10.1 ANTI-FUNGAL AGENTS
Antifungal agents are antimicrobial agents that kill or inhibit the growth of pathogenic fungi. Antimicrobial agents that inhibit the growth of pathogenic fungi are generally known as fungistatic agents while those that kill fungi are known as fungicidal agents. There are fewer antifungal agents than antibacterial agents because fungi like the mammalian cells are eukaryotic organisms, and drugs used for the treatment of fungal infections or human mycoses have profound untoward effects on their recipient human hosts. Since fungi and human cells share similar cellular and metabolic similarities because they are both eukaryotes, it is difficult to find suitable antifungal agents with little or no toxicity. Many antifungal agents are too toxic for the clinical management of human mycoses. Antifungal agents have poor selective toxicity compared to antibacterial drugs which target prokaryotic cells (e.g. bacteria); and a considerable amount of these drugs have adverse pharmacological features which compromises their usage in clinical medicine. They also have low therapeutic index compared to antibacterial agents with higher therapeutic index; and this is because most antifungal agents disrupts metabolic functions found not only in pathogenic fungi but also in the animal or human host cells taking them because of similarities in the cellular makeup of fungi and animal or human cells. Some of the undesirable characteristics of antifungal agents can be clinically managed because of the fact that most fungal infections in humans (e.g. superficial and cutaneous mycoses) are self-limiting in nature, and may heal on their own even without formal antifungal chemotherapeutic measures. A strong immune system is also critical to the effective management of some human mycoses even in the face of potent antifungal agents; and this is why people with compromised immune system (e.g. HIV/AIDS patients or cancer patients receiving chemotherapy) normally fall victims of opportunistic mycoses and other types of fungal infections which will by and large not affect a normal human being with intact immunity. Because of their notable toxicity, most fungal agents (except for those used for the treatment of systemic or deep mycoses) are usually in topical forms and are only used on the skin surfaces as antimicrobial lotions or solutions. Antifungal agents that are too toxic for systemic use (i.e. for treating deep mycoses) are available for topical administration. Antifungal agents like antibacterial agents target specific sites of their target pathogenic fungi including the fungal cell wall, nucleic acid and cell membranes, even though some of these agents have low selective toxicity when used for treatment. The main groups of antifungal agents include the azoles, polyenes, flucytosine, griseofulvin, cycloheximide, the allylamines and the echinocandins amongst others. The azoles whose main function is to inhibit the synthesis of ergosterol in fungi are the largest antifungal agents, and typical examples include fluconazole, ketoconazole, itraconazole, miconazole, voriconazole and clotrimazole. Ergosterol is the sterol that lines the cell membrane of fungi. (The human cell membrane is majorly made up of cholesterol). Polyenes include amphotericin B and nystatin; and their main antimicrobial function is to disrupt the integrity or cellular structure of the fungal cell membrane. Flucytosine is an antifungal agent that inhibits the biosynthesis of nucleic acid molecules (DNA and RNA) in pathogenic fungi. The allylamines are synthetic antifungal agents that interfere with the activities of squalene synthase, enzyme that promote the formation of squalene metabolites in fungi; and typical examples include terbinafine and naftifine. Allylamines generally inhibit fungal squalene metabolism; and they also reduce ergosterol synthesis in fungi. Increased levels of squalene are toxic to fungi. They are mainly used to treat fungal infections caused by dermatophytes. Terbinafine is a broad-spectrum antifungal agent which interferes with the structural and functional integrity of fungal cell membrane by inhibiting the enzyme squalene 2, 3 epoxidase; and this disrupts the synthesis of ergosterol in the target fungi. Terbinafine is used to treat cutaneous mycoses; and the agent can be used systemically or orally to treat some fungal infections (e.g. for treating nail infections). Gastrointestinal upset is usually a common side effect associated with the use of terbinafine. Cycloheximide is an antifungal agent that inhibits the growth of saprophytic fungi (i.e. non-pathogenic fungi); and they are usually incorporated into fungal culture media (e.g. Sabouraud dextrose agar) to prevent the growth of these organisms. Echinocandins are antifungal agents that block the synthesis of glucan (polysaccharide polymers found in fungal cells) layers in pathogenic fungi. Caspofungin is a typical example of an echinocandin, and they have activity against Candida species and Aspergillus species. Griseofulvin is an antifungal agent that inhibits cell division in pathogenic fungi by interfering with mitosis especially at the stage of microtubule development; and they are mainly use for topical antifungal applications. This section shall highlight some of the basic antifungal agents used in clinical medicine for the effective management of human mycoses. The mode of action of these agents, their representative structures as well as their side effects and clinical applications and resistance shall also be discussed.
10.2 NUCLEIC ACID SYNTHESIS INHIBITORS
Some fungal agents are classified as nucleic acid inhibitors because they interfere with the biosynthesis of DNA and RNA which are both critical for the overall development of the target pathogenic fungi. Flucytosine or 5-fluorocytosine is a typical example of antifungal agent that inhibits the synthesis of nucleic acids in pathogenic fungi. Flucytosine or 5-fluorocytosine (5-FC) is a synthetic antifungal agent that is used for the treatment of some fungal infections. Flucytosine (Figure 10.1) is an oral antifungal agent that acts as an antimetabolite. The drug is an analogue of cytosine, and 5-FC inhibits the synthesis of nucleic acids (DNA and RNA) and protein synthesis in fungal cells.
