Localized, tumor-associated osteolysis involves the recruitment and activation of osteoclasts

Intrathecal administration of Resolvin D1 and E1 decreases hyperalgesia in mice with bone cancer pain: Involvement of endocannabinoid signaling

Pain produced by bone cancer is often severe and difficult to treat. Here we examined effects of Resolvin D1 (RvD1) or E1 (RvE1), antinociceptive products of ω-3 polyunsaturated fatty acids, on cancer-induced mechanical allodynia and heat hyperalgesia. Experiments were performed using a mouse model of bone cancer produced by implantation of osteolytic ficrosarcoma into and around the calcaneus bone. Mechanical allodynia and heat hyperalgesia in the tumor-bearing paw were assessed by measuring withdrawal responses to a von Frey monofilament and to radiant heat applied on the plantar hind paw. RvD1, RvE1, and cannabinoid receptor antagonists were injected intrathecally. Spinal content of endocannabinoids was evaluated using UPLC-MS/MS analysis. RvD1 and RvE1 had similar antinociceptive potencies. ED_50s for RvD1 and RvE1 in reducing mechanical allodynia were 0.2 pg (0.53 fmol) and 0.6 pg (1.71 fmol), respectively, and were 0.3 pg (0.8 fmol) and 0.2 pg (0.57 fmol) for reducing heat hyperalgesia. Comparisons of dose-response relationships showed equal efficacy for reducing mechanical allodynia, however, efficacy for reducing heat hyperalgesia was greater for of RvD1. Using UPLC-MS/MS we determined that RvD1, but not RvE1, increased levels of the endocannabinoids Anandamide and 2-Arachidonoylglycerol in the spinal cord. Importantly, Resolvins did not alter acute nociception or motor function in naïve mice. Our data indicate, that RvD1 and RvE1 produce potent antiallodynia and antihyperalgesia in a model of bone cancer pain. RvD1 also triggers spinal upregulation of endocannabinoids that produce additional antinociception predominantly through CB2 receptors.

Spinal miRNA-124 regulates synaptopodin and nociception in an animal model of bone cancer pain

Strong breakthrough pain is one of the most disabling symptoms of cancer since it affects up to 90% of cancer patients and is often refractory to treatments. Alteration in gene expression is a known mechanism of cancer pain in which microRNAs (miRNAs), a class of non-coding regulatory RNAs, play a crucial role. Here, in a mouse model of cancer pain, we show that miR-124 is down-regulated in the spinal cord, the first relay of the pain signal to the brain. Using in vitro and in vivo approaches, we demonstrate that miR-124 is an endogenous and specific inhibitor of synaptopodin (Synpo), a key protein for synaptic transmission. In addition, we demonstrate that Synpo is a key component of the nociceptive pathways. Interestingly, miR-124 was down-regulated in the spinal cord in cancer pain conditions, leading to an up-regulation of Synpo. Furthermore, intrathecal injections of miR-124 mimics in cancerous mice normalized Synpo expression and completely alleviated cancer pain in the early phase of the cancer. Finally, miR-124 was also down-regulated in the cerebrospinal fluid of cancer patients who developed pain, suggesting that miR-124 could be an efficient analgesic drug to treat cancer pain patients.

Dissociation between the relief of skeletal pain behaviors and skin hypersensitivity in a model of bone cancer pain

Recent studies have suggested that in humans and animals with significant skeletal pain, changes in the mechanical hypersensitivity of the skin can be detected. However, whether measuring changes in skin hypersensitivity can be a reliable surrogate for measuring skeletal pain itself remains unclear. To explore this question, we generated skeletal pain by injecting and confining GFP-transfected NCTC 2472 osteosarcoma cells unilaterally to the femur of C3H male mice. Beginning at day 7 post-tumor injection, animals were administered vehicle, an antibody to the P2X3 receptor (anti-P2X3) or anti-NGF antibody. Pain and analgesic efficacy were then measured on days 21, 28, and 35 post-tumor injection using a battery of skeletal pain-related behaviors and von Frey assessment of mechanical hypersensitivity on the plantar surface of the hind paw. Animals with bone cancer pain treated with anti-P2X3 showed a reduction in skin hypersensitivity but no attenuation of skeletal pain behaviors, whereas animals with bone cancer pain treated with anti-NGF showed a reduction in both skin hypersensitivity and skeletal pain behaviors. These results suggest that although bone cancer can induce significant skeletal pain-related behaviors and hypersensitivity of the skin, relief of hypersensitivity of the skin is not always accompanied by attenuation of skeletal pain. Understanding the relationship between skeletal and skin pain may provide insight into how pain is processed and integrated and help define the preclinical measures of skeletal pain that are predictive end points for clinical trials.

Use of Animal Models in Understanding Cancer-induced Bone Pain

Many common cancers have a propensity to metastasize to bone. Although malignancies often go undetected in their native tissues, bone metastases produce excruciating pain that severely compromises patient quality of life. Cancer-induced bone pain (CIBP) is poorly managed with existing medications, and its multifaceted etiology remains to be fully elucidated. Novel analgesic targets arise as more is learned about this complex and distinct pain state. Over the past two decades, multiple animal models have been developed to study CIBP’s unique pathology and identify therapeutic targets. Here, we review animal models of CIBP and the mechanistic insights gained as these models evolve. Findings from immunocompromised and immunocompetent host systems are discussed separately to highlight the effect of model choice on outcome. Gaining an understanding of the unique neuromolecular profile of cancer pain through the use of appropriate animal models will aid in the development of more effective therapeutics for CIBP.

NGF blockade at early times during bone cancer development attenuates bone destruction and increases limb use

Studies in animals and humans show that blockade of nerve growth factor (NGF) attenuates both malignant and nonmalignant skeletal pain. While reduction of pain is important, a largely unanswered question is what other benefits NGF blockade might confer in patients with bone cancer. Using a mouse graft model of bone sarcoma, we demonstrate that early treatment with an NGF antibody reduced tumor-induced bone destruction, delayed time to bone fracture, and increased the use of the tumor-bearing limb. Consistent with animal studies in osteoarthritis and head and neck cancer, early blockade of NGF reduced weight loss in mice with bone sarcoma. In terms of the extent and time course of pain relief, NGF blockade also reduced pain 40% to 70%, depending on the metric assessed. Importantly, this analgesic effect was maintained even in animals with late-stage disease. Our results suggest that NGF blockade immediately upon detection of tumor metastasis to bone may help preserve the integrity and use, delay the time to tumor-induced bone fracture, and maintain body weight.

Dissecting the role of bone marrow stromal cells on bone metastases

Tumor-induced bone disease is a dynamic process that involves interactions with many cell types. Once metastatic cancer cells reach the bone, they are in contact with many different cell types that are present in the cell-rich bone marrow. These cells include the immune cells, myeloid cells, fibroblasts, osteoblasts, osteoclasts, and mesenchymal stem cells. Each of these cell populations can influence the behavior or gene expression of both the tumor cells and the bone microenvironment. Additionally, the tumor itself can alter the behavior of these bone marrow cells which further alters both the microenvironment and the tumor cells. While many groups focus on studying these interactions, much remains unknown. A better understanding of the interactions between the tumor cells and the bone microenvironment will improve our knowledge on how tumors establish in bone and may lead to improvements in diagnosing and treating bone metastases. This review details our current knowledge on the interactions between tumor cells that reside in bone and their microenvironment.

