Download Medicinal Chemistry of Antifungal Agents

Medicinal Chemistry of
Antifungal Agents
Dr. Afshin Fassihi
Faculty of Pharmacy
MUI Azar,1394
1
Introduction
 Long before Pasteur and Köch works on pathogenic
bacteria Gruby and Schönlein have been studying a
pathogenic fungi, Trichophyton schöenleinii (1839).
 The causative microorganism for thrush
Candida albicans was discovered in the same year.
 But medical mycology was neglected under the
shadow of medical bacteriology, maybe for the nonmalignancy of the fungal diseases.
2
Types of Fungal Infections (Mycoses)
 The term fungi is a general term that includes
both yeasts and moulds.
 Fungal infections fall into two distinct categories:
1) Superficial Mycoses, Dermatophytoses.
2) Deep-Seated Mycoses, Systemic Mycoses.
3
Fungal Cell Structure and Targets
Eukaryotes, like mammalian cells, with many similar
biochemical structures:
• DNA is organized into chromosomes within the nucleus
• Distinct cytoplasmic organelles: endoplasmic reticulum,
Golgi apparatus, mitochondria, and storage vacuoles.
• Cell membrane
This homology to mammalian cells also extends to
biosynthetic pathways, where fungi share similar
mechanisms for DNA replication and protein synthesis.
4
Fungal Cell Structure and Targets
The two primary differences between fungi and
higher eukaryotes have been exploited by
medicinal chemists:
 The constitution of the cell membrane and
 The architecture of the cell wall.
5
Fungal Cell Membrane and Cell Wall
6
Mammalian Cell Membrane
7
Fungal Cell Membrane:
 Lipid bilayars form an unstable structure which cannot
retain its shape and functions. Sterols lie within the lipid
bilayer and act as stiffening agents.
 Sterols, account for approximately 25% of the weight of
the cell membrane.
 The predominant sterol in mammalian cell membranes:
cholesterol,
 The predominant sterol in fungal cell membranes:
ergosterol.
 This difference is a target in selective antifungal design.
8
H3C
CH3
CH3
H
CH3
H
H
H3C
H
H3C
HO
CH3
CH3
CH3
H
Cholesterol
CH3
H
H3C
H
HO
Ergosterol
9
Drugs Affecting Ergosterol
Biosynthesis
10
Drugs Affecting Ergosterol
Biosynthesis
Biosynthesis of Ergosterol in Fungi
 Ergosterol is synthesized from squalene, an ethylenic
hydrocarbon through several steps.
 In this biosynthetic pathway there are four target enzymes
for antifungal drug therapy:
1)
2)
3)
4)
H3C
Squalene epoxidase
14α-demethylase
Δ14-reductase
Δ7, Δ8-isomerase
CH3
19
12
18 11
1
2
HO
CH3
9
3
4
CH3
17
13
16
8
H
7
14
H
CH3
H
H3C
15
6
10
5
Ergosterol
11
12
Squalene Epoxidase Inhibitors




E-Allylamines and Related Compounds
Discovered by Random Screening.
Effective only against nail and skin dermatophytes (narrow
spectrum of activity).
Decrease in fungal membrane ergosterol.
Increase in squalene level which is toxic for the fungi: the main
mechanism of antifungal activity.
13
Allylamine and …
 Mammalian cells have the same enzyme in the
cholesterol biosynthesis pathway with less affinity
toward this inhibitors.
 IC50 for squalene epoxidase in Candida albicans is
0.03μM and for rat liver enzymes is 77μM (Drug:
Terbinafine).
 Some are oral antifungal agents.
N
N
N
A tertiary allyl amine
Unusual substituted
tertiary allyl amines
14
 Naftifine: The first antifungal allylamine,
against tinea (ringworm)
CH3
N
Naftifine
CH3
CH3
N
C
C
C
CH3
 Terbinafine: Stronger, against tinea
and onychomycosis (ringworm of the
nail).
CH3
Terbinafine
CH3
C
CH3
CH3
 Butenafine: Against Candida albicans
in tolnaftate resistant cases.
CH3
N
 Tolnaftate: Against dermatophytosis
caused by:
Tricophytone, Microsporum and
Epidermophytone.
In artificial nail preparations.
Butenafine
CH3
S
N
O
CH3
Tolnaftate
15
14α-demethylase Inhibitors
 An enzyme belonging to CYP450 hemoproteins
 It is involved in mammalian cholesterol synthesis too, but it
needs a higher concentration of drug to be inhibited.
