DIAGN MICROBIOL INFECT DIS 57 1990;13:57-61

In vitro Activity of Meropenem (SM-7338), Imipenem, and Five Other Antibiotics Against Anaerobic Clinical Isolates

Patrick R. Murray and Ann C. Niles

The in vitro susceptibility of 513 recent anaerobic clinical iso- lates was evaluated against meropenem (SM-7338), a new car- bapenem, and six other antibiotics. Virtually all Gram-positive and Gram-negative anaerobic bacteria tested were susceptible to meropenem (defined as MICs ~ 8 IJ, g/ml) with 99.8% of the isolates inhibited by. <~ 4 txg/ml. The activity of merope- hem was comparable to imipenem for most clinical isolates.

Minor differences were observed for Ciostridium and Veillo- nella (meropenem more active) and other Gram-positive bacilli (imipenem more active). Meropenem inhibited all anaerobes re- sistant to clindamycm and metronidazole. Bactericidal tests performed with meropenem demonstrated killing activity against all isolates except Clostridium and Lactobacillus.

I N T R O D U C T I O N

Carbapenem antibiotics, such as imipenem, have a broad spectrum of activity against most bacte- rial pathogens, in part due to their resistance to beta-lactamase hydrolysis (Barry et al., 1985; Barza, 1985). Unfortunately, many of these antibiotics are hydrolyzed in vivo by renal dihydropeptidase-1, so metabolism can potentially reduce their activity to nontherapeutic levels. This problem can be cir- cumvented by combining the carbapenem with an inhibitor of dihydropeptidase activity such as ci- lastatin (Kahan et al., 1983). Another approach is the biochemical modification of the carbapenem molecule, the strategy that was used successfully in the development of the new carbapenem, mer-

openem (formerly SM-7338). Substitution of a 5- dimethyl carbamoylpyrrolidin moiety in the car- bapenem nucleus rendered meropenem resistant to dihydropeptidase hydrolysis (Moellering et al., 1989).

Preliminary studies have demonstrated that the antimicrobial activity of meropenem was not abro- gated by the biochemical modification of the carba- penem nucleus. Meropenem has a broad spectrum of in vitro activity against Enterobacteriaceae spp., Pseu- domonas spp., Haemophilus spp., Neisseria spp., oxa- cillin-susceptible staphylococci, Streptococcus spp., and selected anaerobes (Bauernfeind et al., 1989; Clarke and Zemcov, 1989; Edwards et al., 1989; Jones et al., 1989a,b; King et al., 1989; Neu et al., 1989; Schito et al., 1989; Sentochnik et al., 1989). In the study re- ported here, we compared the activity of meropenem with imipenem and five other antibiotics against a di- verse collection of anaerobic clinical isolates.

From the Washington University School of Medicine (P.R.M.), and Barnes Hospital Clinical Microbiology Laboratory (P.R.M., A.C.N.), Saint Louis, Missouri.

Address reprint requests to: Dr. P. R. Murray, Barnes Hospital Clinical Microbiology Laboratory, St. Louis, MO 63110.

Received October 10, 1989; revised and accepted November 16, 1989. © 1990 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NJ 10010 0732-8893/90/$3.50

MATERIALS AND METHODS

Organisms

A total of 513 anaerobic bacteria isolated within 12 months from clinical specimens submitted to the Barnes Hospital Clinical Microbiology Laboratory were evaluated. Organisms were selected to include

58 P.R. Murray and A.C. Niles

a broad representa t ion of all c o m m o n clinical species and were identified by s tandard laboratory proce- dures (Allen et al., 1985). All o rgan isms were sub- cultured a minimal n u m b e r of t imes before the sus- ceptibility tests were per fo rmed . Selected isolates resistant to c l indamycin or met ronidazole were used to determine the activity of meropenem against these prob lem organisms.

Antibiotics

M e r o p e n e m was suppl ied by ICI Pharmaceut icals Group (Wilmington, Delaware). All o ther antibiotics used in these studies ( imipenem, cl indamycin, chloramphenicol , piperacillin, cefoxitin, and me- tronidazole) were furnished by their respect ive man- ufacturers.

TABLE 1. In Vitro Activity of M e r o p e n e m Against Anaerobic Clinical Isolates

MIC (p.g/ml) Organism

(No. Tested) Antibiotic Range MICso MIC~<)

Bacteroides fragilis group (148)

Bacteroides spp. (80)

Fusobacterium spp. (23)

Clostridium perfrin,gens (57)

Clostridium spp. (65)

Peptostreptococcus spp. (72)

Veillonella spp. (15)

