In vitro activities of ceftobiprole and doripenem tested against frequently encountered aerobic and facultative Gram-positive and Gram-negative bacterial pathogens isolated from patients in Canadian hospitals in 2007

BACKGROUND: In 2008, ceftobiprole was approved by Health Canada for the treatment of patients with complicated skin and skin structure infections including diabetic foot infections; approval of ceftobiprole by the United States Food and Drug Administration is pending. Doripenem is currently under review by Health Canada and was approved by the United States Food and Drug Administration in 2007 for the treatment of patients with complicated intra-abdominal infections and complicated urinary tract infections, including pyelonephritis. OBJECTIVES: To determine the in vitro activities of ceftobiprole and doripenem using a collection of frequently isolated aerobic and facultative bacteria cultured from patient blood, urine, respiratory and wound specimens in 12 Canadian hospitals in 2007. METHODS: Isolates were tested for their susceptibility to a panel of antimicrobial agents using the Clinical and Laboratory Standards Institute broth microdilution method. RESULTS: Ceftobiprole inhibited all isolates of methicillin-resistant Staphylococcus aureus (n=385), methicillin-resistant Staphylococcus epidermidis (n=20), methicillin-susceptible S aureus (n=1095) and methicillin-susceptible S epidermidis (n=108) at a minimum inhibitory concentration (MIC) of 2 μg/mL or less; all isolates of Streptococcus pyogenes (n=105) were inhibited by ceftobiprole at 0.06 μg/mL or less. All isolates of S aureus (MIC 4 μg/mL or less) and S pyogenes (MIC 0.5 μg/mL or less) tested were susceptible to ceftobiprole. Greater than 99% of extended-spectrum beta-lactamase (ESBL)-negative Escherichia coli (n=1528) and Klebsiella pneumoniae (n=436) were susceptible to ceftobiprole (MIC 1 μg/mL or less); against other genera/species of Enterobacteriaceae, susceptibility to ceftobiprole ranged from 80.7% for Enterobacter cloacae (n=166) to 99.2% for Proteus mirabilis (n=119). Ceftobiprole was less active against Pseudomonas aeruginosa (n=633) (90% of isolates inhibited at a concentration of 32 μg/mL [MIC90]) than Enterobacteriaceae. Doripenem inhibited 90% of isolates of E coli (n=1577) and K pneumoniae (n=456), including ESBL-producing isolates (n=69), and E cloacae at a concentration of 0.06 μg/mL or less; doripenem and meropenem had MIC90s of 8 μg/mL for the isolates of P aeruginosa tested. Doripenem demonstrated in vitro activity indistinguishable from that of meropenem against Gram-positive pathogens. CONCLUSIONS: All isolates of methicillin-resistant S aureus tested were susceptible to ceftobiprole (MIC 4 μg/mL or less), differentiating it from any other currently marketed beta-lactam. Doripenem demonstrated potent activity (MIC90 0.5 μg/mL or less) against all isolates of Enterobacteriaceae tested, including ESBL-producing E coli and K pneumoniae, and as potent activity as meropenem (MIC90 8 μg/mL) against P aeruginosa. The current study demonstrated both ceftobiprole and doripenem to be promising broad-spectrum antibacterial agents.


