ANTIFUNGAL AGENTS

ANTIFUNGAL AGENTS

Introduction

•      Fungi are eukaryotic, heterotrophic (not self sustaining) organisms that live as saprobes or parasites.

•      They are complex organisms in comparison to bacteria .Thus antibacterial agents are not effective against fungi.

•      Fungal infections are also called as mycoses.

•      They have nucleus and well defined nuclear membrane, and chromosomes.

•      They have rigid cell wall  composed of chitin ( N –acetylglucosamine ) where as bacterial cell wall is composed of peptidoglycan .

•      Fungal cell membrane contains ergosterol, human cell mebmrane is composed of cholesterol.

•      Fungal infections are caused by microscopic organisms that can invade the epithelial tissue. The fungal kingdom includes yeasts, molds, rusts and mushrooms.

•      Fungi, like animals, are hetrotrophic, that is, they obtain nutrients from the environment, not from endogenous sources (like plants with photosynthesis).

•      Most fungi are beneficial and are involved in biodegradation, however, a few can cause opportunistic infections if they are introduced into the skin through wounds, or into the lungs and nasal passages if inhaled.

•      Antifungal drugs are the drugs, which kill or inhibit the growth of fungus in the body of the host and are used for superficial or deep (systemic) mycoses.

•      Pathogenic fungi affecting animals are eukaryotes, generally existing as either filamentous molds (hyphal forms) or intracellular yeasts.

•      Fungal organisms are characterized by a low invasiveness and virulence.

•      Factors that contribute to fungal infection include necrotic tissue, a moist environment, and immunosuppression.

•      Fungal infections can be primarily superficial and irritating (eg, dermatophytosis) or systemic and life threatening (eg, blastomycosis, cryptococcosis, histoplasmosis, coccidioidomycosis).

•      Clinically relevant dimorphic fungi grow as yeast-like forms in a host but as molds in vitro at room temperature; they include Candida spp, Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum, Sporothrix schenkii, and Rhinosporidium.

•      Several factors can lead to therapeutic failure or relapse after antifungal therapy.

•      Drug access to fungal targets is often compromised.

•      Host inflammatory response may be the first barrier, followed by location in sanctuaries (brain, eye, etc) as a second barrier for some infections, and the organisms themselves as a third barrier.

•      The fungal cell wall is rigid and contains chitin, which along with polysaccharides, acts as a barrier to drug penetration.

•      The cell membrane contains sterols such as ergosterol, which influences the efficacy and potential resistance to some drugs.

•      Cryptococcus and occasionally Sporothrix schenckii produce an external coating or slime layer that encapsulates the cells and causes them to adhere and clump together.

•      Finally, regarding drug access, most infections are located inside host cells, the lipid membrane of which can present a final barrier.

•      Discontinuing therapy after clinical signs have resolved but before infection is eradicated also leads to therapeutic failure.

•      Therapy should extend well beyond clinical cure.

•      Once drugs reach the site of action, therapeutic success is impeded by the nature of fungal infections.

•      Fungal growth is slow, yet most antifungal drugs work better in rapidly growing organisms. Likewise, most antifungal agents are fungistatic in action, with clearance of infection largely dependent on host response.

•      As such, the duration of therapy is long, and the "get in quick, hit hard, and get out quick" recommendation for antibacterial therapy is not appropriate for antifungal therapy; care must be taken to not discontinue therapy too early.

•      However, longer duration of therapy contributes to another common cause of therapeutic failure: host toxicity. Because both the antifungal target organism and the host cells are eukaryotic, the cellular targets of fungal organisms are often similar to the host structures.

•      As such, as a class, antifungal drugs tend to be more toxic than antibacterial drugs. Therefore, the number of antifungal drugs approved for use are markedly fewer than the number of antibacterial drugs.

•      Drugs that can be used locally (including topically) or characterized by distribution to sites of infection (eg, liposomal products) may decrease this risk.

•      The slow growth that characterizes fungal infections means that acquired resistance occurs less commonly than in bacterial infections.

•      Therapeutic failure may also reflect the inability of the immunocompromised host to overcome residual fungal populations inhibited by the drug; those antifungals that are also (positive) immunomodulators may be more effective.

•      A number of serious systemic fungal diseases are well recognized in several parts of the world. Antifungal agents have greatly reduced previously recorded human mortality rates due to systemic mycoses. A relatively narrow selection of drugs is used in these cases.

•      Superficial fungal infections

•      Inculde dermatophytoses of the skin, hair  and nails caused by Trichophyton, Microsporum or Epidermophyton spp and candidiasis or moniliasis of the moist skin and mucous membrane (GIT / Genital tract)

•      Systemic mycoses

•      Candidiasis, Cryptococosis, Aspergillosis, Blastomycosis, Histoplasmosis, Coccidioidomycosis, Paracoccidioidomycosis, etc.

Targets of antifungal drugs

CLASSIFICATION OF ANTIFUNGAL DRUGS

Drugs for systemic fungal infections

            Polyene antibiotics

                        Amphotericin B

            Pyrimidine antimetabolites

                        Flucytosine

            Antifungal azoles

                        Ketoconazole

                        Fluconazole

                        Itraconazole

            Echinocandins

                        Caspofungin, micafungin,  and anidulafungin

Drugs for superficial fungal infections

            Systemic drugs

                        Griseofulvin

                        Iodide

            Topical drugs

                        Nystatin

                        Haloprogin

                        Tolnaftate

                        Azoles (miconazole, econazole, clotrimazole, etc.)

