Influence of test conditions on antifungal time-kill curve results: proposal for standardized methods

Expression profiling of the response of Saccharomyces cerevisiae to 5-fluorocytosine using a DNA microarray

5-Fluorocytosine is a commonly used antifungal agent. It acts by inhibiting the synthesis of fungal DNA and protein. In order to understand the global response of Saccharomyces cerevisiae to changes in DNA and protein synthesis caused by 5-fluorocytosine, genome-wide transcript profiling following 5-fluorocytosine exposure was obtained. A total of 96 genes were identified as responsive to 25 microg/ml fluorocytosine treatment for 90 min, which caused approximately 17% specific growth inhibition. The transcript levels of 57 genes were increased more than 2-fold, while it was found that the transcript levels of the other 39 genes decreased to a similar extent. Genes involved in DNA repair, synthesis and replication represented the highest proportion of induced genes identified, which may account for the easily acquired resistance to 5-fluorocytosine. Two enzyme encoding genes CTS1 and EGT2, which function in the separation of daughter cells from their mother cells, were down-regulated by a factor of 3.7 and 10.2, respectively, indicating that 5-fluorocytosine may also inhibit the separation of fungal cells.

Antifungal pharmacodynamic characteristics of amphotericin B against Trichosporon asahii, using time-kill methodology

We determined the MIC of amphotericin B against 45 Trichosporon asahii isolates from various clinical and environmental sources, and used in vitro time-kill methods to characterize the relationship between amphotericin B concentrations and MIC for four representative T. asahii isolates. Amphotericin B had concentration-dependent antifungal activity. MICs ranged from 0.5 to 16 microg/ml, and most T. asahii isolates (76%, 34/45) were inhibited at safely achievable amphotericin B serum concentrations (< or = 2 microg/ml). However, 40% (18/45) of isolates were not killed at these concentrations (MFCs from 1.0 to 32 microg/ml). At concentrations > or = 2 x MIC, amphotericin B exhibited fungicidal activity (< 99.9% reduction in CFU) over a 12-hr time-period; the maximal effect was achieved at > or =4 x MIC. Susceptibility testing confirmed the resistance of T. asahii to amphotericin B, and in vitro pharmacodynamic results also suggest that amphotericin B is not suitable therapy for T. asahii infection.

Effect of the echinocandin caspofungin on expression of Candida albicans secretory aspartyl proteinases and phospholipase in vitro

Although the echinocandin caspofungin primarily inhibits the synthesis of cell wall 1,3-beta-D-glucan, its fungicidal activity could also potentially perturb the expression of virulence factors involved in the ability of Candida albicans to cause infection. Expression of the C. albicans secretory aspartyl proteinase (SAP) and phospholipase B (PLB) virulence genes was determined by reverse transcription-PCR after the addition of caspofungin to cells grown for 15 h in Sabouraud dextrose broth. In cells that remained viable, expression of SAP1 to SAP3, SAP7 to SAP9, and PLB1 was unaltered after exposure to fungicidal concentrations (4 to 16 micro g/ml) of caspofungin over a period of 7 h. However, expression of SAP5 increased steadily beginning 1 h after exposure to caspofungin. These results indicate that caspofungin is rapidly fungicidal against C. albicans, before any suppression of SAP or PLB1 gene expression can occur.

Antifungal activities of fluconazole, caspofungin (MK0991), and anidulafungin (LY 303366) alone and in combination against Candida spp. and Crytococcus neoformans via time-kill methods

The activities of the echinocandins caspofungin and anidulafungin were evaluated alone and in combination with fluconazole using time-kill methods against isolates of Candida albicans, Candida glabrata, Candida tropicalis, Candida krusei, and Cryptococcus neoformans. Antifungal concentrations tested against each isolate were 0.5 microg/mL and 20 microg/mL of fluconazole and 0.007 microg/mL and 2 microg/mL of both caspofungin and anidulafungin. In addition, 20 microg/mL of fluconazole was tested with 2 microg/mL of caspofungin and anidulafungin to test for additive or antagonistic activity. Finally 0.5 microg/mL of fluconazole was tested with 0.007 microg/mL of caspofungin and anidulafungin to test for synergy. Combinations of fluconazole and caspofungin or anidulafungin resulted in indifference. Azole-echinocandin combinations do not produce antagonistic effects; therefore, combinations of these agents may warrant future clinical evaluation.

