Effects of synthesis conditions and release medium pH on release properties of acyclovir - mercaptopropyl modified...

The purpose of the present study was to evaluate the prospect of use of mercaptopropyl modified silica as a platform for development of new oral formulation of antiviral drug acyclovir (ACV) which is able to control release of the drug irrespective of release medium pH.

The composites of ACV with mercaptopropyl modified silica were synthesized using sol-gel technology under different conditions (synthesis pH, drug loading). The composites were characterized using scanning electron microscopy, dynamic light scattering and differential scanning calorimetry methods. The effects of the synthesis conditions on physicochemical properties of the prepared composites and their release properties were studied.

The sol-gel synthesis conditions and release medium pH influence significantly release properties of the composites. The influence was explained by contributions of different factors, such as the drug-silica interactions in the composites, structure of the silica matrix and its stability in release media, hydrophobic nature of ACV, its pH-dependent solubility. It was found that all the synthesized composites followed the zero-order kinetics which is controlled by anomalous diffusion.

The studies showed that the composites exhibited controlled release of ACV up to 80 h. However, the release properties of the drug depend significantly on pH of release medium, i.e. the release properties (the release rate, the amount of released ACV) will change during transition of the composites through various segments of GIT. Therefore, the synthesized composites are not a promising basis for development of new oral dosage form of ACV.