A) Drugs that disrupt fungal cell membrane

i) Polyenes 

                                    Amphotericin, Nystatin, Natamycin

ii) Azoles

            A) Imidazole          

                        Ketoconazole, Butaxonazole, Clotrimazole, Econazole,

                        Miconazole, Oxiconazole, Sulconazole

            B) Triazole  

                                    Fluconazole, Itraconazole, Tioconazole

iii) Allylamines

                                    Terbinafine, Naftifine, Butenafine 

iv) Echinocandins

                                    Caspofungin                                 

B) Drugs that inhibits mitosis

                                    Griseofulvin

C) Drugs that inhibits DNA synthesis

Flucytosine

D) Miscellaneous

                                    Haloprogin, Tolnaftate, Whitefield's ointment

                                    Ciclopiroxolamine

Diagram showing mechanism of action of different anti fungal durgs

Superficial Mycosis

a)  Dermatophyte  infection (ring worm, tinea).

•      Benzoic acid ointment for mild infection.

•      Topical imidazole (like miconazole, clotrimazole) is preferred now a days

•      Tioconazole for nail infection

•      Griseofulvin  orally for extensive scalp or nail tinea infection.

b)                  Candida infection.

•      Cutaneous infection: by  topical amphotericin, clotrimazole ,econazole, miconazole  or nystatin

•      Candidiais of elementary tract mucosa: amphotericin, fluconazole, ketoconazole, 

      miconazole or nystatin.

Vaginal candidiasis: Clotrimazole, econazole, ketoconazole, miconazole or nystatin

POLYENE MACROLIDE ANTIBIOTICS

Amphotericin B is the model polyene macrolide antibiotic and is the sole member of this class used systemically.

            Polyene antifungal antibiotics are large molecules, consisting of a long polyene, lipid-soluble component and a markedly hydrophilic component.

            Amphotericin B acts as both a weak base and a weak acid, and as such is amphoteric.

            The polyene macrolides have been isolated from various strains of bacteria; amphotericin B is an antibiotic product of Streptomyces nodosus.

            Amphotericin B, nystatin, and pimaricin (natamycin) are the only polyene macrolide antibiotics used in veterinary medicine.

            The polyenes are poorly soluble in water and the common organic solvents. They are reasonably soluble in highly polar solvents such as dimethylformamide and dimethyl sulfoxide.

            In combination with bile salts, such as sodium deoxycholate, amphotericin B is readily soluble (micellar suspension) in 5% glucose. This colloidal preparation has been used for IV infusion.

            The polyenes are unstable in aqueous, acidic, or alkaline media but in the dry state, in the absence of heat and light, they remain stable for indefinite periods.

•      They should be administered parenterally (diluted in 5% dextrose) as freshly prepared aqueous suspensions. Lack of stability indicates that labeled expiration dates be adhered to once the product is diluted. Amphotericin B is also prepared as liposomal and lipid-based preparations, enhancing its safety without loss of efficacy.

Antifungal Activity
Mode of Action

            The polyenes bind to sterol components in the phospholipid-sterol membranes of fungal cells to form complexes that induce physical changes in the membrane.

            The number of conjugated bonds and the molecular size of a particular polyene macrolide influence its affinity for different sterols in fungal cell membranes. Amphotericin B has a greater affinity for fungal ergosterol, the major sterol in fungal membranes, than for eukaryotic (host) cell membrane cholesterol.

            The long polyene structure causes the formation of channels in the fungal cell membrane. The resultant loss of membrane permeability results in the loss of critically important molecules.

            Potassium ion efflux from the fungal cell and hydrogen ion influx cause internal acidification and a halt in enzymatic functions.

            Sugars and amino acids also eventually leak from an arrested cell.

            Fungistatic effects are most often evident at usual polyene concentrations.

High drug concentrations and pH values between 6.0 and 7.3 in the surrounding medium may lead to fungicidal rather than fungistatic action.

            In addition to these direct effects on susceptible yeasts and fungi, evidence suggests that amphotericin B may also act as an immunopotentiator (both humoral and cell mediated), thus enhancing the host's ability to overcome mycotic infections.

•      Fungal Resistance

•      Polyene macrolides are inherently resistant to dermatophytes. Acquired resistance to the polyene antifungal macrolides is rare both clinically and in vitro. Pythium, a pseudofungus, is less susceptible, because it contains limited ergosterol in its cell membranes. Resistance has been documented for Candida spp, which are among the more rapidly growing fungal organisms. In general,resistance develops slowly and does not reach high levels, even after prolonged treatment.

•      Antifungal Spectra

•                  The polyene antibiotics have broad antifungal activity against organisms ranging from yeasts to filamentous fungi and from saprophytic to pathogenic fungi, but there are great differences between the susceptibilities of the various species and strains of fungi.

•                  They are ineffective against dermatophytes.

•                  In vitro susceptibilities (both resistant and highly susceptible) do not always correlate well with the clinical response, which suggests that host factors may also play a role.

•                  Many algae and some protozoa (Leishmania, Trypanosoma, Trichomonas, and Entamoeba spp) are sensitive to the polyenes, but these compounds have no significant activity against bacteria, actinomycetes, viruses, or animal cells. Amphotericin B is effective against yeasts (eg, Candida spp, Rhodotorula spp, Cryptococcus neoformans), dimorphic fungi (eg, Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis), dermatophytes (eg, Trichophyton, Microsporum, and Epidermophyton spp), and molds. It also has been used successfully to treat disseminated sporotrichosis, pythiosis, and zygomycosis, although it may not always be effective.

•                  Nystatin is mainly used to treat mucocutaneous candidiasis, but it is effective against other yeasts and fungi. The antimicrobial activity of pimaricin is similar to that of nystatin, although it is mainly used for local treatment of candidiasis, trichomoniasis, and mycotic keratitis.

