Molly A. Sevcik

Selective Blockade of the Capsaicin Receptor TRPV1 Attenuates Bone Cancer Pain

Cancer colonization of bone leads to the activation of osteoclasts, thereby producing local tissue acidosis and bone resorption. This process may contribute to the generation of both ongoing and movement-evoked pain, resulting from the activation of sensory neurons that detect noxious stimuli (nociceptors). The capsaicin receptor TRPV1 (transient receptor potential vanilloid subtype 1) is a cation channel expressed by nociceptors that detects multiple pain-producing stimuli, including noxious heat and extracellular protons, raising the possibility that it is an important mediator of bone cancer pain via its capacity to detect osteoclast- and tumor-mediated tissue acidosis. Here, we show that TRPV1 is present on sensory neuron fibers that innervate the mouse femur and that, in an in vivo model of bone cancer pain, acute or chronic administration of a TRPV1 antagonist or disruption of the TRPV1 gene results in a significant attenuation of both ongoing and movement-evoked nocifensive behaviors. Administration of the antagonist had similar efficacy in reducing early, moderate, and severe pain-related responses, suggesting that TRPV1 may be a novel target for pharmacological treatment of chronic pain states associated with bone cancer metastasis.

Anti-NGF therapy profoundly reduces bone cancer pain and the accompanying increase in markers of peripheral and central sensitization

Bone cancer pain can be difficult to control, as it appears to be driven simultaneously by inflammatory, neuropathic and tumorigenic mechanisms. As nerve growth factor (NGF) has been shown to modulate inflammatory and neuropathic pain states, we focused on a novel NGF sequestering antibody and demonstrated that two administrations of this therapy in a mouse model of bone cancer pain produces a profound reduction in both ongoing and movement‐evoked bone cancer pain‐related behaviors that was greater than that achieved with acute administration of 10 or 30 mg/kg of morphine. This therapy also reduced several neurochemical changes associated with peripheral and central sensitization in the dorsal root ganglion and spinal cord, whereas the therapy did not influence disease progression or markers of sensory or sympathetic innervation in the skin or bone. Mechanistically, the great majority of sensory fibers that innervate the bone are CGRP/TrkA expressing fibers, and if the sensitization and activation of these fibers is blocked by anti‐NGF therapy there would not be another population of nociceptors, such as the non‐peptidergic IB4/RET‐IR nerve fibers, to take their place in signaling nociceptive events.

A Blocking Antibody to Nerve Growth Factor Attenuates Skeletal Pain Induced by Prostate Tumor Cells Growing in Bone

Prostate cancer is unique in that bone is often the only clinically detectable site of metastasis. Prostate tumors that have metastasized to bone frequently induce bone pain which can be difficult to fully control as it seems to be driven simultaneously by inflammatory, neuropathic, and tumorigenic mechanisms. As nerve growth factor (NGF) has been shown to modulate inflammatory and some neuropathic pain states in animal models, an NGF-sequestering antibody was administered in a prostate model of bone cancer where significant bone formation and bone destruction occur simultaneously in the mouse femur. Administration of a blocking antibody to NGF produced a significant reduction in both early and late stage bone cancer pain-related behaviors that was greater than or equivalent to that achieved with acute administration of 10 or 30 mg/kg of morphine sulfate. In contrast, this therapy did not influence tumor-induced bone remodeling, osteoblast proliferation, osteoclastogenesis, tumor growth, or markers of sensory or sympathetic innervation in the skin or bone. One rather unique aspect of the sensory innervation of bone, that may partially explain the analgesic efficacy of anti-NGF therapy in relieving prostate cancer-induced bone pain, is that nearly all nerve fibers that innervate the bone express trkA and p75, and these are the receptors through which NGF sensitizes and/or activates nociceptors. The present results suggest that anti-NGF therapy may be effective in reducing pain and enhancing the quality of life in patients with prostate tumor-induced bone cancer pain.

Intravenous paclitaxel administration in the rat induces a peripheral sensory neuropathy characterized by macrophage infiltration and injury to sensory neurons and their supporting cells.