Increased anandamide uptake by sensory neurons contributes to hyperalgesia in a model of cancer pain

Opioids do not effectively manage pain in many patients with advanced cancer. Because anandamide (AEA) activation of cannabinoid type-1 receptors (CB1R) on nociceptors reduces nociception, manipulation of AEA metabolism in the periphery may be an effective alternative or adjuvant therapy in the management of cancer pain. AEA is hydrolyzed by the intracellular enzyme fatty acid amide hydrolase (FAAH), and this enzyme activity contributes to uptake of AEA into neurons and to reduction of AEA available to activate CB1R. We used an in vitro preparation of adult murine dorsal root ganglion (DRG) neurons co-cultured with fibrosarcoma cells to investigate how tumors alter the uptake of AEA into neurons. Evidence that the uptake of [(3)H]AEA into dissociated DRG cells in the co-culture model mimicked the increase in uptake that occurred in DRG cells from tumor-bearing mice supported the utility of the in vitro model to study AEA uptake. Results with the fluorescent AEA analog CAY10455 confirmed that an increase in uptake in the co-culture model occurred in neurons. One factor that contributed to the increase in [(3)H]AEA uptake was an increase in total cellular cholesterol in the cancer condition. Treatment with the FAAH inhibitor URB597 reduced CAY10455 uptake in the co-culture model to the level observed in DRG neurons maintained in the control condition (i.e., in the absence of fibrosarcoma cells), and this effect was paralleled by OMDM-1, an inhibitor of AEA uptake, at a concentration that had no effect on FAAH activity. Maximally effective concentrations of the two drugs together produced a greater reduction than was observed with each drug alone. Treatment with BMS309403, which competes for AEA binding to fatty acid binding protein-5, mimicked the effect of OMDM-1 in vitro. Local injection of OMDM-1 reduced hyperalgesia in vivo in mice with unilateral tumors in and around the calcaneous bone. Intraplantar injection of OMDM-1 (5μg) into the tumor-bearing paw reduced mechanical hyperalgesia through a CB1R-dependent mechanism and also reduced a spontaneous nocifensive behavior. The same dose reduced withdrawal responses evoked by suprathreshold mechanical stimuli in naive mice. These data support the conclusion that OMDM-1 inhibits AEA uptake by a mechanism that is independent of inhibition of FAAH and provide a rationale for the development of peripherally restricted drugs that decrease AEA uptake for the management of cancer pain.

Chronic administration of the selective P2X3, P2X2/3 receptor antagonist, A-317491, transiently attenuates cancer-induced bone pain in mice

The purinergic P2X3 and P2X2/3 receptors are in the peripheral nervous system almost exclusively confined to afferent sensory neurons, where they are found both at peripheral and central synapses. The P2X3 receptor is implicated in both neuropathic and inflammatory pain. However, the role of the P2X3 receptor in chronic cancer-induced bone pain is less known. Here we investigated the effect of systemic acute and chronic administration of the selective P2X3, P2X2/3 receptor antagonist (5-[[[(3-Phenoxyphenyl)methyl][(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]carbonyl]-1,2,4-benzenetricarboxylic acid sodium salt hydrate) (A-317491) in a murine model of cancer-induced bone pain. Chronic administration of A-317491 (30 μmol/kgs.c., b.i.d.) resulted in a transient attenuation of pain related behaviours in the early stage of the bone cancer model, but had no effect in the late and more progressed stage of bone cancer. Also, acute administration of A-317491 (100 μmol/kgs.c.) had no effect in the progressed stage of the bone cancer pain model. Thus, systemically administered A-317491 did not demonstrate a robust effect in the present mouse model of cancer-induced bone pain.

Location matters: osteoblast and osteoclast distribution is modified by the presence and proximity to breast cancer cells in vivo

Bone metastasis is a common incurable complication of breast cancer affecting around 70% of patients with advanced disease. In order to improve outcomes for these patients, the cellular and molecular mechanisms underlying bone metastasis need to be established. The majority of studies to date have focused on end-stage disease and little is known about the events taking place following initial tumour cell colonisation of bone. Here we report the results of a longitudinal study that provides detailed analysis of the spatial and temporal relationship between bone and cancer cells during progression of bone metastasis. Tumour growth in bone was initiated by intra-cardiac inoculation of MDA-MB-231-GFP breast cancer cells in immunocompromised mice. Differentiating between areas of bone in direct contact with the tumour and areas distal to the cancer cells but within the tumour bearing bone, we performed comprehensive analyses of the number and distribution of osteoclasts and osteoblasts. Tumour colonies were detectable in bone from day 10, while reduced trabecular bone volume was apparent from day 19 onwards. Cancer-induced changes in osteoblast and osteoclast numbers differed substantially depending on whether or not the cells were in direct contact with the tumour. Compared to naïve controls, areas of bone in direct contact with the tumour had significantly reduced osteoblast but increased osteoclast numbers, whereas the reverse was found in distal areas. Our data demonstrate that tumour cells induce substantial changes in the bone microenvironment prior to the appearance of bone lesions, suggesting that early therapeutic intervention may be required to oppose the tumour-induced changes to the microenvironment und thus tumour progression.

P2X7 receptor-deficient mice are susceptible to bone cancer pain

The purinergic P2X7 receptor is implicated in both neuropathic and inflammatory pain, and has been suggested as a possible target in pain treatment. However, the specific role of the P2X7 receptor in bone cancer pain is unknown. We demonstrated that BALB/cJ P2X7 receptor knockout (P2X7R KO) mice were susceptible to bone cancer pain and moreover had an earlier onset of pain-related behaviours compared with cancer-bearing, wild-type mice. Furthermore, acute treatment with the selective P2X7 receptor antagonist, A-438079, failed to alleviate pain-related behaviours in models of bone cancer pain with and without astrocyte activation (BALB/cJ or C3H mice inoculated with 4T1 mammary cancer cells or NCTC 2472 osteosarcoma cells, respectively), suggesting that astrocytic P2X7 receptors play a negligible role in bone cancer pain. The results support the hypothesis that bone cancer pain is a separate pain state compared with those of neuropathic and inflammatory pain. However, the recent discovery of a P2X7 receptor splice variant expressed in the knockout mice used for this study complicates the interpretation of the results. The P2X7 splice variant receptor was detected in the spinal cord but not in osteoclasts of the P2X7R KO mouse. Further experiments are needed to elucidate the exact role of the P2X7 receptors in bone cancer pain.

Advances in the biology of bone metastasis: how the skeleton affects tumor behavior

It is increasingly evident that the microenvironment of bone can influence the cancer phenotype in many ways that favor growth in bone. The ability of cancer cells to adhere to bone matrix and to promote osteoclast formation are key requirements for the establishment and growth of bone metastases. Several cytokine products of breast cancers (e.g. PTHrP, IL-11, IL-8) have been shown to act upon host cells of the bone microenvironment to promote osteoclast formation, allowing for excessive bone resorption. The increased release of matrix-derived growth factors, especially TGF-β, acts back upon the tumor to facilitate further tumor expansion and enhance cytokine production, and also upon osteoblasts to suppress bone formation. This provides a self-perpetuating cycle of bone loss and tumor growth within the skeleton. Other contributing factors favoring tumor metastasis and colonization in bone include the unique structure and stiffness of skeletal tissue, along with the diverse cellular composition of the marrow environment (e.g. bone cells, stromal fibroblasts, immune cells), any of which can contribute to the phenotypic changes that can take place in metastatic deposits that favor their survival. Additionally, it is also apparent that breast cancer cells begin to express different bone specific proteins as well as proteins important for normal breast development and lactation that allow them to grow in bone and stimulate bone destruction. Taken together, these continually emerging areas of study suggest new potential pathways important in the pathogenesis of bone metastasis and potential areas for targeting therapeutics.

Mechanism of cancer pain

Ongoing and breakthrough pain is a primary concern for the cancer patient. Although the etiology of cancer pain remains unclear, animal models of cancer pain have allowed investigators to unravel some of the cancer-induced neuropathologic processes that occur in the region of tumor growth and in the dorsal horn of the spinal cord. Within the cancer microenvironment, cancer and immune cells produce and secrete mediators that activate and sensitize primary afferent nociceptors. Pursuant to these peripheral changes, nociceptive secondary neurons in spinal cord exhibit increased spontaneous activity and enhanced responsiveness to three modes of noxious stimulation: heat, cold, and mechanical stimuli. As our understanding of the peripheral and central mechanisms that underlie cancer pain improves, targeted analgesics for the cancer patient will likely follow.

Bone cancer induces a unique central sensitization through synaptic changes in a wide area of the spinal cord

BACKGROUND

Chronic bone cancer pain is thought to be partly due to central sensitization. Although murine models of bone cancer pain revealed significant neurochemical changes in the spinal cord, it is not known whether this produces functional alterations in spinal sensory synaptic transmission. In this study, we examined excitatory synaptic responses evoked in substantia gelatinosa (SG, lamina II) neurons in spinal cord slices of adult mice bearing bone cancer, using whole-cell voltage-clamp recording techniques.

RESULTS

Mice at 14 to 21 days after sarcoma implantation into the femur exhibited hyperalgesia to mechanical stimuli applied to the skin of the ipsilateral hind paw, as well as showing spontaneous and movement evoked pain-related behaviors. SG neurons exhibited spontaneous excitatory postsynaptic currents (EPSCs). The amplitudes of spontaneous EPSCs were significantly larger in cancer-bearing than control mice without any changes in passive membrane properties of SG neurons. In the presence of TTX, the amplitude of miniature EPSCs in SG neurons was increased in cancer-bearing mice and this was observed for cells sampled across a wide range of lumbar segmental levels. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor- and N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs evoked by focal stimulation were also enhanced in cancer-bearing mice. Dorsal root stimulation elicited mono- and/or polysynaptic EPSCs that were caused by the activation of Adelta and/or C afferent fibers in SG neurons from both groups of animals. The number of cells receiving monosynaptic inputs from Adelta and C fibers was not different between the two groups. However, the amplitude of the monosynaptic C fiber-evoked EPSCs and the number of SG neurons receiving polysynaptic inputs from Adelta and C fibers were increased in cancer-bearing mice.

