Comparative in vitro activity of faropenem against staphylococci

Christof von Eiff*, Sven Schepers and Georg Peters

Institute of Medical Microbiology, University of Münster, Domagkstraße 10, 48149 Münster, Germany

Received 21 December 2001; returned 28 February 2002; revised 9 April 2002; accepted 29 April 2002


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The anti-staphylococcal activity of faropenem, a novel ß-lactam, was examined and compared with that of amoxicillin, cefuroxime, clindamycin and vancomycin using the agar dilution method. A total of 234 staphylococci, including a large number of clonally different methicillin-resistant strains and a representative number of Staphylococcus aureus small colony variants, were tested. While the activity of faropenem was independent of the staphylococcal phenotype, the novel penem was up to eight times more active against methicillin-susceptible strains compared with the other agents tested. In addition, faropenem was active against many methicillin-resistant strains of S. aureus and coagulase-negative staphylococci.


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Staphylococcus aureus and coagulase-negative staphylococci (CoNS) reveal a remarkable propensity for resistance to various antibiotics due to defined resistance mechanisms.1,2 Alternative antimicrobials are needed for the treatment of infections due to multiple resistant staphylococci and to reduce the increasing selection pressure on these pathogens in hospitals.

Faropenem (SUN5555) is a novel ß-lactam with a penem structure that is different from that of carbapenems and other available ß-lactams. It is intended for oral administration as a pro-drug ester, faropenem-daloxate. The compound is stable to most ß-lactamases and has a broad spectrum of antimicrobial activity.35 However, data on its activity against staphylococci are limited.

The aims of this study were: (i) to evaluate the activity of faropenem against a large number of different and well-characterized staphylococcal species, particularly against clonally different methicillin-resistant strains isolated from several geographical locations in Germany; and (ii) to compare the in vitro anti-staphylococcal activity of this novel agent with that of amoxicillin, cefuroxime, clindamycin and vancomycin.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
A total of 234 staphylococcal strains freshly isolated from clinical specimens were tested. A large number of isolates were collected from blood or from other clinical specimens during the course of two multicentre studies, conducted in more than 30 university and community hospitals in Germany. Only CoNS considered clinically relevant were included. Small colony variants (SCVs) of S. aureus, which were more resistant to many antibiotics than the normal phenotype S. aureus, were also included in this study.6 The SCVs were collected in prospective studies, in particular from patients with chronic and antibiotic-resistant infections, such as chronic osteomyelitis.6 Except for the SCVs, which were compared with clonally identical normal strains, only one isolate per patient was tested.

Since methicillin-resistant strains may cause outbreaks, we used several other criteria to avoid including multiple isolates of the same strain. First, isolates were collected over a period of years, thus making a single clone less likely. Secondly, isolates were collected from different geographical locations in Germany, which would also reduce the chance of obtaining a single clone. Finally, when S. aureus isolates with similar antibiograms and phenotypes were obtained, we carried out pulsed-field gel electrophoresis (PFGE) and selected only one example of each strain.

The 151 S. aureus strains included 45 penicillin-susceptible S. aureus (PSSA), 45 methicillin-susceptible S. aureus (MSSA), 31 methicillin-resistant S. aureus (MRSA) and 15 SCVs with their corresponding normal phenotype strains (15 SCVs and 15 strains with normal phenotype, isolated from simultaneous or sequential cultures). Clonal identity of the SCV/normal phenotype strain pair was proved by PFGE (data not shown). The 83 CoNS comprised 16 methicillin-susceptible (MSSE) and 18 methicillin-resistant Staphylococcus epidermidis (MRSE) strains, 17 methicillin-susceptible (MSSH) and 15 methicillin-resistant Staphylococcus haemolyticus (MRSH) strains and 17 other CoNS belonging to the following species: six Staphylococcus lugdunensis, six Staphylococcus schleiferi and five Staphylococcus hominis.

The staphylococci were identified from a variety of conventional phenotypic characteristics and by using the API-Staph system (ATB32 Staph; bioMérieux, Marcy-L’Étoile, France). Methicillin susceptibility was tested by supplementation of the agar with 2% NaCl (read after incubation for 48 h at 30°C using 5 µg oxacillin discs) and resistance was confirmed by detection of the mecA gene in strains with a non-definable resistant phenotype, as described previously.7

The MICs were determined on Mueller–Hinton agar, using the agar dilution technique with a final inoculum of c. 104 cfu/spot. The following antimicrobial agents were obtained from their respective manufacturers: faropenem, amoxicillin, cefuroxime, clindamycin and vancomycin. The compounds were tested in 13 different concentrations ranging from 0.031 to 128 mg/L. The results were read after 18 h incubation at 36°C. Due to their slow growth, the results for the SCVs were read after 48 h incubation at 36°C. Sterility and growth controls were always carried out and the following reference strains were included as controls: S. aureus ATCC 25923, ATCC 29213, ATCC 43300; Enterococcus faecium ATCC 51599; Escherichia coli ATCC 35218; Pseudomonas aeruginosa ATCC 27853.


