Formulation of inhaled medicines: effect of delivery vehicle on immortalized epithelial cells

The small volume of airway lining fluid renders it susceptible to alteration by the deposition of inhaled formulations. The increasing popularity of the pulmonary route for drug delivery has led to an increasing number of pharmaceutical excipients being incorporated into inhaled dosage forms. The effects of drug delivery vehicle on airway epithelial cells can be studied with the aid of cell culture models of the respiratory epithelium. The effects of pH, osmolarity, and lactose on epithelial cell layers were studied using 16HBE14o- cells. Mannitol flux was used to assess epithelial permeability, enzymatic conversion of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was used as a measure of epithelial cell metabolism, and release of the cytosolic enzyme lactate dehydrogenase was used as a measure of cell integrity. The effect of buffer composition on epithelial cell mucus secretion was studied using HT29-clone H cells, with mucus secretion measured using an enzyme-linked lectin assay. The permeability of 16HBE14o- cell layers was increased by apical fluid of pH 5, 6, and 9 as well as osmolarities of 96 and 545 mOsm. MTT conversion was reduced by apical fluid of pH 5 and 6 and osmolarity of 96 mOsm. Lactate dehydrogenase release was only increased by apical fluid of pH 9. No effect of lactose solution (100 mM) on the epithelial cells was observed. Mucus secretion by HT29-clone H cells was lowest in Dulbecco's modified Eagle medium (2.92 +/- 0.23 ng per cell layer) and was increased in phosphate-buffered saline with magnesium and calcium (4.28 +/- 0.38 ng per cell layer) and phosphate-buffered saline without magnesium and calcium (6.56 +/- 0.72 ng per cell layer). These results suggest that the physicochemical properties of inhaled formulations should be carefully controlled. The effect of buffer composition on mucus secretion suggests that even the application of "physiological" solutions may affect the epithelium. These cell models represent an opportunity to investigate the interaction of drug delivery vehicles with the epithelium.