PHARMACEUTICAL CHEMISTRY II (PHCM672)

Lecture 1, Antifungal Drugs (Antimycotics)

Dr. Mohammad Abdel-Halim

Pharmaceutical Chemistry I

Pharmaceutical Chemistry II

Pharmaceutical Chemistry III

Pharmaceutical Chemistry IV

Antibacterial drugs 1) Antifungal Drugs

2) Antihelmintics and

ectoparasiticinfections

3) Antimalarial Agents

4) Antimycobacterial Agents

5) Antiprotozoal Drugs

6) Antiviral Drugs

7) Anticancer Drugs

Drugs affecting CVS,

antihistaminics and NSAIDs

Ch

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oth

era

pe

uti

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ag

en

ts

Drugs affecting CNS, hormones

and related drugs, and vitamins

� Fungal kingdom: yeasts, molds, rusts, and mushrooms (~ 1 million species, ~ 300 pathogenic for men)

� Most fungal infections (mycoses) are caused by yeasts and molds

� Most fungi live on dead organic matter (in the soil, on leaves or wood)

� A few can cause infections through wounds or inhalation

� Only few can only live on mammalian hosts (e.g. Candida albicans is a part of a normal flora of the GI tract and vagina)

Fungi and fungal infections

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� Cutaneous mycoses: include ‘’dermatophytoses ‘’ caused by

dermatophytes also can be by Candida albicans , infection is limited to the

epidermis, (Most common)

� Subcutaneous when the infection penetrates significantly beneath the skin,

� Systemic : The deep-seated, systemic mycoses, when the infection is deep

within the body or disseminated to internal organs (Less common).

Human fungal diseases (mycoses) are classified by the

location on or in the body where the infection occurs:

Systemic Infections can be due to :1) True pathogenic fungi capable of infecting healthy individuals, 2) Opportunistic fungi OPPORTUNISTIC MYCOSES in

individuals who have predisposing conditions such as immunodeficiency or debilitating diseases (for example, AIDS)and use of immunosupressive drugs for organ transplants and cancer chemotherapy.

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Examples of infectins by fungi

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athlete’s foot

onychomycosis

face, arms and shoulders scalp

groin area

toe and finger nails

Dermatophytic infections (Tinea, ringworm)

Fungi use keratin as a

source of nutrition. This

ability allows them to

infect keratinized tissues

and structures, such as

skin, hair, and nails.

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Candida albicans

� Major cause of oral (thrush) and vaginal yeast infections

� Occur when the normal population of flora disturbed by treatment of a bacterial infection with an antibiotic

� Infections of the skin and nails possible

� In persons with healthy immune system only superficial infectiones of the skin and mucosa

� In persons with impaired immune system deep-seated systemic infections possible (death threatening) , “OPPORTUNISTIC”

� Found in bird droppings (pigeons)

� Inhalation of the dust contaminated with spores can cause a minor lung infection often mistaken for a cold

� For immunocompromised persons spreading via the circulatory system possible including CNS infections (fatal without treatment)

Cryptococcus neoformans “OPPORTUNISTIC”

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Blastomyces dermatidus

� Causes Blastomycosis Dimophic : Grow in one form in soil at room temparature and in a different form in the human host at 37oC after inhalation

• The resulting lung infections often mild and self-limiting, severe systemic infections possible)

Aspergillus species

� Cause Aspergillosis: “OPPORTUNISTIC”

Aspergillus species (A. fumigatus, A. niger, A. flavus)

� Dangerous to persons with supressed Immune system , high mortality rate for systemic aspergillosis

� infection through inhalation, wounds, and implanted devices (e.g. catheters)

: Aspergillus species

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� Number of antifungal drugs are limited compared to antibacterial agents

Antifungal Chemotherapy

� Both fungi and mammals are eukaryotes (bacteria are prokaryotes) → small biochemical difference → difficult search for selective agents

� Fungal cells have cell walls (mammalian cells do not) → fungal cell wall a

potentially good target for selective drugs. However, inhibitors of the fungal cell wall biosynthesis became available only recently

� Fungal cell membranes contain ergosterol

mamalian cell mambranes contain cholesterol

HOHOHH HH

Ergosterol Cholesterol

vs.

