Increased Bone Resorption during Lactation in Pycnodysostosis.

Pycnodysostosis, a rare autosomal recessive skeletal dysplasia, is caused by a deficiency of cathepsin K. Patients have impaired bone resorption in the presence of normal or increased numbers of multinucleated, but dysfunctional, osteoclasts. Cathepsin K degrades collagen type I and generates N-telopeptide (NTX) and the C-telopeptide (CTX) that can be quantified. Levels of these telopeptides are increased in lactating women and are associated with increased bone resorption. Nothing is known about the consequences of cathepsin K deficiency in lactating women. Here we present for the first time normalized blood and CTX measurements in a patient with pycnodysostosis, exclusively related to the lactation period. In vitro studies using osteoclasts derived from blood monocytes during lactation and after weaning further show consistent bone resorption before and after lactation. Increased expression of cathepsins L and S in osteoclasts derived from the lactating patient suggests that other proteinases could compensate for the lack of cathepsin K during the lactation period of pycnodysostosis patients.

Denosumab in Patients With Fibrous Dysplasia Previously Treated With Bisphosphonates.

CONTEXT: Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare bone disorder commonly treated with bisphosphonates, but clinical and biochemical responses may be incomplete. OBJECTIVE: To evaluate the efficacy and tolerability of the receptor activator of nuclear factor-κB ligand inhibitor denosumab in the treatment of patients with FD/MAS refractory to bisphosphonate therapy. DESIGN: Case series. SETTING: Academic center of expertise for rare bone diseases. PATIENTS: Data were collected from 12 consecutive patients with FD/MAS with persistent pain and increased biochemical markers of bone turnover (BTMs) after long-term treatment with bisphosphonates (median, 8.8 years) and were treated with subcutaneous denosumab 60 mg at 3- or 6-month intervals with a follow-up for at least 12 months. MAIN OUTCOME(S): Sustained reduction of BTMs and bone pain. RESULTS: A 60 mg dose of denosumab once every 3 months, but not once every 6 months, induced a sustained reduction of BTMs. After a median treatment period of 15.5 months (range, 12 to 19) serum alkaline phosphatase activity and propeptide of type 1 procollagen levels were respectively reduced from 212 ± 39.4 IU/L to 79 ± 6.0 IU/L (P = 0.004) and from 346.2 ± 111.1 ng/mL to 55.7 ± 16.6 ng/mL (P = 0.023) and normalized in 70% and 75% of patients, respectively. Although not quantitavely measured, 10 patients reported a reduction in bone pain of whom 6 reported complete elimination of pain. Treatment with denosumab was well tolerated. CONCLUSION: Our results indicate that 60 mg of denosumab every 3 months is a promising, well-tolerated treatment of most patients with FD/MAS refractory to bisphosphonate therapy. These results together with those of previously published case reports provide the necessary background for the design of a larger, controlled study.

Abaloparatide in patients with mild or moderate renal impairment: results from the ACTIVE phase 3 trial.

Objective: To evaluate, post hoc, the efficacy and safety of abaloparatide by degree of renal impairment.Methods: ACTIVE was a phase 3, 18-month, randomized, double-blind, active-comparator, placebo-controlled study of postmenopausal women with osteoporosis who received subcutaneous abaloparatide 80 µg, placebo, or open-label teriparatide 20 µg daily. Patients with serum creatinine >2.0 mg/dL or 1.5-2.0 mg/dL with an estimated glomerular filtration rate (eGFR) <37 mL/min, calculated by Cockcroft-Gault formula, were excluded.Results: At baseline, 660 patients had eGFR ≥90 mL/min, 1276 had 60 to ˂90 mL/min, and 527 had <60 mL/min. Older age and lower T-scores were associated with greater renal impairment. Among renal-function subgroups, there were no meaningful changes in bone mineral density, fracture risk reduction, or overall incidence of treatment-emergent adverse events in the active-treatment arms. Anemia, nausea, hypercalcemia, and upper-respiratory-tract infection tended to be more frequent with increasing renal impairment. Hypercalcemia measured by albumin-adjusted serum calcium occurred significantly less frequently with abaloparatide than teriparatide in patients with eGFR <60 mL/min (3.6% versus 10.9%; p = .008) and in the overall ACTIVE safety population (3.4% versus 6.4%; p = .006). Computed tomography scans in 376 patients revealed no evidence of increased renal calcification.Conclusion: Increased exposure to abaloparatide and teriparatide in patients with renal impairment led to no meaningful differences in efficacy or safety. These results support the use of abaloparatide without dosage adjustment in patients with renal impairment, provided those with severe renal impairments are monitored for adverse events.

