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. 2004 Mar;48(3):873–878. doi: 10.1128/AAC.48.3.873-878.2004

In Vitro Activities of 3-(Halogenated Phenyl)-5-Acyloxymethyl- 2,5-Dihydrofuran-2-ones against Common and Emerging Yeasts and Molds

Vladimír Buchta 1,*, Milan Pour 2, Petra Kubanová 1, Luis Silva 1, Ivan Votruba 3, Marie Vopršálová 4, Radan Schiller 2, Helena Fáková 2, Marcel Špulák 2
PMCID: PMC353114  PMID: 14982778

Abstract

Three 3-(halogenated phenyl)-5-acyloxymethyl-2,5-dihydrofuran-2-ones were evaluated for activity against 191 strains of common and emerging yeasts and Aspergillus species by the broth microdilution test performed according to NCCLS guidelines. The furanone derivatives displayed broad-spectrum in vitro activity against potentially pathogenic yeasts and molds, especially Aspergillus spp. (MIC ≤ 2.0 μg/ml) and fluconazole-resistant yeast isolates, including Candida glabrata and Saccharomyces cerevisiae. The 4-bromophenyl derivative was the most effective derivative against the majority of species tested, except for the Candida tropicalis and C. glabrata strains, which were more susceptible to the 3-chlorophenyl derivative. The 3,4-dichlorophenyl derivative possessed a lesser in vitro antifungal effect. The potential of further experiments on animal infection and clinical studies is supported by the relatively low cytotoxicity and acute toxicity of the 4-bromophenyl compound. Thus, the halogenated 3-phenyl-5-acyloxymethyl derivatives of 2,5-dihydrofuran-2-one represent a novel, promising group of compounds with significant activity against relevant opportunistic fungi that are pathogenic to humans.

The incidence of and mortality due to fungal infections have increased over the past few decades, particularly in the setting of immunocompromised hosts. In spite of the introduction of several new antifungal drugs, such as caspofungin and voriconazole, the therapeutic management of serious invasive mycoses still remains a challenge for pharmaceutical research and industry (11).

3-(Halogenated phenyl)-5-acyloxymethyl-2,5-dihydrofura-nones (Fig. 1A) represent a novel class of butenolide antimycotics based on (−)incrustoporin (the minus sign means that the compound is a levorotatory enantiomer), obtained from the extract of fermentation of the basidiomycete Incrustoporia carneola, as the lead structure. The natural product displays an antifungal effect toward phytopathogenic molds and some cytotoxic activity (Fig. 1) (15). In agreement with these findings, our antifungal testing revealed that the racemic form of the natural product exhibited only a marginal effect (MIC > 32 μg/ml) on our set of yeast and mold isolates (12). The preparation of synthetic derivatives with halogenated phenyl at C-3 and a methyl group at C-5 led to a significant increase of the in vitro antifungal effect, especially in filamentous fungi (12, 13, 14). The highest in vitro antifungal activity was linked to the substitution of the phenyl group at C-3 by halogens at positions 3 and 4 and to the presence of an acyloxymethyl group at C-5 (13). The conversion of 5-hydroxymethyl derivatives into esters resulted in a broader spectrum of activity and in MICs below 4 μg/ml for most yeasts and molds tested. Interestingly, the effect was not stereospecific, i.e., there were no significant differences in the MICs of the enantiomers and the racemate (13).

FIG. 1.

FIG. 1.

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The structure of (−)incrustoporin and its derivatives. (A) 3-(Halogenated phenyl)-5-acyloxymethyl-2,5-dihydrofuran-2-ones.LNO6-22, Z = 3,4-Cl2; LNO15-22, Z = 3-Cl; LNO18-22, Z = 4-Br.(B) 3-(4-Methoxyphenyl)-5-acetoxymethyl-2,5-dihydrofuran-2-ones(see reference 13 for details). (C) 2-(Substituted aryl)cyclopent-2-enones (see reference 14 for details). Y = H; 4-CH3; 4-OCH3; 4-Cl; 3,4-Cl2. The indicated MICs represent the range of MICs for most of the fungal strains tested.

