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Endocrinology Vol. 144, No. 5 2008-2015
Copyright © 2003 by The Endocrine Society

New Bone Formation with Teriparatide [Human Parathyroid Hormone-(1–34)] Is Not Retarded by Long-Term Pretreatment with Alendronate, Estrogen, or Raloxifene in Ovariectomized Rats

Yanfei L. Ma, Henry U. Bryant, Qingqiang Zeng, Allen Schmidt, Jennifer Hoover, Harlan W. Cole, Wei Yao, Webster S. S. Jee and Masahiko Sato

Lilly Research Laboratories, Lilly Corporate Center (Y.L.M., H.U.B., Q.Z., A.S., J.H., H.W.C., M.S.), Indianapolis, Indiana 46285; and Radiobiology Division, University of Utah (W.Y., W.S.S.J.), Salt Lake City, Utah 84108

Address all correspondence and requests for reprints to: Yanfei L. Ma, M.D, Lilly Research Laboratories, Lilly Corporate Center, Building 98C/B, DC 0403, Indianapolis, Indiana 46285. E-mail: ma_linda{at}lilly.com.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
With the ready availability of several osteoporosis therapies, teriparatide [human PTH-(1–34)] is likely to be prescribed to postmenopausal women with prior exposure to agents that prevent bone loss, such as bisphosphonates, estrogen, or selective estrogen receptor modulators. Therefore, we evaluated the ability of once daily teriparatide to induce bone formation in ovariectomized (Ovx) rats with extended prior exposure to various antiresorptive agents, such as alendronate (ABP), 17{alpha}-ethinyl estradiol (EE), or raloxifene (Ral). Sprague Dawley rats were Ovx and treated with ABP (28 µg/kg, twice weekly), EE (0.1 mg/kg·d), or Ral (1 mg/kg·d) for 10 months before switching to teriparatide 30 µg/kg·d for another 2 months. Analysis of the proximal tibial metaphysis showed that all three antiresorptive agents prevented ovariectomy-induced bone loss after 10 months, but were mechanistically distinct, as shown by histomorphometry. Before teriparatide treatment, ABP strongly suppressed activation frequency and bone formation rate to below levels in other treatment groups, whereas these parameters were not different from sham values for EE or Ral. Trabecular area for ABP, EE, and Ral were greater than that in Ovx controls. However, the trabecular bone effects of ABP were attributed not only to effects on the secondary spongiosa, but also to the preservation of primary spongiosa, which was prevented from remodeling. After 2 months of teriparatide treatment, lumbar vertebra showed relative bone mineral density increases of 18%, 7%, 11%, and 10% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Histomorphometry showed that trabecular area was increased by 105%, 113%, 36%, and 48% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Teriparatide enhanced mineralizing surface, mineral apposition rate, and bone formation rate in all groups. Compression testing of vertebra showed that teriparatide improved strength (peak load) and toughness in all groups to a proportionately similar extent compared with 10 month levels. These data showed a surprising ability of the rat skeleton to respond to teriparatide despite extensive pretreatment with ABP, EE, or Ral. Therefore, the mature skeleton of Ovx rats remains highly responsive to the appositional effects of teriparatide regardless of pretreatment status in terms of cancellous bone area or rate of bone turnover.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
THERAPIES currently available for osteoporosis include antiresorptive agents, such as bisphosphonates, estrogens, calcitonins, and a selective estrogen receptor modulator (SERM). Although antiresorptive agents are powerful tools for preventing bone loss, a bone formation agent that can replace lost bone, restore bone architecture, and enhance bone strength might be especially useful. In contrast to bone resorption inhibitors, human PTH-(1–34) (generic name, teriparatide) has been shown to stimulate new bone formation, resulting in rapid gains in bone mass with improved microarchitecture in osteoporotic patients (1, 2, 3), while reducing the risk for both vertebral and nonvertebral fractures (4, 5). PTH was shown clearly to stimulate mineral apposition onto trabecular, endocortical, and periosteal surfaces in rats (6, 7, 8), rabbits (9, 10), and nonhuman primates (11, 12, 13, 14, 15).

