, terbinafine blocks squalene epoxidase. Two: echinocandins block the wall enzyme glucan synthase, and flucytosine blocks nucleic acid synthesis. Three: match the drug to the map. Membrane, pathway, wall or nucleic acid. Next, bacterial meningitis organisms by age.
Sources
Numbers in the text, e.g. [1], point here- Candida infections and therapeutic strategies: mechanisms of action for traditional and alternative agents. Front Microbiol 2018. pmc.ncbi.nlm.nih.gov/articles/PMC6038711/
- Antifungal therapy: new advances in the understanding and treatment of mycosis. Front Microbiol 2017. pmc.ncbi.nlm.nih.gov/articles/PMC5253656/
- Bondaryk M, et al. Antifungal agents in superficial and mucosal candidiasis: mode of action and resistance. Postepy Dermatol Alergol 2013; Iyer KR, et al. Treatment strategies for cryptococcal infection. Nat Rev Microbiol 2021. pmc.ncbi.nlm.nih.gov/articles/PMC3858657/ · pmc.ncbi.nlm.nih.gov/articles/PMC7868659/
- MSD Manual Professional Version. Antifungal Medications. msdmanuals.com/professional/infectious-diseases/fungi/antifungal-drugs
- LITFL. Amphotericin B; Voriconazole. litfl.com/amphotericin-b/ · litfl.com/voriconazole/
The cell, pathway and membrane drawings are schematic and not to scale. The patient and the practice-question values are invented for teaching.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized): woman figure, Candida albicans, cell membrane, nail, meninges. Icons: Health Icons (healthicons.org), CC0 1.0: pills, syringe. The tongue sketch, cells, pathway map, polyene pore, glucan wall, flucytosine flow, cards and all handwriting are drawn in code (original). No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-012.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
- EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
- EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
- EP 06Staph vs strep: catalase, coagulase and haemolysis5:23
- EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
- EP 08Bacterial genetics: how bacteria swap genes5:13
- EP 09Beta-lactamase and antibiotic resistance: three ways bacteria win5:13
- EP 10DNA vs RNA virus replication: which enzyme each genome needs5:48
- EP 11HIV life cycle and drug targets, step by step5:39
- EP 12Hepatitis B serology: reading the panel5:17
- EP 13Antifungal drugs explained: mechanisms of action5:26
- EP 14Bacterial meningitis organisms by age and risk group5:19