Drug-organic electrolyte complexes as controlled release systems

A water-insoluble complex between diltiazem HCl and Na deoxycholate was prepared to achieve sustained release dosage forms. Physicochemical characterization of the drug complex was carried out with differential scanning calorimetry, (1)H-nuclear magnetic resonance, and Fourier transform infrared spectroscopy. These techniques showed that the characteristic peaks in both the drug and the complexing agent (protonated amine and carboxylate) disappeared and new peaks appeared upon formation of the ionic complex. The release of diltiazem from drug-complex tablets was sustained for a long period of time (>24 h) and was dependent on the pH of the dissolution medium. However, the dependence of drug release on pH was eliminated at pH 6-8 and minimized at pH 1.5 when drug-complex powders were incorporated in hydroxypropylmethylcellulose (HPMC) drug carriers. Unlike the release of diltiazem HCl from HPMC drug carriers, drug release from drug-complex/HPMC tablets was linear or near linear irrespective of the viscosity grade of the polymer (E15 to K4M). This is due to a shift in the controlling mechanism of drug release from drug diffusion to erosion of polymer. Also, drug release kinetics was not significantly affected by the water solubility of cationic drugs (diltiazem HCl, verapamil HCl, propranolol HCl, and labetalol HCl) ranging from 1.6 to 62% and the type of amine (i.e., secondary or tertiary). The same release characteristics were observed from the complexes between anionic drugs (Na salicylate, naproxen Na, and tolmetin Na) and benzathine diacetate as found from the complexes between cationic drugs and Na deoxycholate.