Optimization and characterization of a rat model of prostate cancer-induced bone pain using behavioral, pharmacological, radiological, histological and immunohistochemical methods

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Glycinergic Modulation of Pain in Behavioral Animal Models

Animal models of human pain conditions allow for detailed interrogation of known and hypothesized mechanisms of pain physiology in awake, behaving organisms. The importance of the glycinergic system for pain modulation is well known; however, manipulation of this system to treat and alleviate pain has not yet reached the sophistication required for the clinic. Here, we review the current literature on what animal behavioral studies have allowed us to elucidate about glycinergic pain modulation, and the progress toward clinical treatments so far. First, we outline the animal pain models that have been used, such as nerve injury models for neuropathic pain, chemogenic pain models for acute and inflammatory pain, and other models that mimic painful human pathologies such as diabetic neuropathy. We then discuss the genetic approaches to animal models that have identified the crucial glycinergic machinery involved in neuropathic and inflammatory pain. Specifically, two glycine receptor (GlyR) subtypes, GlyRα1(β) and GlyRα3(β), and the two glycine transporters (GlyT), GlyT1 and GlyT2. Finally, we review the different pharmacological approaches to manipulating the glycinergic system for pain management in animal models, such as partial vs. full agonism, reversibility, and multi-target approaches. We discuss the benefits and pitfalls of using animal models in drug development broadly, as well as the progress of glycinergic treatments from preclinical to clinical trials.

Gait analysis as a robust pain behavioural endpoint in the chronic phase of the monoiodoacetate-induced knee joint pain in the rat

The monoiodoacetate-induced rat model of osteoarthritis knee pain is widely used. However, there are between-study differences in the pain behavioural endpoints assessed and in the dose of intraarticular monoiodoacetate administered. This study evaluated the robustness of gait analysis as a pain behavioural endpoint in the chronic phase of this model, in comparison with mechanical hyperalgesia in the injected (ipsilateral) joint and development of mechanical allodynia in the ipsilateral hind paws. Groups of Sprague-Dawley rats received a single intraarticular injection of monoiodoacetate at 0.5, 1, 2 or 3 mg or vehicle (saline) into the left (ipsilateral) knee joint. An additional group of rats were not injected (naïve group). The pain behavioural methods used were gait analysis, measurement of pressure algometry thresholds in the ipsilateral knee joints, and assessment of mechanical allodynia in the ipsilateral hind paws using von Frey filaments. These pain behavioural endpoints were assessed premonoiodoacetate injection and for up to 42-days postmonoiodoacetate injection in a blinded manner. Body weights were also assessed as a measure of general health. Good general health was maintained as all rats gained weight at a similar rate for the 42-day study period. In the chronic phase of the model (days 9-42), intraarticular monoiodoacetate at 3 mg evoked robust alterations in multiple gait parameters as well as persistent mechanical allodynia in the ipsilateral hind paws. For the chronic phase of the monoiodoacetate-induced rat model of osteoarthritis knee pain, gait analysis, such as mechanical allodynia in the ipsilateral hind paws, is a robust pain behavioural measure.

Pharmacological characterization of the chronic phase of the monoiodoacetate-induced rat model of osteoarthritis pain in the knee joint

For patients with osteoarthritis (OA) of the knee, pain is the most debilitating symptom. Although it has been proposed that the chronic phase of the monoiodoacetate (MIA)-induced rodent model of knee joint pain may be superior to other chronic or acute OA models for assessing the analgesic efficacy of novel molecules, relatively few pharmacological studies have been conducted in the chronic phase of this model. Hence, this study was designed to use pharmacological methods to characterize the chronic phase of the MIA-induced rat model of knee joint OA pain. Rats received a single intraarticular injection of MIA at 2.5 mg or vehicle (saline) into the left (ipsilateral) knee joint. Pain behaviour was assessed by measuring paw withdrawal thresholds (PWTs) in the hindpaws pre-MIA injection and twice-weekly until study completion on day 42. Mechanical allodynia was fully developed in the ipsilateral hindpaws (PWTs ≤6 g) from day 7 and it persisted until day 42. MIA-injected rats with PWTs ≤6 g in the ipsilateral hindpaws received single doses of one of four clinically available drugs that represent four distinct pharmacological classes, viz gabapentin, amitriptyline, meloxicam and morphine, according to a 'washout' protocol with at least 48 hours between successive doses. Gabapentin evoked dose-dependent anti-allodynia as did morphine whereas amitriptyline and meloxicam were inactive. Our findings are aligned with clinical data showing that gabapentin and morphine alleviated OA pain in the knee. The lack of efficacy of amitriptyline is consistent with the loss of descending diffuse noxious inhibitory controls reported by others in this model.

