lec 5- Anti-fungal agents Flashcards
1
Q
Infections caused by yeast
A
- Candida albicans causes infections of mucous membranes (thrush) and blood stream
- Cryptococcus Neoformans causes pneumonia and meningitis
2
Q
Infections caused by moulds
A
- Filamentous fungi (dermatophytes)
- Trichophyton rubrum
3
Q
Human ringworm
A
- Tinea corporis
- Classic circular ringworm inflammatory lesion- spreading outwards/healing in the centre
- Trichophyton sp.
- Athletes foot (Tinea pedis)
- Broken skin and inflammation lesions between toes
- Topical: Azole or Terbinafine
- Allylamine= C=C bond with CH2 then amine
4
Q
Targets for Antifungal drug development
A
- Mannoproteins
- Caspofungin
- b-(1,6)-glucan
- b-(1,3)-glucan
- Chitin
- Phospholipid bilayer- Ergosterol- polyenes
- DNA/RNA synthesis- 5-FC
- Ergosterol synthesis pathway- imidazoles, triazoles
- Squalene- terbinafine
5
Q
Ergosterol/Cholesterol antifungal drugs
A
- Work by exploiting differences between mammalian and fungal cells to kill the fungal organism without dangerous effects on the host
- Both fungi and humans eukaryotes
- Difficult to find or design drugs that target fungi without affecting human cells (side effects)
- Fungal cell membranes have unique sterol (ergo)- which replaces cholesterol in the mammalian cell membrane
6
Q
Sterol chemical structures
A
7
Q
Cholesterol/Ergosterol- Similarities and Differences
A
Similarities
- Ring A/B/C/D tetracyclic
- Template and side chain
- Secondary alcohol
Differences
- The double bond in ring B (diene)
- A side chain Me, double bond
- These Structural differences are key to antifungal chemotherapy
8
Q
Amphotericin B
A
- Target = Ergosterol
- Broad spectrum polyene macrolide antibiotic
- Large ring cyclic ester
- Most potent antifungal agent for systemic mycosis, in clinical use since 1960
- Fungicidal drug at higher concentrations and static at lower levels
Produced by Streptomyses nodosum natural product
- CSF conc= 2-3% of blood conc
- Highest concentration in liver, spleen, bone marrow with less in kidneys and lungs
9
Q
Mechanism of action of polyene antifungal
A
- Amphotericin B
- Binds to ergosterol
- Intercalation of cell membrane (makes pores/holes into the membrane)
- allows ions (Na, K, Ca) as well as proteins to flow out of the cell
- low conc = fungistatic
- High conc= fungicidal
10
Q
Mechanism of action
A
- Binds to fungal cell membrane (ergosterol) =>
- Forms pores =>
- Alters permeability and transport =>
- Cell death
11
Q
Clinical Use
A
- Treatment of nearly all life threatening mycotic infections
- For systemic disease: slow IV
- Local: topical application
- Cholesterol, present in host cell membranes, closely resembles fungal ergosterol
- Result: High toxicity of AM-B in humans
- Highlight- Fungal arthritis = Local injection
12
Q
Liposomal amphotericin B
A
- New lipid formulations
- Amphotericin B is incorporated into lipid formulations to reduce toxicity and enhance efficacy + bioavailability
- This allows higher dose to be used without increasing the toxicity
- Much more expensive than ordinary AM-B
13
Q
Target: Cell wall- antifungal agents
A
- Cyclic lipo-peptide
- Caspofungin (Cancidas, Merck): life-threatening systemic mycoses but not cryptococcal meningitis
- Structure: A semi-synthetic echinocandin Cyclic lipoprotein
- Mechanism: inhibition of synthesis of 1,3,-b-D-glucan in fungal cell walls
- Anidulafungin (Ecalta, Pfizer): similar to caspofungin
14
Q
Caspofungin- structure
A
- cyclic
- Amide bonds
15
Q
Echinocandins
A
- The newest class of antifungal
