Mechanisms and clinical impact of antifungal drug resistance

Current and future approaches to antimycotic treatment in the era of resistant fungi and immunocompromised hosts

Due to the ever-increasing number of immunocompromised patients, both localised and life-threatening systemic fungal infections are on the increase. Conventional treatment is of limited help, not in the least due to a less optimum benefit-to-risk ratio. Moreover, emerging pathogens with reduced antimicrobial susceptibility and the development of resistance in Candida albicans form a new challenge. Fortunately, conventional antimycotics have been improved and entirely new ones are on the horizon as well as alternative approaches such as immunoreconstitution.

A novel multidrug efflux transporter gene of the major facilitator superfamily from Candida albicans (FLU1) conferring resistance to fluconazole

Azole resistance in Candida albicans can be mediated by several resistance mechanisms. Among these, alterations of the azole target enzyme and the overexpression of multidrug efflux transporter genes are the most frequent. To identify additional putative azole resistance genes in C. albicans, a genomic library from this organism was screened for complementation of fluconazole hypersusceptibility in Saccharomyces cerevisiae YKKB-13 lacking the ABC (ATP-binding cassette) transporter gene PDR5. Among the C. albicans genes obtained, a new gene was isolated and named FLU1 (fluconazole resistance). The deduced amino acid sequence of FLU1 showed similarity to CaMDR1 (formerly BEN(r)), a member of the major facilitator superfamily of multidrug efflux transporters. The expression of FLU1 in YKKB-13 mediated not only resistance to fluconazole but also to cycloheximide among the different drugs tested. The disruption of FLU1 in C. albicans had only a slight effect on fluconazole susceptibility; however, it resulted in hypersusceptibility to mycophenolic acid, thus suggesting that this compound could be a substrate for the protein encoded by FLU1. Disruption of FLU1 in a background of C. albicans mutants with deletions in several multidrug efflux transporter genes, including CDR1, CDR2 and CaMDR1, resulted in enhanced susceptibility to several azole derivatives. FLU1 expression did not vary significantly between several pairs of azole-susceptible and azole-resistant C. albicans clinical isolates. Therefore, FLU1 seems not to be required for the development of azole resistance in clinical isolates.

Depletion of the squalene synthase (ERG9) gene does not impair growth of Candida glabrata in mice

Squalene synthase (farnesyl-diphosphate farnesyltransferase, EC 2.5. 1.21) is the first committed enzyme of the sterol biosynthesis pathway. Inhibitors of this enzyme have been intensively studied as potential antifungal agents. To assess the effect of deactivating squalene synthase on the growth of fungi in mice, we isolated the squalene synthase (ERG9) gene from the pathogenic fungus Candida glabrata and generated strains in which the CgERG9 gene was under the control of the tetracycline-regulatable promoter. Depletion of the ERG9 gene by doxycycline (DOX), a derivative of tetracycline, decreased the cell viability in laboratory media, whereas it did not affect cell growth in mice at all. The growth defect caused by DOX in laboratory media was suppressed by the addition of serum. Analyses of the sterol composition of the restored cells in serum-containing media suggest that the defect of ergosterol biosynthesis can be complemented by the incorporation of exogenous cholesterol into the cells. Thus, deactivation of squalene synthase did not affect fungal growth in mice, presumably because the cells were able to incorporate cholesterol from the serum. These results showed that squalene synthase could not be a suitable target of antifungals for the treatment of C. glabrata infection.

In vitro susceptibility of Candida albicans isolates from apical and marginal periodontitis to common antifungal agents

The susceptibility of a total of 70 Candida albicans strains to five common antifungal agents was determined. Thirty-five of the strains were isolated from persistent cases of apical periodontitis and 35 from cases of marginal periodontitis. The susceptibility of the strains to amphotericin B, 5-fluorocytosine and three azoles: fluconazole, miconazole and clotrimazole, was tested. The antifungal agents and yeast inoculums were prepared according to the NCCLS (National Committee for Clinical Laboratory Standards) recommendations. The yeasts were incubated with ten different concentrations of antifungal agents at 35 degrees C for 48 h. Yeast growth was measured spectrophotometrically. All strains from both isolation sources were susceptible to low concentrations of amphotericin B and 5-fluorocytosine, whereas the susceptibility to the three azoles varied, and three of the strains showed azole cross-resistance. These findings are in agreement with recent reports of increased azole resistance in Candida species in general and suggest the possibility that the oral cavity may act as a reservoir of resistant yeast isolates in systemic infections.