Antifungal and cancer cell growth inhibitory activities of 1-(3',4',5'-trimethoxyphenyl)-2-nitro-ethylene

The antifungal and cancer cell growth inhibitory activities of 1-(3',4',5'-trimethoxyphenyl)-2-nitro-ethylene (TMPN) were examined. TMPN was fungicidal for the majority of 132 reference strains and clinical isolates tested, including those resistant to fluconazole, ketoconazole, amphotericin B or flucytosine. Minimum fungicidal concentration/minimum inhibitory concentration (MFC/MIC) ratios were < or = 2 for 96% of Cryptococcus neoformans clinical isolates and 71% of Candida albicans clinical isolates. TMPN was fungicidal for a variety of other basidiomycetes, endomycetes and hyphomycetes, and its activity was unaffected by alterations in media pH. The frequency of occurrence of fungal spontaneous mutations to resistance was <10(-6). Kill-curve analyses confirmed the fungicidal action of TMPN, and demonstrated that killing was concentration- and time-dependent. At sub-MIC exposure to TMPN, C. albicans did not exhibit yeast/hyphae switching. TMPN was slightly cytotoxic for murine and human cancer cell lines (GI50=1-4 microg ml(-1)), and weakly inhibited mammalian tubulin polymerization (IC50=0.60 microg ml(-1)).

Candida lusitaniae catheter-related sepsis

OBJECTIVE

To present a case describing Candida lusitaniae candidemia in an immunocompetent patient successfully treated with fluconazole antifungal therapy. Time-kill studies of the C. lusitaniae isolate using amphotericin B, and an extensive review of the literature are also presented.

CASE SUMMARY

A 52-year-old immunocompetent Latin-American woman was admitted to the special care unit with severe sepsis. Her recent medical history included an exploratory laparotomy for gallstone pancreatitis, requiring cholecystectomy, segmental sigmoid colectomy, drainage of peritoneal abscesses, and a colostomy. In addition, the patient required a central venous catheter (CVC) placement for prolonged broad-spectrum antibiotic therapy and total parenteral nutrition therapy. Yeast was isolated from the abdominal abscess and blood cultures obtained on day 1, and from the catheter tip on day 5. The woman received initial empiric antifungal therapy with fluconazole, which was later changed to amphotericin B. After the yeast was identified as C. lusitaniae on day 8, this was changed to fluconazole for the duration of therapy. C. lusitaniae was not present in blood cultures taken two weeks after the CVC was removed, and the cultures remained negative thereafter. After a prolonged hospitalization, the patient was discharged home.

DISCUSSION

Disseminated infections with C. lusitaniae usually occur in immunocompromised patients, although isolated reports of C. lusitaniae infections in immunocompetent patients have been described. Therapeutic challenges of C. lusitaniae treatment include its primary resistance to amphotericin B and species misidentification. Isolates recovered from our patient were submitted for fungus time--kill studies that suggested unique susceptibility patterns to amphotericin B, indicating a trend toward resistance.

CONCLUSIONS

Based on variable susceptibility patterns of C. lusitaniae to amphotercin B and flucytosine, fluconazole is an appropriate choice as first-line therapy for C. lusitaniae candidemia.

Evaluation of amphotericin B and flucytosine in combination against Candida albicans and Cryptococcus neoformans using time-kill methodology

In this study, time-kill methods were used to evaluate the antifungal activity of amphotericin B and flucytosine, alone and in combination, against six isolates of Candida albicans and Cryptococcus neoformans. Five regimens were tested against each isolate: (1) flucytosine, (2) low-dose amphotericin B, (3) high-dose amphotericin B, (4) low-dose amphotericin B plus flucytosine, and (5) high-dose amphotericin B plus flucytosine. Low-dose amphotericin B and flucytosine, administered alone and simultaneously, demonstrated fungistatic activity against all sample isolates except C. albicans 90028, in which fungicidal activity was detected with the combination. High-dose amphotericin B, alone and in combination, resulted in a rapid fungicidal effect in all isolates. In both the low and high-dose combinations, indifferent activity was demonstrated against all tested isolates. By virtue of the absence of an antagonistic interaction between these two agents, complementary pharmacokinetic profiles, and non-overlapping toxicities, continued clinical use of these agents in combination may be considered.

