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Keywords:

  • cyclic PTH regimen;
  • bone strength;
  • BMD;
  • bone structure;
  • mice

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

We developed a cyclic PTH regimen with repeated cycles of 1-week on and off daily PTH injection and explored its effects on bone strength, BMD, bone markers, and bone structure in mice. Cyclic protocols produced 60–85% of the effects achieved by daily protocols with 57% of the total PTH given, indicating more economic use of PTH. The study supports further exploration of cyclic PTH regimens for the treatment of osteoporosis.

Introduction: To minimize the cost and the catabolic action of hPTH(1-34), a cyclic PTH regimen with repeated 3-month cycles of on-and-off daily injection of hPTH(1-34) was developed in humans and shown to be as effective as a daily regimen in increasing vertebral BMD. However, changes in BMD may not adequately predict changes in bone strength. A murine model was developed to explore the efficacy of a cyclic PTH regimen on bone strength in association with other bone variables.

Materials and Methods: Twenty-week-old, intact, female C57BL/J6 mice (n = 7/group) were treated with (1) daily injection with vehicle for 7 weeks (control); (2) daily injection with hPTH(1-34) (40 μg/kg/day) for 7 weeks (daily PTH); and (3) daily injection with hPTH(1-34) and vehicle alternating weekly for 7 weeks (cyclic PTH). BMD was measured weekly by DXA, and serum bone markers, bone structure, and strength were measured at 7 weeks.

Results: Daily and cyclic PTH regimens increased BMD at all sites by 16–17% and 9–12%, respectively (all p < 0.01). The most dramatic effect of cyclic PTH occurred during the second week of treatment when PTH was off, with femoral and tibial BMD continuing to increase to the same extent as that produced by daily PTH. Both daily and cyclic PTH regimens significantly increased osteocalcin (daily, 330%; cyclic, 260%), mTRACP (daily, 145%; cyclic, 70%), femoral cortical width (daily, 23%; cyclic, 13%), periosteal circumference (daily, 5%; cyclic, 3.5%), and bone strength (max load: daily, 48%; cyclic, 28%; energy absorbed: daily, 103%; cyclic, 61%), respectively. Femoral bone strength was positively correlated with BMD, bone markers, and cortical structure. Neither regimen had an effect on vertebral bone strength. Although actual effects of cyclic PTH were 60–85% of those produced by daily PTH, the effects of cyclic PTH per unit amount administered were slightly greater than those of daily PTH for most measures.

Conclusions: PTH-enhanced femoral bone strength is positively correlated with its effects on femoral BMD, bone markers, and bone structure. Cyclic PTH regimens represent a potential economic use of PTH and warrant further study.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

THE AMINO-TERMINAL 1–34 fragment of human PTH {hPTH(1-34)} is the only anabolic agent currently approved by the U.S. Food and Drug Administration for the treatment of osteoporosis. (1–4) In contrast to conventional antiresorptive agents, which reduce bone resorption and ultimately suppress bone formation, (5–7) PTH stimulates bone formation. (1–4, 8–10) In humans, bone formation markers, including bone-specific alkaline phosphatase (BSALP) and osteocalcin, are increased within a month of initiation of daily injection of hPTH(1-34), whereas PTH-induced increases in resorption markers catch up about 6 months later. (1–3, 8) In addition, histomorphometric analysis of human iliac crest bone biopsies using quadruple tetracycline labeling revealed that daily injection of hPTH(1-34) (25 μg/day) directly stimulated bone formation without any significant increase in bone resorption within 6 weeks. (8) These findings suggest that daily PTH injection induces bone formation without prior resorption on initiation of therapy. (1, 2, 8)

However, there are some disadvantages to PTH treatment. First, the anabolic effect of PTH plateaus over time. The mechanisms for this phenomenon have not yet been defined. Second, the cost of PTH treatment is greater than that for antiresorptive agents. Third, daily injections are inconvenient and may reduce compliance. To dissociate the PTH-induced early bone formation from its later stimulation of bone resorption, we developed a cyclic PTH regimen and compared its efficacy to that of a daily PTH regimen in humans. (9) We found that 15 months of repeated 3-month cycles of on- and off-hPTH(1-34) daily administration in patients, who had prior and ongoing treatment with alendronate, produced an anabolic effect on vertebral BMD similar to that seen in patients given PTH daily for 15 months. (9) Despite the fact that 40% less PTH was used, the cyclic regimen produced the same effects on vertebral BMD as the daily PTH regimen.

