Theoretical analyses of dispersed-drug release from planar matrices with a boundary layer in a finite medium.

Analytical solutions for the kinetics of dispersed-drug release from planar matrices with a boundary layer in a well-stirred finite external medium were derived in a general and a simplified form. The general solutions are applicable for a broad range of the ratio of initial drug loading to drug solubility (e.g. C(0)/C(s)> or =3) till all dispersed drug is dissolved, while the simplified solutions describe the entire release process for higher C(0)/C(s) ratios (e.g. C(0)/C(s)> or =10). As the C(0)/C(s) ratio increased, the general solutions approached the exact solution from the lower bound, and the simplified solution from the upper bound. This property could be useful to find the lower and upper bound of an exact solution for the sink condition without a boundary layer when it is unknown. The current solutions can cover more scenarios than the existing analytical and approximate solutions. The formulas, with explicit expressions, can be readily applied to analyze determinants of release kinetics, including volume of external medium, initial drug loading, and boundary layer thickness. With the criterion established for finding the conditions of drug saturation in a medium, minimal liquid volume and maximal time for refreshing the medium can be determined.