Preparations

•      Amphotericin B is available as an IV solution complexed to bile acids but also as several different preparations complexed to lipid mixtures. Because reticuloendothelial cells phagocytize the lipid component, directed delivery to the site of fungal infection is facilitated, reducing renal exposure. Prolonged antifungal activity (compared with nonliposomal preparations) has been documented.

Pharmacokinetic Features

            Absorption

            The polyene macrolide antibiotics are poorly absorbed from the GI tract. Amphotericin B is usually administered IV or topically and occasionally locally, intrathecally, or intraocularly.

            Nystatin and piramycin are mostly applied topically. Nystatin is given PO to treat intestinal candidiasis. Absorption is minimal from sites of local application.

            Distribution

            Amphotericin B is widely distributed in the body after IV infusion. It associates with cholesterol in host cell membranes throughout the body and is subsequently released slowly into the circulation. Penetration into the CSF, saliva, aqueous humor, vitreous humor, and hemodialysis solutions is generally poor.

            Amphotericin B becomes highly bound to plasma lipoproteins (~95%). Complexing amphotericin B with various lipid-based products alters the distribution.

Biotransformation and Excretion

            The disposition of amphotericin B is not well described in companion animals. Approximately 5% of a total daily dose of amphotericin B is excreted unchanged in the urine.

            Over a 2-wk period, ~20% of the drug may be recovered in the urine.

            The hepatobiliary system accounts for 20%–30% of excretion.

            The fate of the remainder of amphotericin B is unknown.

Pharmacokinetics

•      Amphotericin B has a biphasic elimination pattern.

•      The initial phase lasts 24 hr, during which levels fall rapidly (70% for plasma and 50% for urine).

•      The second elimination phase has a 15-day half-life, during which plasma concentrations decline very slowly.

•      Amphotericin B is usually infused IV, every 48–72 hr, until the total cumulative dosage has been reached.

•      The disposition of the various lipid-complexed amphotericin B products is variable, Because of its small size, AmBisome^® is characterized by the slowest uptake by reticuloendothelial cells and thus the highest plasma drug concentrations of amphotericin B.

•                  However, the amount of free versus complexed amphotericin B is not clear. AmBisome also was able to achieve CNS concentrations and was associated with the least nephrotoxicity in human studies.

•                  AmBisome has been studied in Beagles. Achievable amphotericin concentrations were much higher at equivalent doses of AmBisome compared with other products; further, dogs were able to well tolerate 4 mg/kg for 30 days. Amphotericin concentrations accumulate with multiple dosing when administered as AmBisome.

•      Therapeutic Indications and Dose Rates

•                  Amphotericin B is used principally to treat systemic mycotic infections.

•                  Despite its ability to cause nephrotoxicity, amphotericin B remains a commonly used antifungal agent because of its effectiveness.

•                  Multiple approaches to delivery have been described in an attempt to minimize nephrotoxicity.

•                  In addition, dosing continues until a maximal cumulative dose is reached, with the amount varying with the fungal organism.

•                  Nystatin is primarily indicated for treatment of mucocutaneous (skin, oropharynx, vagina) or intestinal candidiasis; pimaricin is mainly used in therapeutic management of mycotic keratitis.

Adverse Effects and Toxicity

•      Oral administration of nystatin can lead to anorexia and GI disturbances.

•      The IV infusion of amphotericin B can cause an anaphylactoid reaction due to direct mast cell degranulation. A pre-test dose is recommended to detect this reaction, and pretreatment with H₁ antihistamines and short-acting glucocorticoids may be appropriate. Thrombophlebitis may occur with perivascular leakage.

•      The primary toxicity associated with amphotericin B is nephrotoxicity. Within 15 min of IV administration of amphotericin B, renal arterial vasoconstriction occurs and lasts for 4–6 hr. This leads to diminished renal blood flow and glomerular filtration. Because amphotericin B binds to the cholesterol component in the membranes of the distal renal tubules, a change in permeability occurs in these cells, leading to polyuria, polydipsia, concentration defects, and acidification abnormalities. The net result is a distal renal tubular acidosis syndrome.

•      The metabolic acidosis leads to bone buffering, the excessive release of calcium into the circulation, and ultimately nephrocalcinosis due to calcium precipitation in the acidic environment of the distal tubules. Almost every animal treated with amphotericin B develops some degree of renal impairment, which may become permanent depending on the total cumulative dose.

•      The administration of amphotericin B can lead to a number of other adverse effects, including anorexia, nausea, vomiting, hypersensitivity reactions, drug fever, normocytic normochromic anemia, cardiac arrhythmias (and even arrest), hepatic dysfunction, CNS signs, and thrombophlebitis at the injection site.

•                  A number of adjuvant therapies are used to minimize adverse events of amphotericin B.

•                  Pretreatment with antiemetic and antihistaminic agents prevents the nausea, vomiting, and hypersensitivity reactions.

•                  Giving corticosteroids IV also limits severe hypersensitivity reactions. Mannitol (1 g/kg, IV) with each dose of amphotericin B, and sodium bicarbonate (2 mEq/kg, IV or PO, daily) may help prevent acidification defects, metabolic acidosis, and azotemia; however, clinical evidence of efficacy has not been proved.

•                  Saralasin (6–12 mcg/kg/min, IV) and dopamine (7 mcg/kg/min, IV) infusions have prevented oliguria and azotemia induced by amphotericin B in dogs.

•                  Administering IV fluids or furosemide before amphotericin B prevents pronounced decreases in renal blood flow and glomerular filtration rate.

•                  Newer preparations in which amphotericin B is mixed with lipid or liposomal vehicles (particularly liposomes) are safer and have maintained efficacy.

Interactions

•      Amphotericin B may be combined with other antimicrobial agents with synergistic results. This often allows both the total dose of amphotericin B and the length of therapy to be decreased.