However, it is still unclear whether changes in BMD adequately predict bone strength and, ultimately, reduction in fracture risk. The only clinical trial of PTH with a large enough sample size to assess fractures did not show a strong relationship between the increase in BMD and reduction in fracture incidence. (4) To evaluate the effects of a cyclic PTH regimen on bone strength and other bone measures, we developed a cyclic PTH protocol in mice. The following information was considered when determining the appropriate duration of the on/off intervals in mice. In previous animal studies, withdrawal of PTH after continued daily injection resulted in rapid loss of its anabolic effect on bone mass and structure. (10–14) In young male rats, (10) histomorphometric analysis showed that daily injection of PTH for 12 days increased bone mass and osteoblast surface by 2- to 3-fold and that, within 12 days of PTH withdrawal, the PTH-stimulated bone mass was lost and the PTH-increased osteoblast surface (2- to 3-fold) was decreased by 50%. In the same study, it was shown that the increased osteoclast surface was diminished to control level within 8 days of PTH withdrawal. (10) We therefore speculated that the PTH-off periods in a cyclic PTH regimen in mice should be <8 days. We previously showed that virgin, female mice reached peak bone mass in the lumbar vertebrae, with very slight age-related increases in tibial and femoral BMD, at ∼20-22 weeks. (15) Moreover, we recently showed that by 2 weeks, intermittent daily PTH injection markedly increased the bone resorption marker, mouse TRACP (mTRACP), to a level produced by continuous PTH infusion. (16) Thus, in consideration of these data, as well as the differences between mice and humans in life span, body size, bone growth, and responsiveness to PTH based on our previous murine studies, (15–18) we postulated that 1-week on- and off-PTH cycles would best achieve the specified aims and would serve as a reasonable model of our clinical trial. (9) Therefore, we treated mature mice with daily versus weekly on and off cyclic PTH regimens for 7 weeks and compared the effects of these PTH regimens on BMD, bone markers, bone structure, and bone strength.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

Materials

hPTH(1-34) was purchased from Bachem (Torrance, CA, USA). A diagnostic kit for total serum calcium and most of the biochemical agents, including ketamine and xylazine, were purchased from Sigma Chemical (St Louis, MO, USA). An immunoradiometric assay (IRMA) kit for mouse osteocalcin was obtained from Immutopics (San Clemente, CA, USA). The mTRACP assay kit was purchased from Immunodiagnostic Systems (Phoenix, AZ, USA).

Animals

The experimental protocol was reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of Helen Hayes Hospital.

Twenty-one virgin, female C57BL/J6 mice were purchased from the Jackson Laboratory (Bar Harbor, ME, USA) at 8 weeks of age and stabilized at the Animal Research Facility of Helen Hayes Hospital. At 20 weeks of age, animals were weighed and randomly divided into the following three groups (n = 7 each; Table 1): daily injection with vehicle (0.2 mM acetic acid in PBS, pH 7.42) for 7 weeks (control); daily injection with hPTH(1-34) (40 μg/kg/day) for 7 weeks (daily PTH); and a weekly alternating regimen of daily hPTH(1-34) (40 μg/kg/day) and daily vehicle for 7 weeks (3.5 cycles; cyclic PTH). Animals were housed three or four per cage, given free access to water, and fed a standard diet (Purina Mills, St Louis, MO, USA) in a room maintained at 22 ± 1°C with 60–75% humidity on 12-h light/dark cycles. Injections were performed between 9:00 a.m. and 10:00 a.m. Body weight was measured weekly before BMD measurement.

Table Table 1.. Experimental Protocol
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Collection of blood and bone specimens

At 20–24 h after the last injection, animals were killed with a mixture of ketamine (100 mg/kg) and xylazine (3 mg/kg) after the last BMD measurement. Blood was collected by cardiac puncture, and serum was stored at −80°C until use for biochemical assays. Right femurs and L4-L6 vertebrae were carefully excised, gently cleaned with PBS (pH 7.42), wrapped with PBS-soaked gauze, stored at −20°C overnight, and sent to SkeleTech (Bothell, WA, USA) for mechanical testing and pQCT. The identity of the samples was blinded before analysis.

