Transcript What are the targets for antifungal therapy?

An Introduction to Anti-fungal Pharmacology
The following slides were generously supplied by
Professor Russell E. Lewis, Pharm.D., BCPS
University of Houston College of Pharmacy, University of
Texas M.D. Anderson Cancer Center.
With lecture notes written by Hannah Woodcock and Jenny
Bartholomew, University of Manchester, UK.
Types of fungal infections - Mycoses
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Superficial mycoses
 Affect the skin, hair and nails
Subcutaneous mycoses (tropical)
 Affect the muscle and connective tissue immediately
below the skin
Systemic (invasive) mycoses
 Involve the internal organs
 Primary vs. opportunistic
Allergic mycoses
 Affect lungs or sinuses
 Patients may have chronic asthma, cystic fibrosis or
sinusitis
There is some overlap between these groups
What are the targets for antifungal
therapy?
Cell membrane
Fungi use principally ergosterol
instead of cholesterol
DNA Synthesis
Some compounds may be
selectively activated by fungi,
arresting DNA synthesis.
Cell Wall
Unlike mammalian cells, fungi
have a cell wall
Atlas of fungal Infections, Richard Diamond Ed. 1999
Introduction to Medical Mycology. Merck and Co. 2001
Cell Membrane Active Antifungals
Cell membrane
• Polyene antibiotics
- Amphotericin B, lipid
formulations
- Nystatin (topical)
• Azole antifungals
- Ketoconazole
- Itraconazole
- Fluconazole
- Voriconazole
- Miconazole, clotrimazole (and
other topicals)
Azole Antifungals for Systemic
Infections
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Ketoconazole (Nizoril)
Itraconazole (Sporanox)
Fluconazole (Diflucan)
Voriconazole (Vfend)
Fluconazole
Ketoconazole
Imidazole
Triazoles
“2nd generation
triazole”
Azoles - Mechanism
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In fungi, the cytochrome P450enzyme lanosterol 14-a
demethylase is responsible for the
conversion of lanosterol to
ergosterol
Azoles bind to lanosterol 14ademethylase inhibiting the
production of ergosterol
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Some cross-reactivity is seen with
mammalian cytochrome p450
enzymes
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Drug Interactions
Impairment of steroidneogenesis
(ketoconazole, itraconazole)
Effect of azoles on C. albicans
Before exposure
After exposure
Azoles - Pharmacodynamics
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Concentration-independent fungistatic
agents
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Dosage escalation may be necessary
when faced with more resistant fungal
species (e.g. Candida glabrata)
Goal of dosing is to maintain AUC:MIC
>50
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i.e. maintain concentrations 1-2 x MIC for
the entire dosing interval
Ketoconazole
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Spectrum: yeasts and moulds - poor
absorption limits its role for severe infections,
generally used in mucosal infections only
Pharmacokinetics
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Variable oral absorption, dependent on pH (often
given with cola or fruit juice)
T1/2 7-10 hours
Protein binding > 99%
Hepatic, bile and kidney elimination
Ketoconazole - Adverse effects
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Adverse effects
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N&V, worse with higher doses (800 mg/day)
Hepatoxicity (2-8%), increase in transaminases,
hepatitis
Dose related inhibition of CYP P450 responsible
for testosterone synthesis
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Gynecomastia, oligosperma, decreased libido
Dose-related inhibition of CYP P450 responsible
for adrenal cortisol synthesis
Ketoconazole - Drug Interactions
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Potent inhibitor of cytochrome P450 3A4
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Rifampin and phenytoin decrease ketoconazole
levels
Ketoconazole increases cyclosporin, warfarin,
astemizole, corticosteroid, and theophylline levels
 Many of these drug interactions are
severe
Drugs that increase gastric pH will decrease
blood levels of ketoconazole
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Antacids, omeprazole, H2 blockers
Ketoconazole - Dose
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Serious infections 800 mg/day PO
Other: 200-400 mg/day PO
Cost $2.50 per 200 mg tablet
Fluconazole
Advantages
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Well tolerated
IV/PO formulations
Favorable
pharmacokinetics
Disadvantages
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Fungistatic
Resistance is
increasing
Narrow spectrum
(Drug interactions)
Fluconazole - spectrum
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Good activity against C. albicans and
Cryptococcus neoformans
Non-albicans Candida species more
likely to exhibit primary resistance
Always resistant
C. krusei
>
Sometimes resistant
C. glabrata >
C. parapsilosis
C. tropicalis
C. kefyr
Fluconazole - resistance
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Primary resistance (seen in severely ill or
immunocompromised patients)
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Selection of resistant species or subpopulations
Replacement with more resistant strain
Secondary resistance (seen in patients with
AIDS who experienced recurrent
orophayrngeal candidiasis and received longterm fluconazole therapy)
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Genetic mutation
Upregulation of efflux pumps
Mechanisms of antifungal resistance
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Target enzyme
modification
Ergosterol biosynthetic
pathway
Efflux pumps
Drug import
White TC, Marr KA, Bowden RA.