Increased osteoclastogenesis in patients with vertebral fractures following discontinuation of denosumab treatment.

OBJECTIVE: To test the hypothesis that rebound of bone remodeling is responsible for clinical vertebral fractures reported in a few patients with osteoporosis after cessation of denosumab treatment. DESIGN: In this case-control study we compared clinical and biochemical characteristics of postmenopausal women with clinical vertebral fractures 8-16 months after the last injection of denosumab (Dmab/Fx+, n = 5) with those of treatment-naïve women with such fractures (Fx+, n = 5). In addition, 5 women who discontinued denosumab treatment but did not sustain vertebral fractures 18-20 months after the last injection were studied (Dmab/Fx-, n = 5). METHODS: We measured serum microRNAs, gene expression of mRNAs of factors regulating formation and activity of osteoclasts and biochemical markers of bone and mineral metabolism. In Dmab/Fx+ and Fx+ women, blood was taken 4-8 weeks after the fracture. RESULTS: Compared to Fx+ women, Dmab/Fx+ women had higher serum P1NP and CTx levels, and significantly lower serum miR-503 and miR-222-2 that downregulate osteoclastogenesis and osteoclast activity, and higher RANK (13-fold) and CTSK (2.6-fold) mRNA. The respective values of Dmab/Fx- women were in the same direction as those of Dmab/Fx+ women but of a lesser magnitude. CONCLUSIONS: Bone fragility in women with clinical vertebral fractures after stopping denosumab therapy is pathophysiologically different from that of treatment-naïve women with osteoporosis and clinical vertebral fractures and it is associated with upregulation of markers of osteoclast formation and activity. The small number of women with this rare event studied is a limitation.

Sclerostin deficiency in humans.

Sclerosteosis and van Buchem disease are two rare bone sclerosing dysplasias caused by genetic defects in the synthesis of sclerostin. In this article we review the demographic, clinical, biochemical, radiological, and histological characteristics of patients with sclerosteosis and van Buchem disease that led to a better understanding of the role of sclerostin in bone metabolism in humans and we discuss the relevance of these findings for the development of new therapeutics for the treatment of patients with osteoporosis.

From disease to treatment: from rare skeletal disorders to treatments for osteoporosis.

During the past 15 years there has been an expansion of our knowledge of the cellular and molecular mechanisms regulating bone remodeling that identified new signaling pathways fundamental for bone renewal as well as previously unknown interactions between bone cells. Central for these developments have been studies of rare bone disorders. These findings, in turn, have led to new treatment paradigms for osteoporosis some of which are at late stages of clinical development. In this article, we review three rare skeletal disorders with case descriptions, pycnodysostosis and the craniotubular hyperostoses sclerosteosis and van Buchem disease that led to the development of cathepsin K and sclerostin inhibitors, respectively, for the treatment of osteoporosis.

Novel approaches to the treatment of osteoporosis.

Despite the availability of efficacious treatments for fracture reduction in patients with osteoporosis, there are still unmet needs requiring a broader range of therapeutics. In particular, agents that are capable of replacing already lost bone and that also drastically reduce the risk of non-vertebral fractures are needed. Studies of rare bone diseases in humans and animal genetics have identified targets in bone cells for the development of therapies for osteoporosis with novel mechanisms of action. Here, we review these new developments, with emphasis on inhibitors of cathepsin K in osteoclasts and sclerostin in osteocytes, which are currently studied in phase 3 clinical trials.