 IC50 for C. albicans demethylase is 10-9 M and for human
enzyme is 10-6 M (Drug: Clotrimazole)
16
Heme Structure
17
CH3H3C
CH3
CH3
CH3
CH3
HO
CH3
O
N N
Fe
N N
H3C
14 alpha-Demethylase Heme
CH3H3C
CH3
CH3
CH3
CH3
HO
CH2OH
H3C
CH3H3C
CH3
CH3
CH3
CH3
HO
CH(OH)2
H3C
CH3H3C
CH3
CH3
CH3
CH3
HO
C(OH)3
H3C
CH3H3C
CH3
CH3
CH3
CH3
HO
H3C
-HCO2H
18
Mechanism of 14α-Demethylase
Inhibition with Azole Compounds
O
CH3
N
N
O
Cl
Cl
 N1 or N3 of azole
antifungals is basic and
bonds to iron atom in the
heme of CYP450, where
the activated oxygen is
bonded normally.
O
O
N
N
N
N
Fe
N
N
 The other parts of the
drug bond to specific sites
of the enzyme.
Ketoconazole
19
Azole Antifungals
 Broad spectrum activity, superficial and deep-seated
mycoses.
 The azole ring can be diazole or triazole.
Diazole antifungals
(Imidazole derivatives)
 Clotrimazole, Miconazole Econazole: In the treatment
of tinea and candidiasis.
Cl
Cl
Cl
O
O
N
N
N
N
C
N
N
Cl
Cl
Cl
Cl
Clotrimazole
Miconazole
Cl
Econazole
20
Azole Antifungals
Ketoconazole: Broad spectrum.
 Orally for systemic mycoses: systemic candidiasis,
coccidiomycosis, thrush and blastomycosis.
 Inhibits the human demethylase too and causes an decrease
in the concentration of testosterone and corticosterone in
human.
 Inhibits other P450s in human so increases the plasma
concentration of cyclosporin, phenytoin or terfenadine.
Cl
Cl
Ketoconazole
O
O
N
N
O
O
N
N
C
CH3
21
Azole Antifungals
Triazole antifungals
 Fluconazole
 Voriconazole
OH
CH3
OH
N
N
N
N
N
F
F
Fluconazole
F
N
N
N
CH3
N
N
F
N
F
Voriconazole
22
Azole Antifungals
Cl
Itraconazole
Cl
O
O
N
N
O
O
N
N
N
N
N
N
F
Posaconazole
F
O
N
O
N
N
N
O
N
N
N
N
OH
23
SAR of Azole Antifungals
1.
A basic imidazole or 1,2,4-triazole is essential for antifungal
activity, N3 of imidazole and N1 of triazole binds to P450 iron.
2.
The most active ones have two or three aromatic rings, at
least one of them is substituted with halogens.
3.
These substitutions are 2,4-dichloro or 2,4-difluoro.
4.
The most active azoles have flour in the structure.
5.
The big non-polar part resembles the steroid molecule in
binding to the enzyme.
24
Δ14-reductase Inhibitors
 Amorolfine the only morpholino compound is such
an inhibitor.
 In the treatment of dermatophytosis.
H3C
CH3
O
N
C
C2H5
CH3
H3C
Amorolfine
25
Δ7, Δ8-isomerase Inhibitors
Amorolfine inhibits Δ7, Δ8-isomerase too.
H3C
CH3
O
N
C
C2H5
CH3
H3C
Amorolfine
26
Polyene Membrane disrupters

Macrocyclic lactones with:
a) hydrophilic parts; a number of hydroxyl groups on the
ring and a desoxy amino hexose (mycosamine).
b) hydrophobic ring with a number of double bonds.
27
Polyene Membrane disrupters
 They belong to two different groups:
a) With a 26 member ring: Natamycin
b) With a 38 member ring: Amphotericin B and Nystatin
Amphotericin B
Natamycin
28
Polyene Membrane disrupters
 Locally used in the treatment of dermal and GI
infections caused by Candida albicans.
 Treatment of deep-seated mycoses caused by:
Candida, Aspergillus, Cefalosporium, Penicillinum
and Fusarium spp.
29
Mechanism of Antifungal Activity of
Polyenes
 Act by binding to ergosterol in the fungal cell
membrane.
 Hydrogen bonding between hydroxyl, carboxyl and
amino groups of the drug and membrane makes the
channel stable.
 Attach with higher affinity to ergosterol containing
membranes than cholesterol containing ones.
 This forms pores in the membrane and increases its
permeability eventually leading to cell death.
