The endogenous opioid system is involved in key physiological functions such as pain and mood control. It is often assisted in these functions by the endogenous cannabinoid system, which appeared much later in evolution. The primary endogenous effectors of these systems are enkephalins and anandamide (AEA), respectively.
Increasing the concentrations and half-life of enkephalins and AEA, and thereby enhancing the functions of the endogenous opioid system and the endogenous cannabinoid system, respectively, is the core concept that led to the development of the novel analgesics and mood regulators described in this Review.
Exogenous effectors — such as morphine — that indiscriminately overstimulate all central and peripheral opioid receptors may induce serious unwanted effects such as constipation, respiratory depression, tolerance or dependence.
The tonic release of enkephalins occurs only in a limited number of structures involved in pain control, in areas where the noxious stimulus takes place: in nociceptors (which are located at the periphery), in the medulla or in the brain. Therefore, enhancing enkephalin concentrations does not induce the side effects observed with morphine.
Increasing the concentrations and half-life of enkephalins by inhibiting both of the zinc metallopeptidases (enkephalinases) that account for their rapid degradation has analgesic effects that are comparable to those of morphine. Similarly, the protection of endogenous cannabinoids from degradation by fatty acid amide hydrolase (FAAH) elicits analgesic effects.
Dual enkephalinase (DENK) inhibitors that are capable of concomitantly inhibiting both enkephalinases at nanomolar concentrations have been designed based on their structures and using molecular modelling. Similarly, irreversible inhibitors of FAAH have been designed.
Analgesia produced by DENK inhibitors and FAAH inhibitors represents the first new strategy to alleviate pain that has reached clinical development in decades.
The leading DENK inhibitor, PL37, which is entering Phase II clinical trials, has been shown to act at the peripheral level in animal models of neuropathic pain as well as in individuals with neuropathic pain; therefore, considering its favourable safety profile, PL37 is a candidate for the treatment of neuropathic pain.
All current treatments for pain potentiate the analgesic properties of DENK inhibitors, especially on neuropathic pain. In addition, DENK inhibitors have shown promising antidepressant effects in animal models.
Chronic pain remains unsatisfactorily treated, and few novel painkillers have reached the market in the past century. Increasing the levels of the main endogenous opioid peptides — enkephalins — by inhibiting their two inactivating ectopeptidases, neprilysin and aminopeptidase N, has analgesic effects in various models of inflammatory and neuropathic pain. Stemming from the same pharmacological concept, fatty acid amide hydrolase (FAAH) inhibitors have also been found to have analgesic effects in pain models by preventing the breakdown of endogenous cannabinoids. Dual enkephalinase inhibitors and FAAH inhibitors are now in early-stage clinical trials. In this Review, we compare the effects of these two potential classes of novel analgesics and describe the progress in their rational design. We also consider the challenges in their clinical development and opportunities for combination therapies.
Subscribe to Journal
Get full journal access for 1 year
only $21.58 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Apkarian, A. V., Bushnell, M. C., Treede, R. D. & Zubieta, J. K. Human brain mechanisms of pain perception and regulation in health and disease. Eur. J. Pain 9, 463–484 (2005).
Basbaum, A. I., Bautista, D. M., Scherrer, G. & Julius, D. Cellular and molecular mechanisms of pain. Cell 139, 267–284 (2009).
Abrahamsen, B. et al. The cell and molecular basis of mechanical, cold, and inflammatory pain. Science 321, 702–705 (2008).
Lannersten, L. & Kosek, E. Dysfunction of endogenous pain inhibition during exercise with painful muscles in patients with shoulder myalgia and fibromyalgia. Pain 151, 77–86 (2010). This is an interesting clinical study showing that the activation of the endogenous pain inhibitory system is reduced in patients with fibromyalgia.
Willis, W. D. in Neuropathic Pain: Aetiology, Symptoms, Mechanisms, and Management (ed. Campbell, J. N.) 527–531 (IASP Press, Seattle, 1996).
Rashid, M. H., Inoue, M., Toda, K. & Ueda, H. Loss of peripheral morphine analgesia contributes to the reduced effectiveness of systemic morphine in neuropathic pain. J. Pharmacol. Exp. Ther. 309, 380–387 (2004).
