Received: 16 May 2009

Accepted: 1^st referee: 8 June 2009

2^nd referee: 12 July 2009

 

 

Antifungal Drug Resistance in Pathogenic Fungi (Review Article)

Salah Agha, Noha El-Mashad, Douaa EL-Deeb and Mostafa Mansour

Department of Clinical Pathology, Mansoura University, Egypt

                                                                                                                                                                                                         

Abstract

Laboratory tests can help in establishing or confirming the diagnosis of a fungal infection, in providing objective assessments of response to treatment, and in monitoring resolution of the infection. Antifungal agents are chemotherapeutic agents used for controlling fungal infections. They are classified into polyenes, azoles, flucytosine, allylamines, echinocandins and griseofulvin. The standardization of antifungal susceptibility testing methods is crucial for the evaluation of antifungal drugs. Antimicrobial susceptibility testing methods for fungi have been standardized by the Clinical Laboratory Standards Institutes (CLSI). It is clear that in vitro susceptibility tests can predict clinical resistance. However, several other factors, such as the immune status of the host and severity of the infection influence the clinical outcome and should be considered. Over the next decade, antifungal resistance may become an increasingly crucial determinant of the outcome of antifungal treatment. It is important to distinguish in vitro, molecular and clinical resistance. The most striking point is that detection of resistance in vitro does not always mean clinical resistance. This follows from the multifactorial nature of the clinical response. Not only the in vitro activity of the drug but also its pharmacokinetic and pharmacodynamic properties, the immune status of the host, virulence factors of the infecting fungus and existence of indwelling catheters determine the ultimate clinical outcome.

Introduction

Over the next decade, antifungal resistance may become an increasingly crucial determinant of the outcome of antifungal treatment⁽¹⁾. It is important to distinguish in vitro, molecular and clinical resistance⁽²⁾. In vitro resistance is a minimum inhibitory concentration (MIC) value of a drug against a particular strain that is above a predefined limit for that drug. Molecular resistance, on the other hand, is demonstration of resistance at genetic level and may include detection of point mutations, gene conversions, gene amplifications leading to overexperssion, or mitotic recombinations⁽³⁾. Clinical resistance means a clinical failure in response to treatment with an antifungal agent. The most striking point is that detection of resistance in vitro does not always mean clinical resistance⁽⁴⁾. Antifungal resistance is a broad concept describing a relative insensitivity of a fungal infection to respond to antifungal therapy. Resistance has been traditionally classified as either mycological resistance or clinical resistance⁽³⁾. Mycological resistance refers to non susceptibility of a fungus to an antifungal agent by in vitro susceptibility testing, it may be primary (intrinsic, i.e. present before exposure to antifungal) or secondary (acquired, i.e. that which develops after antifungal exposure)⁽⁵⁾. Clinical resistance is defined as the failure to eradicate a fungal infection despite the administration of an antifungal agent with in vitro activity against the organism. Such failures can be attributed to a combination of factors related to the host, the antifungal agent, or the pathogen⁽⁶⁾.

Fungal infections

Fungal infections are classified by Hay⁽⁷⁾ into:

a)   Superficial mycoses: They include dermatophytosis, superficial candidosis andMalassezia infection,

b)  Subcutaneous mycoses: They include sporotrichosis, mycetoma, chromoblastomycosis, subcutaneous zygomycosis and lobomycosis,

c)   Systemic mycoses: They include opportunistic & systemic candidosis, aspergillosis, zygomycosis and cryptococcosis.

d)  Endemic: They include African histoplasmosis blastomycosis, coccidioidomycosis, paracoccidioidomycosis and infection due to Penicillium marneffei.

Antifungal drugs

Antifungal agents are chemotherapeutic agents used for controlling fungal infections. They are classified into polyenes, azoles, flucytosine, allylamines, echinocandins and griseofulvin⁽⁸⁾.

