Assessment of regional changes in skeletal metabolism following 3 and 18 months of teriparatide treatment

Abstract

Teriparatide (TPTD) increases skeletal mass, bone turnover markers, and bone strength, but in vivo effects at individual skeletal sites have not been characterized. Quantitative radionuclide imaging studies reflect bone blood flow and osteoblast activity to assess regional changes in bone metabolism. Changes in bone plasma clearance using technetium-99m methylene diphosphonate (^99mTc-MDP) were quantified and correlated with changes in bone turnover markers in 10 postmenopausal women with osteoporosis. Subjects underwent bone scintigraphy at baseline and 3 and 18 months after initiating TPTD 20 µg/day subcutaneously. Subjects were injected with 600 MBq ^99mTc-MDP, and whole-body bone scan images were acquired at 10 minutes and 1, 2, 3, and 4 hours. Multiple blood samples were taken between 5 minutes and 4 hours after treatment, and free ^99mTc-MDP was measured using ultrafiltration. The Patlak plot method was used to evaluate whole-skeleton ^99mTc-MDP plasma clearance (K_bone) and derive regional bone clearance for the calvarium, mandible, spine, pelvis, and upper and lower extremities using gamma camera counts. Bone turnover markers were measured at baseline and 3, 12, and 18 months. Median increases from baseline in whole-skeleton K_bone were 22.3% (p = .004) and 33.7% (p = .002) at 3 and 18 months, respectively. Regional K_bone values were increased significantly in all six subregions at 3 months and in all subregions except the pelvis at 18 months. Bone markers were increased significantly from baseline at 3 and 18 months and correlated significantly with whole-skeleton K_bone. This is the first study showing a direct metabolic effect of TPTD at different skeletal sites in vivo, as measured by tracer kinetics. © 2010 American Society for Bone and Mineral Research

Introduction

Recombinant human parathyroid hormone fragment rhPTH(1-34) [teriparatide (TPTD)] is approved for use in women with postmenopausal osteoporosis and in men with primary or hypogonadal osteoporosis who are at high risk of fracture.1–4 TPTD acts through an anabolic mechanism to promote bone formation and resorption, with a net positive bone balance.5 Accretion of new bone on trabecular and cortical surfaces with TPTD therapy leads to improved bone microarchitecture6–8 with an associated increase in bone mineral density (BMD).2, 3, 9 Because TPTD increases the rate of bone remodeling,10 early changes in biochemical markers of bone formation and bone resorption may predict the treatment response, as measured by increases in BMD,11 which, in turn, contributes to decreased fracture risk.12

While markers of bone turnover provide integrated information regarding total skeletal formation and resorption, these surrogates do not provide information regarding regional bone metabolism. Radionuclide bone scan imaging using the radiopharmaceutical technetium-99m methylene diphosphonate (^99mTc-MDP)13 is a frequently performed nuclear medicine examination used to investigate regional bone metabolism in a variety of clinical settings.14, 15 The concentration of ^99mTc-MDP in bone is a function of bone blood flow and extraction efficiency.16 The radionuclide tracer ^99mTc-MDP binds to newly mineralized bone, thus serving as a marker of osteoblast activity.15 Microautoradiography studies show enhanced accumulation of ^99mTc-MDP at sites of bone mineralization, suggesting that osteoblastic activity and the subsequent skeletal mineralization influence the quantitative uptake of tracer.16 In clinical practice, isotopic bone scans are reported solely on visual interpretation of the scan images. This qualitative assessment allows for detection of discrete changes in the skeleton in areas of focal uptake of tracer. In the research setting, methods have been developed to translate focal and diffuse visual assessments into quantifiable data.16–19 Skeletal plasma clearance (mL/min) of tracer, denoted K_bone, measures the bone metabolic activity and takes into account both the blood input function and a quantitative determination of the amount of radionuclide tracer cleared from the plasma into bone.