Differential antifungal activity of isomeric forms of nystatin

When nystatin is placed in RPMI and other biological fluids, there is loss of pure nystatin, with the development of two distinguishable chromatographic peaks, 1 and 2. Peak 1 appears identical to commercially prepared nystatin. By nuclear magnetic resonance (NMR) and mass spectral analysis, peak 2 appears to be an isomer of peak 1. The isomers are quantitatively and fully interconvertible. Formation of peak 2 is accelerated at a pH of >7.0 and ultimately reaches a near 55:45 (peak 1/peak 2 ratio) mixture. We sought to determine the relative activities of peaks 1 and 2 against Candida spp. Peak 2 consistently showed higher MICs when it was the predominant form during the experiment. Time-kill analyses showed that peak 2 required > or =8 x the concentration of peak 1 to produce a modest and delayed killing effect, which was never of the same magnitude as that produced by peak 1. In both types of assays, the activity of peak 2 corresponded with intra-assay formation of peak 1. Both MIC measurements and time-kill analysis suggest that peak 2 has considerably less activity, if any at all, against Candida spp. Peak 2 may serve as a reservoir for peak 1.

Antifungal susceptibility testing: practical aspects and current challenges

Development of standardized antifungal susceptibility testing methods has been the focus of intensive research for the last 15 years. Reference methods for yeasts (NCCLS M27-A) and molds (M38-P) are now available. The development of these methods provides researchers not only with standardized methods for testing but also with an understanding of the variables that affect interlaboratory reproducibility. With this knowledge, we have now moved into the phase of (i) demonstrating the clinical value (or lack thereof) of standardized methods, (ii) developing modifications to these reference methods that address specific problems, and (iii) developing reliable commercial test kits. Clinically relevant testing is now available for selected fungi and drugs: Candida spp. against fluconazole, itraconazole, flucytosine, and (perhaps) amphotericin B; Cryptococcus neoformans against (perhaps) fluconazole and amphotericin B; and Aspergillus spp. against (perhaps) itraconazole. Expanding the range of useful testing procedures is the current focus of research in this area.

Antifungal pharmacodynamics: concentration-effect relationships in vitro and in vivo

The pharmacodynamics of antifungal compounds involve relationships among drug concentrations, time, and antimicrobial effects in vitro and in vivo. Beyond better understanding of a drug's mode of action, characterization of these relationships has important implications for setting susceptibility breakpoints, establishing rational dosing regimens, and facilitating drug development. Important advances have been made in the experimental investigation of pharmacokinetics and pharmacodynamics of antifungal drugs; however, much remains to be learned about specific pathogens and specific sites of infection. Increased incorporation of pharmacokinetic and pharmacodynamic principles in experimental and clinical studies with antifungal agents is an important objective that will benefit the treatment and prophylaxis of life-threatening invasive fungal infections in immunocompromised patients.

Comparative bactericidal activities of ciprofloxacin, clinafloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin against Streptococcus pneumoniae in a dynamic in vitro model

Several new quinolones that exhibit enhanced in vitro activity against Streptococcus pneumoniae have been developed. Using a dynamic in vitro model, we generated time-kill data for ciprofloxacin, clinafloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin against three isolates of quinolone-susceptible S. pneumoniae. Three pharmacokinetic profiles were simulated for each of the study agents (0.1, 1, and 10 times the area under the concentration-time curve [AUC]). Target 24-h AUCs were based upon human pharmacokinetic data resulting from the maximal daily doses of each agent. Ciprofloxacin was the least active agent against all three isolates. With regimens that simulated the human 24-h AUC, ciprofloxacin resulted in an initial, modest decline in the numbers of CFU per milliliter; however, by 48 h the numbers of CFU per milliliter returned to or exceeded the starting inoculum. At the AUC, levofloxacin resulted in variable bacteriostatic and bactericidal activities against the isolates. The remaining agents yielded bactericidal (99.9% reduction) activity by 48 h with regimens that simulated the AUC. At 0.1 time the AUC ciprofloxacin and levofloxacin produced no inhibitory effect, grepafloxacin exhibited bacteriostatic activity, trovafloxacin had mixed static and cidal activities, and clinafloxacin and moxifloxacin caused significant reductions in the numbers of CFU per milliliter by 48 h. All six agents produced cidal activity at 10 times the AUC. In this dynamic in vitro model of infection, the quinolones demonstrated various degrees of activity against S. pneumoniae. The rank order of activity, with respect to bactericidal effect, was ciprofloxacin (least active) << levofloxacin < grepafloxacin, trovafloxacin < clinafloxacin and moxifloxacin (most active). The rank order of the agents with respect to the selection of resistance was ciprofloxacin (most likely) > grepafloxacin, moxifloxacin, and trovafloxacin > levofloxacin > clinafloxacin.

