The Src inhibitor AZD0530 reversibly inhibits the formation and activity of human osteoclasts

Current and emerging bone resorption inhibitors for the treatment of osteoporosis

INTRODUCTION

Osteoporosis is a metabolic skeletal disease characterized by low bone mass and strength, and increased risk for fragility fractures. It is a major health issue in aging populations, due to fracture-associated increased disability and mortality. Antiresorptive treatments are first line choices in most of the cases.

AREAS COVERED

Bone homeostasis is complicated, and multiple factors can compromise skeletal health. Bone turnover is a continuous process regulated by the coupled activities of bone cells that preserves skeletal strength and integrity. Imbalance between bone resorption and formation leads to bone loss and increased susceptibility to fractures. Antiresorptives prevent bone loss and reduce fracture risk, by targeting osteoclastogenesis and osteoclast function and survival. Their major drawback is the coupling of osteoclast and osteoblast activity, due to which any reduction in bone resorption is followed by suppression of bone formation.

EXPERT OPINION

During the last couple of decades significant progress has been made in understanding of the genetic and molecular basis of osteoporosis. Critical pathways and key molecules that mediate regulation of bone resorption have been identified. These factors may underpin novel therapeutic avenues for osteoporosis, but their potential for translation into clinical applications is yet to be tested.

Obacunone inhibits RANKL/M-CSF-mediated osteoclastogenesis by suppressing integrin- FAK-Src signaling

Disrupted osteoblastogenesis or aberrant activation of osteoclastogenesis usually results in the break of bone homeostasis thus causing bone-associated diseases like osteoporosis. Obacunone, as a natural compound present in citrus fruits, has been demonstrated for various biological activities including anti-cancer and anti-inflammatory properties. However, the role of obacunone in regulating osteoclastogenesis has not been elucidated so far. Here, using in vitro cell models of RANKL (Receptor activator of nuclear factor-kB ligand) and M-CSF (Macrophage-colony-stimulating factor)-induced osteoclastogenesis, we showed that obacunone inhibited osteoclast differentiation in RAW264.7 cells and bone marrow macrophages (BMMs), as evidenced by obacunone dose-dependent reduction in numbers of osteoclasts and downregulated expressions of osteoclastogenesis-associated key genes. The anti-osteoclastic properties of obacunone were associated with downregulated expressions of Integrin α1 and attenuated activation of Focal adhesion kinase (FAK) and Steroid receptor coactivator (Src) signaling. Functional Integrin α1 blockade or FAK-Src inhibition suppressed RANKL/M-CSF-induced osteoclastogenesis, while Integrin α1 overexpression or FAK/Src activation partially attenuated obacunone's effects on suppressing RANKL/M-CSF-induced osteoclast differentiation. Furthermore, in vivo administration of obacunone displayed super therapeutic effects in attenuating ovariectomy-induced bone loss in mice, as indicated by decreases in serum biomarkers of bone turnover, restoring of femur fracture maximum force, and reversing of the worsened bone-related parameters in ovariectomized animals. Taken together, these findings demonstrate that obacunone has pharmacological activities to suppress osteoclast differentiation through modulating the Integrin-FAK-Src pathway, and suggest that obacunone is a therapeutic candidate for the treatment and prevention of bone diseases such as osteoporosis.

Zika virus infects human osteoclasts and blocks differentiation and bone resorption

Bone-related complications are commonly reported following arbovirus infection. These arboviruses are known to disturb bone-remodeling and induce inflammatory bone loss via increased activity of bone resorbing osteoclasts (OCs). We previously showed that Zika virus (ZIKV) could disturb the function of bone forming osteoblasts, but the susceptibility of OCs to ZIKV infection is not known. Here, we investigated the effect of ZIKV infection on osteoclastogenesis and report that infection of pre- and early OCs with ZIKV significantly reduced the osteoclast formation and bone resorption. Interestingly, infection of pre-OCs with a low dose ZIKV infection in the presence of flavivirus cross-reacting antibodies recapitulated the phenotype observed with a high viral dose, suggesting a role for antibody-dependent enhancement in ZIKV-associated bone pathology. In conclusion, we have characterized a primary in vitro model to study the role of osteoclastogenesis in ZIKV pathogenesis, which will help to identify possible new targets for developing therapeutic and preventive measures.

Mechanisms of disease-modifying effect of saracatinib (AZD0530), a Src/Fyn tyrosine kinase inhibitor, in the rat kainate model of temporal lobe epilepsy

We have recently demonstrated the role of the Fyn-PKCδ signaling pathway in status epilepticus (SE)-induced neuroinflammation and epileptogenesis in experimental models of temporal lobe epilepsy (TLE). In this study, we show a significant disease-modifying effect and the mechanisms of a Fyn/Src tyrosine kinase inhibitor, saracatinib (SAR, also known as AZD0530), in the rat kainate (KA) model of TLE. SAR treatment for a week, starting the first dose (25 mg/kg, oral) 4 h after the onset of SE, significantly reduced spontaneously recurring seizures and epileptiform spikes during the four months of continuous video-EEG monitoring. Immunohistochemistry of brain sections and Western blot analyses of hippocampal lysates at 8-day (8d) and 4-month post-SE revealed a significant reduction of SE-induced astrogliosis, microgliosis, neurodegeneration, phosphorylated Fyn/Src-419 and PKCδ-tyr311, in SAR-treated group when compared with the vehicle control. We also found the suppression of nitroxidative stress markers such as iNOS, 3-NT, 4-HNE, and gp91phox in the hippocampus, and nitrite and ROS levels in the serum of the SAR-treated group at 8d post-SE. The qRT-PCR (hippocampus) and ELISA (serum) revealed a significant reduction of key proinflammatory cytokines TNFα and IL-1β mRNA in the hippocampus and their protein levels in serum, in addition to IL-6 and IL-12, in the SAR-treated group at 8d in contrast to the vehicle-treated group. These findings suggest that SAR targets some of the key biomarkers of epileptogenesis and modulates neuroinflammatory and nitroxidative pathways that mediate the development of epilepsy. Therefore, SAR can be developed as a potential disease-modifying agent to prevent the development and progression of TLE.

A Synthetic Peptide, CK2.3, Inhibits RANKL-Induced Osteoclastogenesis through BMPRIa and ERK Signaling Pathway

The skeletal system plays an important role in the development and maturation process. Through the bone remodeling process, 10% of the skeletal system is renewed every year. Osteoblasts and osteoclasts are two major bone cells that are involved in the development of the skeletal system, and their activity is kept in balance. An imbalance between their activities can lead to diseases such as osteoporosis that are characterized by significant bone loss due to the overactivity of bone-resorbing osteoclasts. Our laboratory has developed a novel peptide, CK2.3, which works as both an anabolic and anti-resorptive agent to induce bone formation and prevent bone loss. We previously reported that CK2.3 mediated mineralization and osteoblast development through the SMAD, ERK, and AKT signaling pathways. In this study, we demonstrated the mechanism by which CK2.3 inhibits osteoclast development. We showed that the inhibition of MEK by the U0126 inhibitor rescued the osteoclast development of RAW264.7 induced by RANKL in a co-culture system with CK2.3. We observed that CK2.3 induced ERK activation and BMPRIa expression on Day 1 after stimulation with CK2.3. While CK2.3 was previously reported to induce the SMAD signaling pathway in osteoblast development, we did not observe any changes in SMAD activation in osteoclast development with CK2.3 stimulation. Understanding the mechanism by which CK2.3 inhibits osteoclast development will allow CK2.3 to be developed as a new treatment for osteoporosis.

Role of the bone microenvironment in bone metastasis of malignant tumors - therapeutic implications

BACKGROUND

Bone is one of the most common sites for solid tumor metastasis. Bone metastasis of a malignant tumor seriously affects the quality of life and the overall survival of patients. Evidence has suggested that bone provides a favorable microenvironment that enables disseminated tumor cells to home, proliferate and colonize, leading to the formation of metastases. In the process of bone metastasis the bone microenvironment may be considered as an orchestra that plays a dissonant melody through blending (e.g. cross-talk between osteoclasts, osteoblasts and/or other cells), adding (e.g. a variety of biological factors) or taking away (e.g. blocking a specific pathway) players.

