Improvement of the Pulmonary Absorption of (Asu1,7)-Eel Calcitonin by Various Protease Inhibitors in Rats

The effects of protease inhibitors, Na-glycocholate, bacitracin, bestatin, nafamostat mesilate and soybean trypsin inhibitor (STI) on the pulmonary absorption of (Asu^1,7)-eel calcitonin (ECT, molecular weight 3363) were investigated in rats. The pulmonary absorption of ECT was estimated by measuring its hypocalcemic effect. When ECT alone was administered into the lung, the pharmacological availability of ECT was 2.7%. Co-administration with STI or bestatin did not change the pharmacological effect of ECT. However, Na-glycocholate, bacitracin and nafamostat mesilate caused a significant hypocalcemic effect following the pulmonary absorption of ECT and a maximal effect was noted in the presence of 20 mM bacitracin, approaching the effect after intravenous administration of ECT. Bacitracin and Na-glycocholate reduced the degradation of ¹¹¹In-ECT in rat lung homogenate. Therefore, protease inhibitors effectively improved the pulmonary absorption of ECT.