No loss of biomechanical effects after withdrawal of short-term PTH treatment in an aged, osteopenic, ovariectomized rat model.

The aim of the study was to assess the biomechanical effects of short-term PTH treatment and withdrawal on bone mass and strength in an aged, osteopenic, ovariectomized (ovx) rat model. Additionally, the effect of sequential therapy with PTH and the bisphosphonate, risedronate, the effect of longterm PTH monotherapy, and the effect of long-term risedronate monotherapy were assessed. 96 4-month-old rats were randomized into nine groups. Eight groups were ovariectomized and one group was sham operated. 12 months after surgery, treatment regimens were initiated (OW) and were continued for either 2 weeks (2 W) or 12 weeks (12 W). The treatment regimens were as follows: (1) baseline ovx (OW); (2) ovx-saline (2 W); (3) ovx-PTH 1-34 (2 W); (4) intact-saline (12 W); (5) ovx-saline (12 W); (6) ovx-risedronate (12 W); (7) ovx-PTH 1-34 (12 W); (8) ovx-PTH 1-34 (2 W), followed by pause (10 W); and (9) ovx-PTH 1-34 (2 W), accompanied by risedronate (12 W). The effect of therapy (endpoint) was measured at three skeletal sites: vertebral bodies; femoral cortical bone; and femoral necks. The results revealed an anabolic, time-dependent effect of PTH 1-34 at all skeletal sites. No loss of anabolic effect was observed 10 weeks after discontinuation of 2 week PTH treatment in this rat model. Risedronate given in sequential therapy with PTH produced no significant effect on biomechanical properties at any skeletal sites when compared with 2 week PTH followed by a 10 week pause. However, when risedronate was given alone, a positive effect was seen at the vertebral site after a 12 week treatment. On the basis of this study with short-term PTH treatment of aged, osteopenic, ovariectomized rats, there seemed to be a significant effect of PTH on the biomechanical properties and no loss of effect even 10 weeks after PTH withdrawal.