Droloxifene does not blunt bone anabolic effects of prostaglandin E2, but maintains prostaglandin E2-restored bone in aged, ovariectomized rats

Abstract Droloxifene (DRO) is a selective estrogen receptor modulator that prevents bone loss by inhibition of bone turnover associated with estrogen deficiency in both growing and aged female rats. The purposes of this study were to test: (a) whether DRO can maintain prostaglandin E 2 (PGE 2 )-restored bone after discontinuation of PGE 2 in aged, ovariectomized (ovx) rats; (b) if an inhibition of bone turnover by DRO reduces bone anabolic effects of PGE 2 ; and (c) whether bone mass restored by PGE 2 plus DRO can be maintained after discontinuation of both agents. Female rats at 12 months of age were sham-operated (sham) or ovx. Three months postsurgery, ovx rats were treated with either PGE 2 (3 mg/kg per day, subcutaneously [s.c.]) alone, or PGE 2 plus DRO (10 mg/kg per day, per os [p.o.]) for 2 months. Thereafter, the PGE 2 or PGE 2 plus DRO treatment was withdrawn and the rats were then treated with either vehicle or DRO for another 1.5 months. Using dual-energy X-ray absorptiometry (DXA), total lumbar vertebral bone mineral density (LV-BMD) was determined in vivo at months 0, 3, 5, and 6.5. At the end of the study, the rats were autopsied, and BMD of total femur, femoral shaft, distal femoral metaphysis, and proximal femur was determined ex vivo by DXA. Standard static and dynamic bone histomorphometric parameters were determined on the fourth lumbar vertebral body (L-4). At 3, 5, or 6.5 months postsurgery, LV-BMD decreased significantly (−15%, −19%, and −19%, respectively) in the vehicle-treated ovx rats compared with sham. Beginning at 3 months post-ovx, PGE 2 alone or in combination with DRO for 2 months completely restored LV-BMD back to the sham level. There was no difference in LV-BMD in PGE 2 alone or PGE 2 plus DRO. Upon cessation of PGE 2 treatment, a significant decrease in LV-BMD was observed in the PGE 2 -alone group (−12%). On the other hand, when DRO treatment was given after discontinuation of PGE 2 , the PGE 2 -restored LV-BMD was completely maintained. In the PGE 2 plus DRO group, no loss in LV-BMD was observed after cessation of either PGE 2 alone or both PGE 2 and DRO. However, treatment with DRO following 2 months of PGE 2 plus DRO further increased LV-BMD (+10%). At the end of the study, ex vivo femoral BMD data confirmed the observation in lumbar vertebrae. Histomorphometric results of L-4 indicated that loss in bone mass after cessation of PGE 2 in PGE 2 alone group was associated with increased bone turnover. Treatment with DRO in the maintenance phase inhibited bone turnover and prevented bone loss induced by withdrawal of PGE 2 . Trabecular bone mass was maintained in the PGE 2 plus DRO followed by vehicle group and further increased in the PGE 2 plus DRO followed by DRO groups. We found that: (a) DRO is efficacious in maintaining PGE 2 -restored bone after discontinuation of PGE 2 ; (b) DRO did not blunt the anabolic effects of PGE 2 ; (c) bone loss occurred after cessation of treatment in the PGE 2 -alone group, whereas it was maintained after cessation of treatment in PGE 2 plus DRO group; and (d) an additional anabolic effect was found in ovx rats treated with PGE 2 plus DRO followed by DRO.