RELEASE OF PROPRANOLOL HCL FROM A TABLET COATED WITH A MACROPOROUS MEMBRANE

The release of propranolol HCl from core tablets coated by compression with ethyl cellulose (EC) or cellulose acetate butyrate (CAB 381 or C AB 500) was studied. The release profile was typically sigmoidal, with a constant release segment. The zero-order release rate constant (K0) for EC was 6.63 +/- 0.45 mg/h compared to 1.98 +/- 0.16 mg/h for CAB 381 and 2.56 +/- 0.27 mg/h for CAB 500. The media transfer coefficient into the EC disks also was much faster (64.50 +/- 4.94 x 10(-5) cm2 / h) than that for CAB 381 (7.57 +/- 0.39 x 10(-5) cm2/h) or CAB 500 (8. 09 +/- 1.31 x 10(-5) cm2/h). The propranolol HCI dissolution rate cons tant at 25 rpm was not significantly affected by the pressure used to make the core tablet (p = 0.712). The decline in K0 with increasing am ounts of polymer used in the coat was accurately modelled by an expone ntial function (r2 = 0.971). The extrapolated value for the intrinsic dissolution rate with no polymer coating (206.10 mg/cm2 per h) was clo se to the determined intrinsic dissolution rate (210.47 mg/cm2 per h). Increasing the compression pressure resulted in an initial decline in the normalized mass transfer coefficient, K(n), followed by an increa se in K(n); the influence of pressure on K(n) diminished with increasi ng polymer amount. Since K(n) incorporates the reduction in coat thick ness and the radial expansion of the core tablet (due to the applied p ressure), the increase in K(n) is probably due to microscopic stress f ractures in the coat.