PubMed · 15635787
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Michael M Ritter. 2005-01-06. A bitter tale.. https://pubmed.ncbi.nlm.nih.gov/15635787/
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Although ion-exchange resins have been used widely as drug delivery systems, their exact release kinetics has not been reported yet. Usually only the rate-limiting step has been taken into account and the rest of the steps have been ignored as instantaneous processes. To investigate the exact release kinetics of polymer-coated drug/ion-exchange resin complexes for sustained drug delivery, the results of new mathematical modeling were compared with experimental results. Drug/resin complexes with a model drug, dextromethorphan, were prepared and used as cores for fluid-bed coating. An aqueous colloidal dispersion of poly(vinyl acetate) was applied for the coating. A comprehensive mathematical model was developed using a mechanistic approach by considering diffusion, swelling, and ion-exchange processes solved by numerical techniques. The rate-limiting factor of the uncoated resin particles was diffusion through the core matrix. Similarly, in the coated particles the rate-limiting factor was diffusion through the coating membrane. The mathematical model has captured the phenomena observed during experimental evaluations and the release dynamics from uncoated and coated (at different coat levels) particles were predicted accurately (maximum RMSE 2.4%). The mathematical model is a useful tool to theoretically evaluate the drug release properties from coated ion-exchange complexes thus can be used for design purposes.
The effect of valence of mobile counter-ions (extracting electrolytes), mobile co-ions, and drug-like compounds was evaluated on drug binding/release in ion-exchange fibers. The experimental results support the Donnan theory and suggest that incorporation of monovalent salicylic acid (SA) and divalent 5-hydroxyisophthalic acid (di-COOH) into the anion-exchange fibers was attained mainly as a result of electrostatic (ionic) interaction, with additional contribution of non-electrostatic interactions. Increasing the capacity of ion-exchanger increased the molar amount of compound loading. More efficient release of model anions was observed at increasing valence or concentration of the extracting counter-ion. Potency to release the compounds decreased in the order of citrate (-3) > sulfate (-2) > chloride (-1). The valence of co-ions (sodium (+1) vs. calcium (+2)) in the external solution had only a slight effect on the release. Due to dual site binding (two ionized carboxylate groups), the amount of di-COOH bound into the fibers was half of that of monovalent SA. Also the release was significantly reduced, as the electrostatic interaction was stronger in the case of divalent compound. Simulations on the effect of valence on the Donnan potential and theoretical modeling of the release efficiencies by the external ions supported successfully the conclusions above.
A novel method was developed for the preparation of highly efficient anion- and cation-exchange microHPLC columns using an on-column polymerization of methacrylates having amine or sulfonic acid functional groups onto monolithic silica capillary columns modified with 3-methacryloxypropyltriethoxysilane as the anchor groups. The chromatographic evaluation of the columns using nucleic acids, nucleotides, and inorganic anions as samples showed the characteristics of the ion-exchange-type stationary phases. These columns exhibited higher separation efficiency when compared with the conventional particle-packed columns. A capillary column for the simultaneous anion- and cation-exchange separation could be prepared by a step-by-step functionalization. The advantages of this column preparation will include: (1) no need of column packing; (2) no need of the preparation of silane reagents possessing anion- and cation-exchange functionalities; (3) the amount of immobilized polymer could be controlled by changing polymerization conditions. These columns should be suitable for the separation of biologically active compounds by the microHPLC modes.