Antifungal Agents

Tutorial 1

= Antimycotics

Pharmaceutical Chemistry II

1) Give examples of some common fungal infections indicating whether they are rather

superficial or systemic.

Fungal infections Superficial/Systemic

Tinea corporis

Tinea capitis

Tinea unguium (onychomycosis)

Tinea pedis (Athlethe’s foot)

Tinea cruris (jockitch)

Superficial

(skin, hair, and nails)

Candida albicans

Cryptococcus neoformans

Superficial (oral, vaginal, skin

and nails )/Systemic

Systemic

Blastomyces dermatidus Systemic

Aspergillosis Systemic

1-Inhibitors of Cell

Membrane and its Function

2- Inhibitors of DNA/RNA Functions

3- Inhibitors of Cell Wall Functions

Echinocandins (Caspofungin)

Flucytosin - Griseofulvin

A-Drugs affecting cell membrane function

Polyenes (Nystatin - Amphotericin B)

B-Inhibitors of ergosterol synthesis 1-Azoles

�Imidazoles (Clotrimazole, Miconazole, Ketoconazole)

�Triazoles (Itraconazole, Posaconazole, Fluconazole, Voriconazole)

2-Allylamines Terbanafine , Tolnaftate

3-Morpholines (Amorolfine) Not explained

Cla

ssificatio

n o

f An

tifun

ga

l

Dru

gs

4

Inhibitors of Cell Membrane

A- Drugs affecting cell membrane function

Polyene antibiotics (Nystatin - Amphotericin B)

Primary amine

Mycosamine

Amnio sugar

Carboxylic

acid

2) Describe the structure of amphotericin B and show how its structure is

amphoteric and amphiphilic. Explain its mode of action.

amphoteric: has acidic

and basic group

Large lactone ring (38 membered)

Hydrophilic

Lipophilic

amphiphilic or amphipathic:

Has hydrophilic part and lipophilic part

Polyene: multiple double bonds

2) Describe the structure of amphotericin B and show how its structure is

amphoteric and amphiphilic. Explain its mode of action.

creates an ion-channelcreates an ion-channel

Tunneling→ loss of essen:al cell cons:tuents Tunneling→ loss of essen:al cell cons:tuents

(e.g. K+) → cell death

Mechanism of action

binds to hydrophobic

region of ergosterol

forming a hydrophilic

channel

binds to hydrophobic

region of ergosterol

forming a hydrophilic

channel

PolyenesPolyenes

� The number of conjugated double bonds (nystatin = 4; amphotericin = 7)

�Amphotericin B: the highest activity and the lowest toxicity

3) What are the main structural, pharmacological, and toxicological differences

between amphotericin B and nystatin?

9

Inhibitors of Cell Membrane

B- Inhibitors of ergosterol synthesis 1- Azoles

� Imidazoles (Clotrimazole, Miconazole, Ketoconazole)

�Triazoles (Itraconazole, Posaconazole, Fluconazole,

Voriconazole)

2- Allylamines Terbanafine &Tolnaftate

3- Morpholines (Amorolfine) Not explained

α

∆

10

Ergosterol

Biosynthesis

Inhibitors

14141414α----CHCHCHCH3333

(3)

Morpholines

(1) allylamines

(2) azoles

� Squalene epoxidase Inhibitors → 1. Allylamines

� Lanosterol 14α-demethylase Inhibitors → 2. Azoles

� ∆14-Reductase Inhibitors → 3. Morpholines

Three enzymes can be

inhibited in this pathway

HO

lanosterolHO

4) a) Explain the mechanism of action of azoles.

� Mechanism of action: Inhibition of CYP450 14α–Demethylase ���� lack of ergosterol

needed for the intact membrane ���� accumulation of lanosterol in the fungal cell

membrane ���� permeability change ���� cell death

CYP450

14α–Demethylase

Azoles

14α-Me

b) Why is the cholesterol synthesis in humans usually not affected?

� Cholesterol synthesis in human cells also employing 14α-demethylase but

not affected due to the reduced strength of inhibition, example Ketoconazole:

IC50 (Cand. albicans enzyme) ≈ 10-9 MIC50 (human enzyme) ≈ 10-6 M

IC50 represents the concentration of a drug needed for 50% inhibition.

The lower the ic50 the more potent the inhibitor .

So it inhibits fungal enzyme

with 1000 times more

Azoles

N

NH

imidazole

N

N

NH

triazole

� Clotrimazole

� Miconazole

� Ketoconazole

� Itraconazole

� Posaconazole

� Fluconazole

� Voriconazole

5) a) Indicate the position of the most basic nitrogen in imidazole and triazole.

Explain why using the appropriate equations.

�Imidazole: N3

�Triazole: N4

1

1

2

2

3

34

Protonation of the most basic nitrogen provides a stabilized conjugate acid by resonance.

� N1 lone pair is involved

in resonance, therefore

not basic.

� N1 lone pair is involved

in resonance.