CONCLUSIONS

Here, we review the normal bone microenvironment, bone microenvironment-related factors that promote bone metastasis, as well as mechanisms underlying bone metastasis. In addition, we elude on directions for clinical bone metastasis management, focusing on potential therapeutic approaches to target bone microenvironment-related factors, including bisphosphonate, denosumab, CXCR4/CXCL12 antagonists and cathepsin K inhibitors.

Osteoclast Signal Transduction During Bone Metastasis Formation

Osteoclasts are myeloid lineage-derived bone-resorbing cells of hematopoietic origin. They differentiate from myeloid precursors through a complex regulation process where the differentiation of preosteoclasts is followed by intercellular fusion to generate large multinucleated cells. Under physiological conditions, osteoclastogenesis is primarily directed by interactions between CSF-1R and macrophage colony-stimulating factor (M-CSF, CSF-1), receptor activator of nuclear factor NF-κB (RANK) and RANK ligand (RANKL), as well as adhesion receptors (e.g., integrins) and their ligands. Osteoclasts play a central role in physiological and pathological bone resorption and are also required for excessive bone loss during osteoporosis, inflammatory bone and joint diseases (such as rheumatoid arthritis) and cancer cell-induced osteolysis. Due to the major role of osteoclasts in these diseases the better understanding of their intracellular signaling pathways can lead to the identification of potential novel therapeutic targets. Non-receptor tyrosine kinases and lipid kinases play major roles in osteoclasts and small-molecule kinase inhibitors are emerging new therapeutics in diseases with pathological bone loss. During the last few years, we and others have shown that certain lipid (such as phosphoinositide 3-kinases PI3Kβ and PI3Kδ) and tyrosine (Src-family and Syk) kinases play a critical role in osteoclast differentiation and function in humans and mice. Some of these signaling pathways shows similarity to immunoreceptor-like receptor signaling and involves important other enzymes (e.g., PLCγ2) and adapter proteins (such as the ITAM-bearing adapters DAP12 and the Fc-receptor γ-chain). Here, we review recently identified osteoclast signaling pathways and their role in osteoclast differentiation and function as well as pathological bone loss associated with osteolytic tumors of the bone. A better understanding of osteoclast signaling may facilitate the design of novel and more efficient therapies for pathological bone resorption and osteolytic skeletal metastasis formation.

Results of a Randomized, Double-Blinded, Placebo-Controlled, Phase 2.5 Study of Saracatinib (AZD0530), in Patients with Recurrent Osteosarcoma Localized to the Lung

Purpose

Osteosarcoma is a rare cancer and a third of patients who have completed primary treatment will develop osteosarcoma recurrence. The Src pathway has been implicated in the metastatic behavior of osteosarcoma; about 95% of samples examined express Src or have evidence of downstream activation of this pathway. Saracatinib (AZD0530) is a potent and selective Src kinase inhibitor that was evaluated in adults in Phase 1 studies. The primary goal of this study was to determine if treatment with saracatinib could increase progression-free survival (PFS) for patients who have undergone complete resection of osteosarcoma lung metastases in a double-blinded, placebo-controlled trial. Patients and Methods. Subjects with recurrent osteosarcoma localized to lung and who had complete surgical removal of all lung nodules were randomized within six weeks after complete surgical resection. Saracatinib, or placebo, was administered at a dose of 175 mg orally, once daily, for up to thirteen 28-day cycles.

Results

Thirty-seven subjects were included in the analyses; 18 subjects were randomized to receive saracatinib and 19 to receive placebo. Intent-to-treat analysis demonstrated a median PFS of 19.4 months in the saracatinib treatment group and 8.6 months in the placebo treatment group (p=0.47). Median OS was not reached in either arm.

Conclusions

Although saracatinib was well tolerated in this patient population, there was no apparent impact of the drug in this double-blinded, placebo-controlled trial on OS, and Src inhibition alone may not be sufficient to suppress metastatic progression in osteosarcoma. There is a suggestion of potential clinical benefit as evidenced by longer PFS in patients randomized to saracatinib based on limited numbers of patients treated.

Emerging therapeutic targets for osteoporosis

Introduction: Osteoporosis is a chronic, skeletal disorder characterized by compromised bone strength and increased fracture risk; it affects 50% of women and 20% of men. In the past two decades, there have been substantial improvements in the pharmacotherapy of osteoporosis which have yielded potent inhibitors of bone resorption or stimulators of bone formation.Areas covered: This review discusses newly identified targets and pathways and conceptual approaches to the prevention of multiple age-related disorders. Furthermore, it summarizes existing therapeutic strategies for osteoporosis.Expert opinion: Our enhanced understanding of bone biology and the reciprocal interactions between bone and other tissues have allowed the identification of new targets that may facilitate the development of novel drugs. These drugs will hopefully achieve the uncoupling of bone formation from resorption and possibly exert a dual anabolic and antiresorptive effect on bone. Alas, limitations regarding adherence, efficacy on nonvertebral fracture prevention and the long-term adverse events still exist for currently available therapeutics. Moreover, the efficacy of most agents is limited by the tight coupling of osteoblasts and osteoclasts; hence the reduction of bone resorption invariably reduces bone formation, and vice versa. This field is very much 'a work in progress.'

An exploratory randomized-controlled trial of the efficacy of the Src-kinase inhibitor saracatinib as a novel analgesic for cancer-induced bone pain

Pain is a major symptom of bone metastases from advanced cancer and represents a clinical challenge to treat effectively. Basic neurobiology in preclinical animal models implicates enhanced sensory processing in the central nervous system, acting through N-methyl-D-aspartate (NMDA) glutamate receptors, as an important mechanism underpinning persistent pain. The non-receptor tyrosine kinase Src is thought to act as a hub for regulating NMDA receptor activity and the orally available Src inhibitor saracatinib has shown promise as a potential analgesic in recent animal studies. Here we tested the efficacy of saracatinib as a novel analgesic in an exploratory phase II randomized controlled trial on cancer patients with painful bone metastases. Twelve patients completed the study, with 6 receiving saracatinib 125 mg/day for 28 days and 6 receiving placebo. Pharmacokinetic measurements confirmed appropriate plasma levels of drug in the saracatinib-treated group and Src inhibition was achieved clinically by a significant reduction in the bone resorption biomarker serum cross-linked C-terminal telopeptide of type I collagen. Differences between the saracatinib and placebo groups self-reported pain scores, measured using the short form of the Brief Pain Inventory, were not clinically significant after 4 weeks of treatment. There was also no change in consumption of maintenance analgesia in the saracatinib-treated group and no improvement in Quality-of-Life scores. The data were insufficient to demonstrate saracatinib has efficacy as analgesic, although it may have a role as an anti-bone resorptive agent.

Non-Viral Delivery System and Targeted Bone Disease Therapy

Skeletal systems provide support, movement, and protection to the human body. It can be affected by several life suffering bone disorders such as osteoporosis, osteoarthritis, and bone cancers. It is not an easy job to treat bone disorders because of avascular cartilage regions. Treatment with non-specific drug delivery must utilize high doses of systemic administration, which may result in toxicities in non-skeletal tissues and low therapeutic efficacy. Therefore, in order to overcome such limitations, developments in targeted delivery systems are urgently needed. Although the idea of a general targeted delivery system using bone targeting moieties like bisphosphonates, tetracycline, and calcium phosphates emerged a few decades ago, identification of carrier systems like viral and non-viral vectors is a recent approach. Viral vectors have high transfection efficiency but are limited by inducing immunogenicity and oncogenicity. Although non-viral vectors possess low transfection efficiency they are comparatively safe. A number of non-viral vectors including cationic lipids, cationic polymers, and cationic peptides have been developed and used for targeted delivery of DNA, RNA, and drugs to bone tissues or cells with successful consequences. Here we mainly discuss such various non-viral delivery systems with respect to their mechanisms and applications in the specific targeting of bone tissues or cells. Moreover, we discuss possible therapeutic agents that can be delivered against various bone related disorders.

