PHARMACOLOGICAL EVALUATION OF THE CALCITONIN ANALOG SB-205614 IN MODELS OF OSTEOCLASTIC BONE-RESORPTION IN-VITRO AND IN-VIVO - COMPARISON WITH SALMON-CALCITONIN AND ELCATONIN

The activity of a novel calcitonin SE 205614 was compared with salmon calcitonin (sCT) and (Asu(1,7))-eel calcitonin (ELC) in six different models of osteoclastic bone resorption in vitro and in vivo. SE 205614 is an ELC analogue that has an acetylenic bridge instead of the natur al disulphide bridge, rendering the molecule more stable biologically than sCT and equally stable to ELC. Our aim was to determine whether t his structural change compromised biologic activity, and if not, wheth er the increased stability could be used to exploit novel modes of adm inistration. In the in vitro assays of pit formation by disaggregated rat osteoclasts on cortical bone slices (DROcA) and PTH stimulation of Ca-45-release from prelabeled fetal rat bone, no significant differen ces in activity were observed between the three calcitonins. In the DR OcA, IC(50)s of 0.003, 0.015 and 0.064 pg/ml for sCT, ELC, and SIE 205 614, respectively, were determined, with total or near complete inhibi tion observed at 1 pg/ml (0.3 pM). In the assay of PTH-stimulation of Ca-45 release, IC(50)s were measured of 5.5, 4.8, and 12.9 pM for sCT, ELC, and SE 205614, respectively; in every case maximal inhibition (c a. 80%) was observed at 30 and 100 pM. The internationally approved U. S. Pharmacopoeia bioassay of hypocalcemia in the rat following intrave nous (IV) administration indicated that SE 205614 had a greater potenc y than ELC or sCT. More important, a full dose-hypocalcemic response c urve demonstrated significantly increased potency compared to sCT or E LC, as the doses causing 15% lowering of serum calcium (approximately 50% of the maximum effect) were 33.9, 25.2, and 12.9 mg/kg for sCT, EL C, and SE 205614, respectively. As a preliminary means of investigatin g alternative delivery forms of calcitonin, the time course of the hyp ocalcemic effect was investigated in the rat and rabbit following TV a dministration, and was compared with that following intranasal (LN) ad ministration (rat and rabbit), and following intracolonic administrati on (rat only). Maximal effects were similar, whereas in general the hy pocalcemic effect of SE 205614 was of a longer duration than the other two calcitonins; this was reflected in a larger area over the curve ( AOC). However, following LN administration in the rabbit, where an aer osol delivery device similar to that used in the clinic was used to ad minister the calcitonins, SE 205614 (100 IU/kg) induced a highly signi ficant two-fold increase in the AOC compared to ELC or sCT. The calcit onins were also compared in assays designed to measure therapeutic eff icacy in the rat. In the prevention of retinoid-induced hypercalcemia, no significant differences were observed between the calcitonins. IC( 50)s were obtained of 3.0, 2.9, and 4.8 IU/kg for sCT, ELC, and SE 205 614, respectively. In the model of osteoporosis (induced by a combinat ion of immobilisation and ovariectomy), the three calcitonins all show ed a similar dose-dependent protection against femoral trabecular bone loss, which was significant at 15 IU/kg. These experiments in the rat suggested that following administration of equipotent doses intramusc ularly, there were no significant differences between the activity of the calcitonins, although only SE 205614 significantly increased bone density in the tibia compared to operated animals at both 2.5 and 15 I U/kg. Thus, SE 205614, a novel structural analogue of ELC, is a highly potent calcitonin, which has some significant advantages over ELC and sCT, and thus may be more effective in the development of IN treatmen t of diseases involving elevated osteoclastic bone resorption, such as osteoporosis.