1) Polyenes

Amphotericin B (AmB) is a polyene antifungal produced by the soil actinomycete Streptomyces nodosus⁽⁹⁾. Amphotericin B remains the most effective, broad-spectrum, fungicidal agent with the greatest experience for the treatment of systemic mycoses. Both intrinsic and acquired resistances are limited. In an effort to enhance antifungal efficacy and reduce toxicity, amphotericin B has been combined with other agents with promising results. The primary antifungal activity of amphotericin B is mediated by its preferential binding to ergosterol in the fungal cell membrane. Amphotericin B is active against most of the common yeasts, moulds, and dimorphic fungi causing human infection including: Candida species, Cryptococcus neoformans, Blastomyces dermatitidis, Histoplasma capsulatum, Coccidioides immitis, Paracoccidioides brasiliensis, Sporothrix schenckii, Aspergillus species, and the agents of mucormycosis. The adverse effects include nephrotoxicity, fever, chills, phlebitis, anaemia, gastrointestinal tract disturbance, hypoxia, hypotension, hypertension, dyspnea and vomiting⁽¹⁰⁾.

2) Azoles

The introduction of the azole class of antifungal drugs with the licensing of miconazole in 1979 was the beginning of a new era in therapy for systemic fungal diseases. The intravenous miconazole is associated with significant toxicity. It is no longer commercially available. Three oral azole drugs, ketoconazole, fluconazole and itraconazole have been developed to overcome this toxicity. For many systemic mycoses, these drugs have been effective and safe alternatives to Am B. A new generation of triazole agents (voriconazole, posaconazole, ravuconazole and albaconazole) represents an alternative to conventional antifungals for the management of serious fungal infection management⁽¹¹⁾.

The primary antifungal effect of the azoles occurs via inhibition of a fungal cytochrome P-450 enzyme involved in the synthesis of ergosterol, the major sterol in the fungal cell membrane. On a molecular level, binding of the free azole nitrogen with the heme moiety of fungal C-14_ demethylase inhibits demethylation of lanosterol, thereby depriving the cell of ergosterol. The net result is a disruption of normal structure and function of the cell membrane and ultimately, the inhibition of cell growth and morphogenesis⁽¹²⁾

 

3) Flucytosine

Flucytosine (5-fluorocytosine; 5-flucytosine; 5-FC) is one of the oldest antifungal agents. Human clinical trials were initiated in the late 1960s for both cryptococcal meningitis and disseminated candidiasis. The rapid emergence of flucytosine resistance was observed, particularly among C. Neoformans isolates, limiting its utility as single agent therapy⁽¹³⁾.

4) Allylamines

Allylamines were discovered in the 1970s and were totally synthetic. Terbinafine is an oral and topical antifungal agent in the allylamine class of antifungal compounds. It is a reversible, noncompetitive inhibitor of squalene epoxidase an enzyme which, together with (2,3)-oxidosqualene cyclase, is rcsponsible for the conversion of squalene to lanosterol⁽¹⁴⁾. The resulting ergosterol depletion and squalene accumulation affect membrane structure and function⁽⁸⁾.

5) Echinocandins

Echinocandins are the newest class of antifungal agents⁽¹⁵⁾.The echinocandins act by noncompetitive inhibition of the synthesis of 1,3-beta-glucan, a polysaccharide in the cell wall of many pathogenic fungi⁽¹⁶⁾.

6) Griseofulvin

Griseofulvin has been used as a first-line drug for treatment of dermatophytosis for many years. However, following the emergence of more effective and less toxic alternatives, itraconazole and terbinafine, the clinical use of griseofulvin is now limited. Griseofulvin exerts antifungal activity via inhibition of fungal mitosis by disrupting the mitotic spindle through interaction with polymerized microtubules⁽³⁾.

