Preparation of the Levo-Tetrahydropalmatine Liposome Gel and Its Transdermal Study

The aim of this study was to develop a liposome gel containing levo-tetrahydropalmatine (l-THP) and evaluate its transdermal properties.

A L₁₆ (4³) orthogonal experiment was conducted to optimize the preparation of l-THP liposomes and assess their characterization and stability in a gel. The transdermal features were analyzed through in vivo and in vitro experiments on rats and Strat-M^® membrane, respectively. The metabolism of l-THP in liver and skin S9 fractions was also studied.

The optimization of the orthogonal experiment revealed that the ideal mass ratio of phosphatidylcholine, cholesterol, and l-THP during preparation was 10:1:3. The resulting liposome exhibited a particle size of 68 nm, a PDI of 0.27, a drug loading of 4.33%, an encapsulation of 18.79%, and a zeta potential of -41.27 mV. Both the l-THP and its liposome-gel formulation were found to be stable for a duration of 45 days at 4 °C and 30 °C. During the in vivo transdermal study, the maximum concentration (C_max) of l-THP from the liposome gel was 0.16 μg/mL, and the time to reach this maximum concentration (t_max) was 1.2 hours. The relative bioavailability of l-THP in the liposome gel was 233.8% compared to the emulsion. The concentration of l-THP (prepared in PBS) decreased at a rate of 0.0067 μg/mL/min in the liver S9 fraction and 0.0027 μg/mL/min in the skin S9 fraction, however, this difference was not observed when l-THP was encapsulated in liposomes. l-THP passed through the Strat-M^® membrane at a rate of 0.0032 mg/cm²/h and 0.002 mg/cm²/h for the emulsion and liposome gel, respectively.

The optimal process for the preparation of l-THP liposomes was obtained. Compared to the emulsion, the liposomes provided greater bioavailability when used transdermally. The liposomes also provided greater stability for l-THP during storage.