30
Mechanism of Antifungal Activity of
Amphotericin B
31
Toxicities of Polyenes
 Stimulation of the host immune cells by amphotericin B:
release of inflammatory cytokines by circulating
monocytes resulting in:
fever, chills, rigor, nausea, vomiting, myalgias,
arthralgias, and headache during intravenous infusions.
 At higher concentrations, amphotericin B binds to
cholesterol in mammalian cell membranes leading to
various organ toxicities, most importantly nephrotoxicity.
32
Fungal Cell Wall:
It is critical for cell viability and pathogenicity:
 A protective shell and provides cell morphology,
 A critical site for exchange and filtration of ions and
proteins,
 A place for metabolism and catabolism of complex
nutrients.
Because mammalian cells lack a cell wall, it represents
an ideal and specific target for antifungal therapy.
33
 It is composed of a complex network of proteins and
polycarbohydrates that varies in composition depending on
the fungal species.
 Disruption of this protein/carbohydrate matrix results in a
structurally-defective cell wall, rendering the fungal cell
sensitive to osmotic lysis.
34
Agents Acting on The Cell Wall:
Glucan synthesis inhibitors
 They do this by inhibiting the enzyme 1,3-beta glucan
synthase.
 It results in depletion of glucan polymers in the fungal
cell, resulting in an abnormally weak cell wall unable
to withstand osmotic stress.
β-1,3 Glucan
35
36
Echinocandins
 Herbal peptide discovered in 1970’s.
 Contains a large cyclic peptide attached to a long
chain fatty acid.
37
Pneumocandin
(Caspofungin)
 Herbal peptide FDA approved which is used to treat
aspergillosis.
38
Nucleoside Antifungals
 Fungal and mammalian cells share remarkable
homology in DNA replication and RNA translation:
DNA and protein synthesis are difficult targets for
developing selective antifungal agents
 The only class of agents in clinical use that target
DNA/RNA synthesis:
Antimetabolites
 This class has only one example:
Flucytosine or 5-fluorocytosine (5-FC).
39
Antimetabolites
Flucytosine
NH2
F
NH
5-FC
N
H
O
Cytosine Permease
Cytosine Deaminase
O
F
NH
N
H
O
5-FU
40
Mechanism of Antifungal Action
O
F
NH
N
H
O
5-FU
O
O
F
F
NH
N
O
NH
O
N
O
O
-
O
P
OH2C
-
O
P
O
P
O
P
O-
5-FdUTP
O
O
P
O-
O
O
P
OH2C
OH
-
OH
O
O
O
O
O-
Blocks DNA synthesis
by inhibiting thymidylate synthetase
O-
O-
O
OH
5-FUTP
Incorporates into RNA,
synthesis of non-sense proteins
41
Mechanism of Adverse Effects of 5-FC
 Developed in the 1950's as a potential antineoplastic
agent but, it was ineffective since mammalian cells
do not have cytosine deaminase but,
Flucytosine can be converted to 5-FU by bacteria
residing in the gastrointenstinal tract.
 The most common adverse effects seen with
flucytosine are similar to 5-FU chemotherapy
(diarrhea, nausea and vomiting, bone marrow
suppression) but at reduced intensity.
42
Miscellaneous Agents:
Griseofulvin
 Isolated from Penicillium griseofulvum.
 Griseofulvin inhibits fungal cell mitosis by disrupting
mitotic spindle formation-a critical step in cellular
division.
 It is used orally for superficial mycoses, enters in the
structure of the precursors of keratin.
43
•
•
•
•
•
•
•
•
•
Tinea is a general term used to describe skin mycoses. The term ringworm is even less
precise, but is usually considered a synonym.
Onychomycosis (also known as "dermatophytic onychomycosis" "ringworm of the nail"
and "tinea unguium") means fungal infection of the nail. It is the most common disease
of the nails and constitutes about a half of all nail abnormalities.
Coccidioidomycosis is a fungal disease caused by Coccidioides immitis or C. posadasii.
The disease is usually mild, with flu-like symptoms and rashes.
Blastomycosis (also known as "North American blastomycosis," "Blastomycetic
dermatitis," and "Gilchrist's disease") is a fungal infection caused by the organism
Blastomyces dermatitidis. can present in one of the following ways:
a flu-like illness with fever, chills, myalgia, headache, and a nonproductive cough which
resolves within days.
an acute illness resembling bacterial pneumonia, with symptoms of high fever, chills, a
productive cough, and pleuritic chest pain. skin lesions, usually asymptomatic, appear as
ulcerated lesions with small pustules at the margins
bone lytic lesions can cause bone or joint pain.
prostatitis may be asymptomatic or may cause pain on urinating.
laryngeal involvement causes hoarseness.
44