Previous Article | Next Article ![]()
Antimicrobial Agents and Chemotherapy, February 2003, p. 524-528, Vol. 47, No. 2
0066-4804/03/$08.00+0 DOI: 10.1128/AAC.47.2.524-528.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Department of Pharmacology, Oklahoma State University Center for Health Sciences, Tulsa, Oklahoma 74107-1898
Received 5 September 2002/ Returned for modification 11 October 2002/ Accepted 31 October 2002
|
|
|---|
|
|
|---|
Mortality among patients with primary amebic meningoencephalitis is greater than 95% (4). This grave prognosis is due to the rapid progression of the disease, the often delayed diagnosis, and the lack of effective therapeutic agents. Since primary amebic meningoencephalitis was identified, a wide range of therapeutic agents against N. fowleri have been evaluated, including many antifungal, antiprotozoal, antibacterial, and antipsychotic agents. Most of these drugs were determined to have little or no efficacy against N. fowleri, either in vitro or in vivo (5-8, 22).
Of the drugs that have been evaluated against N. fowleri, amphotericin B, an antifungal drug, is the only agent with established clinical efficacy. Studies have demonstrated the in vitro and in vivo activity of amphotericin B against various strains of N. fowleri (5-8, 13, 17, 19, 20, 21), and at least seven patients with primary amebic meningoencephalitis have been successfully treated with amphotericin B alone or in combination with other drugs (1, 2, 14, 16, 19, 25).
Although amphotericin B remains the first choice for treatment of primary amebic meningoencephalitis, its use is frequently associated with renal toxicity, manifested as azotemia and hypokalemia. Amphotericin B often causes anemia, and many patients experience chills, fever, nausea, vomiting, and headache. Moreover, not all patients treated with amphotericin B have survived primary amebic meningoencephalitis (21). Therefore, we investigated the activity of newer antimicrobial agents versus N. fowleri in vitro and in a mouse model of primary amebic meningoencephalitis in order to identify other potentially useful agents for treating this infection in humans. Azithromycin, a macrolide antibiotic, was selected for this study on the basis of previous reports that described the in vitro sensitivity of Acanthamoeba spp. to this drug (18) and its activity in experimental toxoplasmosis (3).
This report presents the results of our in vitro and in vivo studies of azithromycin versus N. fowleri and a comparison of its activity with that of amphotericin B.
|
|
|---|
Therapeutic agents. Amphotericin B aqueous solution, 250 µg/ml (Sigma-Aldrich Inc., St. Louis, Mo.), was diluted with sterile deionized water to obtain the final concentrations used for in vitro studies. For in vivo studies, amphotericin B powder, consisting of 45% amphotericin B, 35% deoxycholic acid sodium, and 20% sodium phosphate (Sigma-Aldrich Inc.), was dissolved in sterile deionized water to provide the amphotericin B concentrations needed to administer the indicated doses to mice. Azithromycin for injection (Zithromax; Pfizer Inc., New York, N.Y.), consisting of powdered azithromycin dihydrate, was dissolved in and diluted with sterile deionized water to provide the concentrations and doses employed in this study.
In vitro studies. For each drug study, 30 ml of Mix medium was inoculated with 104 amebae/ml from 72-h stock cultures. An aliquot of diluted amphotericin B or azithromycin solution was added to experimental flasks to obtain the required drug concentrations, while control flasks received the same 0.12-ml volume of deionized water. Agents were tested at three concentrations, with each concentration tested in triplicate. Amphotericin B was serially diluted to provide final concentrations of 0.01, 0.1, and 1 µg/ml, while azithromycin was diluted to final concentrations of 1, 10, and 100 µg/ml.
