Imatinib as a potential antiresorptive therapy for bone disease

Case Report of Renal Calculi in a Child Receiving Imatinib for Acute Lymphoblastic Leukemia

Rationale

Imatinib is used in the treatment of Philadelphia chromosome positive (Ph+) leukemias and has been reported to have a direct effect on bone physiology.

Presentation

To report on a child with Ph+ acute lymphoblastic leukemia who presented with bilateral flank pain and gross hematuria.

Diagnosis

She was diagnosed with obstructive kidney stones 101 days after commencing daily oral imatinib. Stone analysis revealed the presence of calcium phosphate.

Interventions and outcome

The patient passed the stones spontaneously with medical therapy that included the use of thiazide, allopurinol, and potassium citrate, but she required temporary insertion of a double-J stent to relieve an obstruction.

Novel findings

Imatinib inhibits receptor tyrosine kinases and stimulates the flux of calcium from the extracellular fluid into bone, resulting in hypocalcemia with a compensatory rise in parathyroid hormone that may result in phosphaturia and the formation of calcium phosphate stones. Given that kidney stones are rare events in children, we believe that monitoring for kidney stone formation needs to be performed in children receiving imatinib.

Response of Central Giant Cell Granuloma of the Jaw to Imatinib

Central giant cell granuloma of the jaw (CGCJ) can be locally aggressive and result in facial and dental deformity. A child with CGCJ was treated surgically and with denosumab with a response but life-threatening toxicity. Imatinib, a tyrosine kinase inhibitor, was prescribed based on clinical similarities between CGCJ and cherubism, for which Imatinib has been effective. Within 2 months, a computed tomographic scan showed significant ossification, which increased over the following 8 months. This case suggests that tyrosine kinase inhibitors may be an effective option, and one with limited toxicity, for CGCJ.

Bone Metabolism Effects of Medical Therapy in Advanced Renal Cell Carcinoma

The medical therapy of advanced renal cell carcinoma (RCC) is based on the use of targeted therapies, such as tyrosine kinase inhibitors (TKI) and immune-checkpoint inhibitors (ICI). These therapies are characterized by multiple endocrine adverse events, but the effect on the bone is still less known. Relatively few case reports or small case series have been specifically focused on TKI and ICI effects on bone metabolism. However, the importance to consider these possible side effects is easily intuitable because the bone is one of the most frequent metastatic sites of RCC. Among TKI used in RCC, sunitinib and sorafenib can cause hypophosphatemia with increased PTH levels and low-normal serum calcium levels. Considering ICI, nivolumab and ipilimumab, which can be used in association in a combination strategy, are associated with an increased risk of hypocalcemia, mediated by an autoimmune mechanism targeted on the calcium-sensing receptor. A fearsome complication, reported for TKI and rarely for ICI, is osteonecrosis of the jaw. Awareness of these possible side effects makes a clinical evaluation of RCC patients on anticancer therapy mandatory, especially if associated with antiresorptive therapy such as bisphosphonates and denosumab, which can further increase the risk of these complications.

The influence of M-CSF on fracture healing in a mouse model

Macrophage colony-stimulating factor 1 (M-CSF) is known to play a critical role during fracture repair e.g. by recruiting stem cells to the fracture site and impacting hard callus formation by stimulating osteoclastogenesis. The aim of this experiment was to study the impact of systemic M-CSF application and its effect on bony healing in a mouse model of femoral osteotomy. Doing so, we studied 61 wild type (wt) mice (18-week-old female C57BL/6) which were divided into three groups: (1) femoral osteotomy, (2) femoral osteotomy + stabilization with external fixator and (3) femoral osteotomy + stabilization with external fixator + systemic M-CSF application. Further, 12 op/op mice underwent femoral osteotomy and served as proof of concept. After being sacrificed at 28 days bony bridging was evaluated ex vivo with µCT, histological and biomechanical testing. Systemic M-CSF application impacted osteoclasts numbers, which were almost as low as found in op/op mice. Regarding callus size, the application of M-CSF in wt mice resulted in significantly larger calluses compared to wt mice without systemic M-CSF treatment. We further observed an anabolic effect of M-CSF application resulting in increased trabecular thickness compared to wt animals without additional M-CSF application. Systemic M-CSF application did not alter biomechanical properties in WT mice. The impact of M-CSF application in a mouse model of femoral osteotomy was oppositional to what we were expecting. While M-CSF application had a distinct anabolic effect on callus size as well as trabecular thickness, this on bottom line did not improve biomechanical properties. We hypothesize that in addition to the well-recognized negative effects of M-CSF on osteoclast numbers this seems to further downstream cause a lack of feedback on osteoblasts. Ultimately, continuous M-CSF application in the absence of co-stimulatory signals (e.g. RANKL) might overstimulate the hematopoietic linage in favor of tissue macrophages instead of osteoclasts.

Metabolic properties of the osteoclast

Osteoclasts are defined as cells capable of excavating 3-dimensional resorption pits in bone and other mineralised tissues. They are derived from the differentiation/fusion of promonocytic precursors, and are usually large, multinucleated cells. In common with other cells from this myeloid lineage such as macrophages and dendritic cells, they are adapted to function in hypoxic, acidic environments. The process of bone resorption is rapid and is presumably highly energy-intensive, since osteoclasts must actively extrude protons to dissolve hydroxyapatite mineral, whilst secreting cathepsin K to degrade collagen, as well as maintaining a high degree of motility. Osteoclasts are well known to contain abundant mitochondria but they are also able to rely on glycolytic (anaerobic) metabolism to generate the ATP needed to power their activity. Their primary extracellular energy source appears to be glucose. Excessive accumulation of mitochondrial reactive oxygen species in osteoclasts during extended periods of high activity in oxygen-poor environments may promote apoptosis and help to limit bone resorption - a trajectory that could be termed "live fast, die young". In general, however, the metabolism of osteoclasts remains a poorly-investigated area, not least because of the technical challenges of studying actively resorbing cells in appropriate conditions.

