Anti-Fungal and Anti-Viral Agents (Week 4, Mod 7)

What are the 5 different classifications that antifungals are classified by?

• Superficial/Systemic infection treatment

• Topical/Systemic administration of drug

• Natural Antifungals/ Synthetic agents

• Fungicidal/Fungistatic

  • Fungicidal = kill; utilized in immunocompromised patients

  • Fungistatic = stops growth; treats for everything else

• Chemical Subclass

What are some reasons why fungal infections may be on the rise?

• Use of agents that disrupt normal host microflora - antibiotics

• Failure to develop a strong immune system or immunosuppressive drugs

• Patient management that suppresses the immune response

• Chemotherapy, HIV/AIDS, transplants, steroid treatment

• Diabetes

• Antibiotic use 

• Age

• Invasive surgical procedures that introduce fungi – iv lines etc; fungus like plastic; can easily create a biofilm

• Hospital acquired infection

What are the 6 SUBclasses of chemical antifungals?

• Allylamines

• Azoles

• Polyenes

• Glucan Synthesis (cell wall) inhibitors - Echinocandins 

• Antimetabolites

• Griseofulvin - developed at UofG, used for dermatophyte fungal infection of the nail

  • Very toxic; last resort treatment

What is important for these drugs in particular?

SELECTIVE TOXICITY

Need to target features of the fungus that CANNOT be found in the host

• Difficult, due to the fact that fungi are ALSO eukaryotic; need to find a target that is unique to the fungal cell

What is one of the primary (and most common) targets for some of the antifungal agents?

Ergosterol 

• Like cholesterol but not

• Is the main sterol present throughout the fungal cell membrane

Which 3 drug classes target ergosterol in the fungal cell membrane?

• Allylamines

• Azoles 

• Amphotericin B (in the class of Polyenes)

MOST COMMON ANTIFUNGAL DRUGS in human and animal medicine

Briefly describe the biosynthesis of ergosterol… what two enzymes are targets for its production?

Enzymes: 

• Squalene epoxidase 

• 14-sterol demethylase 

Lanosterol: precursor molecule 

Describe the Allylamines … the mechanism of action, the spectrum of what it targets, side effects, routes of transmission, and pharmacokinetics.

Dermatophyte infections only

• Example: Terbinafine (Brand name - Lamisil)

Mechanism of action: inhibits ergosterol biosynthesis via 

    inhibition of squalene epoxidase (fungicidal)

• KILLS fungi; sometimes topical doesn’t work as well, so oral treatment is needed 

• Spectrum: Dermatophytes 

• Side effects: generally transient and mild (GIT & skin)

• Routes: oral and topical

• Pharmacokinetics: highly lipophilic (persists in skin), 

Describe the Topical Azoles … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

Can be used both TOPICALLY and ORALLY depending on kind of azole…

• Imidazoles - Topical use only

• Examples: Clotrimazole; Enilconazole, Miconazole - lots of topical azoles – main use is topical for superficial mucous membrane and skin infection

• Ketoconazole – oral therapy but toxic; CAN CAUSE LIVER FAILURE

• Mechanism of action: inhibition of cytP450-dependent 14-sterol de-methylase

• Spectrum: Broad spectrum (generally fungistatic; prolonged treatment)

• Side effects: GIT; anorexia; hepatotoxicity; suppression of steroid production (ketoconazole); teratogenic, can cause cancer 

Describe the Systemic Azoles … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

• Triazoles - for serious systemic mycoses

• Examples: fluconazole, itraconazole, voriconazole, and posaconazole

• Itraconazole – drug of choice for Histoplasmosis 

• Routes: oral

  • Itraconazole: lipophilic, highly plasma protein bound, hepatic metabolism and excretion in faeces

    • Good against aspergillus 

  • Fluconazole: water soluble and can be given i.v, minimally plasma protein bound, minimally metabolised, 80% excreted by kidney unchanged

    • Good against zygomycetes 

Both FUNGICIDAL, as the fungi they target only infect immunocompromised patients 

How do fungi eventually develop antifungal resistance to azoles?