Van Buchem disease: clinical, biochemical, and densitometric features of patients and disease carriers.

Van Buchem disease (VBD) is a rare bone sclerosing dysplasia caused by the lack of a regulatory element of the SOST gene, which encodes for sclerostin, an osteocyte-derived negative regulator of bone formation. We studied the demographic, clinical, biochemical, and densitometric features of 15 patients with VBD (12 adults and 3 children) and 28 related carriers of the gene mutation. The most common clinical findings in patients were facial palsy (100%) and various degrees of hearing impairment (93%); raised intracranial pressure had been documented in 20%. The clinical course of the disease appeared to stabilize in adulthood, with the majority of patients reporting no progression of symptoms or development of complications with time. Carriers of the disease had none of the clinical features or complications of the disease. Sclerostin could be detected in the serum in all but 1 VBD patients (mean 8.0 pg/mL; 95% confidence interval [CI], 4.9-11.0 pg/mL), and were lower than those of carriers (mean 28.7 pg/mL; 95% CI, 24.5-32.9 pg/mL; p < 0.001) and healthy controls (mean 40.0 pg/mL; 95% CI, 34.5-41.0 pg/mL; p < 0.). Serum procollagen type 1 amino-terminal propeptide (P1NP) levels were also significantly higher in adult patients (mean 96.0; 95% CI, 54.6-137.4 ng/mL versus mean 47.8; 95% CI, 39.4-56.2 ng/mL, p = 0.003 in carriers and mean 37.8; 95% CI, 34.5-41.0 ng/mL, p = 0.028 in healthy controls) and declined with age. Bone mineral density (BMD) was markedly increased in all patients (mean Z-score 8.7 ± 2.1 and 9.5 ± 1.9 at the femoral neck and spine, respectively); BMD of carriers was significantly lower than that of patients but varied widely (mean Z-scores 0.9 ± 1.0 and 1.3 ± 1.5 at the femoral neck and spine, respectively). Serum sclerostin levels were inversely correlated with serum P1NP levels (r = -0.39, p = 0.018) and BMD values (femoral neck r = -0.69, p < 0.001; lumbar spine r = -0.78, p < 0.001). Our results show that there is a gene-dose effect of the VBD deletion on circulating sclerostin and provide further in vivo evidence of the role of sclerostin in bone formation in humans. The small amounts of sclerostin produced by patients with VBD may explain their milder phenotype compared to that of patients with sclerosteosis, in whom serum sclerostin is undetectable.

Serum sclerostin levels in Paget's disease and prostate cancer with bone metastases with a wide range of bone turnover.

Evidence has been accumulating for the role of osteocytes as key players in the regulation of bone remodeling. One of the main products of these cells, sclerostin, inhibits bone formation and may also stimulate bone resorption. Circulating sclerostin has been evaluated in humans, but data are scarce in patients with different rates of bone turnover. To address this issue we evaluated serum sclerostin levels in patients with Paget's disease of bone (PD) and in patients with prostate cancer metastatic to the skeleton (PC). Sclerostin levels were measured in 88 patients with PD, 20 patients with PC and 237 healthy individuals (113 men and 124 women, aged 20 to 77 years). Bone turnover was evaluated by measuring serum levels of procollagen type 1 amino-terminal propeptide (P1NP) in all individuals studied and ß-carboxy-terminal cross-linking telopeptide of type I collagen (ß-CTX) only in patients. Patients were aged between 45 and 88 years and had a wide range of bone turnover: serum P1NP 9.2 to 1872 ng/ml and ß-CTX 50 to 3120 pg/ml. Patients with PD and with PC had significantly higher mean serum sclerostin levels (53.1 ± 22.7 pg/ml and 56.6 ± 25.8 pg/ml, respectively) compared to healthy controls (38.1 ± 12.1 pg/ml) (p<0.001). Serum sclerostin levels were significantly correlated with P1NP in all (n=345) studied subjects (r=0.32, p<0.001). Circulating sclerostin levels are significantly increased in patients with increased bone turnover, regardless of underlying pathology. These increased levels may be due to a compensatory response to the increased number of osteoblasts at affected skeletal sites and may contribute to the increased bone resorption in patients with PC .

