Histological Quantification of Gene Silencing by Intratracheal Administration of Dry Powdered Small-Interfering RNA/Chitosan Complexes in the Murine Lung

The use of small-interfering RNA (siRNA) as an inhalation therapy has recently received much attention. Some reports have confirmed the suppression of gene expression in whole lungs following intratracheal administration of dry powdered siRNA; however, the anatomical location in the lung where gene silencing occurs has not been precisely identified. Here, we aimed to histologically evaluate gene silencing efficacy in murine lungs by intratracheal administration of an siRNA/chitosan complex as a dry powder. Enhanced green fluorescence protein (EGFP)-specific siRNA (EGFP-siRNA)/chitosan powder was prepared and administered intratracheally to EGFP transgenic mice or mice carrying metastatic lung tumors consisting of Lewis lung carcinoma (LLC) cells stably expressing EGFP (EGFP-LLCs). Thereafter, green fluorescence intensities were quantified in the airways, parenchyma, and lung tumors. Intratracheal administration of the EGFP-siRNA/chitosan powder suppressed EGFP expression in the bronchi, bronchioles, and alveolar walls of EGFP transgenic mice. Additionally, EGFP-siRNA/chitosan effectively silenced EGFP expression in lung tumors consisting of EGFP-LLC cells. Pulmonary administration of siRNA/chitosan powder suppressed gene expression throughout the lung and in lung tumors. Therefore, this may become a powerful strategy to target genes expressed in a wide range of respiratory diseases involving the airways, parenchyma, and lung tumors.