Many osteoporotic patients are already on therapy; therefore, one important question is whether pretreatment with antiresorptive drugs can affect teriparatide skeletal efficacy. In particular, bisphosphonates form very stable complexes with bone that do not easily dissociate, suggesting that alendronate (ABP) effects persist well after discontinuation of treatment (16, 17). In addition, histomorphometry has shown antiresorptives to significantly reduce the rate of bone turnover, as bone formation can be strongly suppressed after initial inhibition of bone resorption activity (18, 19). Therefore, drug interactions could arise even when these agents are used sequentially, as suggested in a rat study that showed that pretreatment with ABP blunted the anabolic skeletal effects of an analog to PTH, SDZ PTS 893 (20).

The purpose of this study was to compare the effects of pretreatment with different classes of antiresorptive agents on teriparatide skeletal efficacy. Ovariectomized (Ovx) rats were treated with ABP, estrogen, or raloxifene (Ral) for 10 months before switching to teriparatide treatment for 2 months.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Treatment regimens
One hundred and twenty-seven virgin, 2.5-month-old Sprague Dawley female rats (Harlan Industries, Indianapolis, IN), weighing approximately 220 g, were maintained on a 12-h light/12-h dark cycle at 22 C with ad libitum access to food (TD 89222 with 0.5% calcium and 0.4% phosphorus, Teklad, Madison, WI) and water. Twelve rats were killed as baseline controls, with the remaining rats randomized to either sham-operated (n = 18) or bilateral Ovx groups (n = 97). At 1 wk after operation, Ovx rats were treated with either 20% hydroxypropyl-ß-cyclodextrin [Ovx vehicle (Veh) controls; n = 16] orally, ABP (28 µg/kg, twice weekly; n = 20) sc, Ral (1 mg/kg·d; n = 20) orally, or 17{alpha}-ethinyl estradiol (EE; n = 20) orally for 10 months (Fig. 1Go). Drugs were prepared as previously reported (21) with a dosing volume of 1 ml/kg. These doses of various antiresorptives were demonstrated to be fully effective in Ovx rats in previous studies (20, 21, 22). Six rats from each group were then killed, and the remaining Ovx rats were switched to sc administration of teriparatide [recombinant human PTH-(1–34); Eli Lilly \|[amp ]\| Co., Indianapolis, IN] at 30 µg/kg·d for the following 2 months. Teriparatide was prepared in a vehicle of sterile 0.15 M NaCl, 0.001 N HCl, and 2% heat-inactivated rat serum. Doses were administered in a relative volume of 0.5 ml/kg between 0800 and 1000 h each day. All rats were given sc injections of a double fluorochrome label of calcein (10 mg/kg; Sigma-Aldrich, St. Louis, MO) at 14, 13, 4, and 3 d before death. This protocol was approved by the animal committee of Lilly Research Laboratories to ensure compliance with NIH guidelines.



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  		Figure 1. Experimental study design. Sham Ovx (Sham) or Ovx rats (Ovx) were treated with vehicle, ABP (28 µg/kg, sc, twice weekly), Ral (1 mg/kg·d, orally), or EE (0.1 mg/kg·d, orally) for 10 months. These groups were then either necropsied or continued on with teriparatide [hPTH-(1–34); 30 µg/kg·d, sc) for another 2 months.

 
One day after the last treatment, the animals were anesthetized with isoflurane to permit collection of a blood sample by cardiac puncture. The animals were then euthanized by CO2 inhalation. Tibiae and femora were removed, cleaned of soft tissue, fixed in 10% formalin for 48 h, then stored at 4 C in 70% ethanol. Lumbar vertebra 4 (LV4) through LV6 were removed and stored for later analyses by microcomputed tomography (LV4) or biomechanical testing (LV5).