Development of Orally Applicable, Combinatorial Drug-Loaded Nanoparticles for the Treatment of Fibrosarcoma

Nanoparticulate systems have been receiving a significant attention especially for the treatment of cancer but one of the main hurdles is to produce these developed and high-tech nanosystems in large quantities. Anticancer drug formulations are generally designed for parenteral administrations but oral administration is still the most convenient route. In this study, orally applicable nano-sized chitosan nanoparticles (NPs) were successfully prepared using Nano Spray Dryer. It is possible to produce these NPs in large quantities by simply increasing the processing time using the machine without changing any parameter. A chemotherapeutic agent (imatinib mesylate; IMA) and nonsteroidal anti-inflammatory drug (dexketoprofen trometamol) were loaded together in these NPs. NPs were also functionalized with polyethylene glycol and folic acid to obtain long circulating NPs and tumor targeting. The antitumoral activities of formulations showed that these developed NPs can enhance the effectiveness. Animal experiments were performed on fibrosarcoma-bearing mice model, and the treatment with 0.8 mg/μL/kg IMA-loaded chitosan NPs was found to be successful to slow down the growth of tumors. The tumor tissues were removed from the animals and enzymatic activities were evaluated. The inhibitory effect of tyrosine kinase was found to be enhanced from 36.4% to 68.4% when IMA was used in combination with dexketoprofen trometamol. Furthermore, all dried NPs were found to be stable for more than a year at 25°C. Presented results show that these developed combinatorial drug-loaded NPs can be used for the treatment of fibrosarcoma, and these data can provide an insight, new strategies for productions or alternatives in cancer treatment.

Multiparameter Analysis of Off-Target Effects of Dasatinib on Bone Homeostasis in Patients With Newly Diagnosed Chronic Myelogenous Leukemia

BACKGROUND

We assessed patients with chronic myelogenous leukemia (CML) for serum calcium (Ca), phosphate (PO4), bone alkaline phosphatase, N-telopeptide (NTx), osteoprotegerin (OPG) levels, and trabecular bone area (TBA) in bone marrow (BM) specimens before and after treatment with dasatinib. We identified a significant increase in percentage of TBA in postdasatinib BM (P = .022). This suggests that dasatinib therapy can increase TBA without significant changes in bone and mineral metabolism. Interferences with bone homeostasis and mineral metabolism have been described in patients receiving imatinib for CML or gastrointestinal stromal tumors. Dasatinib is a potent second-generation tyrosine kinase inhibitor designed to inhibit ABL and SRC kinases while also interfering with the c-Kit, platelet-derived growth factor receptor, and STAT5 pathways.

PATIENTS AND METHODS

We used a multiparameter approach to examine the off-target effects of dasatinib in 30 patients with CML treated between 2009 and 2012. We recorded serum Ca and PO4 levels, analyzed markers of bone formation (bone alkaline phosphatase/bone-specific alkaline phosphatase [BAP]) and bone resorption (NTx), measured OPG levels, and digitally analyzed changes in TBA in paired BM biopsy specimens before and after treatment. We correlated all findings with each other and with the results of conventional cytogenetic and molecular analyses.

RESULTS

We identified a significant increase in the percentage of TBA in postdasatinib BM biopsy specimens (P = .022) and noted a decrease in serum OPG levels in 75% of patients. Ca, PO4, BAP, and NTx levels remained steady, without significant changes. There was no correlation between biomarker levels, percentage of TBA, and/or cytogenetic or molecular response.

CONCLUSION

These findings suggest that dasatinib therapy (within the therapeutic range) can increase trabecular bone, without causing significant changes in bone and mineral metabolism. Nonetheless, monitoring of bone health and skeletal integrity should be included into the long-term management of patients treated with dasatinib to further enhance our understanding of its safety profile and its potential role as a treatment modality for other bone diseases.

Osteomacs and Bone Regeneration

PURPOSE OF REVIEW

Mounting evidence supporting the critical contribution of macrophages, in particular osteal macrophages, to bone regeneration is reviewed. We specifically examine the potential role of macrophages in the basic multicellular units coordinating lifelong bone regeneration via remodelling and bone regeneration in response to injury. We review and discuss the distinctions between macrophage and osteoclast contributions to bone homeostasis, particularly the dichotomous role of the colony-stimulating factor 1-colony-stimulating factor 1 receptor axis.

RECENT FINDINGS

The impact of inflammation associated with aging and other hallmarks of aging, including senescence, on macrophage function is addressed in the context of osteoporosis and delayed fracture repair. Resident macrophages versus recruited macrophage contributions to fracture healing are also discussed. We identify some of the remaining knowledge gaps that will need to be closed in order to maximise benefits from therapeutically modulating or mimicking the function of macrophages to improve bone health and regeneration over a lifetime.

Tyrosine Kinase Inhibitors Regulate OPG through Inhibition of PDGFRβ

Nilotinib and imatinib are tyrosine kinase inhibitors (TKIs) used in the treatment of chronic myeloid leukemia (CML) and gastrointestinal stromal tumors (GIST). In vitro, imatinib and nilotinib inhibit osteoclastogenesis, and in patients they reduce levels of bone resorption. One of the mechanisms that might underlie these effects is an increase in the production of osteoprotegerin (OPG). In the current work we report that platelet-derived growth factor receptor beta (PDGFRβ) signaling regulates OPG production in vitro. In addition, we have shown that TKIs have effects on RANKL signaling through inhibition of the PDGFRβ and other target receptors. These findings have implications for our understanding of the mechanisms by which TKIs affect osteoclastogenesis, and the role of PDGFRβ signaling in regulating osteoclastogenesis. Further studies are indicated to confirm the clinical effects of PDGFRβ-inhibitors and to elaborate the intracellular pathways that underpin these effects.

Cellular Hypoxia Promotes Heterotopic Ossification by Amplifying BMP Signaling

Hypoxia and inflammation are implicated in the episodic induction of heterotopic endochondral ossification (HEO); however, the molecular mechanisms are unknown. HIF-1α integrates the cellular response to both hypoxia and inflammation and is a prime candidate for regulating HEO. We investigated the role of hypoxia and HIF-1α in fibrodysplasia ossificans progressiva (FOP), the most catastrophic form of HEO in humans. We found that HIF-1α increases the intensity and duration of canonical bone morphogenetic protein (BMP) signaling through Rabaptin 5 (RABEP1)-mediated retention of Activin A receptor, type I (ACVR1), a BMP receptor, in the endosomal compartment of hypoxic connective tissue progenitor cells from patients with FOP. We further show that early inflammatory FOP lesions in humans and in a mouse model are markedly hypoxic, and inhibition of HIF-1α by genetic or pharmacologic means restores canonical BMP signaling to normoxic levels in human FOP cells and profoundly reduces HEO in a constitutively active Acvr1(Q207D/+) mouse model of FOP. Thus, an inflammation and cellular oxygen-sensing mechanism that modulates intracellular retention of a mutant BMP receptor determines, in part, its pathologic activity in FOP. Our study provides critical insight into a previously unrecognized role of HIF-1α in the hypoxic amplification of BMP signaling and in the episodic induction of HEO in FOP and further identifies HIF-1α as a therapeutic target for FOP and perhaps nongenetic forms of HEO. © 2016 American Society for Bone and Mineral Research.

