1996 Volume 19 Issue 2 Pages 263-267
In vitro skin permeation of buprenorphine (BUP) and three of its alkyl ester prodrugs was evaluated using hairless mouse skin. The three esters selected were the acetyl ester (Ac-BUP), butyl ester (Bu-BUP), and isobutyl ester (Isb-BUP). These drugs were applied on the skin as saturated slurries in three vehicles commonly used to formulate agents for transdermal purposes : propylene glycol, polyethylene glycol 400 (PEG 400), and light mineral oil. Unique solubilities were found for each drug on each vehicle. Fluxes through hairless mouse skin were evaluated for each combination of drug and vehicle using Franz diffusion cells. From PEG 400 formulations, the skin fluxes of BUP, Ac-BUP, Bu-BUP, and Isb-BUP were 0.47±0.08, 1.64±0.31, 0.33±0.05, 0.75±0.20μg/cm2/h, respectively. Thus, among the three potential prodrugs chosen, only Ac-BUP showed significantly higher skin flux than BUP. There were no inter-vehicle differences in the fluxes from saturated slurries between the vehicles. Moreover, all the esters were detected substantially in the form of regenerated parent drug (BUP) in the receptor compartment. Indeed, only Ac-BUP exited the skin in a measurably intact form, but the fraction escaping metabolism in transit was small (approximately 2%). However, based on drug dispositions in the skin, the regeneration of buprenorphine seems to depend on the alkyl chain length of the ester moiety. The molar percentages of regenerated parent drug in whole drug collected from the skin following the permeation experiments were : Ac-BUP, 9.2% ; Bu-BUP, 40.7% ; Isb-BUP, 9.6%, respectively. Thus, only Ac-BUP appears promising as a prodrug of buprenorphine, because it is not overly hydrophilic for skin permeation and is also highly metabolized to the parent compound while in the skin.
