Comparative in vitro antimicrobial activity of a new carbapenem, E1010, and tentative disc diffusion test interpretative criteria

Peter C. Fuchs,*, Arthur L. Barry and Steven D. Brown

The Clinical Microbiology Institute, 9725 SW Commerce Circle, Wilsonville, OR 97070, USA


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
The susceptibility of 705 bacterial isolates representing 46 different species to E1010 (ER-35786), imipenem, meropenem and cefepime was determined by the NCCLS broth microdilution test. The MIC90s for E1010 were <=1.0 mg/L for Enterobacteriaceae, fastidious Gram-negative bacteria, streptococci and anaerobes. E1010 was two- to four-fold more active than imipenem and meropenem against Pseudomonas aeruginosa, and four-fold more active than the other carbapenems against methicillin-resistant Staphylococcus aureus. Vancomycin-resistant enterococci and most Enterococcus faecium were resistant to all four drugs tested. The NCCLS disc diffusion test was performed simultaneously on the non-fastidious organisms. Assuming the MIC breakpoints for E1010 will be the same as for the other carbapenems, the disc diffusion zone diameter breakpoints of imipenem and meropenem would also be applicable to E1010.


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
E1010 (ER-35786) is a new parenteral carbapenem with a broad spectrum of antibacterial activity.1,2 Structurally it is characterized by a methyl group at the 1ß position.1 A favourable feature is its increased in vitro activity against Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus compared with imipenem and meropenem.1,36 Preliminary human pharmacokinetic studies in healthy male volunteers indicated that a 30 min intravenous infusion of 250 mg of E1010 resulted in a mean Cmax of 19.7 mg/L, and higher doses up to 1000 mg yielded correspondingly higher serum concentrations.7 The terminal half-life was 1.7 h, nearly twice that of imipenem. The pharmacokinetic properties of E1010 appear to be comparable to those of the other currently available carbapenems. Consequently, for the purpose of this report, we have assumed that the susceptible and resistant breakpoints for E1010 will be the same as for imipenem and meropenem.

The present study was designed to (i) compare the in vitro antibacterial activity of E1010 with that of imipenem, meropenem and cefepime against a broad range of bacterial pathogens, and (ii) determine preliminary E1010 disc diffusion interpretive criteria for non-fastidious bacteria.


    Materials and methods
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 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
A total of 705 recent clinical bacterial isolates from several North American medical centres (<=6 months in stock) were selected as representative pathogens. These included the 645 aerobic isolates and 60 anaerobes that are listed in the TableGo.


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Table. Susceptibility of 705 bacterial isolates to E1010 and three comparison drugs
 
E1010 was provided as standardized powder by Eisai Co., Ltd, Tokyo, Japan. The comparison drugs imipenem (Merck, Rahway, NJ, USA), meropenem (AstraZeneca, Wilmington, DE, USA) and cefepime (Bristol-Myers Squibb, Princeton, NJ, USA) were acquired from their respective US manufacturers. For the disc diffusion tests, 10 µg E1010 discs were prepared in-house at 110% of stated potency. Commercially prepared 10 µg discs of imipenem and meropenem served as controls.

All aerobic organisms were tested by the broth microdilution method recommended by the NCCLS.8 Drug concentrations tested were serial two-fold dilutions ranging from 128 to 0.004 mg/L for the three carbapenems, and from 64 to 0.25 mg/L for cefepime. Non-fastidious organisms were tested simultaneously by the disc diffusion method outlined by the NCCLS.9 Anaerobic bacteria were tested by the agar dilution method as described by the NCCLS.10 The medium used was Brucella agar supplemented with 1 mg/L vitamin K1 and 5 mg/L haemin, and the drug concentrations tested were the same as listed above.

Quality control organisms were included as appropriate with each day's tests. These included: S. aureus ATCC 29213 and 25923, Enterococcus faecalis ATCC 29212, Streptococcus pneumoniae ATCC 49619, Escherichia coli ATCC 25922, P. aeruginosa ATCC 27853, Haemophilus influenzae ATCC 49247 and 49766, Bacteroides fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741. All results with quality control organisms for which published ranges were available10,11 were within those ranges.