Selective inhibition of Src family kinases by SU6656 increases bone mass by uncoupling bone formation from resorption in mice

Mice deficient in the non-receptor tyrosine kinase Src exhibit high bone mass due to impaired bone resorption and increased bone formation. Although several Src family kinase inhibitors inhibit bone resorption in vivo, they display variable effects on bone formation. SU6656 is a selective Src family kinase inhibitor with weaker activity towards the non-receptor tyrosine kinase Abl and receptor tyrosine kinases which are required for appropriate osteoblast proliferation, differentiation and function. Therefore, we sought to determine whether SU6656 could increase bone mass by inhibiting bone resorption and by stimulating bone formation, and to explore its mechanisms of action. Four-month-old female C57Bl/6J mice received intraperitoneal injections of either 25 mg/kg SU6656 or its vehicle every other day for 12 weeks. SU6656-treated mice exhibited increased bone mineral density, cortical thickness, cancellous bone volume and trabecular thickness. SU6656 inhibited bone resorption in mice as shown by reduced osteoclast number, and diminished expressions of Oscar, Trap5b and CtsK. SU6656 did not affect Rankl or Opg expressions. However, it blocked c-fms signaling, osteoclastogenesis and matrix resorption, and induced osteoclast apoptosis in vitro. In addition, SU6656 stimulated bone formation rates at trabecular, endosteal and periosteal bone envelopes, and increased osteoblast number in trabecular bone. SU6656 did not affect expressions of clastokines favoring bone formation in mice. However, it stimulated osteoblast differentiation and matrix mineralization by specifically facilitating BMP-SMAD signaling pathway in vitro. Knockdown of Src and Yes mimicked the stimulatory effect of SU6656 on osteoblast differentiation. In conclusion, SU6656 uncouples bone formation from resorption by inhibiting osteoclast development, function and survival, and by enhancing BMP-mediated osteoblast differentiation.

Osteogenic and osteoclastogenic potential of jaw bone-derived cells-A case study

Though the stem cell properties of tooth-derived periodontal ligament and gingival cells have been widely documented, surprisingly little is known about both the osteogenic and osteoclastogenic differentiation capacities of the more clinically relevant jaw bone-derived cells. These cells could be considered being recruited during bone healing such as after tooth extraction, after placing an implant, or after surgical or traumatic injury. Here, we compared the osteoblast and osteoclastogenesis features of four consecutive bone outgrowths with periodontal ligament and gingiva cells. For osteogenesis assay, cells were cultured in osteogenic medium, whereas in osteoclastogenesis assays, cells were cultured in the presence of human peripheral blood mononuclear cells (PBMCs) as a source of osteoclast precursors. After osteogenic stimulus, all six cell types responded by an increased expression of osteoblast markers RUNX2 and DMP1. Periodontal ligament cells expressed significantly higher levels of RUNX2 compared to all bone outgrowths. Alkaline phosphatase enzyme levels in periodontal ligament cells reached earlier and higher peak expression. Mineral deposits were highest in periodontal ligament, gingiva and the first bone outgrowth. Osteoclastogenesis revealed a stepwise increase of secreted pro-osteoclastogenesis proteins M-CSF, IL-1β, and TNF-α in the last three consecutive bone cultures. OPG mRNA showed the opposite: high expression in periodontal and gingiva cells and the first outgrowth. Osteoclast numbers were similar between the six cultures, both on bone and on plastic. This first study reveals that jaw bone outgrowths contain bone remodelling features that are slightly different from tooth-associated cells.

Raddeanin A suppresses breast cancer-associated osteolysis through inhibiting osteoclasts and breast cancer cells

Bone metastasis is a severe complication of advanced breast cancer, resulting in osteolysis and increased mortality in patients. Raddeanin A (RA), isolated from traditional Chinese herbs, is an oleanane-type triterpenoid saponin with anticancer potential. In this study, we investigated the effects of RA in breast cancer-induced osteolysis and elucidated the possible mechanisms involved in this process. We first verified that RA could suppress osteoclast formation and bone resorption in vitro. Next, we confirmed that RA suppressed Ti-particle-induced osteolysis in a mouse calvarial model, possibly through inhibition of the SRC/AKT signaling pathway. A breast cancer-induced osteolysis mouse model further revealed the positive protective effects of RA by micro-computed tomography and histology. Finally, we demonstrated that RA inhibited invasion and AKT/mammalian target of rapamycin signaling and induced apoptosis in MDA-MB-231 cells. These results indicate that RA is an effective inhibitor of breast cancer-induced osteolysis.

SRC kinase inhibition with saracatinib limits the development of osteolytic bone disease in multiple myeloma

Multiple myeloma (MM)-associated osteolytic bone disease is a major cause of morbidity and mortality in MM patients and the development of new therapeutic strategies is of great interest. The proto-oncogene SRC is an attractive target for such a strategy. In the current study, we investigated the effect of treatment with the SRC inhibitor saracatinib (AZD0530) on osteoclast and osteoblast differentiation and function, and on the development of MM and its associated bone disease in the 5TGM.1 and 5T2MM murine MM models. In vitro data showed an inhibitory effect of saracatinib on osteoclast differentiation, polarization and resorptive function. In osteoblasts, collagen deposition and matrix mineralization were affected by saracatinib. MM cell proliferation and tumor burden remained unaltered following saracatinib treatment and we could not detect any synergistic effects with drugs that are part of standard care in MM. We observed a marked reduction of bone loss after treatment of MM-bearing mice with saracatinib as reflected by a restoration of trabecular bone parameters to levels observed in naive control mice. Histomorphometric analyses support that this occurs through an inhibition of bone resorption. In conclusion, these data further establish SRC inhibition as a promising therapeutic approach for the treatment of MM-associated osteolytic bone disease.

Effects of Src-kinase inhibition in cancer-induced bone pain

Background

Bone metastases occur frequently in advanced breast, lung, and prostate cancer, with approximately 70% of patients affected. Pain is a major symptom of bone metastases, and current treatments may be inadequate or have unacceptable side effects. The mechanisms that drive cancer-induced bone pain are not fully understood; however, it is known that there is sensitization of both peripheral bone afferents and central spinal circuits. It is well established that the N-methyl-D-aspartate receptor plays a major role in the pathophysiology of pain hypersensitivity. Inhibition of the non-receptor tyrosine kinase Src controls N-methyl-D-aspartate receptor activity and inhibiting Src reduces the hypersensitivity associated with neuropathic and inflammatory pains. As Src is also implicated in osteoclastic bone resorption, we have investigated if inhibiting Src ameliorates cancer-induced bone pain. We have tested this hypothesis using an orally bioavailable Src inhibitor (saracatinib) in a rat model of cancer-induced bone pain.

Results

Intra-tibial injection of rat mammary cancer cells (Mammary rat metastasis tumor cells -1), but not vehicle, in rats produced hindpaw hypersensitivity to thermal and mechanical stimuli that was maximal after six days and persisted for at least 13 days postinjection. Daily oral gavage with saracatinib (20 mg/kg) beginning seven days after intra-tibial injection reversed the thermal hyperalgesia but not the mechanical allodynia. The analgesic mechanisms of saracatinib appear to be due to an effect on the nervous system as immunoblotting of L2-5 spinal segments showed that mammary rat metastasis tumor cells-1 injection induced phosphorylation of the GluN1 subunit of the N-methyl-D-aspartate receptor, indicative of receptor activation, and this was reduced by saracatinib. Additionally, histology showed no anti-tumor effect of saracatinib at any dose and no significant effect on bone preservation.

Conclusions

This is the first demonstration that Src plays a role in the development of cancer-induced bone pain and that Src inhibition represents a possible new analgesic strategy for patients with bone metastases.