Antifungal pharmacodynamics: review of the literature and clinical applications

Invasive fungal infections are seen with growing frequency, likely due to increases in numbers of patients at risk of infection. Optimal selection and dosing of antifungal agents are important, as these infections are often refractory to available therapy. In contrast to antibacterials, studies examining the pharmacodynamic properties of antifungals and their application in treating invasive disease often are lacking. Agents administered for invasive infections are amphotericin B, flucytosine, and azole antifungals. Several drugs are under investigation, such as posiconazole, voriconazole, and the echinocandins, and preliminary pharmacodynamic data likely will help shape dosing regimens. Clinical trials that investigated dosage and administration, as well as the potential benefits of combination and sequential therapy, are addressed. In addition, antifungal susceptibility and animal models of infection are discussed.

In vitro pharmacodynamic characteristics of nystatin including time-kill and postantifungal effect

Four Candida albicans isolates and six non-albicans Candida isolates were evaluated by time-kill methods to characterize the relationship between nystatin concentrations, the rate and extent of fungicidal activity, and the postantifungal effect (PAFE). Against Candida species, nystatin exhibits concentration-dependent fungicidal activity and a pronounced PAFE.

Evaluation of voriconazole pharmacodynamics using time-kill methodology

Voriconazole is an investigational azole antifungal agent with activity against a variety of fungal species, including fluconazole-susceptible and -resistant Candida species and Cryptococcus neoformans. In this study, we employed in vitro time-kill methods to characterize the relationship between concentrations of voriconazole and its fungistatic activity against Candida albicans, Candida glabrata, Candida tropicalis, and C. neoformans. Isolates were exposed to voriconazole concentrations ranging from 0.0625 to 16 times the MIC, and the viable colony counts were determined over time. The 50 and 90% effective concentrations (EC(50) and EC(90), respectively) were determined at 8, 12, and 24 h following the addition of voriconazole. At each time point, near-maximal fungistatic activity, as indicated by the EC(90), was noted at a drug concentration of approximately three times the MIC. Additionally, EC(50) and EC(90) did not change over time, thus suggesting that the rate of activity was not improved by increasing concentrations. Voriconazole exhibits non-concentration-dependent pharmacodynamic characteristics in vitro.

A Time-Kill Evaluation of Fluconazole and Amphotericin B against Candida Isolates

Objective:

To compare the activity of fluconazole and amphotericin B against four isolates of Candida spp. with varying susceptibilities by use of time-kill methodology.

Methods:

Two isolates of Candida albicans and one isolate each of Candida tropicalis and Candida glabrata were used for all experiments. Susceptibilities were determined in duplicate by E-test. Time-kill studies were performed with fluconazole 1, 10, and 20 μg/mL and amphotericin B 0.25, 1, and 2.5 μg/mL. Samples were withdrawn at 0, 2, 8, 12, and 24 hours for fluconazole and at 0, 0.5, 1, 2, 4, 12, and 24 hours for amphotericin B, diluted if necessary, then plated on potato dextrose agar plates. After incubating plates at 35 °C for 24 hours, colony counts were determined.

Results:

The minimum inhibitory concentration of fluconazole and amphotericin B, respectively, were as follows: 0.75 and 0.25 μg/mL for C. albicans ATCC 90028, >256 and 0.25 μg/mL for C. albicans 1378, 2 and 1 μg/mL for C. tropicalis, and 12 and 0.125 μg/mL for C. glabrata. Differences observed in the activity of fluconazole were dependent on the Candida spp. and susceptibility to the drug. All three concentrations of fluconazole were fungistatic against C. albicans ATCC 90028 and ineffective against C. albicans 1378 (the resistant isolate). Fluconazole suppressed the growth of C. tropicalis for the initial eight hours, after which fungal growth resumed. Against C. glabrata, fluconazole was ineffective at 1 μg/mL, whereas the 10- and 20-μg/mL concentrations were fungistatic for the first 12 hours. Amphotericin B displayed a concentration-dependent effect against all four isolates of Candida spp.

Conclusions:

Fluconazole 10 and 20 μg/mL were either fungistatic or suppressive for each of the susceptible and susceptible dose-dependent Candida isolates for the initial eight to 12 hours. In addition, despite resumed fungal growth, final colony counts with these concentrations were decreased by >80% compared with the growth controls at 24 hours. By comparison, the activity of amphotericin B was dependent on concentration against all four isolates, and fungicidal activity was demonstrated with only the concentration of 2.5 μg/mL. These concentrations of fluconazole and amphotericin B are readily available with common, clinically used dosing regimens (i.e., fluconazole 400 mg qd and amphotericin B 1 mg/kg/d).

