Comparative activities of eight quinolones against members of the Bacteroides fragilis group.

The in vitro activities of five new quinolones (clinafloxacin [CI-960 or PD-127391], BAY Y 3118, E-4868, E-5065, and E-5068) against 100 Bacteroides fragilis group bacterial isolates were compared with those of ciprofloxacin, ofloxacin, and sparfloxacin. Overall, E-5068 was the most active in vitro (MIC for 90% of isolates tested [MIC90], 0.25 microgram/ml); this was followed by clinafloxacin and BAY Y 3118 (MIC90, 0.5 microgram/ml). Ciprofloxacin, sparfloxacin, and ofloxacin were the least active (MIC90s, 64, 16, and 16 micrograms/ml, respectively). B. fragilis and Bacteroides caccae were more susceptible than the other members of the B. fragilis group to all of the quinolones tested.

fragilis and Bacteroides caccae were more susceptible than the other members of the B. fragilis group to all of the quinolones tested.
The most important and most frequently isolated anaerobes from clinical infections are those belonging to the Bacteroides fragilis group. In the past few years, several reports have F emphasized the increasing incidence of resistance of Bacte-C H roides species to the currently available antimicrobial agents (1, 3,0 7,9). Consequently, a need has arisen for new, safe, and effective antimicrobial agents that can be used in the treatment of infections caused by anaerobes. H The recent resurgence in the clinical use of quinolones in the 2 treatment of bacterial infections stems from the development of new fluorinated compounds which exhibit both greater and broader ranges of antibacterial activity. These two characteristics allow the use of a single agent in the treatment of E4868 F infections when a mixed bacterial population is present.
In the work described here, a comparison was made between the in vitro activities of ciprofloxacin and ofloxacin and those of sparfloxacin (a difluoroquinolone with a 7 dimethyl-piperazinyl substituted group), BAY Y 3118 and clinafloxacin (monofluo-F roquinolones with new substituents at positions 7 and 8), and H three monoor difluorinated naphthyridine derivatives (E-4868, E-5065, and E-5068), which are characterized chemically by the presence at position 7 of an azetidin ring with different C'-3 substituents (Fig. 1), against 100 B. fragilis group strains in Cl order to assess their antianaerobic activities.
The 100 anaerobic bacteria studied were isolated from the same number of patients at Seville University Hospital during E-5065 1992 and 1993 and were identified by standard criteria (14).
These included 31 B. fragilis, 12 Bacteroides caccae, 16 Bacteroides distasonis, 13 (17) and Barry et al. (4). Plates containing serial doubling dilutions of antimicrobial agents ranging from 0.06 to 128 ,ug/ml were inoculated with a Steer's replicator to give a final inoculum of 105 CFU. They were incubated in an anaerobic chamber (Forma Scientific) for 48 h at 37°C. The MIC was defined as the lowest concentration of drug that inhibited growth. The appearance of a single colony or a barely visible haze was disregarded (17).
The in vitro activities of the quinolones against members of the B. fragilis group are given in Table 1 Since the introduction of the fluoroquinolones in 1982 (15), innovative developments have yielded several antimicrobial agents with increased activities and broader antibacterial spectra. However, those in clinical use, such as ciprofloxacin or ofloxacin, have MICs for anaerobic organisms that approach or exceed the achievable levels of those agents in human serum (6,19).
Because ciprofloxacin is widely used clinically, many recent studies of new fluoroquinolones have used it as a reference agent (3,12) and, as in the present work, have found its in vitro activity against B. fragilis to be very poor.
Ofloxacin possesses an oxazine ring that results in slightly improved activity against anaerobic organisms (8,12). Some investigators have found that strains of B. fragilis were susceptible to ofloxacin (MIC90, 4 ,ug/ml) but that most other species of the B. fragilis group were resistant to this agent. In the current study, it was found that the MIC90 of ofloxacin for B. fragilis was 4 ,ug/ml; the ofloxacin MICs for other species tested in the present study were greater.
Our results, that sparfloxacin is more active than ofloxacin, with overall MIC90s of 4 ,ug/ml, agree with those of Barry and Fuchs (3) and Goldstein and Citron (12). It should be noted that, in our study, the MIC90 of sparfloxacin for B. thetaiotaomicron was 16 times greater (32 ,ug/ml) than those observed for B. caccae and B. fragilis (MIC90s, 2 ,ug/ml).
As reported previously (3,4,8), clinafloxacin has very good activity against anaerobes. King et al. (16) and Goldstein and Citron (13) found interspecies differences, as we did, since the MIC90 of clinafloxacin for B. vulgatus was 4 ,ug/ml, or 33 times higher than that for B. caccae (0.12 ,ug/ml). BAY Y 3118 had the same favorable activity as clinafloxacin against the B. fragilis group, with both compounds having overall MIC90s of 0.5 ,ug/ml, and interspecies differences were observed. These results are in agreement with those of other investigators (5,10,18), even though such interspecies differences were not reported. Clinafloxacin resembles BAY Y 3118 very closely in terms of its potency and range of activity against B. fragilis group strains. Both agents also have identical structures except for the moiety at position 7. The Esteve compounds showed different activities against the members of the B. fragilis group. E-4868 was the least active of the three, with an overall MIC90 greater than that of sparfloxacin. E-5065 had an MIC90 of 2 p,g/ml, and E-5068 had an MIC90 of 0.25 ,ug/ml for all 100 strains. Barrett et al. (2) reported an E-4868 MIC90 for the B. fragilis group of 4 ,ug/ml. The activities of these antimicrobial agents against the different species of the B. fragilis group also varied in our study.
Thus, E-4868 had an MIC90s of 4 ,ug/ml for B. caccae and B. thetaiotaomicron and 8 ,g/ml for B. fragilis and B. ovatus. B. vulgatus and B. distasonis proved to be the most resistant; the MIC90 of E-4868 was 16 ,ug/ml. E-5065 was most active against B. thetaiotoaomicron (MIC90, 0.5 ,ug/ml); this was followed by its activity against B. fragilis and B. caccae (MIC90s, 1 jig/ml) and B. vulgatus and B. distasonis (MIC90s, 4 ,ug/ml). E-5068 was the most active of the three drugs, with MIC90s of between 0.12 and 4 ,ug/ml. The most striking feature was the difference between the MIC90s of E-5068 for B. caccae and B. fragilis (0.12 ,ug/ml) and that for B. vulgatus (4 ,ug/ml). The same phenomenon was observed with clinafloxacin.
Gargallo-Viola et al (11) studied different azetidin derivatives from the Esteve Laboratories and demonstrated that two of them, E-4502 and E-4500, displayed considerable activity against anaerobes. The differences in the activities of the compounds tested in the present work may be due to the different C'-3 substituents of the azetidin group at position 7 of the molecule.
A comparison of the MIC90s of the eight compounds that we studied shows the enhanced activities that were achieved with E-5068, BAY Y 3118, and clinafloxacin in comparison with those of ciprofloxacin and ofloxacin against members of the B. fragilis group. Their very low MICs for these common anaerobic pathogens suggest that they may be useful for the treatment of infections caused by these organisms; anaerobes have previously represented a significant gap in the spectra of the currently available quinolones. The increased levels of activity of these novel compounds suggest that they will have a role to play in the treatment of mixed and anaerobic infections.
Their therapeutic usefulness will depend on the results of additional in vitro susceptibility studies involving a larger number of strains, pharmacokinetic and toxicological studies, and clinical trials.