Meropenem .008-2 .12 [mipenem .008-1 .06 Piperacillin .06- > 128 8 Clindamycin <~.004-> 128 .25 Chloramphenicol .015-8 2 Cefoxitin .06-128 8 Metronidazole .06-> 128 .5 Meropenem <~.004-16 .03 Imipenem <~.004-4 .03 Piperacillin .016->128 2 Clindamycin ~<.004-> 128 .03 Chloramphenicol .008->128 1 Cefoxitin .06->128 2 Metronidazole .06-> 128 .5 Meropenem <~.004-2 .008 Imipenem .008-1 .03 Piperacillin ~.004-128 .03 Clindamycin .015->128 .03 Chloramphenicol .03-16 .12 Cefoxitin .015-32 .06 Metronidazole .008->128 .06 Meropenem ~<. 004-. 008 <~.004 Imipenem .008-.06 .03 Piperacillin <~.004-4 .015 Clindamycin .008-4 .25 Chloramphenicol .5-2 1 Cefoxitin .06-1 .25 Metronidazole .06-4 .5 Meropenem <~.004-4 .25 lmipenem ~<.0044 .5 Piperacillin <~.004->128 1 Clindamycin .008->128 .05 Chloramphenicol .0154 .25 Cefoxitin .015->128 8 Metronidazole .015->128 .25 Meropenem <~.004-1 .008 Imipenem ~<.004-.5 .008 Piperacillin <~ .004-32 .03 Clindamycin .008-64 .06 Chloramphenicol .034 .25 Cefoxitin .008-8 .12 Metronidazole .03-> 128 .25 Meropenem <~.004-. 12 .03 Imipenem .015-.5 .12 Piperacillin .06-64 16

.25

.25 64 64 4

32 1

.12

.25 32

128 4

64 4

.5

.5

.5 128

4 4 1

<~ .004 .06 .06

2 2

.5 1 1 2 4

64 2

64 2

.25

.06

.06 4 2 1

>128 .06 .25

32

Percent Susceptible ~

100 100 96 84

100 88 97 99

100 95 86 94 89 91

100 100 100 78

100 96 91

100 100 100 93

100 100 100 100 100 98 72

100 71 95

100 100 100 88

100 100 78

100 100 100

In vitro Activity of M e r o p e n e m 59

TABLE 1. Cont inued

Organism (No. Tested)

MIC (p.g/ml) Percent

Antibiotic Range MIC~ MICg0 Susceptible"

Lactobacillus spp. (4)

Bifidobacterium spp. (11)

Eubacterium spp. (15)

Actinobacillus spp. (23)

Clindamycin .015--~128 .03 .06 93 Chloramphenicol .015-2 .5 1 100 Cefoxitin .25-4 1 4 100 Metronidazole .12-1 .5 1 100 Meropenem .25-4 4 - - 100 lmipenem .06-1 1 - - 100 Piperacillin .12-4 2 - - 100 Clindamycin .015-128 .12 - - 75 Chloramphenicol 4-16 4 - - 75 Cefoxitin 128- ~ 128 > 128 - - 0 Metronidazole > 128 > 128 - - 0 Meropenem .03-2 .5 1 100 lmipenem .015-.5 .12 .25 100 Piperacillin <~. 004-2 .12 2 100 Clindamycin ~.004-.06 .015 .06 100 Chloramphenicol .25-2 .5 2 100 Cefoxitin 1-16 8 16 100 Metronidazole .5-4 2 2 100 Meropenem .008-.25 .12 .12 100 Imipenem 4.004-.25 .015 .12 100 Piperacillin .008-16 1 8 100 Clindamycin <.004-.25 .015 .25 100 Chloramphenicol .12-2 2 2 100 Cefoxitin .015-4 1 4 100 Metronidazole .03->128 .5 32 87 Meropenem ~.004-2 .06 .25 100 Imipenem .008-1 .03 .06 100 Piperacillin .015-32 .5 2 100 Clindamycin ~.004-16 .06 1 96 Chloramphenicol .06--4 .25 2 100 Ce foxi tin .008-16 .12 1 100 Me tron idazole .5- > 128 64 > 128 43

dSusceptibility was defined by current NCCLS standards (M11-T2, 1989): imipenem, 48 ~g/ml; piperacillin ~ 128 ~.g/ml; clindamycin 42 ~g/ml; chloramphenicol ~ 16 tag/ml; cefoxitin <~ 16 ~g/ml; metronidazole 416 ~.g/ml. lmipenem standards were used for mer- openem tests. This is consistent with the pharmacokinetic properties of meropenem (Bax et al., 1989).

S u s c e p t i b i l i t y Tes t M e t h o d s

MICs and MBCs of the seven antibiotics for the test o rgan i sms were de te rmined by the broth dilution p rocedures r e c o m m e n d e d by the National Commi t - tee for Clinical Laboratory S tandards (NCCLS, 1989) and American Society for Microbiology (Sutter, 1985). MIC tests were pe r fo rmed in broth microdilut ion trays using Wilkins-Chalgren broth prepared in-house and incubat ion in an anaerobic chamber for 48 hr. MBC tests were de te rmined by the macrobro th di- lution procedure . Jones and Gard iner (1989c) re- por ted that m e r o p e n e m , unlike i m i penem (Shungu et al., 1985), is more stable in broths c o m m o n l y used for anaerobic susceptibil i ty testing. Thus, special test ing me thods were unnecessary .