This study is the first to assess the impact of TPTD on the bone metabolic response using ^99mTc-MDP bone scan imaging. The primary objective of this study was to determine whether teriparatide 20 µg/day is associated with a change in skeletal plasma clearance (K_bone) of ^99mTc-MDP in the whole skeleton, measured from baseline to 18 months of treatment, in postmenopausal women with osteoporosis. The secondary aims of the study were to quantify bone plasma clearance and skeletal uptake of ^99mTc-MDP in six subregions, to visually evaluate the focal and diffuse changes in the whole skeleton between baseline and 3 and 18 months of therapy, and to assess adverse events and patient safety with TPTD. Exploratory objectives included examining correlations between changes in skeletal plasma clearance with values and changes in bone turnover markers at 3 and 18 months and changes in BMD at 18 months.

Materials and Methods

Study population

In this prospective, exploratory, phase IV open-label study, 10 postmenopausal white women with osteoporosis were treated with teriparatide 20 µg/day by subcutaneous self-injection. This report describes the observations after 18 months of TPTD therapy. Subjects were required to be between 50 and 85 years of age, ambulatory, and free of other chronically disabling conditions other than osteoporosis. They were required to have a total-hip, femoral neck, or lumbar spine BMD T-score of −2.5 or less20 or less than −2.0 if they had a history of vertebral or nonvertebral fracture. Hip T-scores were calculated using the Third National Health and Nutrition Examination Survey (NHANES III) reference range,21 and lumbar spine T-scores were calculated using the manufacturer's reference range.

Exclusion criteria included fractures in areas of bone affected by diseases (eg, cancer), metabolic bone disorders (eg, Paget's disease of bone) other than osteoporosis, diseases that affect bone metabolism (eg, hyperparathyroidism), or impaired renal or hepatic function. Subjects were excluded if they had received the following treatments prior to enrollment: calcitonin in the past 2 months, corticosteroids in the past 3 months, or fluorides, vitamin D > 50,000 IU/week, androgens, anabolic steroids, or other drugs that affect bone metabolism in the past 6 months. Subjects who used the following agents in the past 3 months or for more than 2 months in the past year prior to enrollment were excluded: oral, transdermal, or injectable estrogens; progestins; selective estrogen receptor modulators (SERMs); or oral bisphosphonates. Subjects treated with intravenous bisphosphonates within 12 months prior to enrolment or treated with prior TPTD, parathyroid hormone (PTH), or PTH analogue at any time also were excluded.

All women received daily calcium (1000 mg) and vitamin D (400 to 1200 IU) supplements for 2 months prior to the baseline visit and for the duration of the study. The trial was conducted in accordance with good clinical practices and the Declaration of Helsinki. The institutional review board or ethics committee approved the protocol prior to study initiation. All enrolled subjects provided written informed consent consistent with all local and regional regulations prior to undergoing any study procedure or receiving any study treatment.

Bone density measurements and biochemical markers of bone turnover

BMD in the lumbar spine (L₁-L₄) and left hip was measured at baseline and 18 months using dual-energy X-ray absorptiometry (DXA; Hologic, Inc., Bedford, MA, USA). The long-term precision was 1.6% for both spine and total-hip BMD.22

Fasting concentrations of bone turnover markers were measured at baseline and 3, 12, and 18 months. The bone-formation markers were serum procollagen type I N-terminal propeptide (PINP; Orion Diagnostica, Espoo, Finland; interassay coefficient of variation 3.1% to 8.2%) and serum bone-specific alkaline phosphatase (BSAP; Hybritech, Beckman-Coulter, Brea, CA, USA; interassay coefficient of variation 7.4% to 7.9%). The bone-resorption marker was urinary excretion of N-terminal telopeptide (NTX; Ostex, Seattle, WA, USA; interassay coefficient of variation 6.7% to 14.8%) normalized to creatinine. The reference ranges for the bone turnover markers in postmenopausal women were specified by the central laboratory (Covance CLS, Geneva, Switzerland), which analyzed the samples.

