Catanionic drug-surfactant mixtures: phase behavior and sustained release from gels.

Purpose: To study mixtures of SDS and the drugs diphenhydramine, tetracaine, and amitriptyline to compile phase diagrams and to investigate the use of interesting phases for sustained release from gels.

Methods: Phase diagrams were composed by studying large numbers of different compositions of negatively charged SDS and positively charged drug compounds visually, rheologically, and by cryotransmission electron microscopy. Drug release from Carbopol 940 and agar gels containing interesting phases, e.g., vesicle and branched micelle phases, was measured in vitro by the USP paddle method.

Results: Vesicles and elongated and branched micelles were formed on the SDS-rich side in all three systems examined. The tetracaine system differed from the other two in that it showed a vesicle area in the drug-rich side. Release of diphenhydramine from Carbopol 940 gels was slowed by at least a factor of 10 when in the form of vesicles or branched micelles. The same delay was found for both drug-rich and SDS-rich tetracaine vesicles.

Conclusions: Mixtures of SDS and positively charged drugs form the same interesting phases as traditional catanionic mixtures. This may prove useful in obtaining functional controlled-release systems when using gels as drug carriers.