Antifungal (Antimycotic drugs)

Antifungal (Antimycotic drugs)

Download Pdf  

Antifungal (Antimycotic) drugs are agents that are used to prevent the growth and multiplication of fungi. The pathogenic fungi are eukaryotes possessing a true membrane-bounded nucleus (with several chromosomes), cytoplasmic organelles (like mitochondria), a rigid cell wall (consisting of chitin) and a cell membrane (containing a lipid bilayer that is mainly composed of ergosterols). Fungi generally reproduce asexually via budding, sporulation or hyphal fragmentation. The pathogenic fungi generally consist of two main groups of organisms i.e., moulds and yeasts. The moulds (e.g., Aspergillus fumigatus) exist in multicellular filamentous forms whereas yeasts (e.g., Candida albicans) have unicellular oval or spherical appearance. Dimorphic fungi (e.g., Blastomyces, Histoplasma) may grow like a yeast inside the host and in vitro conditions (at room temperature) these may grow like a mould.

 

Fungal infections usually called mycoses (singular mycosis) have been divided into two distinct classes: superficial and systemic. The superficial fungal infections generally affect the keratinized layers of skin and its appendages (hair, feathers, horns and nails) or mucous membranes (buccal, gastro-intestinal, ruminal or vaginal). The lesions are frequently ring-shaped and hence the disease is commonly known as ringworm. The superficial fungal infection is generally caused by a homogenous group of keratinophilic fungi called dermatophytes (infection is known as dermatophytosis). These include members of genera Microsporum, Trichophyton and Epidermophyton. The superficial fungal infection is generally irritating with manifestations of alopecia (absence of hair from where it is normally present), erythema (redness), scar formation and exudation etc. The superficial fungal infection is generally easier to treat by topical and systemic medications. The systemic fungal infections (systemic mycoses) affect deeper body tissues and organs. Depending upon the causative agent, systemic fungal diseases have been named as histoplasmosis, blastomycosis, candidiasis and aspergillosis etc. The systemic fungal infections are often serious, life threatening and these are generally difficult to treat. In the last 30-40 years, there is a steady increase in the occurrence of fungal infections attributed mainly to intensive use of corticosteroids and elimination or suppression of non-pathogenic bacteria (that normally compete with fungi) due to indiscriminate use of broad spectrum antibacterial drugs. Fungal infections are more difficult to treat, they are slowly eradicated and antifungal drugs are more toxic to host than the antibacterial drugs.

 

Antifungal drugs are classified into specific and non-specific antifungal drugs.

(A) Specific antifungal drugs: The following drugs are included in this category.

1. Polyene antibiotics: These include Amphotericin B and Nystatin. These drugs interact with membrane ergosterol and form channels/pores in the membrane. This alters the membrane permeability and causes the influx of hydrogen ions and efflux of potassium ions thereby leading to acidosis that finally impairs some important cellular enzymatic reactions. Polyene antibiotics have broad spectrum antifungal activity.

 