Tissue invasion and metastasis: Molecular, biological and clinical perspectives

Cancer is a key health issue across the world, causing substantial patient morbidity and mortality. Patient prognosis is tightly linked with metastatic dissemination of the disease to distant sites, with metastatic diseases accounting for a vast percentage of cancer patient mortality. While advances in this area have been made, the process of cancer metastasis and the factors governing cancer spread and establishment at secondary locations is still poorly understood. The current article summarizes recent progress in this area of research, both in the understanding of the underlying biological processes and in the therapeutic strategies for the management of metastasis. This review lists the disruption of E-cadherin and tight junctions, key signaling pathways, including urokinase type plasminogen activator (uPA), phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene (PI3K/AKT), focal adhesion kinase (FAK), β-catenin/zinc finger E-box binding homeobox 1 (ZEB-1) and transforming growth factor beta (TGF-β), together with inactivation of activator protein-1 (AP-1) and suppression of matrix metalloproteinase-9 (MMP-9) activity as key targets and the use of phytochemicals, or natural products, such as those from Agaricus blazei, Albatrellus confluens, Cordyceps militaris, Ganoderma lucidum, Poria cocos and Silybum marianum, together with diet derived fatty acids gamma linolenic acid (GLA) and eicosapentanoic acid (EPA) and inhibitory compounds as useful approaches to target tissue invasion and metastasis as well as other hallmark areas of cancer. Together, these strategies could represent new, inexpensive, low toxicity strategies to aid in the management of cancer metastasis as well as having holistic effects against other cancer hallmarks.

Emerging drugs for the treatment of bone metastasis

INTRODUCTION

Bone metastases are virtually incurable resulting in significant disease morbidity, reduced quality of life and mortality. Bone provides a unique microenvironment whose local interactions with tumor cells offer novel targets for therapeutic interventions. Increased understanding of the pathogenesis of bone disease has led to the discovery and clinical utility of bone-targeted agents other than bisphosphonates and denosumab, currently, the standard of care in this setting.

AREAS COVERED

In this review, we present the recent advances in molecular targeted therapies focusing on therapies that inhibit bone resorption and/or stimulate bone formation and novel anti-tumoral agents that exerts significant effects on skeletal metastases, nowadays available in clinical practice or in phase of development.

EXPERT OPINION

New emergent bone target therapies radium-223, mTOR inhibitors, anti-androgens have demonstrated the ability to increase overall survival in bone metastatic patients, other compounds, such as ET-1 and SRC inhibitors, up to now failed to clearly confirm in clinical trials their promising preclinical data.

CXCL8 and CCL20 Enhance Osteoclastogenesis via Modulation of Cytokine Production by Human Primary Osteoblasts

Generalized osteoporosis is common in patients with inflammatory diseases, possibly because of circulating inflammatory factors that affect osteoblast and osteoclast formation and activity. Serum levels of the inflammatory factors CXCL8 and CCL20 are elevated in rheumatoid arthritis, but whether these factors affect bone metabolism is unknown. We hypothesized that CXCL8 and CCL20 decrease osteoblast proliferation and differentiation, and enhance osteoblast-mediated osteoclast formation and activity. Human primary osteoblasts were cultured with or without CXCL8 (2–200 pg/ml) or CCL20 (5–500 pg/ml) for 14 days. Osteoblast proliferation and gene expression of matrix proteins and cytokines were analyzed. Osteoclast precursors were cultured with CXCL8 (200 pg/ml) and CCL20 (500 pg/ml), or with conditioned medium (CM) from CXCL8 and CCL20-treated osteoblasts with or without IL-6 inhibitor. After 3 weeks osteoclast formation and activity were determined. CXCL8 (200 pg/ml) and CCL20 (500 pg/ml) enhanced mRNA expression of KI67 (2.5–2.7-fold), ALP (1.6–1.7-fold), and IL-6 protein production (1.3–1.6-fold) by osteoblasts. CXCL8-CM enhanced the number of osteoclasts with 3–5 nuclei (1.7-fold), and with >5 nuclei (3-fold). CCL20-CM enhanced the number of osteoclasts with 3–5 nuclei (1.3-fold), and with >5 nuclei (2.8-fold). IL-6 inhibition reduced the stimulatory effect of CXCL8-CM and CCL20-CM on formation of osteoclasts. In conclusion, CXCL8 and CCL20 did not decrease osteoblast proliferation or gene expression of matrix proteins. CXCL8 and CCL20 did not directly affect osteoclastogenesis. However, CXCL8 and CCL20 enhanced osteoblast-mediated osteoclastogenesis, partly via IL-6 production, suggesting that CXCL8 and CCL20 may contribute to osteoporosis in rheumatoid arthritis by affecting bone cell communication.

Tumor necrosis factor-α antagonist infliximab inhibits osteoclast formation of peripheral blood mononuclear cells but does not affect periodontal ligament fibroblast-mediated osteoclast formation

BACKGROUND AND OBJECTIVE

The inflammatory cytokine tumor necrosis factor-alpha (TNF-α) is elevated in inflamed periodontal tissues and may contribute to periodontitis progression. TNF-α stimulates formation and activity of osteoclasts, the cells that are recruited in periodontitis, that cause alveolar bone degradation and subsequent tooth loss. We previously showed that TNF-α is elevated in co-cultures of periodontal ligament fibroblast (PDLF) and peripheral blood mononuclear cells (PBMC). Hence, TNF-α could be a determining factor in osteoclast formation in these cultures, as osteoclasts are formed despite the fact that prototypical osteoclast differentiation factor receptor activator of nuclear factor kappa-B ligand is outnumbered at least 100-fold by its inhibitor osteoprotegerin in these cultures.

MATERIAL AND METHODS

To assess the role of TNF-α in periodontitis-associated osteoclast formation in vitro, osteoclast formation was analyzed in the presence of the anti-TNF-α therapeutic agent infliximab in two culture systems: (i) PBMC in co-culture with PDLFs from controls and patients with periodontitis, or (ii) with PBMC only. PDLFs from control and patients with periodontitis were exposed to infliximab, PBMCs were added and the formation of osteoclast-like cells was assessed.

RESULTS

TNF-α was highest levels in supernatants at 7 d in co-cultures and declined at 14 and 21 d. TNF-α was undetectable in cultures that received infliximab. The formation and activity of osteoclasts in co-cultures was not affected by infliximab. In contrast, infliximab in cultures of only PBMC significantly reduced the formation of osteoclasts. This reduction was accompanied by a decreased number and size of cell clusters, a step that precedes the formation of osteoclasts. TNF-α was again undetectable in the supernatant of infliximab-treated cultures, but was detectable at similar levels in cell lysates of control and infliximab-treated PBMC cultures.

CONCLUSION

Our study shows that the contribution of TNF-α to osteoclast formation is cell system dependent. It contributes to PBMC-induced osteoclast formation, possibly by establishing stronger cell-cell interactions that precede osteoclast formation.

Perspectives on osteoporosis therapies

Osteoporosis is a skeletal disease which predisposes to fragility fractures with high morbidity and economic impact, and, therefore, the goal of any osteoporosis treatment is to reduce the fracture risk. In the various forms of osteoporosis an imbalance between bone resorption and apposition is present, that generally leads to a reduction of bone mineral density and bone quality, and finally to the increased fracture risk. Nowadays, several drugs are available with a demonstrated anti-fracturative effect obtained by inhibiting bone resorption or stimulating bone formation. However, their use is not free from limitations and side effects. Importantly, to date, the available antiresorptive drugs have also an inhibiting, though to a lesser extent, effect on bone apposition and, similarly, the anabolic drugs lead to an increase also of bone resorption. Advances in our knowledge about bone biology, with molecular insights into mechanisms underlying osteoblast, osteoclast, and osteocyte activity, have led to the recognition of new potential targets and consequently to the formulation of new therapeutic agents to treat osteoporosis. New potential developments among the antiresorptive drugs include cathepsin K inhibitors and among the osteoanabolic drugs those activating the Wnt signaling pathway, such as the monoclonal antibodies against sclerostin. The novelty of these compounds is that their mechanism of action gives the exciting possibility to uncouple bone resorption and bone formation, and data available so far appear to be promising. Finally, several new therapeutic targets are under investigation in preclinical studies which could open further approaches to treat osteoporosis in the future.