Enhanced extracellular production of aspartyl proteinase, a virulence factor, by Candida albicans isolates following growth in subinhibitory concentrations of fluconazole

We examined the production of secreted aspartyl proteinase (Sap), a putative virulence factor of Candida albicans, by a series of 17 isolates representing a single strain obtained from the oral cavity of an AIDS patient before and after the development of clinical and in vitro resistance to fluconazole. Isolates were grown in Sap-inducing yeast carbon base-bovine serum albumin medium containing 0, 0.25, 0.5, or 1 MIC of fluconazole, and cultures were sampled daily for 14 days to determine extracellular Sap activity by enzymatic degradation of bovine serum albumin. Extracellular Sap activity was significantly decreased in a dose-dependent manner for the most fluconazole-susceptible isolate (MIC, 1.0 microg/ml) and significantly increased in a dose-dependent manner for the most fluconazole-resistant isolate (MIC, >64 microg/ml). Enhanced extracellular Sap production could not be attributed to cell death or nonspecific release of Sap, because there was no reduction in the number of CFU and no significant release of enolase, a constitutive enzyme of the glycolytic pathway. Conversely, intracellular Sap concentrations were significantly increased in a dose-dependent manner in the most fluconazole-susceptible isolate and decreased in the most fluconazole-resistant isolate. Enhanced Sap production correlated with the overexpression of a gene encoding a multidrug resistance (MDR1) efflux pump occurring in these isolates. These data indicate that exposure to subinhibitory concentrations of fluconazole can result in enhanced extracellular production of Sap by isolates with the capacity to overexpress MDR1 and imply that patients infected with these isolates and subsequently treated with suboptimal doses of fluconazole may experience enhanced C. albicans virulence in vivo.

Synthesis and antifungal activity of a ferrocene-fluconazole analogue

A novel ferrocene fluconazole analogue was synthesized and its antifungal properties investigated against yeast strains of medical importance, including those intrinsically resistant to fluconazole. In vitro tests revealed a slight increase in fungal growth and a reversal of the effect of fluconazole at minimal inhibitory concentrations.

Suscetibilidade a antifúngicos de cepas de Candida albicans isoladas de pacientes com estomatite protética

Pacientes portadores de próteses totais, apresentam, com freqüência a chamada estomatite protética, com a qual associa-se Candida albicans determinando a chamada candidíase eritematosa. Assim, procuramos avaliar a suscetibilidade dessa levedura a agentes antifúngicos. A suscetibilidade de dezenove cepas de Candida albicans isoladas de pacientes apresentando estomatite protética foi estudada frente a: um derivado poliênico representado, pela anfotericina B (AnB), e dois derivados azóicos, cetoconazol e miconazol. A atividade antifúngica foi estudada a partir da determinação da concentração inibitória mínima (CIM) e da concentração fungicida mínima (CFM), pela técnica de diluição em ágar. Os resultados obtidos, mostraram baixos valores de CIMs e CFMs (< 0,15 <FONT FACE="Symbol">m</font>g/ml) para a AnB frente a todas as leveduras. Para o miconazol e o cetoconazol, foram observadas CIMs invariavelmente <FONT FACE="Symbol">£</font> 4,00<FONT FACE="Symbol"> m</font>g/ml; para as CFMs, foram obtidos valores <FONT FACE="Symbol">³</font> 16,00<FONT FACE="Symbol"> m</font>g/ml frente a maioria das cepas. Conclui-se que a AnB apresentou maior ação fungicida in vitro enquanto os azóis demonstraram ação fungistática mas não fungicida. Acreditamos que a pesquisa de novas drogas, principalmente de uso tópico ainda é necessária, a fim de se tratar, com sucesso, a candidíase eritematosa, comumente observada nas chamadas estomatites protéticas.

In vitro susceptibility of chromoblastomycosis and phaeohyphomycosis agents to antifungal drugs

The in vitro susceptibility of chromoblastomycosis and phaeohyphomycosis agents to antifungal drugs was appraised using the reference macrodilution method proposed by the National Committee for Clinical Laboratory Standards (NCCLS) for yeasts modified for filamentous fungi. The antifungal drugs amphotericin B, 5-fluorocytosine, itraconazole and fluconazole were tested against one environmental and 18 clinical isolates. This work amended the macrodilution methods proposed by NCCLS and suggests that a conidial suspension free of hyphae leads to a more reliable assay and provides for better reproducibility. The macrodilution method was performed with 10(4) conidia ml-1. The MIC values ranged from 1.0 to 16.0 micrograms ml-1 for amphotericin B and 3.12 to 25.0 micrograms ml-1 for 5-fluorocytosine. A MIC range of 0.06 to 1.95 micrograms ml-1 was determined for itraconazole while 2.0 to 64.0 micrograms ml-1 was detected for fluconazole.

Rhodamine 6G efflux for the detection of CDR1-overexpressing azole-resistant Candida albicans strains

We investigated the drug efflux mechanism in azole-resistant strains of Candida albicans using rhodamine 6G (R6G). No significant differences in R6G uptake were observed between azole-sensitive B2630 (9.02 +/- 0.02 nmol/10(8) cells) and azole-resistant B67081 (8.86 +/- 0.03 nmol/10(8) cells) strains incubated in glucose-free phosphate buffered saline. A significantly higher R6G efflux (2.0 +/- 0.21 nmol/10(8) cells) was noted in the azole-resistant strain (B67081) when glucose was added, compared with that in the sensitive strain B2630 (0.23 < or = 0.14 nmol/10(8) cells). A fluconazole-resistant strain C40 that expressed the benomyl resistance gene (CaMDR) also showed a low R6G efflux (0.16 +/- 0.06 nmol/10(8) cells) as did the sensitive strains. Accumulation of R6G in growing C. albicans cells was inversely correlated with the level of CDR1 mRNA expression. Our data also suggest that measurement of intracellular accumulation of R6G is a useful method for identification of azole-resistant strains due to CDR1-expressed drug efflux pump.