• Membrane changes lead to reduced drug up-take

• Mutation of the target enzyme

• Over production of the target enzyme

• Modification of the ergosterol biosynthesis pathway

• Drug efflux due to up-regulation ABC transporters, MFS transporters

• Biofilm formation - can form biofilms in IV lines

Describe the Polyenes … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

Examples: Amphotericin B, Nystatin

• Mechanism of action: binds to ergosterol, disrupts osmotic integrity of the membrane by forming pores – ions leak from cell – cidal and oxidative (H2O2) damage

• Spectrum: broad spectrum

• Side effects: nephrotoxicity (i.v.), hypokalaemia, thrombophlebitis

• Routes: depends on preparation – Nystatin topical

• Pharmacokinetics: poorly water soluble (amphotericin Bs forms a colloid in solution for injection), poor absorption from GIT

** VERY effective against fungi, but also VERY toxic to the patient; lots of side effects

• Mainly used in VERY ill patients 

What is an example of a nystatin product?

Canaural - topical; for yeast infection

Spectrum of nystatin is broad but 

topical version mainly for yeast infection in skin 

For otitis externa in dogs and cats

Also contains a drug that kills ear mites and an anti-inflammatory drug. 

How do fungi manage to resist Polyenes?

Amphotericin B resistance remains rare

• Linked to reduced ergosterol in fungal cell membrane

• Forms cross membrane pores - leads to cell leakage and cell death

• Resistance linked to increased intracellular catalase reducing the oxidative killing mechanism of AmB.

• Innate resistance in some isolates of Candida lusitaniae, Aspergillus terreus, Scedosporium species, Lomentospora prolificans and Purpureocillium lilacinum.

• Fungicidal – emergent resistance rare - cells killed not bathed in fungistatic agent allowing mutation resistance – although observed in C. guilliermondii

What is another primary target for antifungals that is unique to fungal cells?

FUNGAL CELL WALL

• Main component = glucans

• Can inhibit the production of glucans to weaken the cell wall

• Inhibit the enzyme beta (1,3) glucan synthase

Describe the Echinocandins (glucan wall inhibitor) … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

Severe systemic mycoses only

• Examples: Caspofungin; Anidulafungin, Micafungin 

• Mechanism of action: block synthesis of β (1,3) glucan

• Spectrum: Candida  & Aspergillus Species

• Side effects: minimal

• Routes: i.v.

• Pharmacokinetics: water soluble, highly plasma protein bound, eliminated in urine and faeces as metabolites

What is the mechanism of RESISTANCE to echinocandins?

• Inhibit glucan synthase which synthesizes beta glucan, a structural component of fungal cell walls

• Structural integrity lost – cidal in yeast, static in moulds

• Many moulds resistant (not Aspergillus) most Candida species susceptible

• Candida parapsilosis innately less susceptible – higher breakpoints

• Various FKS1 (glucan synthase subunit) mutations identified also FKS2 and 3 genes all of which encode the target enzyme and up-regulation of chitin synthesis  - rescue mechanism

Describe the Antimetabolites … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

• Examples: Flucytosine (5-fluorocytosine – converted to 5-fluorouracil by cytosine deaminase) - Combination therapy for severe yeast infections only

• Mechanism of action: Incorporation of 5-FU into RNA disrupts protein synthesis (fungicidal) and further conversion of 5-FU to fluorodeoxyuridine monophosphate inhibits thymidylate synthase which interferes with DNA synthesis (double hit; reduces protein synthesis and fungus replication)

• Spectrum: Narrow; Cryptococcus; Candida species

• Side effects: generally well tolerated

• Routes: oral

• Pharmacokinetics: excreted unchanged by kidney

• Often combined with Amphotericin B (synergism)

Drug of choice for CRYPTOCOCCUS infection 

- Need to treat with flucytosine AND amphotericin B, never Flucytosine on its own; fungus will become resistant to it very rapidly 

How do fungi resist flucytosine?

