-Teriparatide, a therapeutic agent for osteoporosis, has been reported to increase the incidences of bone neoplasms such as osteosarcoma when administered subcutaneously to Fischer 344 (F344) rats for a long term, but its non-carcinogenic dose level following 2-year daily administration has not been established. Here we report detailed studies on the carcinogenicity of teriparatide following longterm administration. When teriparatide was administered subcutaneously to male and female SpragueDawley (SD) rats daily for 2 years, the incidence of osteosarcoma was increased at 13.6 μg/kg/day. The non-carcinogenic dose level was 4.5 μg/kg/day for both males and females. The development of osteosarcoma in SD rats depends on the dose level of, and treatment duration with, teriparatide. Responses of the bones to teriparatide were similar between F344 and SD rats in many aspects. These results suggested that the carcinogenic potential of teriparatide in SD rats is essentially the same as in F344 rats.

Evidence supports that daily and once-weekly administration of teriparatide, human (h)PTH(1-34), enhance bone mass in osteoporotic patients. However, it is uncertain whether different frequencies of hPTH(1-34) administration would induce bone formation similarly in terms of quantity and quality. To investigate that issue, mice were subjected to different frequencies of PTH administration, and their bones were histologically examined. Frequencies of administration were 1 time/2 days, 1 time a day, and 2 and 4 times a day. Mice were allocated to either to control or to 3 different dosing regimens: 80 μg/kg of hPTH(1-34) per injection (80 μg/kg per dose), 80 μg/kg of hPTH(1-34) per day (80 μg/kg · d), or 20 μg/kg of hPTH(1-34) per day (20 μg/kg · d). With the regimens of 80 μg/kg per dose and 80 μg/kg · d, high-frequency hPTH(1-34) administration increased metaphyseal trabecular number. However, 4 doses per day induced the formation of thin trabeculae, whereas the daily PTH regimen resulted in thicker trabeculae. A similar pattern was observed with the lower daily hPTH(1-34) dose (20 μg/kg · d): more frequent PTH administration led to the formation of thin trabeculae, showing a thick preosteoblastic cell layer, several osteoclasts, and scalloped cement lines that indicated accelerated bone remodeling. On the other hand, low-frequency PTH administration induced new bone with mature osteoblasts lying on mildly convex surfaces representative of arrest lines, which suggests minimodeling-based bone formation. Thus, high-frequency PTH administration seems to increase bone mass rapidly by forming thin trabeculae through accelerated bone remodeling. Alternatively, low-frequency PTH administration leads to the formation of thicker trabeculae through bone remodeling and minimodeling.

Once-weekly preventive administration of hPTH(1-34) increased the total contents of immature and mature enzymatic cross-links, which contributed significantly to vertebral cancellous bone strength.

Intermittent parathyroid hormone (iPTH) treatment induces bone anabolic effects that result in the recovery of osteoporotic bone loss. Human PTH is usually given to osteoporotic patients because it induces osteoblastogenesis. However, the mechanism by which PTH stimulates the expansion of stromal cell populations and their maturation toward the osteoblastic cell lineage has not be elucidated. Mouse genetic lineage tracing revealed that iPTH treatment induced osteoblastic differentiation of bone marrow (BM) mesenchymal stem and progenitor cells (MSPCs), which carried the leptin receptor (LepR)-Cre. Although these findings suggested that part of the PTH-induced bone anabolic action is exerted because of osteoblastic commitment of MSPCs, little is known about the in vivo mechanistic details of these processes. Here, we showed that LepR + MSPCs differentiated into type I collagen (Col1) + mature osteoblasts in response to iPTH treatment. Along with osteoblastogenesis, the number of Col1 + mature osteoblasts increased around the bone surface, although most of them were characterized as quiescent cells. However, the number of LepR-Cre-marked lineage cells in a proliferative state also increased in the vicinity of bone tissue after iPTH treatment. The expression levels of SP7/osterix (Osx) and Col1, which are markers for osteoblasts, were also increased in the LepR + MSPCs population in response to iPTH treatment. In contrast, the expression levels of Cebpb, Pparg, and Zfp467, which are adipocyte markers, decreased in this population. Consistent with these results, iPTH treatment inhibited 5-fluorouracil-or ovariectomy (OVX)-induced LepR + MSPC-derived adipogenesis in BM and increased LepR + MSPC-derived osteoblasts, even under the adipocyte-induced conditions. Treatment of OVX rats with iPTH significantly affected the osteoporotic bone tissue and expansion of the BM adipose tissue. These results indicated that iPTH treatment induced transient proliferation of the LepR + MSPCs and skewed their lineage differentiation from adipocytes toward osteoblasts, resulting in an expanded, quiescent, and mature osteoblast population.

