Metabolism of vasopressin, oxytocin, and their analogues in the human gastrointestinal tract.

The bioavailability from the gastrointestinal tract of peptides as large as nonapeptides is very low, which may be attributed to extensive lumenal and mucosal degradation. The aim of the present study was to investigate the stability of the neurohypophyseal hormones arginine-vasopressin (AVP), oxytocin (OT), and their synthetic analogues in human intestinal contents, small intestinal brush-border membranes, and gastric, rectal, and colonic plasma membranes. Peptides were incubated in gastrointestinal contents from healthy volunteers and in human intestinal mucosa homogenates. The extent of degradation was determined by reversed-phase high performance liquid chromatography (HPLC). AVP was rapidly degraded in the ileum fractions of the intestinal contents whereas 50% of the analogue 1-deamino-8-D-arginine vasopressin (dDAVP) remained intact after 35 min. The degradation was pH dependent, and a concentration-dependent inhibition was observed when aprotinin, a proteinase inhibitor, was preincubated with contents from the ileum. No degradation of AVP, dDAVP, or oxytocin analogues was observed in the mucosa homogenate from the stomach. The peptides were found to be rather slowly degraded by intestinal microvilli membranes and colonic and rectal plasma membranes. This degradation occurred essentially when reduced glutathione 10(-4) M was added to the incubations. In conclusion, the major enzymatic barrier to intestinal absorption of OT, VP, and their analogues is present in the intestinal juice and not in the mucosa, which, however, constitutes a major physical barrier to peptide transport.

Glycosylated peptide hormones: pharmacological properties and conformational studies of analogues of [1-desamino,8-D-arginine]vasopressin.

Two analogues of the antidiuretic drug [1-desamino,8-D-arginine]vasopressin (DDAVP), which have a glycosylated serine at position 4, have been prepared by Fmoc solid phase peptide synthesis. The glycosylated analogues had significantly higher bioavailabilities than the nonglycosylated [D-Tyr2,Ser4]DDAVP and DDAVP on intraintestinal administration in rat. The improved bioavailability resulted from an increased absorption from the small intestine and most likely from an increased stability toward enzymatic degradation, whereas plasma clearance was either unaffected or slightly increased by the glycosylation. The glycosylated analogues displayed only very low agonistic and antagonistic activities at the vasopressin V2-receptor. Conformational studies performed by 1H NMR spectroscopy did not reveal any major influence from glycosylation on the conformation of the peptide backbone. The lack of receptor binding displayed by the analogues is therefore most likely explained by steric repulsion between the carbohydrate moiety and the vasopressin receptor which prevents receptor binding.

Evaluation of the degradation of desamino1,D-arginine8-vasopressin by nasal mucosa.

The nasal route is a convenient and simple way to administer peptides to humans. Absorption of the drug, however, requires passage of the substance through the nasal mucosa. This is a possible site of enzymatic degradation of the peptide. It is shown that rabbit nasal mucosa homogenates rapidly degrade the synthetic anti-diuretic hormone analogue desamino1,D-arginine8-vasopressin in vitro. The metabolite formed has been identified as des-(amino,arginine8,glycineamide9)- vasopressin, which is stable under the prevalent in vitro incubation conditions. It is proposed that this process is catalyzed by intracellular post-proline cleavage enzyme. Reversed phase chromatography in combination with immunological detection has been used to study the possible presence of this metabolite in the circulation after intranasal administration to humans. The metabolite des-(amino,arginine8,glycineamide9)-vasopressin could not be detected in plasma following intranasal administration, possibly indicating a paracellular absorption of desamino1,D-arginine8-vasopressin or absence of this enzymatic activity in humans.