Clin Microbiol Review 1998;11:382-402
Fluconazole - What is not covered
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Candida krusei
+/- Candida glabrata
Aspergillus species and other moulds
Fluconazole - Pharmacokinetics
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Available as both IV and PO
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Bioavailibility > 90%
Linear pharmacokinetics
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t 1/2 = ~24 hours
Cmax (400 mg IV) = 20 µg/ml (steady state)
Protein binding < 12%
Vd 0.85 L/kg (widely distributed)
>90% excreted unchanged through the kidney
Fluconazole - adverse
effects/monitoring
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N&V, rash:
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More likely with high doses and in AIDS
patients
Asymptomatic increase in LFTs (7%)
Drug interactions:
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May increase phenytoin, cyclosporin,
rifabutin, warfarin, and zidovudine
concentrations
Rifampin reduced fluconazole levels to half
(even though FLU is not a major substrate)
Fluconazole - Dosing
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Mucosal candidiasis
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Systemic fungal infections
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100-200 mg/day (150 mg tablet vulvovaginal
candidiasis)
400-800 mg q24h
> 800 mg q24h in unstable patient, S-DD isolate,
or if non-albicans spp. (except C. krusei)
Maintenance for cryptococcal meningitis
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400 mg q24h
Key Biopharmaceutical Characteristics of
the Triazole Antifungals
Fluconazole
Itraconazole
Spectrum vs.
Candida and
Aspergillus
C. albicans, C.
tropicalis +/C. glabrata
No Aspergillus
Similar Candida
coverage as
fluconazole, +
Aspergillus
Broad, includes
most Candida spp.,
Aspergillus,
Fusarium sp. Not
Zygomycoses
Oral formulation
(% bioavailibility)
Tablet (>90%)
Capsule (6-25%)
Solution (20-60%)
Tablet (>90%)
Intravenous
formulation
Available, no
solubilizer
Available, cyclodextrin
Available,
cyclodextrin
Clearance
Renal (80%)
Hepatic 3A4
Hepatic 2C19, 3A4
Serum half life (hr)
24
24-30
6-24
CSF penetration
Excellent
Poor
Excellent
CYP 3A4 inhibition
Weak
Strong
Moderate-Strong
Adverse effects
N&V, hepatic
N&V, diarrhea
(solution), hepatic, CHF
N&V, visual
disturbances,
hepatic, rash
R.E. Lewis 2002. Exp Opin Pharmacother 3:1039-57.
Voriconazole
Itraconazole Solution - Side Effects
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Taste disturbances
Nausea and vomiting
Osmotic diarrhea (especially at doses >
400 mg/day)
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Long-term compliance often difficult
Voriconazole - Side Effects
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Visual disturbances (~ 30%)
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Decreased vision, photophobia, altered color
perception and ocular discomfort
IV > oral
No evidence of structural damage to retina
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Reversible alterations in function of retinal rods and cones
Testing 2 weeks after the end of treatment demonstrates a
return to normal function
Long term effects?..caution against night-time driving
Effects may be intensified by hallucinations (2-5%)
Amphotericin B
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Polyene antibiotic
Fermentation product of Streptomyces
nodusus
Binds sterols in fungal cell membrane
Creates transmembrane channel and
electrolyte leakage.
Active against most fungi except
Aspergillus terreus, Scedosporium spp.
Lipid Amphotericin B Formulations
Abelcet ® ABLC
Ribbon-like particles
Carrier lipids: DMPC,
DMPG
Particle size (µm): 1.611
Amphotec ® ABCD
Disk-like particles
Carrier lipids: Cholesteryl
sulfate
Particle size (µm): 0.120.14
Ambisome ® L-AMB
Unilaminar liposome
Carrier lipids: HSPC,
DSPG, cholesterol
Particle size (µm) : 0.08
DMPC-Dimyristoyl phospitidylcholine HSPC-Hydrogenated soy phosphatidylcholine
DMPG- Dimyristoyl phospitidylcglycerol
DSPG-Distearoyl phosphitidylcholine
Amphotericin B
Classic amphotericin B deoxycholate
(Fungizone™) formulation: serious toxic
side effects.