CONCLUSIONS

These results show that spinal synaptic transmission mediated through Adelta and C fibers is enhanced in the SG across a wide area of lumbar levels following sarcoma implantation in the femur. This widespread spinal sensitization may be one of the underlying mechanisms for the development of chronic bone cancer pain.

Tumor-evoked hyperalgesia and sensitization of nociceptive dorsal horn neurons in a murine model of cancer pain

Pain associated with cancer, particularly when tumors metastasize to bone, is often severe and debilitating. Better understanding of the neurobiological mechanisms underlying cancer pain will likely lead to the development of more effective treatments. The aim of this study was to characterize changes in response properties of nociceptive dorsal horn neurons following implantation of fibrosarcoma cells into and around the calcaneus bone, an established model of cancer pain. Extracellular electrophysiological recordings were made from wide dynamic range (WDR) and high threshold (HT) dorsal horn neurons in mice with tumor-evoked hyperalgesia and control mice. WDR and HT neurons were examined for ongoing activity and responses to mechanical, heat, and cold stimuli applied to the plantar surface of the hind paw. Behavioral experiments showed that mice exhibited hyperalgesia to mechanical and heat stimuli applied to their tumor-bearing hind paw. WDR, but not HT, nociceptive dorsal horn neurons in tumor-bearing mice exhibited sensitization to mechanical, heat, and cold stimuli and may contribute to tumor-evoked hyperalgesia. Specifically, the proportion of WDR neurons that exhibited ongoing activity and their evoked discharge rates were greater in tumor-bearing than in control mice. In addition, WDR neurons exhibited lower response thresholds for mechanical and heat stimuli, and increased responses to suprathreshold mechanical, heat, and cold stimuli. Our findings show that sensitization of WDR neurons contributes to cancer pain and supports the notion that the mechanisms underlying cancer pain differ from those that contribute to inflammatory and neuropathic pain.

Morphine treatment accelerates sarcoma-induced bone pain, bone loss, and spontaneous fracture in a murine model of bone cancer

Metastatic bone cancer causes severe pain that is primarily treated with opioids. A model of bone cancer pain in which the progression of cancer pain and bone destruction is tightly controlled was used to evaluate the effects of sustained morphine treatment. In cancer-treated mice, morphine enhanced, rather than diminished, spontaneous, and evoked pain; these effects were dose-dependent and naloxone-sensitive. SP and CGRP positive DRG cells did not differ between sarcoma or control mice, but were increased following morphine in both groups. Morphine increased ATF-3 expression only in DRG cells of sarcoma mice. Morphine did not alter tumor growth in vitro or tumor burden in vivo but accelerated sarcoma-induced bone destruction and doubled the incidence of spontaneous fracture in a dose- and naloxone-sensitive manner. Morphine increased osteoclast activity and upregulated IL-1 beta within the femurs of sarcoma-treated mice suggesting enhancement of sarcoma-induced osteolysis. These results indicate that sustained morphine increases pain, osteolysis, bone loss, and spontaneous fracture, as well as markers of neuronal damage in DRG cells and expression of pro-inflammatory cytokines. Morphine treatment may result in "add-on" mechanisms of pain beyond those engaged by sarcoma alone. While it is not known whether the present findings in this model of osteolytic sarcoma will generalize to other cancers or opioids, the data suggest a need for increased understanding of neurobiological consequences of prolonged opioid exposure which may allow improvements in the use of opiates in the effective management of cancer pain.

Chemical interactions between fibrosarcoma cancer cells and sensory neurons contribute to cancer pain

In an experimental model of cancer pain, the hyperalgesia that occurs with osteolytic tumor growth is associated with the sensitization of nociceptors. We examined functional and molecular changes in small-diameter dorsal root ganglion (DRG) neurons to determine cellular mechanisms underlying this sensitization. The occurrence of a Ca2+ transient in response to either KCl (25 mM) or capsaicin (500 nM) increased in small neurons isolated from murine L3-L6 DRGs ipsilateral to fibrosarcoma cell tumors. The increased responses were associated with increased mRNA levels for the Ca2+ channel subunit alpha2delta1 and TRPV1 receptor. Pretreatment with gabapentin, an inhibitor of the alpha2delta1 subunit, blocked the increased response to KCl in vitro and the mechanical hyperalgesia in tumor-bearing mice in vivo. Similar increases in neuronal responsiveness occurred when DRG neurons from naive mice and fibrosarcoma cells were cocultured for 48 h. The CC chemokine ligand 2 (CCL2) may contribute to the tumor cell-induced sensitization because CCL2 immunoreactivity was present in tumors, high levels of CCL2 peptide were present in microperfusates from tumors, and treatment of DRG neurons in vitro with CCL2 increased the amount of mRNA for the alpha2delta1 subunit. Together, our data provide strong evidence that the chemical mediator CCL2 is released from tumor cells and evokes phenotypic changes in sensory neurons, including increases in voltage-gated Ca2+ channels that likely underlie the mechanical hyperalgesia in the fibrosarcoma cancer model. More broadly, this study provides a novel in vitro model to resolve the cellular and molecular mechanisms by which tumor cells drive functional changes in nociceptors.

Cellular and humoral mechanisms of osteoclast formation in Ewing's sarcoma

Cellular mechanisms that account for tumour osteolysis associated with Ewing's sarcoma are uncertain. Osteoclasts are marrow-derived multinucleated cells (MNCs) that effect tumour osteolysis. Osteoclasts are known to form from macrophages by both receptor activator for nuclear factor-κB (RANK) ligand (RANKL)-dependent and -independent mechanisms. In this study, our aim has been to determine whether tumour-associated macrophages (TAMs) isolated from Ewing's sarcoma are capable of differentiating into osteoclasts and to characterise the cellular and humoral mechanisms whereby this occurs. Tumour-associated macrophages were isolated from two Ewing's sarcomas and cultured on both coverslips and dentine slices for up to 21 days with soluble RANKL and macrophage colony stimulating factor (M-CSF). Osteoclast formation from TAMs (CD14⁺) was evidenced by the formation of tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR)-positive MNCs, which were capable of carrying out lacunar resorption. This osteoclast formation was inhibited by the addition of bisphosphonates. Both Ewing's sarcoma-derived fibroblasts and some bone stromal cells expressed RANKL and supported osteoclast formation by a contact-dependent mechanism. We also found that osteoclast differentiation occurred when Ewing's TAMs were cultured with tumour necrosis factor (TNF)-α in the presence of M-CSF and that TC71 Ewing's sarcoma cells stimulated osteoclast formation through the release of a soluble factor, the action of which was abolished by an antibody to TNF-α. These results indicate that TAMs in Ewing's sarcoma are capable of osteoclast differentiation by both RANKL-dependent and TNF-α-dependent mechanisms and that Ewing's sarcoma cells produce osteoclastogenic factor(s). Our findings suggest that anti-resorptive and anti-osteoclastogenic therapies may be useful in inhibiting the osteolysis of Ewing's sarcoma.

Osteoclasts direct bystander killing of bone cancer

Primary and metastatic bone cancers are difficult to eradicate and novel approaches are needed to improve treatment and extend life. As bone cancer grows, osteoclasts, the principal bone-resorbing cells of the body, are recruited to and activated at sites of cancer. In this investigation, we determined if osteoclast lineage cells could function as a cell-based gene delivery system to bone cancers. We used the cytosine deaminase (CD) 5-fluorocytosine (5-FC) enzyme/prodrug system and studied bone marrow and bones from transgenic mice expressing a novel CD gene regulated by the osteoclast tartrate-resistant acid phosphatase (TRAP) gene promoter (Tg/NCD). DsRed2-labeled 2472 sarcoma cells were placed in Tg/NCD osteoclastogenic cultures and treated with 5-FC. 5-FC treatment resulted in profound bystander killing (90%; P < 0.05). The effect of 5-FC treatment on osteoclast lineage cells was most dramatic when administered at the beginning of the 7-day cultures, suggesting that mature osteoclasts are less sensitive to 5-FC. Evaluation of osteoclast-directed bystander killing in vivo revealed dramatic killing of bone cancer with only a modest effect on osteoclast number. Specifically, 5-FC treatment of tumor-bearing Tg/NCD mice or Tg/NCD bone marrow transplanted C3H mice (Tg/NCD-C3H) resulted in 92% and 44% reductions in tumor area, respectively (P < 0.05). Eight of ten 5-FC-treated Tg/NCD mice had complete bone tumor killing and five of six 5-FC-treated Tg/NCD-C3H mice had reduced tumor compared with controls. In addition, Tg/NCD osteoclasts were resistant to 5-FC treatment in vivo, a very important feature, as it identifies osteoclasts as an ideal CD gene delivery system.

