Influence of complex solubility on formulations based on lambda carrageenan and basic drugs

The purpose of the present work was to compare the behavior of some drug/carrageenan complexes having different solubility in water, in a controlled release formulation. Diltiazem HCl, bupropion HCl, metoprolol tartrate, and tramadol HCl were used as model drugs. The complexes were characterized by means of solubility measurements, release test at constant surface area, and water uptake measurements, and the results were related to their performance in controlled release formulations. For the more soluble complexes (involving metoprolol and tramadol) the occurrence of gelation after hydration was observed, while diltiazem complex apparently did not gellify; bupropion behavior was intermediate. A correspondence was found between the observed differences in complex solubility and hydration-gelation behavior and the drug release profiles. For all the drugs considered, the release was completed in about 10 to 12 hours, but different kinetics were observed depending on the solubility of the complexes. All the considered complexes seem suitable for controlled release purposes, although the data obtained show the relevance of the complex solubility to drug release profiles.