MECHANISM OF NASAL ABSORPTION OF DRUGS .3. NASAL ABSORPTION OF LEUCINE ENKEPHALIN

The nasal absorption of a model peptide, leucine enkephalin (LE), was studied in rats using an in situ technique in which 4 mL of perfusion solution was circulated. Leucine enkephalin (LE) was found to undergo hydrolysis to its major metabolite des‐tyrosine leucine enkephalin (DTLE). The addition of 1% sodium glycocholate (SGC) to the perfusion solution resulted in an increase in the overall rate of disappearance of LE and a decrease in the rate of formation of DTLE. When LE was added to nasal washings (i.e., Ringer's buffer that was precirculated through the nasal cavity to extract enzymes), LE was found to form DTLE. When SGC or puromycin was added to the nasal washings prior to the addition of LE, the rate of conversion of LE to DTLE was significantly reduced, suggesting that these two agents can inhibit peptidase enzyme activity in the nasal cavity. Since the volume of the solution has been shown to influence the kinetics of absorption of drugs administered nasally, a new experimental technique, the in vivo‐in situ technique, which utilizes small volumes of solution and simulates realistic use of nose drops, was employed to further examine the mechanism of absorption and hydrolysis of LE in rats. Leucine enkephalin (LE) dissolved in 100 μL of Ringer's buffer was placed in the isolated nasal cavities of rats. The disappearance of LE and the appearance of DTLE were followed by rinsing the nasal cavity with fresh buffer. Disappearance of LE was always accompanied by appearance of DTLE, and the fraction of LE converted to DTLE decreased as the concentration of LE increased, suggesting a saturable enzymatic process. In the presence of the small peptides, L‐tyrosyl‐L‐tyrosine and tri‐L‐tyrosine methyl ester, the hydrolysis of LE was reduced and less DTLE was formed, suggesting that the competitive inhibition of the nasal peptidases was caused by these small peptides. The results also suggested that as much as 30% of LE was absorbed in the first 10 min. In the presence of 1% SGC, the formation of DTLE was greatly reduced and the absorption of LE was greatly enhanced. After 10 min, no LE or DTLE remained in the nasal cavity, suggesting that SGC markedly inhibits enzymatic hydrolysis of LE and greatly accelerates absorption of both LE and DTLE into the systemic circulation. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company