Two major differences between fungal and mammalian cells :

1- Inhibitors of Cell

Membrane and its Function

2- Inhibitors of Cell Wall Synthesis

Echinocandins (Caspofungin)

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

Cla

ssificatio

n o

f An

tifun

ga

l

Dru

gs

3- Inhibitors of DNA/RNA Functions/ mitosis inhibitors

Flucytosin - Griseofulvin

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Inhibitors of Cell Membrane

A- Drugs affecting cell membrane function

Polyene antiboitics (Nystatin - Amphotericin B)

Natural products (Antibiotics) isolated from cultures of Streptomyces species

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1) Amphotericin B

� Breakthrough in systemic antifungal therapy (1956)

� Relatively of low toxicity to human cells permitting IV administration

� Named after the amphoteric behaviour (carboxy group + amino group)

� Macrocyclic lactones including conjugated double bonds (polyenes), OH-groups, a carboxyl group, and an aminosugar mycosamine

→ two distinct regions: hydrophobic and hydrophilic

hydrophobic region

hydrophilic region

mycosamine

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� Drug of choice for many systemic life-threatening fungal infections

� Can not cross the blood-brain barier: inthratecal administration (into the spinal cord) for treatment of CNS infections

� Insoluble in water, formulated as a complex with deoxycholic acid

� Nephrotoxic

� New formulations such as liposomal encapsulation decreased toxicity (blood vessels at the site of infection are more permeable for the lipid formulations than those of normal tissue →→selective delivery of the drug to the site of infection, see the next slide.

1) Amphotericin B

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A. Amphotericin B intercalated between the phospholipids of a spherical liposome (AmBisome®).

B. Outcomes of antifungal therapy in cancer patients treated with conventional amphotericin B and liposomal amphotericin B.

1) Amphotericin B

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2) Nystatin (Mycostatin®)

Discovered in the New York State

Health Lab (1951)

O

O

HO

OH

O

OH

OH

OH

OH

OHOHOHO

O O

OH

NH2

HO

� First clinically used polyene antifungal used topically in creams ointments

and oral suspension

� Used for vaginal (vaginal tablets) and cutaneous candidiasis (topically)

� No systemic application (too toxic due to high affinity to membrane bound

cholesterol in human cells)

� No absorption from the GI tract → oral application to treat fungal infections

of the mouth and GI tract e.g GI candidiasis

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binds to hydrophobic

region of ergosterol

forming a hydrophilic

channel

binds to hydrophobic

region of ergosterol

forming a hydrophilic

channel

PolyenesPolyenes

� Polyenes mechanism of action: insertion into cell membranes → tunneling, loss of essential cell constituents (e.g. K+) → cell death

� The number of conjugated double bonds (nystatin and

amphotericin 7) correlates directly with antifungal activity, and inversely with

toxicity to mamallian cells → amphotericin B: the highest activity and the lowest toxicity

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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 will Not to be discussed

CH3HO

lanosterol

HO

HO HO

O

Ergosterol

squalene epoxidase

lanosterol 14 -demethylase

14-reductase

squalene

14α

∆

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ErgosterolBiosynthesis

Inhibitors

14141414α----CCCCHHHH

(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

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1- Azoles: Imidazoles and Triazoles 1- Azoles: Imidazoles and Triazoles

� Tha largest class of antimycotics (> 20 drugs in the market for both superficial and systemic mycoses)

� Broad spectrum of activity, oral bioavailability of some analogs

Azoles Can be

OR

RNX

N

FeN

N

N

N

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The basic imidazole/triazole nitrogen of the drug forms a bond to

the heme iron preventing the enzyme from oxidizing

its normal substrate

14α-demethylase

The 14α-demethylase is CYP450 enzyme, it has heme co factor .

