Fungi are ubiquitous microorganisms that differ from bacteria in their cellular structure, and this makes them naturally resistant to antibacterial agents (Table 42.1). Fungi are broadly divided into yeasts and moulds. Yeasts are typically round or oval shaped microscopically, grow flat round colonies on culture plates and reproduce by forming buds from their cells. Moulds (e.g. Aspergillus, Mucor) appear as a collection or mass (mycelium) of individual tubular structures called hyphae that grow by branching and longitudinal extension. They appear as a fuzzy growth on appropriate conducive medium (e.g. Penicillium colonies on stale bread or Sabourauds agar). The most commonly seen yeast, Candida, occasionally produces pseudohyphae.

Table 42.1 Important characteristics of a fungal cell

Fungi	Bacteria
Eukaryotes	Prokaryotes, eubacteria
Cell and cytoplasm	Cell and cytoplasm
Nucleus with multiple chromosomes enclosed in a nuclear membrane	No nucleus or nuclear membrane, has single chromosome
Contains endoplasmic reticulum, golgi apparatus, mitochondria and ribosomes	Other structures absent except ribosomes
Cytoplasmic membrane	Cytoplasmic membrane
Contains phospholipids and sterols	Contains phospholipids and no sterols
Cell wall	Cell wall
Contains chitins, mannans,+/- cellulose	Contains peptidoglycan, lipids and proteins

There are hundreds of species of fungi found in the environment, but only the important human fungal pathogens and their treatment will be discussed in this chapter. The fungi of medical importance can be divided into four groups (Table 42.2).

Table 42.2 Classification of fungi of medical importance

Group	Examples	Infections caused
Yeast	Candida spp.	Oral and vaginal thrush
	Candida spp.	Deep seated: candidaemia, empyema
	Cryptococcus neoformans	Meningitis
	Saccharomyces cervesiae	Rare systemic infection in immunocompromised host
	Malassezia furfur	Rare systemic infection in immunocompromised host
Yeast-like	Geotrichium candidium
Yeast-like	Trichosporon beigelii
Dimorphic fungi	Blastomyces dermatitidis	For first three: deep systemic organ involvement, more commonly in the immunocompromised host
	Coccidioides immitis	Deep subcutaneous infection following trauma
	Histoplasma capsulatum
	Paracoccidioides brasiliensis
	Sporothrix schenckii
Moulds
1. Hyaline
a. Zygomyces	Rhizopus	Infections in neutropenic patients and those with diabetic ketoacidosis
	Mucor
	Absidia
b. Hyalohyphomycosis	Aspergillus fumigatus and other Aspergillus spp.	Systemic infection: invasive pulmonary or central nervous system involvement
	Fusarium	Fusarium keratitis
	Scedosporium apiospermum	Deep infection in immunocompromised host, for example, transplant patients
2. Dermatophytes	Trichophyton spp.	For all three: various skin (ringworm) hair and nail infections
	Microsporum spp.
	Epidermophyton
3. Dematiaceous	Alternaria spp.	Deep tissue infection with granulomas
3. Dematiaceous	Cladophialora spp.	Chromomycosis, mycetomas

Some fungi like Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis are known as dimorphic fungi (Table 42.2) because they are found in the infected host in yeast form at 35–37 °C temperature but grow as moulds, in vitro, at room temperatures (22 °C incubation).

Fungi mainly reproduce by forming spores through mitosis giving rise to two daughter cells. They are known by names given to this imperfect state (asexual reproduction), but the same fungus, for example, Scedosporium apiospermum (asexual form) is also known as Pseudoallescheria boydii (sexual form). However, for all practical purposes, only the oldest and best-established name for the fungi is used in diagnostic laboratories.

Fungal spore are spread by air, water and direct contact with infected source. Humans usually become infected by inhalation of airborne spores or by inoculation into traumatised skin and mucous membrane.

Laboratory diagnosis

Microscopical examination and culture of fungi is the mainstay of laboratory diagnosis. Appropriate staining of histological sections of affected tissue is helpful in making a diagnosis when culture growth may or may not be positive. Yeast colonies and moulds are characteristic in their appearance on culture plates and can be preliminarily identified by their shape, colour and temperatures at which they grow. For the genus and species identification of yeasts, microscopic examination and biochemical tests are necessary. Moulds are identified by their morphology and the nature of sporulation on agar medium.

Antifungal sensitivity testing for yeast is done by determining the minimum inhibitory concentration (MIC) of the antifungal agent in the E test® strip method which has now replaced the measurement of ‘inhibition zone’ by disc testing. E-test® strips are also available for determining sensitivity of antifungal agents against moulds. Molecular diagnosis utilising polymerase chain reaction (PCR) is not available for use in routine practice. Serological diagnosis to look for antibodies in patient’s blood is of use only in Coccidioides infection. Enzyme-linked immunosorbent assay (ELISA) methods to look for galactomannan antigen in deep Aspergillus infection are available but not fully evaluated. A positive test needs to be interpreted in conjunction with other findings. Antigen detection is useful in disseminated Histoplasmosis and Cryptococcosis.

Fungal infection

It is important to distinguish harmless colonisation with fungi and significant infection, as only the latter would benefit from antifungal treatment.

More often, fungi are a cause of superficial infections of the skin and mucous membranes.

In some susceptible hosts whose immune system is heavily compromised, deep-seated infections involving organs like lungs and brain can manifest as ‘difficult to cure’ infections, for example, pulmonary aspergillosis or cryptococcal meningitis.