X-Ray bone densitometry analysis of LV
Fourth LV were analyzed by high resolution microcomputed tomography (Norland/Stratec, Fort Atkinson, WI) using voxel dimensions of 0.05 x 0.05 x 1 mm. Excised vertebrae in 50% ethanol/saline were wrapped in Parafilm and centered with respect to the quantitative computed tomography (QCT) gantry, and a coronal scout-scan was generated first in two dimensions. LV4 was scanned along the midtransverse plane to measure bone mineral density (BMD), bone mineral content (BMC), and cross-sectional area as previously described (21): BMD = BMC/volume. Volume can be calculated by multiplying area by the slice thickness.

Histomorphometry
Proximal tibiae were first stained for 4 d in Villanueva osteochrome bone stain for osteoid staining (Polysciences, Inc., Warrington, PA), then dehydrated in graded ethanol, defatted in acetone, and embedded in methyl methacrylate. Longitudinal sections of 210 µm thickness were first cut using a diamond wafering saw (Buehler Isomet, Evanston, IL) then further hand ground to 20-µm sections for analysis. Measurements were performed on the entire marrow region within the cortical shell between 1 and 4 mm distal to the growth plate-metaphyseal junction using an Image Analysis System (Osteomeasure, Inc., Atlanta, GA). Trabecular area, perimeter, single- and double-labeling surfaces, eroded surface, osteoid surface, labeling, and wall width were measured and trabecular thickness, mineral appositional rate, bone formation rate-surface reference, and activation frequency were calculated as described previously (23, 24). Bone cell numbers were not measured because of the thickness (20 µm) of these sections.

Biomechanical testing of femur and LV5
Mechanical properties of the midshaft were measured for intact left femora using three-point bending (25). Load was applied midway between two supports that were 15 mm apart. Femora were positioned so the loading point was about 7.5 mm proximal from the distal popliteal space, and bending occurred about the medial-lateral axis. Femoral neck strength was measured by mounting the proximal half of the femur vertically in a chuck and applying downward force at a rate of 1 mm/sec on the femoral head until the neck failed. L5 vertebrae were analyzed after the posterior processes were removed, and the ends of the centrum were made parallel using a diamond wafering saw (Buehler Isomet). Vertebral specimens were compressed using the materials testing device. The compressive load was applied through a pivoting platen to correct for possible nonparallel alignment of the faces of the vertebral body. Parameters analyzed included peak load, Young’s modulus, and modulus of toughness, as described previously (8, 25).

Statistics
Data were presented as the mean ± SEM. Group differences were assessed by ANOVA with pairwise contracts examination. Fisher’s protected least significant difference test was used to compare the differences between the groups, where the significance level for the overall ANOVA was P < 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
As expected, Ovx rats weighed more than sham animals (data not shown). After 10 months, no differences in body weight were observed between ABP and Ovx controls, but Ral and EE lowered body weight by 29–31% compared with Ovx rats at 13 months of age. When treatment was switched to teriparatide (PTH), Ral- and EE-pretreated rats gained proportionally more body weight than other rats, although they (Ral/PTH, EE/PTH) still tended to weigh less than Ovx controls after 12 months and at 15 months of age (data not show). The weight changes were not related to longitudinal skeletal growth, as femoral length was not different between the groups (data not show).

Excised lumbar vertebrae (LV) were evaluated by QCT along the midtransverse plane as shown in Table 1Go. After 10 months of treatment and at 13 months of age, significant increases in vertebral BMC (23%) and BMD (7%) were found in sham rats compared with baseline values, which were determined at 3 months of age at study initiation. Ovariectomy for 10 months resulted in a significantly lower BMC (-15%) and BMD (-19%) compared with sham values. Ovx BMD was also 13% lower than baseline, indicating that ovariectomy inhibited the maturation-induced gain in bone mass (Table 1Go). After 10 months, ABP BMD was higher than that in Ovx rats, but not different from sham rats. Ral and EE prevented the ovariectomy-induced loss of BMD relative to baseline. Ral and EE BMD were 11% and 13% higher than Ovx control values, respectively, but both were 10% and 8% lower than sham values, respectively. BMC and BMD values for ABP were greater than those for Ral or EE groups, whereas Ral and EE parameters were not different (Table 1Go). After 2 months of subsequent treatment, teriparatide increased vertebral BMD by 18%, 7%, 10%, and 11% compared with 10-month levels for animals previously treated with Veh (Veh/PTH), ABP (ABP/PTH), Ral (Ral/PTH), or EE (EE/PTH), respectively.