Acute and chronic administration of the cannabinoid receptor agonist CP 55,940 attenuates tumor-evoked hyperalgesia

Patients with cancer frequently report pain that can be difficult to manage. This study examined the antihyperalgesic effects of a cannabinoid receptor agonist, CP 55,940, in a murine model of cancer pain. Implantation of fibrosarcoma cells into and around the calcaneous bone in mice produced mechanical hyperalgesia (decreased paw withdrawal thresholds and increased frequency of paw withdrawals). On day 13 after implantation, mechanical hyperalgesia, nociception, and catalepsy were assessed. Mice were randomly assigned to receive CP 55,940 (0.01-3 mg/kg, i.p.) or vehicle and behavioral measures were redetermined. CP 55,940 dose-dependently attenuated tumor-evoked mechanical hyperalgesia. To examine the effect of catalepsy on the antihyperalgesic effect of CP 55,940, mice with tumor-evoked hyperalgesia were pretreated with the dopamine agonist apomorphine prior to administration of CP 55,940. Apomorphine attenuated the cataleptic effect of CP 55,940 but did not attenuate its antihyperalgesic effect. In a separate group of mice with tumor-evoked hyperalgesia, administration of the dopamine antagonist spiperone produced catalepsy that was approximately 2.5 fold greater than that produced by CP 55,490. Whereas this dose of CP 55,940 completely reversed tumor-evoked mechanical hyperalgesia, spiperone only attenuated mechanical hyperalgesia by approximately 35%. Thus, the cataleptic effects of CP 55,940 did not fully account for its antihyperalgesic effect. The antihyperalgesic effect of CP 55,940 was mediated via the cannabinoid CB1 but not CB2 receptor. Finally, repeated administration of CP 55,940 produced a short-term and a longer-term attenuation of tumor-evoked hyperalgesia. These results suggest that cannabinoids may be a useful alternative or adjunct therapy for treating cancer pain.

Ability of breast cancer cell lines to stimulate bone resorbing activity of mature osteoclasts correlates with an anti-apoptotic effect mediated by macrophage colony stimulating factor

We compared the effect of conditioned medium (CM) from several human breast carcinoma cell lines on osteoclast bone resorbing activity and osteoclast apoptosis. Our findings indicate that ability of cancer cell line to increase the in vitro bone resorbing activity is linked to their potential to inhibit osteoclast apoptosis. Cancer cells producing the higher level of M-CSF have the higher osteolytic activity, suggesting that M-CSF originating from cancer cells may contribute, at least in part, to the osteoclast activity at the metastatic site by enhancing their survival. Given that M-CSF plays an important role in the anti-apoptotic effect, we speculated that blocking M-CSF pathway would prevent the CM effects. Small interfering RNA (siRNA) targeting M-CSF and imatinib, a protein tyrosine kinase inhibitor targeting M-CSF receptor, almost completely reversed the CM effect on both osteoclast apoptosis and bone resorption. Blockade of M-CSF pathway could be thus of clinical value in the treatment of breast cancer related bone destruction.

Metastatic bone cancer pain: Etiology and treatment options

Painful metastatic bone disease remains a challenge for physicians. The treatment choices available are wide and varied, with each having its appropriate place in the management of painful bone metastases. Radiotherapy remains the mainstay of treatment with or without surgery. Advances in understanding the intricate pathway responsible for pain generation and the addition of agents such as bisphosphonates to the physician's armamentarium further assist in the management of painful bone metastases.

Malignant melanoma and bone resorption

The cellular and humoral mechanisms accounting for osteolysis in skeletal metastases of malignant melanoma are uncertain. Osteoclasts, the specialised multinucleated cells that carry out bone resorption, are derived from monocyte/macrophage precursors. We isolated tumour-associated macrophages (TAMs) from metastatic (lymph node/skin) melanomas and cultured them in the presence and absence of osteoclastogenic cytokines and growth factors. The effect of tumour-derived fibroblasts and melanoma cells on osteoclast formation and resorption was also analysed. Melanoma TAMs (CD14+/CD51-) differentiated into osteoclasts (CD14-/CD51+) in the presence of receptor activator for nuclear factor kappaB ligand (RANKL) and macrophage-colony stimulating factor. Tumour-associated macrophage-osteoclast differentiation also occurred via a RANKL-independent pathway when TAMs were cultured with tumour necrosis factor-alpha and interleukin (IL)-1alpha. RT-PCR showed that fibroblasts isolated from metastatic melanomas expressed RANKL messenger RNA and the conditioned medium of cultured melanoma fibroblasts was found to be capable of inducing osteoclast formation in the absence of RANKL; this effect was inhibited by the addition of osteoprotegerin (OPG). We also found that cultured human SK-Mel-29 melanoma cells produce a soluble factor that induces osteoclast differentiation; this effect was not inhibited by OPG. Our findings indicate that TAMs in metastatic melanomas can differentiate into osteoclasts and that melanoma fibroblasts and melanoma tumour cells can induce osteoclast formation by RANKL-dependent and RANKL-independent mechanisms, respectively.

Osteoclasts direct bystander killing of cancer cells in vitro

Cytosine deaminase (CD) catalyzes the deamination of 5-fluorocytosine (5FC) to produce the highly toxic chemotherapeutic agent 5-fluorouracil (5FU). A unique feature of the CD/5FC enzyme/prodrug system is its ability to kill adjacent cells via bystander killing. Bystander killing of cancer cells can be mediated by non-cancerous accessory cells transduced with the CD gene; one type of non-cancerous accessory cell found in primary bone cancer and breast cancer metastases to bone is the osteoclast. This manuscript determines if osteoclast precursor cells, transduced with the CD gene, can function as a gene delivery system capable of killing cancer cells. An osteoclast precursor cell line (RAW 264.7, RAW) and authentic bone marrow-derived osteoclast precursor cells were transduced with a retroviral vector containing the cytosine deaminase fusion gene (NCD) composed of the human nerve growth factor receptor and CD genes. RAW cells and bone marrow-derived osteoclast precursor cells transduced with NCD expressed NCD protein and converted 5FC to 5FU. Treatment of NCD-transduced osteoclast precursor cells with the 5FC prodrug resulted in significant killing in vitro. NCD-transduced osteoclasts were co-cultured with either DsRed2-labeled sarcoma cells (2472-DSR) or green fluorescent protein (GFP)-labeled breast cancer cells (GFP-4T1). Treatment of the NCD osteoclast/tumor cell co-cultures with 5FC resulted in bystander killing of 2472-DSR cells (P < 0.006) and GFP-4T1 cells (P < 0.004). These findings demonstrate that NCD-transduced osteoclasts can promote killing of cancer cells and introduce the exciting possibility for developing osteoclast-mediated, CD-based treatment of primary bone cancers and breast cancer metastases to bone.

Analgesic effects of nonsteroidal antiinflammatory drugs, acetaminophen, and morphine in a mouse model of bone cancer pain

PURPOSE

Bone metastasis is one of the major causes of cancer-related pain, and not all bone cancer pain can be effectively treated. Recently, a mouse model of bone cancer pain was introduced. To test the analgesic effects of nonsteroidal antiinflammatory drugs on bone cancer pain, the authors examined the effects of oral administration of a cyclooxygenase-1 (COX-1) selective inhibitor (SC560), a COX-2 selective inhibitor (celecoxib), and a nonselective COX inhibitor (indomethacin) on bone cancer pain and compared these effects to the effect of orally administered acetaminophen and morphine.

METHODS

An animal model of bone cancer pain was induced by injecting osteolytic murine sarcoma cells in the mouse femur. Drugs were administered orally 2 weeks after tumor-cell implantation, and the level of bone cancer pain was assessed 30, 60, 90, 120, and 180 min after drug administration.

RESULTS

Oral administration of acetaminophen, indomethacin, and morphine, but not of SC560 or celecoxib, produced an analgesic effect on bone cancer pain. Co-administration of a subanalgesic does of morphine with acetaminophen enhanced the analgesic effect of acetaminophen.

