The efficacy of chitosan as nasal absorption enhancer of salmon calcitonin (sCT) was investigated in rats and the results were compared with that of hydroxypropyl and dimethyl-beta-cyclodextrins (HP-beta-CD and DM-Beta-CD). Two types of chitosans, namely the free base (CS J) and the glutamate salt form (CS G), were evaluated for their absorption enhancing effect using an in vivo absorption technique. The solutions containing sCT and chitosans (0 to 1.25% w/v) in isotonic phosphate buffers (pH 3.0 to 6.0) were nasally administered at the dose of 10 IU/Kg (n = 5 rats/group). The plasma calcium lowering effect in each sCT-treated rat was indicated by the total percent decrease in plasma calcium (%D). At 1% w/v CS J showed an increase in %D as the pH was decreased in accordance with the increased ionization and hydration of free amine chitosan at more acidic pH. However, results from 1% w/v CS G showed an increase in %D with increasing pH, with maximum calcium lowering effect observed at pH 6.0 in contrast to CS J which gave optimal activities at pH 3.0-4.0. At their optimal pH (4.0 for CS J and 6.0 for CS G), both chitosans exhibited the enhancing effect which was concentration dependent from 0.25 to 1.0% w/v and leveled off at 1.25% w/v. At 0.25% w/v the enhancing effect of CS J and CS G was already significant when compared to their control groups receiving only sCT (p<0.05). The ranking of %D in a decreasing order was 1% w/v CS J pH 4.0 (9.85+_1.89)>5% w/v DM-beta-CD pH 7.4 (9.68+_0.31)>1% w/v CS G pH 6.0 8.43+_0.67)>5% w/v HP-beta-CD pH 7.4 (8.05+_0.46). Using specific RIA, the absolute bioavailability of sCT after comparison with intravenous administration was determined to be 2.45, 1.91 and 1.22% for 1% CS J, 5% DM-beta-CD and control group (intranasal sCT alone), respectively. Although the absolute nasal bioavailability seemed to be low, the inclusion of 1% CS J resulted in two-fold increase in the AUC0-180 of plasma sCt relative to that of the control group. This was equivalent to 201.8% relative bioavailability when compared to the control group at the same nasal dose. Addition of 5% DM-beta-CD also led to the relative nasal bioavailability of 156.8% or 1.56 fold increase in absorption over the control group. All the enhancers showed significant absorption enhancement (p<0.05) with the highest effect observed with CS J. In order to estimate further the inhibitory effect of chitosans on proteolytic enzymes, the activities of trypsin and leucine aminopeptidase, two major proteolytic enzymes found in the nasal mucosa, were investigated in vitro. The results show that both CS J and CS G did not have any inhibitory effects on these enzymes as opposed of aprotinin and bestatin, their respective specific inhibitors. As a result, the nasal absorption enhancement of chitosans may not involve protection of the peptide drug against proteolytic degradation by enzymes in the nasal cavity, but it may involve the direct effect of chitosans on the mucosal permeability. In conclusion, cationic polymer chitosans, particularly CS J, may have promising potential as an effective nasal absorption enhancer of sCT.
