N-acetyl oxyntomodulin_30–37: pharmacokinetics and activity on gastric acid secretion

Summary

Oxyntomodulin, an intestinal hormone which inhibits gastric acid secretion, is composed of glucagon and a C-terminal octapeptide. This octapeptide mimics the biological activity of the hormone. We have studied the activity of the N-acetyl octapeptide, partially protected against enzymatic degradation, on pentagastrin-, histamine- and milk meal-stimulated secretion in conscious rats and compared it to that of oxyntomodulin and its derivatives. The N-acetylated octapeptide had a 3-fold higher potency than the octapeptide on pentagastrin-stimulated secretion. On histamine-stimulated secretion, the differences between the acetylated and the free octapeptides were that the former displayed a dose-response curve parallel to that of oxyntomodulin and a 4-fold higher potency. The increase in potency appears to be related in part to a decrease in the metabolic clearance rate in vivo (6-fold) and, in vitro, to an increase in half-life (3-fold) when incubated with rat liver plasma membranes. Similarly to the free octapeptide, the acetylated form decreased acid secretion stimulated by a milk meal, when infused before the meal. Acetylation of the Lysine side chains resulted in a totally inactive molecule. The results indicate that acetylating the N-terminus of the octapeptide of oxyntomodulin increases the similarities with the natural hormone. The still large difference in potency (≈ 40-fold) between the 37-amino acid peptide and its acetylated 8-amino acid derivative is explained by the higher (≈ 40-fold) metabolic clearance rate (MCR) of the latter, indicating that further decreasing this parameter, in particular by protecting the short peptide from the enzymatic degradation, should result in a potent and efficient molecule, usable for pharmacological research and therapeutics.