Micropolarity and microviscosity of amitriptyline and dextran sulfate/carrageenan-amitriptyline systems: the nature of polyelectrolyte-drug complexes.

The polarity and viscosity of the microenvironment of aggregates of the cationic amphiphilic drug amitriptyline and dextran sulfate (DxS)/carrageenan-amitriptyline aggregates in aqueous solution were investigated by means of steady state and time-resolved fluorescence. For the latter systems, equilibrium dialysis and capillary viscometry were also used. The micropolarity as detected by pyrene indicated the formation of amitriptyline aggregates, both with and without polyelectrolyte, having properties similar to "traditional" cationic micelles. The pyrene lifetime in the amitriptyline- and amitriptyline-polyelectrolyte aggregates was long ( approximately 230 and approximately 300 ns, respectively), indicating that pyrene was well protected from oxygen quenching, especially so in the latter case. The microviscosity of the amitriptyline aggregates themselves, and in the presence of polyelectrolyte, was high, as indicated by intramolecular excimer formation of 1,3-di(1-pyrenyl)-propane (P3P), rotational diffusion fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), and intramolecular rotational relaxation about bonds of [p-(dimethylamino)benzylidene]-malonitrile (BMN). These results and the concurrent decrease of the bulk viscosity indicate that the polyelectrolyte, acting as polycounterion, is tightly wrapped around the amitriptyline aggregates. Amitriptyline hence behaves in accordance with accepted models of cationic surfactant-polyelectrolyte interaction. Copyright 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:1665-1677, 2001