Antifungal agents

Topical and systemic antifungal agents are available to treat mucocutaneous candidiasis, various forms of tinea (ringworm) and other dermatophytosis, onychomycosis and deep-seated systemic infections (e.g. candidaemia, mucor mycoses, fungal endocarditis, osteomyelitis). Some infective conditions and their treatment are dealt with in the sections that follow. The side effects of a range of antifungal agents are set out in Table 42.3.

Table 42.3 Side effects of antifungal agents

Drug	Side effects
Griseofulvin	Mild: headache, gastro-intestinal side effects. Hypersensitivity reactions such as skin rashes, including photosensitivity
Griseofulvin	Moderate: exacerbation of acute intermittent porphyria; rarely, precipitation of systemic lupus erythematosus. Contraindicated in both acute porphyria, systemic lupus erythematosus, pregnancy and severe liver disease
Terbinafine	Usually mild: nausea, abdominal pain; allergic skin reactions; loss and disturbance of sense of taste. Not recommended in patients with liver disease
Amphotericin	Immediate reactions (during infusion) include headache, pyrexia, rigors, nausea, vomiting, hypotension; occasionally, there can be severe thrombophlebitis after the infusion
	Nephrotoxicity and hypokalaemia
	Anaemia due to reduced erythropoiesis
	Peripheral neuropathy (rare)
	Cardiac failure (exacerbated by hypokalaemia due to nephrotoxicity)
	Immunomodulation (the drug can both enhance and inhibit some immunological functions)
Flucytosine	Mild: gastro-intestinal side effects (nausea, vomiting). Occasional skin rashes Moderate: myelosuppression (dose related), hepatotoxicity
Fluconazole	Mild: nausea, vomiting and occasional skin rashes; occasionally, elevated liver enzymes (reversible)
Fluconazole	Moderate or severe: rarely, hepatotoxicity and severe cutaneous reactions, especially in AIDS patients
Itraconazole	Mild: nausea and abdominal pain; occasional skin rashes
Itraconazole	Moderate or severe: rarely, hepatotoxicity
Voriconazole	Similar to fluconazole and itraconazole
Voriconazole	Mild: reversible visual disturbances occur in about 30% patients
Caspofungin	Mild: gastro-intestinal side effects; occasional skin rashes

Superficial infection

Candida infections

Guidance on the treatment of topical and systemic therapy (Pappas et al., 2009) are also available for treatment of mild, moderate and severe oropharyngeal and oesophageal candidiasis and suppressive therapy for patients with HIV infection.

Dermatophytosis

Epidemiology

Dermatophytosis, or tinea, is a condition caused by three genera of dermatophyte fungi: Trichophyton, Epidermophyton and Microsporum. Unlike Candida, these are moulds which have a predilection for keratinised tissue such as skin, nail and hair. These fungi are very widely distributed throughout the world and may be acquired from the soil (anthrophilic, e.g. Trichophyton rubrum), from animals (zoophilic) or from humans (geophilic) infected with the fungus.

Some species are prevalent throughout the world, for example, Microsporum canis, while some are area specific, for example, Trichophyton mentagrophyte in Europe and New Zealand.

Clinical presentation

The classical clinical presentation of dermatophyte infection of the skin is ringworm (tinea), a circular, inflamed lesion with a raised edge and associated skin scaling. However, presentation is influenced by the site of infection, for example, tinea pedis (athlete’s foot) between the toes, tinea cruris on the body, and by the actual species of fungus causing the infection. In general, less severe lesions are produced by human fungal strains, while those acquired from animals can produce quite intense inflammatory reactions.

Dermatophytosis of the nail results in thickened, discoloured nails, while in the scalp, infection presents with itching, skin scaling and inflammation, and patchy hair loss (alopecia).

Rarely, deep dermatophytosis may be seen in immunocompromised patients with involvement of subcutaneous tissue (granuloma).

Diagnosis

The diagnosis of dermatophyte infection is confirmed by collecting appropriate specimens such as material from infected nails and skin. The fungi can be seen microscopically and specimens may also be cultured, but antifungal susceptibility testing is not required.

Treatment

Small or medium areas of skin infection can be treated with topical therapy, but nail, hair and widespread skin infection should be systemically treated with oral antifungal agents.

The most commonly used topical agents are the imidazoles, of which a wide variety is available, including clotrimazole, ecoazole, miconazole, sulconazole and tioconazole. There is little to choose between these agents, all of which are usually applied two or three times daily, continuing for up to 2 weeks after the lesions have healed. Side effects are uncommon and usually consist of mild skin irritation. Other topical agents include amorolﬁne, terbinaﬁne and tolnaftate.

The main oral antifungals used for dermatophytosis are terbinafine, itraconazole and fluconazole. Griseofulvin is an alternative treatment for tinea capitis.

Terbinaﬁne

Terbinaﬁne was the first member of a new class of antifungal agents, the allylamines, which became available for systemic use. These agents act by inhibition of the fungal enzyme squalene epoxidase, an enzyme involved in the synthesis of ergosterol, an essential component of the fungal cytoplasmic membrane. Although terbinaﬁne has a very broad antifungal spectrum in the laboratory, its in vivo efficacy does not correspond to its in vitro activity, and it is used only for the treatment of dermatophyte infection.

Terbinafine is the treatment of choice for tinea infections at 250 mg/day for 2–4 weeks, 250 mg/day for 6 weeks for fingernail infection and 12 weeks for toenail infection.