1- Azoles: Imidazoles and Triazoles

� Mechanism of action: Inhibition ofCYP450 14α-demethylase → lack of ergosterol needed for the intact membrane → accumulation of lanosterol in the fungal cell membrane → permeability change and dysfunction of membrane-embedded proteins → cell death

� Cholesterol synthesis in human cells (also employing 14α-demethylase) is muchless affected due to the reduced strengh of inhibition for the human enzyme

e,g,Ketoconazole has 1000 fold higher affinty to bind the fungal enzyme

Imidazole or

triazole

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Selected Imidazole Antimycotics

1) Clotrimazole (Canesten®)

� The simplest prototype structure

� pKa 6.1

� Practically insoluble in water, soluble in dilute mineral acids

�Used only topically for the treatment of many tinea infections and candidiasis (vaginal candidiasis).

� Available as a solution in polyethylene glycol, lotion, cream, and powder.

N

Cl

N

basic

Clotrimazole is not considered suitable for the

teatment of systemic infections.

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2) Miconazole (Daktarin®)

� Bis(dichlorobenzyl)-ether

� pKa 6.9

� Sparingly soluble in water

� Used also as mononitrate salt

� For the treatment of both superficial and systemic infections

� Injectable form of the free base solubilized with polyethylene glycol → treatment of severe systemic infections

� Cream, lotion, powder, and spray for the treatment of tinea and cutaneous infections

dichlorobenzyl

dichlorobenzyl

ether

N

N

O

Cl

Cl

Cl

Cl

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3) Ketoconazole (Nizoral®)

� logP 2.88 → more hydrophilic than clotrimazole (5.76) and miconazole (6.42)

� Slightly soluble in water

� pKa 6.5, bioavailability only at low stomach pH < 4, antacids and H2-histamine antagonists which decrease stomach acidity reduce absorption

� Metabolism Extensively metabolised to inactive metabolites (one step: N-deacetylation)

� Ketal

� The first orally active antifungal azole

N-deacetylation

Ketal

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� Indicated for a broad spectrum of systemic infections

� Not effective against Aspergillus or Cryptococcus

� Also used orally to treat severe cutaneous dermatophytic infections not responsive to topical therapy

�Topically: creams and shampoos against cutaneous candidiasis, tinea infections

� Side effects

1- Hepatotoxic

2- Inhibition of the synthesis of cholesterol and other steroid hormones → anti-androgenic effects (loss of libido, gynecomastia)

- today´s clinical use limited to topical applications

- replaced by itraconazole: a systemic agent with less side-effects and an expanded antifungal spectrum (next slide)

3) Ketoconazole (Nizoral®)

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Selected Triazole Antimycotics

Itraconazole (Sporanox® ,1984)

� Structurally related to ketoconazole

1,2,4-triazol-3-one

1,2,4-triazole

� More effective and better tolerated, orally active, broad-spectrum antifungal agent than ketoconazole (but much more expensive)

ω-1 Hydroxylation

(Active)

1

2

3

4

Both Ketoconazole and itraconazole are powerful CYP3A4 inhibitors →

so, we have to decrease the conc. of coadministrated drugs which are

CYP3A4 substrates, such as the hypnotic triazolam, the immuno

suppresant cyclosporin.

� Advatages over ketoconazole

i. Not hepatotoxic, no anti-androgenic effects

ii. Effective against Aspergillus infections

iii. Longer half-life (20-30 h) than ketoconazole (6-9 h), active hydroxy-metabolite (ω-1 hydroxylation)

� Like ketoconazole : oral absorption requires acidic conditions (see ketoconazole), food doubles oral bioavailability

� No penetration into CNS and CSF (cerebrospinal fluid) →not used to treat meningitis and fungal CNS infections

1) Itraconazole (Sporanox® ,1984)

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2) Posaconazole

� Novel itraconazole analog designed after its active ω-1 hydroxyl metabolite:

tetrahydrofuran

� Used to treat invasive infections by Candida, and Aspergillus species in immunosupressed patients

� Metabolized mainly by phase II glucoronide conjugation (not by CYP450) → fewer drug interactions (adavantage over keoconazole and itraconzole

phase II glucoronide conjugation

Red parts in the structure indicate the differences from itraconazole

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3) Fluconazole (Diflucan®,1984)

