Cyclooxygenase inhibitors and thalidomide ameliorate vincristine-induced hyperalgesia in rats

DRG Voltage-Gated Sodium Channel 1.7 Is Upregulated in Paclitaxel-Induced Neuropathy in Rats and in Humans with Neuropathic Pain

Chemotherapy-induced peripheral neuropathy (CIPN) is a common adverse effect experienced by cancer patients receiving treatment with paclitaxel. The voltage-gated sodium channel 1.7 (Na_v1.7) plays an important role in multiple preclinical models of neuropathic pain and in inherited human pain phenotypes, and its gene expression is increased in dorsal root ganglia (DRGs) of paclitaxel-treated rats. Hence, the potential of change in the expression and function of Na_v1.7 protein in DRGs from male rats with paclitaxel-related CIPN and from male and female humans with cancer-related neuropathic pain was tested here. Double immunofluorescence in CIPN rats showed that Na_v1.7 was upregulated in small DRG neuron somata, especially those also expressing calcitonin gene-related peptide (CGRP), and in central processes of these cells in the superficial spinal dorsal horn. Whole-cell patch-clamp recordings in rat DRG neurons revealed that paclitaxel induced an enhancement of ProTx II (a selective Na_v1.7 channel blocker)-sensitive sodium currents. Bath-applied ProTx II suppressed spontaneous action potentials in DRG neurons occurring in rats with CIPN, while intrathecal injection of ProTx II significantly attenuated behavioral signs of CIPN. Complementarily, DRG neurons isolated from segments where patients had a history of neuropathic pain also showed electrophysiological and immunofluorescence results indicating an increased expression of Na_v1.7 associated with spontaneous activity. Na_v1.7 was also colocalized in human cells expressing transient receptor potential vanilloid 1 and CGRP. Furthermore, ProTx II decreased firing frequency in human DRGs with spontaneous action potentials. This study suggests that Na_v1.7 may provide a potential new target for the treatment of neuropathic pain, including chemotherapy (paclitaxel)-induced neuropathic pain.SIGNIFICANCE STATEMENT This work demonstrates that the expression and function of the voltage-gated sodium channel Na_v1.7 are increased in a preclinical model of chemotherapy-induced peripheral neuropathy (CIPN), the most common treatment-limiting side effect of all the most common anticancer therapies. This is key as gain-of-function mutations in human Na_v1.7 recapitulate both the distribution and pain percept as shown by CIPN patients. This work also shows that Na_v1.7 is increased in human DRG neurons only in dermatomes where patients are experiencing acquired neuropathic pain symptoms. This work therefore has major translational impact, indicating an important novel therapeutic avenue for neuropathic pain as a class.

Novel small molecule α9α10 nicotinic receptor antagonist prevents and reverses chemotherapy-evoked neuropathic pain in rats

BACKGROUND

Peripheral neuropathy is a common dose-limiting side effect of chemotherapy. There are no clinically proven analgesics for the treatment of this condition. Drugs from different classes have been tested with mixed results. Identification of novel molecular targets for analgesic(s) is important. Antagonism of the α9α10 nicotinic acetylcholine receptor (nAChR) subtype (absent in brain) is thought to underlie analgesic efficacy of peptide α-conotoxins. We found novel nonpeptide small molecule analogs from a family of tetrakis-, tris-, and bis-azaaromatic quaternary ammonium salts (high potency with selectivity as antagonists at the α9α10 nAChRs) to produce dose-related analgesia in rat models of nerve injury-evoked neuropathy and persistent inflammatory pain. No tests were done in a model of neuropathy induced by drug administration (ie, chemotherapy).

METHODS

In this study, a lead bis-analog, ZZ1-61c, was characterized in a rat model of vincristine-evoked neuropathy. Male Sprague-Dawley rats were repeatedly dosed with the vinca-alkaloid, vincristine (100 μg/kg/day IP, days 1 to 5 and 8 to 12). ZZ1-61c (100 μg/kg/day IP) was given either along with or after completion of vincristine (commencing by day 15 when neuropathy was maximum). Responsiveness was assessed with von Frey hairs and the paw-pressure test. The effects of ZZ1-61c on motor function (rotarod) and muscle strength (grip test) were characterized in naïve rats.

RESULTS

The development of neuropathy was demonstrated with repeated dosing of vincristine (pain hypersensitivity in response to mechanical stimulation). ZZ1-61c showed both preventive and restorative effects on this condition: (1) vincristine-evoked sensitivity to pressure was reduced by coadministration of ZZ1-61c; (2) established neuropathy was diminished by ZZ1-61c after cessation of chemotherapy. ZZ1-61c did not cause motor dysfunction (rotarod) or muscular weakness (the grip test).