(a) Amphotericin B: Amphotericin B is fungistatic at normal doses but it can become fungicidal at higher concentrations. It is effective against aspergillus, blastomyces, candida, histoplasma, leishmania, trypanosoma, histomonas and entameoba. Amphotericin B also has some immunomodulatory action by potentiating both humoral and cell-mediated immunity. This enhances the host’s ability to combat fungal infection. As it binds preferentially to ergosterol rather than cholesterol (the principal sterol found in mammalian cell membranes), a relative (but not absolute) specificity is conferred. It is an amphoteric drug and is insoluble in water, its solution is unstable and rapidly decomposes when exposed to light. The commercially available reconstituted powder contains Sodium desoxycholate for enhancing solubility. Liposomal preparations of Amphotericin B are also available. Amphotericin B is poorly absorbed from GI tract and is given via oral route only for gastrointestinal mycosis. For systemic infection it is given by repeated daily slow I/V injection (it is usually diluted in 5% Dextrose) as it is not absorbed from parentral sites. For fungal meningitis it has to be given via intrathecal route. Amphotericin B exhibits bi-phasic elimination with an initial plasma half life of 24-48 hours followed by a longer terminal half life of about 15 days. Low level of drug (in the form of metabolites) appears in urine over a longer period of time and in case of humans the complete clearance of Amphotericin B may take more than two months. Amphotericin B being a toxic drug has many side effects especially nephrotoxicity that occurs via two mechanisms –intense renal vasoconstriction (causing reduced renal blood flow and decreased GFR) and binding of drug to membrane cholesterol in renal tubular membrane. This causes alteration of renal tubular permeability leading to altered electrolyte flux, acidification defects (metabolic acidosis) and concentration abnormalities (e.g., polyuria). The distal tubular acidosis may also lead to excessive release of calcium (as a result of bone resorption) into the circulation which can get precipitated in acidic renal environment and can result in nephro-calcinosis. Systemic administration of Amphotericin B is also associated with other adverse effects like nausea, vomiting, anaphylaxis and nervous signs (if given intra-thecally). The incidence of serious side effects can be minimized through certain precautionary measures and prophylactic medication.  Pre-treatment with anti-emetics and antihistaminics can be used to avoid nausea, vomiting and anaphylaxis. Mannitol (an osmotic diuretic) and Sodium bicarbonate (urinary alkalinizer) may help to prevent acidification defects while intravenous fluid therapy minimizes the chances of renal ischemia.

 

Drug interactions associated with Amphotericin B
Combination/Co-administration	Result/Consequences	Remarks
Amphotericin B and Flucytosine	Synergistic effect	Ideal action
Amphotericin B and Minocycline	Synergistic effect	Ideal action
Amphotericin B and Rifampicin	Synergistic effect	Ideal action
Amphotericin B and Miconazole	Antagonistic action	Loss of pharmacological action
Amphotericin B and Aminoglycosides	Enhanced nephrotoxic effects	Enhanced toxic effect
Amphotericin B and Digitalis	Amphotericin B induced hypokalemia aggravates Digitalis induced cardiotoxicity	Enhanced toxic effect
Amphotericin B and Thiazide diuretics	Enhanced Potassium depletion	Enhanced toxic effect
Amphotericin B and Loop diuretics	Enhanced Potassium depletion	Enhanced toxic effect
Amphotericin B and Neuromuscular blockers	Enhanced skeletal muscle relaxation	Enhanced toxic effect
Amphotericin B and antineoplastic drugs	Enhanced cytotoxicity	Ideal action

 

(b) Nystatin: Nystatin is primarily used for cutaneous, oral (e.g., thrush), intestinal or vaginal candidiasis in dogs and cats. However it is not effective against dermatophytes. It is usually combined with corticosteroids and antibacterial agents to enhance efficacy and reduce the chances of Candida superinfection. Nystatin is not absorbed from skin or GIT, so its topical or oral administration is safe. It is commonly available in the form of topical (creams, ointments, powders) and oral preparations (tablets, suspension) as its parentral administration is associated with hemolytic anemia due to interaction of the drug with erythrocyte sterols.

 

2. Griseofulvin: It is a systemic antifungal antibiotic, obtained from Penicillium griseofulvin. It acts by impairing the polymerization of microtubular proteins with microtubules thus perturbing the spindle formation in dividing fungal cells and thereby arresting cell division at the metaphase stage. This leads to production of multinucleate fungal cells that are devoid of viability. It is a narrow spectrum antifungal drug mainly effective against dermatophytes but is not active against deep mycoses, candida and bacteria. Griseofulvin is fungistatic against older and dormant fungi but may kill (shows fungicidal action) actively growing young fungal cells. After oral administration it accumulates into keratin and produces action against superficial fungi. It has high affinity for keratin precursor cells and can be found in keratinized tissues within 4-8 hours of administration. When these precursor cells differentiate, the Griseofulvin remains bound to them and persists in the newly formed keratin layer. Thus the newly formed layers of nails or hairs are the first to become free of fungal infection. Its action is very slow requiring 4-6 weeks of therapy for skin infections and up to one year for nail infections. Griseofulvin is also secreted in perspiration, so sweat and trans-epidermal fluid loss play an important role in the transfer of drug into stratum corneum. Griseofulvin has been proved to be effective when added to drinking water for the prevention of mycotic dermatitis in ostriches and as a feed supplement to treat ringworm outbreak in sheep flock.  Acute adverse effects induced by Griseofulvin are infrequent and generally not serious. At very high doses, if given during first two trimesters of pregnancy, it may cause teratogenic and carcinogenic effects (characterized by cleft palate, skeletal and CNS abnormalities) in cats.

