VJ - Antifungals Flashcards
What are Fungi? (4)
- Eukaryotes (unicellular and multicellular, cell nucleus, etc.)
- Heterotrophs except with lichen (algae/cyanobacteria)
- Mostly opportunistic (secondary) infection
- Cause superficial, (sub)cutaneous and systemic (tissues/organs) = mycoses
What are the 3 problematic classes of fungi for humans?
- Dermatophytes
- Yeast
- Moulds
What are characteristics of Dermatophytes? (2)
- Commensals that metabolize keratin
- Cause common superficial diseases (e.g., ringworm, athlete’s foot, jock itch)
What are characteristics of Yeast? (4)
- Unicellular, reproduces by fission or budding
- Often commensal (pharynx, GI tract, vagina)
- Diseases: e.g., Candida (C. auris, C. albicans) and Cryptococcus (C. neoformans)
- Causes mucosal infections (e.g., thrush) and systemic infections (e.g., meningitis)
What are characteristics of Moulds? (3)
- Multicellular, form colonies, reproduce through sporulation
- Commonly found in soil and organic matter
- Diseases: e.g., Aspergillosis (from Aspergillus sp.) - often begins in airways, can spread systemically
How are fungi becoming a growing problem? (3)
- Usually in immunocompromised (AIDS, immunosuppression, radiotherapy, organ transplants, etc.)
- Hospital acquired infections: pregnancy, diabetes, antibiotic treatment
- Becoming increasingly resistant to antifungals
What makes fungi different to other eukaryotes?
Cell wall containing glucans and chitin, as well as a cell membrane containing ergosterol
Why are fungal infections challenging to treat? (2)
- Fungi grow slowly and infect poorly penetrated tissues
- Require prolonged treatment due to resistance
What is the difference between fungistatic and fungicidal antifungals?
Fungistatic: Inhibits fungal growth
Fungicidal: Kills fungal cells
What are major sites of action for antifungal drugs? (5)
1) Inhibition of nucleic acid synthesis
- Flucytosine
2) Inhibition of proteins synthesis
- Sordarins
- Azasordarins
3) Disruption of microtubules and inhibition of mitosis
- Griseofulvin
4) Disruption of cell wall
- Inhibitors of glucan synthesis (Echinocandins)
- Inhibitors of chitin synthesis (Nikkomycin)
5) Disruption of cell membrane
- Inhibitors of ergosterol synthesis (Azoles/Allylamines)
- Direct membrane damage (Polyenes)
Describe the fungal cell wall structure (2)
Inner layer: β‐1,3‐glucan, β‐1,6‐glucan, chitin
Outer layer: mannose-proteins (involved in cell adhesion, immune evasion)
What are the functions of the fungal cell wall? (4)
- Offers environmental protection against osmotic forces
- Involved in morphogenesis (cell division, budding, filamentous growth)
- Prevents leakage of mannose-proteins by providing an anchoring point
- Mannose-proteins aid in cell adhesion, structure, and immune evasion (e.g., dectin-1 in Candida albicans)
What is the role of Uracil diphosphate (UDP) in fungal cell wall synthesis?