About 70% of an oral dose of terbinafine is absorbed, and the drug appears in high concentrations in the skin. The half-life is about 16–17 h, and therefore the drug can be given once per day. Terbinaﬁne is metabolised in the liver and the metabolites are excreted in the urine so that hepatic or renal dysfunction will prolong the elimination half-life.

Itraconazole is the second preferred agent at 200 mg/day for 1–2 weeks and longer, with repeated courses for finger and toenail involvement.

Griseofulvin

The first orally administered treatment for dermatophytosis was griseofulvin, which has now been available for over 40 years. Griseofulvin is active only against dermatophyte fungi and is inactive against all other fungi and bacteria. In order to exert its antifungal effect, it must be incorporated into keratinous tissue, where levels are much greater than serum levels, and therefore it has no effect if used topically.

The usual adult dose is 10–20 mg/kg/day for 6 weeks for treatment of larger lesions in tinea corporis.

Griseofulvin is well absorbed and absorption is enhanced if taken with a high-fat meal. In children, it may be given with milk. A 1000-mg dose produces a peak serum level of about 1–2 mg/L after 4 h, with a half-life of at least 9 h. An ultra-fine preparation of griseofulvin exists which is almost totally absorbed and permits the use of lower doses (typically 330–660 mg daily). This preparation is not available in the UK. Elimination is mainly through the liver and inactive metabolites are excreted in the urine. Less than 1% of a dose is excreted in urine in the active form, but some active drug is excreted in the faeces.

The duration of treatment with griseofulvin is dependent entirely on clinical response. Skin or hair infection usually requires 4–12 weeks’ therapy, but nail infections respond much more slowly; 6 months’ treatment is often required for fingernails, a year or more for toenail infections. Unfortunately, the rate of treatment failure or relapse in nail infection is high, and may reach up to 60%. Hence, terbinafine and itraconazole may be preferred agents.

Pityriasis versicolor

This is a common superficial skin infection caused by a yeast-like fungus, Malassezia furfur. The organism is a member of the normal skin flora and lives only on the skin because it has a growth requirement for medium-chain fatty acids present in sebum.

Clinical presentation

The condition usually appears as patches scattered over the trunk, neck and shoulders. These patches produce scales and may be pigmented in light-skinned individuals, appearing light brown in colour. In dark-skinned patients, the lesions may lose pigment and appear lighter than normal skin.

In some patients, Malassezia yeast is also associated with dandruff and seborrhoeic dermatitis, although the exact role of the yeast in causing this condition remains uncertain. In AIDS patients, seborrhoeic dermatitis may be quite extensive and sudden in onset.

Malassezia folliculitis can appear as greasy papules or pustules on the trunk or face of an HIV-infected individual.

Diagnosis

The diagnosis is made by microscopy of scrapings from the lesion. The specimen is examined for the presence of yeast cells and short hyphae. Culture is not usually required for diagnosis and, since it requires special culture media, is not routinely attempted.

Treatment

Pityriasis versicolor is treated with topical terbinaﬁne cream or a topical imidazole cream such as clotrimazole, econazole or miconazole. Cheaper topical alternatives are 2% selenium sulphide lotion or 20% sodium thiosulphate applied daily for 10–14 days. Relapses are common and treatment may need to be repeated. In severe cases, oral itraconazole (200 mg once daily for 7 days) may be given. Treatment of seborrhoeic dermatitis and folliculitis is undertaken with topical azole creams and 1% hydrocortisone. This condition can also often relapse.

Ear infection

Fungi sometimes infect the external auditory canal, causing otitis externa, the most common causative organisms being various species of Aspergillus (such as A. niger and A. fumigatus) and Candida albicans and other Candida species. A variety of other fungi found in the environment can also cause this condition. The use of topical antibacterial agents in the ear may predispose to local fungal infection.

Clinical presentation

Fungal infection of the ear usually presents as pain and itching in the auditory canal, sometimes with a reduction in hearing due to blockage of the canal. There may be an associated discharge from the ear. Clinical examination shows a swollen red canal, and the fungal mycelium is sometimes visible as an amorphous white or grey mass.

Diagnosis

The diagnosis of a fungal infection of the external canal can be made by microscopy and culture of material obtained from the ear.

Treatment

Aural toilet with removal of obstructing debris is very important in the management of fungal infections of the external auditory canal. A topical antifungal agent such as nystatin or amphotericin, or an imidazole can also be applied.

Infections with saprophytic fungi

Some saprophytic fungi (normally harmless human commensals) can cause significant infections. An example of such an infection in a host with a normal immune system is fusarium keratitis in contact lens wearers. Penicillium marneffei can cause skin infection and disseminated fatal infection in HIV-infected patients in South East Asia. Scedospermium pulmonary infections are seen in lung transplant recipients.

A large retrospective data from the USA revealed that the three most common non-Aspergillus moulds causing invasive fungal infection among patients receiving haemopoietic stem cell transplants (HSCT) were Fusarium, Scedospermium and Zygomycetes.

Deep-seated fungal infections

Most deep-seated or systemic fungal infections seen in the UK are the result of some breakdown in the normal body defences, which may be due to disease or medical treatment. Fungi that cause superficial infections can also cause deep-seated infection in immunocompromised patients with leukaemia and lymphoma and those in the post-transplant period of immunosuppression. There are, however, a group of fungi, often referred to rather misleadingly as the pathogenic fungi, which are able to cause systemic infection in a previously healthy person. These infections, which are usually due to dimorphic fungi, include diseases such as histoplasmosis, blastomycosis and coccidioidomycosis. They are rare in the UK but rather more common in the USA and other parts of the world.

