Factors affecting drug release from hydroxypropyl methylcellulose matrix systems in the light of classical and percolation theories.

IMPORTANCE OF THE FIELD: Hydroxypropyl methylcellulose (HPMC) is a versatile polymer widely used in the preparation of pharmaceutical dosage forms. The behavior of this polymer is a key factor in designing a variety of controlled release systems, especially hydrophilic matrices in which HPMC can be the only substance responsible for controlling the release rate of the drug. AREAS COVERED IN THIS REVIEW: A new approach, proposed in 2004, based on percolation theory to explain the influence of the main formulation factors on drug release from HPMC matrices has been analyzed, paying attention to the advantages with respect to previous theories. WHAT THE READER WILL GAIN: The influence of especially important factors such as polymer concentration and particle size is now much better known thanks to these new theories. TAKE HOME MESSAGE: To formulate a HPMC matrix, the system must be above the polymer's critical point, that is, allowing HPMC to act as outer phase. In this way, a coherent gel layer will be obtained because the first moment and the drug release will be controlled by this layer. Furthermore, knowing the critical points allows the vicinity of these points to be avoided, which are regions of high variability. In this way, robust dosage forms can be obtained.