Classes and MoA's

Question 1

What are the types of Azoles? Give examples for each subclass

Answer 1

1. Imidazole - miconazole, ketoconazole
2. Triazole - fluconazole
3. Voriconazole
4. Isavuconazole (used when can’t tolerate amphotericin B)
5. Pozaconazole (used when can’t tolerate amphotericin B)

Question 2

What is the mechanism of action of Azoles?

Answer 2

Inhibit C-14 demthylase which catalyses the 14-demethylation of lanosterol to synthesise ergosterol

This is achieved through forming a complex to the iron atom of CYP450 preventing substrate binding

Leads to accumulation of 14-alpha methyl steroids which are much more bulky thus destroys the planarity of the hydrophobic phospholipids

Causes cytoplasmic membrane dysfunction = impairs function of ATPase and electron transport systems = cell death

Question 3

Why can’t miconazole be used systemically while fluconazole can?

Answer 3

Miconazole is susceptible to metabolic inactivation, is very lipophilic and highly plasma protein bound = low drug bioavailability

Fluconazole = opposite to above

Question 4

Are azoles selective to fungi?

Answer 4

Human cell contains sterol C-14 demthylase which biosynthesises cholesterol but the difference in binding pocket of the enzyme is enough for a ole to recognise and select the fungal one

Question 5

How does antifungal resistance occur?

Answer 5

Biofilms become less susceptible to the antifungal
Mutations in the active site = inability for antifungals to bind to the enzyme to inhibit its activity leads to increased ergosterol biosynthesis
Gene amplification = over expression of the sterol synthesis gene which encodes for the 14-demethylase enzyme that converts lanosterol to ergosterol
Decreased accumulation of azoles due to overexpression of the ABC and MER genes which encode for efflux pumps = decreased accumulation of azoles

Question 6

Which class of antifungal do amphotericin B and nystatin belong to?

Answer 6

Polyene

Question 7

Why isn’t nystatin used systemically?

Answer 7

Too toxic

Question 8

MoA of polyene antifungal?

Answer 8

The drug aggregates within plasma membrane binding to ergosterol forming a hydrophilic channel in the sterol.

Once the pore is formed this allows leakage of ions into the internal membrane which disturbs the internal pH = enzymes won’t work = cell dies

Question 9

Why isn’t amphotericin B tolerated well?

Answer 9

Even though it has a stronger binding constant to ergosterol, it can still bind to cholesterol in human cells and exert the same effects as it would in a fungal cell = lots of s/e

Question 10

How can you reduce the toxicity of amphotericin B?

Answer 10

Incorporate into lipsosomes = more lipophilic = slowing down the onset of action = increases selectivity for fungal cells = reduces s/e

But this is expensive

Question 11

MoA of Griseofulvin

Answer 11

Prevents fungal cell division

Binds to tubulin = inhibits function of microtubules (organelle transport) and spindles (cell division / mitosis)

Question 12

MoA of 5-flurocytosine

Answer 12

Activity requires destination by cytosine deaminase

Then gets converted to flurouracil which interacts with RNA biosynthesis = inhibits protein synthesis

Other metabolites interact with DNA synthesis

Question 13

How is 5-flurocytosine selective to fungal cells?

Answer 13

As it requires deamination by cytosine deaminase to become active

Human cells have little/no cytosine deaminase activity

Question 14

What is the MoA of Echinocandin antifungals and give an example of a drug in this class?

Answer 14

Caspofungin

Blocks the synthesis of chitin cell wall component 1-3-beta glucan through inhibiting 1-3-beta-glucan synthase

Question 15

What is the difference between fungistatic and fungicidal?

Answer 15

Static = inhibit growth, cidal = kill