Emerging fungal pathogens, drug resistance and the role of lipid formulations of amphotericin B in the treatment of fungal infections in cancer patients: a review

The incidence of life-threatening invasive fungal infections in immunocompromised patients has increased dramatically in recent years. Candida spp other than C. albicans are increasingly being isolated, and Aspergillus infections also are on the increase, as well as infections due to previously uncommon organisms. It is likely that this phenomenon is multifactorial in origin, although the extensive use of antifungal prophylaxis may have played a role, especially for the emergence of non-albicans Candida. Amphotericin B remains the antifungal agent with the broadest spectrum of action available and is thus the standard treatment for immunocompromised patients with proven or suspected fungal infections, especially aspergillosis. However, its potential for nephrotoxicity limits its usefulness. Lipid formulations of amphotericin B may allow therapy to be administered with reduced renal toxicity. Three different lipid formulations of amphotericin B currently are available. These compounds have different pharmacokinetic properties and seem to achieve higher serum or tissue concentrations than amphotericin B. This statement is based on animal models and scattered human data. At present, there are no studies comparing the lipid formulations with each other and only a few randomized trials comparing them with conventional amphotericin B. However, a number of open clinical trials and compassionate-use protocols suggest that lipid-based forms of amphotericin B can achieve good response rates with minimal toxicity in patients with a variety of invasive mycoses, including those who have proved refractory or intolerant to previous therapy with conventional amphotericin B. Unfortunately, the cost of these compounds remains high and may represent a limiting factor to their use.

Comparison of E-test and broth microdilution methods for antifungal drug susceptibility testing of molds

We compared the E test with a broth microdilution method, performed according to National Committee for Clinical Laboratory Standards document M27-A guidelines, for determining the in vitro susceptibilities of 90 isolates of pathogenic molds (10 Absidia corymbifera, 10 Aspergillus flavus, 10 Aspergillus fumigatus, 10 Aspergillus niger, 10 Aspergillus terreus, 10 Exophiala dermatitidis, 10 Fusarium solani, 10 Scedosporium apiospermum, 5 Scedosporium prolificans, and 5 Scopulariopsis brevicaulis). Overall, there was 71% agreement between the results of the two methods for amphotericin B (E-test MICs within +/-2 log2 dilutions of broth microdilution MICs) and 88% agreement with the results for itraconazole. The overall levels of agreement (within +/-2 log2 dilutions) were >/=80% for 5 of the 10 species tested against amphotericin B and 8 of the 10 species tested against itraconazole. The best agreement between the results was seen with A. fumigatus and A. terreus (100% of results for both agents within +/-2 log2 dilutions). The poorest agreement was seen with S. apiospermum, S. prolificans, and S. brevicaulis tested against amphotericin B (20% of results within +/-2 log2 dilutions). In every instance, this low level of agreement was due to isolates for which the broth microdilution MICs were low but for which the E-test MICs were much higher. The E test appears to be a suitable alternative procedure for testing the susceptibility of Aspergillus spp. and some other molds to amphotericin B or itraconazole.

Membrane fluidity affects functions of Cdr1p, a multidrug ABC transporter of Candida albicans

Earlier, we have shown that the overexpression of an ABC transporter, CDR1, is involved in the emergence of multidrug resistance in Candida albicans. In this study, we checked its function in vivo by expressing it in different isogenic Saccharomyces cerevisiae erg mutants, which accumulated various intermediates of the ergosterol biosynthesis and thus altered the membrane fluidity. Functions like the accumulation of rhodamine 123, beta-estradiol, fluconazole and floppase activity associated with Cdr1p were measured to ascertain their responses to an altered membrane phase. The floppase activity appeared to be favoured by an enhanced membrane fluidity, while the effluxing of substrates and Cdr1p's ability to confer multidrug resistance were significantly reduced. We demonstrate that only some of the functions of Cdr1p were affected by an altered lipid environment.

