Comparative in vitro pharmacodynamics of BO-2727, meropenem and imipenem against Gram-positive and Gram-negative bacteria

Authors


Corresponding author and reprint requests: Inga Odenholt, Department of Infectious Diseases, University Hospital, S-751 85 Uppsala, Sweden Tel: (46)-18-665651 Fax: (46)-18-665650

Abstract

Objective: To investigate and compare the in vitro pharmacodynamics of three carbapenems: imipenem, meropenem and BO-2727.

Method: The following studies were performed: (1) comparative studies of the rate of killing of the three carbapenems of reference strains of Gram-positive and Gram-negative bacteria at a concentration corresponding to the 1-h serum level following 500 mg intravenously in humans; (2) comparative studies of the rate of killing of BO-2727, meropenem and imipenem at different antibiotic concentrations of reference strains of Gram-positive and Gram-negative bacteria; (3) comparative studies of the rate of killing of BO-2727, meropenem and imipenem of bacteria which are phenotypically tolerant; (4) studies of the postantibiotic effect of BO-2727 using viable counts and optical density; (5) studies of the postantibiotic sub-MIC effect (PA SME) of BO-2727 using optical density.

Results: No difference in killing rate was noted between the three carbapenems, and there was no concentration-dependent killing of the Gram-negative strains after 6 h. A pronounced paradoxical effect was seen against Staphylococcus aureus. All three antibiotics were able to kill phenotypically tolerant bacteria. Only very short or no postantibiotic effect of BO-2727 was found against the investigated strains. Very long PA SMEs were noted for the Gram-negative strains, although there was a pronounced variation for the different strains of Pseudomonas aeruginosa.

Conclusions: There was no significant difference between the studied carbapenems in their pharmacodynamic properties. All three antibiotics acted similarly to other β-lactam antibiotics.

INTRODUCTION

The carbapenems, imipenem and meropenem, are broad-spectrum agents with excellent in vitro activity against Gram-positive and Gram-negative bacteria, including strict anaerobes. They are highly resistant to hydrolysis by β-lactamases, with a few exceptions such as the β-lactamases of Stenotrophomonas maltophilia and some Aeromonas strains [1–4]. Imipenem, in contrast to other β-lactam antibiotics, has also been reported to kill non-growing bacteria [5,6] and to show a postantibiotic effect (PAE) not only against Gram-positive bacteria but also against Pseudomonas aeruginosa [7,8]. BO-2727 is a new carbapenem with similar in vitro activity to that of meropenem and imipenem. However, BO-2727 demonstrates even lower MIC values and greater in vivo activity in animal models against Staphylococcus aureus and P. aeruginosa in comparison with meropenem [9,10]. BO-2727, like meropenem, is also more stable than imipenem against human dihydropeptidase and has a longer half-life than the other carbapenems [11].

To further clarify the in vitro pharmacodynamic properties of BO-2727 in comparison with those of imipenem and meropenem, the following experiments were performed: (1) comparative studies of the rate of killing of the three carbapenems of reference strains of Gram-positive and Gram-negative bacteria at a concentration corresponding to the 1-h serum level following 500 mg intravenously in humans; (2) comparative studies of the rate of killing of BO-2727, meropenem and imipenem, at seven different concentrations, of reference strains of Gram-positive and Gram-negative bacteria; (3) comparative studies of the rate of killing of BO-2727, meropenem and imipenem of phenotypically tolerant bacteria; (4) studies of the PAE of BO-2727 using viable counts and optical density determinations; and (5) studies of the postantibiotic sub-MIC effect (PA SME) of BO-2727 using optical density determinations.

MATERIALS AND METHODS

Antibiotics

Imipenem and BO-2727 were provided by Merck, Sharp & Dohme, Starnberg, Germany, and meropenem by Zeneca, Gothenburg, Sweden. The antibiotics were obtained as reference powders with known potency. Imipenem was diluted in phosphate-buffered saline (PBS; pH 7.2), BO-2727 in sodium chloride (0.9%) and meropenem in Sörensen's buffer (pH 7.0). Dilutions were made on the day on which the experiments were performed.