Fungal infections in the compromised host

Common mycoses

Epidemiology and predisposing factors

There are a large number of conditions which may predispose the individual to systemic or deep-seated fungal infection. These are summarised in Table 42.4. A breach in the body’s mechanical barriers may predispose to fungal infection. For example, fungal infection of the urinary tract occurs most commonly in catheterised patients who have received broad-spectrum antibiotics, while total parenteral nutrition (TPN) is strongly associated with fungaemia, sometimes with unusual fungi such as Malassezia furfur. This is due to the use of TPN infusions containing lipids, which are a growth requirement of this organism. Most cases of systemic fungal infection, however, are associated with a defect in the patient’s immune system, and the nature of the organisms encountered is often related to the nature of the immunosuppression. Neutropenia, for example, is usually associated with Candida species, Aspergillus and mucormycosis, while defects of cell-mediated immunity, for example HIV infection, are strongly associated with infection by Cryptococcus neoformans. Prolonged diabetic ketoacidosis is a risk factor for developing rhinocerebral zygomycosis where mortality can be as high as 100% if there is significant underlying disease.

Table 42.4 Conditions predisposing to systemic or deep-seated fungal infection

Infection	Predisposing conditions
Systemic candidiasis	Neutropenia from any cause (disease or treatment)
	Use of broad-spectrum antibiotics which eliminate the normal body flora
	Indwelling intravenous cannulae, especially when used for total parenteral nutrition
	Haematological malignancy and HSCT
	Solid organ transplantation
	AIDS (particularly associated with severe mucocutaneous infection)
	Intravenous drug abuse
	Cardiac surgery and heart valve replacement, leading to Candida endocarditis
	Gastro-intestinal tract surgery
	Oesophagectomy leak leading to pleural space infection (empyema)
Aspergillosis	Neutropenia from any cause, especially if severe and prolonged
	Acute leukaemia
	Solid organ transplantation (mainly lungs)
	Chronic granulomatous disease of childhood (defect in neutrophil function)
	Pre-existing lung disease (usually leads to aspergillomas; fungus balls form in the lung rather than invasive or disseminated infection)
Cryptococcosis	AIDS
	Systemic therapy with corticosteroids
	Renal transplantation
	Hodgkin’s disease and other lymphomas
	Sarcoidosis
	Collagen vascular diseases
Zygomycosis	Diabetic hyperglycaemic ketoacidosis (leading to rhinocerebral infection)
	Severe, prolonged neutropenia
	Burns (leading to cutaneous infection)

HSCT, hematopoietic stem cell transplantation.

Many different fungi have been described as causing systemic fungal infection, but the most common organisms encountered and the conditions they cause are listed in Table 42.5. Of these, Candida and Aspergillus are by far the most common in the UK.

Table 42.5 Common causes of systemic and deep-seated fungal infection in the UK

Condition/organism	Common clinical presentations
Candidiasis (Candida albicans, C. glabrata, C. krusei, C. tropicalis, other Candida species)	Fungaemia
	Colonisation of intravenous cannulae
	Pneumonia
	Meningitis
	Bone and joint infections
	Endocarditis
	Endophthalmitis
	Peritonitis in chronic ambulatory peritoneal dialysis
Aspergillosis (Aspergillus fumigatus, A. flavus, other Aspergillus species)	Invasive pulmonary aspergillosis
	Disseminated aspergillosis
	Aspergilloma
	Endocarditis
Cryptococcosis (Cryptococcus neoformans)	Meningitis
	Pneumonia
	Cutaneous infection
Zygomycosis (various species of the genera Rhizopus, Mucor, Absidia)	Rhinocerebral infection
	Pulmonary mucormycosis
	Surgical wound and burns infection
Malassezia furfur	Cutaneous infection (especially in burns patients)
Malassezia furfur	Fungaemia associated with total parenteral nutrition

Clinical presentation

Symptoms can be non-specific like low-grade fever, night sweats, weight loss, cough, chest pain and septic shock in extreme cases (Table 42.6).

Table 42.6 Clinical presentation of systemic fungal infection

Condition	Clinical presentation
Fungaemia (the presence of fungi in the bloodstream), usually due to Candida species	Fever, low blood pressure and sometimes other features of septic shock, especially in neutropenic patients
	Relatively low-grade fungaemias such as those associated with colonised intravenous cannulae often present only with fever
	Disseminated infection to multiple organ systems is quite common with Candida species, leading to central nervous system disease, endocarditis, endophthalmitis, skin infections, renal disease, bone and joint infection
Pneumonia, most frequently due to Aspergillus species	Fever, chest pain and cough which may be non-productive. May progress rapidly, especially with Aspergillus infection, to severe respiratory distress, necrosis of the lung and pulmonary haemorrhage
Pneumonia, most frequently due to Aspergillus species	Formation of fungal balls in pre-existing lung cavities with or without invasion
Meningitis and other central nervous system infection	Candida infection may present as a typical meningitis, although it is often more insidious.
Meningitis and other central nervous system infection	Aspergillosis is associated with headache, confusion and focal neurological signs due to the presence of brain infarcts. Cryptococcosis most frequently presents as a chronic, insidious meningitis with headache and alteration in mental state
Mucormycosis	Angioinvasive. The most common presentation of mucormycosis is rhinocerebral infection.
	Initially an infection of the sinuses, it then spreads locally to the palate, orbit and eventually into the brain, leading to encephalitis
	Pulmonary disease can present as fungal balls radiologically with symptoms of haemoptysis

Diagnosis

Organ-specific radiological findings backed by laboratory tests, as discussed earlier in this chapter, are the mainstay of diagnosis.