Quantitative measurements of bone metabolism using ^99mTc-MDP

Subjects had gamma camera bone scan imaging performed at baseline and 3 and 18 months after starting TPTD therapy. Each subject was injected intravenously with 600 MBq of ^99mTc-MDP. For quantitative analysis, anterior and posterior whole-body images were acquired using a dual-headed gamma camera (ADAC Forte, ADAC Laboratories, Milpitas, CA, USA) with a scan speed of 25 cm/min at 10 minutes and 1, 2, 3, and 4 hours after injection. A standard diagnostic scan (scan speed 10 cm/min) was performed at 3.5 hours for visual assessment of the bone scan image. Subjects were asked to drink plenty of fluids and to empty their bladder before each whole-body scan. Five-minute right and left lateral spot views of the skull were acquired immediately after the 1-, 2-, 3-, and 4-hour scans.

Whole-body retention of ^99mTc-MDP in bone and soft tissue at 10 minutes and 1, 2, 3, and 4 hours was measured from the geometric mean of anterior and posterior total-body counts after correction for background and urine activity by subtracting counts from regions of interest (ROI) drawn over the bladder and kidneys.19 After correction for radioactive decay, the retention of ^99mTc-MDP in bone and soft tissue was derived by normalizing with respect to the uncorrected (eg, including bladder and kidneys) whole-body count at 10 minutes, which was defined as 100%. The whole-body ^99mTc-MDP images also were analyzed for counts in four subregions of the whole skeleton (ie, spine, pelvis, upper extremities, and lower extremities) by copying the ROI defined on the 4-hour anterior and posterior images onto the 10-minute and 1-, 2-, and 3-hour whole-body images.19 Results were expressed as the percentage of injected dose by taking the geometric mean of anterior and posterior counts and normalizing to the 10-minute whole-body count.

The retention of ^99mTc-MDP in the calvarium and mandible was measured by drawing appropriate ROIs on the lateral images of the skull obtained at 4 hours. The same ROIs were copied onto the 1-, 2-, and 3-hour images. After correction for background counts and ^99mTc-MDP decay, the geometric mean of the counts in the 1-, 2-, 3-, and 4-hour images were normalized to the geometric mean of the counts from a point source of known activity measured with and without transmission through the skull to derive absolute uptake.23

At 5 and 20 minutes and 1, 2, 3, and 4 hours after injection of ^99mTc-MDP, 5-mL blood samples were taken via an indwelling venous cannula in the opposite arm to the injection site. Blood samples were centrifuged, and 2 mL of plasma placed in 10-kDa filters (Amicon Ultra-4, Millipore Corp., Bedford, MA, USA) and spun for 30 minutes at 2000g to measure the free ^99mTc-MDP.24 Aliquots (1 mL) of ultrafiltrate and whole plasma were counted in an automatic gamma counter together with ^99mTc-MDP standards to measure the plasma concentration curves of free and total (free plus protein-bound) ^99mTc-MDP.

The Patlak plot method25, 26 was used to evaluate whole-skeleton ^99mTc-MDP plasma clearance (K_bone) and to derive regional values of bone plasma clearance for the calvarium, mandible, spine, pelvis, and upper and lower extremities using gamma camera counts for each subregion. Qualitative visual scoring of changes in the bone scan images from baseline at 3 and 18 months was performed by three reviewers who classified the changes into three groups (ie, no discernible change in uptake, possible change in uptake, and definite change in uptake).

