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Antimicrobial Agents and Chemotherapy, May 2006, p. 1823-1827, Vol. 50, No. 5
0066-4804/06/$08.00+0 doi:10.1128/AAC.50.5.1823-1827.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Departments of Infectious Diseases, Infection Control and Employee Health,1 Veterinary Medicine and Surgery, The University of Texas M. D. Anderson Cancer Center, Houston, Texas2
Received 9 August 2005/ Returned for modification 6 September 2005/ Accepted 6 February 2006
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Because EDTA, a chelator agent used as the standard treatment for lead poisoning, has been shown to have antifungal activity in vitro (14), we conjectured that it might serve as an adjunct antifungal therapy. Our hypothesis was that EDTA would serve as a chelator of divalent cations and thereby potentiate the activity of the amphotericin B-based antifungal regimen being administered. To test this hypothesis, we examined the efficacy of EDTA alone and in combination with amphotericin B lipid complex (ABLC) (Abelcet; Elan, San Diego, CA) in an immunosuppressed rat model with invasive pulmonary aspergillosis to determine whether the combination improved the outcome in this rat model.
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Antifungal agents. Amphotericin B (AMB) deoxycholate (Fungizone) was purchased from Bristol-Myers Squibb (Princeton, NJ). A 5.0-mg/ml solution was prepared with distilled water immediately prior to its use. ABLC in 5-mg/ml vials was donated by Enzon Pharmaceuticals (Bridgewater, NJ). EDTA (Endrate) was purchased (Abbott Laboratories, North Chicago, IL) in vials of 20 ml at 50 mg/ml. The solution was diluted in distilled water to the desired concentration prior to its use in animals.
Inoculum preparation and inoculation procedure.
Aspergillus fumigatus strain AF293 was used in this infected-animal model. To prepare the inoculum, A. fumigatus was grown on Sabouraud's dextrose agar plates for 1 week at 37°C. Conidia were collected by flooding the plates with 10 ml of sterile normal saline with 0.1% Tween 80. The inocula were scraped off gently with an L-shaped sterile glass (inoculum loop), and then the harvested solutions were filtered through 4- by 4-inch (10- by 10-cm) gauze to eliminate the hyphal branching elements and mycelial fragments. The remaining suspension, consisting mostly of conidia, was concentrated by centrifugation, and the supernatant was removed. Conidia were then suspended to achieve a final concentration of 1 x 109 conidia/ml. The viability of all inocula was confirmed to be
95% by hemocytometer counting, followed by manual counting of the conidia in serial suspensions on plates. The MIC at 48 h for this isolate was 0.5 µg/ml for AMB and 1 µg/ml for ABLC, which was determined by using the NCCLS method adapted for molds (11). For inoculation (300 µl) of the fungi, a suspension of 1 x 107 conidia of A. fumigatus was delivered with a micropipette (Corning, Corning, NY) to the nares of the animal. All the inoculation procedures with the immunosuppressed rats were performed with the animal in an induction box under spontaneous ventilation with a 4% isoflurane and oxygen (5 liters/min) gas mixture.
Time-kill curve study. Aliquots of the suspension of A. fumigatus strain AF293 were prepared in 50 ml of RPMI medium at 1 x 104 conidia/ml in the presence of EDTA, ABLC, and ABLC plus EDTA at concentrations ranging from 0.25 µg/ml to 16 µg/ml after 24 h of drug exposure in a shaker at 35°C. Aliquots (0.1 ml) of the conidial suspension were removed and diluted to obtain 10- to 104-fold dilutions and were plated on Sabouraud dextrose agar plates. The plates were incubated at 35°C for 24 h, and the number of CFU/ml was determined. The same procedure was repeated for the remaining aliquots after 48 h of drug exposure. A time-kill curve (see Fig. 3) was constructed by plotting the mean log number of CFU/ml against the time conidia were exposed to the antifungal drug.
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FIG. 3. The time-kill curves show the effects of EDTA and ABLC alone and in combination on the growth of A. fumigatus strain AF293 after 24 or 48 h of drug exposure.