Acute phase response following intravenous zoledronate in postmenopausal women with low bone mass.

An acute phase response (APR) is frequently observed in patients treated with intravenous (i.v.) zoledronate (ZOL). We aimed to define clinical and laboratory parameters that may predict ZOL-induced APR in women with low bone mass. Fifty-one postmenopausal women with low bone mass were given a single i.v. infusion of ZOL 5mg. APR was clinically defined by the visual analog pain scale (VAS) for the musculoskeletal symptoms and body temperature. White blood cell count (WBC), leucocyte subpopulations, C-reactive protein (CRP), parathyroid hormone (PTH), 25-hydroxyvitamin D [25(OH)D], interleukins (IL)-1b and -6, tumor necrosis factor (TNF)α and interferon (IFN)γ were measured before and 48 h following the infusion. Subsequently, patients were divided into those experiencing APR (APR+) or not (APR-). WBC, granulocytes, CRP, IL-1b and IL-6 were significantly increased, whereas lymphocytes, eosinophils, calcium, phosphate and 25(OH)D decreased 48h after ZOL infusion. Twenty-eight of the 51 patients (54.9%) experienced an APR. APR+ patients were younger and had higher baseline lymphocytes compared to APR- patients. There was no difference (p=0.405) in the development of APR between treatment-naive patients (19/32, 59.4%) and patients previously treated with another oral nitrogen-containing bisphosphonate (9/19, 47.4%). In conclusion, our data suggest that pre-treatment higher lymphocyte number increases the risk of APR while previous treatment with another nitrogen-containing bisphosphonate does not significantly reduce the risk. Serum 25(OH)D concentrations decrease significantly after the infusion, possibly as part of the inflammatory response to ZOL.

Patients with sclerosteosis and disease carriers: human models of the effect of sclerostin on bone turnover.

Sclerosteosis is a rare bone sclerosing dysplasia, caused by loss-of-function mutations in the SOST gene, encoding sclerostin, a negative regulator of bone formation. The purpose of this study was to determine how the lack of sclerostin affects bone turnover in patients with sclerosteosis and to assess whether sclerostin synthesis is decreased in carriers of the SOST mutation and, if so, to what extent this would affect their phenotype and bone formation. We measured sclerostin, procollagen type 1 amino-terminal propeptide (P1NP), and cross-linked C-telopeptide (CTX) in serum of 19 patients with sclerosteosis, 26 heterozygous carriers of the C69T SOST mutation, and 77 healthy controls. Chips of compact bone discarded during routine surgery were also examined from 6 patients and 4 controls. Sclerostin was undetectable in serum of patients but was measurable in all carriers (mean 15.5 pg/mL; 95% confidence interval [CI] 13.7 to 17.2 pg/mL), in whom it was significantly lower than in healthy controls (mean 40.0 pg/mL; 95% CI 36.9 to 42.7 pg/mL; p < 0.001). P1NP levels were highest in patients (mean 153.7 ng/mL; 95% CI 100.5 to 206.9 ng/mL; p = 0.01 versus carriers, p = 0.002 versus controls), but carriers also had significantly higher P1NP levels (mean 58.3 ng/mL; 95% CI 47.0 to 69.6 ng/mL) than controls (mean 37.8 ng/mL; 95% CI 34.9 to 42.0 ng/mL; p = 0.006). In patients and carriers, P1NP levels declined with age, reaching a plateau after the age of 20 years. Serum sclerostin and P1NP were negatively correlated in carriers and age- and gender-matched controls (r = 0.40, p = 0.008). Mean CTX levels were well within the normal range and did not differ between patients and disease carriers after adjusting for age (p = 0.22). Our results provide in vivo evidence of increased bone formation caused by the absence or decreased synthesis of sclerostin in humans. They also suggest that inhibition of sclerostin can be titrated because the decreased sclerostin levels in disease carriers did not lead to any of the symptoms or complications of the disease but had a positive effect on bone mass. Further studies are needed to clarify the role of sclerostin on bone resorption.