CONCLUSIONS

This study suggests that ZZ1-61c, a novel compound with a unique mechanism of antagonistic action at the α9α10 nAChR, may be a potential drug candidate for prevention and attenuation of neuropathic pain resulting from chemotherapy. Such a strategy may provide effective treatment that circumvents toxicity of centrally acting agonists at nAChR.

Chemotherapy-induced neuropathic pain

PURPOSE OF REVIEW

To discuss the importance, clinical features, possible pathology and treatments of chemotherapy-induced neuropathic pain. Newer biological agents such as bortezomib will be considered in greater detail.

RECENT FINDINGS

Chemotherapy-induced peripheral neuropathy (CIPN) is a frequent complication of common anticancer therapies. It may lead to treatment compromise, significantly adds to the symptom burden and interferes with quality of life of cancer survivors. Recent investigations have identified processes involved in CIPN which may give some insight for the development of novel treatments. CIPN induced by different anticancer therapies may be heterogeneous and present as distinct neuropathic pains. Recent work has focussed on the newer anticancer drugs such as bortezomib. Contemporaneous studies have failed to find good evidence for the use of several common antineuropathic agents and further research is required.

SUMMARY

Painful CIPN remains under recognized and undertreated. It is an important cause of pain during cancer treatment and is a common pain in the cancer survivor. Difficulties in assessment and limitations in treatment contribute to management problems. Improvements in education (patient and clinician), assessment and treatment would potentially reduce the often debilitating effects of painful CIPN.

Minocycline prevents impaired glial glutamate uptake in the spinal sensory synapses of neuropathic rats

Activation of glutamate receptors and glial cells in the spinal dorsal horn are two fundamental processes involved in the pathogenesis of various pain conditions, including neuropathic pain induced by injury to the peripheral or central nervous systems. Numerous studies have demonstrated that minocycline treatment attenuates allodynic and hyperalgesic behaviors induced by tissue inflammation or nerve injury. However, the synaptic mechanisms by which minocycline prevents hyperalgesia are not fully understood. We recently reported that deficient glutamate uptake by glial glutamate transporters (GTs) is key for the enhanced activation of N-methyl-d-aspartate (NMDA) receptors in the spinal sensory synapses of rats receiving partial sciatic nerve ligation (pSNL). In this study, we investigated how minocycline affects activation of NMDA receptors in the spinal sensory synapses in rats with pSNL by whole cell recordings of NMDA currents in spinal laminea I and II neurons from spinal slices. The effects of minocycline treatments on the dorsal horn expression of glial GTs and astrocyte marker glial fibrillary acidic protein (GFAP) were analyzed by immunohistochemistry. We demonstrated that normalized activation of NMDA receptors in synapses activated by both weak and strong peripheral input in the spinal dorsal horn is temporally associated with attenuated mechanical allodynia in rats with pSNL receiving intraperitoneal injection of minocycline. Minocycline ameliorated both the downregulation of glial GT expression and the activation of astrocytes induced by pSNL in the spinal dorsal horn. We further revealed that preventing deficient glial glutamate uptake at the synapse is crucial for preserving the normalized activation of NMDA receptors in the spinal sensory synapses in pSNL rats treated with minocycline. Our studies suggest that glial GTs may be a potential target for the development of analgesics.

Impaired glial glutamate uptake induces extrasynaptic glutamate spillover in the spinal sensory synapses of neuropathic rats

Glial cell dysfunction and excessive glutamate receptor activation in spinal dorsal horn neurons are hallmark mechanisms of pathological pain. The way in which glial cell dysfunction leads to excessive glutamate receptor activation in the spinal sensory synapses remains unknown. We and others recently reported the downregulation of glial glutamate transporter (GT) protein expression in the spinal dorsal horn of neuropathic rats. In this study, we showed that excitatory postsynaptic currents originating from N-methyl-d-aspartate receptor activation (NMDA EPSCs) elicited by peripheral synaptic input in the spinal sensory synapses were enhanced in neuropathic rats with mechanical allodynia induced by partial sciatic nerve ligation. The enhanced NMDA EPSCs were accompanied by an increased proportion of NR2B receptor activation. Physically blocking the extrasynaptic glutamate with dextran or chemically scavenging the glutamate with glutamic-pyruvic transaminase ameliorated the abnormal NMDA EPSCs in neuropathic rats. Pharmacological blockade of glial GTs with dihydrokainic acid enhanced NMDA receptor activation elicited by synaptic input or puffed glutamate in normal control rats, but this effect was precluded in neuropathic rats. Thus extrasynaptic glutamate spillover and extrasynaptic NMDA receptor activation induced by deficient glial glutamate uptake in the synapses resulted in the excessive activation of NMDA receptors in neuropathic rats. It is suggested that extrasynaptic glutamate spillover may be a key synaptic mechanism related to phenotypic alterations induced by nerve injury in the spinal dorsal horn and that glial GTs are potential new targets in the development of analgesics.