Candida glabrata: review of epidemiology, pathogenesis, and clinical disease with comparison to C. albicans

Until recently, Candida glabrata was considered a relatively nonpathogenic commensal fungal organism of human mucosal tissues. However, with the increased use of immunosuppressive agents, mucosal and systemic infections caused by C. glabrata have increased significantly, especially in the human immunodeficiency virus-infected population. A major obstacle in C. glabrata infections is their innate resistance to azole antimycotic therapy, which is very effective in treating infections caused by other Candida species. Candida glabrata, formerly known as Torulopsis glabrata, contrasts with other Candida species in its nondimorphic blastoconidial morphology and haploid genome. C. glabrata currently ranks second or third as the causative agent of superficial (oral, esophageal, vaginal, or urinary) or systemic candidal infections, which are often nosocomial. Currently, however, there are few recognized virulence factors of C. glabrata and little is known about the host defense mechanisms that protect against infection. Two established animal models (systemic and vaginal) have been established to study treatment, pathogenesis, and immunity. Treatment of C. glabrata infections can include azoles but often requires amphotericin B or flucytosine. This review summarizes all known clinical and experimental information about C. glabrata infections with comparisons to C. albicans as a means of contrasting the two species commonly observed and emphasizing the many recognized differences.

Organism-dependent fungicidal activities of azoles

We investigated the antifungal activities of itraconazole and voriconazole on Aspergillus species by time kill studies, and the results were compared with those obtained for Candida species. Exposure of Aspergillus fumigatus conidia to varying concentrations (1.25 to 10 microg/ml) of itraconazole and voriconazole resulted in cellular death; the cytocidal effect was time and concentration dependent. In contrast, no killing of Candida albicans occurred in the presence of itraconazole and voriconazole at concentrations as high as 10 microg/ml, although candidal growth was inhibited compared to the drug-free control. Amphotericin B (1.25 to 10 microg/ml), on the other hand, killed both A. fumigatus and C. albicans. Similar results were obtained for non-A. fumigatus aspergilli and non-C. albicans Candida species. These observations indicate that both itraconazole and voriconazole are cytocidal agents for Aspergillus species but not for Candida species, suggesting that azoles possess organism-dependent fungicidal activities.

Azole resistance among oral Candida species isolates from AIDS patients under ketoconazole exposure

This is a prospective study designed to investigate species distribution and azole susceptibility profile among Candida spp. isolated from the oral cavities of AIDS patients. One hundred thirty-two AIDS patients sequentially admitted at a teaching tertiary care hospital were enrolled in this study. Samples were obtained by swabbing the oral cavities of the patients. Yeast isolates were identified by classical methods and the antifungal susceptibility profile was further determined according to the NCCLS microbroth assay. Among all patients with prescriptions of systemic antifungal drugs, ketoconazole had been elected to treat 56% of patients. We found 82% of oral yeast carriage, 22% of them harboring non-albicans species. Overall rates of susceptibility dose dependent/resistance to azoles was 16% for itraconazole, 13% for ketoconazole, and 10% for fluconazole with a high agreement rate among the susceptibility profiles of all isolates tested against the triazoles.

Synergy of nitric oxide and azoles against Candida species in vitro

The candidacidal activity of nitric oxide (NO) as delivered by a class of compounds termed diazeniumdiolates has been investigated. Diazeniumdiolates are stable agents capable of releasing NO in a biologically usable form at a predicted rate, and three such compounds were examined for activity. One compound, (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, 2-diolate (DETA-NO), proved to be most suitable for examining NO activity due to its relatively long half-life (20 h) and because of limited candidacidal activity of the uncomplexed DETA nucleophile. DETA-NO was active against six species of Candida for which the MICs necessary to inhibit 50% growth (MIC50s) ranged from 0.25 to 1.0 mg/ml. C. parapsilosis and C. krusei were the most susceptible to the compound. In addition to a determination of NO effects alone, the complex was utilized to investigate the synergistic potential of released NO in combination with ketoconazole, fluconazole, and miconazole. Activity was investigated in vitro against representative strains of Candida albicans, C. krusei, C. parapsilosis, C. tropicalis, C. glabrata, and C. dubliniensis. Determination of MIC50, MIC80 and MICs indicated that DETA-NO inhibits all strains tested, with strains of C. parapsilosis and C. krusei being consistently the most sensitive. The combination of DETA-NO with each azole was synergistic against all strains tested as measured by fractional inhibitory concentration indices that ranged from 0.1222 to 0.4583. The data suggest that DETA-NO or compounds with similar properties may be useful in the development of new therapeutic strategies for treatment of Candida infections.

Efficacy of LY303366 against amphotericin B-susceptible and -resistant Aspergillus fumigatus in a murine model of invasive aspergillosis

