Extra cancellous bone induced by combined prostaglandin E2 and risedronate administration is maintained after their withdrawal in older female rats

Recovery of trabecular and cortical bone turnover after discontinuation of risedronate and alendronate therapy in ovariectomized rats

Alendronate (ALN) and risedronate (RIS) are bisphosphonates effective in reducing bone loss and fractures associated with postmenopausal osteoporosis. However, it is uncertain how long it takes bone turnover to be re-established after treatment withdrawal, and whether this differs between the two drugs. The objective of this study was to determine the time required to re-establish normal bone turnover after the discontinuation of ALN and RIS treatment in an animal model of estrogen-deficiency osteoporosis. Two hundred ten, 6-mo-old female Sprague-Dawley rats were ovariectomized and 6 wk later were randomized into baseline controls (n = 10) and four treatment groups (n = 50/group): vehicle-treated controls (CON; 0.3 ml sterile water), ALN (2.4 microg/kg), low-dose RIS (RIS low; 1.2 microg/kg), and high-dose RIS (RIS high; 2.4 microg/kg). Treatments were administered 3 times/wk by subcutaneous injection. Baseline controls were killed at the initiation of treatment. Other groups were treated for 8 wk, and subgroups (n = 10/ treatment group) were killed 0, 4, 8, 12, and 16 wk after treatment was withdrawn. Static and dynamic histological analyses were performed for cortical (tibial diaphysis) and trabecular (proximal tibia and L(4) vertebrae) bone. DXA and mechanical testing was performed on the L(5) vertebra. After 8 wk of treatment, trabecular bone turnover rates were significantly suppressed in all drug-treated animals. Trabecular bone formation rate (BFR/BS) remained significantly lower than vehicle in bisphosphonate-treated animals through 12 wk. Sixteen weeks after treatment withdrawal, trabecular BFR/BS in the proximal tibia was re-established in animals treated with RIS but not in animals treated with ALN compared with controls. BMD of the fifth lumbar vertebra remained significantly higher than controls 16 wk after treatment withdrawal in ALN-treated animals but not in RIS-treated animals. Despite reductions in BMD and increases in bone turnover, ultimate force of the fifth lumbar vertebra remained significantly higher in all drug-treated animals through 16 wk after withdrawal.

Intermittent on/off prostaglandin E2 and risedronate are equally anabolic as daily PGE2 alone treatment in cortical bone of ovariectomized rats

In this study, we evaluated the rat cortical bone changes after a two-cycle, 60-day each (ON/OFF/ON/OFF) treatment with either prostaglandin E2 (OVX/c-PGE2) alone or in combination with risedronate (OVX/c-PGE2+Ris), in comparison with daily treatment with PGE2 for 240 days (OVX/PGE2-240d) in ovariectomized (OVX) rats. At the end of the study, we found that: (1) the overall effectiveness of the treatment on bone mass in the tibial shaft indicates the following ranking: OVX/PGE2-240d = OVX/c-PGE2+Ris > OVX/c-PGE2 > OVX/c-Ris > or = OVX = aging; (2) the same bone mass and architecture were produced in the OVX/PGE2-240d and the OVX/c-PGE2+Ris groups, but the histomorphometric profiles differed in that the former exhibited a higher bone turnover and index of resorption; (3) OVX/c-PGE2+Ris treatment prevented endocortical bone loss and minimized trabecular bone loss during the OFF periods; and (4) the OVX/c-PGE2 alone treatment resulted in the accumulation of less total bone than OVX/PGE2-240d and OVX/c-PGE2+Ris because it could not maintain most of the new subendocortical and marrow trabecular bone generated earlier. In summary, both continuous daily PGE2 and two cycles ON/OFF combined PGE2 and Ris treatments result in more bone mass than two cycles ON/OFF PGE2 alone and Ris alone in estrogen-deficient rats. This study showed that the anabolic effects of PGE2 can be induced and maintained either by continuous administration or by cyclical PGE2+Ris.

