The aim of this study was to use surface modification of microparticles to assess whether steric stabilisation can reduce macrophage uptake, hence allowing the possibility of improved targeting of epithelial and dendritic cells in the deep lung. In this project, the level of cellular association between the modified particles and cells most representative of deep lung epithelium was examined. Three different epithelial related cells systems were used: A549 (a model for type II pneumocytes), BEAS-2B (a model for small airway epithelial cells) and undifferentiated primary human bronchial epithelial cells (HBECs ). Statistical comparisons were made using t tests or ANOVA as appropriate.

Poly (D,L lactide-co-glycolide) (PLGA) microparticles were surface modified with polyethylene oxide based surfactants. Spherical and smooth surface microspheres in the size range of 1-4 µm, appropriate for inhalation, were produced under defined conditions using certain combinations of Pluronic (P) and Tetronic (T) surfactants. PLGA-T908, PLGA-PF127, PLGA-T908&T904 formulations (BASF Corporation) were assessed for cellular association using flow cytometry on coumarin-6 labelled microsphere systems. A PLGA-surfactant free formulation was used as a control.

Flow cytometry analysis revealed a reduced level of association for PLGA-T908 (37.0 % ± 6.8), PLGA-PF127 (27.0 % ± 0.8) and PLGA-T908&T904 (33.0 % ± 2.0) particles with A549 cells, relative to the control PLGA-surfactant free sample (50.0 % ± 5.3) (all n=3, p<0.05), indicating a possible steric stabilisation effect. Interestingly, the level of association with A549 cells at 4°C where cell metabolic processes are slowed down was similar to association at 37°C (n=3, p = ns). This suggests that no active process was involved in association of these particles with A549 cells and would be in agreement with the literature demonstrating that A549 cells are not professional phagocytic cells.

With BEAS-2B cells, less association was seen at 4ºC than 37ºC for all formulations (i.e. PLGA-surfactant free 8.0 % ± 1.4 v 46.0 % ± 6.7, p<0.05, n=4). BEAS-2B’s are not believed to be of a phagocytic nature, however the results suggest that an active process is involved and that uptake may have occurred. Studies on undifferentiated HBECs showed similar responses to those seen in BEAS-2B cells with reduced association at 4ºC relative to 37ºC (PLGA-surfactant free 34.0 % ± 5.2 v 68.0 % ± 5.7), again suggesting involvement of an active uptake process. HBECs also showed high association levels with PLGA-T908 (95.0 % ± 3.3) as well as PLGA-PF127 (79 % ± 1.2) particles at 37ºC (n=4).

These findings suggest that there are significant differences in association profiles between microparticles and different model cell systems relevant to the small airways.