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Intra- and extracellular activity of linezolid against Staphylococcus aureus in vivo and in vitro
Anne Sandberg1, Klaus Skovbo Jensen, Pierre Baudoux
1National Center for Antimicrobials & Infection Control, Statens Serum Institut, 5 Artillerivej, DK-2300 Copenhagen S, Denmark.
Background And Aims:
Treatment of Staphylococcus aureus infections remains problematic (slow responses and frequent recurrences). Intracellular persistence of the S. aureus could explain those difficulties because of impaired intracellular efficacy of antibiotics. Our aim was to study linezolid for its intracellular activity.
Methods:
(i) Pharmacodynamic (PD) analysis of intracellular activity using in vitro (THP-1 macrophages) and in vivo (mouse peritonitis) models with determination of key dose-response parameters [maximal relative efficacy (E(max)), relative potency (EC(50)) and static concentration (C(static))] towards methicillin-susceptible S. aureus (ATCC 25923; clinical isolate) with linezolid MICs of 4 mg/L; (ii) pharmacokinetic (PK) analysis in uninfected mice for determination of C(max), AUC and half-life for total and free drug; and (iii) determination of the predictive PK/PD parameter (fT > MIC, fAUC(24)/MIC or fC(max)/MIC) for therapeutic outcome.
Results:
In vitro, linezolid showed an E(max) of approximately 1 log(10) cfu reduction compared with initial inoculum both intra- and extracellularly and an approximately 3-fold increased relative potency (lower EC(50) and C(static)) intracellularly. In vivo, the efficacy of linezolid was impaired (<0.5 log(10) reduction extracellularly; failure to reduce the cfu to less than the initial load intracellularly) with, however, an increased intracellular potency (lower EC(50)). Infection outcome correlated better with the fAUC(24)/MIC (R(2) = 55%) than with the fT > MIC parameter (R(2) = 51%) for the extracellular compartment, but no parameter emerged as significant for the intracellular compartment.
Conclusions:
Linezolid exerts only a weak intracellular activity against the strains of S. aureus tested, even though, in contrast to most other antibiotics, its potency does not appear impaired in comparison with the extracellular activity.
Insights
Linezolid shows limited intracellular effectiveness against Staphylococcus aureus infections, despite maintaining potency within cells. Further research is needed to understand its impact on persistent intracellular bacteria.
Area of Science:
- Pharmacology
- Microbiology
- Infectious Diseases
Background:
- Staphylococcus aureus infections present treatment challenges due to slow responses and recurrences.
- Intracellular persistence of S. aureus may contribute to treatment difficulties by reducing antibiotic efficacy.
- Understanding intracellular antibiotic activity is crucial for effective S. aureus infection management.
Purpose of the Study:
- To evaluate the intracellular activity of linezolid against Staphylococcus aureus.
- To compare the pharmacodynamic and pharmacokinetic properties of linezolid intracellularly versus extracellularly.
- To identify predictive pharmacokinetic/pharmacodynamic parameters for linezolid's therapeutic outcome in S. aureus infections.
Main Methods:
- In vitro pharmacodynamic (PD) analysis in THP-1 macrophages and in vivo models (mouse peritonitis).
- Determination of key dose-response parameters (Emax, EC50, Cstatic) for linezolid against S. aureus.
- Pharmacokinetic (PK) analysis in mice and assessment of predictive PK/PD parameters (fT > MIC, fAUC24/MIC, fCmax/MIC).
Main Results:
- Linezolid demonstrated weak intracellular efficacy (approx. 1 log10 cfu reduction) but maintained potency intracellularly.
- In vivo, linezolid's efficacy was impaired extracellularly and failed to reduce intracellular bacterial load.
- Infection outcome correlated better with fAUC24/MIC (55%) than fT > MIC (51%) for extracellular bacteria; no significant intracellular parameter was identified.
Conclusions:
- Linezolid exhibits limited intracellular activity against Staphylococcus aureus strains.
- While intracellular potency is not impaired, overall intracellular efficacy remains weak.
- Further investigation is required to optimize linezolid therapy for intracellular S. aureus infections.
