Characterization of three hydroxypropylmethylcellulose substitution types : rheological properties and dissolution behaviour

Three substitution types of the same viscosity grade (Methocel® K4M, E4M and F4M) of hydroxypropylmethylcellulose (HPMC), were characterized using rheological measurements. Creep viscosity and oscillation tests were performed on 5% and 7% w/w polymer gels. The erosion rate of gels was measured and was found to correlate well with the results of rheological characterization. The erosion evaluated on tablets made of pure polymer was found to be very slow and the differences between the three polymer types could not be directly related to rheological behaviour. Matrix tablets containing either a freely soluble (diprophylline) or a very slightly soluble drug (acetazolamide) were prepared and tested for drug release. Diprophylline release was diffusion controlled and unaffected by the rheological differences between the three polymers. The release rate of poorly soluble acetazolamide showed the same rank order as the erosion rate of pure polymer tablets.