LY303366 is a novel antifungal echinocandin with excellent in vitro activity against Aspergillus spp. We compared four doses (1, 2.5, 10, and 25 mg/kg of body weight) of LY303366 with amphotericin B (0.5 to 5 mg/kg) in a temporarily neutropenic murine model of invasive aspergillosis against an amphotericin B-susceptible (AF210) and an amphotericin B-resistant (AF65) Aspergillus fumigatus isolate based on in vivo response. Mice were immunosuppressed with cyclophosphamide (200 mg/kg) and infected 3 days later. Treatment started 18 h after infection and lasted for 10 days. LY303366 was given once daily intravenously for 10 days, and amphotericin B (at 0.5, 2, and 5 mg/kg) was given once daily intraperitoneally for 10 days, or only on days 1, 2, 4, and 7 (at 5 mg/kg). Kidneys and lungs from survivors were cultured on day 11. Control mice in both experiments had 90 to 100% mortality. Amphotericin B at 0.5 mg/kg and LY303366 at 1 mg/kg yielded 10 to 20% survival rates for mice infected with either AF210 or AF65. Amphotericin B at 2 and 5 (both regimens) mg/kg yielded a 70 to 100% survival rate for mice infected with AF210 but a 10 to 30% survival rate for mice infected with AF65 (P = 0.01 to 0.04 compared with AF210). Against AF210 and AF65, LY303366 at 2.5, 10, and 25 mg/kg produced a survival rate of 70 to 80%, which was as effective as amphotericin B for AF210, but superior to amphotericin B for AF65 (P < 0.03 to 0.0006). For AF65, LY303366 at 10 and 25 mg/kg/day was superior to amphotericin B at 2 and 5 mg/kg/day in reducing tissue colony counts (P = 0.01 to 0.003), and for AF210, amphotericin B at 5 mg/kg/day and at 5 mg/kg in four doses was more effective than all four regimens of LY303366 in reducing renal culture counts (P = 0.01 to 0.0001). The present study shows, for the first time, that in vivo resistance of A. fumigatus to amphotericin B exists, although this could not be detected by in vitro susceptibility assays. Furthermore, LY303366 appears to be effective against amphotericin B-susceptible and -resistant A. fumigatus infection in this model and should be further evaluated clinically.

Clinical, cellular, and molecular factors that contribute to antifungal drug resistance

In the past decade, the frequency of diagnosed fungal infections has risen sharply due to several factors, including the increase in the number of immunosuppressed patients resulting from the AIDS epidemic and treatments during and after organ and bone marrow transplants. Linked with the increase in fungal infections is a recent increase in the frequency with which these infections are recalcitrant to standard antifungal therapy. This review summarizes the factors that contribute to antifungal drug resistance on three levels: (i) clinical factors that result in the inability to successfully treat refractory disease; (ii) cellular factors associated with a resistant fungal strain; and (iii) molecular factors that are ultimately responsible for the resistance phenotype in the cell. Many of the clinical factors that contribute to resistance are associated with the immune status of the patient, with the pharmacology of the drugs, or with the degree or type of fungal infection present. At a cellular level, antifungal drug resistance can be the result of replacement of a susceptible strain with a more resistant strain or species or the alteration of an endogenous strain (by mutation or gene expression) to a resistant phenotype. The molecular mechanisms of resistance that have been identified to date in Candida albicans include overexpression of two types of efflux pumps, overexpression or mutation of the target enzyme, and alteration of other enzymes in the same biosynthetic pathway as the target enzyme. Since the study of antifungal drug resistance is relatively new, other factors that may also contribute to resistance are discussed.

Amino acid substitutions in the cytochrome P-450 lanosterol 14alpha-demethylase (CYP51A1) from azole-resistant Candida albicans clinical isolates contribute to resistance to azole antifungal agents

The cytochrome P-450 lanosterol 14alpha-demethylase (CYP51A1) of yeasts is involved in an important step in the biosynthesis of ergosterol. Since CYP51A1 is the target of azole antifungal agents, this enzyme is potentially prone to alterations leading to resistance to these agents. Among them, a decrease in the affinity of CYP51A1 for these agents is possible. We showed in a group of Candida albicans isolates from AIDS patients that multidrug efflux transporters were playing an important role in the resistance of C. albicans to azole antifungal agents, but without excluding the involvement of other factors (D. Sanglard, K. Kuchler, F. Ischer, J.-L. Pagani, M. Monod, and J. Bille, Antimicrob. Agents Chemother. 39:2378-2386, 1995). We therefore analyzed in closer detail changes in the affinity of CYP51A1 for azole antifungal agents. A strategy consisting of functional expression in Saccharomyces cerevisiae of the C. albicans CYP51A1 genes of sequential clinical isolates from patients was designed. This selection, which was coupled with a test of susceptibility to the azole derivatives fluconazole, ketoconazole, and itraconazole, enabled the detection of mutations in different cloned CYP51A1 genes, whose products are potentially affected in their affinity for azole derivatives. This selection enabled the detection of five different mutations in the cloned CYP51A1 genes which correlated with the occurrence of azole resistance in clinical C. albicans isolates. These mutations were as follows: replacement of the glycine at position 129 with alanine (G129A), Y132H, S405F, G464S, and R467K. While the S405F mutation was found as a single amino acid substitution in a CYP51A1 gene from an azole-resistant yeast, other mutations were found simultaneously in individual CYP51A1 genes, i.e., R467K with G464S, S405F with Y132H, G129A with G464S, and R467K with G464S and Y132H. Site-directed mutagenesis of a wild-type CYP51A1 gene was performed to estimate the effect of each of these mutations on resistance to azole derivatives. Each single mutation, with the exception of G129A, had a measurable effect on the affinity of the target enzyme for specific azole derivatives. We speculate that these specific mutations could combine with the effect of multidrug efflux transporters in the clinical isolates and contribute to different patterns and stepwise increases in resistance to azole derivatives.

