1. School of Pharmacy, University of Nizwa
Antifungal Drugs
Course Coordinator
Jamaluddin Shaikh, Ph.D.
School of Pharmacy, University of Nizwa
Lecture 9
March 04, 2013

2. Fungal Infections Infectious diseases caused by fungi are called
mycoses, and they are often chronic in nature
Mycotic infections are mostly superficial and only involve the skin (cutaneous mycoses), but fungi may also penetrate the skin, causing subcutaneous infections
The fungal infections that are most difficult to treat are the systemic mycoses, which are often life-threatening
Unlike bacteria, fungi are eukaryotic. They have rigid cell walls composed largely of chitin rather than peptidoglycan
Fungal infections are generally resistant to antibiotics used in the treatment of bacterial infections, and conversely, bacteria are resistant to the antifungal agents
peptidoglycan, a characteristic component of most bacterial cell walls

3. Antifungal Drugs

5. Amphotericin B
A naturally occurring, polyene macrolide antibiotic produced by Streptomyces nodosus
In spite of its toxic potential, it is the drug of choice for the treatment of life-threatening, systemic mycoses.
The drug is also sometimes used in combination with flucytosine so that lower (less toxic) levels of amphotericin B are possible

6. Amphotericin B: Mechanism of Action
Amphotericin B molecules bind to ergosterol in the fungal cell membrane, forming a pore
Electrolytes (particularly potassium) and small molecules are lost through the pore, resulting in cell death

7. Amphotericin B: Antifungal Spectrum
Either fungicidal or fungistatic, depending on the organism and the concentration of the drug
Effective against a wide range of fungi, including Cryptococcus neoformans, Histoplasma capsulatum, Candida albicans, and many strains of aspergillus
Amphotericin B: Pharmacokinetics
Administered by slow, intravenous infusion
Extensively bound to plasma proteins and is distributed throughout the body
Little of the drug is found in the CSF
Does cross the placenta

8. Amphotericin B: Adverse Effects
Low therapeutic index. A total adult daily dose should not exceed 1.5 mg/kg
Fever and chills: Occur most commonly 1 to 3 hrs after iv administration, but they usually subside with repeated administration of the drug
Renal impairment: Patients may exhibit a decrease in glomerular filtration rate and renal tubular function
Hypokalemia: fall of potassium level
Amphotericin B: Resistance
Fungal resistance is associated with decreased ergosterol content of the fungal membrane

9. Flucytosine: Mechanism of Action
Enters fungal cells via a cytosine-specific permease, an enzyme not found in mammalian cells
Then converted by a series of steps to 5-fluorodeoxyuridine 5'-monophosphate,
which eventually disrupt DNA and
protein synthesis
Note: Amphotericin B
increases cell permeability,
allowing more flucytosine
to penetrate the cell. Thus,
flucytosine and amphotericin B
are synergistic

10. Flucytosine: Antifungal Spectrum
Fungistatic
Effective in combination with amphotericin B for treating candidiasis or cryptococcosis
Flucytosine: Pharmacokinetics
Well absorbed by the oral route
Distributes throughout the body water and penetrates well into the CSF
Excreted by glomerular filtration, and the dose must be adjusted in patients with compromised renal function

11. Flucytosine: Adverse Effects
Causes reversible neutropenia, thrombo-cytopenia, and dose-related bone marrow depression
Hepatic dysfunction
Gastrointestinal disturbances, such as nausea, vomiting, and diarrhea, are common
Flucytosine: Resistance
Resistance due to decreased levels of any of the enzymes in the conversion of 5-FC to 5-fluorouracil (5-FU) and beyond, or increased synthesis of cytosine, can develop during therapy
neutropenia, a condition of an abnormally low number of neutrophils
Thrombocytopenia is a relative decrease of platelets in blood