 

3. Flucytosine (5-Fluorocytosine, 5-FC): It is an anti-metabolite that was originally synthesized as an antineoplastic drug but later on it was found to have good antifungal activity. Flucytosine, an analogue of cytosine is as such an inactive substance and requires conversion into active metabolites inside fungal cells. On administration, Flucytosine enters fungal cells via a cytosine-specific permease, an enzyme not found in mammalian cells. Inside the fungal cell, it is rapidly converted into 5-fluorouracil (5-FU) by cytosine deaminase enzyme. 5-FU acts as antimetabolite by competing with uracil. It initially forms 5-fluorouredylate and then 5- fluorouridine triphosphate which interferes with pyrimidine metabolism and eventually inhibits fungal RNA and protein synthesis. 5-fluorouredylate by an alternate pathway is also metabolized to 5-fluorodeoxyuridylate which competes with thymidine and thus inhibits the action of thymidylate synthase enzyme. Inhibition of thymidylate synthase enzyme deprives the organism of thymidylic acid (an essential DNA component), which ultimately leads to disruption of DNA synthesis and cell division.  Many organisms including mammals are resistant to flucytosine because they lack the permease enzyme system required for its activity. Flucytosine is fungistatic, its spectrum of activity is limited and includes cryptococcus, candida, aspergillus while dermatophytes are resistant. The combination of Flucytosine and Amphotericin B is synergistic because Amphotericin B affects fungal cell permeability allowing more Flucytosine to penetrate the cell. It also shows synergistic action with Ketoconazole. Flucytosine should not be combined with other immuno-suppressant drugs as it may enhance the chances of myolsuppression. Flucytosine has relatively low toxicity and is well tolerated over long periods. As it interferes with DNA synthesis so it is likely to affect the body systems with rapidly dividing cells. Bone marrow depression may occur. Teratogenic effects have been reported in laboratory animals. It is contraindicated in patients with pre-existing bone marrow depression and in pregnant animals.

4. Azoles: The azoles are a group of synthetic broad spectrum antifungal agents. Azoles alter the membrane permeability of susceptible fungi through the inhibition of ergosterol synthesis. They inhibit 14-α-demethylase, a fungal microsomal cytochrome 450-dependent enzyme. This enzyme catalyzes demethylation of lanosterol to ergosterol, the major cell membrane component. The reduced ergosterol content in the cell membrane in turn decreases fluidity of membrane and increases the permeability with effects similar to those triggered by Amphotericin B. Inhibition of ergosterol synthesis also results in accumulation of 14-α-methyl sterol, which impairs membrane functions leading to alterations in energy metabolism and growth inhibition. Cholesterol synthesis in mammals is not affected as it does not require 14-α-demethylase, although synthesis of selected steroids and drug metabolizing enzymes may be impaired in the host by some drugs. Azoles are further classified into two subgroups.

(a) Imidazoles: They contain two nitrogen atoms. Ketoconazole, Miconazole, Thiabendazole and Clotrimazole are included in this group.