Bacterial strains and media

The strains used in the study were: Pseudomonas aeruginosa ATCC 27853 and four clinical isolates of the same species (strains 2060, 2057, 5003, 2049); Escherichia coli ATCC 25922 and four clinical isolates of E. coli (2084, 2089, 6023, 6024); Staphylococcus aureus FDA 209P and four clinical isolates of S. aureus (3028, 1003, 2019, 2005); and four clinical isolates of Acinetobacter lwoffi (5030, 5063, 1092, 2035). All the clinical strains were obtained from the Clinical Microbiological Laboratory, Uppsala, Sweden. The Gram-negative strains were grown in Mueller-Hinton (MH) broth (Difco Laboratories, Detroit, MI, USA), supplemented with 50 mg/L of Ca2+ and 25 mg/L of Mg2+, for 6 h at 37°C, yielding an initial inoculum of approximately 109 CFU/mL for the E. coli strain, and 5 × 107 CFU/mL for the P. aeruginosa strain and the A. lwoffi strain. S. aureus was grown in Todd-Hewitt broth (T-H), resulting in approximately 109 CFU/mL. In the experiments with non-growing bacteria, PBS was used as nutrient-depleted medium.

Determination of minimum inhibitory concentrations (MICs)

The MICs for the strains used in experiments 1 to 3 and in experiment 4a were determined by a macro-dilution technique, in MH broth (Gram-negative strains) or in T-H broth (S. aureus) with a final inoculum of approximately 105 CFU/mL of the test strains, and incubation at 37°C for 24 h. The MIC was defined as the lowest concentration of antibiotic allowing no visible growth.

In experiment 4b and in experiment 5, the MICs were determined in the BioScreen C (LabSystems, Helsingfors, Finland), which has been described in detail elsewhere [12]. Two-fold dilutions of the antibiotics were placed in wells in a microtiter plate together with the test strain with a final inoculum of approximately 105 CFU/mL in liquid medium as above. Growth curves were then monitored automatically in the BioScreen C as optical density. The MICs were defined as the lowest concentration of the antibiotic that gave no increase in optical density. The lowest detectable level of optical density corresponded to 5 × 105 CFU/mL. All MIC values are given as geometric means of three separate determinations.

Experiment 1. Determination of the rate of killing

In the first study a concentration corresponding to the 1-h serum level following 500 mg intravenously in humans of the three antibiotics was used (20 mg/L for BO-2727, 18 mg/L for imipenem and 15 mg/L for meropenem). Tubes, containing adequate medium with the addition of the antibiotic, were inoculated with a suspension of the test strain, giving a final bacterial count of approximately 5 × 105 CFU/mL, and incubated at 37°C. Samples were withdrawn at 0, 3, 6, 9, 12 and 24 h and, if necessary, diluted in PBS. Three dilutions of each sample were spread on blood agar plates (Colombia agar base with 5% horse blood), incubated at 37°C and counted after 24 h. Only plates with 50 to 500 colonies were counted. All three antibiotics were tested against S. aureus FDA 209P, E. coli ATCC 25922, P. aeruginosa ATCC 27853 and A. lwoffi 5030; three experiments were performed for each antibiotic–bacterium combination.

Experiment 2. Determination of the rate of killing at different concentrations

In the second study, different concentrations of the three antibiotics were used. Tubes, containing 4 mL of either MH broth or T-H broth with antibiotic at 2, 4, 8, 16, 32, 64 and 128 x MIC respectively, were inoculated with a suspension of the test strain, giving a final bacterial count of approximately 5 × 105 CFU/mL. The tubes were incubated at 37°C and samples were then withdrawn and counted as described above. All three antibiotics were tested against S. aureus FDA 209P, E. coli ATCC 25922 and P. aeruginosa ATCC 27853; three experiments were performed for each antibiotic-bacterium combination.