Disruption of c-Kit Signaling in Kit(W-sh/W-sh) Growing Mice Increases Bone Turnover

c-Kit tyrosine kinase receptor has been identified as a regulator of bone homeostasis. The c-Kit loss-of-function mutations in WBB6F1/J-Kit^W/W-v mice result in low bone mass. However, these mice are sterile and it is unclear whether the observed skeletal phenotype is secondary to a sex hormone deficiency. In contrast, C57BL/6J-Kit^W-sh/^W-sh (W^sh/W^sh) mice, which carry an inversion mutation affecting the transcriptional regulatory elements of the c-Kit gene, are fertile. Here, we showed that W^sh/W^sh mice exhibited osteopenia with elevated bone resorption and bone formation at 6- and 9-week-old. The c-Kit W^sh mutation increased osteoclast differentiation, the number of committed osteoprogenitors, alkaline phosphatase activity and mineralization. c-Kit was expressed in both osteoclasts and osteoblasts, and c-Kit expression was decreased in W^sh/W^shosteoclasts, but not osteoblasts, suggesting an indirect effect of c-Kit on bone formation. Furthermore, the osteoclast-derived coupling factor Wnt10b mRNA was increased in W^sh/W^sh osteoclasts. Conditioned medium from W^sh/W^sh osteoclasts had elevated Wnt10b protein levels and induced increased alkaline phosphatase activity and mineralization in osteoblast cultures. Antagonizing Wnt10b signaling with DKK1 or Wnt10b antibody inhibited these effects. Our data suggest that c-Kit negatively regulates bone turnover, and disrupted c-Kit signaling couples increased bone resorption with bone formation through osteoclast-derived Wnt 10 b.

Management of central giant cell granuloma of the jaws with intralesional steroid injection and review of the literature

PURPOSE

We report the results of the intralesional steroid injections for the management of central giant cell granuloma (CGCG) of the jaws.

METHODS

Seven CGCGs were treated with intralesional injection of corticosteroids. To accomplish this, 3.5 mL of triamcinolone and 3.5 mL of 0.5 % marcaine with 1/200,000 epinephrine (total 7 mL) were mixed. An adequate amount of steroid was injected into different areas of the lesion. This procedure was repeated on a weekly basis for 6 weeks.

RESULTS

Clinical and radiological examination showed complete resolution and ossification of the lesions in four patients. Partial recovery was achieved in two patients. One patient did not respond to the treatment and underwent surgical curettage.

CONCLUSIONS

We suggest that intralesional steroid injection is safe and effective for the treatment of CGCG, especially in non-aggressive lesions.

Impact of germline and somatic missense variations on drug binding sites

Advancements in next-generation sequencing (NGS) technologies are generating a vast amount of data. This exacerbates the current challenge of translating NGS data into actionable clinical interpretations. We have comprehensively combined germline and somatic nonsynonymous single-nucleotide variations (nsSNVs) that affect drug binding sites in order to investigate their prevalence. The integrated data thus generated in conjunction with exome or whole-genome sequencing can be used to identify patients who may not respond to a specific drug because of alterations in drug binding efficacy due to nsSNVs in the target protein's gene. To identify the nsSNVs that may affect drug binding, protein–drug complex structures were retrieved from Protein Data Bank (PDB) followed by identification of amino acids in the protein–drug binding sites using an occluded surface method. Then, the germline and somatic mutations were mapped to these amino acids to identify which of these alter protein–drug binding sites. Using this method we identified 12 993 amino acid–drug binding sites across 253 unique proteins bound to 235 unique drugs. The integration of amino acid–drug binding sites data with both germline and somatic nsSNVs data sets revealed 3133 nsSNVs affecting amino acid–drug binding sites. In addition, a comprehensive drug target discovery was conducted based on protein structure similarity and conservation of amino acid–drug binding sites. Using this method, 81 paralogs were identified that could serve as alternative drug targets. In addition, non-human mammalian proteins bound to drugs were used to identify 142 homologs in humans that can potentially bind to drugs. In the current protein–drug pairs that contain somatic mutations within their binding site, we identified 85 proteins with significant differential gene expression changes associated with specific cancer types. Information on protein–drug binding predicted drug target proteins and prevalence of both somatic and germline nsSNVs that disrupt these binding sites can provide valuable knowledge for personalized medicine treatment. A web portal is available where nsSNVs from individual patient can be checked by scanning against DrugVar to determine whether any of the SNVs affect the binding of any drug in the database.

Impact of long-term exposure to the tyrosine kinase inhibitor imatinib on the skeleton of growing rats

The tyrosine kinase (TK) inhibitor imatinib provides a highly effective therapy for chronic myeloid leukemia (CML) via inhibition of the oncogenic TK BCR-ABL1. However, off-target TKs like platelet-derived growth factor receptors (PDGF-R) and colony-stimulating factor-1 receptor (c-fms), involved in bone remodeling, are also inhibited. Thus, pediatric patients with CML on imatinib exhibit altered bone metabolism, leading to linear growth failure. As TKI treatment might be necessary for a lifetime, long-term effects exerted on bone in children are of major concern. Therefore, we studied the skeletal long-term effects of continuous and intermittent imatinib exposure in a juvenile rat model. Four-weeks-old male Wistar rats were chronically exposed to imatinib via drinking water over a period of 10 weeks. Animals were exposed to a standard and high imatinib dosage continuously and to the high imatinib dose intermittently. Bone mass and strength were assessed using pQCT, micro-computed tomography (μCT), and biomechanical testing at the prepubertal, pubertal, and postpubertal age. Bone length and vertebral height as well as biochemical markers of bone turnover were analyzed. Femoral and tibial bone length were dose-dependently reduced by up to 24% (p<0.0001), femoral and tibial trabecular bone mass density (BMD) were reduced by up to 25% (p<0.01), and femoral breaking strength was lowered by up to 20% (p<0.05). Intermittent exposure mitigated these skeletal effects. Long-term exposure resulted in reduced vertebral height by 15% and lower trabecular BMD by 5%. Skeletal changes were associated with suppressed serum osteocalcin (p<0.01) and non-significantly elevated serum CTX-I and PINP levels. In conclusion, imatinib mainly impaired longitudinal growth of long bones rather than the vertebrae of growing rats. Interestingly, intermittent imatinib exposure has less skeletal side effects, which may be beneficial in pediatric patients taking imatinib.