Development of secondary resistance to fluconazole in Cryptococcus neoformans isolated from a patient with AIDS

Cryptococcus neoformans is the fifth most common opportunistic agent of infection in patients with AIDS in the USA, exceeded only by Candida species, Pneumocystis carinii, cytomegalovirus and Mycobacterium avium1, 2, 6, 10, 11. In Brazil is the sixth, exceeded by Candida species, P. carinii, Mycobacterium species, Toxoplasma gondii, and herpes simplex virus (AIDS, Boletim Epidemiológico, set/nov 96, Ministério da Saúde, Brasil). During 30 years, the treatment of C. neoformans meningitis was based on the use of amphotericin B with or without flucytosine13. Nowadays, with the immunodepression caused by human immunodeficiency virus (HIV) infection and the availability of new antifungal drugs as the triazoles, the concept related to cure and relapses of cryptococcosis has been altered7, 20. Patients are treated with amphotericin B with or without flucytosine as initial therapy, but maintenance therapy is always necessary in AIDS patients with C. neoformans infections

In vitro kill curves of a new semisynthetic echinocandin, LY-303366, against fluconazole-sensitive and -resistant Candida species

In vitro killing by a new semisynthetic echinocandin, LY-303366, was characterized using clinical isolates of fluconazole-sensitive (Y58) and -resistant (Y180) Candida albicans as well as Candida glabrata (Y7) and Candida krusei (Y171). The 24-h kill curves for Y58 and Y180 demonstrated dose-independent killing of between 1 and 2 log10 with LY-303366 at concentrations of 0.1, 1, 10, 50, 100, and 1,000 times the MIC. Regrowth did not occur at 24 h with either C. albicans isolate at the aforementioned LY-303366 concentrations. At their MICs, LY-303366 and amphotericin B produced similar killing kinetics in cultures of Y58, Y180, Y7, and Y171, while all cultures exposed to fluconazole at its MIC demonstrated stasis or growth over 24 h.

Candidemia in a Canadian tertiary care hospital from 1976 to 1996

The incidence of candidemia was reviewed at the Health Sciences Centre in Winnipeg, Canada, over a 21-year period (1976 to 1996). Candida species were identified as blood-stream isolates in significantly (p < 0.05) higher numbers from 1991 to 1996 than in the previous 15 years. Antifungal susceptibilities remained unchanged with Candida albicans isolates tested from 1985 to 1996. Retrospective chart reviews revealed that all patients with candidemia possessed at least two risk factors. The main risk factors identified were recent or concurrent antibiotic therapy (95% of patients), presence of a central line (93% of patients), and immunosuppression (88% of patients). Treatment generally involved amphotericin B therapy (81% of patients), and death occurred in 52% of the patients. Mortality directly attributable to Candida species could be established in 23% of patients.

In vitro antifungal activity of BMS-181184 against systemic isolates of Candida, Cryptococcus, and Blastomyces species

BMS-181184 is a water-soluble derivative of the pradimicin group of antifungal compounds. We determined the in vitro activities of BMS-181184 and comparator agents amphotericin B, 5-fluorocytosine, fluconazole, and ketoconazole against 184 systemic fungal isolates collected at the Health Sciences Centre in Winnipeg, Canada, between 1987 and 1995. BMS-181184 demonstrated MICs of between 1 and 8 micrograms/mL for all Candida albicans, Candida glabrata, Candida tropicalis, Candida krusei, Candida lusitaniae, and Cryptococcus neoformans isolates tested. BMS-181184 was less active against Candida parapsilosis (MIC90 = 16 micrograms/mL) and Blastomyces dermatitidis (MIC90 = 32 micrograms/mL). Isolates of Candida species with fluconazole MICs of > or = 16 micrograms/mL and those with fluconazole MICs of < or = 8 micrograms/mL demonstrated similar BMS-181184 sensitivities.

1,2,3-Thiadiazole: a novel heterocyclic heme ligand for the design of cytochrome P450 inhibitors

