Efficacy of ampicillin combined with ceftriaxone and gentamicin in the treatment of experimental endocarditis due to Enterococcus faecalis with no high-level resistance to aminoglycosides

Joan Gavaldà*, Pedro López Onrubia, María Teresa Martín Gómez, Xavier Gomis, José Luis Ramírez, Oscar Len, Dolors Rodríguez, Manuel Crespo, Isabel Ruíz and Albert Pahissa

Infectious Diseases Research Laboratory, Infectious Diseases Division, Hospital Universitari Vall d’Hebron, Universitat Autònoma de Barcelona, Avda. Vall d’Hebron, 119–129, 08035 Barcelona, Spain

Received 24 March 2003; returned 25 April 2003; revised 4 June 2003; accepted 6 June 2003


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
Objective: This study tests the usefulness of ceftriaxone combined with ampicillin as an alternative to ampicillin plus gentamicin for the treatment of experimental endocarditis due to Enterococcus faecalis without high-level resistance to aminoglycosides. It also determines whether adding ceftriaxone to ampicillin and gentamicin increases the effectiveness against experimental enterococcal endocarditis resulting from E. faecalis.

Methods: Animals with catheter-induced endocarditis were infected intravenously with 108 cfu of the EF91 strain of E. faecalis and were treated for 3 days with ampicillin 2 g every 4 h administered as ‘human-like’ (H-L) pharmacokinetics, plus gentamicin 1 mg/kg every 8 h H-L, or ceftriaxone 2 g every 12 h H-L alone or combined with gentamicin 6 mg/kg every 24 h administered subcutaneously.

Results: The results of therapy for experimental endocarditis resulting from EF91 showed that the combination of ampicillin plus ceftriaxone was as effective as ampicillin plus gentamicin. The triple combination did not improve on the overall efficacies of the two-drug combinations.

Conclusions: Because of its lower nephrotoxicity, ampicillin plus ceftriaxone may be a useful alternative therapy for E. faecalis endocarditis in selected patients.

Keywords: bacteria, intravascular infections, animal models, antibiotic combinations


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
The combination of penicillin or ampicillin plus an aminoglycoside for 4–6 weeks is recommended for the treatment of enterococcal endocarditis.1 Nevertheless, prolonged therapy with aminoglycosides carries a risk of nephro- and oto-toxicity, particularly in the elderly,1 the population most frequently succumbing to this infection. In previous work, our group demonstrated the efficacy of ampicillin plus ceftriaxone for the treatment of experimental endocarditis resulting from Enterococcus faecalis strains highly resistant to aminoglycosides (HLRAg).2 This combination could be an attractive therapeutic alternative, having potentially lower toxicity than the standard therapy for E. faecalis endocarditis. The aims of the present study were to evaluate the efficacy of ampicillin plus ceftriaxone in the treatment of experimental endocarditis due to non-HLRAg E. faecalis, and to test the effectiveness of a triple combination, adding ceftriaxone to ampicillin and gentamicin, to see whether in vivo efficacy was increased.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
The study strain was E. faecalis EF91, recovered from a patient with endocarditis. MICs/MBCs were determined using Mueller–Hinton broth and a standard microdilution method following NCCLS guidelines.3 Qualitative estimation of the synergy between ampicillin, ceftriaxone and gentamicin (bacteriostatic interaction) was obtained using a disc diffusion method (ampicillin 10 µg; gentamicin 30 µg; ceftriaxone 30 µg).4 In addition, time–kill synergy studies were performed in quintuplicate, following accepted methods, in Mueller–Hinton broth, using an inoculum of 107 cfu/mL and an ampicillin concentration of 1 mg/L either alone or in combination with gentamicin (final concentrations: 1, 3 and 10 mg/L) or ceftriaxone (final concentrations: 10, 20 and 40 mg/L).4

Induction of aortic endocarditis, and insertion of a catheter into the inferior vena cava to administer ampicillin, gentamicin and ceftriaxone treatment in New Zealand rabbits (weight: 2–2.1 kg) has been described previously.5,6 Forty-eight hours after inducing infection with a 1 mL inoculum containing 108 cfu of EF91 strain, a 3 day treatment protocol was started in infected rabbits, randomized into the following treatment groups: control, without treatment; ampicillin 2 g every 4 h, using human-like (H-L) pharmacokinetics iv; ampicillin 2 g every 4 h H-L iv + gentamicin 1 mg/kg every 8 h H-L iv; ampicillin 2 g every 4 h H-L iv + ceftriaxone 2 g every 12 h H-L iv; and ampicillin 2 g every 4 h H-L iv + ceftriaxone 2 g every 12 h H-L iv + gentamicin 6 mg/kg every 24 h subcutaneously (sc). Intravenous drugs were administered using a computer-controlled infusion pump system (infusion pump, Alice King, Hawthorne, CA, USA) that reproduces human serum pharmacokinetics in rabbits. The pharmacokinetic data for ampicillin, ceftriaxone and gentamicin administered iv to mimic human pharmacokinetics have been published previously,2,5,6 and are summarized in Table 1. The pharmacokinetic parameters of gentamicin 6 mg/kg every 24 h sc have also been published.5 The experimental protocol was approved by the Ethics Committee for Animal Experimentation of Vall d’Hebron Hospitals and the Agricultura, Ramaderia i Pesca Department of the Generalitat de Catalunya.


