Target site drug determinations are crucial for optimizing treatment of infectious diseases. There is limited knowledge of antibiotic drug penetration into the pulmonary epithelial lining fluid (PELF) and a lack of easily performed methods for continuous drug sampling hereof. The aim of this study was to develop a readily accessible microdialysis (MD) method for antibiotic drug quantification in PELF of pigs. The fluoroquinolone danofloxacin was administered to anaesthetized pigs allocated to three groups: intravenous injection, intravenous infusion and intramuscular injection. MD probes were guided through a tracheostomy into the distal bronchioles using an insertion tube. Intravenously administered inulin served as a marker of extracellular fluid contamination of PELF. Concentrations of free drug in MD fractions were compared to total and non-protein-bound drug concentrations in plasma. Rising and declining danofloxacin plasma concentrations were rapidly reflected in PELF, suggesting an efficient drug transport across the blood bronchial barrier. The AUC FREE DRUG PELF /AUC FREE DRUG PLASMA ratio was 1.8 (S.D. 0.4, 95% CL 1.4-2.3). Although the probes were placed without fiberscopic or other special equipment, the danofloxacin concentrations in PELF were consistent within the different administration groups. The described MD method for drug quantification in PELF is easily accessible and provides repeatable results. However, trace amount of inulin was detected in the MD fractions, suggesting a local tissue reaction induced by the MD membrane. The significance of this finding needs to be clarified in future studies.

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Recent intrabronchial microdialysis data indicate that the respiratory epithelium is highly permeable to drugs. Of concern is whether intrabronchial microdialysis disrupts the integrity of the respiratory epithelium and thereby alters drug penetration into the pulmonary epithelial lining fluid (PELF). The objective of this study was to investigate the effect of intrabronchial microdialysis on the integrity of the bronchial epithelium. Microdialysis sampling in PELF in proximal (n = 4) and distal bronchi (n = 4) was performed after intravenous inulin and florfenicol administration in anaesthetized pigs. Inulin was used as a marker molecule of permeability of the epithelium, and florfenicol was used as test drug. Bronchial tissue was examined by histopathology (distal and proximal bronchi) and gene expression analysis (RT-qPCR, proximal bronchi) at the termination of the experiment (6.5 hr). The microdialysis probe caused overt tissue trauma in distal bronchi, whereas no histopathological lesions were observed in proximal bronchi. A moderate up-regulation of the pro-inflammatory cytokines IL1B, IL6 and acute-phase reactant serum amyloid A was seen in proximal bronchi surrounding the microdialysis probes suggesting initiation of an inflammatory response. The observed up-regulation is considered to have limited impact on drug penetration during short-term studies. Inulin penetrated the respiratory epithelium in both proximal and distal bronchi without any correlation to histopathological lesions. Likewise, florfenicol penetration into PELF was unaffected by bronchial histopathology. However, this independency of pathology on drug penetration may not be valid for other antibiotics. We conclude that short-term microdialysis drug quantification can be performed in proximal bronchi without disruption of tissue integrity.