Animal Models of Cancer-Related Pain: Current Perspectives in Translation

The incidence of pain in cancer patients during diagnosis and treatment is exceedingly high. Although advances in cancer detection and therapy have improved patient prognosis, cancer and its treatment-associated pain have gained clinical prominence. The biological mechanisms involved in cancer-related pain are multifactorial; different processes for pain may be responsible depending on the type and anatomic location of cancer. Animal models of cancer-related pain have provided mechanistic insights into the development and process of pain under a dynamic molecular environment. However, while cancer-evoked nociceptive responses in animals reflect some of the patients’ symptoms, the current models have failed to address the complexity of interactions within the natural disease state. Although there has been a recent convergence of the investigation of carcinogenesis and pain neurobiology, identification of new targets for novel therapies to treat cancer-related pain requires standardization of methodologies within the cancer pain field as well as across disciplines. Limited success of translation from preclinical studies to the clinic may be due to our poor understanding of the crosstalk between cancer cells and their microenvironment (e.g., sensory neurons, infiltrating immune cells, stromal cells etc.). This relatively new line of inquiry also highlights the broader limitations in translatability and interpretation of basic cancer pain research. The goal of this review is to summarize recent findings in cancer pain based on preclinical animal models, discuss the translational benefit of these discoveries, and propose considerations for future translational models of cancer pain.

Assessment of the anti-allodynic efficacy of a glycine transporter 2 inhibitor relative to pregabalin and duloxetine in a rat model of prostate cancer-induced bone pain

BACKGROUND

The pathobiology of prostate cancer-induced bone pain (PCIBP) is underpinned by both inflammatory and neuropathic components. Here, we used a rat model of PCIBP to assess the analgesic efficacy of a glycine transporter 2 (GlyT2) inhibitor (N-(6-((1,3-dihydroxypropan-2-yl)amino)-2-(dimethylamino)pyridin-3-yl)-3,5-dimethoxy-4-(4-(trifluoromethyl)phenoxy) benzamide) relative to two clinically available adjuvant drugs that are recommended for the relief of neuropathic pain, viz, pregabalin and duloxetine.

METHODS

PCIBP was induced in male Wistar Han rats following intra-tibial injection (ITI) of rat prostate cancer (AT3B) cells into the left tibia. Sham-rats received an ITI of heat-killed AT3B cells. PCIBP rats with fully developed mechanical allodynia in the ipsilateral hindpaws as assessed using von Frey filaments, received single oral (p.o.) bolus doses of the GlyT2 inhibitor (3-30 mg/kg), pregabalin (3-100 mg/kg), duloxetine (3-100 mg/kg), or vehicle. Baseline paw withdrawal thresholds (PWTs) were determined in the ipsilateral (injured side) and contralateral hindpaws immediately prior to dosing and at scheduled times for 3 h post dosing in individual animals.

RESULTS

Single oral bolus doses of the GlyT2 inhibitor (3-30 mg/kg) evoked partial pain relief at the doses tested in the ipsilateral hindpaws of PCIBP rats without any discernible behavioural side effects. By contrast, single oral bolus doses of pregabalin at 10-100 mg/kg evoked dose-dependent and complete alleviation of mechanical allodynia. By comparison, single oral bolus doses of duloxetine at doses up to 100 mg/kg lacked efficacy.

CONCLUSION

Oral administration of this GlyT2 inhibitor evoked partial pain relief in PCIBP rats and did not evoke central nervous system side effects in contrast to GlyT2 inhibitors reported by others.