Role of Antifungal susceptibility testing in resistance:

The standardization of antifungal susceptibility testing methods is crucial for the evaluation and development of antifungal drugs. Although these methods have been standardized for a long time for bacteria, they only recently have been adopted for fungal pathogens. Although a long-approved testing method has been implemented for bacterial isolates the same level does not yet exist for fungi. Antimicrobial susceptibility testing methods for fungi have been standardized by the Clinical Laboratory Standards Institutes (CLSI). The antifungal susceptibility testing methods include broth dilution, colorimetric tests, spectrophotometric methods, disk diffusion, flow cytometry and fungicidal testing⁽¹⁷⁾.

A meta-analysis of the available in vitro-in vivo correlation studies found that the percentage of clinical success was 91% for infections due to isolates susceptible to the corresponding antifungal agent while it was 48% for those where the isolate were resistant⁽⁴⁾. These success rates were shown to be similar to the success rates for therapy of susceptible resistant bacterial isolates, and the general concept of a 9060 rule was proposed in which susceptible isolates responded about 90% of the time and the resistant isolates responded about 60% of the time. This analysis makes it clear that in vitro susceptibility tests can predict clinical resistance. However, several other factors, such as the immune status of the host and severity of the infection influence the clinical outcome and should be considered⁽³⁾.

Factors contributing to antifungal resistance include

a)   Fungal factors such as virulence, size of fungal population.

b)  Drug causes related to antifungal resistance include its pharmacokinetic, dose, nature and drug interactions⁽¹⁸⁾.

c)   Host factors play an important role in the outcome of fungal infection. The main host factors are immune status and presence of foreign bodies and hematological malignancy⁽¹⁹⁾.

Cellular factors associated with resistant
fungi:

A common cause of refractory disease is infection with a fungal strain of which the drug MIC is higher than average. Several factors can lead to the presence of a resistant strain in a patient: intrinsic resistance of endogenous strains, replacement with a more resistant species (C. krusei, C. glabrata), replacement with a more resistant strain of C. albicans, genetic alterations that render an endogenous strain resistant, transient gene expression that renders an endogenous strain temporarily resistant, alteration in cell type (yeast/ hypha, switch phenotype), and size and variability of the population⁽²⁾.

To summarize, these mechanisms can be grouped into following categories:

·   Decreased drug import or increased drug export (efflux pumps).

·   Alteration in drug target binding site.

·   Changes in biosynthetic pathways (particularly sterol synthesis) that circumvent or attenuate the effects of antifungal inhibition.

·   Alterations in intracellular drug processing.

·   Up-regulation of homeostatic stress-response pathways to deal with antifungal associated damage⁽²⁰⁾.

Resistance to polyenes

Reports of Amphotericin B resistance are limited. Primary resistance to Am B exists in some isolates of Candida lusitaniae, C. lipolytica and C. guillermondii Trichosporon beigelii, often show intrinsic resistance to AmB. Until now, invasive infections caused by these fungi have been relatively infrequent. Secondary resistance has occasionally been described in Candida spp. as well as in C. neoformans⁽⁵⁾ .

Mechanisms of resistance to polyenes include alterations in membrane sterols, defense mechanisms against oxidative damage, defects in ergosterol biosynthetic genes and alterations in sterol to phospholipids ratio⁽²¹⁾.

Cell membrane damage is due to the formation of reactive oxygen species (ROS), such as superoxide, hydrogen peroxide, and hydroxyl radicals, that result in membrane disruption and cell death through membrane lipid peroxidation. Defense against oxidative damage is involved in the resistance to amphotericin B. In the absence of oxygen, Am B may act as an antioxidant, and therefore as a chain terminator of the peroxidation process, and it may partially protect the fungus against phagocytosis⁽²²⁾.

Young et al.⁽²³⁾ investigated genetic alterations in the ergosterol biosynthetic pathway of C. lusitaniae. An ERG6 mutant strain of C. lusitaniae was designed to investigate Am B resistance in this species. Amphotericin B-resistant isolates of C. lusitaniae were found to have increased levels of ERG6 transcript, as well as reduced ergosterol content.