Hill, R. NK1 (substance P) receptor antagonists — why are they not analgesic in humans? Trends Pharmacol. Sci. 21, 244–246 (2000).
Wallace, M. S. et al. A multicenter, double-blind, randomized, placebo-controlled crossover evaluation of a short course of 4030W92 in patients with chronic neuropathic pain. J. Pain 3, 4227–4233 (2002).
Wallace, M. S. et al. A randomized, double-blind, placebo-controlled trial of a glycine antagonist in neuropathic pain. Neurology 59, 1694–1700 (2002).
Szallasi, A., Cortright, D. N., Blum, C. A. & Eid, S. R. The vanilloid receptor TRPV1, 10 years from channel cloning to antagonist proof-of-concept. Nature Rev. Drug Discov. 6, 357–372 (2007).
Melzack, R. The future of pain. Nature Rev. Drug Discov. 7, 629 (2008).
Roques, B. P. et al. The enkephalinase inhibitor thiorphan shows antinociceptive activity in mice. Nature 288, 286–288 (1980). This seminal study introduced the idea that 'physiological' analgesia can be elicited by increasing extracellular levels of enkephalins via inhibition of their inactivating enzymes.
Moore, R. A. & McQuay, H. J. Prevalence of opioid adverse events in chronic non-malignant pain: systematic review of randomised trials of oral opioids. Arthritis Res. Ther. 7, R1046–R1051 (2005).
Wang, T., Collet, J. P., Shapiro, S. & Ware, M. A. Adverse effects of medical cannabinoids: a systematic review. CMAJ 178, 1669–1678 (2008).
North, R. A., Williams, J. T., Surprenant, A. & Christie, M. J. Mu and delta receptors belong to a family of receptors that are coupled to potassium channels. Proc. Natl Acad. Sci. USA 84, 5487–5491 (1987).
Dhawan, B. N. et al. International Union of Pharmacology. XII. Classification of opioid receptors. Pharmacol. Rev. 48, 567–592 (1996).
Konig, M. et al. Pain responses, anxiety and aggression in mice deficient in pre-proenkephalin. Nature 383, 535–538 (1996). This study reported the first knockout of the PENK gene, and demonstrated the importance of enkephalins in crucial physiological functions such as pain control, reward and adaptation. See also reference 32.
Noble, F., Benturquia, N., Bilkei-Gorzo, A., Zimmer, A. & Roques, B. P. Use of preproenkephalin knockout mice and selective inhibitors of enkephalinases to investigate the role of enkephalins in various behaviours. Psychopharmacology (Berl.) 196, 327–335 (2008). This study demonstrates that the antinociceptive effects of DENK inhibitors at the supraspinal and peripheral levels are absent in PENK-knockout mice, thus underpinning the specificity of these inhibitors for the in vivo protection of endogenous enkephalins. See also reference 32.
Belluzzi, J. D. et al. Analgesia induced in vivo by central administration of enkephalin in rat. Nature 260, 625–626 (1976).
Mosnaim, A. D. et al. In vitro methionine5-enkephalin degradation kinetics by human brain preparations. Neurochem. Res. 33, 81–86 (2008).
Roques, B. P., Noble, F., Dauge, V., Fournié-Zaluski, M. C. & Beaumont, A. Neutral endopeptidase 24.11: structure, inhibition, and experimental and clinical pharmacology. Pharmacol. Rev. 45, 87–146 (1993). This was the first in-depth review on enkephalinases, examining their role in enkephalin metabolism, distribution and cloning, their molecular characteristics, mechanism of action and selectivity, as well as inhibitor design and their pharmacological properties.
Williams, J. T., Christie, M. J., North, R. A. & Roques, B. P. Potentiation of enkephalin action by peptidase inhibitors in rat locus ceruleus in vitro. J. Pharmacol. Exp. Ther. 243, 397–401 (1987). This electrophysiological study demonstrated for the first time the presence of a low-tonic release of enkephalins in some structures rich in opioid receptors. This explains why — unlike morphine — DENK inhibitors have no depressant respiratory effects, and also explains why the actions of DENK inhibitors are primarily related to the extracellular concentrations of enkephalins that activate opioid receptors.
