Regional variation in oral mucosal drug absorption: permeability and degree of keratinization in hamster oral cavity.

The regional permeability of oral mucosa to salicylic acid was investigated in vivo in hamsters along with histological variations, especially the degree of keratinization. Histological sections from six regions, i.e., sublingual mucosa, buccal mucosa, dorsum of tongue, ventral surface of tongue, labial mucosa, and cheek pouch mucosa, were prepared to assess the degree of keratinization. The area under the plasma concentration-time curve of salicylic acid following the administration of salicylic acid to the oral mucosa with a film dosage form and the thickness of stratum corneum of each site were in inverse proportion to each other, suggesting that the stratum corneum layer represents the principle barrier to drug absorption.