Ropivacaine loaded microemulsion and microemulsion-based gel for transdermal delivery: Preparation, optimization, and evaluation

The objective of the present study was to prepare and evaluate a ropivacaine-loaded microemulsion (ME) formulation and microemulsion-based Carbopol gel (ME-gel) for transdermal delivery. Pseudo-ternary phase diagrams and a simplex lattice experiment design were utilized to screen and optimize the ME formulation. In the process, drug solubility and particle size were inspected as dependent variables whilst Capryol (R) 90 (X-1), Smix (X-2, Labrasol (R): absolute ethanol = 1:2 w/w), water (X-3) as independent variables. Following the optimization, the optimal ME formulation was comprised of 15% Capryol (R) 90, 53% Smix, and 32% water, respectively. Ropivacaine loaded ME appeared to be spherical under transmission electron microscope, and the average particle size was 58.79 nm. The results of ex vivo permeation study showed that ropivacaine had a significant higher cumulative amount from ME than that from ME-gel. Histopathology study elucidated that the microstructure of skin surface was significantly changed by the treatment of ME formulation. Skin irritation study indicated that neither ME nor ME-gel caused any irritation responses. Both ME and ME-gel presented a remarkable analgesic activity on acetic acid-induced writhing in mice. In conclusion, ME could be a promising formulation for ropivacaine transdermally administration