Peripheral cannabinoids attenuate carcinoma-induced nociception in mice

Classical cannabinoid receptors as target in cancer-induced bone pain: a systematic review, meta-analysis and bioinformatics validation

To test the hypothesis that genetic and pharmacological modulation of the classical cannabinoid type 1 (CB1) and 2 (CB2) receptors attenuate cancer-induced bone pain, we searched Medline, Web of Science and Scopus for relevant skeletal and non-skeletal cancer studies from inception to July 28, 2022. We identified 29 animal and 35 human studies. In mice, a meta-analysis of pooled studies showed that treatment of osteolysis-bearing males with the endocannabinoids AEA and 2-AG (mean difference [MD] - 24.83, 95% confidence interval [95%CI] - 34.89, - 14.76, p < 0.00001) or the synthetic cannabinoid (CB) agonists ACPA, WIN55,212-2, CP55,940 (CB1/2-non-selective) and AM1241 (CB2-selective) (MD - 28.73, 95%CI - 45.43, - 12.02, p = 0.0008) are associated with significant reduction in paw withdrawal frequency. Consistently, the synthetic agonists AM1241 and JWH015 (CB2-selective) increased paw withdrawal threshold (MD 0.89, 95%CI 0.79, 0.99, p < 0.00001), and ACEA (CB1-selective), AM1241 and JWH015 (CB2-selective) reduced spontaneous flinches (MD - 4.85, 95%CI - 6.74, - 2.96, p < 0. 00001) in osteolysis-bearing male mice. In rats, significant increase in paw withdrawal threshold is associated with the administration of ACEA and WIN55,212-2 (CB1/2-non-selective), JWH015 and AM1241 (CB2-selective) in osteolysis-bearing females (MD 8.18, 95%CI 6.14, 10.21, p < 0.00001), and treatment with AM1241 (CB2-selective) increased paw withdrawal thermal latency in males (mean difference [MD]: 3.94, 95%CI 2.13, 5.75, p < 0.0001), confirming the analgesic capabilities of CB1/2 ligands in rodents. In human, treatment of cancer patients with medical cannabis (standardized MD - 0.19, 95%CI - 0.35, - 0.02, p = 0.03) and the plant-derived delta-9-THC (20 mg) (MD 3.29, CI 2.24, 4.33, p < 0.00001) or its synthetic derivative NIB (4 mg) (MD 2.55, 95%CI 1.58, 3.51, p < 0.00001) are associated with reduction in pain intensity. Bioinformatics validation of KEGG, GO and MPO pathway, function and process enrichment analysis of mouse, rat and human data revealed that CB1 and CB2 receptors are enriched in a cocktail of nociceptive and sensory perception, inflammatory, immune-modulatory, and cancer pathways. Thus, we cautiously conclude that pharmacological modulators of CB1/2 receptors show promise in the treatment of cancer-induced bone pain, however further assessment of their effects on bone pain in genetically engineered animal models and cancer patients is warranted.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

The CB2 cannabinoid receptor as a therapeutic target in the central nervous system

INTRODUCTION

Targeting CB2 cannabinoid receptor (CB2r) represents a promising approach for the treatment of central nervous system disorders. These receptors were identified in peripheral tissues, but also in neurons in the central nervous system. New findings have highlighted the interest to target these central receptors to obtain therapeutic effects devoid of the classical cannabinoid side-effects.

AREAS COVERED

In this review, we searched PubMed (January 1991-May 2021), ClinicalTrials.gov and Cochrane Library databases for articles, reviews and clinical trials. We first introduce the relevance of CB2r as a key component of the endocannabinoid system. We discuss CB2r interest as a possible novel target in the treatment of pain. This receptor has raised interest as a potential target for neurodegenerative disorders treatment, as we then discussed. Finally, we underline studies revealing a novel potential CB2r interest in mental disorders treatment.

EXPERT OPINION

In spite of the interest of targeting CB2r for pain, clinical trials evaluating CB2r agonist analgesic efficacy have currently failed. The preferential involvement of CB2r in preventing the development of chronic pain could influence the failure of clinical trials designed for the treatment of already established pain syndromes. Specific trials should be designed to target the prevention of chronic pain development.