Clinical laboratory and safety assessments

PTH(1-84), 25-hydroxyvitamin D, 24-hour urine calcium, and clinical chemistry were collected at screening. To determine renal function, the glomerular filtration rate (GFR) was measured using plasma clearance of ⁵¹Cr-EDTA and standardized to a body surface area of 1.73 m², as described previously.27 GFR also was estimated using age and serum creatinine values and the Renal Association Web-site calculator (www.renal.org/eGFRcalc/GFR.pl, accessed July 13, 2009). Serum calcium and alkaline phosphatase (AP) were collected prior to TPTD administration at screening and were repeated along with creatinine at each clinic visit. Subjects were questioned at each clinic visit about the occurrence and severity of adverse events. Fractures were collected as adverse events.

Statistical analysis

The percent changes from baseline in skeletal plasma clearance (K_bone), bone turnover markers, and BMD were evaluated using the Wilcoxon signed-rank test. Owing to the small sample size (n = 10), the percent changes were plotted to show the median and interquartile range (IQR). Spearman rank correlation analyses were performed to assess the relationship between changes in K_bone and changes in bone marker after 3 and 18 months of TPTD therapy. Statistical inferences were made based on a two-sided significance level of p < .05.

Results

Subjects in this study had a mean age of 68 years and, on average, had severe osteoporosis with low BMD as well as vertebral fractures (Table 1). However, prevalent vertebral fractures observed visually on the whole-body bone scans performed at the baseline visit had a negligible effect on the global or regional skeletal uptake of ^99mTc-MDP. Creatinine clearance, serum calcium, and PTH concentrations were within the normal range at baseline. Biochemical markers of bone turnover were within the range expected for postmenopausal women.

Table 1. Baseline Characteristics

Characteristic	Mean ± SD	Normal rangea
BCE = bone collagen equivalent; BMD = bone mineral density; SD = standard deviation. a Normal ranges were provided by Covance laboratories (Geneva), which analyzed the samples. Ranges for BSAP, uNTx/Cr, and alkaline phosphatase are for postmenopausal women; all others are for adult females. b GFR was determined by plasma clearance of ^51Cr-EDTA and standardized to a body surface area of 1.73 m^2. Reference range for women aged 50 to 80 years was calculated from a constant GFR of 103.4 mL/min at age 40, with a decline of 9.1 mL/min per decade of age. The estimated GFR was calculated to be 71.9 ± 11.1 (mean ± SD) using mean age and serum creatinine values for this population.
Age (years)	68.3 ± 8.0	—
Years postmenopausal	21.4 ± 7.4	—
Body mass index (kg/m^2)	24.3 ± 5.7	—
Number of prevalent fractures	4.2 ± 2.6	—
Lumbar spine BMD (g/cm^2)	0.70 ± 0.08	—
Lumbar spine T-score	–3.19 ± 0.78	—
Femoral neck BMD (g/cm^2)	0.57 ± 0.08	—
Femoral neck T-score	–2.51 ± 0.76	—
Bone-specific alkaline phosphatase (BSAP) (µg/L)	12.5 ± 3.6	3.8–22.6
N-Terminal propeptide of collagen type I (PINP) (µg/L)	38.9 ± 13.1	19.0–84.0
Urinary N-telopeptide of type I collagen/creatinine (uNTx/Cr) (nmol BCE/mmol Cr)	29 ± 19	0–130
Glomerular filtration rate (GFR) (mL/min)b	79.8 ± 13.2	64.0–91.3b
Serum creatinine (µmol/L)	72.8 ± 6.7	31–101
Serum calcium (mg/dL)	2.47 ± 0.07	2.07–2.64
Intact parathyroid hormone (1-84) (pmol/L)	3.1 ± 1.3	0.6–4.2
Serum alkaline phosphatase (AP) (U/L)	69 ± 15	35–123

At all time points, subjects treated with teriparatide 20 µg/day had statistically significant increases in concentrations of serum BSAP, PINP, and urinary NTX/creatinine ratio (Fig. 1). At 18 months, the median percentage change (IQR) in BMD compared with baseline was +8.7% (1.6% to 9.2%, p = .01) for the spine and +0.9% (0.0% to 3.7%, p = .30) for the total hip.

