Hydrogel-thickened microemulsion for topical administration of drug molecule at an extremely low concentration

In this work, a hydrogel-thickened microemulsion (HTM) was investigated for delivering an extremely low concentration of drug molecule. The pseudo-ternary phase diagrams were constructed using isopropyl myristate (IPM), Tween 80, propylene glycol and water. The various HTM were prepared and characterized. We described that HTM has the combination of o/w microstructure of microemulsion and the three-dimensional gel network of hydrogel in continuous phase using transmission electron microscope. The stability tests showed that HTM had good stability. The influence of the addition of hydrogel into microemulsions on the viscosity and permeation ability is investigated. The abilities of HTM to deliver an extremely low concentration of triptolide as a model drug were evaluated using the in vitro permeation studies. The permeation rates of triptolide from various HTM were 2.2-3.6 times over that from the control hydrogel. The addition of 2% menthol into HTM consisting of 3% IPM, 30% Tween 80, 15% propylene glycol, 0.75% carbomer 940 resulted in the highest permeation rate of 0.105 +/- 0.006 mu g cm(-2) h(-1), which was 5.8 times over control gel. The powerful permeation enhancing ability of HTM with a suitable viscosity makes it promising alternative carrier for transdermal administration of drug molecule at an extremely low concentration. (c) 2007 Elsevier B.V. All rights reserved.