Bourgoin, S. et al. Effects of kelatorphan and other peptidase inhibitors on the in vitro and in vivo release of methionine-enkephalin-like material from the rat spinal cord. J. Pharmacol. Exp. Ther. 238, 360–366 (1986). This was the first in vivo demonstration, using a superfused rat spinal cord, of the relationship between a pain-induced increase in the concentration of endogenous DENK inhibitor-protected enkephalins and the magnitude of the antinociceptive effects. This study demonstrated that DENK inhibitors increase extracellular levels of enkephalins but do not modify their release.
Fields, H. State-dependent opioid control of pain. Nature Rev. Neurosci. 5, 565–575 (2004).
Noble, F. et al. Inhibition of the enkephalin-metabolizing enzymes by the first systemically active mixed inhibitor prodrug RB 101 induces potent analgesic responses in mice and rats. J. Pharmacol. Exp. Ther. 261, 181–190 (1992).
Willer, J. C., Roby, A. & Ernst, M. The enkephalinase inhibitor, GB 52, does not affect nociceptive flexion reflexes nor pain sensation in humans. Neuropharmacology 25, 819–822 (1986).
Fournié-Zaluski, M. C. et al. Analgesic effects of kelatorphan, a new highly potent inhibitor of multiple enkephalin degrading enzymes. Eur. J. Pharmacol. 102, 525–528 (1984). This was the first introduction of the concept that dual inhibition of both NEP and APN is necessary to obtain significant analgesic responses.
Kayser, V., Fournié-Zaluski, M. C., Guilbaud, G. & Roques, B. P. Potent antinociceptive effects of kelatorphan (a highly efficient inhibitor of multiple enkephalin-degrading enzymes) systemically administered in normal and arthritic rats. Brain Res. 497, 94–101 (1989).
Jutkiewicz, E. M. RB101-mediated protection of endogenous opioids: potential therapeutic utility? CNS Drug Rev. 13, 192–205 (2007). This was an elegant confirmation of the potent DOR-dependent antidepressant effects of DENK inhibitors, and the first demonstration that — unlike DOR agonists — they do not induce seizures.
Noble, F. & Roques, B. P. Protection of endogenous enkephalin catabolism as natural approach to novel analgesic and antidepressant drugs. Expert Opin. Ther. Targets 11, 145–159 (2007).
Thanawala, V., Kadam, V. J. & Ghosh, R. Enkephalinase inhibitors: potential agents for the management of pain. Curr. Drug Targets 9, 887–894 (2008).
Gonzalez-Rodriguez, S. et al. Involvement of enkephalins in the inhibition of osteosarcoma-induced thermal hyperalgesia evoked by the blockade of peripheral P2X3 receptors. Neurosci. Lett. 465, 285–289 (2009). This is a demonstration of the peripheral antinociceptive effects of the first orally active DENK inhibitor, PL37, in a model of neuropathic pain. This study also demonstrates the selectivity of PL37 for enkephalinases, as antibodies against enkephalins completely reverse the antinociceptive responses — as observed after knockout of the Penk gene in mice.
Meynadier, J., Dalmas, S., Lecomte, J. M., Gros, C. & Schwartz, J. C. Potent analgesic effects of inhibitors of enkephalin metabolism administered intrathecally to cancer patients. The Pain Clinic 2, 201–206 (1988).
Guindon, J. & Hohmann, A. G. The endocannabinoid system and pain. CNS Neurol. Disord. Drug Targets 8, 403–421 (2009).
Adams, I. B., Compton, D. R. & Martin, B. R. Assessment of anandamide interaction with the cannabinoid brain receptor: SR 141716A antagonism studies in mice and autoradiographic analysis of receptor binding in rat brain. J. Pharmacol. Exp. Ther. 284, 141209–141217 (1998). This is a comparative analysis of the affinities of exogenous and endogenous cannabinoids for cannabinoid receptors.