Animal models of chemotherapy-evoked painful peripheral neuropathies

This review examines recent preclinical research on toxic peripheral neuropathy and potential therapeutic developments. Chemotherapy-induced peripheral neurotoxicity is a major clinical problem because it represents the dose-limiting side effects of a significant number of antineoplastic drugs. Patients are unable to complete full or optimal treatment schedules. The incidence of chemotherapy-induced peripheral neuropathy varies depending on the drugs and schedules used, and this can be quite high, particularly when neurophysiological methods are used to make a diagnosis. However, even when chemotherapy-induced peripheral neuropathy is not a dose-limiting side effect, its onset may severely affect the quality of life of cancer patients and cause chronic discomfort. As such, improved understanding of the pathophysiology of chemotherapy-induced neurotoxicity need for animal models is clinically relevant and will assist in the development of future neuroprotective strategies and also in the design of novel chemotherapies with improved toxicity profiles. In this review, the features of animal models of chemotherapy-induced painful neuropathy developed for 20 years, due to the administration of the most widely used drugs, such as platinum drugs, taxanes, and vinca alkaloids, will be discussed. In a second part, data available on neuroprotectants and treatment strategies, evaluated using these previous animal models in the attempt to prevent neuropathic pain, will be summarized.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Experimental studies of potential analgesics for the treatment of chemotherapy-evoked painful peripheral neuropathies

OBJECTIVE

We investigated potential analgesics for chemotherapy-evoked neuropathic pain using rats treated with paclitaxel.

DESIGN

Drugs were tested in a repeated dosing paradigm (four daily injections). Topiramate was tested with a long-term treatment paradigm (12 days). A literature search was performed to summarize prior data.

MEASURES

Mechanical stimulation of the hind paw was used to assay antiallodynic and antihyperalgesic effects acutely and 24 hours after injection.

RESULTS

Amitriptyline produced significant analgesia, but this was not apparent until after the second injection. Baclofen produced significant effects, but the response varied erratically. Mexiletine and NMED-126 (a mixed N- and T-type calcium channel blocker) produced consistent, significant analgesia when tested acutely, but the pain relief did not persist at 24 hours postinjection. Oxcarbazepine had no effect at any time. Tramadol produced consistent, near-complete analgesia when tested acutely, but the analgesia did not persist to 24 hours postinjection. Topiramate produced significant effects that were first evident after 6-8 days of dosing.

CONCLUSIONS

The present data and data from the literature review suggest that there are several potential treatments for chemotherapy-evoked neuropathic pain. Nonsteroidal anti-inflammatory drugs have little or no efficacy. Opioids have an effect, but probably only with high doses. At least some antidepressants are analgesic in these conditions. Some, but clearly not all, anticonvulsants and sodium channel blockers have efficacy. Tramadol is a particularly promising candidate. Topiramate, acetyl-L-carnitine, carbamazepine, and venlafaxine may have protective or restorative effects. Clinical trials of these candidates are needed to advance the treatment of chemotherapy-evoked pain.

Ghrelin prevents cisplatin-induced mechanical hyperalgesia and cachexia

Complications induced by the chemotherapeutic agent cisplatin, such as neuropathy and cachexia, occur frequently, are often dose limiting, and have an impact on quality of life and survival in cancer patients. The recently discovered hormone ghrelin is a potent GH secretagogue with orexigenic and neuroprotective properties that may prevent or ameliorate these complications. The objective of this study was to determine the effects of ghrelin administration on mechanical hyperalgesia, anorexia, and cachexia induced by cisplatin. Adult male Sprague-Dawley rats were given cisplatin, ghrelin, ghrelin-cisplatin, or vehicle ip. Food intake and body weight were measured daily. Behavioral tests to assess the development of hyperalgesia were conducted by measuring mechanical and thermal sensitivity. Plasma ghrelin and IGF-I levels were also measured. Our results indicate that ghrelin coadministration inhibited the development of cisplatin-induced mechanical hyperalgesia, anorexia, and cachexia induced by cisplatin. Although ghrelin treatment had no effect on plasma IGF-I levels in control rats, it prevented the decrease in IGF-I levels induced by cisplatin. The attenuation of cisplatin-induced mechanical hyperalgesia induced by ghrelin was correlated with the prevention of cisplatin-induced lowering of IGF-I. In conclusion, ghrelin administration may be useful in the treatment or prevention of chemotherapy induced neuropathy and cachexia. Attenuation of mechanical hyperalgesia in the rat by the hormone ghrelin provides a unique model for elucidating the mechanisms involved, which are essential toward our understanding of these complications.