UDP is a carrier molecule for glucose and N-acetyl glucosamine
It activates the following synthases:
- Chitin synthase
- β-1,3-glucan synthase (FKS1p and FKS2p)
- β-1,6-glucan synthase
What are Echinocandins and their role in antifungal treatment? (5)
Echinocandins are β-1,3-glucan synthesis inhibitors (natural and semi-synthetic)
- Cyclic hexapeptides that inhibit β-1,3-glucan synthase (FKS1p subunit) via non-competitive inhibition (binding site unresolved)
- Known as the “penicillin of antifungals,” they are fungistatic and fungicidal
- Effective against Candida and Aspergillus infections
- Good distribution in tissues, including the brain; however, they have poor bioavailability, requiring intravenous administration
What is Nikkomycin Z and its role in antifungal treatment? (3)
Nikkomycin Z is a chitin synthesis inhibitor that competitively inhibits chitin synthase
- It is believed to enter cells via a peptide endocytosis mechanism
- Currently used in veterinary settings but has not advanced beyond Phase I clinical trials in humans due to high costs
What is the role of ergosterol in fungal cell membranes? (3)
- Ergosterol serves a similar function to cholesterol in other eukaryotes
- It is thought to be related to climatic instabilities (varying humidity and moisture) in fungi’s ecological niches (plant and animal surfaces, soil)
- Ergosterol impacts membrane fluidity and functionality
How do polyenes cause direct membrane damage in fungi? (4)
- Polyenes may act as ionophores, binding to ergosterol (more than cholesterol) to form pores
- This increases membrane permeability, particularly to monovalent cations (e.g., K+)
- The result is leakage, cell lysis, and cell death (fungicidal effect)
- Other hypotheses include reactive oxygen species (ROS) generation and functioning as extramembranous sponges that “steal” ergosterol
What is Amphotericin B and how does it function as an antifungal? (3)
- Amphotericin B (AMB) is a polyene antimicrobial agent that is poorly water-soluble, often used as lipid complexes
- It selectively targets ergosterol in fungal cell membranes, as it binds better to ergosterol than cholesterol
- While it has low affinity for cholesterol, this reduces but does not eliminate its toxicity to human cells
How is ergosterol targeted by antifungal drugs? (3)
- Squalene epoxidase: Inhibited by allylamines and thiocarbamates
- Lanosterol α14 demethylase: Inhibited by azoles (major antifungal class)
- Δ14 sterol reductase and Δ8 sterol isomerase: Targeted by morpholines (fungi-specific enzymes)
Reducing ergosterol levels and increasing levels of intermediates = toxicity
How do allylamines inhibit ergosterol synthesis in fungi? (3)
- Allylamines non-competitively inhibit squalene epoxidase, reducing ergosterol levels and increasing toxic squalene levels (fungicidal effect)
- Effective against nail and skin dermatophytes, though they have a narrow spectrum and significant side effects when taken orally
- Mammalian cells share this enzyme in cholesterol synthesis
How do azoles and triazoles inhibit ergosterol synthesis in fungi? (3)
Azoles and triazoles are the largest class of antifungals, with broad-spectrum activity and can be taken orally or intravenously
- They competitively inhibit lanosterol α14 demethylase, blocking ergosterol synthesis
- Mammalian cells also have this enzyme, but azoles have much lower affinity (IC50 of 1 nM in Candida vs. 1 μM in human cells), making them less toxic to humans
What are the structural requirements for azole antifungals and how do they interact with other drugs? (3)
- Azoles require a basic imidazole or 1,2,4-triazole ring to bind the iron atom in the heme group of the enzyme’s active site
- The remaining structure binds to the apoprotein, with a large nonpolar part that mimics a steroid molecule
- Azoles inhibit cytochrome P450 (CYP 3A4), potentially interacting with other drugs metabolized by this enzyme (e.g., cyclosporine)
How do morpholines inhibit ergosterol synthesis in fungi? (2)
- Morpholines inhibit both Δ14 reductase (IC50: 2.93 µM) and Δ8 isomerase (IC50: 1.8 nM), creating a synergistic effect by blocking two steps in the same pathway
- Commonly marketed as Curanail, Loceryl, Locetar, or Odenil, they are available over-the-counter as nail lacquers
How does Flucytosine function as an antifungal agent? (3)
Flucytosine is used as an adjunctive treatment for systemic fungal infections (e.g., Cryptococcal meningitis) due to high resistance rates and its fungistatic nature
It is converted to 5-fluorouracil (5-FU) by cytosine deaminase
- 5-FU inhibits thymidylate synthase and can be incorporated into RNA, disrupting RNA function and protein synthesis
What is Griseofulvin, and how does it work as an antifungal?
Griseofulvin, the first antifungal (from Penicillium), interacts with tubulin to disrupt microtubule and spindle formation, arresting cells in metaphase (fungistatic effect)
- It binds to keratin and selectively targets fungal cells through energy-dependent uptake (100x more than in mammalian cells)
- Administered orally for dermatophyte infections (ineffective topically)
