This work is dedicated to the late Prof. Sa sa Baumgartner who did a great piece of research on HPMC-based formulations and their biopharmaceutical characterization.ABSTRACT: Matrix tablets containing paracetamol and hydroxypropyl methylcellulose (HPMC 2906) of different viscosity grades (50, 400, 1500, and 4000 mPaÁs) were evaluated for drug release and change in matrix tablet mass [DM (%)] after exposure to 0.09, 0.15, 0.31, and 0.52M ionic strengths of dissolution media. At 0.09 and 0.15M ionic strength, drug-release profiles reflected the extended release characteristic; in addition the increase in DM was slow and continuous within first few hours. At 0.31M the higher viscosity grade matrices showed extensive initial swelling and the loss of extended release whereas at 0.52M a similar tablet performance is observed for the matrices of all viscosity grades. Notably, when extensive increase in DM occurs in the very beginning of exposure to medium the loss of extended release from the matrix is expected.

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Visualization techniques are frequently used to provide additional information on dissolution processes. The camera system used here, placed underneath the dissolution bath, enabled a bottom view into each separate vessel with the possibility for continuous video recording throughout dissolution testing. Drug release and swelling of hydroxypropyl methylcellulose (HPMC) matrix tablets containing either diclofenac sodium or paracetamol were simultaneously evaluated in a series of media with pH values of 3.0 to 7.5. Swelling of HPMC tablets was studied using two methods: evaluation of the relative change in tablet volume using the camera system and evaluation of relative change in tablet mass based on tablet weighing. The correlation between evaluated parameters in the two methods was high at all pH values for the tablets studied. Swelling of diclofenac sodium tablets was lowest at pH 3, highest at pH 4, and then decreased with the increasing pH of the medium. In contrast to the swelling results, drug release from diclofenac sodium tablets increased with increasing pH of the medium. At lower media pH values (i.e., 3.0, 4.0, and 5.0), the surface of the swelled layer appeared completely white and nontransparent, whereas at higher pH values, a transparent gel layer was clearly seen using the camera system. It is presumed that because of its low solubility, diclofenac precipitated as an acid on the tablet surface at lower pH values and was seen as a white swelled layer. For paracetamol tablets, drug release and swelling were pH independent, and a gel layer was clearly seen in all media tested (pH 3.0, 5.0, and 7.0). Thus, for HPMC tablets containing diclofenac sodium, the use of cameras provided additional information on the drug release and swelling process.