Living Textbook MC610

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Antifungal Agents

Fungi are plant-like non-photosynthetic Eukaryotes that may exist in colonies of single cells (yeast) or filamentous multicellular aggregates (molds or hyphae).

They are saprophytes that live in soil or on dead plants and are of extreme importance to mineralization of dead biomatter. However a few of these agents are parasites that may cause crop damage and an even smaller number can cause disease in humans and animals.

Up till 1980 only easily treatable non-life threatening topical infections were extensively studied. On the other hand systemic infections that were incurable and fatal were largely neglected because they were rare and there were not a lot of effective drugs against these agents. But since then this has changed, due to an increase in the number of infections especially in the hospital settings, reaching about 5% of all infections. This rise is mainly due to the escalation of one type of patients, immunocompromised patients.

There are two major types of fungal infections:

  1. Contagious skin and hair infection (keratinized tissues), by agents that can digest keratin and infection is propagated by contact with an infected patient. It is estimated that 10% of the population is infected with these types of fungi, and the problem is even larger in tropical areas. Causative agents are mostly dermatophytes such as Tinea and Candida albicans.
  2. Systemic infections caused by soilborne, airborne or foodborne fungi. These may enter the body through skin inoculation, inhalation, food or are part of the natural flora. Foodborne fungi can produce toxins that are very dangerous and can cause various effects ranging from hepatic cirrhosis to carcinomas to hallucinations. Other systemic infections can cause minor skin and mucosal lesions but may lead to serious systemic diseases especially in special class of patients such as pregnant women, AIDS, cancer and organ transplant patients and may lead to meningitis and brain abscesses. Systemic Candida infections are the most common type of infections that afflict HIV patients. It is noteworthy that some of these fungi are not virulent, but are opportunistic agents that turn pathogenic.

Although no immunization is available for these diseases, prevention is important through sanitation.

The cell wall and cell membrane of fungi is a multilayered structure that contains about 85% carbohydrates and the remainder of the cell wall is made up of proteins and lipids, with the sterol ergosterol woven within these elements. Ergosterol belongs to the same family of sterols as Cholestrol, its equivilant in mammalian cells, and differs mainly in the side chain and possessing an extra double bond in ring B, making the molecule slightly flatter.

The biosynthesis of Ergosterol involves the oxidation of Squalene by the enzyme Squalene Epoxidase, which cyclizes to Lanosterol, followed by a CYP450 14α-demethylase reaction that eventually gives rise to Ergosterol. Cholestrol biosyntheis follows a simillar pattern.

Fatty Acids

These agents have been used for a long time against dermatophytes and include Undecylenic acid and its Ca2+, Zn2+ and Cu2+ salts that are used in cream form for athlete's foot, due to Tinea pedis and other ring worm infections, but not on hair, nail or yeast infections. Their mechanism of action is not fully understood but seem to involve disruption of the cell membrane.

Thiocarbamates

These agents are used topically in tinctures and ointments against dermatophytes. They inhibit squalene epoxidase an enzyme that will lead to accumulation of squalene, a precursor of ergosterol and diminish ergosterol biosynthesis. Squalene has a toxic effect on the cell, and the decreased amount of ergosterol will disrupt cell membrane function. Both effects will cause cell death.
An example of these agents is Tolnaftate .

Allylamines:

They are used in the same nature as thiocarbamates and have the same indications and mechanism of action. Examples include Naftifine, Butenafine and Terbinafine. The latter agent has very little affinity for mammalian squalene epoxidase and so is well tolerated when administered orally. It accumulates in keratin in hair, skin and under nail plates and persists after treatment is stopped.

Griseofulvin

A natural product isolated from Penicillium griseofulvin in 1939 and has been used against Dermatophytes since 1951. Its main mechanism of action involves malformation of spindle microtubules important for proper mitosis. It also binds to fungal RNA and may inhibit protein biosynthesis, possibly interering with cell wall biosynthesis. It protects newly formed keratin cells from spread of infection and is said to be aided by formation of keratinized cells that block the fungus from nutrients.