Experiment 3. Determination of the rate of killing of non-growing bacteria

After an incubation of 6 h at 37°C in broth, 40 μL of the bacterial suspension was transferred to a tube with 4 mL of PBS, giving an inoculum of approximately 5 × 105 CFU/mL. The tubes were then incubated at 37°C. Three hours later, antibiotic was added at the same concentrations as in experiment 1 (20, 18 and 15 mg/L respectively). One tube without antibiotic served as a control. The tubes were thereafter reincubated at 37°C and samples were withdrawn and counted as described above. All three antibiotics were tested against S. aureus FDA 209P, E. coli ATCC 25922 and P. aeruginosa ATCC 27853; three experiments were performed for each antibiotic–bacterium combination.

Experiment 4a. Determination of the postantibiotic effects using viable counts

The PAEs of BO-2727 using viable counts were determined against P. aeruginosa ATCC 27853 and four clinical strains of the same species (2060, 2057, 5003, 2049). The PAE was also determined against S. aureus FDA 209 P and four clinical isolates of the same species (3028, 1003, 2019, 2005). All strains were studied at least twice. After incubation for 6 h at 37°C, all strains were diluted 1:10 in order to obtain an inoculum of approximately 5 × 107 CFU/mL at the beginning of the experiments. The strains were then exposed to 10XMIC of BO-2727 for 2 h at 37°C. To eliminate the antibiotics, the strains were washed three times for 5 min at 1400g and diluted in fresh media. The control strains were treated similarly. To allow monitoring of growth, the exposed strains were, after the 2-h exposure, diluted in fresh medium by a factor of 1:10 or 1:100, or left undiluted, depending on the rate of killing. In order to obtain an inoculum as close to that of the exposed cultures as possible, the controls were diluted 1:1000. The cultures were then reincubated at 37°C for another 6 h. Samples were withdrawn at 0 and 2 h (before and after dilution), and at 3, 4, 5, 6 and 8 h, if necessary diluted in PBS, and counted as described above. The PAE was defined according to the following formula [13]: PAE =T-C, where T is the time required for the viable counts of the antibiotic-exposed cultures to increase by one log10 above the counts observed immediately after washing, and C is the corresponding time for the controls.

Experiment 4b. Determination of the postantibiotic effects of BO-2727 in the Bioscreen C

The postantibiotic effect of BO-2727 was investigated against S. aureus FDA 209P, E. coli ATCC 25922, P. aeruginosa ATCC 27853 and A. lwoffi 5030 (two experiments each). Four clinical isolates of each of S. aureus (3028, 1003, 2019, 2005) and E. coli (2084, 2089, 6023, 6024) and three isolates of each of P. aeruginosa (2060, 2057, 5003) and A. lwoffi (5063, 1092, 2035) were also studied (one experiment each). After incubation for 6 h at 37°C, all strains were diluted 1:10 in order to obtain an inoculum of approximately 5 × 107 CFU/mL at the beginning of the experiments. The strains were then exposed to 10 X MIC of BO-2727 for 2 h at 37°C. The bacterial count had by that time declined by 0.5 to one log10 CFU. To eliminate the antibiotic, the strains were washed three times, centrifuged each time for 5 min at 1400g and diluted 1:10 in fresh medium. The unexposed control strains were washed similarly but were also diluted 10−2, 10−3, 10−4 and 10−5 in order to obtain an inoculum as close to that of the exposed strains as possible. Earlier experimental studies had revealed that the shapes of the growth curves of the different dilutions of the controls were parallel, and one log10 difference in initial inoculum corresponded to a constant delay in growth. A control curve for each strain and experiment could therefore be constructed with the same initial inoculum as the corresponding exposed strain. Since killing cannot be measured in BioScreen C, viable counts were used to measure the initial killing of the exposed cultures and after the washing. Viable counts of the controls were also performed at the start of the experiments, before and after washing and dilution at 2 h. Both the exposed strains and the different dilutions of the controls were then diluted 1:10, inoculated in microtiter wells of 400 μL and incubated in the Bioscreen C. Growth curves were measured automatically as optical density in the computer every 10 min for 20 h. The PAE was calculated as the difference in time for the exposed cultures and the corresponding control to grow up to a defined point (A50) on the absorbance curve, where A50 was defined as 50% of the maximal absorbance of the control. A50 was chosen, since this point represented approximately growth of one log10 CFU [12,14].