Cravatt, B. F. et al. Molecular characterization of an enzyme that degrades neuromodulatory fatty-acid amides. Nature 384, 83–87 (1996). This was the first molecular description of FAAH and of its mechanism of action.
Ahn, K., McKinney, M. K. & Cravatt, B. F. Enzymatic pathways that regulate endocannabinoid signaling in the nervous system. Chem. Rev. 108, 1687–1707 (2008).
Fu, J. et al. A catalytically silent FAAH-1 variant drives anandamide transport in neurons. Nature Neurosci. 15, 64–69 (2012). This was the first molecular characterization of a FAAH isotype that is devoid of enzymatic activity and able to act as a shuttle transporter for the reuptake and possible secretion of AEA. This is a major discovery that explains the extracellular actions of AEA.
Freund, T. F., Katona, I. & Piomelli, D. Role of endogenous cannabinoids in synaptic signaling. Physiol. Rev. 83, 1017–1066 (2003). This was the first in-depth review of the mechanisms and roles of endogenous cannabinoids.
Haller, V. L., Stevens, D. L. & Welch, S. P. Modulation of opioids via protection of anandamide degradation by fatty acid amide hydrolase. Eur. J. Pharmacol. 600, 50–58 (2008).
Cravatt, B. F. et al. Functional disassociation of the central and peripheral fatty acid amide signaling systems. Proc. Natl Acad. Sci. USA 101, 10821–10826 (2004).
Kathuria, S. et al. Modulation of anxiety through blockade of anandamide hydrolysis. Nature Med. 9, 76–81 (2003). This study reported the rational design of the first active FAAH inhibitor containing a carbamate group. This irreversible inhibitor is currently used as a standard in pharmacological studies (see reference 54 as well). This study also described the efficient use of [3H]AEA to study AEA secretion and reuptake mechanisms.
Lichtman, A. H., Shelton, C. C., Advani, T. & Cravatt, B. F. Mice lacking fatty acid amide hydrolase exhibit a cannabinoid receptor-mediated phenotypic hypoalgesia. Pain 109, 319–327 (2004).
Massa, F. et al. The endogenous cannabinoid system protects against colonic inflammation. J. Clin. Invest. 113, 1202–1209 (2004).
Hohmann, A. G. et al. An endocannabinoid mechanism for stress-induced analgesia. Nature 435, 1108–1112 (2005).
Petrosino, S. et al. Changes in spinal and supraspinal endocannabinoid levels in neuropathic rats. Neuropharmacology 52, 415–422 (2007).
Khasabova, I. A. et al. A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain. J. Neurosci. 28, 11141–11152 (2008).
Ahn, K. et al. Mechanistic and pharmacological characterization of PF-04457845: a highly potent and selective fatty acid amide hydrolase inhibitor that reduces inflammatory and noninflammatory pain. J. Pharmacol. Exp. Ther. 338, 114–124 (2011).
Clapper, J. R. et al. Anandamide suppresses pain initiation through a peripheral endocannabinoid mechanism. Nature Neurosci. 13, 1265–1270 (2010). These were the first FAAH inhibitors that were shown to act exclusively at the periphery and inhibit nocifensive signals at nociceptor levels. Such compounds could be the most clinically interesting FAAH inhibitors.
Di Marzo, V. Targeting the endocannabinoid system: to enhance or reduce? Nature Rev. Drug Discov. 7, 438–455 (2008).
Bracey, M. H., Hanson, M. A., Masuda, K. R., Stevens, R. C. & Cravatt, B. F. Structural adaptations in a membrane enzyme that terminates endocannabinoid signaling. Science 298, 1793–1796 (2002).
Fournié-Zaluski, M. C. et al. Structure of aminopeptidase N from Escherichia coli complexed with the transition-state analogue aminophosphinic inhibitor PL250. Acta Crystallogr. D Biol. Crystallogr. 65, 814–822 (2009). This was the first structural analysis of Escherichia coli APN complexed with a highly potent APN inhibitor, showing that this bacterial enzyme can be used as a template for the rational design of APN inhibitors that can be used in humans.
Oefner, C., Roques, B. P., Fournié-Zaluski, M. C. & Dale, G. E. Structural analysis of neprilysin with various specific and potent inhibitors. Acta Crystallogr. D Biol. Crystallogr. 60, 392–396 (2004).
