Binding behaviour of hydrophobic drug tetracaine hydrochloride used as organic counterion on ionic surfactants

Abstract Many excipients have been used in the formulation of medications to improve drug properties during the drug delivery process. Current research may show that hydrophobic drugs can be used as organic counterions that improve the excipients properties of the medicine formulation. We have used the anesthetic cationic drug tetracaine hydrochloride (TC) as an organic counterion and anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium dodecyl sulfate (SDS) for this system. In aqueous media, TC significantly reduced the critical micelle concentration (CMC) of AOT and SDS. The counterion binding (B) and binding constant (Kb) values revealed that AOT micelles had a stronger interaction with TC than SDS micelles. Therefore, TC as an organic counterion could improve the micellization of surfactants. The UV spectroscopy experiment has also been used to estimate the TC mean ion occupancy (i0) per AOT/SDS micelles. We found that i0 values were 113.6 and 23.27 for AOT and SDS, respectively. These results indicated the i0 values were controlled by electrostatic interaction between drug and surfactants. Dynamic light scattering (DLS) and Transmission electron microscopy (TEM) measurement provided that the catanionic vesicle was formed with increased TC concentration. Finally, we concluded that the above formulation could be used to enhance interaction properties between the medicine and excipients in the pharmaceutical application.