Transcriptomic characterisation of the optimised rat model of Walker 256 breast cancer cell-induced bone pain

In patients with breast cancer, metastases of cancer cells to the axial skeleton may cause excruciating pain, particularly in the advanced stages. The current drug treatments available to alleviate this debilitating pain condition often lack efficacy and/or produce undesirable side effects. Preclinical animal models of cancer-induced bone pain are key to studying the mechanisms that cause this pain and for the success of drug discovery programs. In a previous study conducted in our laboratory, we validated and characterised the rat model of Walker 256 cell-induced bone pain, which displayed several key resemblances to the human pain condition. However, gene level changes that occur in the pathophysiology of cancer-induced bone pain in this preclinical model are unknown. Hence, in this study, we performed the transcriptomic characterisation of the Walker 256 cell line cultured in vitro to predict the molecular genetic profile of this cell line. We also performed transcriptomic characterisation of the Walker 256 cell-induced bone pain model in rats using the lumbar spinal cord and lumbar dorsal root ganglia tissues. Here we show that the Walker 256 cell line resembles the basal-B molecular subtype of human breast cancer cell lines. We also identify several genes that may underpin the progression of pain hypersensitivities in this condition, however, this needs further confirmatory studies. These transcriptomic insights have the potential to direct future studies aimed at identifying various mechanisms underpinning pain hypersensitivities in this model that may also assist in discovery of novel pain therapeutics for breast cancer-induced bone pain.

Antinociceptive effects of Ginsenoside Rb1 in a rat model of cancer-induced bone pain

Ginsenoside Rb1 (GRb1) is a major ingredient of ginseng, a traditional medicine that has been used for thousands of years. Previous studies have reported that GRb1 had anti-inflammatory, antioxidant and neuroprotective effects. The current study aimed to evaluate the antinociceptive effects of GRb1 in a rat model of cancer-induced bone pain (CIBP) established by intratibial injection of Walker 256 cells. Intraperitoneal injection (i.p.) of GRb1 (5 and 10 mg/kg, but not 1 mg/kg) partially and transiently reversed the mechanical allodynia and thermal hyperalgesia in CIBP rats at 14 days following surgery when the pain behavior is established. Furthermore, repeated administration of GRb1 demonstrated persistent analgesic effect. Additionally, the protein expression and immunoreactivity of iba1, which is the maker of microglia, was significantly suppressed in CIBP rats treated with GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days compared with CIBP rats treated with a vehicle. Furthermore, upregulation of spinal interleukin (IL)-1β, IL-6 and tumor necrosis factor-α were also significantly inhibited by the treatment of GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days. Together, these results indicated that GRb1 may attenuate CIBP via inhibiting the activation of microglia and glial-derived proinflammatory cytokines.

The Somatostatin Receptor-4 Agonist J-2156 Alleviates Mechanical Hypersensitivity in a Rat Model of Breast Cancer Induced Bone Pain

In the majority of patients with breast cancer in the advanced stages, skeletal metastases are common, which may cause excruciating pain. Currently available drug treatments for relief of breast cancer-induced bone pain (BCIBP) include non-steroidal anti-inflammatory drugs and strong opioid analgesics along with inhibitors of osteoclast activity such as bisphosphonates and monoclonal antibodies such as denosumab. However, these medications often lack efficacy and/or they may produce serious dose-limiting side effects. In the present study, we show that J-2156, a somatostatin receptor type 4 (SST4 receptor) selective agonist, reverses pain-like behaviors in a rat model of BCIBP induced by unilateral intra-tibial injection of Walker 256 breast cancer cells. Following intraperitoneal administration, the ED₅₀ of J-2156 for the relief of mechanical allodynia and mechanical hyperalgesia in the ipsilateral hindpaws was 3.7 and 8.0 mg/kg, respectively. Importantly, the vast majority of somatosensory neurons in the dorsal root ganglia including small diameter C-fibers and medium-large diameter fibers, that play a crucial role in cancer pain hypersensitivities, expressed the SST4 receptor. J-2156 mediated pain relief in BCIBP-rats was confirmed by observations of a reduction in the levels of phosphorylated extracellular signal-regulated kinase (pERK), a protein essential for central sensitization and persistent pain, in the spinal dorsal horn. Our results demonstrate the potential of the SST4 receptor as a pharmacological target for relief of BCIBP and we anticipate the present work to be a starting point for further mechanism-based studies.

