The effect of substitution pattern of HPMC on polymer release from matrix tablets

The purpose of this study was to gain further understanding of how the substituent heterogeneity of hydroxypropyl methylcellulose, HPMC, affects the polymer release from hydrophilic matrix tablets. The hypothesis was that the heterogeneous substituent pattern facilitated hydrophobic interactions that increased the viscosity and therefore affected the release rate to a major extent. Polymer tablets were prepared from three heterogeneously substituted HPMC batches of the same substituent (2208) and viscosity (100 cps) grade. To elucidate the hypothesis, fractions of both the dissolved polymer and the tablet residue were collected from the dissolution bath and further characterised. The extensive characterisation showed that, although the dissolved bath fraction and the tablet residue had a similar average degree of substitution, the residue was more heterogeneously substituted. It was further revealed that the heterogeneous substituent pattern of the tablet residue facilitated the formation of soluble gel-like components already at room temperature, which increased the viscosity. The viscosity increased by 150% at temperatures correlated to the dissolution bath, and it was thus concluded that the gel-like components grew in size with temperature. Finally, much lower release rates were obtained by tablets composed of the residue compared to tablets composed of the bath fraction, which clarified the hypothesis.