Based on these experiments, we decided to investigate the in vitro profile of the most promising three esters (LNO6-22, LNO15-22, and LNO18-22) of 3-(halogenated phenyl)-5-acyloxymethyl-2,5-dihydrofuran-2-ones against 191 strains of potentially pathogenic yeasts and molds by using the microdilution broth method. Our results showed that these derivatives are broad-spectrum agents which inhibited not only Aspergillus and common Candida species (such as C. albicans, C. tropicalis, and C. parapsilosis) but also fluconazole-resistant C. albicans isolates and Candida krusei, Candida glabrata, and Saccharomyces cerevisiae, which are intrinsically resistant to fluconazole or whose resistance is easily inducible.

MATERIALS AND METHODS

Fungal organisms

A set of 191 fungal strains was used for the susceptibility testing in the present study (see Table 1). They included clinical isolates obtained from biological materials (blood, urine, sputum, bronchoalveolar lavage fluid, and oral and vaginal swabs) of patients with suspected and proven mycoses. C. albicans ATCC 90028 and C. krusei ATCC 6258 were used as quality controls. The fungal isolates were identified by standard microbiology methods and stored in skim milk medium (Becton Dickinson) for 6 to 10 months at −40°C before use. Prior to antifungal susceptibility testing, each isolate was passaged on Sabouraud dextrose agar (Difco) to ensure optimal growth characteristics and purity.

TABLE 1.

Antifungal susceptibilities of yeasts to 3-(halogenated phenyl)-5-acyloxymethyl-2.5-dihydrofuran-2-ones determined by the microdilution broth method

