Bone turnover in healthy adults measured by whole body retention and urinary excretion of 99mTc-MDP. Normalization by bone mass

Dissociation between global markers of bone formation and direct measurement of spinal bone formation in osteoporosis

Regional bone metabolism measured using 18F-fluoride PET was assessed in 72 postmenopausal women classified as normal, osteopenic, or osteoporotic. Lower values of regional bone formation activity at the lumbar spine were seen in osteoporotic women, whereas global markers of bone formation were significantly increased.

INTRODUCTION

Evaluations of global bone remodeling have revealed that increased bone turnover is a feature of patients with osteoporosis. The noninvasive functional imaging technique of 18F-fluoride positron emission tomography (PET) allows the direct quantitative assessment of bone metabolism at specific sites in the skeleton, including the clinically important site of the lumbar spine.

MATERIALS AND METHODS

The aim of this study was to compare regional skeletal kinetics in 72 postmenopausal women (mean age, 61 years) classified as normal, osteopenic, or osteoporotic according to their BMD T score at the lumbar spine. Each woman had a dynamic PET scan of the lumbar spine after injection of 90 MBq 18F-fluoride ion and measurements of biochemical markers of bone formation and resorption. The arterial plasma input function was derived using aorta arterial activity from the PET image. Time-activity curves were obtained by placing regions of interest over the lumbar vertebrae. A three-compartmental model was used to calculate bone blood flow (K1) and the net plasma clearance of tracer to bone mineral (Ki), reflecting regional osteoblastic activity (ml/minutes/ml). Rate constants k2, k3, and k4, which describe transport between plasma, the extracellular fluid (ECF) compartment, and the bone mineral compartment, were also measured.

RESULTS

The net uptake of fluoride to the bone mineral compartment (Ki) was significantly lower in the osteoporotic group compared with both the osteopenic and normal groups, with a mean difference of 0.005 ml/minutes/ml (16.7%). The fraction of the tracer in the extravascular tissue space that underwent specific binding to bone mineral (k3/k2 + k3) was also significantly reduced in the women classified as osteoporotic. In contrast, levels of bone-specific alkaline phosphatase (BSALP) were significantly higher in the osteoporotic group compared with the normal and osteopenic groups by 35% and 27%, respectively. A significant negative correlation (r = -0.41) was observed between levels of BSALP and the fraction of the tracer that underwent specific binding to bone mineral.

CONCLUSION

Lower values of Ki, a measurement of regional bone formation activity, were seen in women classified as osteoporotic, whereas levels of BSALP, a measure of global bone formation, were significantly increased. These findings are suggestive of increased global skeletal bone turnover in women with postmenopausal osteoporosis but with relatively reduced regional bone formation at the predominantly trabecular site of the lumbar spine.

A prospective study of risedronate on regional bone metabolism and blood flow at the lumbar spine measured by 18F-fluoride positron emission tomography

The effect of risedronate on bone metabolism at the lumbar spine was assessed in 18 women who had a 18F-fluoride PET scan at baseline and after 6 months of therapy. The net plasma clearance of fluoride to bone mineral reflecting osteoblastic activity decreased significantly after therapy.

INTRODUCTION

Quantitative radionuclide studies of bone reflect bone blood flow and regional osteoblastic activity, and the latter should change after treatment with a bisphosphonate, although this has not been previously demonstrated. The aim of this study was to examine regional 18F-fluoride kinetics in the lumbar spine measured by 18F-fluoride positron emission tomography (PET) before and after treatment with risedronate.

MATERIALS AND METHODS

Eighteen women, with a mean age of 67.0 years and a T-score of less than -2 at the spine or hip, had a dynamic PET scan of the lumbar spine after the injection of 90 MBq 18F-fluoride ion at baseline and 6 months after commencing risedronate therapy. The arterial plasma input function was derived using aorta arterial activity from the PET image. Time-activity curves were measured by placing regions of interest over the lumbar vertebrae. A three-compartmental model was used to calculate bone blood flow (K(1)) and the net plasma clearance of tracer to bone mineral (K(i)). Rate constants k(2), k(3), and k(4), which describe transport between plasma, the extracellular fluid (ECF) compartment, and the bone mineral compartment, respectively, were also measured.

RESULTS

Mean vertebral K(i) decreased significantly by 18.4% from baseline (3.32 x 10(-2) ml/min/ml) to 6 months post-treatment (2.71 x 10(-2) ml/min/ml; p = 0.04). This decrease was similar in magnitude to the decrease observed for bone-specific alkaline phosphatase, a marker of bone formation. There was no significant difference in K(1) from baseline (1.49 x 10(-1) ml/min/ml) to 6 months after treatment (1.38 x 10(-1) ml/min/ml; p > 0.05). There was a significant increase in k(2), reflecting the reverse transport of fluoride from the extravascular tissue compartment to plasma, after 6 months of treatment (2.90 x 10(-1)/min versus 4.43 x 10(-1)/min; p = 0.01). No significant changes were seen for k(3) or k(4). There was a significant decrease from baseline in the fraction of tracer in the extravascular tissue space that underwent specific binding to the bone matrix (k(3)/[k(2) + k(3)]), decreasing by 18.1% (p = 0.02).

CONCLUSION

K(i), the net plasma clearance to bone mineral reflecting regional osteoblastic activity, displayed a significant decrease after 6 months of antiresorptive therapy. This is the first study to show a direct metabolic effect of antiresorptive therapy on skeletal kinetics at the clinically important site of the lumbar spine. The use of 18F-fluoride PET may provide a useful noninvasive tool to assess novel treatments currently being developed for osteoporosis.

Whole body and regional retention of Tc-99m-labeled diphosphonates with a whole-body counter: a study with normal males

A collimated whole-body counter was used to measure the retention and distribution of radioactivity along the longitudinal axis of the body at several times during the 24 hours after the intravenous injection of 50 microCi of Tc-99m-diphosphonates. Whole-body retention (WBR) was measured together with regional uptakes in the following four areas: head, chest, bladder, and legs using two structurally related Tc-99m-diphosphonate skeletal imaging agents: 1-hydroxyethylidene diphosphonate (HEDP) and methylene diphosphonate (MDP). The average 24 hour WBR values in young males, reflecting skeletal uptake of these tracers, were 17.7 +/- 2.2% (n = 20) and 31.0 +/- 2.4% (n = 3), respectively. A model of skeletal clearance was developed using the sum of two exponentials. In normal volunteers the initial rapid clearance phase of both tracers had a half-time of about 1 hour, whereas the slower second phase clearance had a half-time of 22 hours with HEDP and 44 hours with MDP. The WBR is usually calculated for the entire body only at 24 hours, but with the improved spatial resolution of a collimated whole-body counter, regional measurements could potentially be done over shorter periods (6-8 hours) in order to simplify the procedure.

A longitudinal study of pre- and postmenopausal changes in calcium metabolism

Longitudinal changes in the serum concentration of calcium, phosphate, alkaline phosphatase, PTH, calcitonin and the renal handling of calcium and phosphate were studied in 19 normal women of the same age before and after the menopause. Significant increase in serum calcium, phosphate and calcitonin and urine calcium/creatinine and TmPO4/GF were shown to precede the premenopause. After cessation of the menstruation, no statistically significant further changes were observed in these variables. Changes in PTH were not observed neither during the premenopausal nor the post-menopausal period. Alkaline phosphatase increased in the postmenopausal period suggesting an increase in bone turn-over.

