Pharmacodynamic activity of ertapenem versus penicillin-susceptible and penicillin-non-susceptible Streptococcus pneumoniae using an in vitro model.
BACKGROUND
Ertapenem is a novel carbapenem with activity against both penicillin-susceptible (MIC < or = 0.06 mg/L) and penicillin-non-susceptible (MIC > or = 0.12 mg/L) Streptococcus pneumoniae. This study assessed the pharmacodynamic activity of ertapenem against penicillin-susceptible and penicillin-non-susceptible S. pneumoniae using an in vitro pharmacodynamic model.


METHODS
Fifteen S. pneumoniae strains including 3 penicillin-susceptible and 12 penicillin-non-susceptible [4 penicillin-intermediate (MIC 0.12-1 mg/L) and 8 penicillin-resistant (MIC > or = 2 mg/L); with different resistance phenotypes including erythromycin-resistant (MIC > or = 1 mg/L), ciprofloxacin-resistant (MIC > or = 4 mg/L) and doxycycline-resistant (MIC > or = 8 mg/L)] were studied. The in vitro pharmacodynamic model was inoculated with 1 x 10(6) cfu/mL and ertapenem was dosed once daily at 0 and 24 h to simulate f (free) Cmax and t(1/2) obtained after a standard 1 g intravenous once daily dose in healthy volunteers (fCmax 15 mg/L, t(1/2) 4 h). Sampling was performed for 48 h to assess viable growth.


RESULTS
Ertapenem T(> MIC) > or = 80% (ertapenem MICs < or = 0.5 mg/L) resulted in bactericidal (> or = 3 log10 killing) activity at 12, 24 and 48 h with complete eradication of penicillin-susceptible and penicillin-non-susceptible S. pneumoniae from the model with no regrowth over the 48 h study period. Ertapenem T(> MIC) < or = 63% (ertapenem MIC > or = 1 mg/L) resulted in bactericidal activity at 12 h with regrowth at 24 and 48 h. The observed MICs for S. pneumoniae of ertapenem studied in the in vitro model did not change during the 48 h period, even for strains where regrowth occurred.


CONCLUSIONS
Ertapenem is bactericidal against both penicillin-susceptible and penicillin-non-susceptible S. pneumoniae (ertapenem MICs < or = 0.5 mg/L) when simulating free drug after 1 g intravenous once daily dosing.
Zhanel
George G
GG
Department of Medical Microbiology, Faculty of Medicine, University of Manitoba, 820 Sherbrook Street, Winnipeg, R3A 1R9, Canada. ggzhanel@pcs.mb.ca
Derkatch
Sheldon
S
Laing
Nancy
N
Noreddin
Ayman M
AM
Hoban
Daryl J
DJ
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D000077727:Ertapenem; D006801:Humans; D008826:Microbial Sensitivity Tests; D010403:Penicillin Resistance; D013296:Streptococcus pneumoniae / Q000187:drug effects*; D047090:beta-Lactams / Q000008:administration & dosage / Q000494:pharmacology*
D000900:Anti-Bacterial Agents; D047090:beta-Lactams; D000077727:Ertapenem