Species (no. of isolates) and antifungal agent IC MIC (μg/ml)a
Species (no. of isolates) and antifungal agent IC MIC (μg/ml)a
Range GMb MIC50 MIC90 Range GMb MIC50 MIC90
Blastoschizomyces capitatus (8)
    Amphotericin B IC95 0.031-0.125 0.113 0.125
    Fluconazole IC80 4-16 9.50 8
IC50 0.125-8 2.89 0.5
    LNO6-22 IC80 1-2 1.63 2
    LNO15-22 IC80 0.5-1 0.688 0.5
    LNO18-22 IC80 0.5-1 0.563 0.5
Candida albicans (25)
    Amphotericin B IC95 0.063-0.25 0.133 0.125 0.25
    Fluconazole IC80 0.125->64 39.3 0.5 >64
IC50 0.125->64 18.4 0.125 >64
    LNO6-22 IC80 1-16 2.88 2 4
    LNO15-22 IC80 1-8 2.16 2 4
    LNO18-22 IC80 0.5-4 1.26 0.5 1
Candida albicans Flcs (16)c
    Amphotericin B IC95 0.063-2 0.156 0.125 0.25
    Fluconazole IC80 0.125-16 1.31 0.25 0.5
IC50 0.125-0.5 0.183 0.125 0.25
    LNO6-22 IC80 1-4 2.13 2 2
    LNO15-22 IC80 1-4 1.75 2 2
    LNO18-22 IC80 0.5-2 1.00 1 2
Candida albicans Flcr (9)d
    Amphotericin B IC95 0.063-2 0.184 0.125 2
    Fluconazole IC80 64->64 120.9 >64 >64
IC50 0.125->64 50.8 8 >64
    LNO6-22 IC80 1-16 4.22 2 16
    LNO15-22 IC80 1-8 2.89 2 8
    LNO18-22 IC80 0.5-4 1.67 2 4
Candida famata (3)
    Amphotericin B IC95 1-2 1.67
    Fluconazole IC80 2->64 64.7
IC50 2-64 32.7
    LNO6-22 IC80 0.5-8 3.5
    LNO15-22 IC80 0.5-4 1.83
    LNO18-22 IC80 0.25-4 1.75
Candida glabrata (12)
    Amphotericin B IC95 0.125-0.25 0.188 0.125 0.25
    Fluconazole IC80 8->64 49.3 32 >64
IC50 4->64 25.3 8 64
    LNO6-22 IC80 0.5-2 1.00 1 2
    LNO15-22 IC80 0.25-1 0.458 0.25 1
    LNO18-22 IC80 0.5-1 0.625 0.5 1
Candida guilliermondii (6)
    Amphotericin B IC95 1-4 2.17 2
    Fluconazole IC80 0.5-32 10.3 4
IC50 0.5-4 2.58 2
    LNO6-22 IC80 0.125-8 2.35 0.5
    LNO15-22 IC80 0.125-4 1.19 0.25
    LNO18-22 IC80 ≤0.063-4 1.39 1
Candida inconspicua (8)
    Amphotericin B IC95 0.063-0.125 0.086 0.031
    Fluconazole IC80 16-64 32.0 32
IC50 4-32 17.0 8
    LNO6-22 IC80 0.5-4 1.69 1
    LNO15-22 IC80 0.25-1 0.688 0.5
    LNO18-22 IC80 0.25-1 0.719 0.5
Candida kefyr (10)
    Amphotericin B IC95 0.063-0.25 0.113 0.125 0.25
    Fluconazole IC80 0.25-1 0.450 0.5 1
IC50 0.25-0.5 0.375 0.25 0.5
    LNO6-22 IC80 0.25-1 0.825 1 1
    LNO15-22 IC80 0.25-1 0.775 1 1
    LNO18-22 IC80 0.5-1 0.550 0.5 1
Candida krusei (9)
    Amphotericin B IC95 0.125-0.25 0.222 0.25 0.25
    Fluconazole IC80 32->64 88.9 64 >64
IC50 32-64 53.3 64 64
    LNO6-22 IC80 2-16 6.00 4 16
    LNO15-22 IC80 2-2 2.00 2 2
    LNO18-22 IC80 1-2 1.78 2 2
Candida lusitaniae (10)
    Amphotericin B IC95 0.031-0.25 0.147 0.125 0.25
    Fluconazole IC80 0.5-32 5.15 1 32
IC50 0.25-8 1.38 0.25 8
    LNO6-22 IC80 1-8 3.2 2 8
    LNO15-22 IC80 1-4 1.9 2 4
    LNO18-22 IC80 1-4 2.1 2 4
Candida norvegensis (6)
    Amphotericin B IC95 ≤0.016-0.063 0.050 0.063
    Fluconazole IC80 16-64 32.0 32
IC50 8-64 21.3 16
    LNO6-22 IC80 0.5->64 28.4 8
    LNO15-22 IC80 0.25-4 2.79 4
    LNO18-22 IC80 0.25-4 1.79 2
Candida parapsilosis (11)
    Amphotericin B IC95 0.031-0.125 0.034 0.031 0.063
    Fluconazole IC80 0.5-64 8.59 4 8
IC50 0.25-16 3.11 2 4
    LNO6-22 IC80 1-8 2.64 2 4
    LNO15-22 IC80 1-4 2.09 2 2
    LNO18-22 IC80 1-4 1.80 2 2
Candida pelliculosa (10)
    Amphotericin B IC95 ≤0.016-0.25 0.067 0.031 0.25
    Fluconazole IC80 0.5->64 17.45 4 >64
IC50 0.25-8 3.325 4 8
    LNO6-22 IC80 0.5-1 0.800 1 1
    LNO15-22 IC80 0.5-1 0.650 0.5 1
    LNO18-22 IC80 0.25-1 0.700 0.5 1
Candida rugosa (4)
    Amphotericin B IC95 1-2 1.25
    Fluconazole IC80 4->64 65.0
IC50 2-64 32.5
    LNO6-22 IC80 2-4 3.00
    LNO15-22 IC80 1-1 1.00
    LNO18-22 IC80 0.5-1 0.875
Candida tropicalis (13)
    Amphotericin B IC95 0.031-0.25 0.113 0.125 0.125
    Fluconazole IC80 0.5->64 51.4 8 >64
IC50 0.25->64 12.4 0.5 16
    LNO6-22 IC80 2-8 3.69 4 8
    LNO15-22 IC80 0.5-4 3.58 4 4
    LNO18-22 IC80 1-4 2.38 2 4
Candida utilis (4)
    Amphotericin B IC95 2-2 2.00
    Fluconazole IC80 1-16 6.25
IC50 1-8 3.75
    LNO6-22 IC80 0.5-2 1.38
    LNO15-22 IC80 0.5-1 0.875
    LNO18-22 IC80 0.5-2 1.13
Cryptococcus spp. (3)
    Amphotericin B IC95 1-2 1.67
    Fluconazole IC80 0.5-2 1.00
IC50 ≤0.063-0.25 0.146
    LNO6-22 IC80 2-4 2.67
    LNO15-22 IC80 0.25-2 1.08
    LNO18-22 IC80 1-1 1.00
Saccharomyces cerevisiae (10)
    Amphotericin B IC95 0.031-0.125 0.060 0.063 0.063
    Fluconazole IC80 2-64 13.4 8 16
IC50 1-64 10.7 4 16
    LNO6-22 IC80 0.5-2 1.20 1 2
    LNO15-22 IC80 0.25-1 0.775 1 1
    LNO18-22 IC80 0.25-2 0.825 0.5 1
Trichosporon beigelii complex (10)
    Amphotericin B IC95 1-4 2.70 2 4
    Fluconazole IC80 0.25-8 2.93 2 8
IC50 0.125-8 1.86 1 8
    LNO6-22 IC80 0.25-4 2.03 2 4
    LNO15-22 IC80 ≤0.063-2 1.03 0.5 2
    LNO18-22 IC80 0.125-2 0.888 0.5 2
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a