CONCLUSION

These data suggest that bone cancer pain is effectively treated by oral administration of indomethacin, acetaminophen, and morphine and that the co-administration of acetaminophen and an opioid provides a beneficial effect when treating of bone cancer pain.

Evaluation of pain-related behavior, bone destruction and effectiveness of fentanyl, sufentanil, and morphine in a murine model of cancer pain

The present study was conducted to evaluate the pain development and bone destruction during bone cancer growth in a murine model of bone cancer pain and to evaluate the analgesic efficacy of fentanyl, sufentanil, and morphine in this model. C3H/HeNCrl mice were inoculated into the intramedullary space of the femur with osteolytic NCTC 2472 fibrosarcoma cells, and followed during a 3-week period to assess pain behaviors (spontaneous lifting and limb-use during forced ambulation on rotarod) and bone destruction (parameters indicative of bone lesions determined by microCT-scans of the tumor-bearing bones) during bone cancer growth. The results showed that in this murine model of cancer-induced bone pain, behavioural manifestations of pain emerge in parallel with the progression of bone destruction. The subcutaneous administration of fentanyl (0.025-0.64 mg/kg), sufentanil (0.005-0.04 mg/kg), and morphine (2.5-40 mg/kg) on the test days 15 and 22 post-inoculation reduced pain-related behaviors in a dose dependent manner. A complete relief from pain-related behaviors was achieved with the following doses: > or =0.16 mg/kg fentanyl, 0.02 mg/kg sufentanil, and 20 mg/kg morphine. In conclusion, the results showed a clear link between tumor growth-induced bone destruction and behavioral pain manifestations, the latter was effectively controlled by the opioids fentanyl, sufentanil, and morphine.

A new experimental rat model of osteosarcoma established by intrafemoral tumor cell inoculation, useful for biology and therapy investigations

Satisfactory experimental models for preclinical cancer studies must follow several criteria: (1) reproducibility of the method used to induce the tumor and (2) clinical, pathological and kinetic similarity with the corresponding human tumors. We developed a model of osteosarcoma locally induced by the intrafemoral injection of osteosarcoma (OSR) cells in Sprague-Dawley rats. This method yields nearly 80% of bone tumors at the injection site. These tumors double their volume fairly slowly (in approximately 20 days) and lung metastases occur in 96% of the animals. The OSR cell-induced tumor is characterized by a direct production of mineralized matrix by the tumor cells themselves, as revealed by histochemical analysis. The microarchitectural parameters which were quantified by a microscanner show an increased trabecular bone volume (+238%) when OSR cells were injected in the femur, as compared to controls injected with vehicle. Osteoblastic markers such as alkaline phosphatase, osteopontin, osteocalcin and bone sialoprotein were expressed by the tumor in vivo, whereas the initially injected OSR cells did not express some of these markers, suggesting that OSR cells reacquired an osteoblastic phenotype in a favorable environment. The clinical, radiological and histological data show that this model shares high similarities with the osteocondensing forms of osteosarcoma in humans.

Re-organization of P2X3 receptor localization on epidermal nerve fibers in a murine model of cancer pain

To determine whether ATP and P2X3 receptors contribute to bone-cancer pain in a mouse model, immunohistochemical techniques were used to identify whether changes in the labeling of P2X3 receptors on epidermal nerve fibers (ENFs) occurred during tumor development. C3H mice were injected with osteolytic fibrosarcoma cells in and around the calcaneus bone. These mice exhibited mechanical hyperalgesia by day 10 post-implantation as assessed using von Frey monofilaments. Biopsies of the plantar skin overlying the tumor were obtained at days 10, 14, and 18 post-implantation. Confocal images were analyzed for the number of PGP 9.5, P2X3, and CGRP immunoreactive (ir) ENFs. The overall ENF population (PGP-ir) decreased progressively over time, whereas the subsets of P2X3-ir fibers demonstrated a modest increase and CGRP-ir nerve fibers remained fairly constant. Importantly, the proportion of CGRP-ir fibers that labeled for P2X3 increased from approximately 6% in control animals to nearly 30% at day 14 following tumor cell implantation. These studies demonstrate increased expression of P2X3 receptors on CGRP-ir ENFs during tumor growth and suggest a role for ATP in cancer-related pain.

Bone cancer pain model in mice: evaluation of pain behavior, bone destruction and morphine sensitivity

The primary aim of the study was to correlate pain development during bone cancer growth with objectively obtained tumor-induced changes in bone morphology. Additionally morphine sensitivity of this bone pain was evaluated. Mice were injected into the femur with osteolytic NCTC2472 cells, and behaviorally followed during a 3-week period. During the observation period increasing pain behavior was observed in tumor-bearing animals. Tumor mice exhibited spontaneous and movement-evoked lifting, the latter evoked through non-noxious palpation of the tumor. Limb use during forced ambulation on a rotarod decreased to substantial non-use of the affected limb by day 23. On day 23, micro-computer tomography scans of the tumor-bearing bones were evaluated for bone destruction. Different bone parameters indicative of osteolysis or fragmentation were significantly correlated with pain behavior. In a separate group of mice the effects of different morphine doses on pain behavior were evaluated on days 17 and 21 of tumor growth. Spontaneous lifting and movement-evoked lifting were sensitive to morphine treatment, although stress-induced analgesia due to repeated restraint might minimize movement-evoked lifting in mice. Limb use during forced ambulation was only slightly ameliorated by high morphine doses.

Bone cancer pain: from mechanism to model to therapy

Although bone cancer pain can be severe and is relatively common, very little is known about the basic mechanisms that generate and maintain this debilitating pain. To begin to define the mechanisms that give rise to bone cancer pain, a mouse model was developed using the intramedullary injection and containment of osteolytic sarcoma cells in the mouse femur. These tumor cells induced bone destruction as well as ongoing and movement-evoked pain behaviors similar to that found in patients with bone cancer pain. In addition, there was a significant reorganization of the spinal cord that received sensory input from the cancerous bone, and this reorganization was significantly different from that observed in mouse models of chronic neuropathic or inflammatory pain. To determine whether this mouse model of bone cancer could be used to define the basic mechanisms giving rise to bone cancer pain, we targeted excessive osteoclast activity using osteoprotegerin, a secreted decoy receptor that inhibits osteoclast activity. Osteoprotegerin blocked excessive tumor-induced, osteoclast-mediated bone destruction, and significantly reduced ongoing and movement-evoked pain, and the neurochemical reorganization of the spinal cord. These data suggest that this model can provide insight into the mechanisms that generate bone cancer pain and provide a platform for developing and testing novel analgesics to block bone cancer pain.

Morphine-Induced Analgesia in the Hot-Plate Test: Comparison between NMRInu/nuand NMRI mice

NMRInu/nu mice are frequently used in cancer research, but their use in behavioural pain tests is unexplored. As behaviour of NMRI mice in pain tests is well-documented, a hot-plate test was performed comparing acute thermal nociception in NMRInu/nu and NMRI mice - untreated and morphine-treated - to estimate the usefulness of NMRInu/nu mice for further research on cancer pain. In both strains, morphine dose-dependently increased response latencies, number of animals reaching cut-off times and AUC values. Yet in NMRInu/nu mice, as compared to NMRI mice, all curves were shifted to the right. In order to be comparable, cut-off times must express a similar degree of baseline response augmentations. NMRInu/nu mice had substantially lower pre-drug latencies, indicating a lowered threshold for painful thermal stimuli, therefore effects of morphine in NMRInu/nu mice were also analysed using a lower cut-off time. Doing so, morphine resulted in similar effects in both strains. The effects were independent of hot-plate temperature, because similar results were obtained using temperatures of 50 and 55 degrees. The different morphine sensitivity of NMRInu/nu compared to NMRI mice primarily seems to depend upon differences in thermal threshold, probably induced by the different genotype of both strains. To determine whether cancer alters pain threshold or morphine analgesia, LoVo tumour-bearing NMRInu/nu mice were also tested. The tumour presence had no influence on withdrawal latencies or morphine efficacy. In general it can be concluded that NMRInu/nu mice with or without tumour can be used for nociceptive testing if baseline sensitivity is properly defined.

Metabolic activation stimulates acid secretion and expression of matrix degrading proteases in human osteoblasts

BACKGROUND

Both cellular and matrix components of healthy bone are permanently renewed in a balanced homoeostasis. Osteoclastic bone resorption involves the expression of vacuolar-type ATPase proton pumps (vATPase) on the outer cell membrane and the secretion of matrix degrading proteases. Osteoblasts modulate the deposition of bone mineral components and secrete extracellular matrix proteins.

