Activities of Garenoxacin (BMS-284756) and Other Agents against Anaerobic Clinical Isolates

A total of 590 clinical isolates consisting of 33 species ofboth gram-positive and gram-negative anaerobes were collectedfrom nine centers in the Chicago area in 1998-1999. The largestnumber of isolates (330 isolates, 56%) belonged to the Bacteroidesgroup. Isolates were tested by agar dilution against garenoxacin(BMS-284756, T-3811 ME), trovafloxacin, moxifloxacin, clindamycin,imipenem, piperacillin-tazobactam, and cefoxitin. All but onespecies (2% of Bacteroides vulgatus isolates) were fully susceptibleto piperacillin-tazobactam and imipenem. A number of specieswere resistant to clindamycin. Among the fluoroquinolones, garenoxacinand trovafloxacin had an MIC at which 90% of the isolates testedwere inhibited of <4 µg/ml for all but two species(Fusobacterium mortiferum/varium and Peptostreptococcus anaerobius).

INTRODUCTION

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Introduction

Anaerobic bacteria cause a variety of infections, in particular,abdominal surgical wound infections and periodontal disease.They also play a role in diabetic foot infections and gynecologicand obstetric infections caused by mixed aerobic and anaerobicbacteria (1). Therapeutic regimens include cephalosporins, aminoglycosides,quinolones, metronidazole, or clindamycin, depending on thesource of the infection (13). However, several species are becomingincreasingly resistant to cephalosporins due to the acquisitionof beta-lactamases. A number of Prevotella species associatedwith periodontal disease have been found to be beta-lactamasepositive (10). In a study of fecal anaerobic strains, resistanceto ampicillin, cefoxitin, or cefuroxime was found in 70% ofstrains, with 19% showing multiple resistance to both ampicillinand cefuroxime (25). Resistance to clindamycin is also increasing(6), and as many as 20 to 40% of Bacteroides fragilis strainsare clindamycin resistant (2, 14). Although metronidazole iseffective against gram-negative anaerobes, 31% of gram-positiveanaerobes have been reported to be resistant (24).

Newer respiratory quinolones such as gatifloxacin, moxifloxacin,trovafloxacin, clinafloxacin, and sitafloxacin show better antianaerobicactivity than older quinolone compounds such as ciprofloxacin(3, 7, 27). Garenoxacin (BMS-284756, T-3811 ME) is a novel des-F(6)-quinolonethat lacks the 6-position fluorine but retains broad-spectrumactivity against gram-positive and gram-negative pathogens aswell as anaerobes (12, 21). It has equivalent activity to trovafloxacinagainst gram-negative anaerobes and often superior activityagainst gram-positive anaerobes (21). It also has activity againstanaerobes overall that is superior to those of levofloxacinand moxifloxacin (15).

In this study, we determined the in vitro activity of garenoxacinand compared it with those of trovafloxacin, moxifloxacin, imipenem,clindamycin, piperacillin-tazobactam, and cefoxitin against33 species of gram-positive and gram-negative anaerobic bacteriacomprising 590 recent clinical isolates obtained from the Chicago,Ill., area.

MATERIALS AND METHODS

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Materials and Methods

Bacterial strains. Clinical isolates (590) collected from 1998 to 1999 from ninemedical centers in the Chicago area were selected for testingand frozen in double skim milk. Selection was from among morethan 1,300 isolates collected during that time period and basedupon species distribution for the purposes of this study. Sitesof infection for isolates tested (when known) included intra-abdominal,pelvic, wound, blood, respiratory, and oral sites. Identificationswere performed by using ANA IDENT II with additional biochemicaltesting as needed.