Experiment 5. Determination of the postantibiotic sub-MIC effects (PA SME) of BO-2727

The PA SMEs of BO-2727 were determined for the reference strains on two different occasions and once for the clinical strains. The postantibiotic phase was induced as described above and the control curves were diluted the same way as in the PAE experiments. Viable counts were also used as described above. The strains in the postantibiotic phase and the different dilutions of the controls were then exposed to 0.1, 0.2, 0.3, 0.4 and 0.5 X MIC of BO-2727 and incubated in the BioScreen C. Growth curves were monitored automatically for 20 h. The PA SME was defined as the difference in time taken between the exposed cultures, later exposed to sub-MICs, and the corresponding controls with the same initial inoculum as the preexposed culture, to reach A50 (defined as above) [12,14].

RESULTS

Minimum inhibitory concentrations

The MIC values for BO-2727, imipenem and meropenem against the strains studied are shown in Table 1. BO-2727 and imipenem had two- to three- fold lower MIC values against S. aureus compared to meropenem measured by visibility. Imipenem showed the highest MIC value against P. aeruginosa. BO-2727 showed overall low MIC values against the studied strains. The difference in MIC values between the two methods (viable counts and BioScreen C) was not more than one dilution step.

Table 1.  Minimum inhibitory concentrations of the carbapenems
 BO-2727  
  1. ND=not done.

StrainsVisibilityOptical densityImipenemMeropenem
S. aureus    
209P0.060.130.030.25
30280.030.03  
10030.060.06  
20190.060.06  
20050.060.06  
E. coli    
ATCC 259220.130.060.250.03
2084ND0.06  
2089ND0.13  
6023ND0.13  
6024ND0.13  
P. aeruginosa    
ATCC 278531.00.54.00.5
20600.50.5  
20570.50.5  
50030.50.25  
20491.02.0  
A. lwoffi    
1–50300.130.060.061.0
5063ND0.13  
1092ND0.03  
2035ND0.25  

Experiment 1. Rate of killing at a concentration reached in human serum 1 h after 500 mg intravenously

No differences in killing rate between the three antibiotics were seen against S. aureus 209P (Figure 1), P. aeruginosa ATCC 27853 and E. coli ATCC 25922. However, a three log10 better killing was noted after 3 h for BO-2727 and imipenem against A. lwoffi 5030 in comparison with meropenem. This difference was no longer apparent after 12 h.

Figure 1.

Killing curves of imipenem, meropenem and BO-2727 at a concentration corresponding to that reached in serum after 1 h against Staphylococcus aureus. Mean of three experiments.

Experiment 2. Rate of killing at different concentrations

Maximal killing was already reached at 4 to 8 X MIC with all three carbapenems against P. aeruginosa, and no further concentration-dependent killing was found thereafter. Against E. coli, all the carbapenems showed a concentration-dependent killing up to 6 h. Thereafter, there was no difference between the killing reached with 8 X MIC and that with the higher concentrations. Figure 2 shows the killing effect of BO-2727 on E. coli ATCC 25922. A notable paradoxical effect was found for all carbapenems against S. aureus, where 4 X MIC gave on average a three log10 better killing in CFUs than 128 X MIC.

Figure 2.

Killing curves of BO-2727 at different concentrations against Escherichia coli ATCC 25922. Mean of three experiments.