- Active against candida and aspergillus (mould + yeast)
Adverse effects
- Extremely well tolerated- specific to fungi cell wall
- Minor GI side effects
- Flushing
- Elevated Liver enzymes (Caspofungin + Cyclosporine)
- Histamine release during IV infusion
16
Q
Mechanism of action
A
- Inhibit the synthesis of b-glycan in the fungal cell wall =>
- Disruption of the fungal cell wall and cell death
17
Q
Molecular target DNA
A
FLUCYTOSINE 5-FC
- 5-fluorocytosine
- pyrimidine
- Compare 5-FU anticancer agent
18
Q
Flucytosine (5-FC)
A
- Pyrimidine Anti-metabolite, narrow-spectrum fungistatic activity
- Water-soluble- good BBB penetration
- Oral only
- poor protein binding
- CSF conc- 75% serum concentration- Excellent CSF
- Treatment of fungal meningitis- BBB penetration
19
Q
5-fluorocytosine inhibits DNA synthesis in fungi
A
- 5-Fluorocytosin =(fungal deaminase)=>
- 5-Fluorouracil ==>
- 5F-dUMP => blocks conversion to dTMP by thmidylate synthetase, DNA synthesis inhibited
- 5-FU = anti-cancer
20
Q
Mode of action
A
- It is converted only in fungus by fungal deaminase to 5-FU- desamination
- 5-FU is then converted to 5-fluorodeoxyuridine monophosphate (FdUMP)- glycosylation
- And then fluorouridine triphosphate (FUTP), phosphorylation/ kinase
- Which then inhibit DNA and RNA synthesis (enzyme inhibitor)
- BONUS: Flucytosine is taken up by fungal cells via the enzyme cytosine permease- Selective uptake
- Not in humans
21
Q
Why doesn’t the drug act on human cells
A
- Human cells are unable to convert the parent drug to its active metabolites
22
Q
Adverse effects
A
- Bone marrow toxicity with anemia, leukopenia thrombocytopenia
- Mammalian bone marrow cell have the capacity to covert 5-FC => 5-FU
- GI disturbance
- Mild and reversible liver dysfunction
23
Q
Griseofulvin
A
- A very insoluble molecule, poor bioavailability of drug (need for new formulations)
- Fungistatic- derived from a species of Penicillium, isolated natural product
- Better absorption when given with fatty foods
- It is deposited in the newly forming skin where it binds to keratin, protecting the skin from new infection
- Interferes with spindle formation in dividing cells and therefore with mitosis, unique mode of action
- Spiro compound
- Chiral
- natural product- benzofuran
24
Q
Overview: antifungal agents
A
- Griseofulvin- oral treatment of dermatophytes
- 5-fluorocytosin (5-FC)- life-threatening systemic mycoses (candidiasis, cryptococcal meningitis)
- Polyenes (amphotericin, nystatin): Life-threatening systemic mycoses (candidiasis, aspergillosis, cryptococcal meningitis)
- Imidazole (miconazole, ketoconazole)
- Triazoles (Fluconazole, intraconazole): candidiasis superficial
- Candidal infection (thrush): topical mycoses
- Allylamines (terbinafine)- dermatophytes (skin, nails)
- Echinocandins- newest class, act on cell wall
25
From metronidazole to imidazole lead structure
* **Metronidazole active**: anaerobic organisms (*Trichomonas vaginalis*, clostridia, *Bacteroides* species)
* Converted to active radical of the nitro group inside the cells which causes DNA strand breakage
26
Azoles- TARGET= Ergosterol synthesis
1. Imidazoles
* Ketoconazole
* Miconazole
* Clotrimazole
2. Triazoles
* Intraconazole
* Fluconazole
* Voriconazole
* Posaconazole
27
Triazoles
* High CSF: Serum conc ration is very high for fluconazole meaning it can be used for fungal meningitis
* Higher half-life means OD administration
*
28
Differences AZOLES
* BROAD SPECTRUM OF ACTIVITY
* Differences in water solubility
* Absorption, membrane penetration