    Results and discussion
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 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
Of the 224 Enterobacteriaceae isolates that were tested, all but one strain of Klebsiella pneumoniae were inhibited by <=4.0 mg/L of E1010 and meropenem, and all but two were susceptible to imipenem (TableGo). The MICs of E1010 were generally two- to four-fold lower than those of imipenem, but meropenem was the most active of the carbapenems against this group, especially against the Proteeae and Serratia spp.

E1010 was the most active agent tested against most species of non-enteric Gram-negative bacilli (TableGo). Notably, 88% of P. aeruginosa were susceptible to E1010, compared with 81% for cefepime, 74% for meropenem and 64% for imipenem. Against Burkholderia cepacia meropenem was more active than E1010. Stenotrophomonas maltophilia was resistant to all four drugs.

Fastidious Gram-negative bacteria were highly susceptible to all four drugs (TableGo). Among the carbapenems, meropenem was most active, followed by E1010 and imipenem. ß-Lactamase production by H. influenzae had no measurable effect on the MICs of any of these drugs.

Methicillin-susceptible staphylococci were susceptible to all four agents, with imipenem being the most active. Although methicillin-resistant staphylococci are considered resistant to all ß-lactam antibiotics, it is of interest that E1010 MICs were four-fold lower than those of the other two carbapenems. Because the number of methicillin-resistant staphylococci tested was small, it will be important to confirm this finding with a larger number of strains.

Streptococci (with the exception of penicillin-resistant pneumococci) were very susceptible to all four drugs (TableGo). The activity of the three carbapenems was comparable, with MIC50s and MIC90s within one two-fold dilution of each other. In the case of penicillin-resistant pneumococci, 91% were susceptible to <=0.5 mg/L of imipenem compared with 46% susceptible to this concentration of E1010.

Vancomycin-susceptible E. faecalis isolates were generally susceptible to the carbapenems. However, vancomycin-resistant E. faecalis and nearly all isolates of Enterococcus faecium were resistant to all four drugs.

Anaerobes were generally susceptible to the carbapenems and resistant to cefepime (TableGo). Against the Gram-negative species imipenem was marginally more active than E1010 and meropenem. E1010 was the most active agent against Clostridium difficile with MICs eight-fold lower than imipenem and two-fold lower than meropenem.

Overall, the in vitro activity of E1010 compared with imipenem and meropenem in this study is quite comparable to previously published data.1 The MIC90s for E1010 were within one two-fold dilution of each other for those organisms tested in both studies. For methicillin-resistant S. aureus (MRSA) the MIC50 for E1010 was substantially lower in the current study (1.0 versus 4.0 mg/L); however, we tested only 10 isolates.

A scattergram of E1010 MICs and disc diffusion zone diameters for 452 non-fastidious isolates is provided in the FigureGo. These included all non-fastidious isolates except methicillin-resistant staphylococci. Species for which there are no NCCLS interpretative criteria for disc diffusion tests (B. cepacia, Pseudomonas fluorescens, Pseudomonas stutzeri, S. maltophilia) were included since they were not amongst the discrepancies and are commonly tested by clinical laboratories using P. aeruginosa NCCLS criteria. The MIC and zone diameter breakpoints currently approved for imipenem and meropenem,11 have been applied. These breakpoints effectively separated susceptible from resistant isolates and yielded discrepancy rates below the limits proposed by the NCCLS:12 zero very major, six (1.3%) major and 20 (4.3%) minor discrepancies. The E1010 discrepancy rate could be lowered slightly by increasing the intermediate zone diameter range from 14–15 to 13–15 mm. We tentatively recommend the 14–15 mm intermediate range to keep it consistent with that of the other carbapenems, but when more isolates with E1010 MICs in the critical intermediate ±1 dilution range have been tested, the larger intermediate zone range may be preferable.