Therapeutic potential of cannabinoids in combination cancer therapy

Derivatives of the plant Cannabis sativa have been used for centuries for both medical and recreational purposes, as well as industrial. The first proof of its medicinal use comes from ancient China, although there is evidence of its earlier utilization in Europe and Asia. In the 19th century, European practitioners started to employ cannabis extracts to treat tetanus, convulsions, and mental diseases and, in 1851, cannabis made its appearance in the Pharmacopoeia of the United States as an analgesic, hypnotic and anticonvulsant. It was only in 1937 that the Marijuana Tax Act prohibited the use of this drug in the USA. The general term Cannabis is commonly used by the scientific and scholar community to indicate derivatives of the plant Cannabis sativa. The word cannabinoid is a term describing chemical compounds that are either derivate of Cannabis (phytocannabinoids) or artificial analogues (synthetic) or are produced endogenously by the body (endocannabinoids). A more casual term "marijuana" or "weed", a compound derived from dried Cannabis flower tops and leaves, has progressively superseded the term cannabis when referred to its recreational use. The 2018 World health organisation (WHO) data suggest that nearly 2.5% of the global population (147 million) uses marijuana and some countries, such as Canada and Uruguay, have already legalised it. Due to its controversial history, the medicinal use of cannabinoids has always been a centre of debate. The isolation and characterisation of Δ⁹ tetrahydrocannabinol (THC), the major psychoactive component of cannabis and the detection of two human cannabinoid receptor (CBRs) molecules renewed interest in the medical use of cannabinoids, boosting research and commercial heed in this sector. Some cannabinoid-based drugs have been approved as medications, mainly as antiemetic, antianorexic, anti-seizure remedies and in cancer and multiple sclerosis patients' palliative care. Nevertheless, due to the stigma commonly associated with these compounds, cannabinoids' potential in the treatment of conditions such as cancer is still largely unknown and therefore underestimated.

Short-Term Medical Cannabis Treatment Regimens Produced Beneficial Effects among Palliative Cancer Patients

In the last decade the use of medical cannabis (MC) for palliative cancer treatment has risen. However, the choice between products is arbitrary and most patients are using Tetrahydrocannabinol (THC)-dominant cannabis products. In this study, we aimed to assess the short-term outcomes of MC treatment prescribed by oncologists in relation to the type of cannabis they receive. A comparative analysis was used to assess the differences in treatment effectiveness and safety between THC-dominant (n = 56, 52%), cannabidiol (CBD)-dominant (n = 19, 18%), and mixed (n = 33, 30%) MC treatments. Oncology patients (n = 108) reported on multiple symptoms in baseline questionnaires, initiated MC treatment, and completed a one-month follow-up. Most parameters improved significantly from baseline, including pain intensity, affective and sensory pain, sleep quality and duration, cancer distress, and both physical and psychological symptom burden. There was no significant difference between the three MC treatments in the MC-related safety profile. Generally, there were no differences between the three MC treatments in pain intensity and in most secondary outcomes. Unexpectedly, CBD-dominant oil treatments were similar to THC-dominant treatments in their beneficial effects for most secondary outcomes. THC-dominant treatments showed significant superiority in their beneficial effect only in sleep duration compared to CBD-dominant treatments. This work provides evidence that, though patients usually consume THC-dominant products, caregivers should also consider CBD-dominant products as a useful treatment for cancer-related symptoms.

Opportunities for cannabis in supportive care in cancer

Cannabis has the potential to modulate some of the most common and debilitating symptoms of cancer and its treatments, including nausea and vomiting, loss of appetite, and pain. However, the dearth of scientific evidence for the effectiveness of cannabis in treating these symptoms in patients with cancer poses a challenge to clinicians in discussing this option with their patients. A review was performed using keywords related to cannabis and important symptoms of cancer and its treatments. Literature was qualitatively reviewed from preclinical models to clinical trials in the fields of cancer, human immunodeficiency virus (HIV), multiple sclerosis, inflammatory bowel disease, post-traumatic stress disorder (PTSD), and others, to prudently inform the use of cannabis in supportive and palliative care in cancer. There is a reasonable amount of evidence to consider cannabis for nausea and vomiting, loss of appetite, and pain as a supplement to first-line treatments. There is promising evidence to treat chemotherapy-induced peripheral neuropathy, gastrointestinal distress, and sleep disorders, but the literature is thus far too limited to recommend cannabis for these symptoms. Scant, yet more controversial, evidence exists in regard to cannabis for cancer- and treatment-related cognitive impairment, anxiety, depression, and fatigue. Adverse effects of cannabis are documented but tend to be mild. Cannabis has multifaceted potential bioactive benefits that appear to outweigh its risks in many situations. Further research is required to elucidate its mechanisms of action and efficacy and to optimize cannabis preparations and doses for specific populations affected by cancer.

