Activity of Pancreatic Endopeptidases Towards Luteinizing Hormone-releasing Hormones

LHRH and its analogues have low oral bioavailability; this is in part due to their degradation by peptidases present in the intestinal lumen. To determine the appropriate inhibitors to co-administer with LHRH oral formulations, the peptidases involved in their digestion have to be identified. Human (hLHRH) and salmon (sLHRH) LHRH analogues contain a number of potential cleavage sites for the lumenal pancreatic secreted serine endopeptidases: chymotrypsin, trypsin and elastase. The rate of LHRH degradation by equimolar concentrations of chymotrypsin, trypsin and elastase were examined separately in vitro, at pH 8.0, 15 degrees C. At a molar ratio of 1:1000 (enzyme:LHRH), both LHRH analogues were rapidly hydrolysed by alpha-chymotrypsin with half-lives of 2.5+/-0.3 and 2.7+/-0.4 min (mean+/-S.D., n=3), respectively, whereas in the presence of elastase both LHRH analogues were slowly hydrolysed with half-lives of 90+/-15 and 114+/-21 min (mean+/-S.D., n=3), respectively. Trypsin had no activity towards either LHRH analogues after 2 h incubation. The degradation of the LHRH analogues by elastase is likely to be a property of the chymotrypsin impurity. It is concluded that protection of the LHRH analogues from alpha-chymotrypsin is a requirement for the development of oral absorbable product.