Establishment and Characterization of a Novel Rat Model of Mechanical Low Back Pain Using Behavioral, Pharmacologic and Histologic Methods

Chronic low back pain (LBP), the leading cause of disability globally, is notoriously difficult to treat. Most rodent models of LBP mimic lumbar radicular pain rather than mechanical LBP. Here, we describe establishment of a new rat model of mechanical LBP that is devoid of a neuropathic component. Groups of adult male Sprague Dawley rats were anesthetized and their lumbar L4/L5 and L5/L6 intervertebral disks (IVDs) were punctured (0.5 mm outer diameter, 2mm-deep) 5 (LPB-5X), or 10 (LBP-10X) times per disk. Sham-rats underwent similar surgery, but without disk puncture. Baseline noxious pressure hyperalgesia of lumbar axial deep tissues, mechanical allodynia in the hindpaws and gait were assessed prior to surgery and once-weekly until study completion on day 49. The model was also characterized using pharmacologic and histologic methods. Good animal health was maintained for ≥ 49 days post-surgery. For LBP- but not sham-rats, there was temporal development of noxious pressure hyperalgesia in lumbar axial deep tissues at days 14–49 post-surgery. Importantly, there were no between-group differences in von Frey paw withdrawal thresholds or gait parameters until study completion. On day 49, significant histologic changes were observed in the L4/L5 and L5/L6 IVDs for LBP-10X rats, but not sham-rats. In LBP-10X rats, single bolus doses of morphine produced dose-dependent relief of primary and secondary mechanical hyperalgesia in lumbar axial deep tissues at L4/L5 and L1, respectively. In conclusion, our new rat model has considerable potential for providing novel insight on the pathobiology of mechanical LBP and for analgesic efficacy assessment of novel compounds.

Optimization and In Vivo Profiling of a Refined Rat Model of Walker 256 Breast Cancer Cell-Induced Bone Pain Using Behavioral, Radiological, Histological, Immunohistochemical and Pharmacological Methods

In the majority of patients with advanced breast cancer, there is metastatic spread to bones resulting in pain. Clinically available drug treatments for alleviation of breast cancer-induced bone pain (BCIBP) often produce inadequate pain relief due to dose-limiting side-effects. A major impediment to the discovery of novel well-tolerated analgesic agents for the relief of pain due to bony metastases is the fact that most cancer-induced bone pain models in rodents relied on the systemic injection of cancer cells, causing widespread formation of cancer metastases and poor general animal health. Herein, we have established an optimized, clinically relevant Wistar Han female rat model of breast cancer induced bone pain which was characterized using behavioral assessments, radiology, histology, immunohistochemistry and pharmacological methods. In this model that is based on unilateral intra-tibial injection (ITI) of Walker 256 carcinoma cells, animals maintained good health for at least 66 days post-ITI. The temporal development of hindpaw hypersensitivity depended on the initial number of Walker 256 cells inoculated in the tibiae. Hindpaw hypersensitivity resolved after approximately 25 days, in the continued presence of bone tumors as evidenced by ex vivo histology, micro-computed tomography scans and immunohistochemical assessments of tibiae. A possible role for the endogenous opioid system as an internal factor mediating the self-resolving nature of BCIBP was identified based upon the observation that naloxone, a non-selective opioid antagonist, caused the re-emergence of hindpaw hypersensitivity. Bolus dose injections of morphine, gabapentin, amitriptyline and meloxicam all alleviated hindpaw hypersensitivity in a dose-dependent manner. This is a first systematic pharmacological profiling of this model by testing standard analgesic drugs from four important diverse classes, which are used to treat cancer induced bone pain in the clinical setting. Our refined rat model more closely mimics the pathophysiology of this condition in humans and hence is well-suited for probing the mechanisms underpinning breast cancer induced bone pain. In addition, the model may be suitable for efficacy profiling of new molecules from drug discovery programs with potential to be developed as novel agents for alleviation of intractable pain associated with disseminated breast cancer induced bony metastases.

