alpha-Chymotrypsin-catalyzed degradation of desmopressin (dDAVP): influence of pH, concentration and various cyclodextrins.

Desmopressin [1-(mercaptopropanoic acid)-8-D-arginine vasopressin; dDAVP] is a vasopressin analogue with a selective antidiuretic effect. The oral bioavailability of desmopressin is limited due both to its high hydrophilicity leading to a low intestinal permeability and to low enzymatic stability. The degradation of desmopressin was investigated in aqueous buffer solutions (pH 6.00-9.00) containing the enzyme alpha-chymotrypsin at a concentration of 0.50 mg/ml at 37 degrees C. The degradation of desmopressin was also studied in solutions containing alpha-chymotrypsin in the concentration range 0.10-1.00 mg/ml (pH 7.40 and 37 degrees C). The rate of degradation was shown to be highly dependent on both enzyme concentration and pH. Maximal alpha-chymotrypsin activity was observed in the pH range 7.40-8.00. It was observed that phenylalanine was formed during the degradation of desmopressin. Phenylalanine was formed in the amount of 20% in 120 min. In the same time period 95% of desmopressin was degraded. The formation of phenylalanine can be explained from the substrate specificity of alpha-chymotrypsin. Cyclodextrins are known to stabilize drugs including peptides against both chemical and enzymatic degradation. In this study it was shown that hydroxypropyl cyclodextrins (alpha, beta and gamma) stabilized desmopressin against alpha-chymotrypsin-catalyzed degradation. The stabilization was by a factor of 3, 9 and 8 at the concentration 12.5% (w/v) for hydroxypropyl-alpha-cyclodextrin, hydroxylpropyl-beta-cyclodextrin and hydroxypropyl-gamma-cyclodextrin.