The methoxy group on the cyclohexene seems to be important for providing lipophilicity for penetration into fungal cells. Changing this methoxy to the larger propoxy or butoxy improves activity, but larger size groups or other substituents will diminish activity.

It is not very effective topically, but mainly used orally. It has very poor solubility and poor absorption, which is improved by the use of the ultramicronized form and taken with fatty meals (such as milk), where sufficient amount will be absorbed and reach the target of action.

Polyenes

These agents contain a macrocyclic lactone ring and are produced by actinomycetes. The ring contains a 4 to 7 double bonds with an aminosugar and a carboxylic acid moiety that may form Zwitterions. They generally have low water solubility, poorly absorbed orally and some are highly toxic.

The best agents among this group of agents are the heptenes (contain 7 conjugated double bonds) that are at least 10 times as active as other polyenes and cause less damage to the cell membrane of host cells.

Their mechanism of action involves being inserted into membrane ergosterol,
leading to changes in membrane characteristics and permeability with loss of intracellular contents and cell death. They are effective against fungi, algae, protozoa, worms and even snails. They will also affect mammalian cells, but clinically useful agents such as Amphotericin B binds 10 times more strongly to ergosterol than to cholesterol.

Systemic application of these drugs has been associated with serious side effects such as hypokalemia and distal tubule acidosis, leading to nephrotoxicity. It is thus usually used in combinations to lower the dose and the associated side effects. The incidence of side effects can be lowered by forming a complex of the drug with lipids through colloidal formation, or through using liposomes as carriers to the site of action and can be administered intravenously. The reason for the lower incidences of toxicity is not exactly known but involves altered distribution, possibly due to the higher permeability of blood vessels at the site of infection that allows for the large lipoid particles to pass through to the infected tissue.

Examples of polyenes include Nystatin , which is too toxic for systemic use and is used for oral thrush caused by Candida in AIDS patients. Amphotericin B is used alone or in combinations for meningitis caused by Cryptococcus in AIDS patients. Although they have a relatively broad spectrum they are not used against many infections such as dermatophytes since there are safer and effective agents available.

5-Fluorocytosine

A fluorinated pyrimidine that was developed first as an antileukemic agent. It is well absorbed orally and can penetrate many tissues including the CSF.

Mechanism of action involves conversion by susceptible organisms to 5-fluorouracil by the enzyme cytosine deaminase and then to the nucleoside 5-fluoro deoxyuridilic acid. The nucleoside inhibits the enzyme Thymidylate Synthase that will suppress DNA synthesis. In addition 5-fluorouracil may be incorporated into RNA and result in inhibition of protein synthesis.

It has a narrow spectrum, effective against some strains of Candida, Cryptococcus and Aspergillus.

Resistance is a major problem, so only use in combination with Amphotericin B that will kill the resistant strains, and may also enhance penetration of Flucytosine into fungal cells by changing permeability of fungal cell membrane (synergism).

Mammalian cells have little Cytosine deaminase and the drug is excreted mostly unchanged, thus shows low toxicity. However when taken with Amphotericin B, its excretion will be lowered and the high plasma level may cause some hemolytic effects, thus plasma levels should be monitored.

Caspofungin

Caspofungin is the first of a unique class of antifungal drugs (Echinocandins) that inhibit the synthesis of ß-1,3-D-glucan, an integral component of the fungal cell wall. The exact mechanism is not understood, but it involves the inhibition of ß-1,3-D-glucan Synthase. Selective toxicity is due to the lack of the glucan component in mammalian cells. Resistance has rarely been seen with these agents.
It is used in the treatment of invasive Aspergillosis and Candidiasis and is indicated in patients that are intolerant to Amphotericin B and Itraconazole . It is given by slow intravenous infusion as the acetate salt over about 1 hour. Side effects are minimal so far.

Azoles

These agents are synthetic compounds that were first introduced in the 1960s. They are now the most versatile and valuable group of antifungal agents for systemic infections. They all have a five-membered ring that contains two ( imidazoles) or three ( triazoles) nitrogen atoms. The N-1 of this ring is attached to other aromatic rings that contain halogens via an aliphatic chain.