Medical treatment of hypercalcaemia.

Hypercalcaemia results from the failure of renal calcium excretion to compensate increased influx of calcium into the circulation from the intestine, the kidneys and the skeleton. Hypercalcaemia is a common metabolic abnormality of varying severity that can be adequately diagnosed and treated. Primary hyperparathyroidism and malignant neoplasms are responsible for >90% of all cases. The management of hypercalcaemia depends on the underlying cause and involves approaches aiming at reducing serum calcium concentrations and correcting associated metabolic disturbances. A number of pharmacological interventions are currently available for the treatment of hypercalcaemia related to malignancy as well as other forms of hypercalcaemia. Knowledge of the pathophysiology of hypercalcaemia and of the properties of these interventions is essential for the successful management of affected individuals. We herein review available therapeutic interventions for hypercalcaemia of varying aetiology.

Bilateral fractures of the femur diaphysis in a patient with rheumatoid arthritis on long-term treatment with alendronate: clues to the mechanism of increased bone fragility.

Unusual fractures of the femur diaphysis have been reported in patients treated with alendronate and, although no causal relationship has been established, excessive suppression of bone turnover and length of treatment with alendronate have been implicated in their pathogenesis. We report here clinical, biochemical, and radiological findings of a patient with rheumatoid arthritis and multiple risk factors for fractures who was treated with alendronate for 8 yr and developed spontaneous bilateral subtrochanteric/diaphyseal fractures. Bone biopsies obtained form the iliac crest and the femur showed decreased bone formation with histomorphometric evidence of markedly increased bone resorption at the femur. These results show for the first time that an imbalance between bone resorption and bone formation at the affected bone is associated with the occurrence of these atypical femur fractures. The cause of this imbalance is currently unknown, and further studies of the epidemiology and pathogenesis of diaphyseal femur fractures are warranted.

Inhibition of bone resorption and growth of breast cancer in the bone microenvironment.

Breast cancer frequently metastasizes to bone, where tumor cells induce osteoclasts to locally destroy bone. During bone resorption, growth factors are locally released that may support bone metastatic growth. Differently from most other tissues, drugs that can limit local turnover, such as bisphosphonates and osteoprotegerin (OPG), are available for bone. We examined the hypothesis that inhibition of bone resorption by two different mechanisms may also affect the growth of cancer cells in bone. For this, we tested the effects of high doses of OPG and zoledronic acid (ZOL) on progression of MDA-231-B/Luc+ breast cancer cells in the bone microenvironment using whole body bioluminescent reporter imaging (BLI). Both treatments significantly inhibited the development of radiographically detectable osteolytic lesions. Histologic examination corroborated the radiographic findings, showing that both treatments preserved the integrity of bone trabeculae and prevented bone destruction (significantly higher trabecular bone volumes vs. vehicle). However, whereas practically no TRAcP-positive osteoclasts were observed in tibiae preparations of animals treated with Fc-OPG, TRAcP-positive osteoclasts were still present in the animals treated with ZOL. Intra-bone tumor burden was reduced with ZOL and Fc-OPG treatment. Although there appeared to be a trend for less overall total tumor burden upon treatment with both compounds, this was not significant as assessed by BLI and histomorphometric analysis due to the extramedullary growth of cancer cells which was not affected by these treatments. Collectively, anti-resorptive agents with different mechanisms of action - ZOL and OPG - significantly reduced cancer-induced osteolysis and intra-osseous tumor burden, but failed to restrain local tumor growth. However, interference with the bone micro-environmental growth support could still be of therapeutic relevance when given to patients early in the course of bone metastatic disease.

Bone morphogenetic protein 7 in the development and treatment of bone metastases from breast cancer.

