Mechanism of sustained‐action medication. Theoretical analysis of rate of release of solid drugs dispersed in solid matrices

Theoretically expected rates of release of solid drugs incorporated into solid matrices have been derived for several model systems. Mathematical relations have been obtained for cases (a) where the drug particles are dispersed in a homogeneous, uniform matrix which acts as the diffusional medium and (b) where the drug particles are incorporated in an essentially granular matrix and released by the leaching action of the penetrating solvent. Release from both planar surface and a sphere is considered. The unidimensional release rates are shown to follow our earlier equation derived for release from ointment bases. Release rates from spherical pellets by both model mechanisms are shown not to follow first‐order relationships. The analyses suggest that for the latter system the time required to release 50 per cent of the drug would normally be expected to be approximately 10 per cent of that required to dissolve the last trace of the solid drug phase in the center of the pellet.