Les activités in vitro du ceftobiprole et du doripénem évaluées contre des pathogènes bactériens gram positifs et gram négatifs aérobiques et facultatifs fréquents, isolés chez des patients d'hôpitaux canadiens en 2007
HISTORIQUE : En 2008, Santé Canada a approuvé le ceftobiprole pour le traitement des patients atteints d'infections complexes de la peau et de la structure cutanée, y compris les infections du pied chez les diabétiques.Son approbation par la Food and Drug Administration des États-Unis est en instance.Santé Canada est à évaluer le doripénem, qui a été approuvé par la Food and Drug Administration des États-Unis en 2007 pour le traitement des patients atteints d'infections intraabdominales et d'infections urinaires complexes, y compris la pyélonéphrite.OBJECTIFS : Déterminer les activités in vitro du ceftobiprole et du doripénem au moyen d'un ensemble de bactéries aérobiques et facultatives souvent isolées, cultivées dans des échantillons sanguins, urinaires, respiratoires et de plaies de 12 hôpitaux canadiens en 2007.MÉTHODOLOGIE : Les isolats ont été vérifiés pour leur susceptibilité à un groupe d'antimicrobiens au moyen de la méthode de microdilution en milieu liquide du Clinical and Laboratory Standards Institute.RÉSULTATS : Le ceftobiprole a inhibé tous les isolats de Staphylococcus aureus méthicillinorésistant (n=385), de Staphylococcus epidermidis méthicillinorésistant (n=20), de S aureus susceptible à la méthicilline (n=1 095) et de S epidermidis susceptible à la méthicilline (n=108), à une concentration minimale inhibitrice [CMI] de 2 µg/mL ou moins.Tous les isolats de Streptococcus pyogenes (n=105) étaient inhibés par le ceftobiprole à 0,06 µg/mL ou moins.Tous les isolats de S aureus (CMI de 4 µg/mL ou moins) et de S pyogenes (CMI de 0,5 µg/mL ou moins) vérifiés étaient susceptibles au ceftobiprole.Plus de 99 % des échantillons d'Escherichia coli (n=1 528) et de Klebsiella pneumoniae (n=426) négatifs à la bêta-lactamase suite page suivante C eftobiprole and doripenem are new broad-spectrum agents available, or soon to be available, to physicians in North America.In 2008, ceftobiprole was approved by Health Canada for the treatment of patients with complicated skin and skin structure infections including non-limb-threatening diabetic foot infections without concomitant osteomyelitis; approval of ceftobiprole by the United States Food and Drug Administration is pending for complicated skin and skin structure infections and nosocomial pneumonia.Doripenem is currently under review by Health Canada and was approved by the United States Food and Drug Administration in 2007 for the treatment of patients with complicated intra-abdominal infections and complicated urinary tract infections, including pyelonephritis.
Complicated skin and skin structure infections are a common occurrence in both outpatient and inpatient settings, with Staphylococcus aureus being the most frequent pathogen (1).Streptococcus pyogenes is also an important pathogen causing skin and skin structure infections as are Gram-negative bacilli, particularly Pseudomonas aeruginosa and Escherichia coli, and anaerobic bacteria for more complicated infections, including diabetic foot infections, where the etiology is frequently polymicrobial (2,3).Important to the treatment of skin and skin structure infections is the observation that the prevalence of methicillin-resistant S aureus (MRSA) is increasing.The SENTRY antimicrobial surveillance program reported an increase in methicillin resistance among North American clinical isolates of S aureus from 26.2% in 1998 to 47.4% in 2004 (3); other investigators showed that 50% of S aureus causing skin and skin structure infections were MRSA (4).Another therapeutic consideration is that many recent anti-MRSA agents with clinical indications for the treatment of skin and skin structure infections (eg, linezolid, daptomycin) have activity only against Gram-positive pathogens, making singleagent therapy incomplete for those infections harbouring Gram-negative bacteria.Hence, the broad-spectrum activity of agents such as ceftobiprole with anti-MRSA activity may allow for the use of monotherapy in situations where a combination of antibacterials may otherwise be required.
Similarly, polymicrobial infections and emerging antimicrobial resistance among many Gram-positive and Gramnegative pathogens causing intra-abdominal and complicated urinary tract infections, support the development and strategic use of broad-spectrum agents including carbapenems.Doripenem is a new carbapenem that demonstrates the favourable characteristics of other carbapenems including broadspectrum activity and beta-lactamase stability (5,6); it has in vitro activity against Gram-positive and Gram-negative human pathogens including anaerobic bacteria (5-7).Doripenem has potency against Gram-positive cocci that is most similar to that of imipenem, and an activity against Gram-negative bacteria that is most like that of meropenem (ie, two-to fourfold greater than that of imipenem) (6).
The objective of the current study was to determine the in vitro activities of ceftobiprole and doripenem using a recent collection of the most frequently isolated aerobic and facultative bacteria cultured from patient specimens by clinical microbiology laboratories in Canadian hospitals.

Bacterial isolates
Isolates were collected and tested as part of the Canadian Ward Surveillance Study (CANWARD 2007).Details regarding isolate management are provided by Zhanel et al (8) in an earlier paper in the present supplement.Testing results are provided for genera/species with 20 or more isolates; this included 2703 Gram-positive isolates and 3726 Gram-negative isolates accounting for 81.6% of the total of 7881 isolates collected by the CANWARD 2007 study.

Antimicrobial susceptibility testing
Antimicrobial susceptibility testing was performed using the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method (9).Minimum inhibitory concentrations (MICs) were interpreted according to CLSI breakpoints where available (10,11).Breakpoints approved by Health Canada are published and were used to interpret ceftobiprole MICs for S aureus, Streptococcus species other than Streptococcus pneumoniae, and Enterobacteriaceae (12).Isolates of MRSA and extended-spectrum beta-lactamase (ESBL)-producing E coli and Klebsiella species were screened for and confirmed using the methods recommended by CLSI (10) and by molecular methods as previously described (8).