Decreased frequency, but normal functional integrity of mesenchymal stromal cells derived from untreated and Imatinib-treated chronic myeloid leukemia patients

In vitro, Imatinib inhibits the proliferation and stimulates the osteogenic and adipogenic differentiation of mesenchymal stromal cells (MSC). However, it is unknown whether Imatinib affects the biology of MSC in vivo. We asked whether MSC from long-term Imatinib-treated CML patients were affected by the in vivo treatment. MSC from untreated and Imatinib-treated patients displayed normal functional properties (i.e. proliferation, immunophenotype, differentiation and hematopoietic supportive capacity) - but a decreased frequency. In vitro, Imatinib lost its effect when discontinued; which suggest that it has a reversible effect on MSC. Therefore it might lose its effect on MSC after discontinuation in vivo.

The tyrosine kinase inhibitor GNF-2 suppresses osteoclast formation and activity

GNF-2, a tyrosine kinase inhibitor, was developed to overcome imatinib-resistant mutations found in CML patients. Osteoclasts are the principal bone-resorbing cells that are responsible for bone diseases, such as osteoporosis, tumor-induced osteolysis, and metastatic cancers. In this study, we investigated the effect of GNF-2 on osteoclast development induced by RANKL and M-CSF. We found that GNF-2 inhibited osteoclast differentiation from BMMs. GNF-2 suppressed RANKL-induced NF-κB transcriptional activity and the induction of c-Fos and NFATc1, which are two key transcription factors in osteoclastogenesis. We also observed that GNF-2 dose-dependently inhibited the proliferation of osteoclast precursors through the suppression of the M-CSFR c-Fms. In addition, GNF-2 accelerated osteoclast apoptosis by inducing caspase-3 and Bim expression. Furthermore, GNF-2 interfered with actin cytoskeletal organization and subsequently blocked the bone-resorbing activity of mature osteoclasts. In agreement with its in vitro effects, GNF-2 reduced osteoclast number and bone loss in a mouse model of LPS-induced bone destruction. Taken together, our data reveal that GNF-2 possesses anti-bone-resorptive properties, suggesting that GNF-2 may have therapeutic value for the treatment of bone-destructive disorders that can occur as a result of excessive osteoclastic bone resorption.

Professor Barrie Vernon-Roberts, AO, MD, BSc, PhD, FRCPath, FRCPA, FAOrthA (Hon), FRS.SA

This issue of Inflammopharmacology contains papers that have been submitted to commemorate the life and work of Professor Barrie Vernon-Roberts, an outstanding clinical scientist in the field of bone pathology and its pharmacological regulation. This review briefly summarizes his major works and achievements as well as a list of his publications.

Use of imatinib in the prevention of heterotopic ossification

BACKGROUND

Heterotopic ossification (HO) is a common complication following orthopedic and trauma surgery, which may have substantial negative effects on the postoperative outcome. Angiogenesis appears to play a critical role in heterotopic ossification. One of the involved signaling molecules is platelet-derived growth factor (PDGF) which may be inhibited by imatinib.

QUESTIONS/PURPOSES

Our goal was to prevent HO by pharmacologically interfering with the molecular signaling pathways involved in the developmental process. We hypothesized that by administering a proven inhibitor of PDGF expression, heterotopic bone formation may be prevented.

METHODS

The effect of imatinib on HO formation was studied in a murine model which reliably produces islets of HO within the soft tissue following Achilles tenotomy. The control group underwent Achilles tenotomy only. The imatinib group received imatinib mesylate. After trial completion, the limbs were harvested and scanned by micro-CT. Heterotopic bone volume was then identified and quantified.

RESULTS

The mean volume of heterotopic bone formed in the control group was 0.976mm(3) compared to 0.221 mm(3) in the imatinib group. The volume of HO in the treatment group was reduced by 85% compared to the control group.

CONCLUSIONS

The administration of imatinib was associated with a significantly reduced volume of HO. This may be due to the inhibitory effect of imatinib on the PDGF signaling pathway during development of HO.

CLINICAL RELEVANCE

The successful reduction of HO formation following imatinib administration has led to further insight concerning the pathogenesis of HO which in the future may lead to new clinical approaches towards the prevention of HO.

Prospective histomorphometric and DXA evaluation of bone remodeling in imatinib-treated CML patients: evidence for site-specific skeletal effects

CONTEXT

Imatinib is a tyrosine kinase inhibitor that has been successfully used to treat Philadelphia chromosome-positive chronic myeloid leukemia (CML) and Kit(+) gastrointestinal stromal tumors. We have previously shown that imatinib therapy is associated with an increase in trabecular bone volume.

OBJECTIVE

In the present study, we performed a prospective analysis of bone indices in imatinib-treated CML patients to determine the mechanism responsible for this altered bone remodeling. DESIGN, PATIENTS, AND INTERVENTION: This study assessed the effects of high-dose (600 mg/d) imatinib on bone parameters in newly diagnosed chronic-phase Philadelphia chromosome-positive CML patients (n = 11) enrolled in the TIDEL II study. At baseline and after 6, 12, and 24 months of treatment, serum markers of bone remodeling were quantitated, dual-energy x-ray absorptiometry analysis of bone mineral density (BMD) was carried out, and a bone biopsy was collected for histological and micro-computed tomography analysis.

RESULTS

Our studies show that the increase in trabecular bone volume and trabecular thickness after imatinib treatment was associated with a significant decrease in osteoclast numbers, accompanied by a significant decrease in serum levels of a marker of osteoclast activity. In contrast, osteoblast numbers were not altered by up to 24 months of imatinib treatment. Notably, we also found that imatinib caused a site-specific decrease in BMD at the femoral neck.

CONCLUSIONS

These data suggest that imatinib therapy dysregulates bone remodeling, causing a generalized decrease in osteoclast number and activity that is not counterbalanced by a decrease in osteoblast activity, leading to increased trabecular bone volume. Further long-term investigations are required to determine the causes and consequences of the site-specific decrease in BMD at the femoral neck.