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Figure. E1010 MICs versus disc diffusion zone diameters for 452 non-fastidious bacteria. Major discrepancies include three S. epidermidis, two P. aeruginosa and one P. putida.

 
Conclusion

In summary, E1010 is a carbapenem that is very active against the Enterobacteriaceae, P. aeruginosa (including imipenem-resistant strains) and S. aureus (including some MRSA). The disc diffusion susceptibility tests appear to be reasonably accurate in determining susceptibility to E1010; tentative zone size criteria (<=13 mm for resistant and >=16 mm for susceptible) are proposed for use until confirmatory clinical and pharmacokinetic data are available.


    Acknowledgments
 
Financial support for this project was provided by Eisai Co., Ltd, Tokyo, Japan.


    Notes
 
* Corresponding author. Tel: +1-503-682-3232; Fax: +1-503-682-2065; E-mail: cmi{at}hevanet.com Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
1 . Ohba, F., Nakamura-Kamijo, M., Watanabe, N. & Katsu, K. (1997). In vitro and in vivo antibacterial activities of ER-35786, a new antipseudomonal carbapenem. Antimicrobial Agents and Chemotherapy 41, 298–307.[Abstract]

2 . Mikamo, H., Sato, Y., Hayasaki, Y., Izumi, K. & Tamaya, T. (1999). In vitro and in vivo antibacterial activity of a new antipseudomonal carbapenem, E1010. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 1999. Abstract 384, p. 294. American Society for Microbiology, Washington, DC.

3 . Inoue, M. & Toyosama, T. (1999). In vitro antibacterial activity of E1010, a new carbapenem, against strains producing extended-spectrum ß-lactamases (ESBL) and imipenem-resistant Pseudomonas aeruginosa. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 1999. Abstract 382, p. 293. American Society for Microbiology, Washington, DC.

4 . Nishino, T., Gotoh, N., Tsujimoto, H. & Otsuki, M. (1999). In vitro antibacterial activity of E1010, a new carbapenem, against various mutants of Pseudomonas aeruginosa. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy. San Francisco, CA, 1999. Abstract 383, p. 294. American Society for Microbiology, Washington, DC.

5 . Satoh, M., Watanabe, N., Hata, K. & Kitoh, K. (1999) E1010, a new parenteral carbapenem: in vitro and in vivo antipseudomonal activity. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy. San Francisco, CA, 1999. Abstract 381, p. 293. American Society for Microbiology, Washington, DC.

6 . Toyosawa, T. & Inoue, M. (1999). In vitro activity of E1010 (ER-35786), a new carbapenem against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy. San Francisco, CA, 1999. Abstract 380, p. 293. American Society for Microbiology, Washington, DC.

7 . Hirasawa, T., Okereke, C., Wheeler, J., Lynn, M. & Hasegawa, J. (2000). Pharmacokinetics (PK), safety and tolerability of E1010, a new parenteral long half-life carbapenem, in healthy male volunteers. In Program and Abstracts of the Fortieth Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, Canada, 2000. Abstract 492, p. 15. American Society for Microbiology, Washington, DC.

8 . National Committee for Clinical Laboratory Standards. (1997). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically: Approved Standard M7-A4. NCCLS, Wayne, PA.

9 . National Committee for Clinical Laboratory Standards. (1997). Performance Standards for Antimicrobial Disk Susceptibility Tests: Approved Standard M2-A6. NCCLS, Wayne, PA.

10 . National Committee for Clinical Laboratory Standards. (1997). Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria: Approved Standard M11-A4. NCCLS, Wayne, PA.

11 . National Committee for Clinical Laboratory Standards. (1999). Performance Standards for Antimicrobial Susceptibility Testing: Tenth Informational Supplement M100-S10. NCCLS, Wayne, PA.

12 . National Committee for Clinical Laboratory Standards. (1998). Development of In Vitro Susceptibility Testing Criteria and Quality Control Parameters: Tentative Guideline M23-T3. NCCLS, Wayne, PA.

Received 8 December 2000; returned 26 February 2001; revised 28 March 2001; accepted 3 April 2001





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