Cannabinoids-induced peripheral analgesia depends on activation of BK channels

The endogenous cannabinoid system is involved in the physiological inhibitory control of pain and is of particular interest for the development of therapeutic approaches for pain management. Selective activation of the peripheral CB1 cannabinoid receptor has been shown to suppress the heightened firing of primary afferents, which is the peripheral mechanism underlying neuropathic pain after nerve injury. However, the mechanism underlying this effect of CB1 receptor remains unclear. The large-conductance calcium-activated potassium (BK) channels have been reported to participate in anticonvulsant and vasorelaxant effects of cannabinoids. We asked whether BK channels participate in cannabinoids-induced analgesia and firing-suppressing effects in primary afferents after nerve injury. Here, using mice with chronic constriction injury (CCI)-induced neuropathic pain, antinociception action and firing-suppressing effect of HU210 were measured before and after BK channel blocker application. We found that local peripheral application of HU210 alleviated CCI-induced pain behavior and suppressed the heightened firing of injured fibers. Co-administration of IBTX with HU210 significantly reversed the analgesia and the firing-suppressing effect of HU210. This result indicated that the peripheral analgesic effects of cannabinoids depends on activation of BK channels.

Cannabinoids and Pain: New Insights From Old Molecules

Cannabis has been used for medicinal purposes for thousands of years. The prohibition of cannabis in the middle of the 20th century has arrested cannabis research. In recent years there is a growing debate about the use of cannabis for medical purposes. The term ‘medical cannabis’ refers to physician-recommended use of the cannabis plant and its components, called cannabinoids, to treat disease or improve symptoms. Chronic pain is the most commonly cited reason for using medical cannabis. Cannabinoids act via cannabinoid receptors, but they also affect the activities of many other receptors, ion channels and enzymes. Preclinical studies in animals using both pharmacological and genetic approaches have increased our understanding of the mechanisms of cannabinoid-induced analgesia and provided therapeutical strategies for treating pain in humans. The mechanisms of the analgesic effect of cannabinoids include inhibition of the release of neurotransmitters and neuropeptides from presynaptic nerve endings, modulation of postsynaptic neuron excitability, activation of descending inhibitory pain pathways, and reduction of neural inflammation. Recent meta-analyses of clinical trials that have examined the use of medical cannabis in chronic pain present a moderate amount of evidence that cannabis/cannabinoids exhibit analgesic activity, especially in neuropathic pain. The main limitations of these studies are short treatment duration, small numbers of patients, heterogeneous patient populations, examination of different cannabinoids, different doses, the use of different efficacy endpoints, as well as modest observable effects. Adverse effects in the short-term medical use of cannabis are generally mild to moderate, well tolerated and transient. However, there are scant data regarding the long-term safety of medical cannabis use. Larger well-designed studies of longer duration are mandatory to determine the long-term efficacy and long-term safety of cannabis/cannabinoids and to provide definitive answers to physicians and patients regarding the risk and benefits of its use in the treatment of pain. In conclusion, the evidence from current research supports the use of medical cannabis in the treatment of chronic pain in adults. Careful follow-up and monitoring of patients using cannabis/cannabinoids are mandatory.

Cannabinoids and cancer pain: A new hope or a false dawn?

The endocannabinoid system is involved in many areas of physiological function and homeostasis. Cannabinoid receptors are expressed in the peripheral and central nervous system and on immune cells, all areas ideally suited to modulation of pain processing. There are a wealth of preclinical data in a number of acute, chronic, neuropathic and cancer pain models that have demonstrated a potent analgesic potential for cannabinoids, especially in patients with cancer. However, although there are some positive results in pain of cancer patients, the clinical evidence for cannabinoids as analgesics has not been convincing and their use can only be weakly recommended. The efficacy of cannabinoids seems to have been 'lost in translation' which may in part be related to using extracts of herbal cannabis rather than targeted selective full agonists at the cannabinoid CB1 and CB2 receptors.

Khat (Catha edulis F.) and cannabinoids: Parallel and contrasting behavioral effects in preclinical and clinical studies

After a brief outline of Catha edulis F. (khat) and the cannabinoid systems, the interactions between the pharmacological effects of khat and cannabinoids will be reviewed. Khat chewing is a widespread habit that has a deep-rooted sociocultural tradition in Africa and the Middle East. Experimental studies conducted to investigate khat's central and peripheral effects have revealed an amphetamine-like mechanism of action mediated through the dopaminergic system. The endocannabinoid system comprises the receptors, the endogenous agonists and the related biochemical machinery responsible for synthesizing these substances and terminating their actions. Endocannabinoids are synthesized "on demand" from membrane phospholipids and then rapidly cleared by cellular uptake and enzymatic degradation. Khat and cannabinoids produce a body of parallel and contrasting behavioral effects. Concurrent consumption of khat and cannabinoids may increase the risk of getting or precipitating psychosis, has rewarding and motivational effect, increases the threshold of pain perception and impairs learning and memory. On the other hand, the action of cannabis to enhance food intake is likely to reduce khat's appetite suppressant effects.