Effects of intermittent administration of low dose human PTH(1-34) on cancellous and cortical bone of lumbar vertebral bodies in adult beagles

This study assessed the effect of low dose human parathyroid hormone [hPTH(1-34)] administration on cancellous and cortical bone of lumbar vertebrae in intact male beagles. 16 19-20-month-old beagle dogs were randomized into four groups: in group 1, the vehicle control group, saline was injected daily; in group 2, the sequential group, 0.375 microg/kg of PTH was injected daily for 4 weeks, then off 8 weeks, and this sequence was once repeated for another 4 and 8 weeks; in group 3, the same dose of PTH was injected once per week for 24 weeks; and, in group 4, PTH was injected three times per week for 24 weeks. Histomorphometric assessment on cancelllous and cortical bone (both ventral and dorsal shell) and two-dimensional node-strut analysis were done on the fifth lumbar vertebral bodies after calcein double bone labeling. In intact adult beagles, on the group treated with 0.375 microg/kg per day three times per week (group 4): (1) had a higher mean value in cancellous bone formation parameters [osteoid surface (+74%), osteoid volume (twofold), mineral apposition rate (+21%), and bone formation rate (twofold)]; (2) exhibited no effect on cortical thickness and porosity in both the ventral and dorsal shell; and (3) showed a lower mean value of node to termini (0.11 +/- 0.02 vs. 0.22 +/- 0.09) and a higher mean value of cortex to node (0.18 +/- 0.06 vs. 0.08 +/- 0.02), but not in trabeculae to trabeculae node, than age-related controls. In conclusion, we found that a low dose of PTH administration: (1) stimulated cancellous bone formation; (2) improved connectivity of trabeculae joined to the cortex; (3) did not decrease cortical thickness; and (4) did not increase cortical porosity in both ventral and dorsal cortexal shell of the lumbar vertebrae during this dosage and period in intact male beagles.

Prostaglandin E2 (PGE2) and risedronate was superior to PGE2 alone in maintaining newly added bone in the cortical bone site after withdrawal in older intact rats

The objects of this study were (1) to determine the effects of risedronate (Ris) and prostaglandin E2 (PGE2) alone and in combination, on tibial diaphyses of older intact female rats; and (2) to observe the fate of any extra bone if formed after withdrawal of the treatment. Nine-month-old female Sprague-Dawley rats were treated with 6 mg of PGE2/kg/day, 1 or 5 micrograms of Ris/kg twice a week, or 6 mg of PGE2/kg/day plus 1 or 5 micrograms of Ris/kg twice a week for the first 60 days and followed by vehicle injections for another 60 days. Cross-sections of double fluorescent labeled, undecalcified tibial diaphyses proximal to the tibiofibular junction were processed for histomorphometry. We found that: (1) neither the 1 microgram nor the 5 micrograms of Ris treatment in the 60-day on/60-day off group showed any histomorphometric differences from age-related controls; (2) while the 60 days of PGE2 treatment added extra cortical bone (6%) on the tibial shaft (due to stimulation of periosteal, endocortical, and marrow trabecular bone formation), the new endocortical and most of the new marrow trabecular bone were lost when treatment was withdrawn; however, the new periosteal bone remained; (3) PGE2 with Ris added the same amount of new bone to tibial diaphysis as did PGE2 alone and upon withdrawal, new marrow trabecular bone was lost but new periosteal and endocortical bones were preserved in PGE2 + 1 microgram of Ris on/off group. In contrast, all the new bone was maintained in the PGE2 + 5 micrograms of Ris on/off group; (4) PGE2 + Ris cotreatment failed to block the increase in cortical bone porosity induced by PGE2; and (5) in the PGE2 alone and PGE2 + 1 microgram of Ris on/off groups bone turnover was higher than that in the PGE2 + 5 micrograms of Ris on/off group. These results indicate that on/off treatment with PGE2 and Ris is superior to PGE2 alone in that it forms the same amount of new bone during treatment, but preserves more cortical bone during withdrawal. Depression of bone resorption and turnover were the tissue mechanisms responsible for this protection.