The application of colloidal liquid aphrons for drug delivery

Abstract

Colloidal Liquid Aphrons (CLAs) have been used in a variety of applications ranging from detergency and solute extraction, to enzyme immobilisation, and have been shown to have very high mass transfer areas due to their size (5-20μm). Also, due to their structure they are very stable while also being naturally buoyant in aqueous solutions, providing natural homogeneity. Over the last few decades their use in the laboratory as an immobilisation support for enzymatic catalysis has been extensive due to their stability, and ability to promote superactivity. Due to these specific characteristics, the possibility of using CLAs as a stable formulation for drug delivery has been particularly attractive. However, in an effort to apply existing research to the area of drug delivery, a new formulation methodology was developed utilizing non-ionic surfactants, nonpolar solvents, and model proteins as active pharmaceutical ingredients with varying molecular weights and isoelectric points (pI). Insights into the chemical forces governing immobilisation showed that hydrophobic interactions were primarily responsible for binding. Adsorption was the main mechanism for immobilisation, with higher affinities being observed with increasing protein concentration and smaller particle size. Superactivity was observed with Lipase and α-chymotrypsin, while aprotinin retained 85% of its inhibitor potency. Evaluation of immobilised enzyme conditions showed that non-ionic CLAs preserved natural pH and temperature optima, while thermodynamic evaluations of activity suggested that the presence of water molecules lead to an active conformation. Characterisation studies on refractive index matched polyaphron systems showed that proteins interacted mainly with the ‘’soapy-shell’’ leading to a hydrated conformation, while calorimetric studies on nonionic surfactant binding proved that surfactant interactions were virtually non-existent. Desorbed enzymes regained their natural conformation illustrating that any small structural effects were reversible upon release. The use of sodium alginate as an additive for enhanced immobilisation proved successful due to induced electrostatic interactions when the pH was lower than the protein pI. Conformational effects upon binding with sodium alginate also proved to be reversible, with an increase in the concentration of the released protein being observed when the pH was >pI. However, the extent of electrostatic interactions on the bioactivity of released proteins was found to be non-denaturing with hydrophilic enzymes, while hydrophobic enzymes were more active upon binding. Finally, insights into proteolytic digestion suggest that electrostatic interactions lead to greater protease vulnerability due to conformational changes induced upon binding.