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  		Table 1. Microcomputed tomography analysis of fourth LV

 
Histomorphometric measurements of proximal tibial metaphyses are presented in Figs. 2Go and 3Go. Significant age-related reductions of trabecular area (36%), mineral appositional rate (75%), bone formation rate (91%), and activation frequency (91%) were observed for 13-month-old sham compared with 3-month-old baseline rats. Ovariectomy for 10 months induced marked reductions of trabecular area (77%) compared with sham values. ABP treatment for 10 months resulted in greater trabecular area compared with sham (59%) and ovx (600%) values. However, ABP also markedly decreased the bone formation rate (84% and 93%), activation frequency (87% and 94%), and eroded surface (74% and 84%) to the levels below both sham and Ovx levels, respectively. ABP trabecular bone effects appeared to be due largely to preservation of the primary spongiosa, which was prevented from remodeling, as primary spongiosa was retained in the metaphyses of only the ABP group (Fig. 4Go). Ral and EE groups also had higher trabecular area (314% and 245%, respectively) relative to the Ovx group, which compared favorably to sham values. Eroded surface was lower in Ral (51%) and EE (54%) compared with Ovx groups. Ral and EE treatments tended to lower dynamic parameters from Ovx toward sham levels.



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  		Figure 2. Histomorphometric analyses of trabecular area (A) and trabecular thickness (B). The proximal tibial metaphysis was analyzed in baseline, Veh-treated sham (Sham), or Ovx rats treated with Veh (Ovx), ABP (28 µg/kg, sc, twice weekly), Ral (1 mg/kg·d, orally), or EE (0.1 mg/kg·d, orally) for 10 months, then switched to teriparatide [hPTH-(1–34); 30 µg/kg·d, sc; Veh/PTH, ABP/PTH, Ral/PTH, and EE/PTH] for another 2 months. Data are presented as the mean ± SE. Significant differences from baseline, age-matched sham, or age-matched Ovx are indicated as b, s, or o. A significant difference for PTH-treated rats from respective 10-month groups of Ovx, ABP, Ral, or EE is indicated by an asterisk (P < 0.05). All three antiresorptive agents prevented Ovx-induced bone loss. Two months of teriparatide treatment increased trabecular area, mainly by thickening the trabeculae in all the groups.

 


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  		Figure 3. Histomorphometric analyses of bone formation, resorption, and turnover in the proximal tibial metaphysis of baseline, Veh-treated sham (Sham) or Ovx rats treated with Veh (Ovx), ABP (28 µg/kg, sc, twice weekly), Ral (1 mg/kg·d, orally), or EE (0.1 mg/kg·d, orally) for 10 months, then switched to teriparatide [hPTH(1–34); 30 µg/kg·d, sc; Veh/PTH, ABP/PTH, Ral/PTH, and EE/PTH] for another 2 months. Data are presented as the mean ± SE. Significant differences from baseline, age-matched sham, or age-matched Ovx are indicated as b, s, or o. A significant difference for PTH-treated rats from respective 10-month groups of Ovx, ABP, Ral, or EE is indicated by an asterisk (P < 0.05). All three antiresorptive agents had lower eroded surface than the Ovx control. ABP decreased the bone formation rate and activation frequency to below both sham and Ovx values. Ral and EE maintained bone formation indexes at sham levels, whereas teriparatide increased these parameters for all groups.

