Intestinal absorption and analgesic activity of aminopeptidase-resistant cellobiose-coupled leucine enkephalinamide.

Leucine enkephalinamide (LEamide), aminopeptidase-degradable opioid peptide, was coupled with cellobiose (cellobiose-coupled LEamide, CcpLEamide). CcpLEamide was absorbed from the rat small intestine in vitro, whereas LEamide was not. CcpLEamide on the mucosal side was more stable than aminopeptidase-resistant cellobiose-coupled leucine enkephalin (CcpLE) in the presence of inhibitors of enkephalinase and angiotensin converting enzyme, and much more stable than LEamide. The absorption rate (clearance) of CcpLEamide was comparable with that of CcpLE in the presence of these peptidase inhibitors. Analgesic activities of these enkephalins were tested by the acetic acid writhing assay and hot-plate assay after subcutaneous administration to mice. Both assays indicated CcpLEamide-induced analgesia. On the other hand, the analgesic activity of LEamide was not observed, but pretreatment with amastatin (a peptidase inhibitor) produced LEamide-induced analgesia. These results indicate that CcpLEamide is stable in the body and has analgesic effects without pretreatment with peptidase inhibitors, and was stable enough to be absorbed from the small intestine. We propose CcpLEamide as an orally active analgesic peptide candidate.