Src inhibition potentiates antitumoral effect of paclitaxel by blocking tumor-induced angiogenesis

The protein kinase Src is frequently over-activated in advanced cancers where it modulates the signaling transduction cascade of several growth factors. The feasibility of combination treatment of Src inhibitors with chemotherapy is currently under investigation. We evaluated the anti-tumoral effect of paclitaxel (PTX) in combination with S13, a tyrosine kinase inhibitor with a prevalent specificity for Src, in a hormone-insensible prostate cancer (PCa) cell model. In vivo, combination treatment with PTX and S13 reduced dramatically PCa tumor growth with a relevant difference in the density of new blood vessels with respect to control and single treatments. This reduction was determined by a concomitant impairment of endothelial cell migration and of VEGF release by cancer cells. In fact, S13, when used alone, was sufficient to reduce tubule formation in vivo, and to inhibit VEGFR2 activation and FAK expression in endothelial cells. In addition, the combination treatment determined a significant reduction in ROS production and HIF-1 stabilization in PCa cells respect to single treatments with S13 or PTX. In conclusion, Src-inhibition could be an effective therapeutic strategy aimed at supporting the anti-angiogenic action of PTX in aggressive PCa.

A challenge with Porphyromonas gingivalis differentially affects the osteoclastogenesis potential of periodontal ligament fibroblasts from periodontitis patients and non-periodontitis donors

AIM

Porphyromonas gingivalis (Pg) may cause an immune-inflammatory response in host cells leading to bone degradation by osteoclasts. We investigated the osteoclast-inducing capacity of periodontal ligament fibroblasts from periodontitis patients and non-periodontitis donors after a challenge with viable Pg.

MATERIALS AND METHODS

PDLFs from periodontitis patients (n = 8) and non-periodontitis donors (n = 7) were incubated for 6 h with or without viable Pg and subsequently co-cultured with osteoclast precursors from peripheral blood mononuclear cells (PBMCs). The number of multinucleated tartrate-resistant acid phosphatase-positive cells was determined at 21 days. Expression of osteoclastogenesis-associated genes was assessed after infection of PDLFs mono-cultures and in PDLFs-PBMCs co-cultures. Resorption activity was analysed on bone slices.

RESULTS

Pg induced the expression of osteoclastogenesis-associated genes by PDLFs. After bacterial challenge the formation of osteoclast-like cell was decreased in co-cultures of PBMCs with non-periodontitis PDLFs, but not with PDLFs from periodontitis patients.

CONCLUSION

PDLFs from sites free of periodontitis respond to an infection with Pg by tempering formation of osteoclast-like cells, probably promoting clearance of the infection. PDLFs from periodontitis sites are desensitized to a Pg challenge in terms of their osteoclast-inducing capacity.

Treatment options in castration-resistant prostate cancer: current therapies and emerging docetaxel-based regimens

BACKGROUND

Docetaxel-based chemotherapy remains the standard of care for metastatic castration-resistant prostate cancer (mCRPC) and it is the only globally approved first-line therapy. Although docetaxel offers improved survival for this patient population, it is also associated with toxicity and resistance in many patients, representing a need for more efficacious therapies. Preclinical advances have led to improved understanding of the molecular biology of prostate cancer, and targeted therapies that exploit the signaling pathways and molecular targets that underscore the disease are being clinically investigated in combination with docetaxel.

DESIGN

This article briefly highlights recent data from phase III trials in mCRPC that have led to agent approval. This article also reviews phase II and III trials in which docetaxel-based regimens have been investigated in mCRPC.

RESULTS

Recently approved agents, including sipuleucel-T, cabazitaxel, abiraterone acetate, and enzalutamide, have diversified the mCRPC treatment landscape. Phase III trials evaluating docetaxel in combination with targeted therapies, including potent oral tyrosine kinase inhibitor, dasatinib, in the READY trial and clusterin inhibitor, custirsen, in the SYNERGY trial, are currently ongoing.

CONCLUSIONS

In combination with docetaxel, targeted agents dasatinib and custirsen will likely expand the existing treatment paradigm for mCRPC if results from phase III trials are positive.

Long-term use of dasatinib in patients with metastatic castration-resistant prostate cancer after receiving the combination of dasatinib and docetaxel

Dasatinib is a potent oral tyrosine kinase inhibitor which targets several kinases, including the SRC family kinases. SRC family kinases have been implicated in androgen therapy resistance that often develops in metastatic castration-resistant prostate cancer (mCRPC), which drives the need for non-androgen targeting therapies. This article describes the preclinical rationale for the use of combination dasatinib and docetaxel therapy in mCRPC, and highlights the results of a phase I–II trial in which 46 patients with mCRPC, treated with a regimen of dasatinib and docetaxel, demonstrated improvements in bone scans, high rates of soft tissue responses, and modulation of markers of bone turnover. This brief report discusses in detail follow-up data on two patients who remain alive after >2.5 years on dasatinib single-agent therapy after discontinuing docetaxel treatment.

Targeted therapies in metastatic castration-resistant prostate cancer: beyond the androgen receptor

Prostate cancer is the most common male cancer and one of the top causes of male cancer-related death in Western countries. Most patients with prostate cancer respond to initial androgen deprivation therapy but eventually progress to castration-resistant prostate cancer (CRPC). Although androgen receptor signaling remains the main driver in CRPC, a growing body of evidence suggests that other pathways are involved in this progression. This article reviews the preclinical data and current status of clinical trials therapeutically targeting tubulin, DNA repair, molecular chaperones such as CLU and Hsp27, tyrosine kinases, and DNA repair.

An update on dual Src/Abl inhibitors

c-Src and Bcr-Abl are two cytoplasmatic tyrosine kinases (TKs) involved in the development of malignancies. In particular, Bcr-Abl is the etiologic agent of chronic myeloid leukemia, where Src is also involved; the latter is hyperactivated in several solid tumors. Because of the structural homology between Src and Abl, several compounds originally synthesized as Src inhibitors have also been shown to be Abl inhibitors, useful in overcoming the onset of some types of chronic myeloid leukemia resistances, which frequently appear in the advanced phases of pathology. In recent years, the development of such compounds has been promoted by both excellent preclinical and clinical results, and by the theory that dual or multi-targeted inhibitors might be more effective than selective inhibitors. This review is an update on the most important dual inhibitors already in clinical trials and includes information regarding compounds that have appeared in the literature in recent years.

Recent developments in treatments targeting castration-resistant prostate cancer bone metastases

BACKGROUND

Prostate cancer is the most common male cancer and one of the top causes of male cancer-related death. Most patients with prostate cancer respond to initial androgen deprivation therapy before progressing to castration-resistant prostate cancer (CRPC) and eventually developing bone metastases. Growth of prostate cancer metastases in the bone microenvironment produces numerous factors that disrupt the dynamic equilibrium of osteogenesis and osteolysis existing in healthy bone, leading to progressive morbidity, poor quality of life, and increased treatment costs.

MATERIALS AND METHODS

Relevant studies of CRPC and targeted therapies were identified from literature and clinical trial databases, websites, and conference abstracts.

RESULTS

Available data on agents potentially targeting bone metastatic CRPC or the bone microenvironment in patients with CRPC are discussed, including inhibitors of tumor growth/survival and bone turnover (SRC family kinase inhibitors, endothelin-1 inhibitors, MET inhibitors, and thalidomide and its derivatives), inhibitors of bone turnover (bisphosphonates and receptor activator of nuclear factor-kB ligand inhibitors), antiangiogenic agents (vascular endothelial growth factor receptor and platelet-derived growth factor blockers), prostate cancer vaccines, and bone-directed radiopharmaceuticals.

CONCLUSIONS

With increasing data availability demonstrating tumor-bone microenvironment interactions and routine incorporation of bone-related end points into CRPC trials, bone microenvironment-targeted agents are likely to become an increasingly important component of CRPC treatment.