Clinical isolates and mutant strains of C. albicans cross-resistant to azoles and polyene have been shown to accumulate sterol intermediates in the cytoplasmic membrane due to a decrease in 5,6 desaturase activity. The altered membrane sterols pattern may provide a common basis for the dual resistance, by preventing polyene binding and by reducing azole inhibition of ergosterol synthesis. Alteration of sterol content and/or composition is not sufficient to explain polyene resistance. Previous work has shown that the type of sterols and phospholipids in cellular membranes were important in polyene resistance, but did not adequately explain resistance⁽²²⁾.

Some polyene-resistant mutants of C. albicans have been shown to have altered fatty acid compositions. The phospholipid composition of sensitive and mutant strains of C. albicans was measured, and noted a slightly higher proportion of saturated fatty acids in the resistant mutants, compared with the sensitive strains. The proportion of long chain fatty acids was similar⁽²²⁾.

Resistance to Azoles

Azole resistance is frequently described in patients with AIDS and mucosal candidiasis and less frequently in invasive infections. Resistance to azole treatment can be stable or transient⁽²⁴⁾.

In addition, there is a growing awareness of the changing epidemiology of fungal infections, with a shift toward species that are intrinsically resistant to the most commonly used antifungal agents (fluconazole)⁽²⁵⁾.

Primary resistance occurs in organisms never exposed to a given drug in that host. This intrinsic resistance is displayed by all, or almost all, isolates of single species to a certain drug, and it could predict clinical failure. The common examples are the resistance of C. krusei and A. fumigatus to fluconazole. In contrast, secondary resistance (also defined as acquired resistance) develops only after exposure of the organism to the drug. An example of secondary resistance is the development of fluconazole resistance in C. albicans strains isolated longitudinally from HIV-infected patients with oropharyngeal candidiasis under long-term treatment with this drug⁽²⁶⁾.

The mechanism of action of azole derivatives is through binding to and inhibiting lanosterol demethylase (Cyp51p or Erg11p), a cytochrome P450 responsible for the 14-α demethylation of lanosterol, thus blocking ergosterol biosynthesis (the major membrane sterol of fungi) and leading to a fungistatic effect in the majority of cases⁽¹²⁾ .

In most instances, resistance to azoles is a multifactorial process involving several mechanisms. Cross-resistance within the azole class of antifungal agents is common, and is becoming an important issue⁽²⁷⁾.

At the molecular level, different mechanisms contribute to resistance against azole agents⁽²¹⁾. These mechanisms include modification of the antifungal target (in the case of azoles lanosteroldemethylase, the product of the ERG11 gene), decreased drug accumulation inside the fungal cells due to the overexpression of multidrug drug efflux pumps, and other alterations in sterol biosynthesis. Deficiency in the uptake of some azole derivatives could also contribute to resistance^(21,26).

1) Alterations in the Target Enzyme

Alterations in the target enzyme (lanosterol 14-α-demethylase), including point mutations and overexpression, of the gene coding for the enzyme lead to decreased susceptibilities to azole drugs, which may also lead to cross-resistance to other azole derivatives. Pathogenic fungi can overcome the inhibition of azoles by increasing the content of the target enzyme molecules, either by gene amplification⁽²⁶⁾. Or by increased expression of the ERG11 gene encoding the drug target enzyme⁽²⁸⁾.

This results in the need for higher intracellular azole concentration to complex all the enzyme molecules present in the cells. However, this mechanism seems to have a limited impact in resistance to azoles, and does not seem to confer high levels of resistance⁽²⁹⁾. Point mutations in the gene encoding the target enzyme for azoles (ERG11) result in amino acid substitutions leading to decreased affinity for azole derivatives⁽³⁰⁾.

Importantly, some of these mutations have been repeatedly identified and may represent “hot spots” for the development of azole resistance. These regions correspond to important functional domains of the enzyme in its interaction with the heme moiety at its active site, and at another region believed to play a role in the entry of the substrate in the substrate pocket⁽³¹⁾.

