Biomarkers and Surrogate Endpoints: Advancing Clinical Research and Applications

Neurosciences III - Biomarkers for Pain Assessment

Objectives

Modification of the function, chemistry, and structure of neurons is a major determinant of inflammatory and neuropathic pain. The challenge is to:

• Characterize the cellular, molecular, and physiological markers that may be involved in pain responses

• Discover the participation of novel genes in plasticity that are relevant to pain mechanisms

• Use imaging techniques to identify pain-activated areas in humans that may provide opportunities to follow effectiveness of new therapeutic approaches

• Utilize this information to improve the diagnosis and initiate novel treatment strategies for pain

Moderators: Gerald Fischbach, M.D., National Institute of Neurological Diseases and Stroke Harold Slavkin, D.D.S., Director, National Institute of Dental and Craniofacial Research Cheryl Kitt, Ph.D., National Institute of Neurological Diseases and Stroke

Discussant: Mitchell Max, M.D., National Institute of Dental and Craniofacial Research

Quantitative Sensory Testing and Scanning Laser Doppler Fleximetry for Peripheral Nerve Injury

Raymond Dionne, D.D.S., Ph.D., National Institute of Dental and Craniofacial Research

ABSTRACTS

Pain is a subjective experience that results from injury to cutaneous, musculoskeletal, visceral, or nervous tissue. Since structural and functional alternations in pain transmission and pain modulation pathways in the central nervous system can emerge from injuries that are not easily recognized or that appear to be minor, identifying the neural basis of altered pain perception is often elusive. Human functional brain imaging is an emerging technique that, with caution, may provide insights into the diagnosis and treatment of pain conditions. Functional magnetic resonance imaging and high‑resolution positron emission tomography have been used to image neural activity related to peripheral and central neuropathic pain conditions. For example, some neuropathic pain states produce reduced thalamic activity, whereas allodynia in central nervous system pain patients leads to activation of pain‑related cortical areas such as the anterior cingulate cortex. Functional brain imaging has begun to be used to compare neural activation patterns related to cutaneous, musculoskeletal, visceral, and neuropathic pain, but data are not yet available to predict the source of pain based on such activation patterns. Competitive radioligand binding studies using opiate agonists such as diprenorphin have identified possible forebrain sites of pain‑related opiate actions. Similar radioligand binding studies may prove useful in screening novel analgesic medications. At the present time, functional brain imaging alone cannot be used to diagnose pain conditions or to determine the effectiveness of analgesic treatments. However, with appropriately designed experiments and cautious interpretation, this method may provide important insights into the neural substrate of different pain conditions and may be useful in evaluating new analgesic treatments.

Coghill RC, Talbot JD, Evans AC, Meyer E, Gjedde A, Bushnell MC, Duncan GH. Distributed processing of pain and vibration by the human brain. J Neurosci 1994;14:4095‑4108.

Hsieh JC, Belfrage M, Stone‑Elander S, Hansson P, Ingvar M. Central representation of chronic ongoing neuropathic pain studied by positron emission tomography. Pain 1995;63:225‑236.

Iadarola MJ, Max MB, Berman KF, Byas‑Smith MG, Coghill RC, Gracely RH, Bennett GJ. Unilateral decrease in thalamic activity observed with positron emission tomography in patients with chronic neuropathic pain. Pain 1995;63:55‑64.

Jones AKP, Qi LY, Fujirawa T, Luthra SK, Ashburner J, Bloomfield P, Cunningham VJ, Itoh M, Fukuda H, Jones T. In vivo distribution of opioid receptors in man in relation to the cortical projections of the medial and lateral pain systems measured with positron emission tomography. Neurosci Lett 1991;126:25‑28.

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Other than reversal of opiate analgesia with naloxone and imaging of regional brain glucose metabolism or blood flow before and after opiates, there has been little investigation into imaging and markers of analgesia or analgesic mechanisms. Development of high‑resolution positron emission tomography (PET) scanners and synthesis of highly selective ligands should allow examination of pharmacologic mechanisms of analgesia and sites of action in humans. For example, we have recently demonstrated spinal cholinergic activation by PET in response to opiate administration, a marker for one site of opiate analgesic activity. In the future, activity at known sites of action could predict analgesic activity in certain pain syndromes and help guide drug development.