� Two triazol rings + hydroxy group → water soluble (logP 0.31) → as a free base suitable for both oral and iv administration

� Excellent oral bioavailability (90%) not affected by the presence of food or pH

� Long half-life 27-34 h

� For the treatment of candidiasis and cryptococcosis

� Penetrates into CNS and CSF → drug of choice for the treatment of cryptococcal meningitis

� Topically against vaginal candidiasis

� Little hepatic metabolism, excreted unchanged in the urine

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4) Voriconazole (VFEND®)

� Fluconazole analog: additional methyl group, one triazol replaced by fluorinated pyrimidine

� Unlike fluconazole, active against Aspergillus and more potent against Candida (new standard in the treatment of invasive Aspergillosis)

� Good oral absorption and penetration of blood-brain barrier

� Metabolism Unlike fluconazole, extensive CYP450 biotransformation → drug interactions

→ methyl hydroxylation,→ N-oxidation)

C CH2CN

F

F

N

N

OHN

N

CH3

HF

� Broader spectrum of activity

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Imidazole

triazoles

Triazoles are superior to imidazoles

Squalene Epoxidase Inhibitors

� Effective only against dermatophytes → to treat skin and nail infections

� Inhibition of squalene epoxidation - an early stage of ergosterol synthesis 1) decrease in total sterol content of the fungal cell membrane leads to disfunction of proteins involved in nutrition transport

2) concentration of toxic squalene ↑

� Cholesterol synthesis in humans unaffected because the drugs are selective for the fungal squalene epoxidase at applied concentrations

Squalene Epoxidase Inhibitors

2) Terbinafine·HCl (Lamisil®)

� Employed topically (t. pedis, t. cruris, t. corporis) and orally against onychomycosis (ringworm of the nails)

Allyl amine

1) Tolnaftate (Tinactin®)

� Thiocarbamate

� Topical treatment of ringworm, jock itch and athlete´s foot

� Old over the counter drug

O

S N

CH3

CH3

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Inhibitors of Cell Wall

Echinocandins (Caspofungin)

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Echinocandins

Echinocandins: “penicillins of antifungal drugs “Expected to be nontoxic for humans as human cells do not have cell walls

� Semi-synthetic cyclic peptide with long lipophilic side chain

� Mode of action Inhibitor of 1,3-β-glucan synthase

� Used in treating life-threatening systemic infections

� Effective against Candida species resistant to other drugs

� Effective against azole-resistant Aspergillus

� Not effective against Cryptococcus neoformans

� No oral bioavailability; IV administration

� Limited hepatic metabolism → no drug interactions

Caspofungin

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H2NNH

O

HO

OH

OH

NH

N

OH

HN

O

HN

O

CH3 CH3

CH3

NH

N O

HO

O

HN

H2N HO

OOH

O

OH

Caspofungin

cyclic peptide

long lipophilic side chain

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Intact

cell wall

After

treatment

with

caspofungin

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Inhibitors of DNA/RNA Functions/Mitosis inhibitors

Flucytosine - Griseofulvin

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Flucytosine

� 5-FU enters the pathways of RNA/DNA-Synthesis ‘‘antimetabolite‘‘→cell death

� Narrow spectrum of activity: indicated to treat severe systemic infections of Candida and Cryptococcus species

� Human cells do not contain cytosine deaminase

� Orally active

� Can be converted to 5-FU by intestinal flora → human toxicity

Inetrferes with DNA/RNA

Prodrug

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Griseofulvin (Grisactin®)

� Antifungal antibiotic from Penicillium griseofulvum

� Spiro structure (two rings having only one common atom)

� Used orally to treat fingernail and toenail infections (topical use ineffective)

� Gets incorporated in newly synthesized keratin

� Does not affect infected keratin → long-term therapy (several months) until the new nail grows

� Mode of action: The drug binds to tubulin, interfering with microtubule function, thus inhibiting mitosis (fungistatic)

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