Zero-order drug release from hydrocolloid matrices.

Matrices are manufactured by direct compression of a powder mixture of a polymer, e.g., methylhydroxypropyl cellulose (MHPC) or polyvinylalcohol (PVAI), and a drug. The following factors that can influence the drug release mode were investigated at constant surface: (i) polymer solution viscosity, glass transition temperature, and swelling; (ii) drug concentration in the matrix and solubility; and (iii) conditions of release experiment (hydrodynamics). In the case of zero-order release profiles (hydrocolloids with low viscosities), only the dissolution of the polymer appears to control the drug release rate. Factors accelerating polymer dissolution resulted in higher release rates. Comparison of swollen and dry hydrocolloid matrices shows that the duration and kinetics of drug release were not controlled by the swelling front moving into the dry polymer, and water penetration and relaxation were not rate controlling. Therefore, the glass transition temperature had no effect on drug release from these hydrocolloids. The higher the hydrodynamic stress exerted on the eroding hydrocolloid, the faster the resulting drug release as a result of accelerated polymer dissolution. With hydrocolloids of very high viscosity the polymer dissolution is slow, and drug release from the swollen gel appears to be controlled by diffusion according to kinetics of the Higuchi type.