Transmucosal delivery of leucine enkephalin: stabilization in rabbit enzyme extracts and enhancement of permeation through mucosae.

Leucine enkephalin (Tyr-Gly-Gly-Phe-Leu; Leu-Enk) is a naturally occurring peptide that has been shown to have pain modulating properties. To evaluate the feasibility of using various absorptive mucosae as a route of systemic delivery, the stability of Leu-Enk and the effect of enzyme inhibitors (e.g., amastatin, EDTA, and thimerosal) on stabilization and permeation of Leu-Enk through rabbit mucosae in the presence of dihydrofusidates were investigated. Enzymes in the nasal, rectal, and vaginal mucosae were extracted and Leu-Enk (50 micrograms/mL) was added to each of the enzyme extracts and incubated to determine the kinetics and mechanism of degradation. The rate of degradation in the extracts in the absence of inhibitors followed the order: rectal > vaginal > nasal. Whereas EDTA had the best stabilizing effect on Leu-Enk, thimerosal was the best stabilizer for the degradation intermediates. A combination of amastatin (50 microM), EDTA (5 mM), and thimerosal (50 microM) had the greatest stabilizing effect on Leu-Enk and its degradation intermediates. For permeation studies, each mucosa was mounted onto a Valia-Chien permeation cell with Leu-Enk (200 micrograms/mL) in isotonic phosphate buffer (as donor solution). The enhancers used for the study were sodium tauro-dihydrofusidate (STDHF), sodium glycodihydrofusidate (SGDHF), and phosphato-dihydrofusidate (PHDHF). The greatest effect was achieved by PHDHF for all the mucosae. STDHF had a significant effect only on the rectal permeation, whereas SGDHF had significant effects on rectal and vaginal mucosae. Mechanisms by which the dihydrofusidates enhance permeating may involve micelle formation.(ABSTRACT TRUNCATED AT 250 WORDS)