Treatment

Compared to the vast array of antibacterial agents available to treat bacterial infections, there are very few systemic antifungal agents available and these comprise four major categories: the polyenes (conventional and lipid formulations of Amphotericin B), the triazoles (fluconazole, itraconazole, voriconazole and posaconazole), the echinocandins (caspofungin, anidulafungin and micafungin) and flucytosine. To provide optimal therapy to the patient, it is necessary to understand the profile, properties and toxicity of these agents. Table 42.7 details the antifungal spectrum of activity against common fungi.

Table 42.7 Antifungal spectrum of activity against common fungi

Antifungal prophylaxis is commonly used to prevent invasive fungal infections in the ‘at risk’ group of patients.

Amphotericin B

Amphotericin, a member of the polyene group, is obtained from various species of Streptomyces. Chemically, it is a large carbon ring of 37 carbon atoms closed by a lactone bond. One side of the molecule contains seven carbon- to-carbon double bonds (polyene) and the other side contains seven hydroxyl groups. It dissolves in organic polar solvents but forms a colloidal suspension of micelles in water which is rendered stable by the addition of the surfactant sodium deoxycholate.

Mode of action

Amphotericin B binds to the ergosterol in fungal cell membrane affecting its integrity by forming pores and therefore cell death. Nystatin, the other polyene, is only used topically because of toxicity associated with its systemic use.

Spectrum of activity

Amphotericin is active against a vast majority of fungi and this is the same for all formulations. Development of resistance is uncommon, although primary resistance has been identified for Aspergillus terreus, Scedosporium spp., Trichosporon spp. and Candida lusitaniae.

Pharmacodynamically, the ratio of the peak serum concentration to the MIC is important for its efficacy.

Amphotericin B deoxycholate (ABD)

Released in 1950, colloidal in nature, amphotericin B deoxycholate is highly protein bound (99%) and insoluble in water. It penetrates poorly into cerebrospinal fluid. Initial elimination of the drug occurs with a half-life of 24–48 h, but this is followed by very slow elimination (half-life about 14 days). As a consequence, it may take 10 weeks for the drug to disappear from the circulation.

Amphotericin B deoxycholate is given by slow IV infusion in 5% dextrose with a dose range of 0.3–1 mg/kg increased to 1.5 mg/kg for serious invasive infections. The duration of treatment can vary from 1 to 2 weeks to longer, depending on the severity of the infection and the organ system involved.

Adverse effects

Nephrotoxicity is the most serious side effect of amphotericin. Renal failure should be monitored regularly at least every other day, and if the serum creatinine exceeds 250 µmol/L, the drug should be discontinued until the creatinine level falls below this limit. Hypokalaemia is also a problem and may be severe, necessitating replacement therapy.

Azotaemia may be seen in patients after the first few infusions of amphotericin B deoxycholate. Chills, fever and tachypnoea may occur but can be avoided by addition of hydrocortisone 25–50 mg to the infusion solution.

The manufacturer recommends that before commencing treatment, a 1 mg test dose should be given in 50 ml of 5% dextrose over a 1- to 2-h period and the patient monitored for fever, rigors and hypotension. True allergic reactions are rare.

Amphotericin B lipid formulations

The advantage of delivering amphotericin encapsulated in liposomes or as a complex with lipid molecules is that a higher unit dose can be given and there is reduction in toxic effects. Three such preparations are currently available: liposomal amphotericin B (AmBisome®), amphotericin B lipid complex (Abelcept®) and amphotericin B colloidal dispersion (Amphocil®).

Liposomal amphotericin B

In liposomal amphotericin B (AmBisome®), the drug is contained in small vesicles each consisting of a phospholipid bilayer enclosing an aqueous environment. This permits the delivery of higher doses (3 mg/kg is recommended, but doses up to 10 mg/kg have been used in some centres) compared to conventional amphotericin, with very little of the immediate toxicity which is such a problem with the conventional formulation. Higher peak serum concentrations are obtained with the liposomal formulation compared to equivalent doses of the conventional drug, although it is not certain if this is clinically relevant. Liposomal amphotericin is concentrated mainly in the liver and spleen, where it is taken up by cells of the reticuloendothelial system. Concentrations in the lung and kidneys are much lower, which may or may not be clinically important.

There is reduced nephrotoxicity with this formulation, and some of the renal dysfunction which has been described in clinical trials of liposomal amphotericin may have been due to concomitant drugs. Three randomised, comparative clinical trials of liposomal amphotericin versus conventional amphotericin B have shown reduced toxicity due to the liposomal preparation, and there is additional evidence for this from open studies. It is this comparative lack of toxicity which accounts for much of the popularity of this agent, despite its expense. These studies also indicate that this agent performs as well as the conventional preparation in febrile neutropenic patients.

Amphotericin B lipid complex and amphotericin B colloidal dispersion

Amphotericin B lipid complex (Abelcet®) is not a liposomal formulation, but consists of large sheets of amphotericin combined with phospholipids. This formulation gives lower peak serum levels compared to the conventional drug because it is rapidly taken up by tissue macrophages, while concentrations in the lungs and the liver are much higher. Patients seem to experience more immediate side effects than with liposomal amphotericin. There is less clinical trial evidence for the use of this agent compared to the liposomal preparation. Amphotericin B colloidal dispersion (Amphocil®) is a formulation consisting of tiny discs of amphotericin and cholesterylsulphate. Like the lipid complex, it too produces low peak serum levels but high liver concentrations compared to the conventional drug. There is less clinical experience with this preparation than with the liposomal preparation, and it appears to have a higher incidence of certain adverse reactions than conventional amphotericin.