OBJECTIVES

To investigate the ability of osteoblasts and osteosarcoma to secrete acid and express matrix degrading proteases upon metabolic activation. To examine also the potential contribution of vATPases to proton secretion expressed on osteoblasts.

METHODS

Osteoblasts were isolated from trabecular bone and characterised by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Proton secretion was analysed by a cytosensor microphysiometer.

RESULTS

Osteoblasts not only express matrix degrading proteases upon stimulation with tumour necrosis factor or with phorbol ester but they also secrete protons upon activation. Proton secretion by osteoblasts is associated partially with proton pump ATPases.

CONCLUSION

These data suggest that, in addition to monocyte derived osteoclasts, cytokine activated mesenchymal osteoblasts and osteosarcoma cells may contribute to the acidic milieu required for bone degradation.

Targeted etanercept for treatment-refractory pain due to bone metastasis: two case reports

BACKGROUND

Parallel bodies of research suggest both a central role for osteoclasts in tumor-induced destruction of bone and the ability of biologic tumor necrosis factor-alpha (TNF-alpha) antagonists to attenuate the osteoclast-mediated bone destruction that accompanies a variety of nonmalignant disorders. Additional studies have implicated TNF-alpha in the promotion of osteoclast-mediated malignant osteolysis and the pathogenesis of neuropathic pain. TNF-alpha antagonists have the potential to interfere in both processes.

OBJECTIVE

This article reviews the cases of 2 patients with treatment-refractory pain due to cancer metastases to bone who were given targeted injections of the biologic anti-TNF agent etanercept based on its potential to interfere directly with both malignant activation of osteoclasts and neuropathic pain.

METHODS

One patient had a diagnosis of non-small cell lung cancer and the other had a diagnosis of breast cancer. Both presented with treatment-refractory pain due to bone metastases. The 2 patients received etanercept 25 mg by targeted SC injection in anatomic proximity to the site of spinal metastasis for relief of their treatment-refractory pain.

RESULTS

Both patients experienced rapid, substantial, and sustained relief of chronic refractory pain at the treatment site after targeted administration of etanercept. Symptomatic improvement was correlated with objective measures of improvement, including weight gain in 1 patient and decreased uptake of radioactive tracer at the targeted site on positron emission tomography in the other.

CONCLUSIONS

Etanercept delivered by targeted SC injection may be of clinical benefit in selected patients with treatment-refractory pain caused by bone metastases. Clinical trials are needed to define the potential benefit of biologic TNF-alpha antagonists in the treatment and prevention of malignant osteolysis.

Breast cancer cells induce osteoclast formation by stimulating host IL-11 production and downregulating granulocyte/macrophage colony-stimulating factor

Breast cancer cells frequently metastasize to the skeleton, where they induce OCL formation and activity, resulting in extensive bone destruction. However, the mechanisms by which breast cancer cells mediate increased osteolysis remain unclear. To elucidate this point, we investigated how 3 human breast cancer cell lines, MDA-MB-231, MDA-MB-435 and MCF-7, induce OCL formation using a murine osteoblast-spleen cell coculture system and compared their effects with a human colorectal cancer cell line, HCT-15; a human lung cancer cell line, HT-1080; and a normal human breast cell line, HME. The breast cancer cell lines supported OCL formation only when osteoblasts were present in spleen cell cocultures, whilst the non-breast cancer cell lines and the normal breast cell line, HME, had no effect. Fractionation of BCCM by ultrafiltration established that osteoclastogenic activity was associated with factors having m.w. >3 kDa. Breast cancer cell lines produced primarily PTHrP, with lesser amounts of IL-6, IL-11 and TNF-alpha. The effect of BCCM on OCL formation in osteoblast-spleen cell cocultures was partially prevented by a neutralising antibody to human PTHrP and completely prevented by a neutralising antibody to either murine IL-11 or the murine IL-11 receptor; neutralising antibodies to human IL-6, IL-11 or TNF-alpha were without effect. BCCM or human PTHrP induced an increase in murine osteoblast IL-11 mRNA and protein production, effects that were prevented in the presence of a neutralising antibody to human PTHrP. The osteoclastogenic activity of IL-11 was mediated by enhancing osteoblast production of PGE(2) effects, which were abrogated by an inhibitor of cyclooxygenase. PGE(2) apparently enhanced OCL formation by downregulating GM-CSF production by spleen cells since recombinant murine GM-CSF inhibited OCL formation and a neutralising antibody to murine GM-CSF blocked these inhibitory effects. We conclude that breast cancer cells induce OCL formation by stimulating osteoblastic production of IL-11. The subsequent release of PGE(2) followed by inhibition of GM-CSF production by cells within the bone microenvironment plays an important part in mediating the effects of breast cancer cells on OCL formation and their resorptive activity.

Bone cancer pain

BACKGROUND

Bone cancer pain is very common, and patients with this type of pain may be difficult to treat. Development of an experimental model for studying this condition is critical to advancing an understanding of the mechanisms that cause pain in patients with malignant disease.

METHODS

A murine model of bone cancer was studied. Combined analysis of the extent of tumor-induced bone destruction, pain, and neurochemical characterization of the peripheral and central nervous systems was performed to investigate bone cancer pain. Disease-induced bone destruction was assessed by radiographs and histomorphometry. Pain was assessed by spontaneous and elicited behaviors, and neurochemical analysis involved immunohistochemical detection of hyperalgesic peptides and neurochemical markers.

RESULTS

Mice with distal femoral sarcomas exhibited behavioral and neurochemical measures of pain. The pain condition created by malignant bone disease was distinct neurochemically from inflammatory and neuropathic pain states. Experimental evidence indicated that both disease-induced osteolysis and tumors themselves contributed to the generation of pain and that peripheral and central sensitization of the nervous system was present.

CONCLUSIONS

Malignant bone disease creates a unique pain state that involves sensitization of the nervous system. Major contributors to the pain state within the bone tissue are osteoclastic bone resorption and the malignant disease itself.

Metastatic bone disease: future directions

Bone cancer is common and difficult to treat. Current bone-targeted therapies include radiation and bisphosphonates. Both treatments can provide pain relief, but neither has been shown to prolong survival. Because bisphosphonates are osteoclast-targeted therapies, they do not exhibit significant tumor-killing properties in humans. Often, patients with asymptomatic skeletal metastases are treated with observation. New treatments are needed for patients known to have bone metastases and those who are at high risk for having bone metastases develop. Enzyme prodrug gene therapy treatment strategies currently are being explored for their potential benefit in designing novel therapies for bone cancer.

Different tumors in bone each give rise to a distinct pattern of skeletal destruction, bone cancer-related pain behaviors and neurochemical changes in the central nervous system

Pain is the most common presenting symptom in patients with bone cancer and bone cancer pain can be both debilitating and difficult to control fully. To begin to understand the mechanisms involved in the generation and maintenance of bone cancer pain, we implanted 3 well-described murine tumor cell lines, 2472 sarcoma, B16 melanoma and C26 colon adenocarcinoma into the femur of immunocompromised C3H-SCID mice. Although each of the tumor cell lines proliferated and completely filled the intramedullary space of the femur within 3 weeks, the location and extent of bone destruction, the type and severity of the pain behaviors and the neurochemical reorganization of the spinal cord was unique to each tumor cell line injected. These data suggest that bone cancer pain is not caused by a single factor such as increased pressure induced by intramedullary tumor growth, but rather that multiple factors are involved in generating and maintaining bone cancer pain.

A cannabinoid agonist differentially attenuates deep tissue hyperalgesia in animal models of cancer and inflammatory muscle pain

Pain associated with cancer and chronic musculoskeletal disorders can be difficult to control. We used murine models of cancer and inflammatory muscle pain to examine whether the cannabinoid receptor agonist WIN55,212-2 reduces hyperalgesia originating in deep tissues. C3H/He mice were anesthetized and implanted with osteolytic NCTC clone 2472 cells into the humeri or injected with 4% carrageenan into the triceps muscles of both forelimbs. At the time of peak hyperalgesia, WIN55,212-2 (1-30mg/kg) or vehicle was administered intraperitoneally and forelimb grip force was measured 0.5-24h later. WIN55,212-2 produced time- and dose-related antihyperalgesia in both models. A 10mg/kg dose of WIN55,212-2 fully reversed carrageenan-evoked muscle hyperalgesia. However, 30mg/kg of WIN55,212-2 attenuated tumor-evoked hyperalgesia only approximately 50%. After controlling for the difference in magnitude of hyperalgesia between the two models, WIN55,212-2 was still more potent at reducing hyperalgesia in the inflammatory model. In the cancer pain model, the antihyperalgesic effect of WIN55,212-2 was partially blocked by pretreatment with the selective CB1 (SR141716A) but not the CB2 (SR144528) receptor antagonist. In contrast, both antagonists blocked antihyperalgesic effects of WIN55,212-2 on carrageenan-evoked muscle hyperalgesia. Catalepsy and loss of motor coordination, known side effects of cannabinoids, did not account for the antihyperalgesia produced by WIN55,212-2. These data show that cannabinoids attenuate deep tissue hyperalgesia produced by both cancer and inflammatory conditions. Interestingly, cannabinoids differentially modulated carrageenan- and tumor-evoked hyperalgesia in terms of potency and receptor subtypes involved suggesting that differences in underlying mechanisms may exist between these two models of deep tissue pain.