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Table 1. Pharmacokinetic parameters of ampicillin, gentamicin and ceftriaxone iv in rabbit, human and rabbit H-L
 
Six hours after completing the final antibiotic infusion, the animals were killed and colony counts in the vegetations determined, as previously described.5,6 Bacterial concentrations in vegetations from the various treatment groups were compared using one-way analysis of variance. Each treatment group was compared with the control group and with each of the other treatment groups using Scheffée’s test. P values <= 0.05 were considered significant.


    Results and discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
The MICs of ampicillin, gentamicin and ceftriaxone were 0.5, 32 and 64 mg/L, respectively; MBCs were 32, 64 and >256 mg/L. Synergistic and bactericidal effects were obtained with combinations of ampicillin and gentamicin (at 1, 3 or 10 mg/L), and ampicillin and ceftriaxone (at 10, 20 or 40 mg/L) after 4 and 24 h of incubation. Susceptibility studies with agar diffusion techniques showed synergy between ampicillin, gentamicin and ceftriaxone.

Results of therapy for EF91 experimental endocarditis are shown in Table 2. After 3 days of treatment, bacterial counts were reduced in vegetations of the treated animals compared with the control group (P < 0.05). Combinations of ampicillin with gentamicin or ceftriaxone were more effective than ampicillin alone (P < 0.05). Treatment with ampicillin plus ceftriaxone was as effective as ampicillin plus gentamicin (P not significant). Ampicillin combined with gentamicin and ceftriaxone was not superior to ampicillin plus gentamicin or ampicillin plus ceftriaxone (P not significant).


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Table 2. Results of the therapeutic groups of rabbit experimental endocarditis due to the Enterococcus faecalis EF91 strain, using H-L pharmacokinetics
 
The objectives of this work were: first, to test the usefulness of ceftriaxone combined with ampicillin as an alternative to the gold standard, ampicillin plus gentamicin, for the treatment of endocarditis caused by an E. faecalis strain with no resistance to ß-lactams or aminoglycosides; and second, to determine whether efficacy against this disease increases with use of a triple combination: ceftriaxone, ampicillin and gentamicin. Our results showed that ampicillin plus ceftriaxone was as effective as the treatment of choice (ampicillin plus gentamicin) for experimental endocarditis. The triple combination did not improve on the overall efficacies of the two-drug combinations.

The present in vitro studies demonstrated synergistic activity between ampicillin and ceftriaxone against EF91. A similar synergistic in vitro effect was detected by Mainardi et al., with amoxicillin and cefotaxime.7 Nevertheless, this combination did not show in vivo efficacy in the treatment of experimental endocarditis resulting from two E. faecalis strains, one with and one without high-level resistance to aminoglycosides. The authors concluded that in vivo efficacy was absent because amoxicillin alone was bactericidal in vivo and the period of time during which both antibiotics were present simultaneously and at adequate concentrations to obtain in vivo synergy, was limited.8

In contrast, our in vivo results show that the combination of ampicillin plus ceftriaxone was as effective as ampicillin plus gentamicin administered in three daily doses. The efficacy of the combination of ampicillin plus ceftriaxone was comparable to the efficacy we found for ceftriaxone 2 g every 12 h H-L combined with ampicillin 2 g every 4 h H-L in the treatment of HLRAg E. faecalis experimental endocarditis.2 The discrepancies between our results and those reported by Join-Lambert et al.8 can be attributed to dissimilarities among the experimental variables. The ß-lactams in the two studies were different and might have had dissimilar activities against E. faecalis, and our experimental model simulated human kinetics of ampicillin 2 g every 4 h iv and ceftriaxone 2 g every 12 h iv. The relationship between the shape of the ß-lactam AUCs and their antimicrobial activity has been established; thus, simulation of human serum pharmacokinetics comes closer to the human clinical situation.

The combination of ampicillin plus ceftriaxone broadens the range of alternative therapies for the treatment of enterococcal endocarditis. Toxicity resulting from aminoglycosides depends mainly on the duration of treatment, the age of the patient and total amount administered.1 Since enterococcal endocarditis appears generally in older patients, and age is associated with a higher risk of nephrotoxicity from aminoglycosides,1 these patients may benefit from less-toxic therapy. Our group recently reported our experience with 20 episodes of enterococcal endocarditis treated with ampicillin combined with ceftriaxone or cefotaxime, including 13 episodes caused by strains with HLRAg and seven resulting from non-resistant strains.9 Sixteen patients received at least 1 month of cephalosporin therapy and all were cured; only three patients underwent surgery and none developed renal failure. We concluded that the combination of ampicillin plus ceftriaxone or cefotaxime was safe and effective in the treatment of enterococcal endocarditis caused by strains with and without HLRAg.