The neurobiology of cancer pain

The global burden of cancer pain is enormous and opioids, despite their side effects, remain the primary therapeutic approach. The cause of cancer pain is unknown. Mechanisms driving cancer pain differ from those mechanisms responsible for inflammatory and neuropathic pain. The prevailing hypothesis put forward to explain cancer pain posits that cancers generate and secrete mediators which sensitize and activate primary afferent nociceptors in the cancer microenvironment. Moreover, cancers induce neurochemical reorganization of the spinal cord, which contributes to spontaneous activity and enhanced responsiveness. The purpose of this review, which covers clinical and preclinical studies, is to highlight those peripheral and central mechanisms responsible for cancer pain. The challenges facing neuroscientists and clinicians studying and ultimately treating cancer pain are discussed.

Combined antiproliferative effects of the aminoalkylindole WIN55,212-2 and radiation in breast cancer cells

The potential antitumor activity of cannabinoid receptor agonists, such as the aminoalklylindole WIN55,212-2 (WIN2), has been studied extensively, but their potential interaction with conventional cancer therapies, such as radiation, remains unknown. In the present work, the influence of WIN2 on the antiproliferative activity of radiation in human (MCF-7 and MDA-MB231) and murine (4T1) breast cancer cells was investigated. The antiproliferative effects produced by combination of WIN2 and radiation were more effective than either agent alone. The stereoisomer of WIN2, WIN55,212-3 (WIN3), failed to inhibit growth or potentiate the growth-inhibitory effects of radiation, indicative of stereospecificity. Two other aminoalkylindoles, pravadoline and JWH-015 [(2-methyl-1-propyl-1H-indol-3-yl)-1-naphthalenyl-methanone], also enhanced the antiproliferative effects of radiation, but other synthetic cannabinoids (i.e., nabilone, CP55,940 [(+)-rel-5-(1,1-dimethylheptyl)-2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]-phenol], and methanandamide) or phytocannabinoids [i.e., Δ⁹-tetrahydrocannabinol (THC) and cannabidiol] did not. The combination treatment of WIN2 + radiation promoted both autophagy and senescence but not apoptosis or necrosis. WIN2 also failed to alter radiation-induced DNA damage or the apparent rate of DNA repair. Although the antiproliferative actions of WIN2 were mediated through noncannabinoid receptor-mediated pathways, the observation that WIN2 interfered with growth stimulation by sphingosine-1-phosphate (S1P) implicates the potential involvement of S1P/ceramide signaling pathways. In addition to demonstrating that aminoalkylindole compounds could potentially augment the effectiveness of radiation treatment in breast cancer, the present study suggests that THC and nabilone are unlikely to interfere with the effectiveness of radiation therapy, which is of particular relevance to patients using cannabinoid-based drugs to ameliorate the toxicity of cancer therapies.

The non-selective cannabinoid receptor agonist WIN 55,212-2 attenuates responses of C-fiber nociceptors in a murine model of cancer pain

Pain from cancer can be severe, difficult to treat, and greatly diminishes patients' quality of life. It is therefore important to gain new information on the mechanisms of cancer pain and develop new treatment strategies. We have used a murine model of bone cancer pain to investigate underlying peripheral neural mechanisms and novel treatment approaches. In this model, implantation of fibrosarcoma cells into and around the calcaneous bone produces mechanical and thermal hyperalgesia in mice. C-fiber nociceptors in tumor-bearing mice develop spontaneous ongoing activity and sensitization to thermal stimuli. However, it is unclear whether sensitization of nociceptors to mechanical stimuli underlies the mechanical hyperalgesia seen in tumor-bearing mice. We therefore examined responses of C-fiber nociceptors to suprathreshold mechanical stimuli in tumor-bearing mice and found they did not differ from those of C-nociceptors in control mice. Thus, sensitization of C-fiber nociceptors to mechanical stimulation does not appear to underlie tumor-evoked mechanical hyperalgesia in this murine model of bone cancer pain. We also examined the effect of the non-selective cannabinoid receptor agonist, WIN 55,212-2, on spontaneous activity and responses evoked by mechanical stimuli of C-fiber nociceptors innervating the tumor-bearing paw. Selective CB1 and CB2 antagonists were administered to determine the contribution of each receptor subtype to the effects of WIN 55,212-2. Intraplantar administration of WIN 55,212-2 attenuated spontaneous discharge and responses evoked by mechanical stimulation of C-fiber nociceptors. These effects were inhibited by prior intraplantar administration of selective CB1 (AM281) or CB2 (AM630) receptor antagonists but not by vehicle. These results indicate that activation of either CB1 or CB2 receptors reduced the spontaneous activity of C-fiber nociceptors associated with tumor growth as well as their evoked responses. Our results provide further evidence that activation of peripheral cannabinoid receptors may be a useful target for the treatment of cancer pain.