The flasks containing amebae and an experimental agent were Vortex shaken, and 10-ml aliquots were distributed to three culture flasks that were incubated at 37°C. Cell growth was determined daily for a period of 96 h and then again at 168 h with a Coulter counter (model ZF; Coulter Electronics, Inc., Hialeah, Fla.). A 0.2-ml aliquot of each cell suspension was added to 9.8 ml of electrolyte solution consisting of 0.5% (vol/vol) formalin and 0.4% (wt/vol) NaCl in deionized water. Cuvettes were Vortex shaken to separate cell aggregates and then read within 5 min. Four successive counts were obtained for each cuvette. The most deviant count for each cuvette was excluded, and the mean of the remaining nine counts (three flasks of three counts each) was determined, and ameba growth was expressed as amebas per milliliter. Ameba concentrations were compared with Student's t test to determine whether differences between control and treated groups were significant. The MIC of each agent was defined as the lowest concentration that significantly (P < 0.05) inhibited ameba growth compared to control cultures throughout the 7-day culture period as determined by Student's t test.
In vivo studies: cell harvesting and inoculation. Amebae were harvested for mouse inoculations after 72 h of culture in Mix medium at 37°C. The amebae were centrifuged at 2,000 x g for 10 min, washed, and resuspended in Page saline to provide a final concentration of 2 x 106 amebae/ml. Male 21-day-old CD-1 mice weighing approximately 23 g (Charles River Laboratories, Inc., Wilmington, Mass.) were used in all in vivo experiments. Prior to experimentation, mice were housed in plastic cages for 3 days and given free access to food and water. Mice were inoculated by intranasal instillation of 10 µl of Page saline containing 2 x 104 amebae into a single naris under isoflurane anesthesia (AErrane; Baxter Caribe Inc., Deerfield, Ill.). All animal studies were conducted in accordance with standard animal experimentation guidelines and with the approval of the Animal Care and Use Committee at the Oklahoma State University Center for Health Sciences.
Treatment of experimental amebic meningoencephalitis. Experimental drug treatments began 72 h after inoculation of amebae and continued once daily for 5 days. For each set of experiments, 40 mice were randomly divided into four groups of 10 mice, with each group receiving a different treatment. The control group received daily 0.1-ml intraperitoneal injections of 0.9% sodium chloride injection (Abbott Laboratories, Chicago, Ill.). The treated groups received 0.1-ml intraperitoneal injections containing the active drug in sufficient concentration to provide the specified dosages of amphotericin B (2.5 or 7.5 mg/kg) or azithromycin (25 or 75 mg/kg). Doses represent the quantity of amphotericin B or azithromycin base administered to mice per kilogram of body weight.
Mortality and mean time to death. Mice were held for 28 days after inoculation, and the cumulative percent dead was recorded on a daily basis. The mean time to death was also determined for each treatment group. In order to verify the cause of death, brain tissue from dead mice was cultured for amebae with 10 ml of Mix medium containing penicillin (500 U/ml) and streptomycin (500 µg/ml) in 25-cm2 polystyrene culture flasks incubated at 37°C. Amebae were observed microscopically in cultures obtained from brain tissue of all infected mice that died during the 28-day observation period.
Statistical analysis. The statistical significance of differences between ameba concentrations in treated and control cultures was determined with Student's two-sample t test. The mean time to death of treated and control mice was compared with the Mann-Whitney U test.
|
|
|---|
![]() View larger version (17K): [in a new window] |
FIG. 1. Effect of amphotericin B (A) and azithromycin (B) on in vitro growth of the Lee (M67) strain of N. fowleri grown in Mix medium at 37°C. Values are means + standard errors of two experiments performed in triplicate. Error bars that are not visible are smaller than the symbols. Student's t test was used to compare means for treated and control samples at 48 and 168 h. **, significantly different from controls, P < 0.01.
|
In vivo studies. Amphotericin B and azithromycin were evaluated for their therapeutic effectiveness against the Lee (M67) strain of N. fowleri in a mouse model of primary amebic meningoencephalitis (15). Preliminary experiments were performed to determine the inoculum that would produce 100% mortality in untreated mice. An inoculum of 104 amebae per mouse produced 80% mortality, whereas an inoculum of 2 x 104 amebae produced 100% mortality and was used in all experimental drug studies.