Determined after 24 h for yeasts and after 48 h for Cryptococcus spp.

b

GM, geometric mean.

c

Fluconazole-susceptible and -intermediate isolates for which the MIC was <64 μg/ml.

d

Fluconazole-resistant isolates for which the MIC was ≥64 μg/ml.

Inoculum preparation

The isolates of yeasts and Aspergillus spp. had been grown for 1 and 4 days, respectively, on Sabouraud dextrose agar at room temperature and then subcultured on the same medium for a further 1 and 4 days, respectively, at 35°C. Yeast and conidial suspensions were prepared in RPMI 1640 medium and sterile water supplemented with 0.01% Tween 80, respectively. Each suspension of a yeast or an Aspergillus sp. was diluted in RPMI 1640 medium at a ratio of 1:50 or 1:10, respectively. The final inoculum sizes ranged from 0.5 × 103 to 2.5 × 103 CFU/ml for yeasts and 0.5 × 104 to 5 × 104 CFU/ml for molds. The inoculum size was determined microscopically by using a Bürker's chamber and verified by plating 100 μl of serial dilutions of each inoculum onto Sabouraud agar plates and incubating them until growth became visible.

Antifungal drugs

The derivatives to be evaluated were prepared according to a procedure described previously (13). The structures of these compounds are shown in Fig. 1A. Amphotericin B (Sigma) and fluconazole (Pfizer, Sandwich, United Kingdom) were used as standard antifungal drugs. The fluconazole was solubilized in sterile distilled water at a starting concentration of 128 μg/ml. The derivatives mentioned above and amphotericin B were dissolved in 100% dimethyl sulfoxide (DMSO; Sigma). The final concentration of DMSO in the test medium did not exceed 1% of the total solution composition.

A stock solution of each antifungal compound was prepared in RPMI 1640 medium with l-glutamine, but without sodium bicarbonate (Sevapharma, Prague, Czech Republic), and then diluted to the required drug concentration. The medium was buffered to pH 7.0 ± 0.1 with 0.165 M morpholinepropanesulfonic acid (MOPS; Sigma). All solutions were prepared immediately before testing.

Antifungal susceptibility testing

MICs were determined by the microdilution broth method with a minor modification (see “Inoculum preparation” above) in sterile, flat-bottom, 96-well microtitration plates according to the NCCLS reference documents M27-A and M38-P for yeasts and filamentous fungi, respectively (9, 10).

Twofold serial dilutions ranging from 64 to 0.063 μg/ml for fluconazole and the furanone derivatives and from 16 to 0.016 μg/ml for amphotericin B were used. To determine the MIC of amphotericin B, RPMI 1640 medium was replaced with antibiotic medium 3 (Becton Dickinson) at pH 7.0 ± 0.2. Wells 1 to 11 contained the drug serial dilutions in 100-μl volumes, and well 12 contained a drug-free medium which served as the growth control for each strain.