The Effect of Gabapentin and Tramadol in Cancer Pain Induced by Glioma Cell in Rat Femur

Preclinical Research The presence of pain as part of the cancer process is variable. Glioblastoma multiform (GBM) can produce bone metastasis, a condition that involves other pathological phenotypes including neuropathic and inflammatory pain. Tramadol and gabapentin are drugs used in the treatment of neuropathic pain. However, there are no studies evaluating their analgesic effects in bone metastasis. We produced a pain model induced by the inoculation of glioma cells (10⁵ ) into the rat femur, by perforating the intercodiloid fossa. Painful behavior was evaluated by measuring mechanical allodynia using the Von Frey test while thermal hyperalgesia was assessed in the plantar test. Histopathological features were evaluated and antinociceptive responses were compared using tramadol and gabapentin. The inoculation of cells inside the right femur produced nociceptive behaviors. Tramadol and gabapentin produced an anti-allodynic effect in this condition, but tramadol did not produce an anti-hyperalgesic response. The development of this model will allow us to perform tests to elucidate the pathology of bone metastasis, cancer pain, and in particular the pain produced by glioma. Drug Dev Res 78 : 173-183, 2017. © 2017 Wiley Periodicals, Inc.

Selective small molecule angiotensin II type 2 receptor antagonists for neuropathic pain: preclinical and clinical studies

Neuropathic pain affects up to 10% of the general population, but drug treatments recommended for the treatment of neuropathic pain are associated with modest efficacy and/or produce dose-limiting side effects. Hence, neuropathic pain is an unmet medical need. In the past 2 decades, research on the pathobiology of neuropathic pain has revealed many novel pain targets for use in analgesic drug discovery programs. However, these efforts have been largely unsuccessful as molecules that showed promising pain relief in rodent models of neuropathic pain generally failed to produce analgesia in early phase clinical trials in patients with neuropathic pain. One notable exception is the angiotensin II type 2 (AT2) receptor that has clinical validity on the basis of a successful double-blind, randomized, placebo-controlled, clinical trial of EMA401, a highly selective, orally active, peripherally restricted AT2 receptor antagonist in patients with postherpetic neuralgia. In this study, we review research to date on target validation, efficacy, and mode of action of small molecule AT2 receptor antagonists in rodent models of peripheral neuropathic pain and in cultured human sensory neurons, the preclinical pharmacokinetics of these compounds, and the outcome of the above clinical trial.

The Walker 256 Breast Cancer Cell- Induced Bone Pain Model in Rats

The majority of patients with terminal breast cancer show signs of bone metastasis, the most common cause of pain in cancer. Clinically available drug treatment options for the relief of cancer-associated bone pain are limited due to either inadequate pain relief and/or dose-limiting side-effects. One of the major hurdles in understanding the mechanism by which breast cancer causes pain after metastasis to the bones is the lack of suitable preclinical models. Until the late twentieth century, all animal models of cancer induced bone pain involved systemic injection of cancer cells into animals, which caused severe deterioration of animal health due to widespread metastasis. In this mini-review we have discussed details of a recently developed and highly efficient preclinical model of breast cancer induced bone pain: Walker 256 cancer cell- induced bone pain in rats. The model involves direct localized injection of cancer cells into a single tibia in rats, which avoids widespread metastasis of cancer cells and hence animals maintain good health throughout the experimental period. This model closely mimics the human pathophysiology of breast cancer induced bone pain and has great potential to aid in the process of drug discovery for treating this intractable pain condition.

Comparison of Burrowing and Stimuli-Evoked Pain Behaviors as End-Points in Rat Models of Inflammatory Pain and Peripheral Neuropathic Pain