Bone morphogenetic protein 7 (BMP7) counteracts the physiological epithelial-to-mesenchymal transition (EMT), a process that is indicative of epithelial plasticity. Because EMT is involved in cancer, we investigated whether BMP7 plays a role in breast cancer growth and metastasis. In this study, we show that decreased BMP7 expression in primary breast cancer is significantly associated with the formation of clinically overt bone metastases in patients with > or = 10 years of follow-up. In line with these clinical observations, BMP7 expression is inversely related to tumorigenicity and invasive behavior of human breast cancer cell lines. Moreover, BMP7 decreased the expression of vimentin, a mesenchymal marker associated with invasiveness and poor prognosis, in human MDA-MB-231 (MDA-231)-B/Luc(+) breast cancer cells under basal and transforming growth factor-beta (TGF-beta)-stimulated conditions. In addition, exogenous addition of BMP7 to TGF-beta-stimulated MDA-231 cells inhibited Smad-mediated TGF-beta signaling. Furthermore, in a well-established bone metastasis model using whole-body bioluminescent reporter imaging, stable overexpression of BMP7 in MDA-231 cells inhibited de novo formation and progression of osteolytic bone metastases and, hence, their metastatic capability. In line with these observations, daily i.v. administration of BMP7 (100 mug/kg/d) significantly inhibited orthotopic and intrabone growth of MDA-231-B/Luc(+) cells in nude mice. Our data suggest that decreased BMP7 expression during carcinogenesis in the human breast contributes to the acquisition of a bone metastatic phenotype. Because exogenous BMP7 can still counteract the breast cancer growth at the primary site and in bone, BMP7 may represent a novel therapeutic molecule for repression of local and bone metastatic growth of breast cancer.

BMP7, a putative regulator of epithelial homeostasis in the human prostate, is a potent inhibitor of prostate cancer bone metastasis in vivo.

Bone morphogenic protein 7 (BMP7) counteracts physiological epithelial-to-mesenchymal transition, a process that is indicative of epithelial plasticity. Because epithelial-to-mesenchymal transition is involved in cancer, we investigated whether BMP7 plays a role in prostate cancer growth and metastasis. BMP7 expression in laser-microdissected primary human prostate cancer tissue was strongly down-regulated compared with normal prostate luminal epithelium. Furthermore, BMP7 expression in prostate cancer cell lines was inversely related to tumorigenic and metastatic potential in vivo and significantly correlated to E-cadherin/vimentin ratios. Exogenous addition of BMP7 to human prostate cancer cells dose-dependently inhibited transforming growth factor beta-induced activation of nuclear Smad3/4 complexes via ALK5 and induced E-cadherin expression. Moreover, BMP7-induced activation of nuclear Smad1/4/5 signaling transduced via BMP type I receptors was synergistically stimulated in the presence of transforming growth factor beta, a growth factor that is enriched in the bone microenvironment. Daily BMP7 administration to nude mice inhibited the growth of cancer cells in bone. In contrast, no significant growth inhibitory effect of BMP7 was observed in intraprostatic xenografts. Collectively, our observations suggest that BMP7 controls and preserves the epithelial phenotype in the human prostate and underscore a decisive role of the tumor microenvironment in mediating the therapeutic response of BMP7. Thus, BMP7 can still counteract the epithelial-to-mesenchymal transition process in the metastatic tumor, positioning BMP7 as a novel therapeutic molecule for treatment of metastatic bone disease.

Interference with the microenvironmental support impairs the de novo formation of bone metastases in vivo.

Interference with the microenvironmental growth support is an attractive therapeutic strategy for repressing metastatic tumor growth. Bone is a highly dynamic tissue that is continuously remodeled by bone resorption and subsequent bone formation. Growth factors supporting bone metastatic growth are released especially during bone resorption. Differently from most other tissues, drugs that can limit local turnover, such as bisphosphonates, are available for bone. In the present study, we tested the hypothesis that inhibition of bone turnover can affect development and growth progression of experimental bone metastasis. Whole-body bioluminescent reporter imaging was used for the detection, monitoring, and quantification in vivo of the growth progression of bone metastases induced by intracardiac or intraosseous injection of luciferase-transfected breast cancer cells (MDA-231-B/luc+) to nude mice. Suppression of bone turnover by bisphosphonates, before bone colonization by cancer cells, inhibited by a great extent the number of developing bone metastasis. Tumor growth in the few, but still developing, bone metastases was affected only transiently. Reduction of bone turnover had no effect on growth progression of bone metastases, which were already established when bisphosphonate treatment was initiated, despite a substantial reduction in osteolysis. Therefore, cancer cells metastatic to bone, after an initial growth phase that depends on the interaction with the local stroma, become independent of microenvironmental growth factor support and progress autonomously. Inhibition of bone turnover may represent a useful adjuvant therapy especially for cancer patients at risk to develop bone metastasis.