Interestingly, some of the new-generation azoles, due to differences in the way they interact with Erg11, may be more insensitive to alterations in the target enzyme. A recent report indicated that posaconazole was active against C. albicans isolates that have mutations in their ERG11 gene causing resistance to other azole derivatives⁽³⁰⁾.

2) Increased Drug Efflux

A second major mechanism leading to azole resistance is by prevention of accumulation of sufficient effective concentrations of the azole antifungal agent in the fungal cells as a consequence of enhanced drug efflux. This mechanism is mediated by two types of multidrug efflux transporters, the Major Facilitators (encoded by MDR genes in C. albicans) and those belonging to the ATP-binding cassette superfamily (ABC transporters, encoded by CDR genes in C. albicans)^(29,32).

ABC transporters, which have been associated with drug resistance in a variety of eukaryotic cells, include a membrane pore composed of transmembrane segments and two ATP-binding cassettes on the cytosolic side of the membrane, which provide the energy source for the pump⁽²⁶⁾.

Upregulation of the CDR genes appears to confer resistance to multiple azoles in C. albicans, whereas upregulation of the MDR1 gene alone leads to fluconazole resistance exclusively⁽³³⁾.

It is not clear how multidrug transporter genes are regulated in pathogenic fungi, including C. albicans, although it is believed that gene up regulation might be caused by alterations in transcription (involving transcription factors)⁽³⁴⁾ .

3) Mutations in other genes in the ergosterol
biosynthetic pathway

Altered sterol Δ(5,6) desaturase is also linked to azole resistance in C. albicans clinical isolates. In azole-sensitive strains treated with azoles, 14-methyl-3,6-diol accumulates and leads to a fungistatic effect, whereas in sterol Δ(5,6) desaturase mutants (due to mutations in the gene ERG3), 14-methylfecosterol, accumulates, which can support growth of the fungal cell. Interestingly, a consequence of this mechanism is that it causes cross-resistance to amphotericin B, due to the fact that ergosterol is absent from cell membranes^(32,35).

4) Combinations of molecular mechanisms of azole resistance

The multiplicity of resistance mechanisms to azole antifungals represents a set of biological tools that enables fungal cells to develop resistance by different combinations. However, the prevalence and relative frequency of resistance mechanisms in a large population of azole-resistant isolates has been investigated in only a few studies. In the study by Perea et al., most of the resistant isolates presented a combination of resistance mechanisms, such as upregulation of efflux transporters (encoded by CDR and MDR genes) and point mutations in the ERG11 alleles^(21,33).

5)Biofilm Resistance

Several studies have demonstrated that the Candida biofilm lifestyle leads to dramatically increased levels of resistance to the most commonly used antifungal agents, particularly azoles⁽²⁶⁾.

Biofilm resistance is a complex multi-factorial phenomenon, which still remains to be fully elucidated and understood. Different mechanisms may be responsible for the intrinsic resistance of Candida biofilms. These include: (a) high density of cells within the biofilm; (b) effects of the biofilm matrix; (c) decreased growth rate and nutrient limitation; (d) expression of resistance genes, particularly those encoding efflux pumps; (e) altered membrane sterols; and (f) presence of “persister” cells⁽³⁶⁾.

Thus, this may be one of the main reasons for the lack of correlation between results of antifungal susceptibility testing, as determined by NCCLS guidelines, and clinical outcome in patients suffering from these types of infections⁽³⁷⁾.

6) Mechanisms of antifungal tolerance

Some important antifungal agents in use (fluconazole, itraconazole) have a fungistatic activity against C. albicans, while other compounds have strong (e.g. amphotericin B) or moderate (e.g. caspofungin) fungicidal activities in C. albicans. The fungistatic nature of azoles limits the efficacy of these substances especially in patients with immunosuppression, since the immune system participates actively with azoles in the elimination of C. albicans from infected sites. Screening of different types of drugs encountered in medical practice able to potentiate the activity of azoles was undertaken. Among these different drugs, cyclosporin A was found to act in synergism with fluconazole and converted in its vitro fungistatic antifungal effect into a potent fungicidal drug⁽³⁸⁾.