Figure 10.1: Chemical structure of 5-fluorocytosine (5-FC).
10.2.1 CLINICAL APPLICATION OF 5-FLUOROCYTOSINE
Flucytosine is rapidly and virtually completely absorbed following oral administration. It has in vitro and in vivo activity against Candida and Cryptococcus. Flucytosine (5-FC) is clinically used for the treatment of systemic mycoses, and the agent is active against Candida species including C. albicans and Cryptococcus neoformans that cause candidiasis and cryptococcosis respectively. It is usually used in combination with other antifungal agents (e.g. amphoteracin B) for treating deep or systemic mycoses. Flucytosine is mainly used for treating yeast infections, and the drug has little or no activity against dimorphic fungi and moulds.
10.2.2 MECHANISM OF ACTION OF 5-FLUOROCYTOSINE
Flucytosine is a nucleoside analogue of cytosine, and 5-FC is deaminated or converted to 5-fluorouracil (a false nucleotide) by cytosine deaminase in the target fungi. The formation of 5-fluorouracil and its incorporation by the pathogenic fungi inhibits the activity of thymidylate synthetase, which is the enzyme that directs DNA synthesis in the organism. Interference of the activities of thymidylate synthetase limits the supply of nucleotides (e.g. thymidine) which is required for the synthesis of DNA in fungi. Flucytosine is a narrow spectrum antifungal agent, and it is only used to treat human mycoses caused by yeasts especially in systemic fungal infections.
10.2.3 SIDE EFFECTS AND FUNGAL RESISTANCE TO 5-FLUOROCYTOSINE
The clinical usage of flucytosine causes the depression of bone marrow development in recipient hosts; and pathogenic fungi develop resistance to the agent especially in cases where the drug is used alone.
10.3 INHIBITORS OF ERGOSTEROL BIOSYNTHESIS
Antifungal agents that inhibit the biosynthesis of ergosterol (fungal sterol), a major component of fungal cell wall are generally known as azoles because they contain the imidazole group from which other agents in this category are chemically derived. And typical examples of antifungal agents that are azoles include fluconazole, ketoconazole, itraconazole, miconazole, voriconazole and clotrimazole. A handful of the azoles or imidazoles as they are often called are used topically as antimicrobial creams or solutions to treat superficial mycoses while the others are either used orally (e.g. ketoconazole) or intravenously (e.g. fluconazole) to treat a wide variety of human mycoses inclusive of superficial mycoses, cutaneous mycoses and systemic mycoses. This group of antimicrobial agent known as azoles or imidazoles also contains agents that have activity against non-fungal organisms such as helminthes (e.g. mebendazole), parasites or protozoa (e.g. metronidazole) and pathogenic bacteria (e.g. metronidazole).
10.3.1 CLINICAL APPLICATION OF KETOCONAZOLE
Ketoconazole (Figure 10.2) is an oral antifungal agent used to treat yeast infections and mycoses caused by dimorphic fungi and Cryptococcus species. They are effective for treating candidiasis, dermatophytosis and some systemic mycoses. Fluconazole is administered orally and/or intravenously; and it is used to treat mycoses caused by Candida species, Cryptococcus species and dimorphic fungi. Itraconazole is administered orally and intravenously; and it is used to treat systemic mycosis including fungal infections caused by dimorphic fungi and invasive moulds such as Aspergillus species. Miconazole is a topical antifungal agent used to treat infections caused by Candida and other yeasts. Voriconazole is administered orally and intravenously; and it is also used to treat fungal infections caused by Candida and other yeasts. Clotrimazole like miconazole is mainly used topically to treat mycoses caused by Candida and other yeasts.
Figure 10.2: Chemical structure of ketoconazole, an azole antifungal agent.
10.3.2 MECHANISM OF ACTION OF KETOCONAZOLE
The azoles or imidazole inhibit the biosynthesis of ergosterol in fungal cell membrane by blocking the activities of cytochrome enzyme or P-450 demethylase which controls an important precursor (particularly cytochrome P-450-dependent demethylation of lanosterol) in fungal cell membrane development. Cytochrome P-450 enzyme is responsible for converting lanosterol to fungal sterol or ergosterol which is an essential component of the cytoplasmic membrane of fungi. By interfering with the cellular and metabolic synthesis of ergosterol in the fungi, the azoles disrupt the integrity of the fungal cell membrane, and this makes the cell to be more permeable to harmful substances including drugs that destroys it. Azoles have a broad spectrum of antimicrobial activity, and they are both fungicidal and fungistatic in action.
