Study of the critical points in combined matrix tablets containing both inert and swelling excipients

This work estimates for the first time critical points in combined matrices containing varying concentrations of the hydrophilic polymer Hydroxypropyl methylcellulose (HPMC) K100 M CR in presence of a constant percentage of the inert matrix forming polymer Eudragit RS-PO as well as varying concentrations of the inert polymer in presence of a constant percentage of the hydrophilic excipient. Drug release assays, water uptake studies and calculation of the Exicipient Efficiency (EE) have been carried out to study the interaction between the polymers. Surprisingly, an increase in the drug release rate occurs as the percentage of the hydrophobic polymer increases in the formulations. This fact is supported by the EE values which indicate a negative interaction between the two excipients. Moreover the HPMC percolation threshold estimated is higher than the one observed in pure HPMC matrices. It can be concluded that the HPMC creates pores in the inert skeleton, destabilizing the system. Moreover, the inert excipient destabilizes the gel layer formed by HPMC, changing its critical point. This information is essential for a rational estimation of the Design Space of a formulation and provides new knowledge on the behavior of the polymers in combined matrices, which contributes to the science based design