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Antimicrobial Agents and Chemotherapy, January 2009, p. 295-297, Vol. 53, No. 1
0066-4804/09/$08.00+0 doi:10.1128/AAC.01023-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Servicio de Dermatología, Hospital Universitario José E. González, Monterrey, NL,1 Departamento de Química Analítica, Facultad de Medicina, UANL, Monterrey, NL,2 Centro de Investigación Biomédica del Noreste, IMSS, Monterrey, NL, México3
Received 31 July 2008/ Returned for modification 2 September 2008/ Accepted 7 October 2008
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(The work presented here was performed in part to fulfill the requirements for the Doctor of Medicine degree [Facultad de Medicina, Universidad Autónoma de Nuevo León] of B.E.C.-M.)
For in vitro susceptibility assays with CIP, we used a previously described broth microdilution method (9) that is based on Clinical and Laboratory Standards Institute (CLSI; formerly the NCCLS) document A-4 to evaluate 30 isolates of N. brasiliensis.
In order to determine how many bacteria are naturally resistant to the quinolones, we calculated the mutation rate of N. brasiliensis HUJEG-1 with the p0 method (5, 7) using Mueller-Hinton medium containing 1x, 4x, and 8x the MIC for each antimicrobial adjusted to contain 109 CFU per plate.
To quantitate the quinolone plasma levels in mice, we injected 8- to 12-week-old female BALB/c mice subcutaneously (s.c.) with CIP at 12.5 and 25 mg/kg and with MXF at 12.5 and 25 mg/kg as previously described (3). SXT was administered by gavage at 50 mg/kg, and blood samples were taken at 0, 20 40 60, 120, 240, 360, 480, and 600 min. SXT was also given to mice after the compound was suspended in the drinking water, and blood samples were obtained at 0, 3, 6, 9, and 12 h. The concentrations of MXF, CIP, and SXT were analyzed by using a high-pressure liquid chromatography method developed in our laboratory.
Experimental mycetomas were produced in 8- to 12-week-old female BALB/c mice using N. brasiliensis HUJEG-1 (3, 4). One week later, therapy was started. Groups of 15 animals were tested. One group of animals was injected with saline solution as a negative control. The rest were treated with MXF at 25 mg/kg s.c. three times a day; with CIP at 25 mg/kg s.c. three times a day; with MXF at 25 mg/kg s.c. three times a day plus SXT at 50 mg/kg in the drinking water twice a day; or with SXT alone at 50 mg/kg in the drinking water twice a day. The degree of infection was determined after 9 weeks (three cycles of 3 weeks of treatment and 1 week of rest) as previously described (3). Two independent evaluators scored the development of lesions, and potential differences among the groups against a control inoculated with saline solution were established by using an analysis of variance test.
The MIC range for CIP was 4 to 64 µg/ml, and the MIC50 and MIC90 were 4 and 64 µg/ml, respectively. The MICs of MXF, CIP, and SXT for N. brasiliensis HUJEG-1 were 0.25, 4, and 2.3/0.12 µg/ml, respectively.
The frequency of the in vitro appearance of antimicrobial-resistant mutants is presented in Table 1. The mutation rate among quinolones was about the same (10–9) for all quinolones at high concentrations of the antimicrobials. The addition of SXT did not modify this value significantly. Some colonies growing in the plates with a high concentration of quinolones (8x) were subcultured, and the MICs were retested. In the case of CIP, a 16-fold increase in the MIC was observed (from 4 to 64 µg/ml); the MIC of MXF increased 64-fold (from 0.25 to 16 µg/ml).
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TABLE 1. Mutation rate of N. brasiliensis HUJEG-1 in response to several quinolones and SXT
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FIG. 1. Plasma levels produced in BALB/c mice after s.c. injection of MXF (left) at 50 and 12.5 mg/kg and of CIP (right) at the same doses. Three mice were bled and sacrificed at each point. Bars represent the standard deviation.
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FIG. 2. Sulfamethoxazole plasma levels produced in BALB/c mice after the application of SXT by gavage (left) and in drinking water (right) in both cases at 50 mg/kg. Three mice were bled and sacrificed at each point. Bars represent the standard deviation.
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FIG. 3. Effect of CIP, MXF, SXT, and the combination MXF-SXT in the development of mycetoma lesions in BALB/c mice infected with N. brasiliensis HUJEG-1. According to the analysis of variance test, significant differences were found for treatment with MXF, SXT, and MXF-SXT, with P values of 0.017, 0.006, and 0.007, respectively.
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Gatifloxacin has been shown to be active in vivo against N. brasiliensis (3). However, since there have been reports of dislycemia with this fluoroquinolone (11), it will be important to properly select young patients without diabetes before using it is used to treat actinomycetoma cases.
MXF has excellent plasma levels in humans, with a maximum plasma concentration of 6.13 at a dose of 400 mg daily and plasma levels greater than 1 µg/ml for about 8 h (6), and it has low toxicity in long-term applications. According to the results obtained here we feel MXF may be useful in the treatment of actinomycetoma using 400 mg every 12 h to keep the plasma levels greater than the MIC and to avoid the selection of naturally resistant bacteria.
Published ahead of print on 13 October 2008. ![]()
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