Enterococcal endocarditis still lacks a completely satisfactory treatment. The standard therapy, ampicillin plus gentamicin, does not cure all cases and its prolonged duration poses a risk to patients. Effort must be made to find combinations that can cure a higher percentage of cases, and efficacious treatments of a shorter duration. In the present study, the efficacy obtained with the addition of ceftriaxone to the combination of ampicillin plus gentamicin was similar to that of ampicillin combined with gentamicin or ceftriaxone. The triple combination did not improve on the overall efficacies of the two-drug combinations. There is little experience with triple-drug combinations for the treatment of enterococcal experimental endocarditis. Caron et al.10 found that combined penicillin, gentamicin and vancomycin was effective for experimental endocarditis caused by E. faecium highly-resistant to penicillin and glycopeptides, whereas combined penicillin or vancomycin plus gentamicin was not effect-ive. These authors detected the emergence of a subpopulation resistant to the combination of penicillin plus vancomycin, and this reduced the utility of this combination.

In conclusion, the efficacy of ampicillin and ceftriaxone combined in our experiments was comparable to that of ampicillin plus gentamicin. Since there is a relatively lower risk of nephrotoxicity with ampicillin plus ceftriaxone, this combination could be valuable for treating enterococcal endocarditis resulting from non-HLRAg strains in elderly patients, or in those with previous renal disease or signs of toxicity. Further studies are needed to establish the true efficacy of this therapeutic approach in humans with enterococcal endocarditis.


    Acknowledgements
 
We thank Celine Cavallo for assistance with our written English.

This work was supported in part by Fondo Investigaciones Sanitarias de la Seguridad Social (FISS grant no. 98/0426).


    Footnotes
 
* Corresponding author. Tel: +34-93-4894033; Fax: +34-93-2746057; E-mail: gavalda{at}hg.vhebron.es Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results and discussion
 References
 
1 . Scheld, W. M. & Sande, M. A. (1995). Endocarditis and intravascular infections. In Mandell, Douglas and Bennett’s Principles and Practice of Infectious Diseases, 4th edn (Mandell, G. L., Bennett, J. E. & Dolin, R., Eds), pp. 740–83. Churchill-Livingstone, New York, NY, USA.

2 . Gavaldà, J., Torres, C., Tenorio, C. et al. (1999). Efficacy of ampicillin plus ceftriaxone in treatment of experimental endocarditis due to Enterococcus faecalis strains highly resistant to aminoglycosides. Antimicrobial Agents and Chemotherapy 43, 639–46.[Abstract/Free Full Text]

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

4 . Eliopoulos, G. M. & Moellering, R. C. (1996). Antimicrobial combinations. In Antibiotics in Laboratory Medicine (Lorian, V. Ed.), pp. 330–96. Williams & Wilkins, Baltimore, MA, USA.

5 . Gavaldà, J., Cardona, P. J., Capdevila, J. A. et al. (1996). Treatment of experimental endocarditis due to Enterococcus faecalis using a once-daily dosing regimen of gentamicin plus simulated profiles of ampicillin in human serum. Antimicrobial Agents and Chemotherapy 40, 173–8.[Abstract]

6 . López, P., Gavaldà, J., Martín, M. T. et al. (2001). Efficacy of teicoplanin-gentamicin given once a day on the basis of pharmacokinetics in humans for treatment of enterococcal experimental endocarditis. Antimicrobial Agents and Chemotherapy 45, 1387–93.[Abstract/Free Full Text]

7 . Mainardi, J. L., Gutmann, L., Acar, J. F. et al. (1995). Synergistic effect of amoxicillin and cefotaxime against Enterococcus faecalis. Antimicrobial Agents and Chemotherapy 39, 1984–7.[Abstract]

8 . Join-Lambert, O., Mainardi, J. L., Cuvelier, C. et al. (1998). Critical importance of in vivo amoxicillin and cefotaxime concentrations for synergy in treatment of experimental Enterococcus faecalis endocarditis. Antimicrobial Agents and Chemotherapy 42, 468–70.[Abstract/Free Full Text]

9 . Gavaldà, J., Miró, J. M., Torres, C. et al. Efficacy of ampicillin (A) plus ceftriaxone (Ctr) or cefotaxime (Cx) in the treatment of endocarditis due to Enterococcus faecalis. In Programs and Abstracts of the Forty-first Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, IL, 2001. Abstract 1342, p. 466. American Society for Microbiology, Washington, DC, USA.

10 . Caron, F., Lemeland, J. F., Humbert, G. et al. (1993). Triple combination penicillin-vancomycin-gentamicin for experimental endocarditis caused by a highly penicillin-and glycopeptide-resistant isolate of Enterococcus faecium. Journal of Infectious Diseases 168, 681–6.[ISI][Medline]





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