            Examples include combinations of 5-flucytosine and amphotericin B for treatment of cryptococcal meningitis, minocycline and amphotericin B for coccidioidomycosis, and imidazole and amphotericin B for several systemic mycotic infections.

            Rifampin may also potentiate the antifungal activity of amphotericin B.

•      Drugs that should be avoided during amphotericin B therapy include aminoglycosides (nephrotoxicity), digitalis drugs (increased toxicity), curarizing agents (neuromuscular blockade), mineralocorticoids (hypokalemia), thiazide diuretics (hypokalemia, hyponatremia), antineoplastic drugs (cytotoxicity), and cyclosporine (nephrotoxicity).

Effects on Laboratory Tests

Treatment with polyene macrolide antibiotics increases plasma bilirubin, CK, AST, ALT, BUN, eosinophil count, and urine protein, and decreases plasma potassium and platelet count.

NYSTATIN

•      Obtained from S. noursei.

•      Its antifungal action and other properties are similar to Amphotericin B.

•      But because of its higher systemic toxicity, its usefulness is limited to Rx of Candida infections of the skin, Mucous membrane and GIT and applied either orally  or topically.

•      Its oral absorption is poor.

•      It is the choice for prevention and Rx of intestinal moniliasis.

•      Dose: Oral: @ 22000 iu/kg/day in 3 divided doses.

NATAMYCIN (PIMARICIN)

•      It is similar to nystatin and primarily used in the Rx of fungal keratitis.

•      A 5% opthalmic solution is applied topically  1 drop instilled in the every eye every 1 – 2 hr.

AZOLES

a. Imidazoles:             Topical: Clotrimazole, Econazole and Miconazole.
                                    Systemic: Ketoconazole and Thiabendazole.
b. Triazoles:   Systemic: Fluconazole and Itraconazole.
Imidazoles & Triazoles have broad spectrum fungistatic activity covering dermatophytes, Candida and some are also effective against systemic mycoses.

            They inhibit fungal CYP450 enzyme, lanosine 14-alpha-demethlase, wcich converts lanosterol to ergosterol (the main sterol in the fungal cell membrane). The interference in ergosterol synthesis leads to structural and functional abnormalities in the fungal cell membrane. Triazoles are less toxic than imidazoles because of their lower affinity for mammalian CYP450 and  cause less interference with mammalian sterol synthesis. Other enzyme systems are also impaired, such as those required for fatty acid synthesis. Because of the drug-induced changes of oxidative and peroxidative enzyme activities, toxic concentrations of hydrogen peroxide develop intracellularly. The overall effect is cell membrane and internal organelle disruption and cell death. The cholesterol in host cells is not affected by the imidazoles, although some drugs impair synthesis of selected steroids and drug-metabolizing enzymes in the host.

Fungal Resistance

•      Sensitivity to the imidazoles varies greatly between various strains of yeasts and fungi, but neither natural nor acquired resistance appears to be prevalent.

Antimicrobial Spectra

•      The antifungal imidazoles also have some antibacterial action but are rarely used for this purpose. Miconazole has a wide antifungal spectrum against most fungi and yeasts of veterinary interest. Sensitive organisms include Blastomyces dermatitidis, Paracoccidioides brasiliensis, Histoplasma capsulatum, Candida spp, Coccidioides immitis, Cryptococcus neoformans, and Aspergillus fumigatus. Some Aspergillus and Madurella spp are only marginally sensitive.

•      Ketoconazole has an antifungal spectrum similar to that of miconazole, but it is more effective against C immitis and some other yeasts and fungi. Itraconazole and fluconazole are the most active of the antifungal imidazoles. Their spectrum includes dimorphic fungal organisms and dermatophytes. They are also effective against some cases of aspergillosis (60%–70%) and cutaneous sporotrichosis. Clotrimazole and econazole are used for superficial mycoses (dermatophytosis and candidiasis); econazole also has been used for oculomycosis. Thiabendazole is effective against Aspergillus and Penicillium spp, but its use has largely been replaced by the more effective imidazoles. Voriconazole is approved for human use in treatment of Aspergillus but is effective against many other fungal organisms. Posaconazole may be more effective than itraconazole or fluconazole but may be associated with more adverse effects.

Pharmacokinetic Features
Absorption and Distribution

•      The imidazoles are rapidly but sometimes erratically absorbed from the GI tract; plasma levels peak within 2 hr after administration PO. Fluconazole is an exception, being close to 100% bioavailable after administration PO. Except for fluconazole, an acidic environment is required for dissolution of the imidazoles, and a decrease in gastric acidity can reduce bioavailability after administration PO. The rate of absorption appears to be increased when the drug is given with meals, but reports are conflicting. Because oral bioavailability can be very poor with noncommercial imidazole products, caution is recommended with compounded products, and monitoring is recommended if a compounded preparation is used.

•      Imidazoles appear to be widely distributed in the body, with detectable concentrations in saliva, milk, and cerumen. CSF penetration is poor except for fluconazole, which reaches 50%–90% of plasma concentrations. Most imidazoles (except fluconazole) are highly protein bound in the circulation (>95%), most to albumin. The highest concentrations of imidazoles are found in the liver, adrenal glands, lungs, and kidneys.

Biotransformation and Excretion

•      Hepatic metabolism is the primary route of elimination. Metabolism of ketoconazole and most other imidazoles by oxidative pathways is extensive. Only ~2%–4% of a dose administered PO appears unchanged in the urine. Itraconazole is metabolized to an active metabolite that may contribute significantly to antimicrobial activity. The biliary route is the major excretory pathway (>80%); ~20% of the metabolites are eliminated in the urine. Fluconazole (in people) is eliminated (≥90%) unchanged in the urine. The kinetics of voriconazole have not yet been evaluated in animals.

