Development of a new experimental model of penicillin-resistant Streptococcus pneumoniae pneumonia and amoxicillin treatment by reproducing human pharmacokinetics

Abstract

The increase of penicillin-resistant Streptococcus pneumoniae (PRSP) pneumonia results in a greater risk of antibiotic treatment failure. In vitro data are not sufficient predictors of clinical efficacy, and animal models may be insufficiently contributive, since they often use immunocompromised animals and do not always respect the human pharmacokinetics of antibiotics. We developed an experimental PRSP pneumonia model in immunocompetent rabbits, by using intrabronchial instillation of PRSP (MIC = 4 mg/liter), without any adjuvant. This reproducible model was used to assess amoxicillin efficacy by reproducing human serum pharmacokinetics following 1-g oral or intravenous administrations of amoxicillin every 8 h. Evaluation was performed by using clinical, CT scan, macroscopic, histopathologic, and microbiological criteria. Experimental pneumonia in untreated rabbits was similar to untreated severe human bacteremic untreated pneumonia; in both rabbits and humans, (i) cumulative survival was close to 50%, (ii) red or gray lung congestion and pleuritis were observed, and (iii) lung and spleen concentrations reached 5 and 4 log(10) CFU/g. A 48-h treatment resulted in a significant bacterial clearance in the lungs (1.53 versus 5.07 log(10) CFU/ml, P < 0.001) and spleen (1.00 versus 4.40 log(10) CFU/ml, P < 10(-6)) and a significant decrease in mortality (0% versus 50%, P = 0.02) in treated versus untreated rabbits. No difference was observed on macroscopic and histopathologic lesions between treated and untreated rabbits (P = 0.36 and 0.78, respectively). Similar results were obtained by using a fully penicillin-susceptible S. pneumoniae strain (MIC = 0.01 mg/liter). Our findings suggest that (i) this new model can be contributive in the evaluation of antibacterial agents and (ii) 1 g of amoxicillin three times a day may be sufficient to treat PRSP pneumonia in immunocompetent humans.

FIG. 1

Cumulative survival of rabbits with penicillin-resistant (strain 16089) and penicillin-susceptible (strain 195) S. pneumoniae experimental pneumonia, according to inoculum concentration (10 log₁₀ versus 7 or 8.5 log₁₀ CFU/ml, P < 0.05, log-rank test). Symbols: -----, strain 16089 (inoculum, 10 log₁₀); ——, strain 195 (inoculum, 10 log₁₀); –-–-, strain 16089 (inoculums, 7 and 8.5 log₁₀).

FIG. 2

Pulmonary lesion scores and pneumococcal concentration evolution in rabbits with penicillin-resistant S. pneumoniae (strain 16089) experimental pneumonia by using 10 log₁₀ CFU/ml inoculum. Symbols: , macroscopic score; , histopathologic score; ---○---, lung pneumococcal concentrations; ---●---, spleen pneumococcal concentrations.

FIG. 3

CT scan examination of penicillin-resistant S. pneumoniae (strain 16089) experimental pneumonia at 24 h. The alveolar condensation of the entire left lower lobe (∗) is due to active pneumonia, in contrast with normal aspects of the right lower and median lobes.

FIG. 4

CT bidimensional coronal reconstruction of transaxial sections. Shown is the evolution of penicillin-resistant S. pneumoniae (strain 16089) experimental pneumonia at 12 h (A), 36 h (B), and 60 h (C). Also shown is alveolar condensation of the left lower lobe (∗) 12 h after inoculation, with a progressive extension to the entire left lung at 60 h. In this case, the right lung does not present radiographic signs of pneumonia.

FIG. 5

Lung histopathologic examination of penicillin-resistant S. pneumoniae (strain 16089) experimental pneumonia at 24 h (A) and 48 h (B). Pneumonia with polymorphonuclear leukocytes and fibrinous exudate fill up the alveoli. Hematoxylin-eosin-safranin stain was applied to the sections. Original magnification, ×250.

FIG. 6

Pulmonary lesions and pneumococcal concentration evolution in rabbits with penicillin-susceptible S. pneumoniae (strain 195) experimental pneumonia by using 10 log₁₀ CFU/ml. Symbols: , macroscopic score; , histopathologic score; ---○---, lung pneumococcal concentrations; ---●---, spleen pneumococcal concentrations.

FIG. 7

Pharmacokinetics of amoxicillin in rabbits and open bicompartmental modelization. Symbols: ---□---, calculated concentrations; —●—, measured concentrations.

FIG. 8

Human amoxicillin pharmacokinetics simulation in rabbits, reproducing human serum profiles following 1-g dose administered intravenously (A) or orally (B). Symbols: , obtained concentrations under the human pharmacokinetics simulation; ---▾---, native concentrations without the controlled infusions; ---▵---, expected human concentrations.