Experiment 3. Rate of killing of non-growing bacteria

All three carbapenems were able to kill non-growing bacteria. Imipenem induced a five log10 decrease in CFU of E. coli ATCC 25922 after 12 h compared to a three log10 decrease for meropenem and a 2.5 log10 decrease for BO-2727 (Figure 3). Against P. aeruginosa ATCC 27853 both imipenem and BO-2727 induced a two log10 decrease in CFUs compared to a one log10 decrease for meropenem. Imipenem was the only antibiotic that was able to kill non-growing S. aureus 209P.

Figure 3.

Killing curves of imipenem, meropenem and BO-2727 against non-growing Escherichia coli ATCC 25922. Mean of three experiments.

Experiments 4a, 4b and 5. Postantibiotic effects and postantibiotic sub-MIC effects of BO-2727

No or only very short PAEs (less than 1 h) were found for all strains investigated (P. aeruginosa 2057, 2049; S. aureus FDA 209 P, 1003, 2019, 2005) with viable counts. In the BioScreen C, a short PAE was noted for all strains of S. aureus except for strain 3028. No PAE could be demonstrated against the Gram-negative strains except for one clinical strain of E. coli (2084) and one of A. lwoffi (2035), 0.9 h and 3.1 h respectively. All strains of A. lwoffi and E. coli were extremely sensitive to sub-MICs in the postantibiotic phase with a delay in regrowth up to more than 20 h at 0.4 X MIC. The PA SMEs of P. aeruginosa and S. aureus were usually shorter and varied among the strains investigated (Table 2).

Table 2.  The postantibiotic effect (PAE) and the postantibiotic sub-MIC effect (PA SME) of BO-2727
 PAE (h)PA SME (h)
     (x MIC)  
 VCOP0.10.20.30.40.5
  1. VC=viable counts; OP=optical density (mean values); ND=Not done.

S. aureus       
209P0.10.90.94.3>20>20>20
3028001.01.94.06.9>20
100300.80.91.83.25.28.8
20190.30.90.61.21.93.97.0
200500.50.71.53.25.711.1
P. aeruginosa       
ATCC 27853−0.30.20.63.38.910.613.4
2060−0.9−3.0−2.61.44.86.66.3
20570.5−0.62.41012.6>20>20
5003−0.7−3.0−2.7−2.1−0.94.97.0
20490.4NDNDNDNDNDND
E. coli       
ATCC 25922ND−0.308.917.7>20>20
2084ND0.92.75.29.0>20>20
2089ND01.43.39.0>20>20
6023ND00.1>20>20>20>20
6024ND0.210>20>20>20>20
A. lwoffi       
5030ND−0.3>20>20>20>20>20
5063ND012.3>20>20>20>20
1092ND02.55.0>20>20>20
2035ND3.1>20>20>20>20>20

DISCUSSION

Several pharmacodynamic parameters, such as the rate of bacterial killing, whether the killing rate is concentration-dependent or not and postantibiotic effects have been recognized as important factors that may influence the optimal dosing regimens for antibiotics [13,15–17]. In the present study, we have compared these pharmacodynamic parameters for imipenem, the first carbapenem to be introduced on the market, meropenem and BO-2727, a new carbapenem which, like meropenem, is stable to renal dehydropeptidase. In this study, no significant difference was found between the three carbapenems in their killing rate, except that BO-2727 showed a better initial killing of A. lwoffi. All three carbapenems exhibited rapid killing of the Gram-negative strains, with a decrease of approximately four log10 CFUs during the first 6 h. A maximal killing was reached already at 4–8 X MIC with all three carbapenems against P. aeruginosa, and no further concentration-dependent killing was found thereafter. An initial concentration-dependent killing up to 6 h was, however, seen against E. coli. For S. aureus, the killing was much slower, with a decrease of only approximately 0.5 log10 CFUs after 24 h, and a notable paradoxical effect was also found for all three carbapenems against this species. This so-called Eagle effect was described early in the literature but the clinical implications of this effect are not clear [18–21]. The lack of faster killing in spite of higher concentrations of the carbapenems in our study is in agreement with studies on other β-lactam antibiotics [22,23].