Each well was inoculated with 100 μl of the inoculum suspension and 100 μl of the medium with a given concentration of the drug being tested, and the microtitration plates were incubated at 35°C in a humid atmosphere. The MICs were determined spectrophotometrically (iEMS Reader MF; Labsystems) as the lowest concentrations that showed 50% growth inhibition (IC50; for fluconazole), 80% growth inhibition (IC80; for all compounds), or 95% growth inhibition (IC95; for amphotericin B) compared with the growth in the drug-free control wells. The MICs were read after 24 and 48 h of incubation without agitation for yeasts and after 48 and 72 h of incubation for Cryptococcus and Aspergillus spp.

Quality control determinations of the MICs of amphotericin B and fluconazole were ensured by testing C. albicans ATCC 90028, C. parapsilosis ATCC 22019, and C. krusei ATCC 6258. The results were within the recommended limits.

Germ tube inhibition test

The in vitro antifungal effects of subinhibitory concentrations of all three furanone derivatives (LNO6-22, LNO15-22, and LNO18-22) were evaluated by the inhibition of the germ tube formation of C. albicans in NYP medium (N-acetylglucosamine [Sigma; 10−3 mol/liter], yeast nitrogen base [Difco; 3.35 g/liter], proline [Roanal; 10−3 mol/liter]) with NaCl (4.5 g/liter) adjusted to pH 6.7 ± 0.1 (7). The suspensions of four C. albicans strains in the NYP medium were prepared from overnight cultures on Sabouraud dextrose agar (Difco) to obtain a final density of (1.0 ± 0.2) × 106 CFU/ml. The compounds were dissolved and diluted in DMSO and added in a volume of 20 μl to the yeast suspension in NYP medium (1.98 ml) to obtain 1/10 and 1/50 of the MIC (MIC/10 and MIC/50, respectively). After a 3-h incubation at 37°C at both concentrations, the percentage of germ tube formation of 100 cells was determined by using a Bürker's chamber. The germ tubes were counted when they were as long as the diameter of the blastospore.

Cytostatic activity assays of LNO18-22

Cell growth inhibition was estimated for mouse lymphocytic leukemia L1210 cells (ATCC CCL 219), CCRF-CEM T lymphoblastoid cells (human acute lymphoblastic leukemia cells; ATCC CCL 119), human promyelocytic leukemia HL-60 cells (ATCC CCL 240), and human cervix carcinoma HeLa S3 cells (ATCC CCL 2.2) (4). L1210, CCRF-CEM, and HL-60 cells were cultivated in RPMI 1640 medium supplemented with fetal calf serum in 24-well tissue culture plates. The end point of the cell growth was 72 h after the addition of LNO18-22. HeLa S3 cells were seeded into 24-well dishes in an RPMI 1640-HEPES modification medium with fetal calf serum. Forty-eight hours after the addition of the drug, the cultivation was stopped and the cell growth was evaluated. In parallel, the cell viability was quantified by using an MTT [3-(4,5-dimethyl-2-thiazyl)-2,5-diphenyl-2H-tetrazolium bromide] standard spectrophotometric assay (1). The inhibitory potencies of the compounds tested were expressed as IC50s.

Animal toxicity experiments for LNO18-22

To determine the 50% lethal dose, 10 male mice (NMRI) were administered doses of 50, 100, 150, and 200 mg of LNO18-22/kg of body weight intraperitoneally as boluses of 0.05 ml per 10 g of body weight. Before the administration, this compound was dissolved in a vehicle composed of 8 parts of propylene glycol and 2 parts of polyethylene glycol. The maximum concentration tested (200 mg/kg) was limited by the solubility of the compound in the vehicle. A control consisting of the vehicle alone was used. The mice were sacrificed after a 2-week follow-up period, and the organs were submitted for pathological examination.