 


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  		Figure 4. Photomicrographs of proximal tibial sections from baseline, Veh-treated sham (sham), or Ovx rats treated with vehicle (Ovx), ABP (28 µg/kg, sc, twice weekly), Ral (1 mg/kg·d, orally), or EE (0.1 mg/kg·d, orally) for 10 months, then switched to teriparatide [hPTH-(1–34); 30 µg/kg·d, sc; Veh/PTH, ABP/PTH, Ral/PTH, and EE/PTH] for another 2 months. Ovariectomy caused significant bone loss and deleterious bone structure. ABP, Ral, and EE prevented these changes. After 10 months, ABP retained primary spongiosa in a manner that closely resembled the metaphysis of baseline. Teriparatide treatment increased bone mass mainly by thickening the trabeculae. Original magnification, x12.

 
Two months of teriparatide administration in animals pretreated with vehicle or antiresorptive agents resulted in marked stimulation of mineral apposition (Figs. 2–4GoGoGo). Teriparatide induced significant gains in trabecular area (105%, 113%, 48%, and 36%) and trabecular thickness (112%, 86%, 50%, and 87%) for Veh/PTH, ABP/PTH, Ral/PTH, and EE/PTH, respectively, compared with their 10-month levels. Teriparatide increased bone formation rate-surface reference by 370%, 6029%, 687%, and 867% for Veh/PTH, ABP/PTH, Ral/PTH, and EE/PTH, respectively, relative to their 10-month levels. Mineral appositional rate and activation frequency also significantly increased in all teriparatide-treated groups.

Biomechanical parameters are detailed in Table 2Go. Ovariectomy for 10 months induced a 50% reduction in toughness of LV relative to sham. Although all three treatments tended to increase vertebral strength (peak load) when compared with Ovx, the changes were not statistically significant. ABP-treated rats had a 24% lower Young’s modulus, but 67% greater toughness relative to Ovx rats. Ral increased Young’s modulus to 41% greater than sham values, but was not different from Ovx values. EE increased toughness by 67% compared with Ovx. Except for a significant increase in femoral shaft toughness (30% and 35%) by ABP compared with sham and Ovx groups, respectively, there were no differences in other biomechanical parameters for the midshaft. No differences in femoral neck strength (peak load) were observed between groups (Table 2Go).


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  		Table 2. Mechanical test analyses of LV, femoral shaft, and femoral neck

 
After 2 months teriparatide treatment, vertebral strength (fracture force) and toughness were significantly increased to largely similar levels for all groups compared with their respective 10-month levels. Young’s modulus was also improved by teriparatide for ABP/PTH and EE/PTH groups relative to their respective 10-month levels. Thus, teriparatide increased vertebral strength, toughness, and, to a lesser extent, Young’s modulus to levels above both Ovx and sham values for all groups regardless of the pretreatment status. Biomechanical testing of the femoral midshaft showed that fracture force was higher for Veh/PTH, ABP/PTH, and Ral/PTH relative to respective 10-month values and sham and Ovx levels for all three groups. Toughness for the ABP/PTH group declined relative to previous ABP levels, whereas toughness for the Ral/PTH group increased relative to sham and Ovx levels (Table 2Go). Teriparatide treatment significantly strengthened the femoral neck of all groups relative to Ovx controls (Table 2Go).


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Results from our study are consistent with previous reports demonstrating a bone protective effect with various antiresorptive agents, including bisphosphonates, estrogens, and SERMs, in Ovx rats. All three antiresorptive agents were confirmed to prevent ovariectomy-induced loss of trabecular bone in the proximal tibia and LV. This protective effect was accompanied by preservation of trabecular bone architecture, as evidenced by the maintenance of trabecular area, trabecular number, and trabecular separation to sham levels. This study also provided new insights regarding the bone anabolic efficacy of teriparatide in Ovx animals pretreated with different antiresorptive agents. Teriparatide significantly enhanced bone mass and bone strength in all groups regardless of trabecular architecture, bone turnover rates, or pretreatment status. Our in vivo model was intended to mimic individuals with advanced osteoporosis or with previous exposure to antiresorptive therapies. In Ovx rats, once-daily teriparatide treatment induced new bone formation in all rats despite previous exposure to ABP, estrogen, or Ral.

