ANTIZYMOTICS - Complete Exam Answer

DEFINITION

Key Point: Antifungal drugs are inherently more toxic than antibacterials because both the target fungi and the host cells are eukaryotes, meaning they share many cellular structures. This limits the number of selective targets available.

CLASSIFICATION OF ANTIZYMOTICS

CLASS I - POLYENES

CLASS II - AZOLES

• Ketoconazole, Miconazole, Clotrimazole, Econazole, Thiabendazole

• Fluconazole, Itraconazole, Voriconazole, Posaconazole, Isavuconazole

CLASS III - ALLYLAMINES

CLASS IV - NUCLEOSIDE ANALOGUES (Antimetabolites)

CLASS V - ECHINOCANDINS

CLASS VI - MISCELLANEOUS / OTHER ANTIFUNGALS

MECHANISM OF ACTION

1. Polyenes - Membrane Disruption by Ergosterol Binding

Amphotericin B / Nystatin
       ↓
Binds to ERGOSTEROL in fungal cell membrane
       ↓
Forms aqueous pores/channels in the membrane
       ↓
Increased membrane permeability
       ↓
Leakage of intracellular ions (K+, Mg2+), amino acids, sugars
       ↓
Cell lysis and FUNGICIDAL action

• Mammalian cells contain cholesterol instead of ergosterol, providing some selectivity
• Amphotericin B has higher affinity for ergosterol than cholesterol, but can still bind cholesterol at high doses - explaining its nephrotoxicity
• Amphotericin B is considered FUNGICIDAL

2. Azoles - Inhibition of Ergosterol Synthesis

Azoles (Ketoconazole, Fluconazole, Itraconazole)
       ↓
Inhibit CYP450-dependent enzyme: LANOSTEROL 14-α-DEMETHYLASE
       ↓
Blocks conversion of Lanosterol → Ergosterol
       ↓
Ergosterol depletion + Accumulation of toxic methylated sterols
       ↓
Cell membrane dysfunction, altered permeability, growth inhibition
       ↓
Primarily FUNGISTATIC (fungicidal at high concentrations)

• The enzyme lanosterol 14-α-demethylase is also called CYP51
• Azoles have higher affinity for fungal CYP than mammalian CYP, but side effects occur due to partial inhibition of mammalian CYP (reduced synthesis of testosterone, cortisol, aldosterone, cholesterol)
• Azoles are generally FUNGISTATIC - clearance depends heavily on host immune response

3. Allylamines - Squalene Epoxidase Inhibition

Terbinafine
       ↓
Inhibits enzyme: SQUALENE EPOXIDASE
       ↓
Blocks conversion of Squalene → Squalene epoxide (early step in ergosterol synthesis)
       ↓
Two effects:
  (a) Ergosterol DEPLETION → cell membrane dysfunction
  (b) Squalene ACCUMULATION (intracellular) → cell toxicity
       ↓
FUNGICIDAL action (squalene accumulation is directly toxic)

• Does NOT inhibit mammalian CYP - fewer drug-drug interactions
• Note: Agents that only deplete ergosterol = fungistatic; agents that also cause squalene accumulation = fungicidal

4. Nucleoside Analogue - Disruption of Nucleic Acid Synthesis

Flucytosine (5-FC)
       ↓
Enters fungal cells via cytosine permease
       ↓
Converted intracellularly to 5-Fluorouracil (5-FU) by cytosine deaminase
       ↓
5-FU → 5-Fluorouridine triphosphate (FUTP) → Incorporated into RNA → Disrupts protein synthesis
5-FU → 5-Fluorodeoxyuridine monophosphate (FdUMP) → Inhibits thymidylate synthetase → Disrupts DNA synthesis
       ↓
FUNGISTATIC to FUNGICIDAL

• Mammalian cells lack cytosine permease and cytosine deaminase, providing excellent selectivity
• Used only in combination with Amphotericin B (to prevent resistance development)

5. Echinocandins - Cell Wall Synthesis Inhibition

Caspofungin / Micafungin
       ↓
Inhibit enzyme: β-(1,3)-D-GLUCAN SYNTHASE
       ↓
Blocks synthesis of β-1,3-D-glucan (major structural polysaccharide of fungal cell wall)
       ↓
Weakened cell wall → osmotic instability → cell lysis
       ↓
FUNGICIDAL against Candida; FUNGISTATIC against Aspergillus

• Mammalian cells have NO cell wall - so echinocandins are highly selective and minimally toxic
• This is a unique target not shared with the host

6. Griseofulvin - Mitotic Spindle Disruption

Griseofulvin
       ↓
Accumulates in keratin-rich tissues (skin, hair, nails) via binding to keratin
       ↓
Binds to TUBULIN, disrupting microtubule polymerization
       ↓
Arrests fungal mitosis at metaphase (spindle poison)
       ↓
Fungistatic - inhibits fungal cell division

• Active only against growing cells and only against dermatophytes (Trichophyton, Microsporum, Epidermophyton)
• NOT effective against Candida or systemic fungi

