Antifungal prophylaxis: to prevent or not

Identification and characterization of a Cryptococcus neoformans ATP binding cassette (ABC) transporter-encoding gene, CnAFR1, involved in the resistance to fluconazole

Resistance to fluconazole is a possible event during prolonged suppressive drug therapy for cryptococ-cal meningitis, the most frequently encountered life-threatening manifestation of cryptococcosis. The knowledge of this resistance at the molecular level is important for management of cryptococcosis. In order to identify genes involved in azole resistance in Cryptococcus neoformans, a cDNA subtraction library technique was chosen as a strategy. First, a fluconazole-resistant mutant BPY22.17 was obtained from a susceptible clinical isolate BPY22 by in vitro exposure to the drug. Then, a subtractive hybridization procedure was used to compare gene expression between the obtained strains. We identified a cDNA overexpressed in the fluconazole-resistant strain BPY22.17 that was used as a probe to isolate the entire gene in a C. neoformans genomic library. Sequence analysis of this gene identified an ATP Binding Cassette (ABC) transporter-encoding gene called C. neoformans AntiFungal Resistance 1 (CnAFR1). Disruption of CnAFR1 gene in the resistant isolate (BPY22.17) resulted in an enhanced susceptibility of the knock-out mutant cnafr1 against fluconazole, whereas reintroduction of the gene in cnafr1 resulted in restoration of the resistance phenotype, thus confirming that CnAFR1 is involved in fluconazole resistance of C. neoformans. Our findings therefore reveal that an active drug efflux mechanism can be involved in the development of azole resistance in this important human pathogen.

Fungal colonisation and fluconazole therapy in acute liver disease

BACKGROUND/AIMS

Fungal infection, particularly with Candida spp., has been identified as an important cause of morbidity and mortality in patients with acute liver failure. Fungal colonisation of superficial mucosal sites usually precedes invasive infection. We investigated colonisation patterns in patients with acute liver disease receiving fluconazole therapy in order to investigate the possibility of emergence of fluconazole-resistant C. albicans or other species.

METHODS

During a 6-month study period, we studied all patients referred to our unit with acute liver disease by twice-weekly sampling and mycological analysis of specimens from superficial mucosal and other sites as appropriate. Patients were treated with prophylactic antimicrobials including 100 mg fluconazole daily in accordance with our usual protocol.

RESULTS

Twenty-two patients with acute liver disease were studied, eight of whom underwent transplantation. Eighteen patients were colonised by fungi at presentation, and six developed secondary colonisation during fluconazole therapy. Four of these patients (all transplanted) became colonised by resistant species; one of these was Aspergillus fumigatus, which led to death. There were no other invasive fungal infections identified during the study period, and no fluconazole-resistant C. albicans were identified.

CONCLUSIONS

Resistance to fluconazole is unlikely to develop in C. albicans during short-term fluconazole prophylaxis in acute liver disease, and in this study we did not find evidence of invasive disease from other Candida spp. during fluconazole therapy. However, in patients at particularly high risk, other strategies are required to prevent infection with Aspergillus spp.

The compromised traveler

Compromised travelers represent a diverse and challenging group of individuals. They include HIV-infected patients who are at risk for potentially adverse reactions to immunizations, and new exposures to enteric water-borne opportunistic pathogens associated with chronic infections. Such travelers may encounter unfamiliar opportunistic fungi and classical tropical infections, such as leishmaniasis, whose pathogenesis can be enhanced by the presence of prior HIV infection. Other immunocompromised groups include those who are functionally or anatomically asplenic, and patients who are iatrogenically immunosuppressed from medications utilized for solid organ transplantation, chemotherapy, or treatment of malignancies. This population of travelers also includes those with diabetes mellitus who may require adjustments in their dosing, administration, and possibly even the types of insulin used on their trips. These patients are also at greater risk for acquisition of tuberculosis, severe community-acquired pneumonia, urinary tract infections, and pyomyositis. Older travelers present both the infectious disease and travel medicine specialist with issues such events, malignancy-related infections, myocardial infarction, and other forms of cardiopulmonary compromise, which the authors address in this article.

Antifungal resistance trends towards the year 2000. Implications for therapy and new approaches

Medical advances have led to increased numbers of immunocompromised patients living longer. Coinciding with this increase in the immunocompromised patient population is an increase in the number of clinically significant fungal infections. Unfortunately, widespread use of the limited numbers of antifungal agents to treat these infections has led to the development of drug resistance. Thus, in an attempt to sort out the mechanisms of resistance for each of the systemically useful antifungal agents, a comprehensive review of the literature has been carried out. The most common mechanisms for the development of resistance involve changes in the enzymatic pathways which serve as the drug targets. For instance, changes in enzymes responsible for the biosynthesis of ergosterol, the target of azole activity, lead to azole resistance. Another common mechanism used by fungi to avoid drug toxicity includes reduced intracellular accumulation of the drug through both decreased permeability and energy-dependent efflux pumps. Using our current understanding of the mechanisms of drug resistance as a template, several strategies to overcome resistance have been identified. These include improvement of host immune function, the use of adjuvant surgery, the development of new drug delivery systems for currently available drugs and the development of new classes of antifungal agents. Also, clinical trials to establish appropriate drug doses and duration of therapy are needed, as well as the benefits of antifungal prophylaxis explored and the use of combination therapies entertained. The war against drug resistant fungi has been identified as we approach the year 2000. With careful and cogent investigations, we do have the tools to fight back against these opportunists. Of all the strategies reviewed, however, in our opinion, the development of new antifungal drugs is likely to have the most significant future impact on our management of drug resistance in fungal infections.