Data analysis

The ranges and geometric means of the MICs were determined for each drug and species. Furthermore, the MIC50 and MIC90 were calculated by using the criteria for MIC determinations described above. The significance of the differences in the MICs of the potential drug candidates (the LNOs) was determined after conversion to the logarithmic scale by using Student's t test with the statistical Microsoft Excel package (version 9.0). P values of <0.05 were considered statistically significant.

RESULTS

The MICs of the three furanone derivatives and the reference compounds (fluconazole and amphotericin B) for our set of clinical yeast strains and Aspergillus species are presented in Tables 1 and 2. When tested against NCCLS quality control strains, the MICs of LNO6-22, LNO15-22, and LNO18-22 were 2.0, 1.0, and 0.5 μg/ml, respectively, for C. albicans ATCC 90028; 2.0 μg/ml each for C. parapsilosis ATCC 22019; and 8.0, 2.0, and 2.0 μg/ml, respectively, for C. krusei ATCC 6258. All three furanone derivatives displayed significant in vitro effects against the most clinically important fungi tested, with especially valuable results against the majority of fluconazole-resistant yeasts, including C. glabrata, Candida inconspicua, Candida famata, S. cerevisiae, and Aspergillus spp. The comparison of these derivatives showed the following order of in vitro antifungal potency: LNO18-22 ≥ LNO15-22 > LNO6-22. The differences in the MICs of the furanones were just marginal for Aspergillus spp., C. parapsilosis, C. tropicalis, Candida kefyr, and Candida pelliculosa but more striking for C. krusei, Candida norvegensis, Candida rugosa, Geotrichum spp., and fluconazole-resistant C. albicans (Tables 1 and 2). The MIC90 of LNO18-22 was <2.0 μg/ml. On the other hand, the highest MICs were mostly obtained with the 3,4-dichlorophenyl derivative (LNO6-22) against C. norvegensis (MIC of >64 μg/ml for one strain, MIC of 8.0 μg/ml for two strains), fluconazole-resistant C. albicans (one strain), and C. krusei (one strain). In regard to LNO15-22 and LNO18-22, the MIC was not above 4.0 μg/ml for any of the fungal strains, with the exception of one fluconazole-resistant C. albicans strain.

TABLE 2.

Antifungal susceptibilities of filamentous fungi to 3-(halogenated phenyl)-5-acyloxymethyl-2,5-dihydrofuran-2-ones determined by the microdilution broth method

Species (no. of isolates) and antifungal agent IC MIC (μg/ml)a
Range GMb MIC50 MIC90
Absidia corymbifera (1)
    Amphotericin B IC95 2
    Fluconazole IC80 >64
    LNO6-22 IC80 4
    LNO15-22 IC80 2
    LNO18-22 IC80 4
Aspergillus fumigatus (12)
    Amphotericin B IC95 0.063-0.25 0.193 0.25 0.25
    Fluconazole IC80 >64 >64 >64 >64
    LNO6-22 IC80 1-2 1.83 2 2
    LNO15-22 IC80 0.5-2 1.25 1 2
    LNO18-22 IC80 0.5-1 0.958 1 1
Aspergillus flavus (3)
    Amphotericin B IC95 1-1 1.00
    Fluconazole IC80 >64 >64
    LNO6-22 IC80 2-2 2.00
    LNO15-22 IC80 1-2 1.67
    LNO18-22 IC80 1-1 1.00
Aspergillus niger (7)
    Amphotericin B IC95 0.063-0.125 0.116 0.125
    Fluconazole IC80 >64 >64 >64
    LNO6-22 IC80 1-2 1.714 2
    LNO15-22 IC80 1-2 1.86 2
    LNO18-22 IC80 0.5-2 1.07 1
Aspergillus terreus (1)
    Amphotericin B IC95 1
    Fluconazole IC80 >64
    LNO6-22 IC80 1
    LNO15-22 IC80 2
    LNO18-22 IC80 0.25
Aspergillus sp. (1)
    Amphotericin B IC95 0.125
    Fluconazole IC80 >64
    LNO6-22 IC80 2
    LNO15-22 IC80 1
    LNO18-22 IC80 1
Geotrichum spp. (4)
    Amphotericin B IC95 2-2 2.00
    Fluconazole IC80 0.5-32 12.1
    LNO6-22 IC80 2-4 3.00
    LNO15-22 IC80 0.5-2 1.38
    LNO18-22 IC80 0.5-1 0.875
Microsporum gypseum (1)
    Amphotericin B IC95 2
    Fluconazole IC80 16
    LNO6-22 IC80 0.25
    LNO15-22 IC80 0.5
    LNO18-22 IC80 0.5
Trichophyton mentagrophytes (1)
    Amphotericin B IC95 1
    Fluconazole IC80 16
    LNO6-22 IC80 0.5
    LNO15-22 IC80 0.5
    LNO18-22 IC80 0.5
Wangiella dermatitidis (1)
    Amphotericin B IC95 0.5
    Fluconazole IC80 4
    LNO6-22 IC80 0.5
    LNO15-22 IC80 0.5
    LNO18-22 IC80 0.5
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a