Src-signaling interference impairs the dissemination of blood-borne tumor cells

Although solid tumors continuously shed cells, only a small fraction of the neoplastic cells that enter the blood stream are capable of establishing metastases. In order to be successful, these cells must attach, extravasate, proliferate and induce angiogenesis. Preclinical studies have shown that small-molecule ATP-competitive Src kinase inhibitors can effectively impair metastasis-associated tumor cell functions in vitro. However, the impact of these agents on the metastatic cascade in vivo is less well understood. In the present studies, we have examined the ability of saracatinib, a dual-specific, orally available inhibitor of Src and Abl protein tyrosine kinases, to interfere with the establishment of lung metastases in mice by tumor cells introduced into the blood stream. The results demonstrate that Src inhibition most effectively interferes with the establishment of secondary tumor deposits when treatments are administered while tumor cells are in the initial phases of dissemination.

Effects of Src kinase inhibition by saracatinib (AZD0530) on bone turnover in advanced malignancy in a Phase I study

Saracatinib (AZD0530) is an orally active once-daily Src kinase inhibitor which modulates key signaling pathways in cancer cells. In a Phase I study in patients with advanced solid malignancies resistant to standard treatment we assessed the effect of saracatinib on bone turnover. Fifty-one patients were randomized into three parallel groups to receive saracatinib 50, 125 or 175 mg/day. After a single dose followed by a 7-day washout, patients received once-daily doses for 21 days. Bone turnover markers were measured in serum and urine samples collected before dosing on days 1, 2, 3, 17 and 28. Samples were available at baseline and more than one other time point for 44 patients. Bone resorption markers were significantly decreased by saracatinib. Serum cross-linked C-terminal telopeptide of type I collagen (sCTX) changed in the 50, 125 and 175 mg/day groups by -36% (95% CI -58, -4), -64% (95% CI -75, -48) and -75% (95% CI -83, -61), respectively, at day 28. Urinary cross-linked N-terminal telopeptide of type I collagen/creatinine ratio (uNTX/Cr) changed in the 50, 125 and 175 mg/day groups by; -13% (95% CI -33, 13), -48% (95% CI -59, -34) and -50% (95% CI -62, -35), respectively, at day 28. The significant decreases in bone resorption markers indicate that suppression of Src kinase inhibits osteoclast activity in patients with advanced cancer. This result suggests that saracatinib may have therapeutic benefit in metastatic bone disease.

Osteogenic effects of a potent Src-over-Abl-selective kinase inhibitor in the mouse

Src-null mice have higher bone mass because of decreased bone resorption and increased bone formation, whereas Abl-null mice are osteopenic, because of decreased bone formation. Compound I, a potent inhibitor of Src in an isolated enzyme assay (IC(50) 0.55 nM) and a Src-dependent cell growth assay, with lower activity on equivalent Abl-based assays, potently, but biphasically, accelerated differentiation of human mesenchymal stem cells to an osteoblast phenotype (1-10 nM). Compound I (≥0.1 nM) also activated osteoblasts and induced bone formation in isolated neonatal mouse calvariae. Compound I required higher concentrations (100 nM) to inhibit differentiation and activity of osteoclasts. Transcriptional profiling (TxP) of calvaria treated with 1 μM compound I revealed down-regulation of osteoclastic genes and up-regulation of matrix genes and genes associated with the osteoblast phenotype, confirming compound I's dual effects on bone resorption and formation. In addition, calvarial TxP implicated calcitonin-related polypeptide, β (β-CGRP) as a potential mediator of compound I's osteogenic effect. In vivo, compound I (1 mg/kg s.c.) increased vertebral trabecular bone volume 21% (microcomputed tomography) in intact female mice. Increased trabecular volume was also detected histologically in a separate bone, the femur, particularly in the secondary spongiosa (100% increase), which underwent a 171% increase in bone formation rate, a 73% increase in mineralizing surface, and a 59% increase in mineral apposition rate. Similar effects were observed in ovariectomized mice with established osteopenia. We conclude that the Src inhibitor compound I is osteogenic, presumably because of its potent stimulation of osteoblast differentiation and activation, possibly mediated by β-CGRP.

Src inhibitors in the treatment of metastatic bone disease: rationale and clinical data

Src is a nonreceptor tyrosine kinase essential for the activation of osteoclasts, the cells that degrade bone. Src also regulates normal cell functions, cancer cell growth and metastasis to organs, including bone where tumor cells induce bone destruction by osteoclasts. Src inhibitors prevent bone destruction and tumor cell growth in animal models of metastatic bone disease, and some are being investigated in clinical trials, particularly in patients with prostate cancer, which has high bone metastatic potential. Here, we review how Src regulates osteoclast formation, activation and survival and the results of preclinical and clinical trials of Src inhibitors, which show some promise in inhibiting the effects of tumor cells on the skeleton.

Role of Src in breast cancer cell migration and invasion in a breast cell/bone-derived cell microenvironment

The preferential metastasis of breast cancer cells to bone comprises a complex set of events including homing and preferential growth, which may require unique factors produced by bone or other cells in the immediate microenvironment. In this study, an in vitro co-culture system composed of bone mesenchymal stem cells and breast cancer cell lines is used to examine the role of Src kinase on breast cancer cell migration and invasion in the presence of bone-derived cells. This research shows that Src kinase activity in breast cancer cell lines with either high or low levels of endogenous Src activity is increased by bone-derived cell-conditioned medium but not HS68 fibroblast-conditioned medium. Breast cancer cells exhibit enhanced migration in co-culture with bone-derived cells but not HS68 fibroblasts or no co-cultured cells. Inhibition of Src kinase activity using the inhibitors PP2 or saracatinib or using siRNA abrogates the preferential migration of the breast cancer cell lines in response to bone-derived cells. Inhibition of Src activity with saracatinib does not have any significant effect on breast cancer cell invasion in the presence of bone-derived cells. Factors are identified that are produced preferentially by bone-derived cells over HS68 cells that may impact breast cancer cell behavior. This research implicates Src kinase as an important effector of bone-derived cell signals on breast cancer cell migration.

Paracrine-mediated differentiation and activation of human haematopoietic osteoclast precursor cells by skin and gingival fibroblasts

OBJECTIVE

Fibroblasts appear to modulate osteoclastogenesis, but their precise role in this process remains unclear. In this work, paracrine-mediated osteoclastogenic potential of different human fibroblasts was assessed.

MATERIALS AND METHODS

Fibroblast-conditioned media (CM) from foetal skin (CM1), adult skin (CM2) and adult gingiva (CM3) were used to promote osteoclastogenesis of osteoclast precursor cells. Cultures supplemented with macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL) were used as controls.

RESULTS

All fibroblast cultures expressed FSP-1, M-CSF and RANKL and produced osteoprotegerin (OPG); gingival fibroblasts presented lowest expression of osteoclastogenic genes and higher production of OPG. All fibroblast CM were able to induce osteoclastogenesis. CM1 showed behaviour similar to positive controls, and slightly higher osteoclastogenic potential than CM, from adult ones. Gingival fibroblasts revealed lowest osteoclastogenic ability. Presence of anti-MCSF or anti-RANKL partially inhibited osteoclastogenesis promoted by CM, although the former antibody revealed higher inhibitory response. Differences among the osteoclastogenic effect of CM were noted, mainly in expression of genes involved in differentiation and activation of osteoclast precursor cells, c-myc and c-src, and less regarding functional related parameters.

CONCLUSIONS

Fibroblasts are able to induce osteoclastogenesis by paracrine mechanisms, and age and anatomical location affect this ability. Other factors produced by fibroblasts, in addition to M-CSF and RANKL, appear to contribute to observed osteoclastogenic potential.

