Novel Drug Carriers for Pulmonary Administration Utilising a Template-Assisted Approach
The template technique was selected for the generation of monodisperse fibres, intended for pulmonary administration. The deposition site in the inhalation tract is strongly governed by the geometry (size and shape) of the particle, whereas the precision of targeting is linked to their homogeneity....
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|The template technique was selected for the generation of monodisperse fibres, intended for pulmonary administration. The deposition site in the inhalation tract is strongly governed by the geometry (size and shape) of the particle, whereas the precision of targeting is linked to their homogeneity. Since conventional carrier systems are not formed within precisely defined templates, such as the track-etched membranes with cylindrical pores used herein, the geometry is less defined. Despite their largely irregular shape, conventional carriers are described as spherical. Two major benefits of fibrous shape have been identified for pulmonary administration, promising advantages over conventional drug carriers. Firstly, the residence time of the therapeutic in the target region, the deep lung, is extended because of the shape and orientation dependent delay of cellular uptake. Secondly, the load of peripheral delivery is increased through fibrous shape; more material is transported per filament in comparison a spherical particle with identical diameter due to alignment in the airstream.
Experiments confirm that the engulfment exclusively occurred from the tips of the cylindrical particles, delaying the uptake until this orientation was reached by the phagocyte. The aerodynamic properties of the cylindrical particles depend on the diameter of the filaments and not on the length, which was constant for all tested filaments. Cylinders with lower diameter proceed to deeper stages in the impactor, implying alignment with the airstream.
The physiological conditions in the peripheral lung with scarce lining fluid, acting as the solvent, and low enzymatic activity of the fragile tissue largely restrict the selection of compounds for the design of pulmonary carriers. Only a few substances have been approved for this route of administration. Filamentous particles were formed from the FDA-approved excipient lactose, APIs and blends of various ratios. These cylinders dissolved instantaneously upon contact with aqueous media. In contrast, longer residence time is desired for prolonged release systems. This can be achieved by the incorporation of hydrogels into the matrix of the cylindrical particles. The biocompatible hydrogel alginate, degrading as a function of the phosphate concentration, was utilized in order to form the backbone of the carrier system. This mode of degradation reduces the likelihood of detrimental long-term accumulation in the peripheral lung because phosphate is ubiquitous in the body.
The template technique allows for the embedding of NPs into the cylinders, too. These hierarchical microfibres were formed from silica NPs and were interconnected with biocompatible hydrogels (alginate and agarose). As a proof of concept, macrophage uptake experiments were performed in order to verify the paradigm of shape and orientation dependent uptake; this could be confirmed for fibres formed with the template technique.
Uptake was quantified using the novel correlative light and electron microscopy (CLEM). Through the combination of high resolution of EM and specificity of fluorescence, misleading quantification based upon the single techniques SEM and FLM could be corrected.
Additionally, the adaptation of the preparation protocol allows for a straightforward generation of hydrogel surfaces carrying fibres in high abundance and fidelity in various diameters and compositions. Literature reports about implications of surface structure on fundamental cell behaviour and functions. Consequently, the adhesion of murine alveolar fibroblasts was scrutinized on hairy alginate sheets with various dimensions and quantities of the filaments. The more abundant and more delicate the filaments were, the more adhesion was observed; in addition to a preferential alignment along the filaments. Without these fibres fibroblasts did not adhere to the alginate hydrogel surface.
Furthermore, the hairy sheets could be loaded with small molecules, as well as macromolecules; a fact that might proof beneficial for potential applications as a surrogate for the ECM, loading growth factors for instance. The release of these model compounds was quantified. It was depending on the molecular size and the phosphate concentration.