Dynamic changes to the endocannabinoid system in models of chronic pain

The analgesic effects of cannabinoid ligands, mediated by CB1 receptors are well established. However, the side-effect profile of CB1 receptor ligands has necessitated the search for alternative cannabinoid-based approaches to analgesia. Herein, we review the current literature describing the impact of chronic pain states on the key components of the endocannabinoid receptor system, in terms of regionally restricted changes in receptor expression and levels of key metabolic enzymes that influence the local levels of the endocannabinoids. The evidence that spinal CB2 receptors have a novel role in the modulation of nociceptive processing in models of neuropathic pain, as well as in models of cancer pain and arthritis is discussed. Recent advances in our understanding of the spinal location of the key enzymes that regulate the levels of the endocannabinoid 2-AG are discussed alongside the outcomes of recent studies of the effects of inhibiting the catabolism of 2-AG in models of pain. The complexities of the enzymes capable of metabolizing both anandamide (AEA) and 2-AG have become increasingly apparent. More recently, it has come to light that some of the metabolites of AEA and 2-AG generated by cyclooxygenase-2, lipoxygenases and cytochrome P450 are biologically active and can either exacerbate or inhibit nociceptive signalling.

Effects of gonadal hormones on the peripheral cannabinoid receptor 1 (CB1R) system under a myositis condition in rats

In this study, we assessed the effects of peripherally administered cannabinoids in an orofacial myositis model, and the role of sex hormones in cannabinoid receptor (CBR) expression in trigeminal ganglia (TG). Peripherally administered arachidonylcyclopropylamide (ACPA), a specific CB1R agonist, significantly attenuated complete Freund's adjuvant (CFA)-induced mechanical hypersensitivity in the masseter muscle in male rats. The ACPA effect was blocked by a local administration of AM251, a specific CB1R antagonist, but not by AM630, a specific CB2R antagonist. In female rats, a 30-fold higher dose of ACPA was required to produce a moderate reduction in mechanical hypersensitivity. CFA injected in masseter muscle significantly upregulated CB1R mRNA expression in TG in male, but not in female, rats. There was a close correlation between the CB1R mRNA levels in TG and the antihyperalgesic effect of ACPA. Interleukin (IL)-1β and IL-6, which are elevated in the muscle tissue following CFA treatment, induced a significant upregulation of CB1R mRNA expression in TG from male rats. The upregulation of CB1R was prevented in TG cultures from orchidectomized male rats, which was restored by the application of testosterone. The cytokines did not alter the CB1R mRNA level in TG from intact as well as ovariectomized female rats. Neither estradiol supplement nor estrogen receptor blockade had any effects on CB1R expression. These data indicate that testosterone, but not estradiol, is required for the regulation of CB1Rs in TG under inflammatory conditions, which provide explanations for the sex differences in the antihyperalgesic effects of peripherally administered cannabinoids.

CB1 and CB2 receptor agonists promote analgesia through synergy in a murine model of tumor pain

In light of the adverse side-effects of opioids, cannabinoid receptor agonists may provide an effective alternative for the treatment of cancer pain. This study examined the potency and efficacy of synthetic CB1 and CB2 receptor agonists in a murine model of tumor pain. Intraplantar injection of the CB1 receptor agonist arachidonylcyclopropylamide (ED(50) of 18.4 μg) reduced tumor-related mechanical hyperalgesia by activation of peripheral CB1 but not CB2 receptors. Similar injection of the CB2 receptor agonist AM1241 (ED50 of 19.5 μg) reduced mechanical hyperalgesia by activation of peripheral CB2 but not CB1 receptors. Both agonists had an efficacy comparable with that of morphine (intraplantar), but their analgesic effects were independent of opioid receptors. Isobolographic analysis of the coinjection of arachidonylcyclopropylamide and AM1241 determined that the CB1 and CB2 receptor agonists interacted synergistically to reduce mechanical hyperalgesia in the tumor-bearing paw. These data extend our previous findings that the peripheral cannabinoid receptors are a promising target for the management of cancer pain and mixed cannabinoid receptor agonists may have a therapeutic advantage over selective agonists.

Biologic mechanisms of oral cancer pain and implications for clinical therapy

Cancer pain is an ever-present public health concern. With innovations in treatment, cancer patients are surviving longer, but uncontrollable pain creates a poor quality of life for these patients. Oral cancer is unique in that it causes intense pain at the primary site and significantly impairs speech, swallowing, and masticatory functions. We propose that oral cancer pain has underlying biologic mechanisms that are generated within the cancer microenvironment. A comprehensive understanding of key mediators that control cross-talk between the cancer and peripheral nervous system, and possible interventions, underlies effective cancer pain management. The purpose of this review is to explore the current studies on oral cancer pain and their implications in clinical management for cancer pain in general. Furthermore, we will explore the endogenous opioid systems and novel cancer pain therapeutics that target these systems, which could solve the issue of opiate tolerance and improve quality of life in oral cancer patients.

