Transbuccal delivery of doxepin: studies on permeation and histological investigation

According to previous studies reporting the anesthetic/analgesic action of oral topical doxepin administration, this study evaluated a model of buccal permeation to determine the depth of delivery of doxepin into excised porcine buccal mucosa following topical application of a saturated aqueous doxepin solution. Buccal mucosa permeation studies were performed using Franz diffusion cells. Cumulative amounts of doxepin permeated were plotted as a function of time. Kinetic permeation parameters as flux (Js), lag time (Tl) and permeability coefficient (Kp) were calculated. Theoretical human plasmatic steady-state doxepin concentration and drug retained in the tissue were also determined in order to evaluate its potential therapeutic use, central or peripheral. Finally, a histological evaluation of the buccal mucosa was performed to test potential damage due to the permeation phenomenon. Obtained results showed a poor aqueous doxepin permeation through buccal mucosa membrane (median parameters Js=34.79 μg/h, Kp=0.49×10(-3)cm/h and Tl=2.8h). Predicted doxepin plasma concentrations would reach 46 ng/mL, far from the required to have central nervous system activity as tricyclic agent. However, median doxepin amount remaining in the mucosa membrane was 0.24 μg/cm(2)/μg tissue, which evidenced a reservoir function of the buccal mucosa. Histologically, no structural damage was observed in the tissues. This study lays the foundation for further research within this area with a view to potentially adopting alternative strategies for enhanced buccal absorption of doxepin in clinical practice.