BMD measurement by DXA

BMD of the femur, tibia, and lumbar vertebrae was measured at baseline (week 0) and weekly thereafter by in vivo DXA using PIXImus (GE Lunar Corp., Madison, WI, USA), as previously described. (17, 18) Briefly, mice were anesthetized with a mixture of ketamine (100 mg/kg) and xylazine (3 mg/kg) and placed prone on the platform of the PIXImus, and BMD was measured with mouse-specific software (Version 1.47). The machine was calibrated daily with the phantom provided by the manufacturer. The scans were analyzed independently by two observers, one blinded to treatment assignment, using the same regions of interest (ROIs) previously described. (18) The two sets of analyzed data were almost identical.

Mechanical loading and bone strength

Frozen right femurs and L5 vertebrae were thawed in PBS, carefully cleaned of any remaining adherent soft tissue, and subjected to four-point bending and compression tests, respectively. (19, 20) The structure of the femoral midshaft was examined by pQCT before the four-point bending test as described below.

Four-point bending test on the midshaft femur

The whole femur was placed on the lower supports of a four-point bending fixture with the posterior side facing downward in an Instron Mechanical Testing Machine (Instron 4465 retrofitted to 5500). The upper loading device was aligned to the center of the femoral shaft. The load was applied at a constant displacement rate of 3 mm/min until failure. The load and displacement were recorded by Testing Machine Software (Merlin II; Instron). The locations for maximum load (N) at failure, stiffness (N/mm), and energy absorbed (mJ) were selected manually from the load and displacement curve and calculated by Testing Machine Software.

Compression test on the fifth lumbar vertebra

For each thawed vertebral sample, the posterior pedicle arch and spinous process were removed using a low-speed diamond saw. Additionally, the cranial and caudal ends of each vertebral body were cut off to obtain a vertebral body specimen with two parallel surfaces and height of ∼2 mm, using a customized pivoting device to accommodate nonperfect parallel surfaces. Each specimen was placed between two platens in an Instron Mechanical Testing Machine (Instron 4465 retrofitted to 5500), and a load was applied at a constant displacement rate of 0.6 mm/min until failure. As described above, maximum load, stiffness, and energy absorbed were selected manually from the load and displacement curve and calculated by the machine's software. (20)

pQCT

The femoral midshafts were subjected to pQCT using a Stratec XCT-RM and associated software (software version 5.40; Stratec Medizintechnik, Pforzheim, Germany). Femoral midshafts were scanned at 50% of the total femoral length measured from the distal end of the femur. The positions were verified using scout views, and one 0.5-mm slice perpendicular to the long axis of the femoral shaft was acquired from each site. The scans were analyzed using a threshold for delineation of the external boundary. The total and cortical BMC, bone area, and BMD, as well as periosteal and endosteal circumferences and cortical width, were obtained from the scan.

Biochemical assays

Serum total calcium was determined in triplicate by colorimetric reaction. (16) Mean CV% for intra-assay variation was 7.9%. Serum levels of osteocalcin and mTRACP were measured in duplicate as bone formation and resorption markers, respectively, according to the manufacturer's instructions. (16) Mean CV% for intra-assay variations for osteocalcin and mTRACP was 2.4% and 6.7%, respectively.

Statistical analysis

Values for BMD, biochemical markers, bone structural variables, and mechanical strength are means ± SE. The significance of differences in BMD among time/treatment groups was determined using general linear model (GLM) analysis of repeated measures (Duncan) using SAS (version 9.1; SAS Institute, Cary, NC, USA). The significance of differences in all other bone variables among the groups was determined by two-way ANOVA. Correlations between variables were determined by Pearson's coefficient analysis.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

Effects of daily and cyclic hPTH(1-34) on BMD at 7 weeks

At 7 weeks, daily and cyclic PTH produced 15.8% and 9.8% increases (both p < 0.001) in femoral BMD (Fig. 1A) and 16.7% and 8.9% increases (both p < 0.001) in tibial BMD (Fig. 1B), respectively. There were significant differences in the anabolic effects between the daily and cyclic PTH regimens in the femur and the tibia at this time-point (p < 0.05). Similarly, daily and cyclic PTH regimens increased vertebral BMD (Fig. 1C) by 15.7% and 11.8% (both, p < 0.01), respectively. However, there was no statistically significant difference in the lumbar vertebrae between the two regimens.

