Drug loading onto ion-exchange microspheres: modeling study and experimental verification.

A new mathematical model was developed and an exact analytical solution without approximations of previous work was derived for the description of the kinetics and equilibrium characteristics of drug loading from a finite external solution onto ion-exchange microspheres. The influence of important parameters pertinent to material properties and loading conditions on the kinetics, efficiency, and equilibrium of drug loading was analyzed using the developed model and equations. The numerical results showed that the rate of drug loading increased with increasing initial drug concentration in the solution or with the relative volume of the external solution and the microsphere. The maximum binding capacity of the micrsophere and the association rate constant had positive effects on the loading rate and the equilibrium loading. A decrease in microsphere radius or an increase in drug diffusion coefficient accelerated the loading process but did not influence the equilibrium drug loading. The model prediction agreed with experimental results of verapamil hydrochloride loading onto sulfopropyl dextran microspheres. The usefulness of the model in the design of loading experiments for desired drug loading efficiency and equilibrium loading was demonstrated by numerical analysis.