Pharmacokinetics

•      The rate of elimination of ketoconazole appears to be dose dependent—the greater the dose, the longer the elimination half-life. There is also a biphasic elimination pattern, with rapid elimination in the first 1–2 hr, then a slower decline over the next 6–9 hr. Ketoconazole is usually administered bid. The half-life of itraconazole is longer (up to 48 hr in cats), thus allowing treatment once to twice daily. Because of the long half-life and mechanism of action (impaired synthesis of the fungal cell membrane), time to efficacy may take longer than drugs that have more rapid actions (such as amphotericin B).

Therapeutic Indications and Dose Rates

•      The imidazoles are used to treat systemic fungal diseases, dermatophyte infections that have not responded to griseofulvin or topical therapy, Malassezia infection in dogs, aspergillosis, and sporotrichosis in animals that cannot tolerate or do not respond to sodium iodide. For serious infections, combination with amphotericin B is strongly recommended. Among the imidazoles, fluconazole may be more likely to distribute into tissues that are tough to penetrate. Both itraconazole and fluconazole are generally preferred to other imidazoles for treatment of systemic fungal infections, including aspergillosis and sporotrichosis. Topically applied imidazoles (clotrimazole, miconazole, econazole) are used for local dermatophytosis. Thiabendazole is included in some otic preparations for treatment of yeast infections.

•      Enilconazole is an imidazole that can be applied topically for treatment of dermatophytosis and aspergillosis. It has been used safely in cats, dogs, cattle, horses, and chickens and is prepared as a 0.2% solution for treatment of fungal skin infections. When infused into the nasal turbinates of dogs with aspergillosis, enilconazole treated and prevented the recurrence of fungal disease. When applied topically to dog and cat hairs, enilconazole inhibits fungal growth in 2 rather than 4–8 treatments, as is necessary with other topically administered antifungal agents.

Dosages of Imidazoles          
            Imidazole                    Dosage, Route, and Frequency       

•      Enilconazole                10 mg/kg in 5–10 mL, bid for 7–14 days      

•      Fluconazole                 5–10 mg/kg, PO, once to twice daily

•      Itraconazole                5–10 mg/kg, PO, once to twice daily

•      Ketoconazole              5–20 mg/kg, PO, bid (dogs)  

•      Thiabendazole             44 mg/kg/day, PO, or 22 mg/kg, PO, bid      

Adverse Effects and Toxicity

•      The imidazoles given PO result in few adverse effects, but nausea, vomiting, and hepatic dysfunction can develop. Ketoconazole in particular is associated with hepatotoxicity, especially in cats. Because imidazoles also inhibit cytochrome P450 associated with steroid synthesis, as a result, sex steroids, including testosterone and adrenal steroid (cortisol), metabolism is inhibited. Adrenal responsiveness to ACTH will be decreased, particularly with ketoconazole. Reproductive disorders related to ketoconazole administration may be seen in dogs. Voriconazole is associated with a number of adverse effects in people, including vision disturbances.

Interactions

•      Imidazoles, in general, inhibit the metabolism of many drugs. Although ketoconazole has the broadest inhibitory effects, fluconazole followed by itraconazole also inhibit metabolism. Concurrent administration of these drugs with other drugs metabolized by the liver and potentially toxic should be done only with extreme caution. Imidazoles also are substrates for P-glycoprotein transport protein and may compete with other substrates, causing higher concentrations. Many of the substrates for P-glycoprotein are also substrates for cytochrome P450. Rifampin, which is a P-glycoprotein substrate, decreases serum ketoconazole because of microsomal enzyme induction. The absorption of the imidazoles, except for that of fluconazole, is inhibited by concurrent administration of cimetidine, ranitidine, anticholinergic agents, or gastric antacids. The risk of hepatotoxicity is increased if ketoconazole and griseofulvin are administered together. Imidazoles might be used concurrently with other antifungals to facilitate synergistic efficacy.

Effects on Laboratory Tests

•      Treatment with imidazoles increases AST, ALT, plasma bilirubin, and plasma cholesterol. Adrenal responsiveness is altered.

KETOCONAZOLE

•      It is the 1^st orally effective BS antifungal drug, useful in both dermtophytoses and deep/systemic mycoses.

•      Administered orally, it is effective in the Rx of dermatophytoses (alternative to griseofulvin) and also very effective in histoplasmosis, balstomycosis, coccidioidiomycosis, cryptococosis and mucocutaneous candidisis.

•      It is less potent, but much less potent than Amphotericin B.

•      Orally it is well absorbed and absorption is facilitated by gastric acidity (largely bound to albumin), except fluconazole, undergo extensive hepatic metabolism nd excreted in urine and faeces.

•      Keto. causes microsomal enzyme inhibition and thus inhibits the metabolism of many drugs, also inhibits steroidogenesis.

•      It can be used along with Amphotericin B or 5-Flucytosine to potentite its antifungal activity.

•      It causes GI disturbances, hormonal and reproductive disorders in dogs.

•      It should not be given to pregnant animals (teratogenic).

•      Dose: Dogs: @ 5-20 mg/kg, twice daily.

FLUCONAZOLE AND ITRACONAZOLE

•      Their pharmacological properties are similar to Keto., but have wider range of antifungal activity and are less toxic;

•      Hence preferred for the Rx of systemic mycoses.

•      Fluconazole has high tissue (including CSF) penetrability and low nephrotoxicity (combined with Amphotericin B in treating crutococcl meningitis).

•      Dose: Fluconzole and Itraconazole: @ Dogs: @ 5-10 mg/kg . Twice daily.

