While interest in Streptococcus pneumoniae has always been high because of its role as a frequent pathogen, the increasing prevalence of penicillin-resistant isolates is of growing concern (2, 9). Rising resistance rate coupled with the failure of monotherapy for meningitis due to these isolates now warrant that empirical therapy consist of ceftriaxone and vancomycin (5, 7, 8). Trovafloxacin, a new fluoroquinolone, has displayed exceptional activity against pneumococci and penetrates well into the cerebrospinal fluid (CSF) (1, 4). Although its role in treatment of pneumococcal meningitis is currently under investigation, its synergistic potential with currently utilized therapies is unknown.
Forty-seven clinical isolates of S. pneumoniae with various susceptibilities to penicillin were studied (penicillin-susceptible, MIC of ≤0.06 μg/ml [n = 15]; intermediate, MIC of 0.1 to 1 μg/ml [n = 16]; resistant, MIC of ≥2 μg/ml [n = 16]). All tests were conducted with cation-adjusted Mueller-Hinton broth (Becton Dickinson Microbiology Systems, Cockeysville, Md.) plus 5% lysed horse blood (Remel Inc., Lenexa, Kans.) and the following agents: trovafloxcin (CP-99,219; Pfizer Laboratories, New York, N.Y.), ceftriaxone (Hoffman-LaRoche Pharmaceuticals, Nutley, N.J.), and vancomycin (Sigma Chemical Company, St. Louis, Mo.).
The MIC for each drug-organism combination was determined in duplicate by using standard microdilution methods in ambient air. Synergy studies were undertaken by using the microdilution checkerboard technique. Bacterial dilutions in the logarithmic growth phase were prepared and pipetted into microtiter trays containing concentrations of each antibiotic which ranged from four to five times below the MIC to two times above the MIC. The final inoculum size, confirmed by colony count determinations, for all experiments was 5 × 105 CFU/ml. Prepared microtiter trays were incubated at 37°C for 24 h, after which the trays were read for inhibition of bacterial growth. Fractional inhibitory concentration (FIC) indices were calculated according to the method of Eliopoulos and Moellering (3). Individual checkerboard runs were repeated five times for each isolate-drug combination, an FIC index was calculated for each, and an overall mean FIC index was determined. Synergy was defined as an FIC of ≤0.5, indifference was defined as an FIC of >0.5 to ≤4, and antagonism was defined as an FIC of >4.
The median and range of MICs of each agent are displayed in Table 1. MICs of trovafloxacin and vancomycin were unaffected by penicillin susceptibility, whereas ceftriaxone MICs increased with increasing penicillin resistance. The in vitro interactions between trovafloxacin and ceftriaxone or trovafloxacin and vancomycin against the isolates are characterized in Table 2. Synergy ws observed in 55 and 15% of the isolates treated with the trovafloxacin and ceftriaxone or trovafloxacin and vancomycin, respectively. Antagonism was not observed. In addition, synergy was observed for 13 of 17 isolates in the trovafloxacin and ceftriaxone studies when organisms were intermediate or resistant (i.e., MIC of >0.5 μg/ml) to ceftriaxone.
TABLE 1.
MICs of individual agents against penicillin-susceptible, -intermediate, and -resistant S. pneumoniae
| S. pneumoniae penicillin susceptibility (n) | Median MIC (range) (μg/ml)
|
|||
|---|---|---|---|---|
| Penicillin | Trovafloxacin | Vancomycin | Ceftriaxone | |
| Susceptible (15) | 0.045 (0.03–0.06) | 0.06 (0.015–0.125) | 0.5 (0.5–1.0) | 0.03 (0.03–0.125) |
| Intermediate (16) | 0.25 (0.125–0.5) | 0.06 (0.015–0.25) | 0.5 (0.5–1.0) | 0.06 (0.03–2.0)a |
| Resistant (16) | 2.0 (2.0–4.0) | 0.06 (0.015–0.5) | 0.5 (0.5–1.0) | 2.0 (0.06–8.0)b |
Includes three isolates resistant to ceftriaxone (two isolates with intermediate susceptibility [MIC of >0.5 to 1 μg/ml] and one resistant isolate [MIC of ≥2 μg/ml]).
Includes 14 isolates resistant to ceftriaxone (1 isolate with intermediate susceptibility [MIC of >0.5 to 1 μg/ml] and 13 resistant isolates [MIC of ≥2 μg/ml]).
TABLE 2.
In vitro interactions between trovafloxacin and ceftriaxone or vancomycin against penicillin-susceptible, -intermediate, and -resistant S. pneumoniae
| S. pneumoniae penicillin susceptibility (n) | No. (%) of isolates
|
|||
|---|---|---|---|---|
| Trovafloxacin and Ceftriaxone
|
Trovafloxacin and Vancomycin
|
|||
| Synergy | Indifference | Synergy | Indifference | |
| Susceptible (15) | 7 (47) | 8 (53) | 3 (20) | 12 (80) |
| Intermediate (16) | 7 (44) | 9 (56) | 3 (19) | 13 (81) |
| Resistant (16) | 12 (75) | 4 (25) | 1 (6) | 15 (94) |
Trovafloxacin has been shown to penetrate well into the CSF of healthy subjects, and concentrations that exceed the MICs for the most common organisms implicated in bacterial meningitis are maintained at this site (1). In addition, trovafloxacin has displayed excellent penetration and bactericidal activity within the CSF of rabbits with experimental meningitis due to a highly penicillin-resistant S. pneumoniae isolate (6). Although the role of trovafloxacin in the treatment of meningitis is currently under study, our data may be of particular clinical relevance in the treatment of meningitis caused by penicillin-resistant S. pneumoniae, since treatment options for this life-threatening disease remain limited.
Acknowledgments
We acknowledge Christina Turley for her technical assistance and Pfizer Laboratories for the support of this study.
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