The influence of concentration on the release of drugs from gels and matrices containing Methocel

The release of propranolol hydrochloride from hydroxypropylmethylcellulose and methylcellulose matrices has been examined at constant cellulose ether contents. As the drug content decreased, the release rate of propranolol became disproportionately higher. HPMC K4M, HPMC F4M and HPMC E4M all performed similarly. However, with methylcellulose matrices, a burst release at low drug levels was apparently due to a failure of the matrix to maintain integrity. Explanations were sought on the basis of diffusional studies. Apparent diffusion coefficients were in the order of 3.1–3.8 × 10⁻⁶ cm² s⁻¹ for propranolol hydrochloride. Each of the four grades performed similarly. Using similar diffusional studies, but HPMC K15M as the polymer, an apparent diffusion coefficient of 3.6 × 10⁻⁶ cm² s⁻¹ was derived, indicating that the coefficient was independent of molecular weight. The coefficient was dependent on HPMC content decreasing from approx. 5.5 × 10⁻⁶ to 3 × 10⁻⁶ cm² s⁻¹ as the HPMC content was increased from 5 to 15% w/w. The diffusion coefficients of tetracycline hydrochloride were lower and conversion to the free base is postulated as the explanation of previously described anomolous release for this drug from matrix tablets. The tortuosity of the gels was independent of the included drug.