Resistance to 5 fluorocytosine

5-FC monotherapy is effective against infections caused by C.neoformans, Candida species including C. glabrata and in chromoblastomycosis and phaeohyphomycosis. However, its use as a single agent is limited due to its high tendency for rapid development of resistance⁽³⁹⁾.

1) Host factors of resistance

A number of factors, both drug and host-related, can contribute to 5-FC resistance in candidal and cryptococcal infections. One such factor is impaired drug absorption/penetration, which can be modulated by the route and vehicle of drug administration. Oral administration of 5-FC can lead to impaired absorption and inadequate serum/tissue drug concentration due to the unique oral environment, where the flushing effect of saliva tend to reduce the drug concentration to sub-therapeutic levels causing treatment failure⁽⁴⁰⁾.

The host factor that can affect drug resistance is the rate of drug metabolism – both for singly used drug and drug combinations. In this regard, properties of the second drug used in combination with 5-FC can also influence its metabolism⁽⁴¹⁾.

Other host factor influencing drug activity is the immune status of the infected host. A C. neoformansstrain from cutaneous lesions of a patient with thrombotic thrombocytopenia purpura was tested and was found to be resistant to 5-FC⁽⁴²⁾.

2) Drug factors of resistance

Some clinical trials have suggested that low concentrations of 5-FC used for therapy are associated with risk of treatment failure. Maximal absorption of certain drugs is achieved by administration of the drug in a particular solution and if this is not done then it can cause low absorption and treatment failure⁽⁴³⁾.

Antagonism is another factor for 5-FC treatment failure when it is used with AmB or 5-FC during combination therapy. Numerous studies conducted have reported antagonism between drugs, in particular, in the case of the combination of AmB and FLU and the combination of 5-FC and azoles⁽⁴⁴⁾.

3) Cellular factors of resistance

Two mechanisms of 5-FC resistance can be distinguished:

a)   Decreased cellular transport or uptake of 5-FC due to the loss of enzymatic activity (loss of permease activity) responsible for conversion to 5-fluoridine monophosphate (FUMP). The resistance due to decreased uptake is found in S. cerevisiae and C. glabrata; this mechanism does not seem to be important in C. albicans or Cryptococcus neoformans.

b)  Resistance of 5-FC may also result from increased synthesis of pyrimidines, which compete with the fluorinated antimetabolites of 5-FC and thus decrease its anti-mycotic activity⁽⁴¹⁾.

Resistance to echinocandins

Acquired resistance to echinocandins in susceptible fungal yeast species has been extremely rare to date. The majority of mutations conferring resistance have been associated with the FKS genes mutation⁽¹⁶⁾.

Several laboratory and clinical C. albicans strains and S. cerevisiae strains were examined for the role of FKS1, whose gene product is a component of the 1,3-β-D-glucan synthase complex, in reduced susceptibility to caspofungin⁽¹⁵⁾.

A corresponding amino acid change was identified in four laboratory strains of C. albicans. In addition, a copy of the mutated FKS1 allele was integrated into a C. albicans laboratory strain, which resulted in reduced susceptibility. The clinical C. albicans with reduced susceptibility was found to have amino acid changes at the same residue as in the laboratory strains⁽³⁹⁾.

Resistance to allylamine

Although resistance to terbinafine appears to be rare in clinical yeast isolates, it has been shown that some azole resistant strains which over-express either CDRl or MDRl are cross-resistant to terbinafine. Terbinafine resistance among the dermatophytes is rare with no reports of resistance prior to 2003. Investigators reported clinical as well as microbial resistance in six Trichophytonrubrum isolates obtained sequentially from a single onychomycosis patient who failed oral terbinafine therapy⁽⁴⁵⁾.