ABP is a potent, highly effective inhibitor of bone resorption that forms stable complexes with bone, is taken up directly by osteoclasts, and inhibits farnesyl disphosphate synthase in the mevalonate pathway, leading to apoptosis of osteoclasts (26, 27). Previous studies showed that ABP prevents bone loss and maintains BMD in osteopenic rats (28, 29), consistent with the reduction of fractures observed in postmenopausal osteoporotic women (30, 31, 32). In Ovx rats, ABP was more efficacious than estrogen or Ral in preventing bone loss associated with aging and estrogen deficiency after 10 months. In addition, histomorphometry confirmed substantial reduction in bone resorption activity, but also a greater than 90% reduction of bone formation rate and activation frequency to well below both sham and Ovx controls, not unlike biopsy data from women (18). However, some significant differences between rat and human histomorphometry were also observed, including a large increase in trabecular area, which has not been observed in women (18). Previously, bisphosphonates were shown to prevent the remodeling of primary spongiosa formed during longitudinal growth of long bones, which leads to clubbing of the metaphysis (33). Similar inhibition of remodeling and preservation of calcified cartilage in long bones were shown for ABP (34, 35), indicating that the trabecular bone effects of ABP were largely due to preservation of the primary spongiosa, which was prevented from remodeling in rats. Because postmenopausal women have ceased longitudinal skeletal growth, and treatment may be initiated when substantial bone loss has already occurred, our rat data may not be entirely relevant to humans. Moreover, higher trabecular area in the proximal tibia metaphysis and higher vertebral BMD for the ABP-treated animals did not translate into superior mechanical properties, which were not different from those of estrogen or Ral after long-term treatment.

Estrogen and the SERM Ral have also been shown to prevent bone loss in Ovx rats (21, 22, 36, 37) and to reduce fracture incidence in postmenopausal women (38, 39, 40, 41, 42). In this study estrogen and Ral were shown to prevent bone loss in the proximal tibia, vertebra, and femoral neck after 10 months in Ovx rats. Estrogen and Ral were largely similar in skeletal efficacy, but neither prevented remodeling of the primary spongiosa, because clubbing was not observed, unlike with ABP. Estrogen and Ral prevented Ovx-induced bone loss by inhibiting osteoclast-mediated bone resorption, resulting in preservation of normal trabecular bone architecture similar to that in sham controls. Unlike ABP, both compounds maintained bone formation activity at about sham levels. Similar remodeling effects of Ral were observed in biopsies from postmenopausal women (19). Vertebral strength for estrogen-treated animals tended to be greater than that for ABP-treated rats, but this difference was not significant, as shown previously (21). Vertebral strength after Ral treatment was also not different from that after ABP treatment, which is contrary to a previous report (21). This discrepancy may be explained by the lower dose of Ral used in this study (1 mg/kg) compared with the previous study (3 mg/kg) (21). We selected 1 mg/kg Ral for this study because it better approximates the clinical dose of 60 mg used to treat osteoporotic women.

Teriparatide was shown previously to stimulate mineral apposition onto trabecular, endocortical, and periosteal surfaces, resulting in improved trabecular and cortical bone architecture and enhanced bone strength to an extent beyond what has been observed with ABP, estrogen, or Ral in rats (8, 43, 44, 45, 46, 47). Processes responsible for PTH efficacy were shown to include recruitment of osteoblasts to the bone surface by the activation of quiescent bone lining cells (48), stimulation of osteoblast differentiation from precursors (49), and inhibition of osteoblast apoptosis (50, 51), with little or no stimulation of bone resorption activity (7, 47). Therefore, substantial net gains in bone mass were appreciated because mineral apposition proceeds at a greater rate than resorption activity. Recently, a large, randomized, placebo-controlled study showed that teriparatide significantly increased bone mass and reduced the risk of vertebral and nonvertebral fractures in osteoporotic women (5).