Bone cancer pain

BACKGROUND

Bone cancer pain is very common, and patients with this type of pain may be difficult to treat. Development of an experimental model for studying this condition is critical to advancing an understanding of the mechanisms that cause pain in patients with malignant disease.

METHODS

A murine model of bone cancer was studied. Combined analysis of the extent of tumor-induced bone destruction, pain, and neurochemical characterization of the peripheral and central nervous systems was performed to investigate bone cancer pain. Disease-induced bone destruction was assessed by radiographs and histomorphometry. Pain was assessed by spontaneous and elicited behaviors, and neurochemical analysis involved immunohistochemical detection of hyperalgesic peptides and neurochemical markers.

RESULTS

Mice with distal femoral sarcomas exhibited behavioral and neurochemical measures of pain. The pain condition created by malignant bone disease was distinct neurochemically from inflammatory and neuropathic pain states. Experimental evidence indicated that both disease-induced osteolysis and tumors themselves contributed to the generation of pain and that peripheral and central sensitization of the nervous system was present.

CONCLUSIONS

Malignant bone disease creates a unique pain state that involves sensitization of the nervous system. Major contributors to the pain state within the bone tissue are osteoclastic bone resorption and the malignant disease itself.

Tumor implantation in mouse humerus evokes movement-related hyperalgesia exceeding that evoked by intramuscular carrageenan

In this paper we compare two innovative models of movement-related pain: tumor-induced nociception following implantation of fibrosarcoma cells into bone and muscle inflammation-induced nociception following injection of the irritant carrageenan into muscle. Importantly, using the grip force test, an assay of movement-related hyperalgesia, both non-malignant and malignant pain are examined in parallel. Movement-related hyperalgesia, known clinically as a specific type of 'breakthrough pain', is a common feature of bone cancer and is thought to be a predictor of poor response to conventional analgesic pharmacotherapy (Bruera et al., 1995, J. Pain Symptom. Manage. 10 (1995) 348; Mercadaute et al., 1992, Pain 50 (1992) 151; Pain 81 (1999) 129). Implantation of NCTC 2472 sarcoma cells in both humeri or injection of carrageenan (4%) in both triceps of C3H/He mice produced apparent forelimb hyperalgesia that was not associated with mechanical hyperalgesia in the forepaw, whereas carrageenan at 6 and 8% did evoke significant cutaneous hyperalgesia of the forepaw as well. Control groups receiving implants of vehicle or no treatment at all did not manifest this forelimb hyperalgesia. B6C3/F1 mice implanted with non-lysis-inducing G3.26 melanoma cells or vehicle did not manifest significant hyperalgesia when compared to B6C3/F1 mice receiving fibrosarcoma cells, indicating a dependence on bone involvement for induction of hyperalgesia in this model. Histological examination at days 3, 7, and 10 post-implantation showed a clear correlation of tumor growth-induced bone destruction with behavioral hyperalgesia. Morphine was more potent in decreasing the maximal hyperalgesia induced by carrageenan than that induced by tumor implantation. Acutely administered morphine (3-100mg/kg, i.p.) attenuated peak hyperalgesia of carrageenan-injected mice (ED(50) 6.9 mg/kg) and tumor-bearing mice (ED(50) 23.9 mg/kg) in a dose-related manner with a difference in potency of 3.5. Tumor-implanted mice with a level of hyperalgesia comparable to that induced by carrageenan required almost three times more morphine (ED(50) 18.5mg/kg) for equivalent attenuation of forelimb hyperalgesia. These animal models of movement-related hyperalgesia may aid in discerning the peripheral and central mechanisms underlying pain that accompanies bone metastases and distinguishing it from the pain associated with muscular inflammation. Importantly, they may also aid in predicting differences in analgesic efficacy in different types of musculoskeletal pain.

Morphological, histomorphometric, and microstructural alterations in human bone metastasis from breast carcinoma

Bone is one of the most common sites of breast cancer metastasis. Metastases are often associated with bone destruction and are a major cause of morbidity. We examined structural bone changes induced by metastatic tumor in bone biopsies from 33 patients with metastatic breast carcinoma (20 from patients with pathological femoral fracture and 13 with no fracture) and 20 normal controls. In all metastatic biopsies bone remodeling was shown to be tumor volume-dependent. Bone resorption and bone formation were biphasic with both increasing at earlier stages of metastatic bone disease and decreasing later on. A comparison of patients with fracture and no fracture did not reveal statistically significant differences in the extent of bone destruction or trabecular thinning. Bone histomorphometry showed limited ability to explain the higher bone volume loss in fracture patients (decreases of 42% and 25%, respectively, in fracture and nonfracture patients compared with controls). However, changes in bone quality, including increased disconnectivity and decreased connectivity, as evaluated by node-strut analysis, suggested that there were more structural changes in the fracture compared with the nonfracture group. The nonfracture group included six patients with no radiological evidence of bone metastasis (occult metastasis). They showed a higher tumor volume and a twofold lower eroded surface compared with the rest of the group. The decrease in bone volume (14% lower than controls) was below the limit of X-ray detection. Because we observed no increase in osteoclast-related parameters and no correlation between osteoclast surface and eroded surface, we believe that, in occult metastasis, osteoclastic bone resorption is not an important factor in overall bone resorption. Quantitatively, the eroded surface in direct contact with tumor cells was threefold higher than the osteoclast surface in occult metastasis, whereas the rest of the metastatic group (27 of 33) showed predominantly osteoclast-mediated eroded surface. Node-strut analysis on occult metastasis revealed a significant increase in disconnectivity without a concomitant significant decrease in bone volume and trabecular thinning. We conclude that, in occult metastasis, bone resorption may be more osteoclast-independent and other mechanisms involving the tumor cells may be more prevalent.

Functional interactions between tumor and peripheral nerve: changes in excitability and morphology of primary afferent fibers in a murine model of cancer pain

We used a murine model to investigate functional interactions between tumors and peripheral nerves that may contribute to pain associated with cancer. Implantation of fibrosarcoma cells in and around the calcaneus bone produced mechanical hyperalgesia of the ipsilateral paw. Electrophysiological recordings from primary afferent fibers in control and hyperalgesic mice with tumor revealed the development of spontaneous activity (0.2-3.4 Hz) in 34% of cutaneous C-fibers adjacent to the tumor (9-17 d after implantation). C-fibers in tumor-bearing mice exhibited a mean decrease in heat threshold of 3.5 +/- 0.10 degrees C. We also examined innervation of the skin overlying the tumor. Epidermal nerve fibers (ENFs) were immunostained for protein gene product 9.5, imaged using confocal microscopy, and analyzed in terms of number of fibers per millimeter of epidermal length and branching (number of nodes per fiber). Divergent morphological changes were linked to tumor progression. Although branching of ENFs increased significantly relative to control values, in later stages (16-24 d after implantation) of tumor growth a sharp decrease in the number of ENFs was observed. This decay of epidermal innervation of skin over the tumor coincided temporally with gradual loss of electrophysiological activity in tumor-bearing mice. The development of spontaneous activity and sensitization to heat in C-fibers and increased innervation of cutaneous structures within the first 2 weeks of tumor growth suggest activation and sensitization of a proportion of C-fibers. The decrease in the number of ENFs observed in later stages of tumor development implicates neuropathic involvement in this model of cancer pain.