Mileni, M. et al. Crystal structure of fatty acid amide hydrolase bound to the carbamate inhibitor URB597: discovery of a deacylating water molecule and insight into enzyme inactivation. J. Mol. Biol. 400, 743–754 (2010). This study investigated the mechanism of AEA hydrolysis at the atomic level to provide a structure-dependent explanation for the irreversible or reversible binding of FAAH inhibitors.
Noble, F. et al. First discrete autoradiographic distribution of aminopeptidase N in various structures of rat brain and spinal cord using the selective iodinated inhibitor [125I]RB 129. Neuroscience 105, 479–488 (2001). This autoradiographic study of APN distribution in the rat brain shows that APN localization is not restricted to blood vessels — it is also present in all structures involved in the main functions of enkephalins and colocalizes in these regions with NEP, MORs and DORs.
Jardinaud, F. et al. Ontogenic and adult whole body distribution of aminopeptidase N in rat investigated by in vitro autoradiography. Biochimie 86, 105–113 (2004).
Sales, N., Dutriez, I., Maziere, B., Ottaviani, M. & Roques, B. P. Neutral endopeptidase 24.11 in rat peripheral tissues: comparative localization by 'ex vivo' and 'in vitro' autoradiography. Regul. Pept. 33, 209–222 (1991).
Roques, B. P. Novel approaches to targeting neuropeptide systems. Trends Pharmacol. Sci. 21, 475–483 (2000). This paper describes how the diffusion of neuropeptides over an extended area may explain why enkephalins are suitable targets for treating inflammatory and neuropathic pain.
Mitrirattanakul, S. et al. Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain. Pain 126, 102–114 (2006).
Wenk, H. N., Brederson, J. D. & Honda, C. N. Morphine directly inhibits nociceptors in inflamed skin. J. Neurophysiol. 95, 2083–2097 (2006).
Stein, C. & Zöllner, C. Opioids and sensory nerves. Handb. Exp. Pharmacol. 194, 495–518 (2009).
Di Marzo, V., Bisogno, T. & De Petrocellis, L. Endocannabinoids and related compounds: walking back and forth between plant natural products and animal physiology. Chem. Biol. 14, 741–756 (2007).
Agarwal, N. et al. Cannabinoids mediate analgesia largely via peripheral type 1 cannabinoid receptors in nociceptors. Nature Neurosci. 10, 870–879 (2007).
Richardson, J. D., Kilo, S. & Hargreaves, K. M. Cannabinoids reduce hyperalgesia and inflammation via interaction with peripheral CB1 receptors. Pain 75, 111–119 (1998).
Schreiter, A. et al. Protection of opioid peptide catabolism for pain control in peripheral inflamed tissue. in: International Narcotic Research Conference (Portland, 2009; Abstract 62). This was the first demonstration, using enkephalin-specific antibodies (that combine thiorphan plus bestatin, and the DENK inhibitor PL253 (P8B)), that all components of the endogenous opioid system are present in inflamed tissues.
Maldonado, R., Valverde, O., Turcaud, S., Fournié-Zaluski, M. C. & Roques, B. P. Antinociceptive response induced by mixed inhibitors of enkephalin catabolism in peripheral inflammation. Pain 58, 77–83 (1994). This was the first demonstration, using methylnaloxonium (an opioid antagonist acting predominantly outside the CNS), that enkephalins have a major role in the peripheral control of nociception.
Stein, C., Schafer, M. & Machelska, H. Attacking pain at its source: new perspectives on opioids. Nature Med. 9, 1003–1008 (2003). This study provided the first convincing arguments for the interest in alleviating pain at the nociceptor level. See also reference 69.
Guindon, J. & Beaulieu, P. The role of the endogenous cannabinoid system in peripheral analgesia. Curr. Mol. Pharmacol. 2, 134–139 (2009).
Joseph, E. K. & Levine, J. D. Mu and delta opioid receptors on nociceptors attenuate mechanical hyperalgesia in rat. Neuroscience 171, 344–350 (2010).