• Sensitive – cascade of active enzymes, cytosine permease to take up the drug, cytosine deaminase and phosphorylase to metabolize it to its toxic form – disrupts RNA and DNA synthesis

• 5-FC converted to 5-FU – Fluorouracil – miscoded RNA or DNA 

• Loss or reduction in activity in any of the enzymes (but most commonly the phosphorylase) leads to primary emergent resistance

• Common during treatment – rarely used as monotherapy, always combined with AmB

Describe the Griseofulvin … the mechanism of action, the spectrum of what it targets, side effects, route(s) of transmission, and pharmacokinetics.

• Dermatophyte infections only

• Mechanism of action: selectively deposited in newly formed keratin, inhibits mitosis, disorganises the spindle microtubules (fungistatic)

• Spectrum: Narrow spectrum (dermatophytes)

• Side effects: idiosyncratic reaction (unexpected side effects, cannot be predicted) in cats, teratogenic (can’t give to pregnant animals)

• Routes: Oral (with high fat diet)

• Pharmacokinetics: Poorly water soluble, hepatic metabolism and faecal elimination

• Note: No longer licenced for food producing animals in UK. Only horses. **

What are some other agents that can be used as antifungals?  What are their mechanisms?

• Iodides – may enhance the immune response of the host

• Propionic, salicylic and undecanoic acids

• Whitfield’s ointment: benzoic acid and salicylic acid in an emulsifying base) have been used traditionally for treating dermatophyte infections of the skin. Though old-fashioned and a little messy, they are cheap and effective.

• Phenolic antiseptics

  • e.g. thymol

  • Hexachlophene

What is an example of a medication that has a combination of antifungal, antibacterial, and steroid?

Aurizon - a medication for ear infections

• Contains:

  • Marbofloxacin (antibacterial)

  • Clotrimazole (antifungal)

  • Dexamethasone (steroid)

What are the 5 antiviral agents that are mentioned in this lecture, and what are they used to treat?

1) Aciclovir - targets herpes simplex

2) Amantadine and Rimantadine - treat Influenza A

3) AZT - treats retroviruses 

4) Tamiflu - targets Influenza A and B

Describe Aciclovir and its mechanism of action, how its administered, side effects, etc.

Aciclovir – nucleoside analogue – substrate for HSV thymidine kinase – phosphorylated  only in HSV infected cells to form acyclovir triphosphate, a competitive inhibitor of viral DNA polymerase – stops viral replication 

Inhibits viral DNA polymerase

• High specificity for herpes simplex

• oral, iv, topical – wide distribution

• minimal side effects

• Uses FHV, viral eye infection

Describe Amantadine and Rimantadine and its mechanism of action, how its administered, side effects, etc.

Blocks viral M₂ ion channel

• Influenza A

• oral – well absorbed

• minimal side effects

• high rates of resistance in Influenza A viruses 

Describe Zidovudine (AZT) and its mechanism of action, how its administered, side effects, etc.

Looks like thymidine – a nucleoside - NRTI Viral RT makes a DNA copy of viral RNA genome – inserts into host chromosome – host enzymes phosphorylate the drug to triphosphate form – inhibits viral replication through competitive binding to viral RT – terminates DNA chain elongation

• nucleoside reverse transcriptase inhibitor

• retroviruses

• oral, iv.

• short term: minor reversible side effects

• long term: anaemia, GIT disturbances

• FLV and FIV

Describe Zanamivir (Relenza) and Oseltamivir (Tamiflu) and its mechanism of action, how its administered, side effects, etc.

Treatment and prophylaxis of Influenza A and B

• Neuraminidase inhibitor; stops new viruses emerging

• Recently used against H1N1 and H5N1

  • Also active against canine parvovirus, feline panleukopenia, kennel cough, and ‘canine flu’.