FLUCYTOSINE

Flucytosine (5-fluorocytosine) is a fluorinated pyrimidine related to fluorouracil that was initially developed as an antineoplastic agent. It should be stored in airtight containers protected from light. Solutions for infusion are unstable and should be stored at 15°–20°C. Usually, it is given PO in capsules.

It is a synthetic narrow spectrum fungistatic drug, active against few strains of Cryptococcus, Candida, Aspergillus and Chromoblastomyces spp.

The other fungai causing systemic mycoses, dermatomycoses and bacteria are insensitive to flucytosine.

It is a pyirmidine antimetabolite and in active as such. It is converted to the antimetabolite 5-Fluuorouracil (5-FU) by cytosine deaminse in the fungl but not in mammalian cells.

Mode of Action

Flucytosine is converted by cytosine deaminase in fungal cells to fluorouracil, which then interferes with RNA and protein synthesis. Fluorouracil is metabolized to 5-fluorodeoxyuridylic acid, an inhibitor of thymidylate synthetase. DNA synthesis is then halted. Mammalian cells do not convert large amounts of flucytosine to fluorouracil and, thus, are not affected at usual dosage levels.

5-FU inhibits thymidylte synthetase and thus DNA synthesis.

Mammalian cells are not affected because of very limited ability to convert flucytoosine to 5-FU.

It is rapidly absorbed orally, widely distributed in the body and has excellent penetration to tissues and fluids including CSF.

Its main indictaions are cryptococcal meningitis (in combination with Amphotericin B), candidisis, aspergillosis and chromomycosis. It is less toxic than mphotericin B.

Dose: oral: dogs: 25 – 5- mg/kg , thrice  day; Cats: 30-40 mg/kg thrice a day.

Fungal Resistance

•      Resistance to flucytosine can develop rapidly even during the course of treatment; this precludes its use as the sole treatment for mycotic infections. The mechanisms of resistance are not completely understood.

Antifungal Spectra

•      The following are the main organisms usually sensitive to flucytosine: Cryptococcus neoformans, Candida albicans, other Candida spp, Torulopsis glabrata, Sporothrix schenckii, Aspergillus spp, and agents of chromoblastomycosis (Phialophora, Cladosporium). The other fungi responsible for systemic mycoses and dermatophytes are resistant to flucytosine.

Pharmacokinetic Features
Absorption and Distribution

•      Flucytosine is rapidly and well absorbed from the GI tract, with plasma levels peaking in 1–2 hr in animals that have received the drug for several days. The drug is widely distributed in the body, with a volume of distribution approximating the total body water. Flucytosine is minimally bound to plasma proteins. There is excellent penetration into body fluids such as the CSF, synovial fluids, and aqueous humor.

Biotransformation and Excretion

•      Nearly all (85%–95%) of an oral dose is excreted unchanged. Flucytosine is principally excreted by glomerular filtration (>80%). The clearance of flucytosine is approximately equivalent to that of creatinine. In renal failure, elimination of flucytosine is markedly impaired.

Pharmacokinetics

•      With normal renal function, the plasma half-life of flucytosine is usually 2–4 hr but may be up to 200 hr with oliguria. Serum levels of 50–100 mcg/mL are usually in the therapeutic range.

Therapeutic Indications and Dose Rates

•      The more common indications for flucytosine include cryptococcal meningitis, used together with amphotericin B (~30% of the isolates develop resistance during the course of treatment); candidiasis (~90% of isolates are usually sensitive); aspergillosis (some strains are sensitive at <5 mcg/mL); chromomycosis (some strains are very sensitive); and sporotrichosis (some cases may respond).

•      General dosages for flucytosine are 25–50 mg/kg and 30–40 mg/kg, PO, tid-qid in dogs and cats, respectively. The dose rate and frequency should be adjusted as needed for the individual animal. Dosage modification is essential in renal failure. Flucytosine serum levels should be monitored if possible.

Adverse Effects and Toxicity

•      Flucytosine is often well tolerated over long periods, but toxic effects may be seen when serum levels are high (>100 mcg/mL). These include GI signs (nausea, vomiting, diarrhea) and reversible hepatic and hematologic effects (increased liver enzymes, anemia, neutropenia, thrombocytopenia). In dogs, erythemic and alopecic dermatitis may be seen but subsides when the drug is discontinued.

Interactions

•      There is synergistic antifungal activity between amphotericin B and ketoconazole, and the combination may retard the emergence of strains resistant to flucytosine. The renal effects of amphotericin B prolong elimination of flucytosine. If flucytosine is used together with immunosuppressive drugs, severe depression of bone marrow function is possible.

Effects on Laboratory tests

•      Treatment with flucytosine increases alkaline phosphatase, AST, ALT, and other liver leakage enzymes, and decreases RBC, WBC, and platelet counts.

GRISEOFULVIN

Griseofulvin is a systemic antifungal agent effective against the common dermatophytes. It is practically insoluble in water and only slightly soluble in most organic solvents. Particle sizes of griseofulvin vary from 2.7 μm (ultramicrosized) to 10 μm (microsized).

Antifungal Activity
Mode of Action

            Dermatophytes concentrate griseofulvin through an energy-dependent process. Griseofulvin then disrupts the mitotic spindle by interacting with the polymerized microtubules in susceptible dermatophytes. This leads to production of multinucleate fungal cells. The inhibition of nucleic acid synthesis and the formation of hyphal cell wall material also may be involved. The result is distortion, irregular swelling, and spiral curling of the hyphae. Griseofulvin is fungistatic rather than fungicidal, except in young active cells.

Fungal Resistance

            Dermatophytes can be made resistant to griseofulvin in vitro.