� N2 lone pair is less

available due to the

electron-withdrawing

effect of the adjacent

electronegative N1.

5) b) Explain how the basicity of imidazole and triazole is essential for:

i) the mechanism of action of azoles.

� The 14α-demethylase is a CYP450 enzyme. It has heme (iron)

as co-factor. The basic imidazole/triazole nitrogen in azoles

forms a bond to the heme iron, preventing the enzyme from

oxidizing (demethylating) lanosterol (available lone pair of

nitrogen required for coordinate bond formation with heme).

ii) the solubility of azoles in dilute mineral acids. Explain using the appropriate

equation.

� Protonation of the basic nitrogen gives ionized form with

higher water solubility.

Imidazole or

triazole

heme

Ionization increases water solubility

6) Draw the structure of clotrimazole and outline its synthesis.

Clotrimazole

16

Clotrimazole Synthesis

Clotrimazole

� Oral bioavailability only at low stomach pH < 4, antacids and

H2-histamine antagonists which decrease stomach acidity reduce

absorption. As acid medium is required for protonation and

solublization of the drug.

7)a) Explain the advantages of ketoconazole

over miconazole and clotrimazole.

� The first orally active

antifungal azole.

b) Explain why the oral bioavailability of ketoconazole is decreased when

coadministrated with antacids or H2-antagonists.

Ketoconazole and amphotericin B should NOT be used together, because the decrease in ergosterol in the fungal membrane reduces the fungicidal action of amphotericin B.

c) Explain why ketoconazole and amphotericin B should NOT be used together.

8)a) Encircle and explain two structural differences between ketoconazole

and itraconazole.

Triazole

Imidazole

Active hydroxy-

metabolite (omega-1

hydroxylation)

extensively metabolised to inactive

metabolites (N-deacetylation )

Longer half-life

Itraconazole Ketoconazole

Better tolerated, not hepatotoxic, no

anti-androgenic effects.

Generally triazole derivatives

are safer

Inhibits the synthesis of cholesterol

and other steroid hormones → an:-

androgenic effects (loss of libido

gynecomastia).

More effective, Broad spectrum

antifungal agent than ketoconazole

Also effective against Aspergillus

infections.

Generally triazole derivatives

are more active

not effective against Aspergillus .

Longer half-life (20-30 h) than

ketoconazole (6-9 h), active hydroxy-

metabolite

Ketoconazole is extensively

metabolized to the inactive

deacetylated product

b) Give 3 advantages of itraconzole over ketoconazole.

9)a) Encircle and explain three structural differences between

itraconazole and posaconazole. Which of the structural changes has the

highest influence on metabolism?

� Metabolized mainly by phase II glucoronide conjugation (not by CYP450)

fewer drug interactions (no competition with other drugs for the CYP450)

Tetrahydrofuran

Dioxolane

Posaconazole

b) Why is posaconazole expected to have less drug interactions than

itraconazole and ketoconazole?

CYP 450 (phase I) CYP3A4

KetoconazoleItraconazole Other drugs

-

Drug – Drug Interactions

Posaconzaole

Phase II (conj.)

direct phaseII metabolism

no competition

with other

drugs for the

CYP450

10) In the structure of Itraconazole, encircle the triazole ring responsible for

its mechanism of action as antifungal drug.

Triazole:

basic, binding to heme

part of 14α-

demethylase via N4

1,2,4-triazol-3-one:

N4 in 1,2,4-triazol-3-one

has no available lone pair

for coordination with

heme iron, no binding to

14α-demethylase

4

4

Not basic

Lone pair is involvedin resonance

F

F

HO

N

N

N

N

N

N

11) Draw the structure of fluconazole.

�Encircle the structural elements responsible for its water

solubility.

� What are two major therapeutic advantage of fluconazole

when compared to other azoles.

� Penetrates into CNS and CSF ���� drug of choice for the

treatment of cryptococcal meningitis

� Excellent oral bioavailability (90%), not affected by the

presence of food or pH

� Long half-life 27-34 h

13) What is the major difference between fluconazole and

voriconazole in terms of metabolism and activity?

Fluconazole Voriconazole

Treatment of candidiasis and

cryptococcosis

Active against Aspergillus, more

potent against Candida

Little hepatic metabolism,

excreted unchanged in the

urine

Extensive CYP450

biotransformation ���� drug

interaction (methyl hydroxylation,

N-oxidation)

Study Guide (tut 1)� The first few lecture slides describing different types of fungal

infections and their characteristics are background preparatory

slides to understand the treatment but not to memorize.

� The exact species of Tinea infections are not to be memorized.

� Structures to be memorized : Clotrimazole and fluconazole.

� All the other structures in the tutorial and the lecture slides are

to be recognized (you will be given the drug structure and

you have know the name).

� The steps of ergosterol synthesis (slide 10) are not to be

memorized but you have to know only the target enzymes

names which are inhibited by the drugs.

� The lecture still has many drugs which are not discussed here