Self-medication of a cannabinoid CB2 agonist in an animal model of neuropathic pain

Drug self-administration methods were used to test the hypothesis that rats would self-medicate with a cannabinoid CB(2) agonist to attenuate a neuropathic pain state. Self-medication of the CB(2) agonist (R,S)-AM1241, but not vehicle, attenuated mechanical hypersensitivity produced by spared nerve injury. Switching rats from (R,S)-AM1241 to vehicle self-administration also decreased lever responding in an extinction paradigm. (R,S)-AM1241 self-administration did not alter paw withdrawal thresholds in sham-operated or naive animals. The percentage of active lever responding was similar in naive groups self-administering vehicle or (R,S)-AM1241. The CB(2) antagonist SR144528 blocked both antiallodynic effects of (R,S)-AM1241 self-medication and the percentage of active lever responding in neuropathic (but not naive) rats. Neuropathic and sham groups exhibited similar percentages of active lever responding for (R,S)-AM1241 on a fixed ratio 1 (FR1) schedule. However, neuropathic animals worked harder than shams to obtain (R,S)-AM1241 when the schedule of reinforcement was increased (to FR6). (R,S)-AM1241 self-medication on FR1, FR3, or FR6 schedules attenuated nerve injury-induced mechanical allodynia. (R,S)-AM1241 (900μg intravenously) failed to produce motor ataxia observed after administration of the mixed CB(1)/CB(2) agonist WIN55,212-2 (0.5mg/kg intravenously). Our results suggest that cannabinoid CB(2) agonists may be exploited to treat neuropathic pain with limited drug abuse liability and central nervous system side effects. These studies validate the use of drug self-administration methods for identifying nonpsychotropic analgesics possessing limited abuse potential. These methods offer potential to elucidate novel analgesics that suppress spontaneous neuropathic pain that is not measured by traditional assessments of evoked pain.

Cannabinoids attenuate cancer pain and proliferation in a mouse model

We investigated the effects of cannabinoid receptor agonists on (1) oral cancer cell viability in vitro and (2) oral cancer pain and tumor growth in a mouse cancer model. We utilized immunohistochemistry and Western blot to show that human oral cancer cells express CBr1 and CBr2. When treated with WIN55,212-2 (non-selective), ACEA (CBr1-selective) or AM1241 (CBr2-selective) agonists in vitro, oral cancer cell proliferation was significantly attenuated in a dose-dependent manner. In vivo, systemic administration (0.013M) of WIN55,212-2, ACEA, or AM1241 significantly attenuated cancer-induced mechanical allodynia. Tumor growth was also significantly attenuated with systemic AM1241 administration. Our findings suggest a direct role for cannabinoid mechanisms in oral cancer pain and proliferation. The systemic administration of cannabinoid receptor agonists may have important therapeutic implications wherein cannabinoid receptor agonists may reduce morbidity and mortality of oral cancer.

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.

Spinal and peripheral analgesic effects of the CB2 cannabinoid receptor agonist AM1241 in two models of bone cancer-induced pain

BACKGROUND AND PURPOSE

The activation of CB(2) receptors induces analgesia in experimental models of chronic pain. The present experiments were designed to study whether the activation of peripheral or spinal CB(2) receptors relieves thermal hyperalgesia and mechanical allodynia in two models of bone cancer pain.

EXPERIMENTAL APPROACH

NCTC 2472 osteosarcoma or B16-F10 melanoma cells were intratibially inoculated to C3H/He and C57BL/6 mice. Thermal hyperalgesia was assessed by the unilateral hot plate test and mechanical allodynia by the von Frey test. AM1241 (CB(2) receptor agonist), AM251 (CB(1) receptor antagonist), SR144528 (CB(2) receptor antagonist) and naloxone were used. CB(2) receptor expression was measured by Western blot.

KEY RESULTS

AM1241 (0.3-10 mg.kg(-1)) abolished thermal hyperalgesia and mechanical allodynia in both tumour models. The antihyperalgesic effect was antagonized by subcutaneous, intrathecal or peri-tumour administration of SR144528. In contrast, the antiallodynic effect was inhibited by systemic or intrathecal, but not peri-tumour, injection of SR144528. The effects of AM1241 were unchanged by AM251 but were prevented by naloxone. No change in CB(2) receptor expression was found in spinal cord or dorsal root ganglia.

CONCLUSIONS AND IMPLICATIONS

Spinal CB(2) receptors are involved in the antiallodynic effect induced by AM1241 in two neoplastic models while peripheral and spinal receptors participate in the antihyperalgesic effects. Both effects were mediated by endogenous opiates. The use of drugs that activate CB(2) receptors could be a useful strategy to counteract bone cancer-induced pain symptoms.