Adler LJ, Gyulai FE, Diehl DJ, Mintun MA, Winter PM, Firestone LL. Regional brain activity changes associated with fentanyl analgesia elucidated by positron emission tomography. Anesth Analg 1997;84(1):120‑126.

Iadarola MJ, Berman KF, Zeffiro TA, Byas‑Smith MG, Gracely RH, Max MB, Bennett GJ. Neural activation during acute capsaicin‑evoked pain and allodynia assessed with PET. Brain 1998;121(5):931‑947.

Mach RH, Voytko ML, Ehrenkaufer RLE, Nader MA, Tobin JR, Efange SMN, Parsons SM, Gage HD, Smith CR, Morton TE. Imaging of cholinergic terminals using the radiotracer [18F](+)‑4‑fluorobenzyltrozamicol: In vitro binding studies and positron emission tomography studies in nonhuman primates. Synapse 1997;25:368‑380.

Pain is an unpleasant sensory and emotional experience that is typically assessed through self‑report measures such as numeric or adjective rating scales. Although pain is a personal experience, it does have behavioral correlates. People who have pain may talk about their pain, reduce their activity level, take medication, or exhibit pain‑related body postures or facial expressions. Over the past decade, there has been a growing recognition that an assessment of such pain‑related behaviors may provide a useful supplement to the more typical self‑report measures of pain. The purpose of this presentation is to provide an overview of recent research on the behavioral assessment of pain. The presentation is divided into three parts. In the first part, the clinical and theoretical rationale for the measurement of pain behavior is discussed. The second part describes and evaluates behavioral observation protocols that are increasingly being used to assess pain behavior. These protocols feature the use of trained observers who systematically observe and code nonverbal pain behaviors that occur in standardized situations or the natural environment. The reliability and validity of behavioral observation protocols have been supported by numerous studies carried out in patients suffering from arthritis pain, back pain, and cancer pain. The third part of this presentation focuses on important future directions for pain behavior assessment. Innovative approaches will be highlighted, including the use of electronic event monitoring to assess pain medication intake and hand‑held electronic diaries to record day‑to‑day variations in pain behavior. Taken together, findings from recent studies suggest that behavioral methods can play an important role in the assessment of pain. These methods provide a useful adjunct to self‑reports of pain and yield information that can be quite helpful in understanding and treating pain.

Neuropathic pain is made up of heterogeneous mechanisms. Such heterogeneous mechanisms are usually clinically assessed only on the basis of subjective measures of overall pain intensity and unpleasantness. The use of surrogate markers to distinguish between pain mechanisms may facilitate the conduct of clinical trials and the development of new treatment strategies. Study groups or subgroups that address pain mechanisms may be distinguished ad hoc by using surrogate endpoints, and surrogate endpoints may be used to evaluate patients’ responses to treatment. However, the validity of such endpoints, the clear relationship between the surrogate endpoint and the true endpoint, is critical to the validity of the data from clinical trials that use these markers. We will discuss the validation of pain biomarkers using clinical trials to evaluate both the marker itself and the true endpoint, using intensity and spread of allodynia as a specific example.

Eisenach JC, Hood DD, Curry R, Tong C. Alfentanil, but not amitriptyline, reduces pain, hyperalgesia, and allodynia from intradermal injection of capsaicin in humans. Anesthesiology 1997;86(6):1279‑1287.

Liu M, Max MB, Parada S, Rowan JS, Bennett GJ. The sympathetic nervous system contributes to capsaicin‑evoked mechanical allodynia but not pinprick hyperalgesia in humans. J Neurosci 1996;16(22):7331‑7335.

Max MB, Byas-Smith MG, Gracely RH, Bennett GJ. Intravenous infusion of the NMDA antagonist, ketamine, in chronic posttraumatic pain with allodynia: A double‑blind comparison to alfentanil and placebo. Clin Neuropharmacol 1995;18(4):360‑368.

Park KM, Max MB, Robinovitz E, Gracely RH, Bennett GJ. Effects of intravenous ketamine, alfentanil, or placebo on pain, pinprick hyperalgesia, and allodynia produced by intradermal capsaicin in human subjects. Pain 1995;63(2):163‑172.

