The effect of enzyme inhibitor and absorption site following [D‐ala², D‐leu⁵]enkephalin oral administration in rats

Abstract

The effects of enzyme inhibitor, amastatin, and absorption site following intravenous (i.v.) oral (p.o.), jejunal and ileal administration of [D-ala(2), D-leu(5)]enkephalin (YdAGFdL) were investigated in rats. Model dependent and independent pharmacokinetic parameters were obtained and compared. Linear pharmacokinetics of YdAGFdL were evaluated at 0.28 and 500 microg doses for i.v. and at 1, 500, and 1000 microg for p.o. and ileal routes. Plasma samples were collected and assayed for intact YdAGFdL using a radiometric thin layer chromatography. The clearance (CL) and half lives of the distribution and elimination phases following the 0.28 microg (n=6) i.v. dose were 42.7+/-26.2 (S.D.) ml/min, 0.48+/-0.17 min, and 3.98+/-0.92 min, while those of the 500 microg dose (n=6) were 48.0+/-23.3 ml/min, 0.59+/-0.25, and 6.81+/-3.12 min, respectively, suggesting apparent linear kinetics. The CL values were close to the cardiac output of rats (50 ml/min) indicating very rapid elimination from the body. Mean bioavailability (F) values following p.o. (n=15), jejunal (n=4), and ileal (n=16) administration were 0.40+/-0.24% (S.E.), 1.25+/-0.39, and 1.78+/-0.40, respectively, and were not significantly different (p<0.05) among three doses (1, 1000, 5000 microg). The F value of YdAGFdL following ileal administration in the presence of amastatin was 8.76+/-4.47% (n=6), a 22 fold increase over po administration and a five fold increase over ileal administration without an inhibitor. These results indicate that 'effective' oral delivery of small peptides may be achievable.

Release of Endogenous Opioids From Duodenal Enteroendocrine Cells Requires Trpm5

“…Peptide hormones from 5–166 amino acids (including Leu-enkephalin LHRH, insulin, calcitonin, and interferon beta), introduced into the intestinal lumen were effective in crossing the intestinal epithelium and entering the bloodstream. 55–59…”

Formulation Strategies to Improve Oral Peptide Delivery

“…As an oral example, desmopressin contains a deaminated amino terminal and contains a more stable D-arg at position eight, which increases stability and lipophilicity. Substitution with D-ala and D-leu in enkephalin also improved oral bioavailability by 20-fold in rats when delivered with the aminopeptidase inhibitor, amastatin [48]. An alkylated form of exendin-4 has also been disclosed, where a long chain fatty alcohol at position 20 (Lys) or position 32 (lys) increases lipophilicity [41].…”

Approaches for enhancing oral bioavailability of peptides and proteins

“…Sodium glycocholate, camostat mesilate, and bacitracin were found to be more efficient in improving the physiological availability of insulin in the large intestine. A profound increase (22-fold) in the oral bioavailability of pentapeptide enkephalin YAGFL [Tyr-Ala-Gly-Phe-Leu], with D-conformation of alanine and leucine amino acids was shown in the presence of peptidase inhibitor amastatin (Lee and Amidon, 2002). Most natural inhibitors have to be co-administrated excessively in large amounts because these compounds are susceptible to enzymatic degradation in gut.…”