Emerging disease issues and fungal pathogens associated with HIV infection

Fungal diseases are increasing among patients infected with human immunodeficiency virus (HIV) type 1. Infections due to Candida and Cryptococcus are the most common. Although mucocutaneous candidiasis can be treated with oral antifungal agents, increasing evidence suggests that prolonged use of these drugs results in both clinical and microbiologic resistance. The optimal therapy for cryptococcal meningitis remains unresolved, although initial treatment with amphotericin B, followed by life-long maintenance therapy with fluconazole, appears promising. Most cases of histoplasmosis, coccidioidomycosis, and blastomycosis occur in regions where their causative organisms are endemic, and increasing data suggest that a significant proportion of disease is due to recent infection. Aspergillosis is increasing dramatically as an opportunistic infection in HIV-infected patients, in part because of the increased incidence of neutropenia and corticosteroid use in these patients. Infection due to Penicillium marneffei is a rapidly growing problem among HIV-infected patients living in Southeast Asia. Although the advent of oral azole antifungal drugs has made primary prophylaxis against fungal diseases in HIV-infected patients feasible, many questions remain to be answered before the preventive use of antifungal drugs can be advocated.

Targeting fungi: a challenge

Invasive fungal infections are more commonly identified in various categories of patients, mainly in cancer patients but also in those undergoing organ transplantation, patients in intensive care units, and those with AIDS. There is a great need to increase the awareness of practitioners who are still underestimating the morbidity and mortality relating to invasive fungal infections, and to stress the economic burden for the society and healthcare systems of invasive fungal infections. The list of fungal pathogens causing life-threatening complications has also increased recently, with the emergence of unusual fungi being more frequently identified in such settings. Early diagnosis of invasive fungal infections is still a major challenge for the clinician at the bedside. Identification of state-of-the-art management is also a difficult task for the clinical scientist involved in the assessment of optimal strategies to prevent and to treat those invasive fungal infections, although major progress has occurred in the last 5 years with the development of new, safe, and effective antifungal agents. Empiric therapy remains a very controversial issue that should be further investigated in high-quality clinical trials. Overall, clinical research in this difficult field requires independent and objective analysis; only large multicenter clinical trials can address these critical issues and rapidly provide convincing results leading to a better prognosis of patients with invasive fungal infections. These complications still represent too often an obstacle to successful control of severe underlying diseases. Clinical research on the appropriate ways to target fungi will not only define state-of-the-art management but also identify ineffective or redundant treatments. Such an approach will make a substantial contribution to the care of the high-risk patients within the next decade and will preserve our capacity for medical excellence.

Prevention and therapy of fungal infections in cancer patients. A review of recently published information

This review of recent publications in the field of fungal infections in cancer patients clearly confirms that protracted severe granulocytopenia is a major risk factor for their development. Because severe and prolonged granulocytopenia plays such a major predisposing role for fungal infections, it is likely that the use of the colony-stimulating factors, which are able to reduce the duration and the severity of granulocytopenia, might prove effective in decreasing the frequency and the severity of these infections. Another conclusion is that certain categories of patients with granulocytopenia might benefit from antifungal prophylaxis and empiric therapy. Conversely, there are other populations who will benefit only marginally from such strategies. Imidazoles, namely fluconazole, for the prevention of local and systemic Candida infections have been shown to be effective in granulocytopenic patients. So far, the development of resistance has not been a major problem. In patients at the greatest risk of developing severe fungal infections, such as those receiving high-dose corticosteroid therapy for GVHD after allogeneic bone marrow transplantation, early administration of low doses of amphotericin B seems to be effective in reducing the development of systemic fungal infection. In terms of therapy, amphotericin B is still the standard approach, especially for empiric treatment, prior to the recognition of a specific pathogen.

Fluconazole for primary prophylaxis of AIDS-associated cryptococcosis: a case-control study

In order to verify whether fluconazole has a prophylactive effect against the occurrence of cryptococcosis in HIV-infected patients and to identify other factors capable of increasing or reducing the risk of this infection, we arranged a case-control study of 17 patients with cryptococcal infection. 34 controls were selected, matched by presence of an AIDS-defining event, CD4 cell count, and date of T-cell phenotyping. No significant difference in exposure to fluconazole, in total days of treatment, or in total dose administered was observed between cases and controls. However, control patients took a significantly higher average daily dosage of fluconazole and a linear tendency in risk reduction (p = 0.04) in relation to increasing dosage was observed. Antiretroviral therapy and an average daily fluconazole dose exceeding 150 mg both each reduced the risk of a cryptococcal infection.

Chemoprophylaxis of fungal infections in neutropenic patients

BACKGROUND

In recent years there has been an increasing incidence and awareness of mucosal candidiasis and invasive fungal infections in neutropenic patients. Early diagnosis (before serious morbidity and mortality) is often difficult, emphasizing the continuing need for adequate prophylaxis.

DESIGN

A review of studies on the chemoprophylaxis of fungal infections in neutropenic patients.

RESULTS

There is a limited number of large, prospective, well-designed studies using proper criteria and end points. No antifungal drug or drug combination has been shown to prevent invasive fungal infection with the exception of fluconazole (mainly Candida Albicans infections) in certain high-risk patients groups.

CONCLUSIONS

Prophylaxis strategies are dependent upon local conditions, patient populations, types of therapies, available resources, etc. Future improvement will be based upon a) improved study quality, b) new strategies for established drugs, c) development of new and safer drugs, d) new strategies to eliminate or reduce immunosuppression and e) cost-benefit studies.