Chemotherapy-evoked neuropathic pain: Abnormal spontaneous discharge in A-fiber and C-fiber primary afferent neurons and its suppression by acetyl-L-carnitine

Cancer patients treated with antimitotic drugs in the taxane and vinca alkaloid classes sometimes develop a chronic painful peripheral neuropathy whose cause is not understood. In animal models of painful peripheral neuropathy due to nerve trauma or diabetes there is obvious axonal degeneration accompanied by an abnormal incidence of spontaneous discharge in A-fiber and C-fiber nociceptors. But animals with paclitaxel- and vincristine-evoked neuropathic pain do not have axonal degeneration at the level of the peripheral nerve. However, recent data show that they do have a partial degeneration of the primary afferent neurons' terminal arbors in the epidermis. It is not clear as to whether this relatively minor degeneration is accompanied by abnormal spontaneous discharge. We surveyed primary afferent axonal activity in the sural nerve of rats with the paclitaxel- and vincristine-evoked pain syndromes at the time of peak pain severity. Compared to vehicle-injected controls, we find a significant increase in spontaneously discharging A-fibers and C-fibers. Moreover, we show that prophylactic treatment with acetyl-l-carnitine (ALC), which blocks the development of the paclitaxel-evoked pain, causes a significant decrease (ca. 50%) in the incidence of A-fibers and C-fibers with spontaneous discharge. These results suggest that abnormal spontaneous afferent discharge is likely to be a factor in the pathogenesis of chemotherapy-evoked painful peripheral neuropathy, and that the therapeutic effects of ALC may be due to the suppression of this discharge.

Chronic Pain in the Cancer Survivor: A New Frontier

OBJECTIVE

This monograph is intended to clarify the clinical problem of chronic pain in cancer patients.

DESIGN

A pertinent literature review on chronic pain syndromes in cancer patients was undertaken using Medline. Further, the treatment strategies for cancer versus chronic pain are contrasted and clarified.

RESULTS

With increasing cancer survivorship come new challenges in patient care. In the United States, the cancer-related death rate has dropped by 1.1% per year from 1993-2002. Seventy-five percent of children and two out of three adults will survive cancer, whereas 50 years ago just one out of four survived. The net effect of these trends and opportunities is a large and rapidly growing population of persons living longer with cancer and/or as cancer survivors. While agreement exists on the best strategies for assessment and treatment of most acute cancer pain syndromes, little consensus exists on the treatment of chronic pain in the patient with slowly progressive cancer or the cancer survivor.

CONCLUSIONS

The landscape of "cancer pain" is shifting quickly into a chronic pain situation in many instances, thereby blurring previous lines of distinction in treatment strategies most suited for "chronic" versus "malignant" pain. Adopting chronic pain treatment strategies including pharmacologic and other pain control techniques, rehabilitation care, and psychological coping strategies may lead to optimal outcomes. Lastly, as cancer evolves into a chronic illness, with co-morbid conditions, recurrent cancer, and treatment toxicities from repeated antineoplastic therapies, pain management challenges in the oncologic patient continue to increase in complexity.

Dysfunction in multiple primary afferent fiber subtypes revealed by quantitative sensory testing in patients with chronic vincristine-induced pain

Vincristine is one of the frontline chemotherapy drugs for the treatment of numerous lymphoid neoplasias. The main dose-limiting complication of vincristine is the development of painful peripheral neuropathy. Although clinical reports have appeared in the literature detailing the symptoms of vincristine neuropathy, quantitative sensory testing data that might yield insight to dysfunction in subsets of primary afferents are lacking. In this report, pain descriptors and anatomical distributions of sensory abnormalities were collected in each patient. Touch detection threshold, sharpness detection threshold, the thresholds for the detection of skin warming, heat pain, skin cooling, and the perception of cooling-induced pain were measured in patients with chronic vincristine-induced pain in each area of sensory abnormality and in skin perceived as outside the affected areas. Elevated touch detection thresholds were observed both within and outside areas affected by pain and sensory abnormality. Elevated sharpness and warm detection thresholds were noted only in areas affected by pain. These data suggest that chronic vincristine-induced pain is associated with dysfunction in Abeta, Adelta, and C caliber primary afferent fibers. Deficits in Abeta fibers appear to precede and presage deficits in the other fiber types, whereas deficits in Adelta- and C-fiber function appear to be specifically associated with the generation of pain.