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To confirm the duration of neutropenia, blood samples obtained by serial tail venophlebotomy (20-µl samples) from three rats in each group were sent for hematologic and chemistry studies on day 0 (baseline, 10 min before inoculation), on day +7 (at the end of therapy), on day +14, and on day +20 (at the end of the study). Absolute neutrophil counts showed that most of the rats tested remained neutropenic, with an absolute neutrophil count of <500 until day 14. However, on day 20, most of the rats showed some immune recovery from neutropenia, with absolute neutrophil counts ranging between 500 and 3,000.
Toxicity determinations. Serum creatinine and calcium concentrations were assessed in the same three rats from each group to rule out toxicity. For the purposes of this study, an abnormal creatinine or calcium level was indicated by a 25% increase from the baseline value (see data presented in Table 2). During the histopathologic analysis of lung specimens, tissue was also examined for damage attributable to the EDTA.
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TABLE 2. Assessment of drug impact on rat serum calcium and creatinine concentrations
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To assess the effect of treatment on the lungs of the animals, four rats from each group were sacrificed on day 7. At sacrifice, anesthetized animals were bled by cardiac puncture and euthanized with carbon dioxide gas. The lung tissues were removed aseptically, weighed, and homogenized in 5 ml of sterile saline for uniform times at the same speed. One milliliter of the lung homogenate was placed onto Sabouraud's dextrose agar to quantify the fungal burden in the lungs in terms of numbers of CFU/g of lung tissue. Another lung tissue sample was sent for histopathologic analysis. To assess the effect of treatment on survival, the remaining 12 rats in each group were followed for a total of 20 days, at which time they were sacrificed.
Analysis of data. We compared the median numbers of CFU/g of lung tissue among the different treatment groups using the Wilcoxon rank sum test normal approximation with continuity correction. Analyses were conducted for rats that survived 1 week or less and for rats that survived for more than 1 week. Histopathologic grading of the angioinvasive lung lesions was done semiquantitatively, using a scoring system ranging from 0 to 4+, which indicated both the severity of the pulmonary lesions and the abundance of organisms. Fisher's exact test was used to compare the grades among the five groups.
The Kaplan-Meier product limit method was used to examine the effects of treatment on survival. Median survival and the corresponding 95% confidence interval values for survival were calculated for rats in each treatment group. The survival rates for the treatment groups were compared by the log rank test. All tests were two-tailed, and an alpha value of 0.05 was considered to indicate significance.
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TABLE 1. Comparative efficacies of EDTA alone and in combination with amphotericin B preparations in the treatment of invasive pulmonary aspergillosis in severely immunocompromised rats
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The effect of the study drugs on the survival of rats with pulmonary aspergillosis is shown in Fig. 1. The overall test for equality over strata indicated that the survival curves for the five groups differed significantly (P < 0.0001). That is, all four treatment groups fared significantly better in terms of survival than did the control group. However, survival duration was significantly better in the ABLC-plus-EDTA treatment group rats than in the rats in either the AMB treatment group or the EDTA treatment group. In addition, the survival duration in the ABLC-plus-EDTA group showed a trend toward being significantly longer than that of the ABLC group. The EDTA-treated group was comparable to the AMB-treated group in terms of showing a significantly prolonged survival compared with that of the control group (P
0.02 and 0.03, respectively).
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FIG. 1. Percentages of survival of immunosuppressed rats infected with A. fumigatus. Treatments were started 18 h after the inoculation and continued for 7 days.
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FIG. 2. Sections of lungs from immunosuppressed rats infected with invasive A. fumigatus (Grocott-Gomori methenamine-silver nitrate stain; magnification, x100). (A) Untreated lung tissue from a control infected rat showing severe granulomatous pneumonia and large numbers of branching fungal hyphae; (B) lung tissue from an infected rat treated for 7 days with EDTA alone showing mild consolidation of the parenchyma and accumulation of fungal hyphae; (C) lung tissue from an infected rat treated for 7 days with ABLC alone showing mild pneumonia and an abundance of organisms; (D) lung tissue from an infected rat treated for 7 days with ABLC plus EDTA showing no significant lesions and no organisms.