Mechanism of Action is by inhibiting a Cytochrome P-450 that catalyzes the conversion of Lanosterol to ergosterol. The N-1 substituent of the azole will bind to the apoprotein of the enzyme, while the N-3 will bind to the ferric atom on the heme prosthetic group of the enzyme and thus prevents the introduction of the oxygen atom onto the sterol. The other aromatic rings will bind to unspecified areas on the enzyme. This will result in the accumulation of Lanosterol and the disruption of the cell membrane, causing leakage and disorganization and ultimately cell death.

Potency of the azoles is dependent on the affinity of the N-1 substituent to the apoprotein and the strength of binding to the heme iron.

They have a broad spectrum but it is variable according to the specific agent.

Resistance is rare towards all azoles.

Azoles will slow down the metabolism of other drugs such as Cyclosporin, Sulfonylureas, Terfenadine and Phenytoin.

Individual Agents:

Clotrimazole can be orally absorbed, but it induces liver microsomal enzymes that will very rapidly metabolize it and render it ineffective. Thus it is reserved for topical use against dermatophytes and many strains of Candida.

Miconazole is also well absorbed, but not metabolized as easily. However it shows many side effects attributed to the castor oil used in its preparation for colloidal stabilization. Mostly used topically against dermatophyte and yeast, although available for IV use. It does not cross CSF.

Ketoconazole is used both topically and orally. It shows very low water solubility, but after oral administration it is converted in the stomach to the hydrochloric acid salt that is soluble and much better absorbed. Absorption will thus be blocked by antacids and H2 antagonists due to lowering of gastric acidity. It shows poor penetration into the CSF and although has a broad spectrum, it is ineffective against certain strains of Candida albicans.

Abnormal elevated liver function is observed in 5 - 10% of patients on Ketoconazole that may result in hepatitis. In addition at the higher end of the dose range (800 mg/day) it may inhibit several enzymes in human steroidogenensis especially of androgens, which may lead to a loss of male libido and sexual potency. On the other hand it has been utilized in cases where elevated androgen is undesirable, such as female acne, hirsutism, male breast cancer and Cushing's syndrome.

It is extensively metabolized by mammalian Cytochrome P-450, in particular CYP3A4, which may lead to several drug-drug interactions. Co-administration of this agent with drugs that are metabolized by the same isozyme, will lead to higher blood level of such drugs. Examples include Terfenadine and Cisapride, both were withdrawn from the market due to this interaction. Other examples include Isoniazid and Rifampin. Its interaction with Cyclosporine has actually been used to lower the dose of the latter, which usually shows low bioavailability.

Fluconazole and Itraconazole contain the triazoles ring and show increased affinity for fungal over human cytochrome P-450 compared to Ketoconazole and no reports of serious side effects have been reported for these agents.

Fluconazole is used both orally and intravenously, shows good water solubility and is well absorbed independent of gastric acidity. It has enhanced activity due to the fluorine atoms, higher plasma concentration and longer duration of action. It can penetrate the CSF and is thus used in meningitis caused by susceptible fungi and in AIDS patients with life threatening systemic fungal infections. It shows little metabolism (around 10%), but inhibits Mammalian Cytochrome P-450, in particular CYP2C9, which leads to dangerous drug-drug interactions with drugs such as warfarin and phenytoin.

Itraconazole shows poor penetration into the CSF, but is a better agent against certain Candida and Aspergillus species that are seen frequently in AIDS patients. It is also extensively metabolized by Cytochrome P450, which can lead to drug-drug interaction with HMG CoA reductase Inhibitors.

The newest agent in this class is Voriconazole . It has the same properties and spectrum as Fluconazole, but is also active against Aspergillus infections where Fluconazole is ineffective and also utilized in Fluconazole-resistant species.

Other examples of Azole Antifungals include Butoconazole (Gynazole®), Sulconazole (Exelderm®), Sertaconazole (Ertaczo®), Tioconazole (Vagistat®) and Terconazole (Terazol®).