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Figure FIG. 1.. Effects of daily vs. cyclic PTH on BMD at 7 weeks. Values are mean ± SE of six (control) and seven (PTH groups) replicates.ap < 0.05 vs. control;bp < 0.01 vs. control;cp < 0.05 vs. daily PTH.20

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Temporal patterns of the anabolic effects of daily and cyclic hPTH(1-34) on BMD

Considering the weekly changes in BMD, the most dramatic effect of PTH treatment occurred during the initial 2 weeks in the femur and the tibia. In long bones, the anabolic effect of daily injection of hPTH(1-34) was significant within 1 week and linearly increased up to 2 weeks (Figs. 2A and 2B). Interestingly, both femoral and tibial BMD continued to increase at the same rate in both PTH groups up to 2 weeks, despite the fact that PTH was absent in the cyclic PTH group during the second week. After 2 weeks, femoral BMD continued to increase at a lower rate in the daily PTH group, and there were no further significant increases in femoral BMD in the cyclic PTH group. The temporal pattern of daily versus cyclic PTH regimens on vertebral BMD was somewhat different from that observed in femoral BMD. Between 3 and 7 weeks, both daily and cyclic PTH increased vertebral BMD in a similar fashion, and at 7 weeks, there was no significant difference between the anabolic effects of the two regimens on vertebral BMD (Fig. 2C).

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Figure FIG. 2.. Temporal changes in (A) femoral, (B) tibial, and (C) vertebral BMD after daily vs. cyclic PTH treatment. Values are mean ± SE of six (control) or seven (PTH groups) replicates.20

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Effects of daily and cyclic hPTH(1-34) on serum total calcium, osteocalcin, and mTRACP

There were no significant differences in serum total calcium levels (Fig. 3A). The serum osteocalcin level increased by 3.3- and 2.6-fold (both p < 0.001) in the daily and cyclic groups, respectively (Fig. 3B). In addition, the resorption marker mTRACP increased by 145% and 70% (both p < 0.05) in the daily and cyclic groups, respectively (Fig. 3C). There were no significant differences in either osteocalcin or mTRACP levels between the treatment groups.

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Figure FIG. 3.. Effects of daily vs. cyclic PTH on (A) serum calcium, (B) bone formation (osteocalcin), and (C) resorption (mTRACP) markers. Values are mean ± SE of six (control) or seven (PTH groups) replicates.20

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Effects of daily and cyclic hPTH(1-34) on femoral bone structure as assessed by pQCT

Total BMC, total bone area, BMD, cortical BMC, cortical bone area, and cortical BMD were significantly greater in both PTH groups than controls, with more prominent effects for all variables in the daily PTH group (Table 2). Femoral cortical width (Fig. 4A) was increased by 23% and 13% (both p < 0.01) in the daily and cyclic PTH groups, respectively. The femoral periosteal circumference was increased by 5.04% (p < 0.01) and 3.47% (p = 0.064) in the daily and cyclic PTH groups, respectively (Fig. 4B), whereas neither regimen had an effect on endosteal circumference (Fig. 4C).

Table Table 2.. Effects of Daily and Cyclical PTH Regimens on Cortical Bone Structure Assessed by pQCT
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Figure FIG. 4.. Effects of daily vs. cyclic PTH on femoral midshaft bone structure (A: cortical width; B: periosteal circumference; C: endosteal circumference). Values are mean ± SE of six (control and cyclic PTH group) or seven (daily PTH group) replicates. One of the femurs in the cyclic group was excluded from pQCT and mechanical testing because it was found broken after excision and shipment.20

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Effects of daily and cyclic hPTH(1-34) on femoral and vertebral strength

At 7 weeks, the femoral maximum load, one of the indices of femoral bone strength, increased by 48% (p < 0.001) and 28% (both p < 0.02) in the daily and cyclic PTH groups, respectively, compared with control (Fig. 5A). Another bone strength index, energy absorbed, increased by 103% (p < 0.001) and 61% (p < 0.02) in the daily and cyclic PTH groups (Fig. 5B). Neither PTH regimen, however, significantly changed vertebral strength for either index (Figs. 5C and 5D), although there was a slight, nonsignificant increase in vertebral maximum load and energy absorbed in the daily PTH regimen.

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Figure FIG. 5.. Effects of daily vs. cyclic PTH on (A and B) femoral and (C and D) vertebral bone strength assessed by (A and C) maximum load and (B and D) energy absorbed. Values are mean ± SE of six or seven replicates as described in Fig. 3. For femurs, N = 6 for control and cyclic PTH and N = 7 for daily PTH. For lumbar vertebrae, N = 6 for control and N = 7 for PTH groups.20

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Correlations among bone strength and other bone measures

To determine whether there were any significant correlations between bone strength and other bone parameters, Pearson coefficient analyses were performed. Femoral bone strength showed a significant, positive correlation with femoral BMD, bone markers, cortical width, and periosteal circumference among all groups, including controls (Table 3). To explore the possibility that this significant, positive correlation might be caused by two cluster distribution patterns from nontreated and treated groups, correlations between bone strength and other variables within the PTH-treated groups were also analyzed. Highly positive correlations were still observed between femoral strength and femoral BMD or bone structure, but correlations with bone markers were no longer statistically significant (Table 3). In contrast, there was no correlation between vertebral strength and other bone variables.