Digital image analysis as a tool to assess the effects of imatinib on trabecular bone in patients with chronic myelogenous leukemia

Skeletal integrity is sustained by osteoblast-osteoclast interactions, controlled by several signaling pathways that include tyrosine kinases. Imatinib is a tyrosine kinase inhibitor with an extended therapeutic range based on its ability to differentially bind to receptor and nonreceptor tyrosine kinases. In this study, we used digital image analysis to assess changes in trabecular bone surface area within bone marrow biopsy specimens of 34 patients with chronic phase chronic myelogenous leukemia treated with single-agent imatinib. These patients were 25 men and 9 women with a median age of 59 years. We selected representative areas of paired bone marrow biopsy specimens obtained at baseline and within the subsequent 48 months. Computer-assisted analysis was performed to calculate trabecular bone area (TBA) within the sample by using the equation TBA% = sum of trabecular area/total biopsy specimen area. Percentage changes were defined as ΔTBA% and were arbitrarily subdivided into marked (>50%), moderate (10%-50%), and mild (<10%). During the study interval, TBA% increased in 24 patients (71%) and decreased in 10 patients (29%). Overall, there was a significant increase in TBA% (P = .02). No correlation was found between changes in trabecular bone area and either clinical or cytogenetic response (P = .25). The results show that imatinib therapy can alter trabecular bone in bone marrow biopsy specimens of chronic myelogenous leukemia patients, most often resulting in an increase in TBA%.

Suppression of PDGF-induced PI3 kinase activity by imatinib promotes adipogenesis and adiponectin secretion

Improved glucose and lipid metabolism is a unique side effect of imatinib therapy in some chronic myeloid leukaemia (CML) patients. We recently reported that plasma levels of adiponectin, an important regulator of insulin sensitivity, are elevated following imatinib therapy in CML patients, which could account for these improved metabolic outcomes. Adiponectin is secreted exclusively from adipocytes, suggesting that imatinib modulates adiponectin levels directly, by transcriptional upregulation of adiponectin in pre-existing adipocytes, and/or indirectly, by stimulating adipogenesis. In this report, we have demonstrated that imatinib promotes adipogenic differentiation of human mesenchymal stromal cells (MSCs), which in turn secrete high-molecular-weight adiponectin. Conversely, imatinib does not stimulate adiponectin secretion from mature adipocytes. We hypothesise that inhibition of PDGFRα (PDGFRA) and PDGFRβ (PDGFRB) is the mechanism by which imatinib promotes adipogenesis. Supporting this, functional blocking antibodies to PDGFR promote adipogenesis and adiponectin secretion in MSC cultures. We have shown that imatinib is a potent inhibitor of PDGF-induced PI3 kinase activation and, using a PI3 kinase p110α-specific inhibitor (PIK-75), we have demonstrated that suppression of this pathway recapitulates the effects of imatinib on MSC differentiation. Furthermore, using mitogens that activate the PI3 kinase pathway, or MSCs expressing constitutively activated Akt, we have shown that activation of the PI3 kinase pathway negates the pro-adipogenic effects of imatinib. Taken together, our results suggest that imatinib increases plasma adiponectin levels by promoting adipogenesis through the suppression of PI3 kinase signalling downstream of PDGFR.

Dasatinib as a bone-modifying agent: anabolic and anti-resorptive effects

BACKGROUND

Bone loss, in malignant or non-malignant diseases, is caused by increased osteoclast resorption and/or reduced osteoblast bone formation, and is commonly associated with skeletal complications. Thus, there is a need to identify new agents capable of influencing bone remodeling. We aimed to further pre-clinically evaluate the effects of dasatinib (BMS-354825), a multitargeted tyrosine kinase inhibitor, on osteoblast and osteoclast differentiation and function.

METHODS

For studies on osteoblasts, primary human bone marrow mensenchymal stem cells (hMSCs) together with the hMSC-TERT and the MG-63 cell lines were employed. Osteoclasts were generated from peripheral blood mononuclear cells (PBMC) of healthy volunteers. Skeletally-immature CD1 mice were used in the in vivo model.

RESULTS

Dasatinib inhibited the platelet derived growth factor receptor-β (PDGFR-β), c-Src and c-Kit phosphorylation in hMSC-TERT and MG-63 cell lines, which was associated with decreased cell proliferation and activation of canonical Wnt signaling. Treatment of MSCs from healthy donors, but also from multiple myeloma patients with low doses of dasatinib (2-5 nM), promoted its osteogenic differentiation and matrix mineralization. The bone anabolic effect of dasatinib was also observed in vivo by targeting endogenous osteoprogenitors, as assessed by elevated serum levels of bone formation markers, and increased trabecular microarchitecture and number of osteoblast-like cells. By in vitro exposure of hemopoietic progenitors to a similar range of dasatinib concentrations (1-2 nM), novel biological sequelae relative to inhibition of osteoclast formation and resorptive function were identified, including F-actin ring disruption, reduced levels of c-Fos and of nuclear factor of activated T cells 1 (NFATc1) in the nucleus, together with lowered cathepsin K, αVβ3 integrin and CCR1 expression.

CONCLUSIONS

Low dasatinib concentrations show convergent bone anabolic and reduced bone resorption effects, which suggests its potential use for the treatment of bone diseases such as osteoporosis, osteolytic bone metastasis and myeloma bone disease.

Osteosclerotic lesions in patients treated with gefitinib for lung adenocarcinomas: a sign of favorable therapeutic response

OBJECTIVE

To assess the frequency of osteosclerotic changes on CT that appeared after treatment with gefitinib in patients with lung adenocarcinoma and the relationship between the osteosclerotic changes and the response to the therapy.

MATERIALS AND METHODS

Our study included 41 patients with lung adenocarcinoma who underwent chest CT both before (CTpre) and after (CTpost) starting treatment with gefitinib. The presence or absence of bone metastases was assessed on the CTpre, and the interval bony change after the therapy was classified as lytic, sclerotic, or no changes on the CTpost. The relationship between treatment results of primary lung cancer and interval bony changes was evaluated.