Agar dilution susceptibility testing. The NCCLS-recommended reference agar dilution method for anaerobes(M11-A4) was used for susceptibility testing (19). Brucellaagar supplemented with blood, hemin, and vitamin K₁ was thetest medium. Each inoculum was prepared and standardized byusing a Vitek colorimeter to deliver approximately 10⁵ CFU/spot.All antibiotic-containing plates were prepared and tested onthe same day. The antimicrobials tested included trovafloxacin(0.015 to 64 µg/ml), moxifloxacin (0.015 to 64 µg/ml),garenoxacin (0.015 to 64 µg/ml), imipenem (0.015 to 64µg/ml), clindamycin (0.015 to 64 µg/ml), piperacillin-tazobactam(0.03 to 128 µg/ml), and cefoxitin (0.03 to 128 µg/ml).Antibiotics were obtained from their respective manufacturersas follows: garenoxacin from Bristol Myers Squibb (New York,N.Y.), trovafloxacin from Pfizer Pharmaceuticals (New York,N.Y.), moxifloxacin from Bayer (West Haven, Conn.), imipenemand cefoxitin from Merck (West Point, Pa.), piperacillin-tazobactamfrom Wyeth Ayerst (St. Davids, Pa.), and clindamycin from Pharmacia-Upjohn(Kalamazoo, Mich.). The interpretation of endpoints was doneaccording to NCCLS guideline M11-A4 (19).

RESULTS

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Results

The 590 selected isolates tested in this study belonged to 33species; 56 (9%) were gram-positive anaerobes and the remainderwere gram-negative, with Bacteroides spp. comprising 56% ofthe latter. Among the 303 isolates selected from the Bacteroidesgroup, 103 (34%) were B. fragilis and 200 (66%) were from othermembers of the B. fragilis group. Of the compounds tested, piperacillin-tazobactam,imipenem, garenoxacin, and trovafloxacin demonstrated good activityagainst almost all species (Table 1). Only 2% of isolates, allbelonging to Bacteroides vulgatus species, were highly resistantto piperacillin-tazobactam and imipenem. Garenoxacin demonstratedexcellent in vitro activity against all isolates, based uponthe MICs at which 50% of the isolates tested were inhibited(MIC₅₀s) and MIC₉₀s. The MIC₉₀s for only two species, Fusobacteriummortiferum/varium and Peptostreptococcusanaerobius, were >4µg/ml. Garenoxacin activity was most similar to that oftrovafloxacin, and it was somewhat more active (according toMIC₉₀s) than moxifloxacin. Based on the recommended NCCLS susceptibilitybreakpoint for trovafloxacin ( $<=$ 2 µg/ml), garenoxacin demonstratedsomewhat greater activity for four members of the B. fragilisgroup (B. fragilis, Bacteroidesthetaiotaomicron, B. vulgatus,and Bacteroidesuniformis) than did trovafloxacin and for allmembers of the B. fragilis group than did moxifloxacin. Fornon-B. fragilis group anaerobes, the activities of all threequinolones were excellent and very similar. Clindamycin demonstratedreduced activity against a number of species, notably thosebelonging to the Bacteroides fragilis group (Table 1). Resistancesof B. fragilis isolates and non-B. fragilis members of the B.fragilis group to clindamycin were 23 and 33%, respectively.Seventy-seven and 64% of B. fragilis and non-B. fragilis membersof the B. fragilis group, respectively, were fully susceptibleto clindamycin. Cefoxitin resistances among B. fragilis andnon-B. fragilis members of the B. fragilis group were only 1and 7%, respectively. However, while 95% of B. fragilis isolateswere fully susceptible to cefoxitin, only 64% of the non-B.fragilis members of the B. fragilis group were fully susceptibleat the breakpoint of $<=$ 16 µg/ml.

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TABLE 1. Ranges, MIC₅₀s, and MIC₉₀s for garenoxacin and other agents against 590 anaerobic isolates

The species with the largest number of isolates (n = 103) testedwas B. fragilis. Piperacillin-tazobactam had the best activityagainst B. fragilis, with all isolates showing susceptibilityto this compound (Table 2). Imipenem with an MIC₉₀ of 0.25 µg/mlalso showed good activity. Among the quinolones, garenoxacinhad a fourfold-lower MIC₉₀ than either trovafloxacin or moxifloxacin.A single isolate showed nonsusceptibility to garenoxacin, whereaswith trovafloxacin and moxifloxacin, 18 and 21 isolates, respectively,were nonsusceptible (Table 2).