Determined after 48 h for the Aspergillus spp. and A. corymbifera and after 72 h for T. mentagrophytes and M. gypseum.

b

GM, geometric mean.

The inhibition of the germ tube formation of four C. albicans strains by LNO6-22 at MIC/10 was similar to that produced by amphotericin B (Fig. 2 and Table 3). Amphotericin B interfered with the filamentation at MIC/50, while all three experimental compounds and fluconazole had no or only a slight effect on this process at MIC/50 (data not shown).

FIG. 2.

FIG. 2.

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Inhibition of germ tube formation (measured as the percentage of germ tube formation of 100 C. albicans cells after a 3-h incubation at 37°C in NYP medium [pH 6.7]) by the furanone derivatives. CA, C. albicans strain; AMB, amphotericin B; FLC, fluconazole; C, control (NYP medium); C+DMSO, control (NYP medium) with 1% DMSO. See Table 3 for data.

TABLE 3.

Inhibition of C. albicans germ tube formation by the furanone derivatives

Antifungal compound % Formation of germ tubes by C. albicans straina:
4 5 7 8
LNO6-22 1.7 ± 0.9 1.7 ± 0.5 1.3 ± 1.2 3.0 ± 1.6
LNO15-22 58.7 ± 2.9 30.0 ± 2.2 5.7 ± 1.2 30.3 ± 2.1
LNO18-22 70.3 ± 3.7 62.7 ± 3.1 6.7 ± 1.2 75.0 ± 4.5
Amphotericin B 1.0 ± 0.8 2.0 ± 1.6 0.3 ± 0.5 1.0 ± 0.8
Fluconazole 74.3 ± 1.7 51.0 ± 2.2 29.7 ± 2.5 98.0 ± 0.8
Control 88.5 ± 5.3 78.5 ± 17.3 57.8 ± 8.9 95.0 ± 2.1
Control with DMSOb 87.8 ± 12.5 79.3 ± 14.8 49.8 ± 22.6 97.5 ± 2.2
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a

Strain numbers correspond to CA4, CA5, CA7, and CA8 in Fig. 2. Values for strains 4 and 5 are shown with standard deviations. All values are geometric means of three experiments and were obtained at MIC/10.

b

Control (NYP medium) with 1% DMSO.

The selected furanone derivative (LNO18-22) exhibited moderate cytotoxic activity against the cell lines tested, with the highest activity being that against the CCRF-CEM T lymphoblastoid cells (MIC = 1.5 μg/ml).

Animal experiment results showed no significant acute toxicity after the intraperitoneal application of LNO18-22, even at the maximum dosage of 200 mg/kg. No deaths of experimental animals occurred during the follow-up period, and no noticeable macroscopic changes of internal organs were observed.