A phenomenon discovered early in the antibiotic era was that penicillin did not exert a bactericidal effect on non-growing bacteria [24]. This characteristic was later shown to be common to most β-lactam antibiotics, with the exception of imipenem, which was shown to be able to kill non-growing bacteria [5,6]. In our study we could show that all three carbapenems were able to kill non-growing Gram-negative bacteria, but imipenem was the only agent able to kill non-growing S. aureus 209 P.

A pharmacodynamic factor that has attracted great interest during the last 10 years is the postantibiotic effect (PAE), i.e. the inhibition of bacterial growth after short exposure to antibiotics [7,13,25–27]. In general, β-lactam antibiotics do not exert any PAE against Gram-negative bacteria, with the exception of carbapenems, which may produce a PAE on some Gram-negative organisms such as P. aeruginosa, depending on the method used [8, 28–30]. In this study, the PAE of BO-2727 was investigated with two different methods, viable counts and optical density. As shown earlier, there were only minor differences between the two methods [12,14]. No or a negative PAE could be demonstrated against the Gram-negative strains except for one clinical strain of E. coli (2084) and one of A. lwoffi (2035), 0.9 h and 3.1 h respectively. The negative PAE seen in the study is explained by the formation of filamentous bacteria during the exposure to BO-2727. At the time of drug removal and during the postantibiotic phase, cell division occurs, yielding a more rapid increase in the number of bacterial cells than in the control cultures. Only with lower inoculum at the induction of the PAE (inoculum of 103 CFU/mL) could a PAE of over 1 h be seen against P. aeruginosa with viable counts (data not shown). This is in agreement with earlier studies on imipenem and meropenem [8,11]. A short PAE against most of the S. aureus strains was noted in the BioScreen.

When PAE is determined, the bacteria are exposed to the antibiotic for a limited period of time at a given constant concentration, followed by removal of the antibiotic. This is in contrast to the clinical situation, where bacteria will be exposed to suprainhibitory concentrations followed by subinhibitory levels (sub-MICs). We have previously studied the influence of the effect of sub-MICs on bacteria in the postantibiotic phase, and found a very long delay in bacterial regrowth for many antibiotic classes and different bacterial species (postantibiotic sub-MIC effect; PA SME) [31–35]. In the present study, we showed that all strains of A. lwoffi and E. coli were extremely sensitive to sub-MICs of BO-2727 in the postantibiotic phase, with a delay in regrowth up to more than 20 h at 0.4 X MIC. The PA SMEs for P. aeruginosa and S. aureus varied among the strains but were, in general, shorter. The values of PA SME for P. aeruginosa corresponded well with those found with meropenem in an earlier study [14].

Studies of the pharmacodynamics of antibiotics in vitro, in animal models and in humans have shown that the optimal dosing regimen varies with different antibiotics, different bacterial species and the status of the host defense [36–41]. Experimental studies in immunocompromised animals and clinical studies in immunocompromised patients have shown that the time for which free serum levels exceed the MIC is the major determinant of efficacy of β-lactam antibiotics [36–38]. In the present study, no difference between the three carbapenems in the studied pharmacodynamic parameters was noted. This class of antibiotics seems to show variable concentration-dependent killing depending on the bacterial species studied, a paradoxical effect against S. aureus, a short PAE or no PAE at all, and a variable PA SME. Therefore, it seems that the most important parameter predicting efficacy in vitro is the time above the MIC. This has also been shown in vivo for imipenem in an animal model [36,38] but clinical trials with different dosing regimens must be performed to confirm the hypothesis.

Acknowledgment

This study was supported by a grant from Merck, Sharp and Dohme, Germany.

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