An ex-vivo pharmacodynamic study comparing bactericidal activity of amoxicillin/sulbactam, azithromycin, doxycycline and levofloxacin against Streptococcus pneumoniae.

We designed a 4-way crossover, ex-vivo pharmacodynamic study to compare the bactericidal rate of amoxicillin/sulbactam (AMX-SUL), azithromycin (AZM), doxycycline (DOX) and levofloxacin (LVX) against Streptococcus pneumoniae ATCC 49619. Six volunteers were randomized to receive alternatively a single tablet of the above drugs. Venous blood samples were obtained immediately before and at 2, 4 and 6 h after dose to perform time-kill studies and to determine antibiotic levels in serum. AMX-SUL was the only drug showing bactericidal activity with the sera obtained at every time after dose, as defined by a > or = 3-log10 cfu/ml decrease in the viable cell counts compared to the original inoculum after a 24-h incubation. AZM was only inhibitory at 2h after dose (i.e. a 1.3-log10 cfu/ml decrease in the viable cell counts) and proved bactericidal at 4 and 6 h post-dose. LVX proved bactericidal with the 2-h serum, was only inhibitory with the 4-h serum (e.g. a 1.5-log10 cfu/ml decrease) and was unable to avoid bacterial growth at 6 h post-dose. Bacterial growth was observed with DOX at every time after dose. This study may shed light on the understanding of breakthrough pneumococcal bacteremia during the course of oral therapy with AZM in patients with community-acquired nia (CAP), as well as the increasing treatment failures observed with LVX, and the selection of bacterial resistance during therapy reported with both drugs. It also provides the basis for a "warning signal" on the use of oral DOX and confirms the efficacy of AMX-SUL.