Choosing a lipid preparation

Unfortunately, there are limited comparative trial data between the different lipid formulations. At present, the greatest clinical experience is with liposomal amphotericin B and by reason of this and the reduced incidence of side effects, it is the preferred agent of the three in many centres in the UK. In an ideal world, all patients requiring amphotericin would receive the conventional preparation initially, being changed to a lipid formulation only if they fail to respond to or cannot tolerate the side effects of the conventional form. However, the incidence of side effects and the difficulty in administration of conventional amphotericin have in practice led to its replacement in most centres with a lipid formulation.

Flucytosine

Amphotericin B and griseofulvin were the only systemic antifungal agents available until the early 1970s, when flucytosine became available for patient use.

Mode of action

Flucytosine (5-fluorocytosine) is a synthetic fluorinated nucleotide analogue. The mode of action is twofold. Following uptake by the cell, which is dependent on the presence of cytosine permease, flucytosine is deaminated to 5-fluorouracil by cytosine deaminase. This in turn is incorporated into fungal RNA in place of uracil, leading to impairment of protein synthesis. Further metabolism of 5-fluorouracil leads to a metabolite that inhibits the enzyme thymidylate synthetase, leading to inhibition of DNA synthesis. Mammalian cells have absent or weak cytosine deaminase activity which accounts for the selective toxicity of flucytosine.

For all practical purposes, flucytosine is only active against yeasts and yeast-like fungi. Inherent resistance occurs in approximately 10% of clinical isolates of Candida species, and acquired resistance develops rapidly if the drug is used alone. There are several resistance mechanisms, some of which result from a single-step mutation giving a high frequency of acquired resistance in organisms exposed to the drug. For this reason, flucytosine should always be given in combination with another agent such as amphotericin, with which it is synergistic.

Pharmacokinetics

Flucytosine is highly soluble in water and over 90% of an oral dose is absorbed from the gastro-intestinal tract. Virtually all the absorbed dose is excreted unchanged in the urine by glomerular filtration. The elimination half-life is about 4 h, but this is greatly prolonged in renal failure and dosage modification is required in patients with renal dysfunction. The degree of protein binding is very low and flucytosine penetrates well into all tissues, including the aqueous humour of the eye, where about 10% of the serum level is achieved, and the CSF, where about 80% of the serum level is achieved.

Administrations

Flucytosine is given orally or by short intravenous infusion. The dose administered by either route in patients with normal renal function is 100–200 mg/kg/day in four divided doses. This must be reduced in renal failure, but the degree of reduction depends on the degree of renal impairment and it is obligatory to monitor the serum levels of flucytosine. Unfortunately, flucytosine assay is now rarely carried out routinely in UK laboratories. Flucytosine is usually given in conjunction with amphotericin which will probably cause some degree of renal dysfunction, so requiring modification of the flucytosine dose. To avoid dose-related marrow toxicity, the peak serum level, obtained at 1 h after an intravenous dose or 2 h after an oral dose, should be maintained in the range 70–80 mg/L and should not be allowed to exceed 100 mg/L.

Side effects

Some of the side effects of flucytosine are given in Table 42.3. The most important toxic effect is a dose-related myelosuppression with neutropenia and thrombocytopenia. This is usually reversible and can be avoided by monitoring serum levels of flucytosine and adjusting the dose accordingly. Hepatotoxicity is also probably a result of high serum levels, and liver function tests should be performed regularly. The drug is teratogenic in some animals and is not recommended in pregnant women for relatively trivial infections such as a fungal urinary tract infection. In cases of a life-threatening fungal infection, which is very rare in pregnancy, the potential benefits of flucytosine must be weighed against the possible risks.

Imidazoles

The imidazoles clotrimazole, econazole, fenticonazole, sulconazole and tioconazole are now principally used for the local treatment of vaginal candidiasis and for dermatophyte infections. Ketoconazole, which first became available in 1979, can be administered for systemic use, but even this agent has been superseded by the triazoles.

Triazoles

Three triazoles are licensed in the UK: fluconazole, itraconazole and voriconazole. They differ substantially from one another in their pharmacokinetic and antifungal activity (Table 42.8). Their main side effects are listed in Table 42.3.

Table 42.8 Comparative pharmacokinetics of antifungal agents

The basic chemical structure of the triazoles is the azole ring, a ﬁve-membered ring containing three nitrogen atoms. Their principal mode of action involves one of the nitrogen atoms of the azole ring binding to fungal cytochrome P450 enzymes. This inhibits the demethylation of lansterol and leads to a reduced concentration of ergosterol necessary for a normal fungal cytoplasmic membrane. The propensity, in man, to also inhibit the metabolism of drugs by cytochrome P450 results in a considerable number of drug interactions.

Fluconazole

Clinical use

Fluconazole is available both orally and parenterally and is used only in the treatment and prophylaxis of infections due to yeasts and yeast-like fungi. It is not used for the treatment of infections due to moulds. It is highly effective in the treatment of Cryptococcus infection, but the first-line treatment of cryptococcosis of the central nervous system is the combination of amphotericin B plus flucytosine for CNS infection due to this organism. This may be followed by fluconazole, which in HIV-infected patients will be required life long as suppressive treatment to prevent relapse. In immunocompetent hosts, fluconazole may be used as the primary treatment for disease not involving the CNS, such as pulmonary infection.