Serine proteases and protease-activated receptor 2-dependent allodynia: a novel cancer pain pathway

Mediators involved in the generation of pain in patients with cancer are poorly understood. Using a combined molecular, pharmacologic, behavioral, and genetic approach, we have identified a novel mechanism of cancer-dependent allodynia induced by protease-activated receptor 2 (PAR2). Here we show that human head and neck carcinoma cells have increased levels of proteolytic activity compared to normal human cell controls. Supernatant from human carcinoma cells, but not controls, caused marked and prolonged mechanical allodynia in mice, when administered into the hindpaw. This nociceptive effect was abolished by serine protease inhibition, diminished by mast cell depletion and absent in PAR2-deficient mice. In addition, non-contact co-culture of trigeminal ganglion neurons with human head and neck carcinoma cells increased the proportion of neurons that exhibited PAR2-immunoreactivity. Our results point to a direct role for serine proteases and their receptor in the pathogenesis of cancer pain. This previously unrecognized cancer pain pathway has important therapeutic implications wherein serine protease inhibitors and PAR2 antagonists may be useful for the treatment of cancer pain.

The analgesic potential of cannabinoids

Historically and anecdotally cannabinoids have been used as analgesic agents. In recent years, there has been an escalating interest in developing cannabis-derived medications to treat severe pain. This review provides an overview of the history of cannabis use in medicine, cannabinoid signaling pathways, and current data from preclinical as well as clinical studies on using cannabinoids as potential analgesic agents. Clinical and experimental studies show that cannabis-derived compounds act as antiemetic, appetite modulating, and analgesic agents. However, the efficacy of individual products is variable and dependent upon the route of administration. As opioids are the only therapy for severe pain, analgesic ability of cannabinoids may provide a much-needed alternative to opioids. Moreover, cannabinoids act synergistically with opioids and act as opioid sparing agents, allowing lower doses and fewer side effects from chronic opioid therapy. Thus, rational use of cannabis-based medications deserves serious consideration to alleviate the suffering of patients due to severe pain.

Do cannabinoids have a role in cancer pain management?

PURPOSE OF REVIEW

Historically cannabinoids have been used for both therapy and recreation, yet the elucidation of the endocannabinoid system and their chemistry has been relatively recent. Prohibition of cannabis has meant few clinical trials, especially in cancer pain. This review will consider previous animal and clinical data and assess more recent investigations of clinical effectiveness of cannabinoids in pain and specifically cancer pain.

RECENT FINDINGS

Meta-analyses based on historical studies question the utility of cannabinoids in pain due to modest analgesia and problematic central side effects. However, there has been a resurgence in clinical trials of cannabis extracts and analogues. New data have contributed to the understanding of how cannabinoids work and proposed how to obtain analgesia unfettered by adverse effects. Moreover, recent clinical trials have demonstrated the current role of cannabinoids may be to attain small but significant benefit in refractory chronic and cancer pain.

SUMMARY

Cannabinoids may be a useful addition to current analgesic treatments. The evidence supports a possible role for cannabinoids in refractory cancer pain. However, to realize the full potential of cannabinoids suggested by preclinical data, it is likely that peripheral CB1 or CB2 receptors or modulation of endocannabinoids will have to be targeted to achieve analgesia without dose limiting side effects.

Mode of action of cannabinoids on nociceptive nerve endings

In recent years, cannabinoids have emerged as attractive alternatives or supplements to therapy for chronic pain states. However, in humans the activation of cannabinoid receptors in neurons of the central nervous system is associated with psychotropic side effects, temporary memory impairment and dependence, which arise via the effects of cannabinoids on forebrain circuits. For clinical exploitation of the analgesic properties of cannabinoids, a major challenge is to devise strategies that reduce or abolish their adverse effects on cognitive, affective and motor functions without attenuating their analgesic effects. The cannabinoid receptor family currently includes two cloned metabotropic receptors: CB1, CB2 and possibly GPR55 which are distributed widely across many key loci in pain-modulating pathways, including the peripheral terminals of primary afferents. Modulation of transducer ion channels expressed at nociceptive terminals occurs upon activation of metabotropic cannabinoid receptors, but direct cannabinoid action on ion channels involved in sensory transduction or regulation of neuron excitability likely contributes to the peripheral cannabinoid effects.