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In this invasive-pulmonary-aspergillosis model, the rats were maintained in a neutropenic state with cyclophosphamide and cortisone acetate for at least 2 weeks in order to closely mimic the clinical situation that is typically so difficult to treat. In this lethal invasive-pulmonary-infection model, the majority of untreated rats died by day 6 after inoculation. However, all the rats treated with EDTA, AMB, or ABLC alone showed longer survival times (Fig. 1) and a lower tissue burden of Aspergillus organisms (Table 1) than did the untreated controls. In addition, the combination of EDTA plus ABLC resulted in longer survival than did either agent alone. Similarly, the combination treatment resulted in a reduced tissue fungal burden and minimal angioinvasion, as shown by histopathologic studies (Fig. 2). These findings suggest that the two drugs have an additive effect and that concurrent therapies with EDTA and ABLC may be of value in patients with hematologic malignancies, especially in patients with persistent neutropenia.
In a recent study that included 232 hematologic malignancy patients with invasive aspergillosis treated at our institution with either ABLC or amphotericin B liposome (AmBisome) as single agents, results showed a response rate of only 11% in patients who received 7 days of therapy, and 89% to 97% of the patients died with aspergillosis in 12 weeks (R. Hachem et al., 43rd Ann. Meet. Infect. Dis. Soc. Am., abstr. 743, 2005). Novel antifungal agents such as echinocandins (caspofungin) and azoles (voriconazole) have recently been introduced and approved for the treatment of invasive aspergillosis in high-risk patients. In two studies by Herbrecht and Maertens (6, 8), the response rate of neutropenic patients with invasive aspergillosis to either caspofungin or voriconazole used as a single-agent therapy did not exceed 30% (6, 8). Furthermore, a recent study from our institution showed that the combination of the lipid formulation of amphotericin B and caspofungin is of limited benefit in improving the outcome of invasive aspergillosis in patients with hematologic malignancy (7). Hence, the results of the current animal study could have substantial clinical implications related to the improvement of the response of invasive aspergillosis to amphotericin B lipid formulation through the addition of EDTA as an antifungal-enhancing agent.
The current study is the first study to show the beneficial in vivo activity of EDTA against invasive pulmonary aspergillosis. Our study is also unique in showing the enhanced activity of a conventional antifungal agent such as ABLC in combination with EDTA. In terms of the mechanism of action of EDTA, EDTA likely inhibits aspergillus growth and causes fungal death by competing with aspergillus siderophores for any of the trace iron and calcium ions that are essential to the maintenance of the life cycle of fungi. This inhibitory effect of EDTA was clearly demonstrated in our study by the test animals' prolonged survival, the reduced tissue fungal burden, and the decreased angioinvasion noted in histopathologic preparations seen in all treatment groups but especially in the group receiving both EDTA and ABLC. Wei and Bobek recently demonstrated that EDTA enhanced the antifungal activity in human salivary mucin peptides mainly by chelating the divalent cations (such as Ca+) (18). In another study, Sen et al. showed that EDTA demonstrates the highest antifungal activity against Candida albicans, compared with those of routine antifungal drugs (15).
To verify the safety of EDTA, we randomly checked the serum calcium and creatinine levels at different times during therapy. Our observations of the rats and the data in Table 2 are consistent with an absence of significant direct toxic effects of EDTA. All of the tested animals showed normal levels. Furthermore, no tissue damage was seen in the histopathologic specimens that could be attributed to the EDTA. The safety of EDTA was also assessed with a rat model by Sanchez-Fructuoso et al. (14), who similarly found that EDTA at doses equivalent to those used in clinical practice appeared to slow the progression of the lesions caused by chronic lead poisoning without causing the damage that can occur with chelating agents.
Our infection model was comparable to other animal models that have been used to show the beneficial activity of ABLC, echinocandin, or a triazole in comparisons with untreated control animals (4, 10, 13, 16). However, in our study, we used a novel approach in which we added the chelator EDTA to potentiate the activity of the antifungal agent being administered.
We concluded from our findings that the combination of EDTA and ABLC given once daily for 7 days is the optimal treatment in this animal model. In addition, the combination therapy showed greater antifungal activities than did AMB, EDTA, or ABLC alone. Further studies are needed to more fully examine the safety, tolerance, efficacy, and optimal dosing of EDTA in the treatment of various fungal infections.
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