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TABLE 2. MIC distribution of antimicrobials tested against B. fragilis (n = 103)

The second-largest group of isolates belonging to a single specieswere Clostridium perfringens strains (n = 40). Most compoundsshowed good activity against these isolates, and no resistantstrains were identified. Garenoxacin showed twofold-better activitythan moxifloxacin for the majority of strains tested and wasslightly less active than trovafloxacin.

DISCUSSION

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Discussion

Appropriate antimicrobial therapy, particularly in the treatmentof surgical wound infections, has been found to have a majorimpact on the outcome. The mortality rate is more than doubledin cases of inappropriate therapy (18). In patients with bacteremiacaused by Bacteroides spp., patients receiving inactive therapyhad a mortality rate of 45% compared with 16% in those receivingactive therapy and clinical failure was 82 and 22% in the twogroups, respectively (20). Consequently, the choice of an effectiveagent for treatment of both single-anaerobe infections as wellas polymicrobial infections is critical.

In this study, clindamycin showed reduced activity against anumber of species compared with other agents. Of 303 Bacteroidesspp. isolates tested, 220 (28%) showed high-level resistanceto clindamycin. Increased resistance of B. fragilis to clindamycinhas been noted in other studies, with rates of 23 and 19%, respectively,previously reported in the United States and Canada (2, 16,22). In a study of Spanish isolates, 34% were clindamycin resistant(5), and a similar rate of resistance (32%) was found in SouthAfrica (17). Resistance to cefoxitin was also noted in someof these studies, although it was low among the Bacteroidesspp. tested in this study. However, the number of isolates withreduced susceptibility to cefoxitin among non-B. fragilis membersof the B. fragilis group reflects a large number of isolatesnear the breakpoint.

Piperacillin-tazobactam, imipenem, and the quinolones showedgood activity against most species, as has been noted in otherstudies (6, 12, 15, 21, 25). Reduced activity to all of theseagents was only found among isolates of B. vulgatus and B. thetaiotaomicronspecies. Of interest, one B. thetaiotaomicron isolate was resistantto piperacillin-tazobactam (MIC = 128 µg/ml), intermediateto imipenem (MIC = 8 µg/ml), but sensitive to cefoxitin(MIC = 16 µg/ml). Similarly, one B. vulgatus isolate wasresistant to both imipenem (MIC > 64 µg/ml) and piperacillin-tazobactam(MIC = 128 µg/ml) but only intermediate to cefoxitin (MIC= 32 µg/ml). These results were confirmed upon repeattesting. Imipenem-resistant isolates that express metalloenzymesare typically resistant to all beta-lactam antibiotics. Themechanism of resistance in these two organisms has not beencharacterized but may be different from reported metalloenzymebeta-lactamases.

Among the quinolones, garenoxacin showed comparable activityto trovafloxacin and activity superior to that of moxifloxacinagainst these species, and this trend was evident for most ofthe species tested. Interestingly, garenoxacin had superioractivity to these two agents against B. fragilis strains, whichare often beta-lactamase positive (8, 23) and clindamycin resistant(2).

Garenoxacin has been shown to have good activity not only againstanaerobes but also against gram-positive and gram-negative aerobesof clinical importance (4, 9, 12, 26). It is effective in vitroagainst Staphylococcus aureus, Legionella sp., Campylobacterjejuni, and Helicobacter pylori (11, 21). Among the quinolones,it has comparable activity to trovafloxacin and activity superiorto those of other quinolones against anaerobes as demonstratedin this and other studies (15, 21). Consequently, it could proveto be a useful agent for the treatment of anaerobic and polymicrobialinfections, particularly in the nosocomial setting.

ACKNOWLEDGMENTS

This work was supported by the Bristol Myers Squibb Company.

We thank Gayatri Vedantam for assistance in data analysis. Wealso thank Joyce de Azavedo for help in preparing the manuscript.

FOOTNOTES

* Corresponding author. Mailing address: Loyola University Medical Center, 2160 S. First Ave., Maywood, IL 60153. Phone: (708) 216-3232. Fax: (708) 216-8198. E-mail: dhecht{at}lumc.edu.

REFERENCES

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