Teriparatide [human parathyroid hormone (1–34)], which exerts an anabolic effect on bone, is used for the treatment of osteoporosis in patients who are at a high risk for fracture. That the once-daily administration of teriparatide causes an increase in cortical porosity in animal models and clinical studies has been a matter of concern. However, it is not well documented that the frequency of administration and/or the total dose of teriparatide affect the cortical porosity. The present study developed 4 teriparatide regimens [20 μg/kg/day (D20), 40 μg/kg/day (D40), 140 μg/kg/week (W140) and 280 μg/kg/week (W280)] in the rabbit as a model animal with a well-developed Haversian system and osteons. The total weekly doses were equivalent in the low-dose groups (D20 and W140) and in the high-dose groups (D40 and W280). After the short-term (1 month) administration of TPDT, micro-CT, histomorphometry and three-dimensional second harmonic generation (3D-SHG) imaging to visualize the bone collagen demonstrated that daily regimens but not weekly regimens were associated with the significant development of cortical porosity and endosteal naïve bone formation by marrow fibrosis. We concomitantly monitored the pharmacokinetics of the plasma teriparatide levels as well as the temporal changes in markers of bone formation and resorption. The analyses in the present study suggested that the daily repeated administration of teriparatide causes more deleterious changes in the cortical microarchitecture than the less frequent administration of higher doses. The findings of the present study may have some implications for use of teriparatide in clinical treatment.

Effects of intermittent teriparatide depend on the dose and frequency of administration. Daily dosing, particularly the higher dose, but not weekly dosing, increased cortical porosity. Work is needed to investigate the effects of the regimens on bone formation.

To investigate whether the administration frequency of parathyroid hormone (PTH) is associated with the development of cortical porosity, this study established 15 dosage regimens of teriparatide [human PTH(1–34), TPTD] with four distinct concentrations and four distinct administration frequencies of TPTD to 16-week-old ovariectomized rats. Our analyses demonstrated that the bone mineral density, mechanical properties, and bone turnover were associated with the total amount of TPTD administered. Our observations further revealed that the cortical porosity was markedly developed as a result of an increased administration frequency with a lower concentration of total TPTD administration in our setting, although the highest concentration also induced cortical porosity. Deconvolution fluorescence tiling imaging on calcein-labeled undecalcified bone sections also demonstrated the development of cortical porosity to be closely associated with the bone site where periosteal bone formation took place. This site-specific cortical porosity involved intracortical bone resorption and an increased number and proximity of osteocytic lacunae, occasionally causing fused lacunae. Taken together, these findings suggested the involvement of local distinctions in the rate of bone growth that may be related to the site-specific mechanical properties in the development of cortical porosity induced by frequent and/or high doses of TPTD.

Daily and weekly administration of teriparatide (PTH1–34) reduces the risk of osteoporotic bone fractures. However, their effects on markers of bone formation and bone resorption differ. These results indicate that the dosing frequency of teriparatide may affect bone metabolism and bone structure, with different effects on bone strength. In the present study, to evaluate the dose-related effects of a low administration frequency of teriparatide on bone status, we investigated the effects of three-times-weekly administration of teriparatide (1.1, 5.6, or 28.2 µg/kg) for 12 months on bone parameters, including bone metabolism markers, bone mineral density (BMD), micro-computed tomography, and bone strength, using 6-month-old ovariectomized (OVX) rats. Three-times-weekly administration of teriparatide dose-dependently increased the BMD of the lumbar vertebra and femur in OVX rats, and increased serum osteocalcin (a marker of bone formation), but not type I collagen C-telopeptide (a marker of bone resorption). The trabecular number and thickness increased in the vertebrae and femur, as in prior reports of daily teriparatide administration in OVX rats. Cortical thickness increased only toward the endocortical side of the femur, unlike with daily administration. Bone strength of the vertebrae and proximal and shaft of the femur was correlated with the changes in BMD and bone structure. These results demonstrate the effects of low frequency, intermittent administration of teriparatide on the biomechanical, and microstructural properties of bone in OVX rats.Electronic supplementary materialThe online version of this article (doi:10.1007/s00223-015-9998-0) contains supplementary material, which is available to authorized users.