Rowbotham MC, Fields HL. The relationship of pain, allodynia and thermal sensation in post‑herpetic neuralgia. Brain 1996;119(Pt 2):347‑354.

Sang CN, Hostetter MP, Gracely RH, Chappell AS, Schoepp DD, Lee G, Whitcup S, Caruso R, Max MB. AMPA/kainate antagonist LY293558 reduces capsaicin‑evoked hyperalgesia but not pain in normal skin in humans. Anesthesiology 1998;89(5):1060‑1067.

Segerdahl M, Ekblom A, Sjolund KF, Belfrage M, Forsberg C, Sollevi A. Systemic adenosine attenuates touch evoked allodynia induced by mustard oil in humans. Neuroreport 1995;6(5):753‑756.

Whitehead WE, Palsson OS. Is rectal pain sensitivity a biological marker for irritable bowel syndrome: Psychological influences on pain perception. Gastroenterology 1998;115(5):1263‑1271.

There have been many developments in the understanding of pain on the molecular and cellular levels. With these developments have come specific markers of the presence of mechanisms highly specific to pain. These objective markers provide the possibility for unique therapies as well as detection of disruption of what might be presumed to be necessary mechanisms for the experience of pain. However, our understanding of pain is far removed from placing it in the objective realm, and the traditional subjective assessments of which the visual analog scale is the gold standard will undoubtedly continue to be in the forefront in clinical pain research. Nevertheless, the existence of markers such as the tetrodotoxin‑resistant sodium channel, vanilloid receptor, and specific second messenger isoforms provide an opportunity to explore further the objectification of pain transmission and present significant potential for pharmacological targeting and as pain transmission markers.

Recent neuropathology research has identified mechanistic links between nerve injury and pain that have been useful in understanding neural biomarkers of pain. There are several clinical studies of historical importance that have sought to link the form of neuropathy with pain conditions, but structure‑function relationships have been too general to provide much prognostic significance. This is due in part to the retrospective protocols of the studies, which have compared sural nerve biopsies with descriptions of clinical complaints. More recent prospective studies demonstrate a strong relationship between injury of small sensory nerve fibers and spontaneous neuropathic pain (Griffin et al.). The strongest correlation in spontaneous neuropathic pain in feet was not with changes in the morphology of sural nerve biopsies, although small fiber loss was evident in many cases, but with small fiber loss in distal biopsies of epidermal tissue of the feet. These studies reinforce the thought that sensory fiber loss is a common feature of neuropathies with spontaneous neuropathic pain, and that the small sensory fiber loss is greatest distally. Laboratory work has demonstrated quite convincingly that the neuropathies with rapid axonal degeneration are the most painful, and that the cytokine‑driven process of Wallerian degeneration is the link that unites painful neuropathies of different causes (Myers et al. 1993, 1999). Although predominantly demyelinating neuropathies can be painful, many of these also involve axonal injury, and it is this component of the neuropathy that best relates to pain. We believe that the direct effect of tumor necrosis factor alpha, upregulated in response to endoneurial release of nociceptive neuropeptides and Schwann cell activation, is the key factor that links nerve injury, pain, and sensitization of the afferent sensory pathway. Thus, markers for cytokine upregulation and axonal degeneration could provide important diagnostic, prognostic, and treatment endpoint measures without the need for a more invasive nerve biopsy. In this regard, the recent work of Helena Brisby in Gothenburg, Sweden, is important (Brisby et al., in press). She has shown that human patients with acute, painful disk herniation in the lumbar spine have significant increases in cerebrospinal fluid (CSF) neurofilaments from injured nerve root axons and increased S‑100 protein in CSF, presumably as a consequence of Schwann cell injury during Wallerian degeneration. It is proposed that continued research to identify molecular markers of axonal injury shed into the blood or CSF will be instrumental in designing a relatively noninvasive biomarker for pain.

Brisby H, Olmarker K, Rosengren L, Cederlund C‑G, Rydevik B. Markers of nerve tissue injury in the cerebrospinal fluid in patients with lumbar disc herniation and sciatica. Spine, in press.