Radionuclide imaging in metabolic and systemic skeletal diseases

Radionuclide imaging with Tc-99m diphosphonates is not an effective method for detecting or ruling out most osteoporotic diseases including senile osteoporosis or accelerated postmenopausal osteoporosis, and the slow loss of bone tissue generally remains undetected by this modality. Nonetheless, it frequently surpasses or supplements radiographic findings in evaluating the focal complications of metabolic bone disease, including fractures, microfractures, stress fractures, vertebral compressions, Milkman-Looser zones, aseptic necrosis, and acute infarction. In contrast to its secondary role in osteoporosis, bone imaging is of prime importance in investigating hypercalcemia, because the major cause of this abnormality is skeletal metastatic malignancy. In defective bone mineralization due to hyperparathyroidism or osteomalacia, a general increase in diphosphonate skeletal uptake is detected more frequently than radiographic abnormalities. However, normal skeletal images do not rule out metabolic bone disease. Biochemical testing is more reliable in detecting primary hyperparathyroidism. On the other hand, in renal osteodystrophy, biochemical abnormalities are variable and bone imaging is helpful in assessing the severity of skeletal involvement, but not its etiology. Many methods of quantitating the kinetics of Tc-99m diphosphonates have been explored, such as plasma clearance, bone-to-soft-tissue ratios, 24-hour total body retention and 24-hour urinary excretion. None of these have been widely accepted. The value of bone imaging is established in other systemic diseases, most notably in Paget's disease, hypertrophic pulmonary osteoarthropathy, sickle cell disease, fibrous dysplasia, and sympathetic dystrophy.

Bone turnover in postmenopausal women after withdrawal of estrogen/gestagen replacement therapy

Bone turnover before and after withdrawal of estrogen/gestagen treatment was studied in a randomized trial with 110 healthy female volunteers, who had passed a natural menopause 6 months to 3 years before the start of the study. Urinary excretion of intravenously injected 99m-technetium diphosphonate was measured as an index of bone turnover; plasma bone Gla protein and serum alkaline phosphatase were measured as indices of bone formation; and fasting urinary excretion of hydroxyproline and calcium were measured as estimates of bone resorption. During 2 years of hormone treatment, all variables decreased highly significantly (p less than 0.001) to a constant low level. Three months after withdrawal all variables increased highly significantly (p less than 0.001) towards, but not above, pretreatment and placebo levels. We conclude that withdrawal of estrogen/gestagen replacement therapy in postmenopausal women increases bone turnover, but not in excess of pretreatment values. This indicates that bone loss (after withdrawal) is similar to that seen in the placebo group and that a rebound phenomenon is unlikely.

Does calcium supplementation prevent postmenopausal bone loss? A double-blind, controlled clinical study

During a two-year study, we examined the effect of calcium supplementation on postmenopausal bone loss in 43 women in the early postmenopausal period who were assigned to one of three treatment groups: percutaneous 17 beta-estradiol (combined with progesterone during the second year), oral calcium (2000 mg daily), and placebo. All participants were examined every three months. Bone mineral content in the forearm (measured by single-photon absorptiometry) and in the entire body and spine (measured by dual-photon absorptiometry) remained constant in the estrogen-treated group but decreased significantly in the groups receiving calcium and placebo. In the calcium-treated group, we observed a tendency toward a slowed loss of compact bone (in the proximal forearm and total skeleton) as compared with the placebo group, while the rate of loss of trabecular bone (the distal forearm and spine) was the same as in the placebo group. Our preliminary data suggest that calcium supplementation in the dosage we used is not as effective as estrogen therapy for the prevention of early postmenopausal bone loss. Calcium supplementation may have had a minor effect on the loss of cortical bone, but it had no effect on the trabecular bone.

Effect of estrogen/gestagen and 24R,25-dihydroxyvitamin D3 therapy on bone formation in postmenopausal women

The effect of two different estrogen/gestagen regimens and 24R,25-(OH)2-cholecalciferol on bone formation was studied in a randomized trial with 144 healthy postmenopausal women. Urinary excretion (UE) of 99m-technetium-diphosphonate and serum alkaline phosphatase (AP) was determined before and then once a year for 2 years of treatment. Both estimates of bone formation showed highly significant decreases (p less than .001) to normal premenopausal levels in women receiving unopposed 17 beta-estradiol or in a sequential combination with progestagen, whereas unchanged high values were found in the groups receiving 24R,25-(OH)2D3 and placebo. The data show that bone turnover increases in early postmenopausal women concomitantly with the loss of bone mass, and that hormonal substitutional therapy normalizes the total skeletal turnover as well as preventing bone loss.