Paclitaxel-induced peripheral neuropathy (PN) can be a significant problem for patients receiving chemotherapeutic regimens for the treatment of breast, ovarian, and lung cancer as PN can influence the quality of life and survivorship in these patients. To begin to understand the cellular changes that occur within the peripheral and central nervous system as PN develops, we intravenously infused rats with clinically relevant doses of paclitaxel. Ten days later, behavioral changes indicative of PN became evident that included mechanical allodynia, cold hyperalgesia, and deficits in ambulation/coordination. These behaviors were accompanied by increased expression of activating transcription factor 3 (ATF3; a marker of cellular injury) in a population of large>medium>small diameter sensory neurons, a population of satellite cells in the lumbar dorsal root ganglia (DRG) and in myelinating Schwann cells in the sciatic nerve. In addition, there was an increase in the expression of glial fibrillary acidic protein (GFAP) in DRG satellite cells and an increase in the number of CD68 positive activated macrophages within the DRG and peripheral nerve. Within lamina III-IV of the lumbar spinal cord, there was an increase in OX42 positive microglia. These data suggest that intravenous infusion of paclitaxel induces a peripheral neuropathy characterized by injury of neuronal and non-neuronal cells in the peripheral nervous system, macrophage activation in both the DRG and peripheral nerve, and microglial activation within the spinal cord. An understanding of the factors involved in the development and maintenance of PN may lead to mechanism based therapies that prevent/treat PN and thus improve the survival and quality of life of patients receiving chemotherapy.

Tumor-induced injury of primary afferent sensory nerve fibers in bone cancer pain

Bone is the most common site of chronic pain in patients with metastatic cancer. What remains unclear are the mechanisms that generate this pain and why bone cancer pain can be so severe and refractory to treatment with opioids. Here we show that following injection and confinement of NCTC 2472 osteolytic tumor cells within the mouse femur, tumor cells sensitize and injure the unmyelinated and myelinated sensory fibers that innervate the marrow and mineralized bone. This tumor-induced injury of sensory nerve fibers is accompanied by an increase in ongoing and movement-evoked pain behaviors, an upregulation of activating transcription factor 3 (ATF3) and galanin by sensory neurons that innervate the tumor-bearing femur, upregulation of glial fibrillary acidic protein (GFAP) and hypertrophy of satellite cells surrounding sensory neuron cell bodies within the ipsilateral dorsal root ganglia (DRG), and macrophage infiltration of the DRG ipsilateral to the tumor-bearing femur. Similar neurochemical changes have been described following peripheral nerve injury and in other non-cancerous neuropathic pain states. Chronic treatment with gabapentin did not influence tumor growth, tumor-induced bone destruction or the tumor-induced neurochemical reorganization that occurs in sensory neurons or the spinal cord, but it did attenuate both ongoing and movement-evoked bone cancer-related pain behaviors. These results suggest that even when the tumor is confined within the bone, a component of bone cancer pain is due to tumor-induced injury to primary afferent nerve fibers that innervate the tumor-bearing bone. Tumor-derived, inflammatory, and neuropathic mechanisms may therefore be simultaneously driving this chronic pain state.

Endothelin and the tumorigenic component of bone cancer pain

Tumors including sarcomas and breast, prostate, and lung carcinomas frequently grow in or metastasize to the skeleton where they can induce significant bone remodeling and cancer pain. To define products that are released from tumors that are involved in the generation and maintenance of bone cancer pain, we focus here on endothelin-1 (ET-1) and endothelin receptors as several tumors including human prostate and breast have been shown to express high levels of ETs and the application of ETs to peripheral nerves can induce pain. Here we show that in a murine osteolytic 2472 sarcoma model of bone cancer pain, the 2472 sarcoma cells express high levels of ET-1, but express low or undetectable levels of endothelin A (ETAR) or B (ETBR) receptors whereas a subpopulation of sensory neurons express the ETAR and non-myelinating Schwann cells express the ETBR. Acute (10 mg/kg, i.p.) or chronic (10 mg/kg/day, p.o.) administration of the ETAR selective antagonist ABT-627 significantly attenuated ongoing and movement-evoked bone cancer pain and chronic administration of ABT-627 reduced several neurochemical indices of peripheral and central sensitization without influencing tumor growth or bone destruction. In contrast, acute treatment (30 mg/kg, i.p.) with the ETBR selective antagonist, A-192621 increased several measures of ongoing and movement evoked pain. As tumor expression and release of ET-1 has been shown to be regulated by the local environment, location specific expression and release of ET-1 by tumor cells may provide insight into the mechanisms that underlie the heterogeneity of bone cancer pain that is frequently observed in humans with multiple skeletal metastases.