Molecular characterization of the isolates demonstrated that resistance to terbinafine was due to alterations in the squalene epoxidase gene or a factor essential for its activity⁽⁴⁶⁾. Overexpression of the target enzyme might also play a role in terbinafine resistance⁽⁴⁷⁾.

Strategies to overcome antifungal resistance

1) Pharmacological

Several strategies have been developed as a means to overcome antifungal resistance such as:

a) Dose intensity

Dose intensity has not been studied in a controlled fashion in organized clinical trials. However, preclinical evidence indicates that the efficacy of some antifungal drugs e.g. amphotericin B can be improved with increasing doses over the range of clinically achieved concentrations⁽⁴⁸⁾.

The lipid formulations of amphotericin B have an improved therapeutic index and allow the delivery of higher daily doses than conventional Am B⁽⁴⁹⁾.

b) Combination therapy

Because of the refractory nature of many fungal infections, combination therapy is increasingly proposed as means to enhance antifungal efficacy, decrease resistance, and potentially reduce toxic effects. With the possible exception of Cryptococcal meningitis, however, no clinical studies have supported the benefits of combination antifungal therapy over monotherapy for refractory fungal infections⁽⁵⁰⁾.

Few data show that combination therapy can slow the development of secondary resistance. Of the few published studies most have shown that combinations of amphotericin B and flucytosine can decrease flucytosine resistance. The use of antifungal combinations could overcome or potentially decrease the probability of resistance⁽⁴⁴⁾.

Combination antifungal therapy, however, could be a useful approach to improving the spectrum of empirical antifungal therapy, particularly in institutions where uncommon but emerging fungal pathogens such as Fusarium spp, Trichosporon beigelli, non-fumigatus Aspergillus spp, or other resistant molds are encountered⁽⁵⁰⁾.

c) Immunomodulators

Quantitative defects in host immunity alone (i.e. neutropenia) are not the sole reasons behind clinical failure associated with antifungal therapy. Subtle qualitative deficiencies in cell function (circulation life, antibody-mediated cytotoxicity, oxidative metabolism) and dysregulation of the adaptive host immune response to fungal infection could be improved with the administration of cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon gamma⁽⁵¹⁾.

Despite some interesting preclinical data, the clinical usefulness of cytokines as means to overcome primary, secondary, or clinical antifungal resistance are to be defined⁽⁵⁰⁾.

d) New antifungals

Different types of mycoses, especially invasive mycoses caused by yeasts and molds, are a growing problem in health care. New generations of triazole agents and echinocandins agents have become available, and represent an alternative to conventional antifungal drugs for serious fungal infection management⁽¹¹⁾.

The echinocandins represent the newest class of antifungals⁽¹⁵⁾. Caspofungin was released in 2001. This was followed by micafungin and anidulafungin⁽⁵²⁾.

2) Non-pharmacological

Early diagnosis is critical to a good outcome in invasive fungal infection⁽⁵³⁾. New diagnostic approaches are being developed and are being re- evaluated in the light of effective prophylaxis⁽⁵⁴⁾. The use of CT scanning of the lung now has an established place in the early diagnosis of invasive aspergillosis. New diagnostic tests are required, real-time PCR is now entering the clinical era and the results of PCR on samples in immunocompromised patients are excellent⁽⁵⁵⁾.

 

Strategies to reduce the risk of invasive fungal infections (IFIs) include: isolation, special diets and gut sterilization⁽⁵⁶⁾. Other preventive strategies include tapering corticosteroids, which are risk factors for developing IFIs, use of post-transplant GM-CSF to accelerate neutrophil recovery and prophylactic use of fluconazole to prevent Candida albicans invasion⁽⁵⁷⁾.

Conclusion

Over the past decade there has been a considerable increase in the knowledge of the mechanisms of anti fungal resistance. Antifungal resistance is a complex, gradual and multifactorial issue. Antifungal drug resistance is a prominent feature in the management of invasive mycoses. Fungi will continue to develop new resistance mechanisms to the available antifungal drugs.

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