Plasticisation of amylodextrin by moisture: consequences for drug release from tablets

Moisture influences the consolidation behaviour of amylodextrin powders and the porosity and mechanical strength of compacts thereof. The aim of this study is to relate moisture content and compact properties to drug release characteristics of amylodextrin tablets. Therefore, amylodextrin tablets containing theophylline monohydrate were prepared and their release characteristics were studied as a function of moisture content and initial porosity. Drug release from amylodextrin tablets occurs through a leaching mechanism in which cracks are progressively formed in the hydrated part of the matrix leading to almost constant release rates. Small variations in moisture content resulted in large changes of the release rate. A unique relationship between porosity and release rate, which was independent on moisture content and compaction pressure, was observed. Above a critical porosity of 0.075 crack formation was followed by disintegration and fast release. Below this critical porosity, tablets stayed intact despite of the formation of cracks, and sustained release was observed. It is concluded that control over moisture content is essential for the production of amylodextrin tablets with reproducible release characteristics. Using amylodextrin containing 10-17%, moisture, tablets with a constant release behaviour can be obtained if sufficient compaction pressure ( > 300 MPa) is applied. Lubrication of amylodextrin powders reduces the effect of porosity significantly and improves the robustness of amylodextrin tablets as a release controlling excipient in tablets largely.