* Half-life/ elimination
* ADME not MOA
* Formulation, route of administration
29
Feature
* **Fluconazole**
* High water solubility
* High membrane penetration
* Half-life
* Imidazole, triazole, extra nitrogen atom, N polar
* Small, low MW
* F atom in para position blocks metabolism, hydroxylation
30
Adverse effects
* Relatively nontoxic- allow slight toxicity because they are only acute treatment
* Minor GI upset
* Abnormalities in liver enzymes- Inhibit cytochrome P450 enzymes
* Very rarely, clinical hepatitis
31
Mechanism of Action
* Inhibition of fungal cytochrome P450 enzymes ==\>
* Reduction of ergosterol synthesis
32
ChE synthesis
* Lanosterol-Demethylase= P450 enzyme
33
Ergosterol synthesis
34
AZOLES
* Inhibition of biosynthesis, ergosterol
* In humans, lanosterol gets converted into ChE
* In micro-organism, it gets converted into ergosterol
* P450, oxidising enzymes, catalyse the oxidation
* Special case, demethylation, 14-alpha-demethylase
* Newly formed double bond, in conjugation, to double bond
35
Ketoconazole
* Older **Imidazole**, more toxic, replaced by itraconazole, but less costly
* The **first** oral AZOLE introduced into clinical use
* It is **less selective** for fungal P450 than are the newer azoles
* Absorption variable (better in acidic medium)
* Penetration in brain & CSF is poor
* In high doses inhibits adrenocortical steroids and testosterone synthesis, resulting in gynecomastia in some males
36
Itraconazole
* Triazole
* Broad-spectrum antifungal with fungistatic action
* MOA: inhibits fungal ergosterol synthesis like other azoles
* Drug absorption is increased by good and by low gastric pH
* Pentration of drug in brain& CSF in poor
* Much more selective than ketoconazole
37
Posaconazole
* The newest triazole
* It is the broadest spectrum member of the azole family
* Selectivity: it is the only azole with significant activity against the agents of zygomycosis and mucormycosis
38
Fluconazole
* Broad-spectrum fungicidal drug
* It is also effective against some Gram-positive & anaerobic bacteria
* Good oral bioavailability, 94% is absorbed
* Penetration in brain & CSF is good, hence used for cryptococcal **meningitis**
*
39
Topical AZOLES
* Clotrimazole, miconazole
* Vulvovaginal candidiasis, oral thrush, dermatophyte infections, including tinea corporis, tinea pedis and tinea cruris
* **Absorption** is negligible, and adverse effects are rare
* Topical and shampoo forms of **ketoconazole** for seborrheic dermatitis and pityriasis Versicolor
40
Topical Allylamines
* Terbinafine & Naftifine
* **Naftifine:** Synthetic allylamine
* Both are effective for treatment of tinea cruris and tinea corporis
* Terbinafine concentrates in skin and especially at nail beds, making it quite useful for fungal nail infections
* **Inhibition biosynthesis ergosterol**
* MOA: Terbinafine inhibits squalene epoxidase in the early stage of ergosterol synthesis in fungi
* Accumulation of intracellular squalene (toxic to the organism) & deficient ergosterol synthesis subsequent fungal cell death
* Keratophillic, fungicidal
* Dermatophytoses, Human ring worm
41
MOA of allylamines
* Like the azole drugs, allylamines interfere with ergosterol biosynthesis
* But rather than interacting with the P450 system/ 14-Alpha-demethylase
* Terbinafine inhibits the fungal enzyme squalene epoxidase (similar structure to squalene)
* This leads additionally to the accumulation of the sterol **Squalene** which is toxic to the organism
42
Overview biosynthesis ergosterol
43
Targets for antifungal drug development