Antifungal Spectra

            Griseofulvin is active against Microsporum, Epidermophyton, and Trichophyton spp. It has no effect on bacteria (including Actinomyces and Nocardia spp), other fungi, or yeasts.

Pharmacokinetic Features
            Absorption

            Plasma levels peak ~4 hr after administration PO, but absorption from the GI tract continues over a prolonged period.

            Absorption is highly variable and influenced by a number of factors .

            The rates of disaggregation and dissolution in the GI tract limit the bioavailability of griseofulvin; thus, microsized and ultramicrosized particles are usually used. High-fat meals, margarine, or propylene glycol significantly enhance GI absorption of griseofulvin and are indicated if the microsized particles are used.

            Distribution

            Griseofulvin is deposited in keratin precursor cells within 4–8 hr of administration PO.

            Sweat and transdermal fluid loss appear to play an important role in griseofulvin transfer in the stratum corneum. When these cells differentiate, griseofulvin remains bound and persists in keratin, making it resistant to fungal invasion. For this reason, new growth of hair, nails, or horn is the first to become free of fungal infection. As the fungus-containing keratin is shed, it is replaced by normal skin and hair. Only a small fraction of a dose of griseofulvin remains in the body fluids or tissues.

Biotransformation and Pharmacokinetics

            Depending on the species, 10%–50% of a griseofulvin dose is excreted almost exclusively as metabolites in the urine, and the remainder in the feces for ~4–5 days after administration.

            The elimination half-life of griseofulvin is ~24 hr in several species. The drug can be detected in 48–72 hr at the base level of the skin, in 6–12 days in the lower quarter, and in 2–19 days in the middle section of the horny layer.

Therapeutic Indications and Dose Rates

            Griseofulvin is used for dermatophyte infections in dogs, cats, calves, horses, and other domestic and exotic animal species. Most dermatophytes are sensitive, but certain species present greater therapeutic challenges than others. Several may require higher dose rates for satisfactory control.

Adverse Effects and Toxicity

            Adverse effects induced by griseofulvin are rare. Nausea, vomiting, and diarrhea have been seen. Hepatotoxicity has also been reported. Animals with impaired liver function should not be given griseofulvin, because its biotransformation will be reduced and toxic levels may be reached. Idiosyncratic (Type B or Type II adverse reaction) toxicity in cats has been reported. Clinical signs are neurologic, GI, and hematologic. Griseofulvin is contraindicated in pregnant animals (especially mares and queens) because it is teratogenic.

Interactions

            Lipids increase GI absorption of griseofulvin. Barbiturates decrease its absorption and antifungal activity. Griseofulvin is a microsomal enzyme inducer and promotes the biotransformation of many concurrently administered drugs. The combined use of ketoconazole and griseofulvin may lead to hepatotoxicity.

Effects on Laboratory Tests

            Treatment with griseofulvin increases alkaline phosphatase, AST, and ALT. Proteinuria may be detected.

ALLYLAMINES

•      The allylamines include terbinafine, naftifine, and the much older thiocarbamate tolnaftate.

•      Their mechanism is competitive inhibition of squalene epoxidase, blocking conversion of squalene to lanosterol, leading to squaline accumulation and ergosterol depletion in the cell membrane.

•      Terbinafine has a much higher affinity for fungal than for mammalian squaline epoxidase. Avid uptake of terbinafine into body fat and epidermis presumably enhances treatment for dermatophytes of superficial yeast pathogens of the skin. However, data are emerging to potentially support its use for systemic fungal infections. Terbinafine is also active against yeasts (eg, Blastomyces dermatitidis, Cryptococcus neoformans, Sporothrix schenckii, Histoplasma capsulatum, Candida, and Pityrosporum spp). Terbinafine increasingly is used in combination with other antifungal drugs to enhance efficacy. Effects are fungicidal.

•      The allylamines appear to be more efficacious than griseofulvin for treatment of dermatophyte infections. Efficacy has also been demonstrated against S schenckii and Aspergillus.

•      Terbinafine may enhance efficacy of other antifungal drugs for a variety of fungal disorders and pythiosis. In contrast to terbinafine, tolnaftate is limited to treatment of dermatophytes.

•      Resistance to the allylamines is rare, but the drugs potentially can be affected by multidrug resistance efflux mechanisms. Terbinafine, available in oral and topical preparations, is well absorbed (80% in people) after PO administration. Fat facilitates absorption. High concentrations occur in the stratum corneum, sebum, and hair. Terbinafine is metabolized by the liver in people; the elimination half-life is sufficiently long to allow once-daily administration, with steady state not occurring for 10–14 days in people. Adverse effects of terbinafine after PO administration are limited to GI and skin signs; hepatobiliary dysfunction is a rare adverse event. Because inhibition of ergosterol synthesis occurs at a step before cytochrome P450 involvement, the allylamines do not affect steroid synthesis as do the imidazoles.

IODIDES

•      Sodium and potassium iodide have both been used to treat selected bacterial, actinomycete, and fungal infections, although sodium iodide is preferred. The in vivo effects of iodides against fungal cells are not well understood. Iodide is readily absorbed from the GI tract and distributes freely into the extracellular fluid and glandular secretions. Iodide concentrates in the thyroid gland (50 times corresponding plasma level) and to a much lesser degree in salivary, lacrimal, and tracheobronchial glands. Longterm use at high levels leads to accumulation in the body and to iodinism.

•      Clinical signs of iodinism include lacrimation, salivation, increased respiratory secretions, coughing, inappetence, dry scaly skin, and tachycardia. Cardiomyopathy has been reported in cats. Host defense systems, such as decreased immunoglobulin production and reduced phagocytic ability of leukocytes, are also impaired. Iodinism may also lead to abortion and infertility.