Postantifungal effects of echinocandin, azole, and polyene antifungal agents against Candida albicans and Cryptococcus neoformans

The postantifungal effect (PAFE) of fluconazole, MK-0991, LY303366, and amphotericin B was determined against isolates of Candida albicans and Cryptococcus neoformans. Concentrations ranging from 0. 125 to 4 times the MIC were tested following exposure to the antifungal for 0.25 to 1 h. Combinations of azole and echinocandin antifungals (MK-0991 and LY303366) were tested against C. neoformans. Fluconazole displayed no measurable PAFE against Candida albicans or Cryptococcus neoformans, either alone or in combination with either echinocandin antifungal. MK-0991, LY303366, and amphotericin B displayed a prolonged PAFE of greater than 12 h against Candida spp. when tested at concentrations above the MIC for the organism and 0 to 2 h when tested at concentrations below the MIC for the organism.

In vitro pharmacodynamic characteristics of flucytosine determined by time-kill methods

Two Candida albicans isolates, three non-albicans Candida isolates (Candida glabrata, Candida krusei, and Candida tropicalis), and one Cryptococcus neoformans isolate were evaluated by time-kill methods to characterize the relationship of flucytosine concentrations to antifungal activity and the duration of the post-antifungal effect (PAE). Against Candida and Cryptococcusisolates, flucytosine at concentrations > 1 x MIC exhibited fungistatic (</=99% reduction in CFU) activity over a 24-h time-period. The rate and extent of fungistatic activity of flucytosine against all isolates was generally not increased when 5-FC concentrations exceeded 4 x MIC. A notable PAE was detected for flucytosine against both Candida and Cryptococcus species that persisted 2 to 4 h. These in vitro data suggest that flucytosine is predominately a concentration-independent fungistatic agent at clinically achieved serum concentrations. This pharmacodynamic characteristic coupled with the persistent PAE and the relatively long half-life of flucytosine in humans (> 5 h), suggests lower daily dosing may possible without loss of antifungal efficacy.

In vitro pharmacodynamic properties of MK-0991 determined by time-kill methods

MK-0991 has demonstrated activity against a variety of fungal pathogens. We evaluated the MIC endpoint for MK-0991 by reading the endpoint using three methods and comparing these results with minimum fungicidal concentrations and electron micrographs. The concentration that resulted in 80% inhibition of fungal growth compared with control, similar to the endpoint for the azole antifungal agents, provided the most consistent results. Additionally, we investigated the time-kill properties of this agent against two isolates each of Candida albicans, Candida glabrata and Candida tropicalis at concentrations ranging from 0.125 x MIC to 16 x MIC. Kill curves were performed using RPMI buffered with morpholine propanesulfonic acid as growth media. Samples were obtained at predetermined time points over 24 h and plated for colony counting. Fungicidal activity was observed with one isolate of C. albicans, two isolates of C. glabrata, and one isolate of C. tropicalis. MK-0991 displayed concentration-dependent activity, which was fungicidal or fungistatic depending on the isolate tested.

Therapy of Candida infections: susceptibility testing, resistance, and therapeutic options

OBJECTIVE

Review the epidemiology of fungal infections, approved susceptibility testing methods, the scope of antifungal resistance, and advances in the treatment of fungal infections.

DATA SOURCES

MEDLINE databases (from 1966 to March 1998) were searched for literature pertaining to the epidemiology and management of fungal infections.

STUDY SELECTION AND DATA EXTRACTION

Articles were selected to assist in providing the reader an understanding of the epidemiology and management of fungal infections.

DATA SYNTHESIS

Fungi have emerged as an important class of pathogens. Even though fungi rank as the fourth most commonly encountered nosocomial bloodstream pathogen, and are associated with the highest mortality of commonly encountered pathogens, only within the past year have methods for conducting and guidelines for interpreting in vitro susceptibility tests been approved. Under the guidance of these standards, we have begun to understand important issues regarding fungi such as the scope and mechanisms of antifungal resistance. Although there has not been a significant addition to our antifungal armamentarium since 1992, advances in antifungal therapy have been realized with the reformulation of available agents and the delineation of the pharmacodynamic characteristics of several antifungals. Additionally, several new agents, including a new class of antifungals, probably will enter into clinical use within the next 5 years.

CONCLUSIONS

We have entered an era in which our understanding of fungi is increasing tremendously. Clinicians need to familiarize themselves with the current concepts surrounding the management of fungal infections in order to provide optimal care for their patients.