Synthesis of 3-phenylpyrazolopyrimidine-1,2,3-triazole conjugates and evaluation of their Src kinase inhibitory and anticancer activities

A series of two classes of 3-phenylpyrazolopyrimidine-1,2,3-triazole conjugates were synthesized using click chemistry approach. All compounds were evaluated for inhibition of Src kinase and human ovarian adenocarcinoma (SK-Ov-3), breast carcinoma (MDA-MB-361), and colon adenocarcinoma (HT-29). Hexyl triazolyl-substituted 3-phenylpyrazolopyrimidine exhibited inhibition of Src kinase with an IC(50) value of 5.6 μM. 4-Methoxyphenyl triazolyl-substituted 3-phenylpyrazolopyrimidine inhibited the cell proliferation of HT-29 and SK-Ov-3 by 73% and 58%, respectively, at a concentration of 50 μM.

DMSO regulates osteoclast development in vitro

Dimethyl sulfoxide (DMSO) is routinely used in the laboratory as a solvent and vehicle for organic molecules. Although it has been used in previous studies involving myeloid cells and macrophages, we are unaware of data demonstrating the effects of DMSO alone on osteoclast development. Recently, we were using DMSO as a vehicle and included a non-vehicle control. Surprisingly, we observed a marked change in osteoclast development, and therefore designed this study to examine the effects of DMSO on osteoclast development. Osteoclasts were generated from two sources: bone marrow macrophages and an osteoclast progenitor cell line. Cells were cultured with DMSO for various durations and at differing concentrations and mature, multinucleated (>3 nuclei) TRAP(+) cells were assessed in terms of cell number, cell surface area, and number of nuclei/cell. Osteoclast surface area increased in 5 μM DMSO to a mean of 156,422 pixels from a mean of 38,510 pixels in control culture, and subsequently decreased in 10 μM DMSO to a mean of 18,994 pixels. With serial addition of DMSO over 5 d, a significant increase in mean surface area, and number of nuclei/cell was also observed, while the opposite was true when DMSO was serially removed from culture. These findings show that DMSO exerts a marked effect on osteoclast differentiation. Since many investigators use DMSO to solubilize compounds for treatment of osteoclasts, caution is warranted as altering DMSO concentrations may have a profound effect on the final data, especially if osteoclast differentiation is being assessed.

Castration-resistant prostate cancer: targeted therapies and individualized treatment

Various molecular mechanisms have been implicated in the progression from hormone-sensitive to castration-resistant prostate cancer (CRPC). Novel targeted agents to treat CRPC have been developed that inhibit either androgen receptor (AR)-mediated signaling (AR antagonists and inhibitors of androgen synthesis) or non-AR-mediated signaling (inhibitors of Src, mammalian target of rapamycin, chaperone proteins, insulin-like growth factor-1 receptor, vascular endothelial growth factor, and endothelin-A receptor) pathways. However, variable efficacy has been observed in clinical trials, most likely because of the biologic heterogeneity of CRPC. To account for potential differences in disease biology, a more individualized approach to treatment, based on genomic and/or proteomic analyses of individual tumors, is being investigated. By identifying tumors with a characteristic molecular subtype and assigning treatment accordingly, it is hoped that a higher proportion of patients will benefit from targeted therapy. Additionally, lessons learned through the application of these technologies to prostate cancer may subsequently influence therapeutic development in other solid tumors.

Evolving role of bone biomarkers in castration-resistant prostate cancer

The preferential metastasis of prostate cancer cells to bone disrupts the process of bone remodeling and results in lesions that cause significant pain and patient morbidity. Although prostate-specific antigen (PSA) is an established biomarker in prostate cancer, it provides only limited information relating to bone metastases and the treatment of metastatic bone disease with bisphosphonates or novel noncytotoxic targeted or biological agents that may provide clinical benefits without affecting PSA levels. As bone metastases develop, factors derived from bone metabolism are released into blood and urine, including N- and C-terminal peptide fragments of type 1 collagen and bone-specific alkaline phosphatase, which represent potentially useful biomarkers for monitoring metastatic bone disease. A number of clinical trials have investigated these bone biomarkers with respect to their diagnostic, prognostic, and predictive values. Results suggest that higher levels of bone biomarkers are associated with an increased risk of skeletal-related events and/or death. As a result of these findings, bone biomarkers are now being increasingly used as study end points, particularly in studies investigating novel agents with putative bone effects. Data from prospective clinical trials are needed to validate the use of bone biomarkers and to confirm that marker levels provide additional information beyond traditional methods of response evaluation for patients with metastatic prostate cancer.

Oestrogen inhibits osteoclast formation induced by periodontal ligament fibroblasts

OBJECTIVE

Since tooth-associated fibroblasts are taken to participate in the formation of osteoclasts and it is unknown whether oestrogen affects this process, the effects of 17β-estradiol (17β-E(2)) were studied on osteoclastogenesis induced by human periodontal ligament fibroblasts (PLFs) and gingival fibroblasts (GFs).

METHODS

Human peripheral blood mononuclear cells (PBMCs) were seeded on monolayers of PLFs and GFs and cocultured for 14 days in the presence or absence of various concentrations of 17β-E(2). The number of tartrate resistant acid phosphatase (TRACP)-positive osteoclast-like cells (OCs) was assessed. In addition, we analysed the PBMC-induced withdrawal of the fibroblasts. mRNA expression was determined of oestrogen receptor (ER)-α, ER-β, receptor activator nuclear factor kappa B ligand (RANKL), and osteoprotegerin (OPG) by PLFs and GFs.

RESULTS

PBMCs induced a higher number and larger fibroblast-free areas if cocultured with PLFs than with GFs. Concomitantly, the number of TRACP-positive OCs was significantly higher in PLF cocultures. 17β-E(2) inhibited the formation of OCs in PLF cocultures. 17β-E(2) did not alter the expression of RANKL, OPG, and ER-α mRNAs in either fibroblast cell population.

CONCLUSION

Our data indicate that PLFs may promote osteoclastogenesis more strongly than GFs. 17β-E(2) inhibits the PLF-induced formation of osteoclast-like cells. Thus, the inhibitory effect of oestrogen on osteoclast formation appears to be cell type dependent.

Src inhibitors in lung cancer: current status and future directions

Src tyrosine kinases regulate multiple genetic and signaling pathways involved in the proliferation, survival, angiogenesis, invasion, and migration of various types of cancer cells They are frequently expressed and activated in many cancer types, including lung cancer. Several Src inhibitors, including dasatinib, saracatinib, bosutinib, and KX2-391, are currently being investigated in clinical trials. Preliminary results of the use of single-agent Src inhibitors in unselected patients with lung cancer show that these inhibitors have a favorable safety profile and anticancer activity. Their combination with cytotoxic chemotherapy, other targeted therapy, and radiation therapy is currently being explored. In this review, we summarize the rationale for and the current status of Src inhibitor development and discuss future directions based on emerging preclinical data.

Functional osteoclast attachment requires inositol-1,4,5-trisphosphate receptor-associated cGMP-dependent kinase substrate

Osteoclast activity is central to balanced bone turnover to maintain normal bone mass. A specialized osteoclast attachment to bone localizes acid secretion to remove bone mineral; in some cases, attachment is functionally impaired despite normal attachment proteins. The inositol-1,4,5-trisphosphate receptor-1 (IP3R1) is an intracellular calcium channel required for regulation of reversible osteoclast attachment by nitric oxide (NO), an important regulator of both normal and pathological bone degradation. In studies using human osteoclasts produced in vitro, we found that IP3R1 binds an endosomal isoform of the IP3R-associated cGMP-dependent kinase substrate (IRAG). IRAG is a substrate of cGMP-dependent kinase-1 (PKG1) and binds the PKG1 isoform PKG1β, which was the predominant form of PKG1 in human osteoclasts. Western blots of IRAG were consistent with NO-dependent serine phosphorylation of IRAG. An additional effect of PKG1β activity in osteoclasts was disassociation of IP3R1-IRAG complexes, as shown by analysis of IP3R1 complexes and by localization of the proteins within cells. IP3R1-IRAG complexes were stabilized by PKG or Src antagonists, Src activity being a requirement for IP3R1 calcium release downstream of PKG. IP3R1-mediated calcium release regulates cellular detachment in part through the calcium-dependent proteinase μ-calpain. In osteoclasts with IRAG suppressed by siRNA, activity of μ-calpain was increased relative to cells with normal IRAG, and regulation of μ-calpain by NO was lost. Furthermore, cells deficient in IRAG detached easily from substrate and had smaller attached diameters and randomly distributed podosomes, although IRAG knockdown did not affect cell viability. Our results indicate that IRAG is required for PKG1β-regulated cyclic calcium release during motility, and that disruption of the IP3R1-IRAG calcium regulation system is a novel cause of dysfunctional osteoclasts unrelated to defects in attachment proteins or acid secretion.