In this study the skeletal efficacy of teriparatide was observed in all groups despite varying trabecular bone status and previous treatment history. In vehicle-treated osteopenic rats, 10 months of ovariectomy eliminated nearly 80% of the trabecular bone compared with age-matched sham controls. Teriparatide doubled the trabecular bone in these animals (Veh/PTH) after 2 months of treatment. Teriparatide nearly restored trabecular architecture and bone strength to normal levels. However, not every skeletal parameter was completely restored to sham levels, but this may reflect the relatively short treatment duration of 2 months in these very osteopenic rats. Therefore, teriparatide has the ability to induce bone formation to increase bone mass and strength regardless of previous experience with antiresorptive agents and independent of the level of previous bone mass and bone turnover rate at the initiation of teriparatide treatment. Pretreatment with antiresorptives resulted in animals with bone turnover similar to sham levels in the case of estrogen and Ral, but also with little bone turnover, as with ABP. No evidence was observed for an adverse pharmacological drug interaction between ABP, estrogen, or Ral and teriparatide on the rat skeleton. Our data showed that teriparatide increased mineralizing surface and mineral appositional rate in all pretreated animals, indicating that teriparatide increased both osteoblast number as well as osteoblast activity. In fact, bone formation indexes were elevated to similar levels in all groups regardless of previous bone formation activities. Teriparatide treatment also improved bone strength and material properties in all groups compared with 10-month levels. These data taken together show that the bone anabolic efficacy of teriparatide was not adversely affected by long-term antiresorptive pretreatment or severe osteopenia.

These data appear to conflict with the report by Gasser et al. (20), who observed a blunted PTH-bone anabolic response in ABP-pretreated rats. The apparent discrepancy may be explained by noting that the magnitude of the blunting effect of ABP in the previous report was modest in size (i.e. 7.4–9.5% decrease in PTH-induced increase in BMC of the proximal tibia metaphysis). Second, the animal models used in the two studies were different. Previously, aged (16 months old at start of study) retired breeder rats were used with a relatively short (8 or 16 wk) ABP treatment period. Thus, Gasser et al. (20) used an animal model that exhibited a very low degree of bone turnover during the ABP exposure period with limited longitudinal growth of long bones. In our study younger, 3-month-old, rats were used with a 10-month treatment period. Our animals still had relatively high bone turnover with significant longitudinal growth during the ABP exposure period. Thus, ABP effects on bone modeling were probably minimal in the previous study but were significant in our study. Third, Gasser et al. (20) employed a PTH analog (SDZ PTS 893), whereas our study used recombinant human PTH-(1–34) (teriparatide).

In summary, the rat skeleton was shown to respond to teriparatide despite extensive pretreatment with ABP, Ral, or estrogen. We conclude that the mature skeleton of Ovx rats remains highly responsive to the appositional effects of teriparatide regardless of pretreatment status in terms of cancellous bone area or rate of bone turnover. The bone-forming effect of once-daily teriparatide treatment appears to be capable of reversing the suppressive effect of antiresorptives on bone turnover. Extrapolation of these results to human use is important, but should be considered with caution, because rat data may not always be relevant to postmenopausal women. Therefore, additional clinical trials will be necessary to clarify the human implications of these animal data.


   Footnotes
 
Abbreviations: ABP, Alendronate; BMC, bone mineral content; BMD, bone mineral density; EE, 17{alpha}-ethinyl estradiol; hPTH, human PTH-(1–34) (generic name, teriparatide); LV, lumbar vertebra; Ovx, ovariectomized; QCT, quantitative computed tomography; Ral, raloxifene; SERM, selective estrogen receptor modulator; Veh, vehicle.

Received October 25, 2002.

Accepted for publication January 8, 2003.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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