Establishment and validation of ethologically-relevant, non-evoked behavioral end-points as surrogate measures of spontaneous pain in rodent pain models has been proposed as a means to improve preclinical to clinical research translation in the pain field. Here, we compared the utility of burrowing behavior with hypersensitivity to applied mechanical stimuli for pain assessment in rat models of chronic inflammatory and peripheral neuropathic pain. Briefly, groups of male Sprague-Dawley rats were habituated to the burrowing environment and trained over a 5-day period. Rats that burrowed ≤ 450 g of gravel on any 2 days of the individual training phase were excluded from the study. The remaining rats received either a unilateral intraplantar injection of Freund's complete adjuvant (FCA) or saline, or underwent unilateral chronic constriction injury (CCI) of the sciatic nerve- or sham-surgery. Baseline burrowing behavior and evoked pain behaviors were assessed prior to model induction, and twice-weekly until study completion on day 14. For FCA- and CCI-rats, but not the corresponding groups of sham-rats, evoked mechanical hypersensitivity developed in a temporal manner in the ipsilateral hindpaws. Although burrowing behavior also decreased in a temporal manner for both FCA-and CCI- rats, there was considerable inter-animal variability. By contrast, mechanical hyperalgesia and mechanical allodynia in the ipsilateral hindpaws of FCA- and CCI-rats respectively, exhibited minimal inter-animal variability. Our data collectively show that burrowing behavior is altered in rodent models of chronic inflammatory pain and peripheral neuropathic pain. However, large group sizes are needed to ensure studies are adequately powered due to considerable inter-animal variability.

Randall Selitto pressure algometry for assessment of bone-related pain in rats

BACKGROUND

Deep pain is neglected compared with cutaneous sources. Pressure algometry has been validated in the clinic for assessment of bone-related pain in humans. In animal models of bone-related pain, we have validated the Randall Selitto behavioural test for assessment of acute and pathological bone pain and compared the outcome with more traditional pain-related behaviour measures.

METHODS

Randall Selitto pressure algometry was performed over the anteromedial part of the tibia in naïve rats, sham-operated rats, and rats inoculated with MRMT-1 carcinoma cells in the left tibia, and the effect of morphine was investigated. Randall Selitto measures of cancer-induced bone pain were supplemented by von Frey testing, weight-bearing and limb use test. Contribution of cutaneous nociception to Randall Selitto measures were examined by local anaesthesia.

RESULTS

Randall Selitto pressure algometry over the tibia resulted in reproducible withdrawal thresholds, which were dose-dependently increased by morphine. Cutaneous nociception did not contribute to Randall Selitto measures. In cancer-bearing animals, compared with sham, significant differences in pain-related behaviours were demonstrated by the Randall Selitto test on day 17 and 21 post-surgery. A difference was also demonstrated by von Frey testing, weight-bearing and limb use tests.

CONCLUSION

Our results indicate that pressure applied by the Randall Selitto algometer on a region, where the bone is close to the skin, may offer a way to measure bone-related pain in animal models and could provide a supplement to the traditional behavioural tests and a means to study deep pain.

Optimization and pharmacological characterization of a refined cisplatin-induced rat model of peripheral neuropathic pain

Chemotherapy-induced peripheral neuropathy (CIPN) is the major dose-limiting side-effect of many front-line anticancer drugs. This study was designed to establish and pharmacologically characterize a refined rat model of cisplatin-induced CIPN. Adult male Sprague-Dawley rats received four (n=18) or five (n=18) single intraperitoneal bolus doses of cisplatin at 3 mg/kg, or saline (control group), once-weekly. Body weight and general health were assessed over a 49-day study period. von Frey filaments and the Hargreaves test were used to define the time course for the development of mechanical allodynia and thermal hypoalgesia in the hindpaws and for efficacy assessment of analgesic/adjuvant agents. The general health of rats administered four cisplatin doses was superior to that of rats administered five doses. Mechanical allodynia was fully developed (paw withdrawal thresholds≤6 g) in the bilateral hindpaws from day 32 to 49 for both cisplatin dosing regimens. They also showed significant thermal hypoalgesia in the bilateral hindpaws. In cisplatin-treated rats with paw withdrawal thresholds of up to 6 g, single bolus doses of gabapentin and morphine produced dose-dependent analgesia, whereas meloxicam and amitriptyline lacked efficacy. We have established and pharmacologically characterized a refined rat model of CIPN that is suitable for efficacy profiling of compounds from analgesic discovery programmes.