R E S U L T S

M e r o p e n e m had excellent activity against the an- aerobic isolates tested in this s tudy, with 512 (99.8%) of the 513 isolates inhibited by ~ 4 ~g/ml. This ac- tivity is comparab le to that seen with i m i p e n e m and super ior to the other antibiotics tested (Table 1). The MIC values for m e r o p e n e m and im ipenem were es- sentially identical for all major o rgan ism groups with the exception of Clostridium perfringens and other clostridial species and Veillonella spp. ( m e r o p e n e m more active) and the nonspore - fo rming Gram-pos - itive bacilli ( imipenem more active).

Both m e r o p e n e m and i m i p e n e m inhibited all 69 c l indamycin resistant isolates, including 25 f rom the Bacteroides fragilis group (Table 2). Likewise, these

60 P.R. Murray and A.C. Niles

TABLE 2. Activity of Meropenem Against 69 Clindamycin-Resistant Anaerobic Bacteria

Range of MIC Values (ixg/ml) Organism

(No. Tested) Meropenem lmipenem

Bacteroides fragilis group (25) Bacteroides spp. (7) Fusobacterium spp. (5) Clostridium spp. (20) Peptostreptococcus spp. (9) Other anaerobes (3)

0.015-0.5 0.06-2 0.12-2

<~0.004-1 ~0.004-1

0.06-4

0.03-0.5 0.03-4 0.5-1 0.008-4 0.008-0.5 0.03-1

two carbapenem antibiotics were very active against metronidazole-resistant anaerobes (Table 3).

The bactericidal activity of meropenem against 34 anaerobic bacteria was measured. MBC:MIC ratio was ~<4 for 13 of 15 B. fragilis group isolates, four of five fusobacteria, zero of five clostridia, two of five peptostreptococci, and three of four lactobacilli. With the exceptions of the Clostridium spp. and Lac- tobacillus spp., all anaerobic bacteria were killed by <~4 ixg/ml of meropenem (Table 4). However, despite MICs of 0.5-1 lag/ml for five isolates of Clostridium, the meropenem MBCs were t>64 txg/ml. Similarly, the MBCs for three of the four lactobacilli were /> 16 txg/ml.

DISCUSSION

Previous studies demonstrated meropenem had ex- cellent activity against most major groups of bacte- ria. Meropenem inhibited 90% of Enterobacteriaceae at ~0.25 ~,g/ml as well at H. influenzae, Branhamella catarrhalis, and Neisseria gonorrhoeae. Inhibition of methicillin-resistant staphylocci generally required

higher concentration of meropenem compared with methicillin-susceptible staphylococci, and merope- nem had modest activity against Pseudomonas spp. and Enterococcus spp.

Preliminary studies indicated that meropenem had excellent activity against all common groups of an- aerobic bacteria (Edwards et al., 1989; Jones et al., 1989b; Neu et al., 1989; Sentochnik et al. 1989), as has also been reported for imipenem (Nasu et al., 1981; Tally and Jacobus, 1983). This is consistent with the data observed herein. Particularly noteworthy was the impressive activity of both carbapenems against clinical isolates resistant to clindamycin (Ta- ble 2) or metronidazole (Table 3). A total of 16% of the Bacteroides fragilis group isolates in this study were resistant to clindamycin (MIC />4 }xg/ml), and 3% were resistant to metronidazole (MIC/>32 ixg/ml). All of these isolates were inhibited by <~0.5 txg/ml of meropenem.

In view of the excellent in vitro activity of mer- openem against both aerobic and anaerobic bacteria, as well as its chemical stability in vivo and in vitro (Jones et al., 1989b), we believe the clinical efficacy of this carbapenem should be examined.

TABLE 3. Activity of Meropenem Against 50 Metronidazole-Resistant Anaerobic Bacteria

Range of MIC Values (txg/ml) Organism

(No. Tested) Meropenem Imipenem

Bacteroides fragilis group (5) Bacteroides, other spp. (6) Fusobacterium spp. (2) Clostridium spp. (3) Peptostreptococcus spp. (16) Other anaerobes (18)

0.03-0.5 0.06-4 1-2 0.03-4

<~0.004-1 0.015-4

0.03-0.5 0.015-4 0.5 0.008-2

<~0.004-0.5 0.008-1

In vitro Activity of Meropenem 61

TABLE 4. Bacteriostatic and Bactericidal Activity of Meropenem

Organisms MIC MBC (No. Tested) (~g/ml) (~g/ml)

Bacteriodes fragilis group ( 1 5 ) 0.03(1)% 0.06(3), 0.12(7), 0.06(1), 0.12(1), 0.25(6), 0.25(2), 0.5(2) 0.5(2), 1(3), 2(1), 4(1)

0.008(4), 0.06(1) 0.03(3), 0.06(2) 0.5(4), 1(1) 64(4), 128(1) 0.008(3), 0.03(1), 0.06(1) 0.008(2), 0.5(2), 1(1) 0.25(1), 4(2), 8(1) 1(1), 16(2), 32(1)

Fusobacterium spp. (5) Clostridium spp. (5) Peptostreptococcus spp. (5) Lactobacillus spp. (4)

aNumber with indicated MIC or MBC in parentheses.

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