DISCUSSION

Our results confirm that the furanone derivatives have a broad spectrum of activity against a variety of pathogenic yeasts and molds, including the fungi with decreased susceptibility to fluconazole, such as C. krusei, C. glabrata, S. cerevisiae, and Aspergillus spp. (Tables 1 and 2). A comparison of the three derivatives showed relatively negligible differences in in vitro antifungal activity. Nevertheless, the 4-bromophenyl derivative LNO18-22 seemed to be the most potent, particularly against C. albicans, C. inconspicua, C. krusei, Blastoschizomyces capitatus, the Trichosporon beigelii complex, and Aspergillus species, while no differences in the potencies of LNO18-22 and the other two derivatives were found against C. parapsilosis, C. tropicalis, Candida lusitaniae, C. kefyr, C. pelliculosa, Candida guilliermondii, C. norvegensis, and S. cerevisiae (Student's t test, P < 0.05). C. tropicalis and C. glabrata were significantly more susceptible to the 3-chlorophenyl derivative (LNO15-22) than to the other two derivatives (Student's t test, P < 0.01).

However, the results for the inhibition of the germ tube formation of C. albicans at a subinhibitory concentration (1/10 of the MIC) indicated that LNO6-22 may have the highest antifungal potential, in regard to the interference with the dimorphic conversion of C. albicans. The percentages of germ tube formation were comparable to those resulting from the same concentration of amphotericin B (Fig. 2).

The promising inhibitory effect on fluconazole-resistant yeasts (at drug MICs of ≥64 μg/ml, based on the IC80 criterion at an optical density at 540 nm) should be pointed out. The MICs of all three compounds for nine fluconazole-resistant C. albicans strains, five C. tropicalis strains, one C. parapsilosis strain, and one C. pelliculosa strain were only about 1 or 2 dilutions higher than the MICs for susceptible strains. The furanone derivatives exhibited a more selective activity against filamentous fungi. The MIC was not above 2.0 μg/ml for any of the mold strains tested, with the exception of the zygomycete Absidia corymbifera. All Aspergillus fumigatus strains were inhibited with 1-μg/ml concentrations of LNO18-22. These findings may be especially valuable in regard to the growing problem with fluconazole-resistant yeasts, particularly in AIDS patients with refractory candidiasis, and a relatively high incidence of invasive aspergillosis in transplant recipients and neutropenic patients (2, 6).

It is well known that butenolides possess a wide range of substitution-dependent biological activities, including antifungal activity (3, 5, 8). The mechanism of the action of these furanone antifungals has not been thoroughly investigated. Our results have shown that these compounds display a strong inhibitory effect, with the differences between the MICs after 24 and 48 h for yeasts and 48 and 72 h for molds and cryptococcal strains being no more than 2 concentrations up the dilution scale.

The substitution of bromine or chlorine in positions 3 and 4 on the phenyl moiety at C-3 of the furanone core seems to contribute to the antifungal effect more than other substituents (13). In this regard, it is worthy to note that the activity of 3-(4-methoxyphenyl)-5-acetoxymethyl-2,5-dihydrofuran-2-one (Fig. 1B) with a 4-methoxyphenyl group at C-3 is significantly lower than those of the derivatives with a halogenated phenyl substitution (13). A recent paper (14) has also shown that the pharmacophoric element must include the five-membered lactone ring, as 3-(substituted aryl)cyclopent-2-enones were found to be completely inactive (Fig. 1C). Overall, a relative uniformity of all 3-(halogenated phenyl)-5-acyloxymethyl derivatives in terms of the spectrum of activity and the low MICs suggests either an easy access of these compounds to the target biomolecules in general or the formation of a highly reactive intermediate. The possible mode of action is currently being investigated, and the results will be reported in due course.

The experiments with the four tumor cell lines revealed that LNO18-22 is moderately cytotoxic, with its cytotoxicity being about 1 order of magnitude higher (MIC, 1.5 to 6.1 μg/ml) than its in vitro activity against most fungal strains tested. In the acute toxicity test, this compound showed no toxic effects in mice up to a dose of 200 mg/kg.

In conclusion, the halogenated 3-phenyl-5-acyloxymethyl derivatives of 2,5-dihydrofuran-2-one represent a novel, promising group of compounds with significant activities against relevant opportunistic fungi that are pathogenic to humans. Given the results described above, these substances deserve further investigation and development as potential systemic antifungals.

Acknowledgments

This work was supported by research project no. 259/2001 of the Grant Agency of Charles University and grant FRVS no. 2979 from the Ministry of Education, Youth and Sports of the Czech Republic.

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