Functional interactions between tumor and peripheral nerve: changes in excitability and morphology of primary afferent fibers in a murine model of cancer pain

We used a murine model to investigate functional interactions between tumors and peripheral nerves that may contribute to pain associated with cancer. Implantation of fibrosarcoma cells in and around the calcaneus bone produced mechanical hyperalgesia of the ipsilateral paw. Electrophysiological recordings from primary afferent fibers in control and hyperalgesic mice with tumor revealed the development of spontaneous activity (0.2-3.4 Hz) in 34% of cutaneous C-fibers adjacent to the tumor (9-17 d after implantation). C-fibers in tumor-bearing mice exhibited a mean decrease in heat threshold of 3.5 +/- 0.10 degrees C. We also examined innervation of the skin overlying the tumor. Epidermal nerve fibers (ENFs) were immunostained for protein gene product 9.5, imaged using confocal microscopy, and analyzed in terms of number of fibers per millimeter of epidermal length and branching (number of nodes per fiber). Divergent morphological changes were linked to tumor progression. Although branching of ENFs increased significantly relative to control values, in later stages (16-24 d after implantation) of tumor growth a sharp decrease in the number of ENFs was observed. This decay of epidermal innervation of skin over the tumor coincided temporally with gradual loss of electrophysiological activity in tumor-bearing mice. The development of spontaneous activity and sensitization to heat in C-fibers and increased innervation of cutaneous structures within the first 2 weeks of tumor growth suggest activation and sensitization of a proportion of C-fibers. The decrease in the number of ENFs observed in later stages of tumor development implicates neuropathic involvement in this model of cancer pain.

Functional interactions between tumor and peripheral nerve: morphology, algogen identification, and behavioral characterization of a new murine model of cancer pain

This paper describes a model of tumor-induced bone destruction and hyperalgesia produced by implantation of fibrosarcoma cells into the mouse calcaneus bone. Histological examination indicates that tumor cells adhere to the bone edge as early as post-implantation day (PID) 3, but osteolysis does not begin until PID 6, correlating with the development of hyperalgesia. C3H/He mice exhibit a reproducible hyperalgesia to mechanical and cold stimuli between PID 6 and 16. These behaviors are present but significantly reduced with subcutaneous implantation that does not involve bone. Systemic administration of morphine (ED(50) 9.0 mg/kg) dose-dependently attenuated the mechanical hyperalgesia. In contrast, bone destruction and hypersensitivity were not evident in mice implanted with melanoma tumors or a paraffin mass of similar size. A novel microperfusion technique was used to identify elevated levels of the putative algogen endothelin (ET) in perfusates collected from the tumor sites of hyperalgesic mice between PID 7 and 12. Increased ET was evident in microperfusates from fibrosarcoma tumor-implanted mice but not from melanoma tumor-implanted mice, which are not hyperalgesic. Intraplantar injection of ET-1 in naive and, to a greater extent, fibrosarcoma tumor-bearing mice produced spontaneous pain behaviors, suggesting that ET-1 activates primary afferent fibers. Intraplantar but not systemic injection of the ET-A receptor antagonist BQ-123 partially blocked tumor-associated mechanical hyperalgesia, indicating that ET-1 contributes to tumor-induced nociception. This model provides a unique approach for quantifying the behavioral, biochemical, and electrophysiological consequences of tumor-nerve interactions.

Cellular mechanisms of bone resorption in breast carcinoma

The cellular mechanisms that account for the increase in osteoclast numbers and bone resorption in skeletal breast cancer metastasis are unclear. Osteoclasts are marrow-derived cells which form by fusion of mononuclear phagocyte precursors that circulate in the monocyte fraction. In this study we have determined whether circulating osteoclast precursors are increased in number or have an increased sensitivity to humoral factors for osteoclastogenesis in breast cancer patients with skeletal metastases (+/- hypercalcaemia) compared to patients with primary breast cancer and age-matched normal controls. Monocytes were isolated and cocultured with UMR 106 osteoblastic cells in the presence of 1,25 dihydroxyvitamin D3[1,25(OH)2D3] and human macrophage colony stimulating factor (M-CSF) on coverslips and dentine slices. Limiting dilution experiments showed that there was no increase in the number of circulating osteoclast precursors in breast cancer patients with skeletal metastases (+/- hypercalcaemia) compared to controls. Osteoclast precursors in these patients also did not exhibit increased sensitivity to 1,25(OH)2D3or M-CSF in terms of osteoclast formation. The addition of parathyroid hormone-related protein and interleukin-6 did not increase osteoclast formation. The addition of the supernatant of cultured breast cancer cell lines (MCF-7 and MDA-MB-435), however, significantly increased monocyte-osteoclast formation in a dose-dependent fashion. These results indicate that the increase in osteoclast formation in breast cancer is not due to an increase in the number/nature of circulating osteoclast precursors. They also suggest that tumour cells promote osteoclast formation in the bone microenvironment by secreting soluble osteoclastogenic factor(s).

Human breast-cancer cells stimulate the fusion, migration and resorptive activity of osteoclasts in bone explants

A central event in bone resorption is the recruitment of osteoclasts to future resorption sites. Breast-cancer cells invariably metastasise to the skeleton and induce extensive bone destruction by osteoclasts. However, our understanding of the mechanisms by which cancer cells interact with osteoclasts remains unclear. Consequently, we compared the effects of conditioned medium (CM) from 2 human breast-cancer cell lines, MB-MDA-231 and MCF-7, with those of a normal human breast epithelial cell line, HME, on osteoclastic fusion, resorptive activity and migration from the periosteum to the developing marrow cavity of fetal mouse metatarsals in culture. Osteoclastic resorptive activity was assessed by pre-labelling 17-day-old fetal metatarsal explants with 45Ca, whilst fusion and migration were monitored by histomorphometry and osteoclasts were identified by their tartrate-resistant acid phosphatase activity. CM from TPA-stimulated breast-cancer cell lines produced a significant increase in osteoclastic resorptive activity, whilst the normal breast cell line produced a minimal increase. The breast-cancer cell lines also stimulated osteoclastic fusion and migration in the metatarsal explants, but the normal breast cell line was without effect. The stimulatory effect of CM from MDA-MB-231 cells on osteoclastic fusion, but not migration, was partially inhibited by preventing prostaglandin and leukotriene synthesis by cells within the bone explants. In contrast, a synthetic matrix metalloproteinase (MMP) inhibitor, but not a cysteine proteinase inhibitor, prevented the migration of osteoclasts to the calcified centre of the metatarsal explants in response to CM from MDA-MB-231 cells. MDA-MB-231 cells also induced an increase in the expression of MMP-9 by migrating osteoclasts. Fractionation of the TPA-stimulated breast cancer cell CM established that the resorptive activity was associated with factors of m.w. >3 kDa. We determined by immuno-assay that human breast-cancer cells secrete parathyroid hormone-related protein (PTH-rP), tumour necrosis factor-alpha (TNF-alpha) and interleukins (ILs) 6 and 11. Neutralizing experiments with human antibodies to these cytokines established that PTH-rP and TNF-alpha production by MDA-MB-231 cells were responsible for mediating their effects on osteoclastic migration and ultimately bone resorption in the metatarsal explants.

Murine models of inflammatory, neuropathic and cancer pain each generates a unique set of neurochemical changes in the spinal cord and sensory neurons

The aim of this investigation was to determine whether murine models of inflammatory, neuropathic and cancer pain are each characterized by a unique set of neurochemical changes in the spinal cord and sensory neurons. All models were generated in C3H/HeJ mice and hyperalgesia and allodynia behaviorally characterized. A variety of neurochemical markers that have been implicated in the generation and maintenance of chronic pain were then examined in spinal cord and primary afferent neurons.Three days after injection of complete Freund's adjuvant into the hindpaw (a model of persistent inflammatory pain) increases in substance P, calcitonin gene-related peptide, protein kinase C gamma, and substance P receptor were observed in the spinal cord. Following sciatic nerve transection or L5 spinal nerve ligation (a model of persistent neuropathic pain) significant decreases in substance P and calcitonin gene-related peptide and increases in galanin and neuropeptide Y were observed in both primary afferent neurons and the spinal cord. In contrast, in a model of cancer pain induced by injection of osteolytic sarcoma cells into the femur, there were no detectable changes in any of these markers in either primary afferent neurons or the spinal cord. However, in this cancer-pain model, changes including massive astrocyte hypertrophy without neuronal loss, increase in the neuronal expression of c-Fos, and increase in the number of dynorphin-immunoreactive neurons were observed in the spinal cord, ipsilateral to the limb with cancer. These results indicate that a unique set of neurochemical changes occur with inflammatory, neuropathic and cancer pain in C3H/HeJ mice and further suggest that cancer induces a unique persistent pain state. Determining whether these neurochemical changes are involved in the generation and maintenance of each type of persistent pain may provide insight into the mechanisms that underlie each of these pain states.