Figure 1. Median percent changes from baseline of the biochemical markers of bone turnover after 3 and 18 months of treatment with TPTD. Error bars show the interquartile range. p Values are calculated using the two-sided Wilcoxon signed-rank test. BSAP = serum bone specific alkaline phosphatase; PINP = serum procollagen type I N-terminal propeptide; NTX/Cr = urinary excretion of N-terminal telopeptide normalized to creatinine.

Qualitative visual scoring of bone scans at 3 and 18 months showed variations in skeletal changes within and between subjects. However, overall changes in ^99mTc-MDP uptake with TPTD therapy were noted frequently in the calvarium and to a lesser extent in the long bones of the lower extremities. Figure 2 shows anterior-view whole-body gamma camera bone scans at baseline and 3 and 18 months of TPTD therapy in a subject with pronounced visual changes in skeletal uptake of ^99mTc-MDP. The values for bone markers and K_bone at 3 and 18 months for this subject were higher than the baseline values (Table 2). However, the BMD changes at the lumbar spine (−0.30%) and hip (0.44%) were not significantly different from baseline at 18 months in this subject.

Figure 2. ^99mTc-MDP bone scan images at baseline and 3 and 18 months for one subject with a robust change in K_bone.

Table 2. Values for Bone Turnover Markers and K_bone at Baseline and 3 and 18 Months in One Subject With a Robust Response

Bone Markera	Baseline	3 Months	18 Months
BCE = bone collagen equivalent. a Bone-formation markers included BSAP and PINP. Bone-resorption marker was urinary N-terminal telopeptide (NTX) normalized to creatinine (Cr).
BSAP (µg/L)	7.5	15.5	14.4
PINP (µg/L)	38	216	190
uNTx/Cr (nmol BCE/nmol Cr)	20	169	202
Alkaline phosphatase (U/L)	47	71	71
Kbone (mL/min)
Whole skeleton	42.5	74.6	67.8
Calvarium	3.22	8.10	9.27
Mandible	0.17	0.38	0.56
Spine	5.71	7.99	9.04
Pelvis	5.78	10.05	10.02
Upper extremities	3.65	7.59	9.82
Lower extremities	8.33	24.23	20.35

For other subjects, discernible visual changes were less pronounced and more variable. At 3 months, six subjects had definite increases in ^99mTc-MDP uptake, and two subjects had possible increase in uptake, whereas two subjects showed no visual uptake at any site at 3 months. Visual changes between the baseline and 18-month bone scan images were more marked than those at 3 months. At 18 months, eight subjects showed a definite increase in uptake, one subject had possible increased uptake, whereas one subject showed no increase in uptake at any site.

The Patlak plot measurements of whole-skeleton K_bone were increased in 9 subjects at 3 months and in all 10 subjects at 18 months. After 3 months of TPTD therapy, the median percent increase (IQR) from baseline in whole-skeleton plasma clearance of ^99mTc-MDP was 22.3% (14.2% to 30.7%, p = .004), and after 18 months, the increase was 33.7% (30.0% to 50.9%, p = .002; Fig. 3A). The increase in whole-skeleton K_bone between 3 and 18 months also was statistically significant (p = .027). The changes in K_bone values for the skeletal subregions are shown in Fig. 3B–F. After 3 months of treatment with TPTD, the median increases from baseline (IQR) in regional skeletal plasma clearance values were statistically significant in each of the six subregions studied [ie, calvarium, 72.2% (42.2% to 106.6%); mandible, 65.9% (26.6% to 106.8%); spine, 17.3% (0.5% to 35.5%); pelvis, 20.3% (5.0% to 43.6%); upper extremities, 42.5% (8.9% to 91.1%); and lower extremities, 21.0% (3.7% to 50.2%)]. After 18 months of treatment, the changes from baseline were statistically significant for five subregions [ie, calvarium, 128.4% (97.4% to 162.6%); mandible, 61.0% (31.3% to 127.8%); spine, 33.8% (7.5% to 58.0%); upper extremities, 95.5% (62.5% to 146.1%); and lower extremities, 34.9% (16.6%–67.1%)]. The change at the pelvis (8.4%, 4.2% to 28.2%) did not reach statistical significance at 18 months. Subregional K_bone values also were statistically significantly increased between 3 and 18 months at the calvarium (p = .008), spine (p = .049), and upper extremities (p = .027).