RESULTS

Osteosclerotic lesions were identified in 11 patients (27%) on CTpost; in 6 of 11 patients osteosclerotic lesions newly appeared where the CTpre showed no bone metastasis before the gefitinib therapy. There were significant differences in the therapeutic response of the primary cancers (P < 0.001) and in the survival rate (P < 0.01) in patients with osteosclerotic changes versus those without osteosclerotic changes.

CONCLUSION

Osteosclerotic changes on CT, observed after gefitinib treatment in patients with lung adenocarcinomas, may be an indicator of a good therapeutic response.

Imatinib inhibits proliferation of human mesenchymal stem cells and promotes early but not late osteoblast differentiation in vitro

Altered bone metabolism has been reported in patients with chronic myeloid leukemia treated with the tyrosine kinase inhibitor imatinib. Several studies have shown that imatinib inhibits the differentiation and activity of osteoclasts in vitro, whereas the effects of imatinib on osteoblast differentiation are less clear. In this study osteoblast differentiation was induced in human mesenchymal stem cells (hMSCs) by treatment with bone morphogenetic protein 2 in vitro. Imatinib inhibited proliferation of hMSCs in a dose-dependent manner. Even though imatinib promoted early osteoblast differentiation assessed by alkaline phosphate activity, mineralization measured by Alizarin Red staining (ARS) was reduced by imatinib. Moreover, the inhibitory effect of imatinib on mineralization was most prominent at low concentrations of imatinib. When we measured the relative mRNA expression levels of Runx2, we found that Runx2 expression was higher in imatinib-treated (5 μM) cultures at early time points during differentiation. On the other hand, the expression of Osterix late during differentiation was lower in imatinib-treated (5 μM) cultures, corresponding to the ARS results. Thus, the effect of imatinib on osteoblast differentiation is not only dependent on the drug concentration, but indeed also on the maturation stage of the cells. This finding might partly explain why previous studies on the effects of imatinib osteoblast differentiation have shown different results.

Distinct impact of imatinib on growth at prepubertal and pubertal ages of children with chronic myeloid leukemia

OBJECTIVE

To determine the extent of growth impairment resulting from imatinib treatment in children with chronic myeloid leukemia (CML).

STUDY DESIGN

Clinical records of 48 chronic-phase CML children administered imatinib as the first-line therapy between 2001 and 2006 were analyzed retrospectively. Cumulative change in height was assessed using the height height-SDS and converted height data from age- and sex-adjusted Japanese norms.

RESULTS

A decrease in height-SDS was observed in 72.9% of children, with a median maximum reduction in height-SDS of 0.61 during imatinib treatment. Median follow-up time was 34 months (range, 10-88 months). Growth impairment was seen predominantly in children who started imatinib at a prepubertal age compared with those who started at pubertal age. Growth velocity tended to recuperate in prepubertal children with growth impairment, as they reached pubertal age, suggesting that imatinib had little impact on growth during puberty.

CONCLUSIONS

Growth impairment was a major adverse effect of long-term imatinib treatment in children with CML. We report the distinct inhibitory effect of imatinib on growth in prepubertal and pubertal children with CML. We should be aware of growth deceleration in children, especially in young children given imatinib before puberty and subjected to prolonged exposure.

The skeletal effects of the tyrosine kinase inhibitor nilotinib

Nilotinib is a tyrosine kinase inhibitor (TKI) developed to manage imatinib-resistance in patients with chronic myeloid leukemia (CML). It inhibits similar molecular targets to imatinib, but is a significantly more potent inhibitor of Bcr-Abl. Nilotinib exhibits off-target effects in other tissues, and of relevance to bone metabolism, hypophosphataemia has been reported in up to 30% of patients receiving nilotinib. We have assessed the effects of nilotinib on bone cells in vitro and on bone metabolism in patients receiving nilotinib for treatment of CML. We firstly investigated the effects of nilotinib on proliferating and differentiating osteoblastic cells, and on osteoclastogenesis in murine bone marrow cultures and RAW264.7 cells. Nilotinib potently inhibited osteoblast proliferation (0.01-1uM), through inhibition of the platelet-derived growth factor (PDGFR). There was a biphasic effect on osteoblast differentiation such that it was reduced by lower concentrations of nilotinib (0.1-0.5uM), with no effect at higher concentrations (1uM). Nilotinib also potently inhibited osteoclastogenesis, predominantly by stromal-cell dependent mechanisms. Thus, nilotinib decreased osteoclast development in murine bone marrow cultures, but did not affect osteoclastogenesis in RAW264.7 cells. Nilotinib treatment of osteoblastic cells increased expression and secretion of OPG and decreased expression of RANKL. In 10 patients receiving nilotinib, levels of bone turnover markers were in the low-normal range, despite secondary hyperparathyroidism, findings that are similar to those in patients treated with imatinib. Bone density tended to be higher than age and gender-matched normal values. These data suggest that nilotinib may have important effects on bone metabolism. Prospective studies should be conducted to determine the long-term effects of nilotinib on bone density and calcium metabolism.

3BP2-deficient mice are osteoporotic with impaired osteoblast and osteoclast functions

A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2-/-mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cell-intrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2-/- osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2-/- osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages.

Effects of second-generation tyrosine kinase inhibitors towards osteogenic differentiation of human mesenchymal cells of healthy donors

The BCR-ABL inhibitor imatinib is a standard first-line therapy for patients with chronic myeloid leukemia. However, it has been demonstrated that this long-term treatment is associated with altered bone metabolism. The mechanisms of this effect are not fully understood, but an inhibition of the platelet-derived growth factor receptor (PDGF-R) β axis has been suspected on the basis of some in vitro findings. We evaluated the osteoblastic differentiation of mesenchymal stem cells derived from bone marrow (hBM-MSCs) after in vitro treatment with dasatinib, nilotinib or bosutinib. Human bone marrow mesenchymal stem cells were induced to differentiate in osteoblastic cells by treatment with osteogenic medium with or without dasatinib, nilotinib or bosutinib. We found that the addition of dasatinib, and to a greater extend nilotinib, induced expression of osteogenic mRNA markers as compared with cultures with standard medium or osteogenic medium only. However, treatment with bosutinib did not induce an increase of osteogenic markers. In conclusion, we show that besides imatinib, other tyrosine kinase inhibitors (TKIs) such as dasatinib and nilotinib, but not bosutinib, increase osteogenic markers in hBM-MSCs. Because bosutinib differs from the other TKIs because of its low affinity to other kinases such as PDGF-R, these experiments suggest that inhibition of PDGF-R may be involved in the induction of osteoblastogenesis by TKIs.

