Reexamination of convective diffusion/drug dissolution in a laminar flow channel: accurate prediction of dissolution rate.

PURPOSE: The convective diffusion/dissolution theory applied to flowthrough dissolution in a laminar channel was reexamined to evaluate how closely it can predict release rate for a model compound on an absolute basis--a comparison that was lacking from the original literature observations reported from this technique.
METHODS: The theory was extended to allow for a finite flux of dissolving material, replacing the fixed concentration by a flux condition on the dissolving surface. The derivation introduces a new parameter, k(s), an area-independent analog of the dissolution rate constant defined in the USP intrinsic dissolution procedure.
RESULTS: The release rate for ethyl-p-aminobenzoate originally observed fell within 10% of the absolute prediction assuming a solubility limited situation, and deviated from this prediction in a manner possibly consistent with a finite flux-limited condition, with k(s) approximately 10(-4) M s(-1). For materials exhibiting lower k(s) values, the derivation suggests that at high flow rates, a limit occurs where dissolution rate becomes independent of shear rate and merely a function of solubility and surface area.
CONCLUSIONS: The new parameter k(s) may be deduced from any set of geometric and flow conditions, provided the fluid velocity can be determined everywhere in the domain.