Labuz, D., Mousa, S. A., Schafer, M., Stein, C. & Machelska, H. Relative contribution of peripheral versus central opioid receptors to antinociception. Brain Res. 1160, 30–38 (2007).
Schlosburg, J. E., Kinsey, S. G. & Lichtman, A. H. Targeting fatty acid amide hydrolase (FAAH) to treat pain and inflammation. AAPS J. 11, 39–44 (2009).
Roques, B. & Fournié-Zaluski, M. C. Amino acid derivatives containing a disulfanyl group in the form of mixed neprilysin and aminopeptidase N inhibitors. WO 2007/048787 (2007).
Menendez, L. et al. Inhibition of osteosarcoma-induced thermal hyperalgesia in mice by the orally active dual enkephalinase inhibitor PL37. Potentiation by gabapentin. Eur. J. Pharmacol. 596, 50–55 (2008). This was the first demonstration, using isobolographic analysis, of the strong synergy between a DENK inhibitor (PL37) and gabapentin in a murine model of neuropathic pain; this study confirmed that PL37 acts at the nociceptor level through selective stimulation of MORs.
Thibault, K. et al. Antinociceptive and anti-allodynic effects of oral L37, a complete inhibitor of enkephalin-catabolizing enzymes in a rat model of peripheral neuropathic pain induced by vincristine. Eur. J. Pharmacol. 600, 71–77 (2008).
Johnson, D. S. et al. Discovery of PF-04457845: a highly potent, orally bioavailable, and selective urea FAAH inhibitor. ACS Med. Chem. Lett. 2, 91–96 (2011). This paper describes the discovery of an orally administrated FAAH inhibitor, which was tested in patients with knee osteoarthritis, but was found to lack efficacy.
Howlett, A. C. et al. International Union of Pharmacology. XXVII. Classification of cannabinoid receptors. Pharmacol. Rev. 54, 161–202 (2002).
Nature Reviews Drug Discovery GPCR Questionnaire Participants. The state of GPCR research in 2004. Nature Rev. Drug Discov. 3, 577–626 (2004).
Schlicker, E. & Kathmann, M. Modulation of transmitter release via presynaptic cannabinoid receptors. Trends Pharmacol. Sci. 22, 565–572 (2001).
Matthes, H. W. et al. Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene. Nature 383, 819–823 (1996). This was the first demonstration of the crucial role of MORs in pain control and dependence mechanisms.
Nieto, M. M., Guen, S. L., Kieffer, B. L., Roques, B. P. & Noble, F. Physiological control of emotion-related behaviors by endogenous enkephalins involves essentially the delta opioid receptors. Neuroscience 135, 305–313 (2005). This study shows that knockout of the gene encoding MOR does not alter the emotional responses of mice subjected to stressful conditions, thus demonstrating the predominance of DORs in the control of these physiological functions.
Le Guen, S. et al. Further evidence for the interaction of μ- and δ-opioid receptors in the antinociceptive effects of the dual inhibitor of enkephalin catabolism, RB101(S): a spinal c-Fos protein study in the rat under carrageenin inflammation. Brain Res. 967, 106–112 (2003).
Sullivan, A. F., Dickenson, A. H. & Roques, B. P. Delta-opioid mediated inhibitions of acute and prolonged noxious-evoked responses in rat dorsal horn neurones. Br. J. Pharmacol. 98, 1039–1049 (1989).
Wang, H. B. et al. Coexpression of δ- and μ-opioid receptors in nociceptive sensory neurons. Proc. Natl Acad. Sci. USA 107, 13117–13122 (2010).
Dickenson, A. H., Le Bars, D. & Besson, J. M. Endogenous opiates and nociception: a possible functional role in both pain inhibition and detection as revealed by intrathecal naloxone. Neurosci. Lett. 24, 161–164 (1981).
Julius, D. & Basbaum, A. I. A neuropeptide courier for δ-opioid receptors? Cell 122, 496–498 (2005).
Roques, B. P. in Encyclopedia of Neuroscience (ed. Squire, L. R.) 789–799 (Oxford Academic Press, Oxford, 2009).
Alexander, S. P. & