Antifungal Agents - Download as DOC

							            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 used 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 Cyclosporin has actually been used to lower the dose of the latter,
which usually shows low bioavailability.

Fluconazole and Itraconazole contain the triazole 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®).

To listen to the lecture, click on one of the following links:

mp3*:            Lecture

Real Player:      Lecture

*Download the lecture by right clicking on the mp3 link and select
"save link as".