Repeated dosing of ABT-102, a potent and selective TRPV1 antagonist, enhances TRPV1-mediated analgesic activity in rodents, but attenuates antagonist-induced hyperthermia

ABSTRACT Transient receptor potential vanilloid type 1 (TRPV1) is a ligand‐gated ion channel that functions as an integrator of multiple pain stimuli including heat, acid, capsaicin and a variety of putative endogenous lipid ligands. TRPV1 antagonists have been shown to decrease inflammatory pain in animal models and to produce limited hyperthermia at analgesic doses. Here, we report that ABT‐102, which is a potent and selective TRPV1 antagonist, is effective in blocking nociception in rodent models of inflammatory, post‐operative, osteoarthritic, and bone cancer pain. ABT‐102 decreased both spontaneous pain behaviors and those evoked by thermal and mechanical stimuli in these models. Moreover, we have found that repeated administration of ABT‐102 for 5–12 days increased its analgesic activity in models of post‐operative, osteoarthritic, and bone cancer pain without an associated accumulation of ABT‐102 concentration in plasma or brain. Similar effects were also observed with a structurally distinct TRPV1 antagonist, A‐993610. Although a single dose of ABT‐102 produced a self‐limiting increase in core body temperature that remained in the normal range, the hyperthermic effects of ABT‐102 effectively tolerated following twice‐daily dosing for 2 days. Therefore, the present data demonstrate that, following repeated administration, the analgesic activity of TRPV1 receptor antagonists is enhanced, while the associated hyperthermic effects are attenuated. The analgesic efficacy of ABT‐102 supports its advancement into clinical studies.

Bone cancer pain: The effects of the bisphosphonate alendronate on pain, skeletal remodeling, tumor growth and tumor necrosis

Abstract Patients with metastatic breast, lung or prostate cancer frequently have significant bone cancer pain. In the present report we address, in a single in vivo mouse model, the effects the bisphosphonate alendronate has on bone cancer pain, bone remodeling and tumor growth and necrosis. Following injection and confinement of green fluorescent protein‐transfected murine osteolytic tumor cells into the marrow space of the femur of male C3H/HeJ mice, alendronate was administered chronically from the time the tumor was established until the bone cancer pain became severe. Alendronate therapy reduced ongoing and movement‐evoked bone cancer pain, bone destruction and the destruction of sensory nerve fibers that innervate the bone. Whereas, alendronate treatment did not change viable tumor burden, both tumor growth and tumor necrosis increased. These data emphasize that it is essential to utilize a model where pain, skeletal remodeling and tumor growth can be simultaneously assessed, as each of these can significantly impact patient quality of life and survival.

Similarities and differences in tumor growth, skeletal remodeling and pain in an osteolytic and osteoblastic model of bone cancer

More than 1.3 million cases of cancer will be diagnosed in 2006 in the United States alone, and 90% of patients with advanced cancer will experience significant, life-altering cancer-induced pain. Bone cancer pain is the most common pain in patients with advanced cancer as most common tumors including breast, prostate, and lung have a remarkable affinity to metastasize to bone. Once tumors metastasize to bone they are a major cause of morbidity and mortality as the tumor induces significant skeletal remodeling, pain and anemia, which reduce the survival and quality of life of the patient. Currently, the factors that drive cancer pain are poorly understood; however, several recently introduced models of cancer pain are not only providing insight into the mechanisms that drive bone cancer pain but are guiding the development of novel mechanism-based therapies to treat the pain and skeletal remodeling that accompanies metatstatic bone cancer. As analgesics can also influence disease progression, findings from these studies may lead to therapies that have the potential to improve the quality of life and survival of patients with skeletal malignancies.

Pancreatic cancer pain and its correlation with changes in tumor vasculature, macrophage infiltration, neuronal innervation, body weight and disease progression

Abstract To begin to understand the relationship between disease progression and pain in pancreatic cancer, transgenic mice that develop pancreatic cancer due to the expression of the simian virus 40 large T antigen under control of the rat elastase‐1 promoter were examined. In these mice precancerous cellular changes were evident at 6 weeks and these included an increase in: microvascular density, macrophages that express nerve growth factor and the density of sensory and sympathetic fibers that innervate the pancreas, with all of these changes increasing with tumor growth. In somatic tissue such as skin, the above changes would be accompanied by significant pain; however, in mice with pancreatic cancer, changes in pain‐related behaviors, such as morphine‐reversible severe hunching and vocalization only became evident at 16 weeks of age, by which time the pancreatic cancer was highly advanced. These data suggest that in mice as well as humans, there is a stereotypic set of pathological changes that occur as pancreatic cancer develops, and while weight loss generally tracks disease progression, there is a significant lag between disease progression and behaviors indicative of pancreatic cancer pain. Defining the mechanisms that mask this pain in early and mid‐stage disease and drive the pain in late‐stage disease may aid in earlier diagnosis, survival, and increased quality of life of patients with pancreatic cancer.