The 1,2,3-thiadiazole heterocycle has been explored as a heme ligand and mechanism-based inactivator for the design of cytochrome P450 inhibitors. One 4,5-fused bicyclic and three 4,5-disubstituted monocyclic 1,2,3-thiadiazoles have been examined for their spectral interactions, inhibition, mechanism-based inactivation, and oxidation products by the versatile microsomal P450s 2B4, 2E1, and 1A2. The compounds generally show heteroatom coordination to the heme iron; however, the binding mode is influenced by the architecture of the active site. For example, 4,5-diphenyl-1,2,3-thiadiazole shows type I and type II difference spectra with P450s 2B4 and 2E1, respectively, and no spectral perturbation with P450 1A2. Except for the fused bicyclic compound, the spectral dissociation constants are in the 2-50 microM range. The effectiveness as an inhibitor depends on the substituents at the 4- and 5- positions and on the P450 examined. Inhibition of the P450-catalyzed 1-phenylethanol oxidation to acetophenone by the thiadiazoles does not correlate with either the type of binding spectra or the spectral dissociation constants of the compounds. P450s 2E1 and 2B4 are inactivated by the 4,5-fused bicyclic 1,2,3-thiadiazole in a mechanism-based manner. Inactivation of the P450 correlates with loss in absorbance at 450 nm for the ferrous-CO complex. The monocyclic 1,2,3-thiadiazoles do not inactivate any of the P450s examined. The 1,2,3-thiadiazole ring is oxidized by the P450 system. Oxidation of the monocyclic compounds results in extrusion of the three heteroatoms and formation of the corresponding acetylenes, whereas oxidation of the fused bicyclic compound does not yield an acetylenic product.

In vitro susceptibility testing of Fonsecaea pedrosoi to antifungals

Based on the difficulties experienced in the treatment of chromoblastomycosis, 12 primary human isolates of F. pedrosoi, were tested for their in vitro susceptibility to various antimycotics. We adapted the recommendations of the NCCLS for yeasts and followed the indications for mold testing from other authors in order to determine their MIC's and the MLC's. It was found that a significant proportion of the isolates were resistant to 3 of the 4 antimycotics tested, as revealed by high MIC values, as follows: 33% were resistant to amphotericin B (AMB), 58.3% to 5 fluocytosine (5 FC) and 66.7% to fluconazole (FLU). Contrarywise, none of the isolates proved resistant to itraconazole (ITZ). Determination of the MLC's revealed that a larger proportion of the isolates were not killed by AMB, 5 FC (91.7%), FLU (100%) or even, ITZ (41.7%). These data indicate that it would be desirable to determine the susceptibility of F. pedrosoi before initiating therapy, in order to choose the more effective antifungal and avoid clinical failure.

In vitro activity of a new semisynthetic echinocandin, LY-303366, against systemic isolates of Candida species, Cryptococcus neoformans, Blastomyces dermatitidis, and Aspergillus species

The in vitro activities of LY-303366, a new semisynthetic echinocandin, and comparators amphotericin B, 5-fluorocytosine, fluconazole, and ketoconazole against 205 systemic isolates of Candida species, Cryptococcus neoformans, Blastomyces dermatitidis, and Aspergillus species were determined. LY-303366 had MICs of < or = 0.32 microg/ml for all Candida albicans (n = 99), Candida glabrata (n = 18), and Candida tropicalis (n = 10) isolates tested. LY-303366 was also active against Aspergillus species (minimum effective concentration at which 90% of the isolates are inhibited, 0.02 microg/ml) (n = 20), was less active against Candida parapsilosis (MIC at which 90% of the isolates are inhibited [MIC90], 5.12 microg/ml) (n = 10), and was inactive against C. neoformans (MIC90, >10.24 microg/ml) (n = 15) and B. dermatitidis (MIC90, 16 microg/ml) (n = 29).

Cloning of Candida albicans genes conferring resistance to azole antifungal agents: characterization of CDR2, a new multidrug ABC transporter gene

Resistance to azole antifungal agents in Candida albicans can be mediated by multidrug efflux transporters. In a previous study, we identified at least two such transporters, Cdr1p and Benp, which belong to the class of ATP-binding cassette (ABC) transporters and of major facilitators, respectively. To isolate additional factors potentially responsible for resistance to azole antifungal agents in C. albicans, the hypersusceptibility of a Saccharomyces cerevisiae multidrug transporter mutant, delta pdr5, to these agents was complemented with a C. albicans genomic library. Several new genes were isolated, one of which was a new ABC transporter gene called CDR2 (Candida drug resistance). The protein Cdr2p encoded by this gene exhibited 84% identity with Cdr1p and could confer resistance to azole antifungal agents, to other antifungals (terbinafine, amorolfine) and to a variety of metabolic inhibitors. The disruption of CDR2 in the C. albicans strain CAF4-2 did not render cells more susceptible to these substances. When the disruption of CDR2 was performed in the background of a mutant in which CDR1 was deleted, the resulting double delta cdr1 delta cdr2 mutant was more susceptible to these agents than the single delta cdr1 mutant. The absence of hypersusceptibility of the single delta cdr2 mutant could be explained by the absence of CDR2 mRNA in azole-susceptible C. albicans strains. CDR2 was overexpressed, however, in clinical C. albicans isolates resistant to azole antifungal agents as described previously for CDR1, but to levels exceeding or equal to those reached by CDR1. Interestingly, CDR2 expression was restored in delta cdr1 mutants reverting spontaneously to wild-type levels of susceptibility to azole antifungal agents. These data demonstrate that CDR2 plays an important role in mediating the resistance of C. albicans to azole antifungal agents.