Figure 3. Median percent change from baseline of the K_bone values for the whole skeleton and five of the six skeletal subregions studied after 3 and 18 months of treatment with TPTD. Data from the mandible are not shown. Error bars show the interquartile range. p Values are calculated using the one-sided Wilcoxon signed-rank test. All results are based on 10 subjects, except the calvarium, which is based on 8 subjects.

The Spearman rank correlation coefficients between changes in K_bone and changes in bone turnover markers after 3 and 18 months of TPTD therapy are shown in Table 3. At both 3 and 18 months, the increases in K_bone in the whole skeleton were significantly correlated with the increases in BSAP and PINP (p < .05, two-sided test). The increases in regional K_bone at 18 months in the lower extremities were significantly correlated with BSAP and urinary NTX. The Spearman correlation coefficients between the changes in whole skeleton K_bone and the changes in BMD were not significant at either the spine (r = 0.03) or hip (r = 0.31). None of the correlations between K_bone values in the six subregions and BMD at the spine or hip were statistically significant.

Table 3. Spearman Rank Correlation Coefficients Between Changes in K_bone and Changes in Bone Turnover Markers (BSAP, NTX, PINP) After 3 and 18 Months of Teriparatide Therapya

Kbone	3 Months			18 Months			Alkaline phosphatase at 3 and 18 months
	BSAP	PINP	NTX	BSAP	PINP	NTX
BSAP = bone-specific alkaline phosphatase; NTX = urinary N-terminal telopeptide; PINP = serum procollagen type I N-terminal propeptide. a Bone-formation markers included BSAP and PINP. Bone resorption marker was NTX normalized to creatinine. Alkaline phosphatase also was measured in this study, and its correlations with Kbone are shown here for comparison with the bone turnover markers. b All values based on 10 subjects except for calvarium and mandible, which are based on 8 subjects. * p < .05; † p < .10 (two-sided test)
Whole skeleton	0.67*	0.60†	0.04	0.60†	0.78*	0.25	0.60†, 0.20
Calvariumb	0.52	0.29	−0.12	0.43	0.19	−0.02	0.02, 0.31
Mandibleb	−0.05	−0.10	0.00	−0.07	0.14	−0.07	−0.31, −0.07
Spine	0.22	0.36	0.58†	0.55	0.18	0.38	0.32, 0.42
Pelvis	0.27	0.44	0.18	0.54	0.43	0.20	0.41, 0.77*
Upper extremities	0.52	0.43	0.10	0.13	0.54	−0.01	0.21, 0.21
Lower extremities	0.42	0.55	0.19	0.71*	0.38	0.71*	0.64†, 0.73*

Alkaline phosphatase concentrations were within the normal range at baseline (Table 1), with median percent change (IQR) from baseline of 5.4% (-1.5% to 14.5%) at 3 months (p = .106), 16.2% (−3.3% to 23.4%) at 12 months (p = .038), and 6.2% (1.6% to 16.9%) at 18 months (p = .064). Alkaline phosphatase concentrations were significantly correlated with K_bone values for the whole skeleton at 3 months, the pelvis at 18 months, and the lower extremities at both time points (Table 3).