• Ketoconazole: It is a systemic antifungal drug. It was the first azole that could be given orally to treat systemic fungal infections and is still considered the prototype drug. Introduction of some newer agents, however, has reduced its clinical usefulness in human and veterinary medicine. Ketoconazole has broad spectrum of antifungal activity which includes candida, crytococcus, blastomyces, histoplasma and dermatophytes. Ketoconazole produces some endocrine adverse effects in host animals mainly by interfering with steroid synthesis via inhibition of P-450 enzyme systems. It produces measurable reduction in testosterone or hydrocortisone synthesis at dosage used for antifungal therapy. But its clinical implications are insignificant. High doses have been tried to reduce testosterone or hydrocortisone levels in clinical cases of prostatic carcinoma or hyperadrenocorticism with success. It may suppress testosterone or hydrocortisone synthesis and may decrease libido and produce reproductive disorders, but the effects are transient at normal doses. Ketoconazole inhibits certain cytochrome P-450 enzyme systems and affects metabolism of other drugs. It is considered a terartogen in laboratory animals. It inhibits cytochrome P-450 enzyme system and potentiates the toxicity of several drugs like Warfarin and Phenytoin. Co-administration of Ketoconazole and Griseofulvin or Amphotericin B is not recommended due to high incidence of toxic effects. The absorption of Ketoconazole is inhibited by concurrent administration of gastric antacids, H₂-blockers and anticholinergic drugs.
• Miconazole: It is primarily used as topical antifungal drug. It penetrates well into skin after topical administration and persists for about 4 days in the stratum corneum. It is effective against trichophyton and microsporum species. It is available as 2% cream and 1% lotion for treating local dermatophytosis in dogs and cats.
• Clotrimazole: It is a topical antifungal drug that is used to treat candidiasis, trichomoniosis and dermatophyte infections of skin and vagina.
• Thiabendazole: It is primarily an anthelmintic agent but also possesses potent antifungal action and is included in some otic or nasal preparations for the treatment of yeast infections.

(b) Triazoles: They contain three nitrogen atoms. Fluconazole and Itraconazole are members of this category.

v  Fluconazole: It is recently introduced synthetic triazole and is effective against local and systemic candidiasis, histoplasmosis, blastomycosis and dermatophytosis but its efficacy against aspergillosis is doubtful. Unlike Ketoconazole and other Imidazoles, Fluconazole has lesser affinity for mammalian cytochrome P-450 enzyme systems responsible for host steroid synthesis but it can inhibit drug metabolizing cytochrome P-450 enzymes to some extent.

v  Itraconazole: It has broader spectrum of antifungal activity and is also active against trypanosome cruzi. Like Fluconazole, it also lacks endocrine side effects associated with Ketoconazole.

 

5. Allylamines: The most frequently used member of this category is Terbinafine.

Terbinafine: It is a synthetic drug and is highly lipophillic in nature. This fungicidal drug acts by selectively inhibiting squalene epoxidase enzyme involved in the synthesis of ergosterol from squalene inside fungal cell wall. Fungal cell death usually results from disruption of cell membrane. The mammalian cell wall enzyme is inhibited only by very high concentrations of Terbinafine. It is effective against yeast, dermatophytes including aspergillus, trichophyton and microsporum. It is well absorbed after oral administration but undergoes significant first pass metabolism that decreases its bioavailability to about 40%. It is highly protein bound (99%). It may be used orally or topically. The adverse effects are usually mild and self-limiting.

 

(B) Non-specific antifungal drugs: The following drugs are included in this category.

1. Sodium iodide: It is occasionally used in the treatment of fungal and bacterial (like actinobacilosis) infections. Use of iodides interferes with thyroid function tests and can lead to iodinism after prolonged administration. Sodium iodide is used as a 20% solution for oral/intravenous administration.

 

2. Benzoic acid and Salicylic acid: Benzoic acid has both bacteriostatic and fungistatic actions. For fungal infections it is mostly combined with Salicylic acid to prepare Whitfield ointment. It is considered useful against superficial dermatophytosis. Salicylic acid is used topically as keratolytic, antiseborrhoeic (prevents or relieves excessive secretion of sebum), antiseptic and fungistatic agent. Its keratolytic action helps in the penetration of concurrently administered drugs.

 

3. Gentian violet: It is a dye that is used as topical antibacterial, antifungal and antiseptic agent.

 

4. Sulpher: It is externally used as ointment, powder or lotion for various skin diseases such as ringworm and eczema (a chronic skin disorder that involves scaly and itchy rashes). It is also used externally as parasiticide, insecticide and disinfectant.

 

5. Copper sulphate: It is applied topically against ringworm infection. The fungicidal action is produced partly by virtue of its astringent (drug that reduces tissue irritation and arrests abnormal secretions) nature and partly by antifungal action of copper ions.