Comparative resistance of Candida albicans clinical isolates to fluconazole and itraconazole in vitro and in vivo in a murine model

Relationships between azole susceptibility and in vivo response to antifungal therapy in a murine model of candidiasis were investigated for Candida albicans isolates sampled from human immunodeficiency virus type 1-positive patients with oropharyngeal candidiasis. The susceptibilities of seven clinical isolates and two reference strains to fluconazole (FCZ) and itraconazole (ITZ) were determined in vitro by the broth microdilution method. Four isolates were resistant to FCZ and ITZ, two were susceptible to both azoles, and three were resistant to FCZ and susceptible to ITZ (dissociated resistance). CD1 mice were inoculated with each isolate and treated with either FCZ or ITZ (drug regimen, 5 mg/kg of body weight twice daily for 5 days). Quantitative cultures of kidneys were performed at the end of the treatment. On the other hand, the survival rates of the mice were followed daily. These two parameters were clearly correlated with in vitro susceptibility. Thus, the phenomenon of a dissociation of resistance to FCZ and ITZ may be found in vivo as well as in vitro.

Fluconazole treatment of Candida albicans infection in mice: does in vitro susceptibility predict in vivo response?

A series of fluconazole-susceptible and-fluconazole resistant Candida albicans fungal isolates were used to infect mice intravenously. Mice were treated with varying doses of fluconazole beginning one day after infection. For all of the 6 fluconazole-susceptible isolates, fluconazole was highly effective at <0.25 mg/kg of body weight twice daily. By contrast, fluconazole was less effective in at least 6 of 10 fluconazole-resistant isolates and was ineffective at > or = 40 mg/kg twice daily in 4 fluconazole-resistant isolates. Although the correlation is not precise, in vitro susceptibility testing of C. albicans can predict in vivo response to fluconazole.

Patterns of in vitro activity of itraconazole and imidazole antifungal agents against Candida albicans with decreased susceptibility to fluconazole from Spain

Two groups of recent clinical isolates of Candida albicans consisting of 101 isolates for which fluconazole MICs were < or = 0.5 microgram/ml (n = 50) and > or = 4.0 micrograms/ml (n = 51), respectively, were compared for their susceptibilities to fluconazole, clotrimazole, miconazole, ketoconazole, and itraconazole. Susceptibility tests were performed by a photometer-read broth microdilution method with an improved RPMI 1640 medium supplemented with 18 g of glucose per liter (RPMI-2% glucose; J. L. Rodríguez-Tudela and J. V. Martínez-Suárez, Antimicrob. Agents Chemother. 38:45-48, 1994). Preparation of drugs, basal medium, and inocula was done by the recommendations of the National Committee for Clinical Laboratory Standards. The MIC endpoint was calculated objectively from the turbidimetric data read at 24 h as the lowest drug concentration at which growth was just equal to or less than 20% of that in the positive control well (MIC 80%). In vitro susceptibility testing separated azole-susceptible strains from the strains with decreased susceptibilities to azoles if wide ranges of concentrations (20 doubling dilutions) were used for ketoconazole, miconazole, and clotrimazole. By comparison with isolates for which fluconazole MICs were < or = 0.5 microgram/ml, those isolates for which fluconazole MICs were > or = 4.0 micrograms/ml were in general less susceptible to other azole drugs, but different patterns of decreased susceptibility were found, including uniform increases in the MICs of all azole derivatives, higher MICs of several azoles but not others, and elevated MICs of fluconazole only. On the other hand, decreased susceptibility to any other azole drug was never found among strains for which MICs of fluconazole were lower.

Origin of differences in susceptibility of Candida krusei to azole antifungal agents

Two Candida krusei isolates were used to compare the effects of fluconazole, ketoconazole and itraconazole on growth and ergosterol synthesis, and to measure intracellular drug contents. Fifty per cent inhibition (IC50) of growth was achieved at 0.05-0.08 microM itraconazole and 0.56-1.2 microM ketoconazole, whereas 91-->100 microM fluconazole was needed to reach the IC50 value. Similar differences in sensitivity to these azole antifungal agents were seen when their effects on ergosterol synthesis from [14C]acetate were measured after 4 h and 24 h of growth. However, when the effects of the azoles on ergosterol synthesis from [14C]mevalonate by subcellular fractions were measured, fluconazole was only 2.3-6.1 times less active than itraconazole, and the IC50 values for ketoconazole were almost similar to those obtained with itraconazole. These results indicate that differences in susceptibility to itraconazole and ketoconazole are unrelated to differences in affinity for the C. krusei cytochrome P450. The much lower growth-inhibitory effects of fluconazole can also be explained partly only by a lower affinity for the P450-dependent 14 alpha-demethylase. The differences in sensitivity of both C. krusei isolates appeared to arise from differences in the intracellular itraconazole, ketoconazole and fluconazole contents. Depending on the experimental conditions, these isolates accumulated 6-41 times more itraconazole than ketoconazole and the intracellular ketoconazole content was 3.0-19.0 times higher than that of fluconazole.