Prolonged treatments with antiresorptive agents and PTH have different effects on bone strength and the degree of mineralization in old estrogen-deficient osteoporotic rats

Abstract

Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree than predicted from change in BMD in women with postmenopausal osteoporosis. We hypothesize that bone active agents improve bone strength in osteoporotic bone by altering different material properties of the bone. Eighteen-month-old female Fischer rats were ovariectomized (OVX) or sham-operated and left untreated for 60 days to induce osteopenia before they were treated with single doses of either risedronate (500 microg/kg, IV), zoledronic acid (100 microg/kg, IV), raloxifene (2 mg/kg, PO, three times per week), hPTH(1-34) (25 microg/kg, SC, three times per week), or vehicle (NS; 1 ml/kg, three times per week). Groups of animals were killed after days 60 and 180 of treatment, and either the proximal tibial metaphysis or lumbar vertebral body were studied. Bone volume and architecture were assessed by muCT and histomorphometry. Measurements of bone quality included the degree of bone mineralization (DBM), localized elastic modulus, bone turnover by histomorphometry, compression testing of the LVB, and three-point bending testing of the femur. The trabecular bone volume, DBM, elastic modulus, and compressive bone strength were all significantly lower at day 60 post-OVX (pretreatment, day 0 study) than at baseline. After 60 days of all of the bone active treatments, bone mass and material measurements agent were restored. However, after 180 days of treatment, the OVX + PTH group further increased BV/TV (+30% from day 60, p < 0.05 within group and between groups). In addition, after 180 days of treatment, there was more highly mineralized cortical and trabecular bone and increased cortical bone size and whole bone strength in OVX + PTH compared with other OVX + antiresorptives. Treatment of estrogen-deficient aged rats with either antiresorptive agents or PTH rapidly improved many aspects of bone quality including microarchitecture, bone mineralization, turnover, and bone strength. However, prolonged treatment for 180 days with PTH resulted in additional gains in bone quality and bone strength, suggesting that the maximal gains in bone strength in cortical and trabecular bone sites may require a longer treatment period with PTH.

FIG. 5

Localized elastic modulus of the PTM trabecular surface measured at day 180. OVX + vehicle induced a significant reduction in elastic modulus and increased the heterogeneity. OVX + Ris, Zol, or Ral and sham had similar elastic modulus, which had a greater percentage of high elastic modulus compared with OVX + PTH. The heterogeneity index of antiresorptive treatments and PTH was similar to that of sham. However, PTH had the highest overall Elastic modulus compared with all the other groups at 180 days. ^ap < 0.05 from all the other groups.

FIG. 1

Proximal tibial trabecular bone microarchitecture changes measured by μCT. Mean values and SD for (A) trabecular bone volume and (B) trabecular bone thickness from the right proximal tibial metaphyses obtained from groups of sham-operated (Sham) or OVX animals treated with vehicle (OVX), risedronate (Ris), zoledronic acid (Zol), raloxifene (Ral), and PTH(1-34) from days 0 to 180. ^ap < 0.05 vs. sham-operated animals at the same time point; ^bp < 0.05 vs. OVX + vehicle-treated animals at the same time point.

FIG. 2

Serum biochemical and surface based assessments of bone turnover. Mean values and SD for (A) urine DPD (nmol) cross-links and creatinine (Cr, mmol) measured by ELISA; (B) percent osteoclast surface (%); (C) percent mineralizing surface (%); and (D) percent surface-based bone formation rate (μm/d). (B-D) Measured from bone histomorphometry obtained from the proximal tibial metaphysis for groups of sham-operated (Sham) or OVX animals treated with vehicle (OVX), risedronate (Ris), zoledronic acid (Zol), raloxifene (Ral), and PTH(1-34) (PTH) from days 0 to 180. ^ap < 0.05 vs. sham-operated animals at the same time point; ^bp < 0.05 vs. OVX + vehicle-treated animals at the same time point.

FIG. 3

Tibial shaft cortical bone histomorphometry. Unstained cross-sections of the midtibial shafts from (A) day 60 and (B) day 180 animals treated with vehicle, vehicle (OVX), risedronate (Ris), zoledronic acid (Zol), raloxifene (Ral), and PTH(1-34) (PTH). Note the enlarged marrow cavities in OVX and Ral groups at day 180 and the markedly reduced marrow cavities in Ris, Zol, and PTH groups at day 180. At day 60, Zol and PTH had higher intracortical remodeling (labeled osteon) in the inner third of the cortex. PTH increased total cross-sectional area and decreased endocortical area at day 180 (C). (D) Treatment with PTH increased the cortical thickness at both days 60 and 180, whereas with Ris and Zol increased cortical thickness at day 180 (E).

FIG. 4

(A) Degree of bone mineralization (DBM) and mineral distribution generated by high-resolution μCT for trabecular bone of the fourth lumbar vertebral bodies at days 60 and 180. The distribution of mineralization shifted to the left with OVX at day 60, with lower values of DBM. All the treatment groups had similar trabecular mineral distribution curves as the sham controls at day 60. At day 180, Ris, Zol, and PTH treatment shifted the mineral distribution to the right, with more bone of higher mineral concentration. (B) Lumbar vertebral compression strength and femoral maximum bending strength at days 60 and 180. At day 60, antiresorptive treatments and PTH had similar lumbar compression strength that was significantly higher than the OVX group. At day 180, PTH further increased lumbar compression strength, which was higher than all the other groups. Femoral bending strengths were increased with Ris and PTH treatments compared with the OVX at day 180. ^ap < 0.05 vs. sham at the same time point; ^bp < 0.05 vs. OVX at the same time point; ^cp < 0.05 vs. the antiresorptive groups.