Griffin JW, Herrman D, Scott L, Cornblath DR, Hauer P, McArthur JC. Fiber loss in painful neuropathies. J Periph Nervous Sys, in press.

Myers RR, Wagner R, Sorkin LS. Hyperalgesia actions of cytokines on the peripheral nervous system. In: Cytokines and Pain: Progress in Inflammation Research. Watkins L (ed). Basel: Birkhauser Verlag 1999;133‑158.

Myers RR, Yamamoto T, Yaksh TL, Powell HC. The role of focal nerve ischemia and Wallerian degeneration in peripheral nerve injury producing hyperesthesia. Anesthesiology 1993;78:308‑316.

Damage to peripheral somatosensory nerves by disease or trauma sometimes results in a chronic syndrome of abnormal pain sensation. Examples of such painful peripheral neuropathies include diabetic neuropathy, posttherapeutic neuralgia, causalgia, and the toxic neuropathy caused by chemotherapy. These abnormal pain conditions respond poorly or not at all to standard nonsteroidal anti-inflammatory drugs and opiate analgesics. Testing new analgesics in these patients is a difficult and expensive undertaking, and testing in normal animals seems pointless because neuropathic pain has no obvious counterpart in normal physiology. Recently developed rat models of painful peripheral neuropathies can serve as biomarkers of clinical conditions, which has led to a rapid increase in research for new drug therapies. Several models are available, but only two have been used extensively in pharmacological research. The chronic constriction injury (CCI) model of Bennett and Xie (1988) is produced by tying loosely constrictive ligatures around the rat's sciatic nerve at midthigh level. The ligatures evoke intraneural edema, the swelling is opposed by the ligatures, and the nerve self‑strangulates. The spinal nerve transection (SNT) model developed by Kim and Chung (1992) involves tight ligation (and hence transection) of the L5 and L6 spinal nerves close to their respective ganglia. Both models produce signs of abnormal pain that resemble those found in patients—heat‑hyperalgesia, mechanohyperalgesia, mechanoallodynia, and cold‑allodynia—plus signs of spontaneous pain. The animal models have been accurate in predicting clinical efficacy. Drugs with efficacy in the rats have efficacy in human patients and vice versa. Moreover, drugs that are weakly effective in rats (e.g., carbamazepine and nonspecific Ca²⁺ channel blockers) are also weakly effective in patients, and drugs that do not work in rats (e.g., benzodiazepines) are also clinically ineffective. Research with these biomarkers has already led to the discovery of at least three entirely new classes of drugs that have clinical efficacy: N-methyl-D-aspartate receptor blockers, N‑type calcium channel blockers, and gabapentin‑like drugs. These drugs have little or no effect on normal acute pain sensation; thus, they would have been impossible to discover using the animal models traditionally used to discover new analgesics.

Bennett GJ, Xie Y‑K. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 1988;33(1):87‑107.

Kim SH, Chung JM. An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat. Pain 1992;50(3):355‑363.