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The MIC distribution data for the S. aureus strains tested are shown in Table 1. Compared with amoxicillin, cefuroxime and vancomycin, faropenem was four to eight times more active against penicillin- and methicillin-susceptible strains of S. aureus. Clindamycin showed similar in vitro activity against these strains (MIC90 0.25 mg/L). However, whereas all strains were inhibited by 0.5 mg/L faropenem, two of these strains were not inhibited by 128 mg/L clindamycin. Regarding the methicillin-resistant strains, faropenem was much more active than the other ß-lactam antibiotics, with 18 of 31 MRSA inhibited by concentrations of <=2 mg/L faropenem. However, nine MRSA strains showed faropenem MICs > 128 mg/L. Strains with reduced susceptibility to vancomycin were not detected. Faropenem was as active against SCVs of S. aureus as against their corresponding parent strains with normal phenotype, indicating in vitro activity of faropenem independent of the phenotype. MICs for both phenotypes were within the range as shown for the other S. aureus strains, with MICs up to 0.25 mg/L for 12 methicillin-susceptible S. aureus SCVs and >128 mg/L for three methicillin-resistant S. aureus SCVs.


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Table 1.  In vitro activity of faropenem and four other antibiotics against S. aureus strains
 
The MIC distribution for the 34 S. epidermidis strains, the 32 S. haemolyticus strains and for 17 CoNS belonging to three other species are shown in Table 2. Among all compounds tested, faropenem was the most active agent against CoNS. All S. epidermidis strains and methicillin-susceptible S. haemolyticus strains tested were inhibited by <=0.25 mg/L faropenem. Thus, the activity of faropenem against S. epidermidis (n = 34) remained unchanged, irrespective of the methicillin-resistant phenotype. Methicillin-resistant S. haemolyticus strains were inhibited by faropenem concentrations similar to those of vancomycin (MIC90 2 mg/L). With regard to other staphylococcal species, faropenem was also as active as vancomycin, with all strains inhibited by <=2 mg/L faropenem.


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Table 2.  In vitro activity of faropenem and four other antibiotics against CoNS
 
Quality control of all MIC determinations was carried out using the reference strains mentioned above. The MICs for these strains were within acceptable limits throughout testing; e.g. for S. aureus ATCC 25923 the MICs were: faropenem 0.125 mg/L; cefuroxime 1 mg/L; clindamycin 0.125 mg/L; amoxicillin 0.125 mg/L and vancomycin 2 mg/L.


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
In recent years, renewed interest in staphylococcal infections has followed the emergence of multidrug-resistant infections despite advances in antibacterial therapy.1,2,8 Glycopeptides have been the mainstay of treatment for methicillin-resistant staphylococci, but the emergence of staphylococcal strains with intermediate resistance to glycopeptides has aroused concern about the potential for resistance to all available antibiotics. To reduce the increasing selection pressure on staphylococci, alternatives to available antimicrobials are still needed.

Faropenem revealed good activity against staphylococci, including a considerable number of S. aureus SCVs. While faropenem was highly active against methicillin-susceptible strains, its MICs for many methicillin-resistant strains of S. aureus, S. epidermidis and S. haemolyticus were low. Recent studies have shown that newer penems, such as LJC 11, 036 and faropenem, are characterized by improved binding affinities to both wild-type and modified PBPs of MRSA and penicillin-resistant pneumococci.9,10 These findings might explain why newer penems exhibit a more marked antibacterial activity against methicillin-resistant strains than other ß-lactam antibiotics; however, it is noteworthy that other ß-lactams developed in the past and shown to be active in vitro against MRSA had limited clinical efficacy.

In previous studies of the activity of faropenem, multiple isolates of the same strain were not excluded by phenotypic or genotypic methods. This is particularly important for MRSA, where clonal spread may cause widespread outbreaks. In addition, only a few staphylococcal species, especially methicillin-resistant strains, were studied or isolates were not categorized according to their resistance to methicillin.3,10,11 Thus, our results for MRSA differed from those of previous studies in which most of the strains tested were inhibited either by high concentrations (MIC50 and MIC90 >100 mg/L)4,5,10 or by low concentrations of faropenem (range 0.06–2 mg/L).3,11 In contrast to some reports, particularly to those testing isolates from Japan,4,5,10 but consistent with the study by Woodcock et al.,3 we found that the activity of faropenem was high against different species of CoNS, especially against S. epidermidis.