Future directions of bone-targeted therapy for metastatic breast cancer

Bone is the most common metastatic site for breast cancer, and bone metastases can cause pain as well as risk of pathological fractures. Emerging treatments for metastatic bone disease have arisen from advances in our understanding of the unique cellular and molecular mechanisms that contribute to bone metastasis. The interaction between tumor cells and the bone microenvironment results in a 'vicious cycle' that increases both bone destruction and tumor burden. The tumor secretes factors, such as parathyroid hormone-related peptide, that stimulate osteoclastogenesis. Similarly, the bone stroma produces growth factors, such as transforming growth factor β, that promote tumor growth in bone. Therapeutic targeting of these microenvironmental factors is under intensive investigation. Other attractive therapeutic targets include signaling molecules, such as receptor activator of nuclear factor κB ligand, Src kinase, and cathepsin K, all of which regulate osteoclast function, and chemokine receptor 4, which is involved in the homing of tumor cells to bone. In this Review, we describe the progress and future directions of novel bone-targeted therapies that may reduce or prevent destructive bone metastasis from breast cancer. Novel modalities for predicting and monitoring treatment response will also be described.

Src kinase inhibitors: an emerging therapeutic treatment option for prostate cancer

IMPORTANCE OF THE FIELD

Once prostate cancer becomes castration-resistant, bone metastases are a significant problem and treatment options are limited. As a result, there is a need for more effective therapies that have antitumor and anti-bone metastatic effects. Because Src and Src-family kinases (SFKs) are involved in multiple signaling pathways central to prostate cancer development, progression, and metastasis, in addition to normal and pathologic osteoclast activities, Src inhibition represents a valid therapeutic strategy for investigation.

AREAS COVERED IN THIS REVIEW

Here, current treatment options for advanced prostate cancer, the preclinical rationale behind using Src inhibitors, emerging data from clinical trials of Src inhibitors in prostate cancer, and future therapeutic directions are described. Data published in peer-reviewed journals within the last 20 years or presented at recent European or American Society of Clinical Oncology conferences have been reviewed.

WHAT THE READER WILL GAIN

Readers will gain an insight into the development of therapeutic Src inhibitors, including dasatinib and saracatinib; an understanding of their effects on prostate cancer cells and the bone microenvironment; and emerging clinical data.

TAKE HOME MESSAGE

Src is implicated in prostate cancer progression and metastasis, therefore treatment with Src inhibitors warrants further investigation.

The Src inhibitor dasatinib accelerates the differentiation of human bone marrow-derived mesenchymal stromal cells into osteoblasts

BACKGROUND

The proto-oncogene Src is an important non-receptor protein tyrosine kinase involved in signaling pathways that control cell adhesion, growth, migration and differentiation. It negatively regulates osteoblast activity, and, as such, its inhibition is a potential means to prevent bone loss. Dasatinib is a new dual Src/Bcr-Abl tyrosine kinase inhibitor initially developed for the treatment of chronic myeloid leukemia. It has also shown promising results in preclinical studies in various solid tumors. However, its effects on the differentiation of human osteoblasts have never been examined.

METHODS

We evaluated the effects of dasatinib on bone marrow-derived mesenchymal stromal cells (MSC) differentiation into osteoblasts, in the presence or absence of a mixture of dexamethasone, ascorbic acid and beta-glycerophosphate (DAG) for up to 21 days. The differentiation kinetics was assessed by evaluating mineralization of the extracellular matrix, alkaline phosphatase (ALP) activity, and expression of osteoblastic markers (receptor activator of nuclear factor kappa B ligand [RANKL], bone sialoprotein [BSP], osteopontin [OPN]).

RESULTS

Dasatinib significantly increased the activity of ALP and the level of calcium deposition in MSC cultured with DAG after, respectively, 7 and 14 days; it upregulated the expression of BSP and OPN genes independently of DAG; and it markedly downregulated the expression of RANKL gene and protein (decrease in RANKL/OPG ratio), the key factor that stimulates osteoclast differentiation and activity.

CONCLUSIONS

Our results suggest a dual role for dasatinib in both (i) stimulating osteoblast differentiation leading to a direct increase in bone formation, and (ii) downregulating RANKL synthesis by osteoblasts leading to an indirect inhibition of osteoclastogenesis. Thus, dasatinib is a potentially interesting candidate drug for the treatment of osteolysis through its dual effect on bone metabolism.

Effect of the specific Src family kinase inhibitor saracatinib on osteolytic lesions using the PC-3 bone model

The hematogenous metastatic spread of prostate cancer is preferentially to bone and can result in significant patient morbidity. Although these metastatic lesions are typically osteoblastic, bone resorption is believed to have a prerequisite role in their development. Src kinase has been identified to contribute to prostate cancer tumor growth and metastasis. In addition, Src is also essential in bone metabolism, especially in bone resorption. We hypothesized that inhibiting Src activity with the specific Src family kinase inhibitor saracatinib (AZD0530) would inhibit tumor cell growth and osteoclast differentiation in the tumor-bone interface, thus providing a new approach for advanced prostate cancer. We found that saracatinib inhibited PC-3 cell growth and invasion in a dose-dependent manner. Phosphorylation of Src, focal adhesion kinase, and P38 kinases was inhibited by saracatinib at the submicromolar range. Saracatinib also inhibited the expression and secretion of invasion-related molecules interlukin-8, urokinase-type plasminogen activator, and matrix metalloprotease-9. Receptor activator of NF-kappaB ligand (RANKL)-induced osteoclastogenesis and signaling were inhibited by saracatinib in both macrophages and PC-3 cells. In in vivo studies, control mice developed more severe osteolytic lesions compared with the treatment group. Immunohistochemical and biochemical assays of bone metabolites confirmed that saracatinib preserved bone architecture in the presence of prostate cancer tumor cells. In summary, we have shown the inhibition of PC3 cell growth and invasion by saracatinib. Src inhibition also blocked the RANKL stimulatory pathway in osteoclasts and PC3 cells. The inhibition of Src thus targets multiple sites involved in prostate cancer bone metastasis, which may offer a therapeutic advantage in treating advanced prostate cancer.

Combining Src inhibitors and aromatase inhibitors: a novel strategy for overcoming endocrine resistance and bone loss

Aromatase inhibitors have largely replaced tamoxifen as the first-line treatment for postmenopausal women with metastatic, hormone receptor-positive (HR+) breast cancer. However, many patients develop clinical resistance with prolonged treatment, and oestrogen deprivation following aromatase inhibition can result in loss of bone mineral density. Furthermore, most patients with metastatic breast cancer develop bone metastases, and the resulting adverse skeletal-related events are a significant cause of patient morbidity. Src, a non-receptor tyrosine kinase, is a component of signalling pathways that regulate breast cancer cell proliferation, invasion and metastasis as well as osteoclast-mediated bone turnover. Preclinical evidence also suggests a role for Src in acquired endocrine resistance. As such, Src inhibition represents a logical strategy for the treatment of metastatic breast cancer. In vitro, combination therapy with Src inhibitors and endocrine agents, including aromatase inhibitors, has been shown to inhibit the proliferation and metastasis of both endocrine-responsive and endocrine-resistant breast cancer cell lines more effectively than either of the therapy alone. Src inhibition has also been shown to suppress osteoclast formation and activity. Combination therapy with aromatase inhibitors and Src inhibitors therefore represents a novel approach through which the development of both acquired resistance and bone pathology could be delayed. Data from clinical trials utilising such combinations will reveal if this strategy has the potential to improve patient outcomes.