 Less toxic preparations:
1) Liposomal amphotericin B
2) Amphotericin B colloidal dispersion
3) Amphotericin B lipid complex
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Amphotericin B - Pharmacokinetics
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Absorption from the GI tract is negligible
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High
Oral solution sometimes used to decontaminate
gut; few side effects
Only reliable method of administration is IV
Selective distribution into deep tissue sites,
with slow release of drug
Low
kidney > liver > spleen > lung > heart > skeletal muscle > brain > bone > CSF > eye
Amphotericin B - Metabolic elimination
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Metabolic fate is unknown, drug accumulates
in tissues and then is slowly released
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Drug levels can be measured in the kidney, liver,
and spleen up to 1 year after receiving drug
Dosages of amphotericin B are generally not
altered due to decreased elimination of the
drug in kidney dysfunction
Hemodialysis does not alter serum drug
concentrations except in hyperlipidemic
patients
Amphotericin B - Elimination
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Inverse correlation between patient age
and elimination of AmB,
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 Age,  elimination,  side effects
Paediatric patients often tolerate
amphotericin B better than adults
Amphotericin B - Nephrotoxicity
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Most significant delayed toxicity
Renovascular and tubular mechanisms
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Vascular-decrease in renal blood flow leading to
drop in GFR, azotemia
Tubular-distal tubular ischemia, wasting of
potassium, sodium, and magnesium
Enhanced in patients who are volume
depleted or who are on concomitant
nephrotoxic agents
Amphotericin B - Manoeuvers
employed to blunt nephrotoxicity…
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Sodium loading-> blunt the
vasoconstriction and tubular-glomerular
feedback
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Administration of 500 ml -1000 ml of NaCl
before and after amphotericin B infusion
Amphotericin B - Drug Interactions
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Enhanced nephrotoxicity
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Nephrotoxic drugs
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Cyclosporine, aminoglycosides, foscarnet,
pentamidine
Antineoplastic agents
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Cisplatin, nitrogen mustards
Amphotericin B - Clinical Uses
The drug of choice for:
 Cryptococcal meningitis
 Mucormycosis (zygomycosis)
 Invasive fungal infection, not responding
to other therapy
Amphotericin B - Dosing and
Administration
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“Test dose” 1.0 mg in 25-100ml 5% dextrose
infused over 10 minutes used to evaluate
possibility of anaphylactic reaction
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No longer recommended, current product has
fewer impurities
Current recommendation- Start with ~30% of
target dose, infuse for 15 minutes, stop infusion,
and monitor patient for adverse effects before
resuming infusion
Rapidly escalate to full dosages within 48-72
hours
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Delay in giving full dose = worse clinical outcome
Cell Wall Active Antifungals
Cell membrane
• Polyene antibiotics
• Azole antifungals
DNA/RNA synthesis
• Pyrimidine analogues
- Flucytosine
Cell wall
• Echinocandins
-Caspofungin acetate
(Cancidas)
Flucytosine
Fluorinated pyrimidine
related to flurouracil.
Flucytosine
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Restricted spectrum of activity.
Acquired Resistance.
> result of monotherapy
> rapid onset
Due to:
1) Decreased uptake (permease activity)
2) Altered 5-FC metabolism (cytosine deaminase
or UMP pyrophosphorylase activity)
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Flucytosine - pharmacokinetics
Oral absorption
complete
Plasma half-life
3-6 hrs
Volume of distribution
0.7-1l/kg (low)
Plasma protein binding ~12%
Flucytosine - side effects
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Infrequent – include D&V, alterations in
liver function tests and blood disorders.
Blood concs need monitoring when used
in conjunction with Amphotericin B.
Flucytosine – Clinical uses
Monotherapy : now limited
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Candidiasis
Cryptococcosis
?Aspergillosis
}
In combination with
amphotericin B or
fluconazole.