Imatinib mesylate does not increase bone volume in vivo

Imatinib mesylate is a tyrosine kinase inhibitor used in the management of disorders in which activation of c-Abl, PDGFR, or c-Kit signaling plays a critical role. In vitro, imatinib stimulates osteoblast differentiation, inhibits osteoblast proliferation and survival, and decreases osteoclast development. Patients treated with imatinib exhibit altered bone and mineral metabolism, with stable or increased bone mass. However, recovery from the underlying disease and/or weight gain might contribute to these effects. We therefore investigated the skeletal effects of imatinib in healthy rats. We evaluated the effects of imatinib on bone volume, markers of bone turnover, and bone histomorphometry in mature female rats treated for 5 weeks with either vehicle, imatinib 40 mg/kg daily, or imatinib 70 mg/kg daily. Compared to vehicle, imatinib reduced trabecular bone volume/tissue volume (mean [SD]: vehicle 26.4% [5.4%], low-dose imatinib 24.8% [4.9%] [P = 0.5], high-dose imatinib 21.1% [5.7%] [P = 0.05]), reduced osteoblast surface (mean [SD]: vehicle 12.8% [5.8%], low-dose 6.8% [1.9%] [P < 0.01], high-dose 7.8 [3.1%] [P < 0.05]), and reduced serum osteocalcin (mean change from baseline [95% CI]: vehicle -8.2 [-26.6 to 10.2] ng/ml, low dose -79.7 [-97.5 to -61.9] ng/ml [P < 0.01 vs. vehicle], high-dose -66.0 [-82.0 to -50.0] ng/ml [P < 0.05 vs. vehicle]). Imatinib did not affect biochemical or histomorphometric indices of bone resorption. These results suggest that, in healthy animals, treatment with imatinib does not increase bone mass and that the improvements in bone density reported in patients receiving imatinib may not be a direct effect of the drug.

Insufficient bilateral femoral subtrochanteric fractures in a patient receiving imatinib mesylate

We present a case of insufficient bilateral femoral subtrochanteric fractures in a patient who was treated with imatinib mesylate, an anticancer drug, for 1 year after a diagnosis of chronic myelogenous leukemia (CML). A 60-year-old woman presented with bilateral thigh pain for 6 months. A plain radiograph revealed bilateral progressive insufficient fractures on the subtrochanteric areas of the femurs. MRI of the femurs revealed incomplete stress fractures and no evidence of bone metastasis on either femur. Bone densitometry showed normal T-scores around the hip joint and spine. The patient had normal serum levels of calcium, vitamin D derivatives, and thyroid hormones. Serum phosphate levels were decreased, and parathyroid hormone levels were increased. Serum osteocalcin and urinary N-telopeptide of collagen cross-links (NTx) were both decreased. A bone biopsy demonstrated normocellular marrow without leukemic cells. A histomorphometric evaluation of her bones revealed reduced bone turnover despite secondary hyperparathyroidism. The serum markers for bone metabolism and histomorphometric evaluations in this patient suggest that the drug may have an effect on bone metabolism. These effects could be seen for both bone formation and resorption: this could result in impaired bone mineralization, a severely suppressed bone turnover rate, insufficient fractures, and bone necrosis, which are sometimes seen with long-term use of bisphosphonates. To our knowledge, this is the first case of an insufficient bilateral femoral shaft fracture that is potentially related to the use of imatinib mesylate in a patient with CML. Careful examination of bone metabolism should be performed in patients with CML because imatinib mesylate treatment is a lifelong process.

CML in pregnancy and childhood

With the improved survivals offered by the tyrosine kinase inhibitors has come the necessity to address issues relating to quality of life and one such area is that of fertility and parenting. Animal data suggest that imatinib at standard dosages is unlikely to impair fertility in either adult males or females but human data remain limited. Children born to men who are actively taking imatinib at the time of conception appear healthy and current advice is not to discontinue treatment. In contrast the data relating to children born to women exposed to imatinib during pregnancy are less encouraging. Although numbers are small there has been a disturbing cluster of rare congenital malformations such that imatinib cannot be safely recommended, particularly during the period of organogenesis. The appropriate management of children with CML has also been radically changed by the advent of imatinib. The features of the disease at presentation, the natural history and the response to therapy seem to be identical in children to that seen in adults. Now that imatinib has been in clinical use for almost ten years without severe long-term side effects, most physicians are now comfortable advising a trial of imatinib prior to consideration of transplant. Data relating to the efficacy and safety of second generation tyrosine kinase inhibitors in childhood is entirely absent and transplant remains the first choice for patients failing imatinib and perhaps also for young patients with sub-optimal responses.

Dysregulation of bone remodeling by imatinib mesylate

Imatinib mesylate is a rationally designed tyrosine kinase inhibitor that has revolutionized the treatment of chronic myeloid leukemia and gastrointestinal stromal tumors. Although the efficacy and tolerability of imatinib are a vast improvement over conventional chemotherapies, the drug exhibits off-target effects. An unanticipated side effect of imatinib therapy is hypophosphatemia and hypocalcemia, which in part has been attributed to drug-mediated changes to renal and gastrointestinal handling of phosphate and calcium. However, emerging data suggest that imatinib also targets cells of the skeleton, stimulating the retention and sequestration of calcium and phosphate to bone, leading to decreased circulating levels of these minerals. The aim of this review is to highlight our current understanding of the mechanisms surrounding the effects of imatinib on the skeleton. In particular, it examines recent studies suggesting that imatinib has direct effects on bone-resorbing osteoclasts and bone-forming osteoblasts through inhibition of c-fms, c-kit, carbonic anhydrase II, and the platelet-derived growth factor receptor. The potential application of imatinib in the treatment of cancer-induced osteolysis will also be discussed.