Sclerostin is a delayed secreted product of osteocytes that inhibits bone formation.

Osteocytes are the most abundant cells in bone and are ideally located to influence bone turnover through their syncytial relationship with surface bone cells. Osteocyte-derived signals have remained largely enigmatic, but it was recently reported that human osteocytes secrete sclerostin, an inhibitor of bone formation. Absent sclerostin protein results in the high bone mass clinical disorder sclerosteosis. Here we report that within adult iliac bone, newly embedded osteocytes were negative for sclerostin staining but became positive at or after primary mineralization. The majority of mature osteocytes in mineralized cortical and cancellous bone was positive for sclerostin with diffuse staining along dendrites in the osteocyte canaliculi. These findings provide for the first time in vivo evidence to support the concept that osteocytes secrete sclerostin after they become embedded in a mineralized matrix to limit further bone formation by osteoblasts. Sclerostin did not appear to influence the formation of osteocytes. We propose that sclerostin production by osteocytes may regulate the linear extent of formation and the induction or maintenance of a lining cell phenotype on bone surfaces. In doing so, sclerostin may act as a key inhibitory signal governing skeletal microarchitecture.

Skeletal retention of bisphosphonate (pamidronate) and its relation to the rate of bone resorption in patients with breast cancer and bone metastases.

UNLABELLED: Bisphosphonate pharmacokinetics may affect individual responses. Skeletal retention of pamidronate infused monthly to patients with bone metastases was highly variable (12-98%) and did not diminish with time, showing the capacity of the skeleton to retain large amounts of bisphosphonate. Relationships between skeletal retention of pamidronate and rate of bone resorption are complex and depend on previous treatment and the total amount of retained bisphosphonate. INTRODUCTION: Bisphosphonates (BPs) given intravenously every 3-4 weeks are effective in the management of metastatic bone disease from breast cancer, but responses among patients vary, and it is not known whether current dose and dose intervals are appropriate for an individual patient. An influence of pharmacokinetics of BPs on antiresorptive action may contribute to this variation in response. To test this hypothesis, we determined the skeletal retention of intravenous pamidronate and its association to the rate of bone resorption in patients with bone metastases from breast cancer. MATERIALS AND METHODS: In a cross-sectional study, 24-h urinary excretion of pamidronate and the biochemical marker of bone resorption N-terminal telopeptide of type 1 collagen and serum alkaline phosphatase were measured in 40 patients with bone metastases from breast cancer at the beginning, after 3-6 months, and after 1 year of treatment with intravenous pamidronate 90 mg every 3-4 weeks. RESULTS AND CONCLUSIONS: Skeletal retention (dose--amount excreted into urine) 2 days after infusion varied between 12% and 98% (mean, 62%) of the administered dose, but there were no differences in retention between patients receiving pamidronate for the first time or after 3-6 months or after 1 year of treatment. Retention of pamidronate was related to the prevalent rate of bone turnover in previously untreated patients, whereas no such relationship was found in previously treated patients. Rate of bone resorption after treatment seemed to be related to the amount of pamidronate retained. During 1 year of treatment, retention of pamidronate remained constant, indicating no saturation of skeletal binding sites with treatment. The variability in retention among individual patients can be attributed to the number of available binding sites. This is related, however, to bone turnover only before the start of treatment. The apparent relationships between skeletal retention and antiresorptive effect could have implications for the design of optimal therapeutic regimens with BPs in patients with bone metastases from breast cancer.