In the past, ß-lactam antibiotics have often seemed to have higher MICs for SCVs. It was assumed that the slow growth of these organisms might reduce the effectiveness of cell-wall antibiotics such as ß-lactams.6 However, in our study, the activity of faropenem was not affected by the phenotype of the staphylococci tested.

In summary, our data indicate that faropenem exhibits a broad spectrum of anti-staphylococcal activity, justifying further evaluation for the therapy of staphylococcal infections. However, methicillin-resistant strains with elevated MICs of faropenem may be found.


    Acknowledgements
 
We sincerely thank S. Weber and S. Hubrich for expert technical assistance. The study was supported by a grant from Bayer, Wuppertal, Germany.

This study was presented in part at the Interscience Conference of Antimicrobial Agents and Chemotherapy, Chicago, December 2001.


    Footnotes
 
* Corresponding author. Tel: +49-251-835-5360; Fax: +49-251-835-5350; E-mail: eiffc{at}uni-muenster.de Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 . Fluit, A. C., Wielders, C. L., Verhoef, J. & Schmitz, F. J. (2001). Epidemiology and susceptibility of 3,051 Staphylococcus aureus isolates from 25 university hospitals participating in the European SENTRY Study. Journal of Clinical Microbiology 39, 3727–32.[Abstract/Free Full Text]

2 . Paradisi, F., Corti, G. & Messeri, D. (2001). Antistaphylococcal (MSSA, MRSA, MSSE, MRSE) antibiotics. Medical Clinics of North America 85, 1–17.[ISI][Medline]

3 . Woodcock, J. M., Andrews, J. M., Brenwald, N. P., Ashby, J. P. & Wise, R. (1997). The in-vitro activity of faropenem, a novel oral penem. Journal of Antimicrobial Chemotherapy 39, 35–43.[Abstract]

4 . Miyazaki, S., Hosoyama, T., Furuya, N., Ishii, Y., Matsumoto, T., Ohno, A. et al. (2001). In vitro and in vivo antibacterial activities of L-084, a novel oral carbapenem, against causative organisms of respiratory tract infections. Antimicrobial Agents and Chemotherapy 45, 203–7.[Abstract/Free Full Text]

5 . Okuda, J., Otsuki, M., Oh, T. & Nishino, T. (2000). In vitro activity of DU-6681a, an active form of the new oral carbapenem compound DZ-2640, in comparison with that of R-95867, faropenem and oral cephalosporins. Journal of Antimicrobial Chemotherapy 46, 101–8.[Abstract/Free Full Text]

6 . Proctor, R. A., Kahl, B., von Eiff, C., Vaudaux, P. E., Lew, D. P. & Peters, G. (1997). Staphylococcal small colony variants have novel mechanisms for antibiotic resistance. Clinical Infectious Diseases 27, Suppl. 1, S68–74.[ISI]

7 . Murakami, K., Minamide, W., Wada, K., Nakamura, E., Teraoka, H. & Watanabe, S. (1991). Identification of methicillin-resistant strains of staphylococci by polymerase chain reaction. Journal of Clinical Microbiology 29, 2240–4.[ISI][Medline]

8 . Hamilton, D. C. & Ludlam, H. (2001). New anti-Gram-positive agents. Current Opinion in Critical Care 7, 232–7.[Medline]

9 . Dalhoff, A. & Nasu, T. (2001). Target affinity and beta-lactamase stability of faropenem in Staphylococcus aureus, Escherichia coli, Proteus vulgaris and Serratia marcescens. Eleventh European Congress of Clinical Microbiology and Infectious Diseases, Istanbul, Turkey. Clinical Microbiology and Infection 7, Suppl. 1, Abstract P1285, p. 271.

10 . Hikida, M., Itahashi, K., Igarashi, A., Shiba, T. & Kitamura M. (1999). In vitro antibacterial activity of LJC 11,036, an active metabolite of L-084, a new oral carbapenem antibiotic with potent antipneumococcal activity. Antimicrobial Agents and Chemotherapy 43, 2010–6.[Abstract/Free Full Text]

11 . Mortensen, J. E. & Egleton, J. H. (1995). Comparative in vitro activity of furopenem against aerobic bacteria isolated from pediatric patients. Diagnostic Microbiology and Infectious Diseases 22, 301–6.[ISI][Medline]





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