The doses of amphotericin B used in this study were based on previous studies with N. fowleri (5, 9, 23). In our experiments, amphotericin B at 2.5 mg/kg/day protected 30% of mice, with a mean time to death of 7.0 days. A higher dose, 7.5 mg/kg/day, resulted in 50% survival and a mean time to death of 7.8 days (Fig. 2). All untreated control mice died, with a mean time to death of 8.1 days. The mean time to death of amphotericin B-treated mice was not significantly different from that of controls.
![]() View larger version (17K): [in a new window] |
FIG. 2. Survival of mice after inoculation with 2 x 104 N. fowleri, followed by treatment with amphotericin B (A) or azithromycin (B) injected once daily on days to 4 to 8 after infection. Control mice received sterile saline. There were 10 animals per group.
|
|
|
|---|
Amphotericin B is the most potent agent showing activity against N. fowleri, with a MIC ranging from 0.018 to 1.0 µg/ml in various studies (5, 7, 8, 13, 17, 20, 21). The MIC of amphotericin B in our studies (0.1 µg/ml) is close to the median of values reported by other investigators. We found that azithromycin was more effective than amphotericin B in experimental primary amebic meningoencephalitis despite having an MIC that was approximately 123 times greater than that of amphotericin B. The greater in vivo efficacy of azithromycin may be related to the unique pharmacokinetics of this drug, which include a long elimination half-life and high tissue levels (10). Amphotericin B exhibits poor penetration of the blood-brain barrier, whereas azithromycin was widely distributed into human brain tissue following systemic administration to humans (11).
The doses of azithromycin and amphotericin B used in this study were the same doses used by other investigators in experimental primary amebic meningoencephalitis (amphotericin B) or toxoplasmosis (azithromycin). The amphotericin B doses used in mice (2.5 and 7.5 mg/kg/day) are larger than the doses used in the treatment of primary amebic meningoencephalitis in humans (1 to 1.5 mg/kg/day), as reported by John (12). However, Carter (5) stated that the high dose of amphotericin B (7.5 mg/kg) is lower than that required to protect mice from many fungal infections. Based on body surface area rather than weight, this dose is comparable to that customarily used in humans, and the level of amphotericin B in mouse serum with this dose (1.2 µg/ml) is much the same as found in human subjects receiving the usual intravenous doses (5). Carter also showed that 100% of mice survived treatment with amphotericin B at 7.5 mg/kg/day for 10 days. Nevertheless, the high doses of amphotericin B employed in our study may potentially cause kidney toxicity or even contribute to lethality in mice with experimental primary amebic meningoencephalitis.
Likewise, the doses of azithromycin employed in experimental toxoplasmosis (25 to 200 mg/kg) are larger than those required for the successful treatment of human infections. It was reported that 100% of mice with experimental toxoplasmosis survived treatment with 200 mg of azithromycin per kg for 10 days (3). We did not observe any obvious toxicity in mice treated with azithromycin at 75 mg/kg for 5 days.
The protocol used in our mouse model of primary amebic meningoencephalitis employed treatments that began 72 h after inoculation of mice with N. fowleri, whereas most investigators began drug therapy prior to or immediately after inoculation (5, 9, 24). We believe that our protocol more closely resembles the clinical setting, in which patients do not present with severe symptoms until several days after exposure to N. fowleri. This model is therefore more likely to indicate which therapeutic agents might be effective against primary amebic meningoencephalitis in a clinical setting. The only other study of delayed treatment of experimental primary amebic meningoencephalitis found that 37.5% of animals survived after treatment with amphotericin B at 2.5 mg/kg/day for 7 days, which is similar to the 30% survival obtained in our study (23).
Further studies are needed to determine whether azithromycin demonstrates additive, synergistic, or antagonistic effects in combination with amphotericin B in mouse models. Little is known about the mechanisms of action of amphotericin B or other drugs against N. fowleri. Azithromycin inhibits bacterial protein synthesis by binding to the bacterial 50S ribosomal subunit and blocking peptide bond formation and translocation, but its mechanisms of action in Acanthamoeba, Naegleria, and Toxoplasma spp. have not been identified and deserve investigation.
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»