Primary tumour growth in an orthotopic osteosarcoma mouse model is not influenced by analgesic treatment with buprenorphine and meloxicam

Little is known about the treatment of bone pain in animal models of bone cancer. In the present study, the orthotopic 143-B human osteosarcoma xenotransplantation model was used to address the following questions: (1) Can repetitive analgesic treatment extend the experimental period by prolonging the time to reach humane endpoints and (2) Does repetitive analgesic treatment affect bone tumour development and metastasis? The analgesics, buprenorphine and meloxicam, were either applied individually or in combination at 12 h intervals as soon as the animals began to avoid using the tumour cell injected leg. While control mice treated with NaCl showed continuous body weight loss, the major criterion previously for terminating the experiments, animals treated with analgesic substances did not. The control mice had to be sacrificed 26 days after tumour cell injection, whereas the groups of animals with the different pain treatments were euthanized after an additional eight days. Importantly, primary intratibial tumour growth was not affected in any of the experimental groups by any of the pain treatment procedures. Between days 26 and 34 after tumour cell injection an increase of about 100% of the number of lung metastases was found for the groups treated with buprenorphine alone or together with meloxicam, but not for the group treated with meloxicam alone. In summary, the results indicated that both buprenorphine and meloxicam are suitable analgesics for prolonging the experimental periods in an experimental intratibial osteosarcoma mouse model.

Hypernociceptive responses following the intratibial inoculation of RM1 prostate cancer cells in mice

BACKGROUND

Pain due to bone metastases of prostatic origin is a relevant clinical issue. We study here the nociceptive responses obtained in mice receiving the intratibial inoculation of RM1 prostate cancer cells.

METHODS

10(2) -10(5) RM1 cells were inoculated to C57BL/6 mice and tumor development was analysed histologically and with luciferase-expressing RM1 cells. Spinal astroglial (GFAP) or microglial (Iba-1) expression was assessed with immunohistochemical methods and hypernociception was measured by the unilateral hot plate, the paw pressure and the von Frey tests. The analgesic effect of morphine, zoledronic acid or the CCR2 antagonist RS504393 was measured. Levels of the chemokines CCL2, CCL3, and CCL5 were determined by ELISA.

RESULTS

The inoculation of 10(3) RM1 cells induced tumoral growth in bone with a mixed osteoclastic/osteoblastic pattern and evoked astroglial, but not microglial, activation in the spinal cord. Hyperalgesia and allodynia were already established four days after inoculation and dose-dependently inhibited by the s.c. administration of morphine (1-5 mg/kg) or zoledronic acid (1-3 mg/kg). CCL2 and CCL5, but not CCL3, were released by RM1 cells in culture whereas only an increased presence of CCL2 was found in bone tumor homogenates. The administration of the CCR2 antagonist RS504393 (0.3-3 mg/kg) inhibited RM1 induced thermal hyperalgesia without modifying mechanical allodynia.

CONCLUSION

The intratibial inoculation of RM1 cells in immunocompetent mice induces hypernociceptive responses and can be useful to perform studies of bone cancer induced pain related to androgen-independent prostate cancer. The antinociceptive role derived from the blockade of the CCR2 chemokine receptors is further envisaged.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Targeting angiotensin II type 2 receptor pathways to treat neuropathic pain and inflammatory pain

INTRODUCTION

Neuropathic pain and chronic inflammatory pain are large unmet medical needs. Over the past two decades, numerous 'pain targets' have been identified for analgesic drug discovery. Despite promising results in rodent pain models, many compounds modulating such targets lacked efficacy in clinical trials. An exception is oral EMA401, a small-molecule angiotensin II type 2 receptor (AT2R) antagonist.

AREAS COVERED

Herein, angiotensin II/AT2R signaling-induced hyperexcitability and abnormal sprouting of cultured dorsal root ganglion neurons, together with radioligand binding, pharmacokinetics, analgesic efficacy and mode of action of small-molecule AT2R antagonists in rodent models of peripheral neuropathic and chronic inflammatory pain, are reviewed. The findings of a successful Phase IIa clinical trial of EMA401 in patients with neuropathic pain are presented in brief.