In patients with candidaemia due to colonised intravenous cannulae, the most important treatment is removal of the infected cannula, but it is common practice to give a short course of antifungal therapy to prevent disseminated infection elsewhere. Although not proven in randomised clinical trials, changing potentially infected central-line catheters in candidaemia patients is probably the most important part of therapy.

In non-neutropenic patients, studies have shown fluconazole and caspofungin to be as efficacious as, and less toxic than, conventional amphotericin B. In such patients, where the infecting organism and its susceptibility to fluconazole are known, it would be reasonable to commence treatment with fluconazole; caspofungin is a potential alternative. In neutropenic patients, and in patients infected with fluconazole-resistant organisms, amphotericin continues to be the treatment of choice. Fluconazole has also been successfully used as prophylaxis against Candida infections in neutropenic patients and patients with AIDS, but this in turn has been associated with an increasing incidence of systemic infections with fluconazole-resistant strains.

Resistance

Some units that use fluconazole extensively have noted an increase in the isolation of yeasts resistant to the drug, with the prevalence of resistance related to the extent of use of fluconazole. Resistance in Candida species is mainly seen in patients who are given long-term prophylactic fluconazole. This selects out those Candida species such as C. krusei and C. glabrata that are inherently less susceptible to fluconazole. Resistance in C. albicans, the most common species infecting humans, is seen mainly in AIDS patients, partly due to the extensive use of fluconazole in treating severe oral and pharyngeal candidiasis in such patients and partly due to the very large numbers of yeasts in the oropharynx of AIDS patients with candidiasis, which increases the chance of resistance due to spontaneous mutation.

Itraconazole

Itraconazole is available for both oral and intravenous use. The drug was originally available in capsules, but a newer liquid formulation gives better absorption and pharmacokinetic profile than the original capsule preparation, leading to significantly greater bioavailability and higher serum levels. Systemic bioavailability of itraconazole oral solution is around 55% optimised under fasting conditions. Itraconazole is extensively metabolised by the liver, predominantly by the CYP 3A4 isoenzyme system and is known to undergo enterohepatic circulation. This is a broad-spectrum antifungal which is effective against yeasts, dermatophytes, the ‘pathogenic’ fungi and some filamentous fungi, such as Aspergillus.

Clinical use

In deep-seated infection, itraconazole is used to treat infections due to the ‘pathogenic’ fungi, but there is less published evidence of its use in the treatment of systemic candidiasis and it cannot be recommended for this purpose (Maertens and Boogaerts, 2005). However, it may be useful in patients who are infected with strains resistant to fluconazole, some of which may remain sensitive to itraconazole, and in patients who are for some reason unable to tolerate fluconazole. It has also been used to treat cryptococcosis, despite its poor penetration of cerebrospinal fluid, and in that condition, it is an alternative to fluconazole for patients who cannot take the latter drug. However, one study comparing fluconazole and itraconazole as maintenance treatment for cryptococcosis was discontinued due to the high rate of relapse in the itraconazole arm.

There is now considerable evidence to support the use of itraconazole as a prophylactic agent in immunocompromised patients. It has been shown to be effective in reducing the incidence of systemic fungal infection compared to placebo and to be more effective than fluconazole, although this is due to a greater reduction in infections due to filamentous fungi, including Aspergillus.

Voriconazole

Voriconazole is available for both oral and intravenous administration. It has advantages over itraconazole in that its absorption from the gastro-intestinal tract is significantly better and is not affected by reductions in gastric acidity due to disease or concomitant medication. Its spectrum of activity is similar to that of itraconazole, but it is more active against Fusarium species, a mould which causes superficial infection of the nails and cornea, and occasionally systemic infection in immunocompromised patients.

Pharmacokinetics

Voriconazole given orally is rapidly and almost fully absorbed (oral bioavailability > 90%) with a maximum serum concentration being achieved in about 2 h after administration under fasting conditions. It is extensively distributed into tissue and penetrates well into the CSF and into vitreous and aqueous humour. It is cleared by hepatic cytochrome P450 metabolism and is involved in many clinically relevant drug–drug interactions. Therapeutic drug monitoring may be indicated in some clinical settings.

Clinical use

The main clinical indication for the use of voriconazole is aspergillosis. Studies have shown improved efficacy compared to conventional amphotericin B in systemic Aspergillus infection. The largest randomised control trial demonstrates that voriconazole is superior to amphotericin B deoxycholate as primary therapy of Aspergillosis (Walsh et al., 2008). One particular indication is cerebral aspergillosis. Although rare, this carries a high mortality rate (90% or higher), and one study has shown this to be reduced by voriconazole, presumably due to its better penetration into the central nervous system. In addition to aspergillosis, voriconazole is also licensed for the treatment of Fusarium infection and for the management of patients infected with strains of Candida resistant to fluconazole. It has been shown to be as efficacious as amphotericin B followed by fluconazole in the treatment of candidaemia in patients who were not neutropenic, but it is not licensed for this indication.

Administration

For invasive pulmonary aspergillosis, a loading dose of 6 mg/kg IV every 12 h for 2 doses was given followed by 4 mg/kg every 12 h and converted to oral therapy 200 mg every 12 h depending on clinical review to decide duration of treatment.