SUMMARY TABLE - Mechanism of Action

VETERINARY APPLICATIONS

1. POLYENES

• Gold standard for systemic (deep) mycoses in animals
• Used for: Blastomycosis, Histoplasmosis, Coccidioidomycosis, Cryptococcosis, Aspergillosis, Candidiasis, Phycomycosis in dogs, cats, horses, and exotic animals
• Route: Slow IV infusion (diluted in 5% dextrose - NOT saline, as it causes precipitation)
• Liposomal Amphotericin B (AmBisome) used to reduce nephrotoxicity
• Adverse effects: Nephrotoxicity, hypokalemia, thrombophlebitis, fever, anemia

• Used topically for oral and gastrointestinal candidiasis in dogs, cats, birds, and reptiles
• Available as oral suspension, powder, or cream
• Not absorbed from GI tract - acts locally

• Used for ocular fungal infections (fungal keratitis) in horses
• Topical ophthalmic preparation

2. AZOLES

• Used in dogs and cats for: Blastomycosis, Histoplasmosis, Coccidioidomycosis, Dermatophytosis, Malassezia dermatitis
• Also used as an adrenal steroidogenesis inhibitor to manage Hyperadrenocorticism (Cushing's disease) in dogs - due to its inhibition of cortisol synthesis
• Caution: Hepatotoxicity; NOT recommended in cats

• Preferred azole in cats and dogs for systemic mycoses
• Used for: Dermatophytosis, Sporotrichosis, Blastomycosis, Histoplasmosis, Aspergillosis, Candidiasis
• Available as capsules and oral solution
• Half-life up to 48 hours in cats

• Excellent CNS and ocular penetration - drug of choice for Cryptococcal meningitis in cats and dogs
• Also used for urinary tract candidiasis (good urinary excretion)
• Generally well tolerated; fewer drug interactions than ketoconazole

• Used for Aspergillus infections resistant to itraconazole
• Used in dogs; cats show idiosyncratic neurotoxicity - caution required
• Used in birds and reptiles for aspergillosis

• Primarily topical use for superficial mycoses, otitis externa (Malassezia, Candida), ringworm
• Clotrimazole used intranasally in dogs for nasal aspergillosis

3. ALLYLAMINES

• Used orally in dogs and cats for dermatophytosis (ringworm) and Malassezia infections
• Effective against dermatophytes (Trichophyton, Microsporum) and yeasts
• Concentrates in skin, hair, and nails
• Fewer drug interactions than azoles - preferred in patients on multiple drugs

4. NUCLEOSIDE ANALOGUES

• Used in combination with Amphotericin B for Cryptococcal infections (meningitis) in cats and dogs
• Combination prevents development of resistance
• Adverse effects: Bone marrow suppression (leukopenia, thrombocytopenia), GI signs

5. ECHINOCANDINS

• Caspofungin and micafungin have limited veterinary use
• Used experimentally/off-label in dogs with refractory Candida or Aspergillus infections
• Not routinely available or approved for veterinary species

6. GRISEOFULVIN

• Only antifungal approved by FDA for systemic veterinary use in the USA
• Used extensively in dogs, cats, and horses for dermatophytosis (ringworm caused by Trichophyton mentagrophytes, Microsporum canis, M. gypseum)
• Must be given with fatty meal to enhance absorption
• Deposited in keratin-precursor cells; new infected keratin is gradually replaced
• Adverse effects: Teratogenic (contraindicated in pregnancy), bone marrow suppression, hepatotoxicity
• Cats (especially Siamese) are particularly sensitive - monitor CBC

7. POTASSIUM IODIDE

• Specific for Sporotrichosis (Sporothrix schenckii) in horses and cats
• Mechanism: Not fully understood; possibly enhances iodide-mediated phagocytosis and immune response
• Oral administration; cats may show iodism (lacrimation, salivation, nasal discharge)

SPECIES-SPECIFIC CONSIDERATIONS (Exam Important!)

IMPORTANT POINTS FOR EXAM

1. Ergosterol is the main target of antifungals - it is present in fungal membranes but absent in mammalian cells (which have cholesterol).
2. Polyenes bind ergosterol; Azoles and Allylamines block ergosterol synthesis (at different steps).
3. Amphotericin B is the broadest-spectrum and most potent antifungal - but also most toxic (nephrotoxic).
4. Fluconazole has the best CNS/CSF penetration - drug of choice for Cryptococcal meningitis.
5. Griseofulvin is the only FDA-approved systemic antifungal for veterinary use.
6. Echinocandins are the only class targeting the fungal cell wall - unique and highly selective target.
7. Flucytosine is never used alone - always combined with Amphotericin B.
8. Terbinafine (allylamine) is fungicidal due to squalene accumulation and does NOT inhibit mammalian CYP.
9. Antifungal treatment must continue well beyond clinical cure due to slow fungal growth and fungistatic nature of most drugs.
10. Liposomal Amphotericin B reduces nephrotoxicity while maintaining efficacy.