DISCUSSION
Ceftobiprole is a broad-spectrum cephalosporin with demonstrated in vitro activity against Gram-positive and Gramnegative human pathogens, including MRSA and MRSE, as well as some species of anaerobic bacteria (2,(13)(14)(15)(16)(17)(18).The data presented in the current study confirm previously published ceftobiprole MIC results for the Gram-positive and Gramnegative pathogens tested (13)(14)(15)(16).Importantly, all isolates of S aureus (MIC 4 µg/mL or less) and S pyogenes (MIC 0.5 µg/mL or less) tested in the current study were susceptible to ceftobiprole.Ceftobiprole also demonstrates more potent activity than ceftriaxone against penicillin-resistant S pneumoniae (19).Against Enterobacteriaceae and P aeruginosa isolates, the potency of ceftobiprole was similar to that of cefepime (Table 2) (2).Ceftobiprole has been previously reported to be as potent as ceftazidime against P aeruginosa (18).
As in vitro testing of ceftobiprole enters clinical laboratories, it will be crucial to ensure that laboratorians are aware not to automatically convert ceftobiprole MIC interpretations to resistant for isolates of MRSA as is done for every other betalactam (10).Ceftobiprole is distinct from all other beta-lactam agents because of its unique mechanism of action.Ceftobiprole demonstrates high affinity binding to penicillin-binding protein (PBP)2a of methicillin-resistant staphylococci, including MRSA (16,20).For this reason, ceftobiprole is indicated to treat infections caused by MRSA (12,15), and laboratories should report ceftobiprole MIC interpretations as they test and not apply an expert rule to the result as is done for all other beta-lactams when an isolate of S aureus is identified as an MRSA.In addition to its high affinity binding to PBP2a in staphylococci, ceftobiprole also has high affinity for PBP2x in penicillin-resistant S pneumoniae, and PBP2, PBP3 and other essential PBPs in methicillin-susceptible S aureus, E coli and P aeruginosa (20).
The pharmacodynamic profile of ceftobiprole is similar to that of other compounds within the cephalosporin class (22).Ceftobiprole exhibits concentration-independent killing, and animal infection models have established the time the free drug concentration exceeds the MIC as the principal pharmacodynamic parameter predictive of efficacy (22).In single-step and serial passage in vitro and in vivo resistance development studies, ceftobiprole demonstrated a low propensity to select for resistant subpopulations (16,23,24).
The current study confirmed previously published results that reported that doripenem was a broad-spectrum carbapenem with in vitro activity against Gram-positive and Gram-negative pathogens (5)(6)(7).Some previous studies have shown doripenem to be more active than other carbapenems against P aeruginosa (6,7); however, this was not observed in the current study.Doripenem has been reported to display the favourable characteristics of other carbapenems, and appears to offer certain advantages in terms of potency, spectrum and beta-lactamase stability when compared with some carbapenems used currently to treat nosocomial infections (5).Carbapenems have broad-spectrum activity against most Grampositive and Gram-negative pathogens, and their enhanced stability to most Ambler class A, C and D beta-lactamases (6).Carbapenems have generally been reserved for the most severe infections, or for infections caused by organisms known to be resistant to other available beta-lactams, especially those with an extended spectrum of activity.Doripenem is also not subject to inactivation by renal dehydropeptidases, a characteristic of imipenem (6).

CONCLUSIONS
The current study demonstrated both ceftobiprole and doripenem to be broad-spectrum antibacterial agents.In the past, empirical antimicrobial coverage of both Gram-positive and Gram-negative infections has generally necessitated the use of at least two antimicrobial agents.Several qualities make ceftobiprole uniquely suited for early empiric use of infections likely to be caused by Gram-positive organisms and mixed Grampositive and Gram-negative organisms (22).Ceftobiprole represents an effective alternative therapy for complicated skin and skin structure infections, including those caused by MRSA and many Gram-negative aerobic and facultative bacilli.Ceftobiprole has an enhanced anti-Gram-positive spectrum, including MRSA, while maintaining broad anti-Gram-negative activity, and possesses stability to many of the beta-lactamases produced by both Gram-positive and Gram-negative pathogens (13).All isolates of MRSA tested were susceptible to ceftobiprole (MIC 4 µg/mL or less), differentiating it from any other currently marketed beta-lactam.Doripenem also demonstrated potent activity (MIC 90 0.5 µg/mL or less) against all isolates of Enterobacteriaceae tested, including ESBL-producing E coli and K pneumoniae, and as potent activity as meropenem (MIC 90 8 µg/mL) against P aeruginosa.ACKNOWLEDGEMENTS: Funding for the CANWARD 2007 study was provided by Affinium, Astellas Pharma Canada, Janssen Ortho, Oryx, Pfizer Canada, TaiGen, Targanta, Wyeth, University of Manitoba, Winnipeg Health Sciences Centre and the National Microbiology Laboratory-Health Canada.The authors would like to thank Nancy Laing, Barb Weshnoweski, Ravi Vashisht, Lisa Bittner and Haley Butcher for their technical assistance.The authors would also like to thank the investigators and laboratory site staff at each medical centre that participated in the CANWARD 2007 study.The investigators of the CANWARD 2007 study are listed in another paper in the present supplement by Zhanel et al (25).CANWARD data are also displayed at www. can-r.ca, the official Web site of the Canadian Antimicrobial Resistance Alliance.