In vitro activity of linezolid against Gram-positive isolates causing infection in continuous ambulatory peritoneal dialysis patients

K. E. Bowker,*, M. Wootton, H. A. Holt and A. P. MacGowan

Bristol Centre for Antimicrobial Research and Evaluation, North Bristol NHS Trust, Southmead Hospital, Westbury on Trym, Bristol BS10 5NB, UK

Sir,

Continuous ambulatory peritoneal dialysis (CAPD) is a common means of support for patients with end stage renal disease. The most important complication of CAPD is infection of the peritoneal cavity or the catheter exit site, usually caused by Gram-positive organisms such as coagulase-negative staphylococci (CNS), Staphylococcus aureus, enterococci, corynebacteria and viridans streptococci. Linezolid, which has recently been licensed for a range of clinical indications, has activity against a wide range of Gram-positive bacteria and also offers the possibility of oral therapy.

In this study we tested the in vitro activity of a range of antimicrobials against Gram-positive organisms isolated from patients undergoing CAPD and investigated the possible effects of used and unused peritoneal dialysis (PD) fluid on the activity of linezolid. One hundred and fifty-one Gram-positive isolates were tested for susceptibility to linezolid, vancomycin, ciprofloxacin, teicoplanin, netilmicin, gentamicin, rifampicin, moxifloxacin and quinupristin– dalfopristin using NCCLS methodology. The MIC50, MIC90 and percentage susceptible, intermediate and resistant according to NCCLS breakpoints and the licensing authorities were determined.1

In addition, the bactericidal activity of linezolid was assessed using time–kill curve methodology; IsoSensitest Broth (ISB), pooled used PD fluid from several patients and unused PD fluid were used. Initial experiments were performed to ensure that the pooled PD fluid did not have any antibacterial effect. Linezolid concentrations of 1, 10 and 100 mg/L were tested and an antibiotic-free control broth was included for each strain. Three strains each of S. aureus, Enterococcus spp. and CNS were employed (linezolid MICs 0.5, 2 and 0.5 mg/L). Viable counts were determined using spiral plater methodology with the limit of detection being 102 cfu/mL. Time–kill kinetics were plotted as log10 cfu/mL versus time. The antibacterial effect was measured by log reduction in viable count at 24 h and the area under the bacterial time–kill curve 0–24 h (AUBKC0–24).

The in vitro activity of linezolid in comparison with the other antimicrobial agents tested is shown in the TableGo. Linezolid had a narrow range of MICs for all strains of S. aureus and CNS (0.5–2 mg/L) and was equipotent with vancomycin. All strains were susceptible using a breakpoint of <=4 mg/L (MIC90 1 mg/L).2 Enterococcus spp. including vancomycin-resistant strains, Corynebacterium spp. and viridans streptococci were all susceptible to linezolid with MIC90 values of 2 and 1 mg/L, respectively. Of the other agents tested, quinupristin–dalfopristin was the most potent with >94% of all strains being susceptible. Moxifloxacin, representing the newer fluoroquinolones, was more potent than linezolid against S. aureus and Enterococcus spp. but was less potent against CNS and corynebacteria/viridans streptococci.


View this table:
[in this window]
[in a new window]
 
Table. In vitro activity of linezolid against Gram-positive isolates
 
The time–kill experiments confirmed the bacteriostatic action of linezolid; increasing the linezolid concentration from 1 to 100 mg/L did not significantly increase the rate of killing. There was no significant difference observed between the used pooled and the unused PD fluids at any of the linezolid concentrations (P >0.05). However, in general there was more bacterial killing when ISB was used. At a linezolid concentration of 1 mg/L, a 0.5 log reduction in viable count was observed for all strains in PD fluid at 24 h. Increasing the concentration to 10 mg/L did not significantly increase bacterial killing in any of the matrices. These results were confirmed when bacterial killing was measured by the AUBKC0–24; no significant differences occurred between the matrices and concentrations, with the exception of S. aureus where a 2 log reduction in count was noted with ISB at 100 mg/L.

Linezolid is cleared by both renal and non-renal routes, and the limited pharmacokinetic data available from patients with renal disease who are on dialysis indicate that these patients should have their dose delayed until after dialysis or be given a supplementary dose.3 Phase I trials have shown that after oral administration (375 mg bd) Cmin values were >4 mg/L, exceeding the MIC90 in this study by at least two-fold. Animal and in vitro studies have indicated that T > MIC is the pharmacodynamic parameter that determines efficacy, with a T > MIC target value of 40–60% optimizing outcome.4 The data produced so far indicate that this figure should be reached comfortably with oral dosing for patients with renal deficiency.

With the increase in the incidence of resistance in Gram-positive isolates over recent years the requirement for effective oral treatment has become more acute. Treatment is currently limited to intravenous glycopeptides, and glycopeptide resistance has been reported in CNS, S. aureus and enterococci. Unfortunately, linezolid resistance has recently been reported for both enterococci and S. aureus from patients with renal disease undergoing peritoneal dialysis.5,6

In conclusion, linezolid has clinically useful in vitro activity against a wide range of Gram-positive isolates causing CAPD peritonitis. Furthermore, neither used nor unused PD fluid adversely affects its activity against these isolates.

Notes

* Corresponding author. Tel: +44-117-959-5654; Fax: +44-117-959-3217; E-mail: Bowker_K{at}southmead.swest.nhs.uk Back

References

1 . 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, Villanova, PA.

2 . Noskin, G. A., Siddiqui, F., Stosor, V., Hacek, D. & Peterson, L. R. (1999). In vitro activities of linezolid against important Gram-positive pathogens including vancomycin-resistant enterococci. Antimicrobial Agents and Chemotherapy 43, 2059–62.[Abstract/Free Full Text]

3 . Brier, M. E., Stalker, D. J., Aronoff, G. R., Batts, D. H., Ryan, K. K., O'Grady, M. A. et al. (1998). Pharmacokinetics of linezolid in subjects with various degrees of renal function and on dialysis. In Program and Abstracts of the Thirty-eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, 1998. Abstract A-54, p. 17. American Society for Microbiology, Washington, DC.

4 . Andes, D., Van Ogtrop, M. L. & Craig, W. A. (1998). Pharmacodynamic activity of a new oxazolidinone, linezolid, in an animal infection model. In Program and Abstracts of the Thirty-eighth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, 1998, Abstract A9. American Society for Microbiology, Washington, DC.

5 . Gonzales, R. D., Schreckenberger, P. C., Graham, M. B., Kel-kar, S., DenBesten, K. & Quinn, J. P. (2001). Infections due to vancomycin-resistant Enterococcus faecium resistant to linezolid. Lancet 357, 1179.[ISI][Medline]

6 . Tsoides, S., Gold, H. S., Sakoulas, G., Eliopoulous, G. M., Wennersten, C., Venkataramanm L. et al. (2001). Linezolid resistance in a clinical isolate of Staphylococcus aureus. Lancet 358, 207.[ISI][Medline]