EXPERT OPINION

The functional importance of angiotensin II/AT2R signaling has remained enigmatic for decades, and there are no clinically available medications that target the AT2R. However, on the basis of preclinical findings and recent clinical trial data showing that the peripherally restricted, small-molecule AT2R antagonist, EMA401, successfully alleviated neuropathic pain in a Phase II clinical trial, the AT2R is receiving considerable attention as a new therapeutic target with human validation for the relief of peripheral neuropathic and chronic inflammatory pain conditions.

Analgesic efficacy and mode of action of a selective small molecule angiotensin II type 2 receptor antagonist in a rat model of prostate cancer-induced bone pain

OBJECTIVE

The pathobiology of prostate cancer (PCa)-induced bone pain (PCIBP) has both inflammatory and neuropathic components. Previously, we showed that small molecule angiotensin II type 2 receptor (AT2 R) antagonists with >1,000-fold selectivity over the angiotensin II type 1 receptor produced dose-dependent analgesia in a rat model of neuropathic pain. Here, we assessed the analgesic efficacy and mode of action of the AT2 R antagonist, EMA200, in a rat model of PCIBP.

METHODS

At 14-21 days after unilateral intratibial injection of AT3B PCa cells, rats exhibiting hindpaw hypersensitivity received single intravenous bolus doses of EMA200 (0.3-10 mg/kg) or vehicle, and analgesic efficacy was assessed. The mode of action was investigated using immunohistochemical, Western blot, and/or molecular biological methods in lumbar dorsal root ganglia (DRGs) removed from drug-naïve and EMA200-treated PCIBP rats relative to sham-control rats.

RESULTS

Intravenous bolus doses of EMA200 produced dose-dependent analgesia in PCIBP rats. Lumbar DRG levels of angiotensin II, nerve growth factor (NGF), tyrosine kinase A (TrkA), phospho-p38 mitogen-activated protein kinase (MAPK), and phospho-p44/p42 MAPK, but not the AT2 R, were increased significantly (P < 0.05) in PCIBP rats, c.f. the corresponding levels for sham controls. EMA200 produced analgesia in PCIBP rats by reducing elevated angiotensin II levels in the lumbar DRGs to attenuate augmented angiotensin II/AT2 R signaling. This in turn reduced augmented NGF/TrkA signaling in the lumbar DRGs. The net result was inhibition of p38 MAPK and p44/p42 MAPK activation.

CONCLUSION

Small molecule AT2 R antagonists are worthy of further investigation as novel analgesics for relief of intractable PCIBP and other pain types where hyperalgesia worsens symptoms.

Pathobiology and management of prostate cancer-induced bone pain: recent insights and future treatments

Prostate cancer (PCa) has a high propensity for metastasis to bone. Despite the availability of multiple treatment options for relief of PCa-induced bone pain (PCIBP), satisfactory relief of intractable pain in patients with advanced bony metastases is challenging for the clinicians because currently available analgesic drugs are often limited by poor efficacy and/or dose-limiting side effects. Rodent models developed in the past decade show that the pathobiology of PCIBP comprises elements of inflammatory, neuropathic and ischemic pain arising from ectopic sprouting and sensitization of sensory nerve fibres within PCa-invaded bones. In addition, at the cellular level, PCIBP is underpinned by dynamic cross talk between metastatic PCa cells, cellular components of the bone matrix, factors associated with the bone microenvironment as well as peripheral components of the somatosensory system. These insights are aligned with the clinical management of PCIBP involving use of a multimodal treatment approach comprising analgesic agents (opioids, NSAIDs), radiotherapy, radioisotopes, cancer chemotherapy agents and bisphosphonates. However, a major drawback of most rodent models of PCIBP is their short-term applicability due to ethical concerns. Thus, it has been difficult to gain insight into the mal(adaptive) neuroplastic changes occurring at multiple levels of the somatosensory system that likely contribute to intractable pain at the advanced stages of metastatic disease. Specifically, the functional responsiveness of noxious circuitry as well as the neurochemical signature of a broad array of pro-hyperalgesic mediators in the dorsal root ganglia and spinal cord of rodent models of PCIBP is relatively poorly characterized. Hence, recent work from our laboratory to develop a protocol for an optimized rat model of PCIBP will enable these knowledge gaps to be addressed as well as identification of novel targets for drug discovery programs aimed at producing new analgesics for the improved relief of intractable PCIBP.