The aim of this study was to develop and characterize lipid nanoparticle systems for the transdermal delivery of buprenorphine and its prodrugs. A panel of three buprenorphine prodrugs with ester chains of various lengths was synthesized and characterized by solubility, capacity factor (log K'), partitioning between lipids and water and the ability to penetrate nude mouse skin. Colloidal systems made of squalene (lipid emulsion, LE), squalene + Precirol (nanostructured lipid carriers, NLC) and Precirol (solid lipid nanoparticles, SLN) as the lipid core material were prepared. Differential scanning calorimetry showed that the SLN had a more-ordered crystalline lattice in the inner matrix compared to the NLC. The particle size ranged from 220-300 nm, with NLC showing the smallest size. All prodrugs were highly lipophilic and chemically stable, but enzymatically unstable in skin homogenate and plasma. The in vitro permeation results exhibited a lower skin delivery of drug/prodrug with an increase in the alkyl chain length. SLN produced the highest drug/prodrug permeation, followed by the NLC and LE. A small inter-subject variation was also observed with SLN carriers. SLN with soybean phosphatidylcholine (SLN-PC) as the lipophilic emulsifier showed a higher drug/prodrug delivery across the skin compared to SLN with Myverol, a palmitinic acid monoglyceride. The in vitro permeation of the prodrugs occurred in a sustained manner for SLN-PC. The skin permeation of buprenorphine could be adjusted within a wide range by combining a prodrug strategy and lipid nanoparticles.