Side effects

Voriconazole has a side-effect profile similar to that of other triazoles. Two adverse effects associated with voriconazole are visual disturbance (appearance of bright lights, colour changes or wavy lines) in 45% of patients and cutaneous phototoxicity (rash) in 8% patients. Both side effects are reversible after discontinuation of therapy.

Posaconazole

Posaconazole is the latest triazole to be licensed in the UK. Like itraconazole, it is absorbed slowly, is a highly protein bound (>98%) and reaches a steady state after a period of 7–10 days. Optimal absorption is achieved when taken with a high fatty meal.

Clinical use

Posaconazole is used 200 mg three times daily for prophylaxis of invasive fungal infection during neutropenia or moderate to severe graft versus host disease (GVHD).

Caspofungin

Caspofungin was the first of the echinocandins to become available for routine use, and others, such as micafungin and anidulafungin, have only recently become available. These agents interfere with the production of the fungal cell wall by inhibiting the synthesis of an important component, 1,3-ß-d-glucan. This is a target which does not exist in mammalian cells, providing selective toxicity against fungi. Caspofungin has a significant advantage over the triazoles in that it does not inhibit the cytochrome P450 system and therefore is not associated with such a wide range of drug interactions.

Susceptible fungi

The drug has a rather unusual spectrum of activity. It is active against most species of Candida, although some are less susceptible than others, but Cryptococcus is resistant. The commonly encountered species of Aspergillus are susceptible, but the drug is inactive against the dermatophytes and activity against other fungi is variable.

Clinical use

Caspofungin is only available for administration via the intravenous route and does not penetrate into the cerebrospinal fluid. Due to its spectrum of activity, caspofungin is indicated only for candidiasis and aspergillosis. A comparative study showed caspofungin to be as efficacious as conventional amphotericin B in invasive candidiasis. It has been shown to be effective in patients with aspergillosis who failed to respond to, or who could not tolerate, other antifungal agents. Finally, caspofungin was also shown to be as effective as liposomal amphotericin B in the empirical treatment of fungal infection in neutropenic patients and had a lower incidence of unwanted effects (Walsh et al., 2004). In the UK, the drug is licensed for this indication.

Choice of treatment

A recent development has been the use of combinations of antifungal agents to improve on the results of single agents. At the present time, there is little firm evidence to support the use of such combinations. Amphotericin B plus flucytosine in the treatment of cryptococcosis is the only combination where evidence exists of increased efficacy over either agent alone. However, faced with a seriously ill patient not responding to single agents, it is not surprising that many clinicians attempt the use of a combination of antifungals, even though the evidence is that the results are no better than monotherapy.

Practice points regarding the drug toxicity in systemic antifungal agents are detailed in Table 42.9.

Table 42.9 Practice points

Drug toxicity in systemic antifungal agents
Infusion-related side effects	• Particularly with conventional amphotericin B
Infusion-related side effects	• Lipid-based preparations also show these, but to a lesser extent
Nephrotoxicity	• Particularly with conventional amphotericin B
	• Results in renal dysfunction
	• Cessation of treatment may be required
	• Drug-level monitoring is not helpful in prevention
	• Potassium loss and hypokalaemia are a serious complication
	• Renal toxicity may be potentiated by concomitant nephrotoxic agents
Hepatotoxicity	• Associated with the azole antifungals
	• Was particularly severe with ketoconazole
	• The newer triazoles may also cause serious liver damage
Bone marrow suppression	• Associated with flucytosine
	• Dose-related problem, so drug-level monitoring may help to prevent it
	• Tends to preclude the use of flucytosine in patients whose marrow is already damaged (e.g. in haematological malignancy or following bone marrow transplant)
Drug interactions	• Associated particularly with the azoles
	• Due to their mode of action in inhibiting the cytochrome P450 system
	• A wide range of drugs may be affected, some with serious interactions
Difficulties in drug administration
Drug precipitation	• Amphotericin B will precipitate out if given in electrolyte-containing infusions
Drug precipitation	• This may also happen in 5% dextrose due to acidity resulting from the manufacturing process
Need for a test dose	• Required for all amphotericin B preparations
Long infusion times and/or large infusion volumes	• A particular problem with conventional amphotericin
	• Long infusion times mean reduced access to intravenous cannulae for other purposes
	• Large infusion volumes may be undesirable in patients with renal or cardiac dysfunction
Variable absorption when taken by mouth	• A known problem with itraconazole
	• Absorption is reduced in the presence of raised gastric pH (e.g. following the use of antacids or drugs such as omeprazole)
	• Absorption is increased in the presence of food or if taken with a cola drink
	• Subtherapeutic levels may occur due to poor absorption
	• Therapeutic drug monitoring is recommended to avoid low levels
Resistance to antifungal agents
Amphotericin B	• Usually seen as a very broad-spectrum antifungal, but inherent resistance is seen in several clinically significant species:
	Aspergillus terreus
	Candida lusitaniae
	Scedosporium apiospermum
	• Acquired resistance developing during treatment is very uncommon
	• Lipid preparations have identical in vitro antifungal activity to the conventional form
Flucytosine	• Acquired resistance during treatment is very common in Candida species
	• Monotherapy promotes the rapid development of resistance
	• Combination therapy with amphotericin will reduce the possibility of acquired resistance developing during treatment
Fluconazole	• Some species of Candida are inherently resistant or less susceptible to fluconazole:
	C. krusei
	C. glabrata
	• Long-term use of fluconazole may result in increased infections with these more resistant strains
	• Long-term use may also result in reduced susceptibility in strains of C. albicans