Table Table 3.. Correlations Between Bone Strength and BMD, Biomarkers, or Bone Structure
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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

This study showed that both daily and cyclic PTH regimens increase bone strength in proportion to their effects on BMD and cortical bone structure in the femur of mature, female C57BL/J6 mice. In contrast, there were no significant effects on vertebral bone strength by either PTH regimen, although both regimens significantly and almost equally increased vertebral BMD at 7 weeks. These data suggest that the effect of PTH on murine bone strength may be site-specific and are consistent with our previous reports showing that the effects of PTH on murine BMD are site-specific, with long bones exhibiting more rapid and marked increases than the lumbar vertebrae during the earlier phase of treatment. (17) The skeletal site-specificity on bone strength in response to PTH was further confirmed by significant, positive correlations between bone strength and BMD, as well as biochemical markers and bone structure in the femur, but not in the lumbar vertebrae. On the other hand, the lumbar spine is the site where PTH produces the most significant response in humans. (1, 2) This species difference may in part be associated with the differences in posture, with the human lumbar spine being a more mechanically loaded site. Because it has been suggested that mechanical stress enhances the anabolic effect of PTH on bone, (21, 22) it seems reasonable to postulate that vertebral bones would respond better to PTH than nonvertebral bones in humans, whereas femoral and tibial bones would be more responsive to PTH than vertebral bones in mice. Consistent with our findings, Tobias et al. (23) showed that IGF-1, which is thought to be a mediator of the anabolic effects of PTH on bone, (24, 25) has opposite effects on formation of trabecular and cortical bone in adult female rats. IGF-1 inhibited trabecular bone formation at the proximal tibia by reducing labeling surface and bone formation rate (BFR), whereas it increased cortical bone formation at the periosteal tibial diaphysis. (23) Thus, if the anabolic effects of PTH on bones are primarily mediated through IGF-1, the effects of PTH on bone strength of the lumbar vertebrae, a trabecular bone-enriched site, could be less than those on bone strength of the femoral cortex.

By using two different PTH administration protocols, we were able to correlate the effects of PTH on bone strength with changes in other bone measures in the femur. The differential efficacy of the two regimens on bone strength was parallel to their effects on all other measures tested, indicating that the bone strength assessed by resistance against bending force in long bones can be accurately predicted by the changes in BMD, changes in a bone formation marker, and changes in cortical bone structure. Although BMD has been considered to be a major determinant of bone strength in clinical studies for more than a decade, (26–28) increases in BMD do not always reflect the effects on fracture incidence. (29, 30) Neer et al. (4) showed that daily injection of hPTH(1-34) at a dose 40 μg/day increased BMD to a greater degree than did 20 μg/day, but resulted in similar reductions in both vertebral and nonvertebral fracture risk. However, in this study, at least in the femur, PTH increased bone strength in proportion to its effects on BMD, cortical width, and periosteal circumference. Thus, the effects of PTH on femoral bone strength seemed to depend mainly on its effects on cortical bone mass and structure. The importance of cortical width and periosteal apposition on bone strength in humans has also been also emphasized. (31, 32)

Conversely, vertebral bone strength was not improved by either PTH regimen, nor did it correlate with vertebral BMD. This raises a question as to why neither PTH regimen exerted any effects on vertebral bone strength, despite the fact that both regimens significantly increased BMD. The first, most likely, answer could be technical problems in mechanical testing on murine vertebrae, which are very small and difficult to clearly excise without damage that might affect the test results. These include imprecision in sampling the middle half of the original specimens as well as variability in the extent of removal of the lateral processes. (20) Because the height of the measured samples (2 mm) is less than one-half of the intact specimen, it is possible that it may not be representative of all of the structural elements that determine vertebral strength. (20) In addition, the compression tests were performed only on one vertebra, whereas vertebral BMD was measured as collective segments of two (L4-L5) or more together. (16–18) Therefore, we speculate that our current compression test on a single vertebra may not adequately reflect the true vertebral strength in mice. Also, our sample size may have been too small to see significant effects. Another possible explanation may lie in lack of loading as described above. This, however, still could not explain the discrepancy that both PTH regimens significantly and equally increased BMD but failed to improve bone strength. Third, because the PTH-induced increases in vertebral BMD were mainly caused by its effects on trabecular bone, (33, 34) the PTH-induced increases in vertebral cortical bone mass and structure might not have been large enough to enhance bone strength at this time-point. Further studies using static and dynamic histomorphometric analyses are currently underway to better understand the effects of PTH on bone structure and bone cell activity in the lumbar vertebrae as well as the femur.