Potential roles of growth factor PDGF-BB in the bony repair of injured growth plate

Injured growth plate cartilage is often repaired by bony tissue resulting in impaired bone growth in children. Using a rat injury model, our previous studies show that following the injury-induced initial inflammatory response, an influx of mesenchymal-like cells occurs within the growth plate injury site prior to formation of bony tissue. As platelet-derived growth factor (PDGF-BB) is a potent chemotactic factor of mesenchymal cells during skeletal tissue repair, we examined its role during the early fibrogenic response and the subsequent bony repair of injured growth plate. Following growth plate injury, rats received daily injection of the PDGF receptor (PDGFR) inhibitor, Imatinib, for 7 days. Immunohistochemical analysis of injured growth plate at day 1 showed the presence of PDGF-BB expression in some inflammatory cells, while at day 4 PDGFR was expressed by a proportion of the infiltrating mesenchymal cells at the injury site. By day 4, PDGFR inhibition reduced mesenchymal infiltrate (P<0.05); by day 14, Imatinib-treated rats exhibited less bony trabeculae and cartilaginous repair tissues, fewer osteoclasts and less bone marrow (BM) at the injury site, compared to vehicle controls (P<0.01). In vitro "scratch" migration assays with rat BM mesenchymal cells revealed that recombinant PDGF-BB increased cell migration into the "wound" (P<0.05), while Imatinib inhibited this chemotactic response. Quantitative RT-PCR analysis showed that Imatinib treatment decreased expression of the cartilage and bone related genes, Col2a1 and osteocalcin, respectively. These results suggest that PDGF-BB contributes to growth plate injury repair by promoting mesenchymal progenitor cell infiltration, the chondrogenic and osteogenic responses, and remodelling of the repair tissues.

Decreased bone turnover despite persistent secondary hyperparathyroidism during prolonged treatment with imatinib

CONTEXT

The tyrosine kinase inhibitor imatinib mesylate has an established role in the management of a number of malignant and proliferative conditions. Cross-sectional and short-term prospective studies have demonstrated secondary hyperparathyroidism during imatinib therapy, and variable changes in markers of bone turnover.

OBJECTIVE

Our objective was to determine the biochemical and skeletal effects of imatinib during long-term therapy.

DESIGN

This was a 2-yr prospective study.

SETTING

The study was performed at an academic clinical research center.

PATIENTS OR OTHER PARTICIPANTS

Nine patients with bcr-abl positive chronic myeloid leukemia were included in the study.

INTERVENTIONS

Patients received Imatinib mesylate 400 mg/d.

MAIN OUTCOME MEASURES

Serum and urine biochemistry, markers of bone turnover, and bone mineral density were measured.

RESULTS

Participants developed mild secondary hyperparathyroidism, with significant decreases in serum calcium and phosphate (P < 0.05 and P < 0.0001 vs. baseline, respectively) and an increase in PTH (P < 0.0001 vs. baseline). Biochemical markers of bone turnover demonstrated a biphasic response, with an initial increase in markers of bone formation being followed by a decrease in markers of both formation and resorption. Bone density at the lumbar spine increased [mean (95% confidence interval) change from baseline 3.6% (1.6, 5.5); P = 0.003] as did that at the total body [1.4% (0.2, 2.5); P = 0.065], whereas that at the proximal femur did not change [-0.12% (-3.0, 2.7); P = 0.93]. Body weight and fat mass increased significantly (P < 0.0001 vs. baseline).

CONCLUSIONS

Long-term treatment with imatinib leads to persistent mild secondary hyperparathyroidism. Despite this, bone turnover is decreased, and bone density is stable or increased. Evaluation of the skeletal actions and safety of imatinib during longer-term therapy is warranted.

Hypolipemiant besides antileukemic effect of imatinib mesylate

The use of imatinib mesylate (IM) (Gleevec, Novartis) in chronic myeloid leukemia (CML) and other neoplastic disorders is in a dramatic increase, inducing long-standing survival. Therefore, the interest in the associated events with this treatment is more and more manifest. We describe a case of CML in which, at the usual antileukemic dose, IM induced a rapid and persistent normalization of the levels of serum cholesterol, triglycerides, low- and high-density lipoproteins and glucose. In the present case report we confirm other observations on the hypolipemiant and antidiabetic effects, concomitant with that antileukemic, of IM.

Innovative approaches of targeted therapy for CML of childhood in combination with paediatric haematopoietic SCT

Allogeneic haematopoietic SCT (HSCT) induces CRs in most patients with CML. With the excellent short-term treatment results induced by imatinib (IMA), attitudes have changed and only a minority of children are now transplanted upfront. This review addresses the role of IMA in children with CML, focusing on the starting dose of IMA, possible adverse effects, timing of HSCT in children, duration of IMA treatment and monitoring of treatment efficacy to unravel failure of early treatment of IMA as well as treatment of CML relapse after HSCT. As the paediatric experience with IMA is still very limited, many answers and algorithms are adapted from CML in adults. Basically, HSCT should be postponed to achieve an optimal tumour cell reduction by IMA treatment. Children with a low-risk EBMT score should undergo HSCT within 2 years after diagnosis to avoid prolonged exposure and unknown late effects of IMA. Without a perfectly HLA-matched donor, HSCT may be postponed until CML becomes refractory to IMA. As realized in the presently activated international trial CML-paed II, this approach represents a risk-adapted therapy with the benefit of being tailored to the needs and profile of an individual patient.

Cathepsin K inhibitors as treatment of bone metastasis

PURPOSE OF REVIEW

Cancer cells that metastasize to the skeleton are, on their own, rarely able to destroy bone. Instead, they stimulate the function of bone-degrading cells, the osteoclasts, leading to the formation of osteolytic lesions. The purpose of this review is to consider cathepsin K, a cysteine protease produced by osteoclasts, as a therapeutic target for the treatment of patients with osteolytic bone metastases.

RECENT FINDINGS

Cathepsin K plays a key role in osteoclast-mediated bone degradation. It is also produced by cancer cells that metastasize to bone where it functions in proteolytic pathways that promote cancer cell invasion. Highly selective and potent cathepsin K inhibitors have been recently developed and shown to be useful antiresorptive agents to treat osteoporosis. Moreover, preclinical studies show that cathepsin K inhibitors reduce breast cancer-induced osteolysis and skeletal tumor burden. This reduction of skeletal tumor burden is due to the antiresorptive activity of cathepsin K inhibitors, which in turn, deprive cancer cells of bone-derived growth factors that are required for tumor growth.

SUMMARY

Cathepsin K inhibitors are appropriate drugs to treat diseases associated with increased bone loss. However, their chronic use in treating osteoporosis may result in adverse effects because basic nitrogen-containing cathepsin K inhibitors accumulate within acidic organelles such as lysosomes, thereby inhibiting the activity of other cathepsins. These adverse effects should not, however, preclude the use of these drugs in life-threatening diseases such as bone metastasis.