12. Ketoconazole: Mechanism of Action
Inhibits C-14 α-demethylase (a cytochrome P450 enzyme), thus blocking the demethylation of lanosterol to ergosterol, the principal sterol of fungal membranes
This inhibition disrupts membrane structure and function and, thereby, inhibits fungal cell

13. Ketoconazole: Antifungal Spectrum
Active against many fungi, including Histoplasma, Blastomyces, and Candida, but not Aspergillus species
Although itraconazole has largely replaced ketoconazole in the treatment of most mycoses because of its broader spectrum, greater potency, and fewer adverse effects, ketoconazole, as a second-line drug, is a less expensive alternative for the treatment of mucocutaneous candidiasis

14. Ketoconazole: Pharmacokinetics
Only administered orally
Requires gastric acid for dissolution and is absorbed through the gastric mucosa
Drugs that raise gastric pH, such as antacids, or that interfere with gastric acid secretion, such as proton-pump inhibitors, impair absorption. Administering acidifying agents, such as cola drinks, before taking the drug can improve absorption
The drug does not enter the CSF
Extensive metabolism occurs in the liver, and excretion is primarily through the bile

15. Ketoconazole: Adverse Effects
In addition to allergies, dose-dependent GI disturbances, including nausea, anorexia, and vomiting, are the most common adverse effect
Endocrine effects, such as gynecomastia, decreased libido, impotence, and menstrual irregularities
Ketoconazole: Resistance
Identified mechanisms of resistance include mutations in the C-14 α-demethylase gene, which cause decreased azole binding. Additionally, some strains of fungi have developed the ability to pump the azole out of the cell
Anorexia: eating behaviors

16. Ketoconazole: Drug Contraindications
By inhibiting cytochrome P450, ketoconazole can potentiate the toxicities of drugs such as cyclosporine, phenytoin, tolbutamide, and warfarin
Rifampicin, an inducer of the cytochrome P450 system, can shorten the duration of action of ketoconazole
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

17. Fluconazole
Clinically important because of its lack of the endocrine side effects of ketoconazole and its excellent penetrability into the CSF
MOA is similar to ketoconazole
Drug of choice for Cryptococcus neoformans
Administered orally or intravenously. Its absorption is excellent and, is not dependent on gastric acidity
Unlike ketoconazole, it is poorly metabolized
Excreted via the kidney
Less adverse effects than ketoconazole
Nausea, vomiting, and rashes are common

18. Itraconazole Broad antifungal spectrum
Mechanism of action, metabolism, and adverse effects are same as that of other azoles
Well-absorbed orally, but it requires acid for dissolution. Food increases the bioavailability
Extensively bound to plasma proteins and distributes well throughout most tissues, except CSF
Should be avoided in pregnancy
Inhibits the metabolism of many drugs, including oral anticoagulants, and statins
Statin: drugs which decreases cholesterol level; quinidine: class I antiarrhythmic agent

20. Terbinafine: Mechanism of Action
Inhibits fungal squalene epoxidase, thereby decreasing the synthesis of ergosterol
Decrease synthesis of ergosterol and accumulation of toxic amounts of squalene result in the death of the fungal cell

21. Terbinafine: Antifungal Spectrum
Primarily fungicidal, antifungal activity is limited to dermatophytes and Candida albicans
Terbinafine: Pharmacokinetics
Orally active, mostly bound to plasma proteins
Deposited in the skin, nails, and fat. Accumulates in breast milk, thus, should not be given to nursing mothers
Extensively metabolized prior to urinary excretion. Patients with either moderate renal impairment or hepatic cirrhosis have reduced clearance
Terbinafine: Adverse Effects
Most common adverse effects are gastrointestinal disturbances, headache, and rash

22. Nystatin
Its structure, chemistry, mechanism of action, and resistance resemble those of amphotericin B
Its use is restricted to topical treatment of Candida infections because of its systemic toxicity
Adverse effects are rare because of its lack of absorption, but nausea and vomiting occasionally occur