10.3.3 SIDE EFFECTS AND FUNGAL RESISTANCE TO KETOCONAZOLE
The azoles like other antifungal agents have some untoward effects when used for the treatment of human mycoses. Mammalian cells also contain cytochrome P-450 enzymes, and the use of imidazoles interferes with the activities of these enzymes in human cells. Cytochrome P-450 enzyme in human cells is a microsomal enzyme or haemoprotein that help in the oxidation of drugs in the liver and other human tissues. Azoles inhibit the conversion of lanosterol to cholesterol (human sterol) in human cells the same way they inhibit ergosterol biosynthesis in fungi; and they may also interfere with the synthesis of the male sex hormone (e.g. testosterone). The inhibition of mammalian cytochrome P-450 enzymes results in the interference of biosynthesis of cortisone, oestrogen and androgen, and this may lead to infertility in human hosts. The resistance of pathogenic fungi to the antimicrobial onslaught of the azoles or imidazoles is rare.
10.4 CELL MEMBRANE INHIBITORS
The cytoplasmic membrane of fungal cell is vital to the sustenance and development of the fungal organism because this part of the cell helps to maintain a constant internal environment and it also help to regulate the inflow and outflow of materials from the cell. Antifungal agents that interfere with the synthesis of fungal cytoplasmic membrane include amphoteracin B and nystatin. Drugs that perform this vital function are generally known as polyenes. Amphotericin B (Figure 10.3) is a widely used polyene that is naturally produced by Streptomyces species; and with nystatin (an analogous polyene), both of these antifungal agents disrupt the structural integrity of fungal cytoplasmic membrane. Nystatin is also synthesized naturally by Streptomyces species.
10.4.1 CLINICAL APPLICATION OF AMPHOTERICIN B
Amphotericin B is an intravenous antifungal agent which is clinically used to treat systemic mycosis. It has a broad spectrum of activity and it is clinically used to treat infections caused by Coccidioides species, Histoplasma species and Blastomycosis species amongst other fungal agents that causes endemic or deep mycoses. Nystatin is a topical antifungal agent used topically to treat some yeast infections especially those caused by Candida species. Nystatin is also applied at the vaginal area and on the skin surfaces to control the multiplication of yeast cells (e.g. Candida) in those regions. Amphotericin B is also used in combination with 5-FC to treat some systemic mycoses; and such antimicrobial combinations achieves a synergistic antifungal effect.
Figure 10.3: Chemical structure of amphoteracin B, a polyene.
10.4.2 MECHANISM OF ACTION OF AMPHOTERICIN B
The polyenes (inclusive of amphoteracin B and nystatin) are antifungal agents with strong affinity for fungal sterol (i.e. ergosterol), and they generally disrupt the cell membrane of fungi. While amphoteracin B is administered intravenously (IV), nystatin is mainly available as creams or solutions and is used for topical treatment of fungal infections because of their toxicity which limits their usage for systemic administration. The binding of the cell membrane of fungi by the polyenes (i.e. amphoteracin B and nystatin) leads to the formation of channels or holes on the cytoplasmic membranes through which important cell molecules exit the fungal cell. Amphotericin and nystatin are fungicidal in action since their antimicrobial effect can lead to the death of the fungal organism.
10.4.3 SIDE EFFECTS AND FUNGAL RESISTANCE TO AMPHOTERICIN B
Polyenes (inclusive of nystatin and amphotericin B) have untoward effects on human host cells upon usage. Nystatin is the most toxic and this limits its use to topical applications in the form of antifungal creams or solutions. Fever, chills, headache and dyspnea are some of the untoward effects of amphotericin administration. Their toxicity is also extended to the renal system causing nephrotoxicty, but this can be controlled when amphotericin B is co-administered with a lipid carrier such as liposomes. The development of resistance to the polyenes by pathogenic fungi is rare.
Griseofulvin (Figure 10.4) is a naturally synthesized antifungal agent that is produced by Penicillium species (particularly P. griseofulvin). Griseofulvin is available both for topical and oral administration, but the drug is mainly used for topical treatment of superficial mycoses. The drug is poorly absorbed by the gastrointestinal tract (GIT), and this limits their usage for oral administration.
Figure 4: Chemical structure of griseofulvin.
10.5.1 CLINICAL APPLICATION OF GRISEOFULVIN
Griseofulvin is mainly used to treat superficial mycoses caused by dermatophytes. And the antifungal agent exists only as creams or solutions which are applied topically on the site of the infection. Fungal infections of the hairs, skin, scalp and ringworm are best treated with griseofulvin. However, griseofulvin is also available for oral administration but the drug is poorly absorbed by the GIT.
10.5.2 MECHANISM OF ACTION OF GRISEOFULVIN
The mechanism of action of griseofulvin is mainly based on its ability to interfere with cell division (particularly mitosis) in fungal cells. Particularly, griseofulvin disrupts microtubule formation or mitotic spindle and this interferes with mitosis in fungi, thus affecting the biosynthetic machinery of other molecules such as the synthesis of DNA, RNA and proteins in fungi.
10.5.3 SIDE EFFECTS AND FUNGAL RESISTANCE TO GRISEOFULVIN
Hypersensitivity is one of the notable side effects of the topical application of griseofulvin for treating fungal infections. Other untoward effects associated with the clinical use of griseofulvin include headache and stomach upset. Resistance to griseofulvin is rare.
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