•      Sodium iodide has been used successfully to treat cutaneous and cutaneous / lymphadenitis forms of sporotrichosis; attempts to control various other mycotic infections with iodides yield equivocal results.

•      The dosage for sodium iodide (20% solution) is 44 mg/kg/day, PO, for dogs, and 22 mg/kg/day, PO, for cats. The dosage for horses is 125 mL of 20% sodium iodide solution, IV, daily for 3 days, then 30 g, PO, daily for 30 days after clinical remission. The dosage rate for treating actinomycosis and actinobacillosis in cattle is 66 mg/kg, by slow IV, repeated weekly. Potassium iodide should never be injected IV.

Topical Antifungal Agents

•      A number of agents that have antifungal activity are applied topically, either on the skin, in the ear or eye, or on mucous membranes (buccal, nasal, vaginal) to control superficial mycotic infections. Concurrent systemic therapy with griseofulvin is often helpful for therapeutic management of dermatophyte infections. The hair should be clipped from affected areas and the nails trimmed to fully expose the lesions before antifungal preparations are applied. Bathing the animal may also be helpful. Isolation or restricted movement of infected animals is wise, especially when dealing with zoonotic fungi.

•      Preparations may be used in the form of solutions, lotions, sprays, powders, creams, or ointments for dermal application, or in the form of irrigant solutions, ointments, tablets, or suppositories for intravaginal use. The concentration of active principle in these preparations varies and depends on the activity of the specific agent.

•      The clinical response to local antifungal agents is unpredictable. Resistance to many of the available drugs is common. Spread of infection and reinfection add to the difficulty of controlling superficial infections. Perseverance is often an essential element of therapy.

•      Some topical antifungal agents that have been used with success in various conditions and species include iodine preparations (tincture of iodine, potassium iodide, iodophors), copper preparations (copper sulfate, copper naphthenate, cuprimyxin), sulfur preparations (monosulfiram, benzoyl disulfide), phenols (phenol, thymol), fatty acids and salts (propionates, undecylenates), organic acids (benzoic acid, salicylic acids), dyes (crystal [gentian] violet, carbolfuchsin), hydroxyquinolines (iodochlorhydroxyquin), nitrofurans (nitrofuroxine, nitrofurfurylmethyl ether), imidazoles (miconazole, tioconazole, clotrimazole, econazole, thiabendazole), polyene antibiotics (amphotericin B, nystatin, pimaricin, candicidin, hachimycin), allylamines (naftifine, terbinafine), thiocarbamates (tolnaftate), and miscellaneous agents (acrisorcin, haloprogin, ciclopirox, olamine, dichlorophen, hexetidine, chlorphenesin, triacetin, polynoxylin, amorolfine).

•      Amorolfine is a topical antifungal agent used to treat onychomycosis and dermatophytosis. It is prepared as a cream or nail lacquer. Amorolfine is a morpholine derivative that appears to interfere with the synthesis of sterols essential for the functioning of fungal cell membranes. In vitro, activity has been shown against some yeasts and dimorphic, dematiaceous, and filamentous fungi (Blastomyces dermatitidis, Candida spp, Histoplasma capsulatum, Sporothrix schenckii, and Aspergillus spp). Despite its in vitro activity, amorolfine is inactive when given systemically and thus is limited to topical use in treatment of superficial infections. Its role in treatment of fungal infection in animals is not clear.

•      Undecylenic acid: (10% powder or alcoholic solution): Has potent fungistatic activity and used in superficial fungal infections and to treat fungal mastitis. Other fatty acids like caprylic acid and propionic acid also has fungistatic action.

•      Salicylic acid: It has fungistatic action and is a component of many topical antifungal preparations because of its good keratolytic activity. It is mainly used to treat chronic superficial dermatomycoses.

•      Benzoic acid: Both Salicylic acid (3%) and benzoic acid (6%) are used to prepare Whitfield ointment, an antifungal ointment. Benzoic acid has fungistatic as well as keratolytic activity. It is used in the dermatomycoses.

•      Tolnaftate (1% lotion): It is active against dermatophytes, but not against candida and bacteria. It is used topically with oral griseofulvin.

•      Ciclopiroxolamine (1% lotion): It is a broad spec. Antifungal agent, active against all dermatophytes and candidia. It is used to treat dermatomycoses and candida infections.

•      Haloprogin (1% cream): a synthetic antifungal agent used to treat dermatomycoses in dogs.

•      Candidicin: A fungistatic and fungicidal antibiotic, active against candida and mainly used in moniliasis. Also used in ringworms.

•      Iodochlorhydroxyquin (3% cream / ointment / powder): Possess both antibacterial and antifungal activity and used topically to treat mixed infections.

•      Cuprimyxin: It has antifungal as well as BS antibacterial activity (active myxin is released on application) and used topically.

•      Imidazoles: These include Clotrimazole (1% cream/lotion) and Miconazole (2% cream/lotion) are used topically against dermatophytes and candidiasis. Econazole (1% cream) most active topical imidazole, used especially in candidiasis. Thiabendazole is given orally incalves to treat ringworm infection @ 22mg/kg,twice a day orally.

•      Amorolfine: it is used topically totreat dermatophytoses.

•      Terbinafine: It is occassionally used in dogs and cats topically (as 1% cream) or orally (@ 5 mg/kg/day for 7 weeks) against dermatophytes and candida. It decreases ergosterol synthesis in fungus.

•      Other agents: Phenols, iodine and mercurials have antibacterial as well as antifungal activity; but they may cause normal tissue damage. Carbol-fuschin solution is also applied topically, twice daily for 7 days. Gentian violet is used in candidiasis.

•      Topical antifungal preparations generally contain an antipruritic, a corticosteroid and an antihistamine in addition to the antifungal agent.