The intent of this meeting is to make development of therapies more efficient by identifying surrogate markers that will predict the clinical outcome of interest but occur much earlier. In pain research, however, no one has a problem with using the patient's pain rating as the major outcome (supplemented by related self-reported dimensions such as mood and function). These outcomes are clinically meaningful and change quickly with effective therapy. The search for laboratory tests of pain, from spinal neurotransmitters to evoked potentials to positron emission tomography scans, has made little progress toward providing a useful substitute for the chronic pain patient's verbal responses. A different question relevant to this meeting is whether simple, inexpensive, homogeneous models of clinical pain such as dental extraction pain or experimental pain can serve as "biomarkers" or "surrogates" for response of the many types of chronic pain to a putative analgesic drug. Some basic pain researchers claim that mechanisms of various classes of chronic pain syndromes (e.g., inflammatory, neuropathic, muscle, visceral) are quite distinct; others emphasize pathophysiological similarites across many categories. The practical clinical and economic question is whether a new class of analgesic needs to be studied in only two or three prototypical pain conditions or whether studies in a dozen different conditions are needed to determine its spectrum of clinical usefulness for chronic pain. Current approaches of the National Institutes of Health (NIH), industry, and the U.S. Food and Drug Administration (FDA) are unlikely to resolve this. First, NIH researchers tend be "splitters," focusing on a single disease, rather than studying drug effects across diseases. Moreover, NIH-supported analgesic clinical trials have been concentrated in a few clinical areas (mainly postoperative pain, neuropathic pain, cancer pain, back pain, dental pain, and headache), with few trials in common conditions such as gastrointestinal, urological, gynecological, cardiac, muscle, and neck pain. Second, industry has been even more conservative because of the financial risk of doing clinical studies in unvalidated models and have restricted most trials of new drugs to postoperative pain, headache, and diabetic and postherapeutic neuropathic pain. Third, FDA guidelines on analgesic development say nothing about repeated dose efficacy studies or claims for chronic pain indications. I propose that NIH, industry, and FDA work out novel mechanisms to promote analgesic trials across neglected pain syndromes. For example, NIH might help fund a set of centers with cohorts of patients with common but poorly studied chronic pain syndromes. Once a new drug is found to be effective in at least one pain condition (e.g., dental surgery), the drug could be studied in highly efficient crossover studies in these many chronic pain models. In addition, industry and FDA should find ways to provide data on newly approved drugs so academics can scrutinize these for correlations between the response of various pain syndromes. The knowledge gained from these programs would provide a basis for more efficient drug development and better choice of drugs for particular patient groups, leading to faster, more effective treatment at lower cost.

Max MB. Divergent traditions in analgesic clinical trials. Clin Pharmacol Ther 1994;56:237-241.

Max MB, Portenoy RK, Laska EM, eds. The Design of Analgesic Clinical Trials. Advances in Pain Research and Therapy, vol. 18. New York: Raven Press, 1991.

Woolf CJ, Bennett GJ, Doherty M, Dubner R, Kidd B, Koltzenburg M, Lipton R, Loeser JD, Payne R, Torebjork E. Towards a mechanism-based classification of pain? Pain 1998 77:227-229.

Recent neuropathology research has identified mechanistic links between nerve injury and pain that have been useful in understanding neural biomarkers of pain. Several clinical studies of historical importance have sought to link the form of neuropathy with pain conditions, but structure-function relationships have been too general to provide much prognostic significance. This is due in part to the retrospective protocols of the studies, which have compared sural nerve biopsies with descriptions of clinical complaints. More recent prospective studies demonstrate a strong relationship between injury of small sensory nerve fibers and spontaneous neuropathic pain. The strongest correlation in spontaneous neuropathic pain in feet was not with changes in the morphology of sural nerve biopsies, although small fiber loss was evident in many cases, but with small fiber loss in distal biopsies of epidermal tissue of the feet. These studies reinforce the idea that sensory fiber loss is a common feature of neuropathies with spontaneous neuropathic pain and that the small sensory fiber loss is greatest distally. Laboratory work has demonstrated quite convincingly that neuropathies with rapid axonal degeneration are the most painful and that the cytokine-driven process of Wallerian degeneration is the link that unites painful neuropathies of different causes. Although predominantly demyelinating neuropathies can be painful, many of these also involve axonal injury, and it is this component of the neuropathy that best relates to pain. We believe that the direct effect of tumor necrosis factor alpha, upregulated in response to endoneurial release of nociceptive neuropeptides and Schwann cell activation, is the key factor that links nerve injury, pain, and sensitization of the afferent sensory pathway. Thus, markers for cytokine upregulation and axonal degeneration could provide important diagnostic, prognostic, and treatment endpoint measures without the need for a more invasive nerve biopsy. In this regard, the recent work of Brisby in Gothenburg, Sweden, is important. She has shown that human patients with acute, painful disc herniation in the lumbar spine have significant increases in cerebrospinal fluid (CSF) neurofilaments from injured nerve root axons and increased S-100 protein in CSF, presumably as a consequence of Schwann cell injury during Wallerian degeneration. It is proposed that continued research to identify molecular markers of axonal injury shed into the blood or CSF will be instrumental in designing a relatively non-invasive biomarker for pain.