Cell Wall Active Antifungals
Cell membrane
• Polyene antibiotics
• Azole antifungals
DNA/RNA synthesis
• Pyrimidine analogues
- Flucytosine
Cell wall
• Echinocandins
-Caspofungin acetate
(Cancidas)
Atlas of fungal Infections, Richard Diamond Ed. 1999
Introduction to Medical Mycology. Merck and Co. 2001
The Fungal Cell Wall
mannoproteins
b1,3
b1,6
glucans
Cell
membrane
b1,3 glucan
synthase
chitin
ergosterol
Atlas of fungal Infections, Richard Diamond Ed. 1999
Introduction to Medical Mycology. Merck and Co. 2001
Echinocandins - Pharmacology
Cyclic lipopeptide antibiotics that
interfere with fungal cell wall
synthesis by inhibition of ß-(1,3) Dglucan synthase
 Loss of cell wall glucan results in
osmotic fragility
Spectrum:
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HO
H
O
H
HO
NH
H
NH
O
N
H
HO
O
H
H
H3 C
OH
NH
O
CH3
H
H
H
H2N
NH
H
O
N
O
HO
H
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OH
H
OH
NH
H
OH
H
H
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O
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OH
Candida species including nonalbicans isolates resistant to
fluconazole
Aspergillus spp. but not activity
against other moulds (Fusarium,
Zygomycosis)
No coverage of Cryptococcus
neoformans
Echinocandins - spectrum
Highly active
Very active
Some activity
Candida albicans,
Candida glabrata,
Candida tropicalis,
Candida krusei
Candida kefyr
Pneumocystis carinii
Candida parapsilosis
Candida gulliermondii
Aspergillus fumigatus
Aspergillus flavus
Aspergillus terreus
Candida lusitaniae
Coccidioides immitis
Blastomyces
dermatididis
Scedosporium species
Paecilomyces variotii
Histoplasma
capsulatum
Low MIC ,with
fungicidal activity and
good in-vivo activity.
Low MIC, but without
fungicidal activity in
most instances.
Detectable activity,
which might have
therapeutic potential for
man (in some cases in
combination with other
Echinocandins act at the apical tips of
Aspergillus hyphae
DiBAC
Bowman et al. Antimicrob Agent Chemother 2002;46:3001-12
Echinocandins-Spectrum vs. Moulds
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Active against Aspergillus
species
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Glucan synthase localized in
apical tips
Activity against other yeast
and moulds is less well
described or variable
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Staining with
antisera to glucan
synthase subunit
(Fks1p)
Mycelial forms of endemic
mycoses?
Aniline blue staining
of β (1-3) glucans –
stains only at apex
Beauvais et al. J. Bacteriol 2001;183:2273-79
Caspofungin - Pharmacokinetics
Absorption
< 2%
Distribution (Vd)
9.67 L
Protein binding
97% albumin
Major metabolic pathway
Peptide hydrolysis, slow
N-acetylation
t 1/2 ß
9-11 hours
CNS penetration
Probably poor
Dosage adjustment
Moderate-severe hepatic dysfunction
Drug-Drug interactions
Significant interactions CSA? FK506, mycophenolate? Inducers of
3A4?
Caspofungin acetate
IV only
Indication:
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Invasive candidiasis
Invasive aspergillosis refractory to other therapies
Dosage and administration
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70 mg day 1, followed by 50 mg daily
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Increase to 70 mg per day in non-responders
Decrease to 35 mg per day in moderate-severe hepatic
dysfunction (Child-Pugh 7-9)
Antiviral Drug Products Advisory Committee, January 10, 2001- www.FDA.gov
Caspofungin - Adverse effects
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Most common AEs are infusion related:
 Intravenous site irritation (15-20%)
 Mild to moderate infusion-related AE including
fever, headache, flushing, erythema, rash (5-20%)
 Symptoms consistent with histamine release (2%)
Most AEs were mild and did not require treatment
discontinuation
Most common laboratory AE
 Asymptomatic elevation of serum transaminases
(10-15%)
Clinical experience to date suggests that these drugs
are extremely well-tolerated
Antiviral Drug Products Advisory Committee, January 10, 2001- www.FDA.gov
Drug
Dosage
Polyenes
Amphotericin B
deoxycholate
Lipo-AMB (AmBisome)
ABLC (Abelcet)
Amphocil
AWP Cost/day for
70kg Patient
1 mg/kg/day IV
3 mg/kg/day IV
5 mg/kg/day IV
3 mg/kg/day IV
£7
£554
£246
£380
Triazoles
Fluconazole
Itraconazole
Voriconazole
400/800 mg IV
400 mg IV
4 mg/kg IV
£56/£112
£72
£80
Echinocandins
Caspofungin
50 mg IV x 1 day,
£334
(Medical Letter 2002;44:63-65; Lancet 2003;362:1142-1151)