A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain

Tumors in bone are associated with pain in humans. Data generated in a murine model of bone cancer pain suggest that a disturbance of local endocannabinoid signaling contributes to the pain. When tumors formed after injection of osteolytic fibrosarcoma cells into the calcaneus bone of mice, cutaneous mechanical hyperalgesia was associated with a decrease in the level of anandamide (AEA) in plantar paw skin ipsilateral to tumors. The decrease in AEA occurred in conjunction with increased degradation of AEA by fatty acid amide hydrolase (FAAH). Intraplantar injection of AEA reduced the hyperalgesia, and intraplantar injection of URB597, an inhibitor of FAAH, increased the local level of AEA and also reduced hyperalgesia. An increase in FAAH mRNA and enzyme activity in dorsal root ganglia (DRG) L3-L5 ipsilateral to the affected paw suggests DRG neurons contribute to the increased FAAH activity in skin in tumor-bearing mice. Importantly, the anti-hyperalgesic effects of AEA and URB597 were blocked by a CB1 receptor antagonist. Increased expression of CB1 receptors by DRG neurons ipsilateral to tumor-bearing limbs may contribute to the anti-hyperalgesic effect of elevated AEA levels. Furthermore, CB1 receptor protein-immunoreactivity as well as inhibitory effects of AEA and URB597 on the depolarization-evoked Ca(2+) transient were increased in small DRG neurons cocultured with fibrosarcoma cells indicating that fibrosarcoma cells are sufficient to evoke phenotypic changes in AEA signaling in DRG neurons. Together, the data provide evidence that manipulation of peripheral endocannabinoid signaling is a promising strategy for the management of bone cancer pain.

Cannabinoid modulation of cutaneous Adelta nociceptors during inflammation

Previous studies have demonstrated that locally administered cannabinoids attenuate allodynia and hyperalgesia through activation of peripheral cannabinoid receptors (CB(1) and CB(2)). However, it is currently unknown if cannabinoids alter the response properties of nociceptors. In the present study, correlative behavioral and in vivo electrophysiological studies were conducted to determine if peripheral administration of the cannabinoid receptor agonists arachidonyl-2'-chloroethylamide (ACEA) or (R)-(+)-methanandamide (methAEA) could attenuate mechanical allodynia and hyperalgesia, and decrease mechanically evoked responses of Adelta nociceptors. Twenty-four hours after intraplantar injection of complete Freund's adjuvant (CFA), rats exhibited allodynia (decrease in paw withdrawal threshold) and hyperalgesia (increase in paw withdrawal frequency), which were attenuated by both ACEA and methAEA. The antinociceptive effects of these cannabinoids were blocked by co-administration with the CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophen yl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) but not with the CB(2) receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-y l](4-methoxyphenyl)methanone (AM630). ACEA and methAEA did not produce antinociception under control, non-inflamed conditions 24 h after intraplantar injection of saline. In parallel studies, recordings were made from cutaneous Adelta nociceptors from inflamed or control, non-inflamed skin. Both ACEA and methAEA decreased responses evoked by mechanical stimulation of Adelta nociceptors from inflamed skin but not from non-inflamed skin, and this decrease was blocked by administration of the CB(1) receptor antagonist AM251. These results suggest that attenuation of mechanically evoked responses of Adelta nociceptors contributes to the behavioral antinociception produced by activation of peripheral CB(1) receptors during inflammation.

The cannabinoid receptor agonist, WIN 55, 212-2, attenuates tumor-evoked hyperalgesia through peripheral mechanisms

Several lines of evidence suggest that cannabinoids can attenuate various types of pain and hyperalgesia through peripheral mechanisms. The development of rodent cancer pain models has provided the opportunity to investigate novel approaches to treat this common form of pain. In the present study, we examined the ability of peripherally administered cannabinoids to attenuate tumor-evoked mechanical hyperalgesia in a murine model of cancer pain. Unilateral injection of osteolytic fibrosarcoma cells into and around the calcaneus bone resulted in tumor formation and mechanical hyperalgesia in the injected hindpaw. Mechanical hyperalgesia was defined as an increase in the frequency of paw withdrawals to a suprathreshold von Frey filament (3.4 mN) applied to the plantar surface of the hindpaw. WIN 55, 212-2 (1.5 to 10 microg) injected subcutaneously into the tumor-bearing hindpaw produced a dose-dependent decrease in paw withdrawal frequencies to suprathreshold von Frey filament stimulation. Injection of WIN 55,212-2 (10 microg) into the contralateral hindpaw did not decrease paw withdrawal frequencies in the tumor-bearing hindpaw. Injection of the highest antihyperalgesic dose of WIN 55,212-2 (10 microg) did not produce catalepsy as determined by the bar test. Co-administration of WIN 55,212-2 with either cannabinoid 1 (AM251) or cannabinoid 2 (AM630) receptor antagonists attenuated the antihyperalgesic effects of WIN 55, 212-2. In conclusion, peripherally administered WIN 55,212-2 attenuated tumor-evoked mechanical hyperalgesia by activation of both peripheral cannabinoid 1 and cannabinoid 2 receptors. These results suggest that peripherally-administered cannabinoids may be effective in attenuating cancer pain.