TPTD was well-tolerated in these 10 subjects, with adverse events similar to those seen in the larger clinical trials. A total of four serious adverse events were reported, three of which occurred in one subject who experienced fractures at the hip and elbow after a fall at the 12-month visit. This subject completed the study, and a negligible increased uptake of ^99mTc-MDP was observed at the two fracture sites at the 18-month time point. No subject had elevated serum calcium values, defined as greater than 11 mg/dL (>2.8 mmol/L), at any time during the study. The subject shown in Fig. 2 reported randomly occurring mild joint/bone pain in the left leg after taking TPTD for 2 months. This adverse event continued for 8 months while the subject was receiving TPTD, but the severity did not increase or affect any other part of the skeleton during this period. After the adverse event stopped, the subject continued to receive TPTD for a further 9 months without reporting any bone or joint pain in any part of the skeleton.

Discussion

Previous studies with TPTD have used biochemical markers of bone turnover as surrogates of total-skeletal bone turnover.3, 7, 8, 11 This is the first study to report the effect of TPTD treatment on global and regional bone metabolism, as assessed by nuclear scintigraphy.

We have shown that the quantitative measurement of whole-skeleton plasma clearance (K_bone) was a more sensitive indicator of response to treatment than the visual changes on the bone scan images. Measurements of whole-skeleton K_bone were increased in all subjects at 18 months, with the value nearly doubling in the most responsive subject (Fig. 2 and Table 2). TPTD treatment resulted in statistically significant increases in K_bone in the different subregions, with the largest changes in the calvarium and the upper and lower extremities. Biochemical markers of bone turnover and BMD at the lumbar spine were increased significantly from baseline at all time points studied, similar to previous clinical trials.2, 3, 7, 8 Quantification of the skeletal plasma clearance of tracer (K_bone), a marker of osteoblast activity, allowed for comparisons between K_bone and bone turnover markers, with some statistically significant correlations seen at the whole skeleton and lower extremities. However, the small number of subjects and the variability in measurements of bone turnover (Fig. 1) and K_bone (Fig. 3) may have limited the statistical power to test the strength of the correlations (Table 3).

Of the 10 subjects treated with TPTD, 1 subject had obvious visual increases in uptake of tracer throughout the whole skeleton (Fig. 2). In the 8 subjects reported as visually showing a definite increase in uptake with 18 months of treatment, the changes were most evident in the calvarium and long bones of the lower extremities, with the change in contrast between these areas and the rest of the skeleton being the clearest visual evidence of a treatment response. In one subject increased uptake was not discernible in any subregion in two subjects, which may reflect the variability of TPTD effects on bone metabolism within and between subjects. Thus the visual changes in bone scan images during TPTD treatment appear to be variable and range from marked global increases in uptake, to increases in the calvarium and lower extremities, to increases in the calvarium alone, to no discernible change. A previous case report found the calvarium to have the most obvious visual change on bone scans following initiation of TPTD treatment.28 Although bone scan images provide a sensitive method of visually detecting focal areas of increased uptake, the results from this study suggest that it may be more difficult to reliably detect changes that affect the whole skeleton, probably because the visual clues are subtler.

The diffuse symmetric skeletal uptake pattern seen with TPTD treatment differs from that seen in Paget's disease of bone, which often manifests as focal asymmetric skeletal uptake29 and may uniformly involve the entire bone of the scapula, vertebrae, or pelvis or be seen as a V-shaped leading edge in affected long bones.15, 30 The skeletal uptake pattern in subjects treated with TPTD is similar to the pattern reported in patients with hyperparathyroidism. In one study, four patients with primary hyperparathyroidism had increased symmetric skeletal uptake in the calvarium, mandible, sternum, acromioclavicular joint, and distal epiphyses of the femur, with two patients also having increased uptake in the lumbar vertebrae and iliac crests.29 In patients with secondary hyperparathyroidism from renal osteodystrophy, bone scans showed increased radionuclide uptake in the axial skeleton (ie, calvarium, mandible, sternum, vertebrae, and pelvis) and in the joints and epiphyses of the long bones of the lower extremities.30–32 However, patients with hyperparathyroidism have continuous exposure to high levels of endogenous PTH, which leads to catabolic effects on bone, whereas low doses of PTH given intermittently produces bone anabolic effects.5 Possible contributing factors to the observed pattern of bone uptake with TPTD therapy include differences in the bone structure (eg, lamellar bone in the calvarium), regional blood flow, and differential bone affinity for tracer or differences in the bone turnover rate, the stage of the remodeling cycle, or density of PTH receptors between skeletal subregions.