In this study, the total amount of hPTH(1-34) administered in the cyclic group was 57% of that in the daily group and, at 7 weeks, the effects of cyclic PTH on most variables were 60–85% of those of daily PTH. This implies that allowing an off-period after PTH treatment may produce more efficient anabolic effects. This concept is clearly shown during the second week of the experiment by the fact that there was an apparent continued anabolic response in femoral BMD in the cyclic PTH group, in which BMD increased even when PTH was absent. This could be partly explained by the fact that mineral continues to be deposited within newly formed bone during the second week of treatment without further increases in bone volume, but resulting in a significant increase in BMD. (35–37) Another possible explanation could lie in differences in osteoblast and osteoclast activity in response to PTH withdrawal. In young male rats, Gunness-Hey et al. (10) reported that the PTH-stimulated osteoblast surface remained at 50% of its maximally activated status even at 12 days after PTH discontinuation, whereas the PTH-stimulated osteoclast surface returned to the control level by 8 days after discontinuation. Thus, osteoblasts and osteoclasts may respond differentially to discontinuation of PTH injections in rats. (10, 12) In this study, it could be that, during the second week when PTH was withheld, PTH-stimulated osteoblastic activity remained high at 7 days after withdrawal, whereas osteoclastic activity might have quickly returned to control levels, resulting in a net increase in bone formation in the absence of PTH during the second week.

At the end of 7 weeks, the cyclic PTH regimen did not further increase femoral BMD beyond the level achieved at 2 weeks. We speculate that the initial, profound increase in femoral BMD might return to the control level or below if mice were maintained without PTH for the remaining 6 weeks after a 1-week treatment with PTH. It remains to be determined whether femoral bone strength and other measures at 2 weeks are the same as those at 7 weeks in the cyclic PTH group.

The animals in this study were not ovariectomized, but no additional agent was given to maintain PTH-induced BMD increments. Circulating estradiol might be adequate to retard bone loss induced by PTH withdrawal. In our recent clinical trial, alendronate was present during both PTH-on and PTH-off periods. (9) In studies using ovariectomized older rats, it was shown that PTH further increased bone strength and BMD after long-term treatment with alendronate. (11) Thus, it still remains to be determined whether continuous or alternating use of a bisphosphonate and PTH in repeated cycles would improve anabolic responses in all measures and whether improved bone strength outcomes could be obtained in our murine model.

This study ended on a PTH-on period. Therefore, the outcomes in biochemical markers, bone structure, and bone strength ending after a PTH-off period remain to be determined. In particular, it will be useful to determine how the bone markers change after repeated on- and off-PTH periods in parallel to changes in BMD. Development of better assay systems that can measure biochemical markers in smaller blood samples from live mice will be helpful for future longitudinal studies of the cellular effects of cyclic PTH protocols.

The total increments in BMD, bone markers, bone structure, and bone strength induced by the cyclic PTH regimen were 60–85% of those produced by the daily regimen after 7 weeks of treatment. However, when the effect was expressed per unit PTH administered, the anabolic effects of the cyclic PTH regimen on most bone measures were identical to, or slightly greater than, those produced by the daily PTH regimen (Table 4). This was most clearly seen at 2 weeks, when the effects of the cyclic PTH regimen on BMD at all sites were almost identical to those in the daily group, despite the fact that the total PTH dose administered in the cyclic PTH group was only one-half that in the daily group.

Table Table 4.. Comparison of Cyclical vs. Daily PTH Regimen on the Increment of Each Bone Parameter per Unit PTH Administered
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In conclusion, this study supports the potential use of cyclic PTH regimens for the treatment of osteoporosis. Such approaches would have obvious financial benefits and could also improve adherence to PTH therapy.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References

The work was supported in part by a Helen Hayes Hospital Foundation grant, unrestricted research funds from Bristol-Myers Squibb Foundation and NS Pharma Co., Ltd., and NIH/NIAMS Grants AR39371 and PO1-AR049363.

References

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. References
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