Brisby H, Olmarker K, Rosengren L, Cederlund C-G, Rydevik B. Markers of nerve titssue injury in the cerebrospinal fluid in patients with lumbar disc herniation and sciatica. Spine, in press.

Griffin JW, Herrman D, Scott L, Cornblath DR, Hauer P, McArthur JC. Fiber loss in painful neuropathies. J Periph Nervous Sys, in press.

Myers RR, Wagner R, Sorkin LS. Hyperalgesia actions of cytokines on the peripheral nervous system. In: Cytokines and Pain: Progress in Inflammation Research. L Watkins, ed. Basel: Birkhauser Verlag, 1999; pp. 133-158.

Myers RR, Yamamoto T, Yaksh TL, Powell HC: The role of focal nerve ischemia and Wallerian degeneration in peripheral nerve injury producing hyperesthesia. Anesthesiology 1993;78:308-316.

The assessment of acute pain in humans relies heavily on the subjective report of subjects using a variety of scales for pain intensity and unpleasantness, comparison to parallel control groups receiving a standard treatment or placebo, and relatively large samples (N=20-50 per group) to overcome the substantial variability in pain responsiveness that exists across patients (Hargreaves and Dionne 1991). Measurement of a biomarker characterized for pain should reduce reliance on such cumbersome methods and permit individualized assessment of pain mechanisms, the perception of pain, and response to analgesic treatments. Plasma beta-endorphin is released in response to pain in animals and humans due to a variety of painful stimuli and is readily measured by radioimmunoassay (RIA) of plasma samples, making it a potential canditate as a biomarker for pain. A series of carefully controlled clinical trials in the oral surgery model of acute pain and inflammation demonstrate the release of beta-endorphin during surgical stress and postoperative pain, supportive of a role as a biomarker for pain. Blockade of beta-endorphin release with a low dose of dexamethasone results in increased pain following oral surgery, whereas increasing plasma levels via administration of corticiptrophin- releasing hormone is analgesic. Administration of ibuprofen following pain onset also suppresses both pain and beta-endorphin over a similar time course, with the most effective dose resulting in the greatest decrease in plasma endorphin levels. Taken together, these data strongly suggest a functional relationship between increased plasma endorphin levels and pain, with lowering of pain-evoked increases by analgesic manipulations. Conversely, measurement of beta-endorphin in cerebrospinal fluid associated with electrical stimulation of the periventricular gray area in humans was demonstrated to be an artifact of concurrent administration of a radio-opaque dye that mimics beta-endorphin in an RIA, indicative of the dangers of reliance on a biochemical marker for a subjective response. The results of another series of studies evaluating levels of prostaglandin E2 at the surgical site as a possible marker for inflammatory pain will also be reviewed.

Dionne RA, McCullough L. Analgesic efficacy of the S(+) isomer of ibuprofen in comparison to racemic ibuprofen. Clin Pharmacol Therap 1998;63:694-701.

Dionne RA, Mueller GP, Young RF, Greenberg RP, Hargreaves KM, Gracely R, Dubner R. Contrast medium causes the apparent increase in beta-endorphin levels in human cerebrospinal fluid following brain stimulation. Pain 1984;20:313-321.

Gordon SM, Dionne RA, Brahim J, Jabir F, Dubner R. Blockade of peripheral neuronal barrage reduces postoperative pain. Pain 1997;70:209-215.

Hargreaves KM, Dionne RA. Evaluating endogenous mediators of pain and analgesia in clinical studies. In: The Design of Analgesic Trials; M. Max, R. Portenoy, E. Laska, eds, New York: Raven Press, 1991, pp. 579-598.

Hargreaves KM, Dionne RA, Mueller GP, Goldstein DS, Dubner R. Naloxone, fentanyl and diazepam modify beta-endorphin levels during surgery. Clin Pharmacol Therap 1986;40:165-171.

Hargreaves KM, Mueller GP, Dubner R, Goldstein DS, Dionne RA. Corticotropin releasing factor (CRF) produces analgesia in humans and rats. Brain Res 1987;422:154-157.

Hargreaves KM, Schmidt EA, Mueller GP, Dionne RA. Dexamethasone alters plasma levels of beta-endorphin and postoperative pain. Clin Pharmacol Therap 1987;42:601-607.