Concentrations of bone turnover markers showed the greatest increase between baseline and 3 months, with unchanged or decreased levels between 3 and 18 months (Fig. 1), consistent with previous clinical trials.3, 7, 11 Such observations have led researchers to suggest that the most robust bone-formation activity occurs early in the course of treatment.5 Skeletal plasma clearance of tracer in the whole skeleton was significantly increased at 3 and 18 months of TPTD treatment compared with baseline. Furthermore, the whole skeleton and every subregion except the pelvis showed relatively greater increases in K_bone after 18 months of TPTD treatment than after 3 months. Additionally, BMD and bone mass continue to increase with longer periods of treatment.2, 3, 7 These observations are not consistent with an attenuation of the anabolic effects of TPTD between 3 and 18 months.

This study has several strengths. Since blood samples were taken to measure tracer input function and results were expressed in terms of whole-skeleton and regional plasma clearance,18, 19 this study used the counts on the gamma camera images to obtain a more physiologic measure of change than conventional measurements of bone uptake.33–35 Another strength was the concurrent assessment of bone scans, bone turnover markers, and BMD, which allowed for quantitative comparisons between the visual images and surrogate markers with well-characterized responses to TPTD treatment. The most important limitation of this study was the small number of subjects enrolled. For this reason, it is unclear how frequently a bone scan showing the very obvious changes seen in Fig. 2 may occur during TPTD treatment or whether a response to treatment in terms of biochemical markers of bone turnover can occur without a corresponding change in bone plasma clearance. The subjects in this study were all white postmenopausal women, so information regarding the effects of TPTD on bone scans in other populations was not collected.

In conclusion, this is the first study to show a direct metabolic effect of TPTD therapy on bone as measured by tracer kinetics at individual clinically important skeletal sites. Postmenopausal women with osteoporosis treated with TPTD had increased skeletal uptake of ^99mTc-MDP, indicative of increased bone formation, which is supported by increases in bone turnover markers and BMD. These data may provide physicians with useful clinical insights into the total and regional skeletal effects of TPTD as measured by radionuclide bone scans. Further, if a bone scan is ordered as a diagnostic tool for other conditions, results from this study may offer practical information to aid the radiologic evaluation of patients who happen to be on TPTD therapy.

Disclosures

AEBM, GMB, and IF were paid by Eli Lilly and Company as clinical trial investigators. KAT, AER, MW, and PC are employees of Lilly USA, LLC, a subsidiary of Eli Lilly and Company.

Coauthors' roles in study conduct and manuscript preparation: Study design: KAT, PC, and IF. Study conduct: AEBM, GMB, KAT, anf IF. Data acquisition: AEBM and GMB. Data analysis: GMB, PC, and AER. Data interpretation: AEBM, GMB, KAT, AER, MW, PC, and IF. Drafting manuscript: AEBM, GMB, KAT, and MW. Reviewing manuscript content: AEBM, GMB, KAT, ARR, MW, PC, and IF. Approving final version of manuscript: AEBM, GMB, KAT, AER, MW, PC, and IF.

Acknowledgements

We would like to thank John H Krege, MD, of Lilly USA, LLC, for insightful comments and critical review of this manuscript. Ewa Rogos, MD, of Lilly Poland, and Roma Jhalli, BSc (Hons), of Lilly UK, assisted with study coordination. Eli Lilly and Company sponsored this study. Clinical trials registration: http://www.clinicaltrials.gov. Trial number NCT00259298; trial registration date: November 28, 2005.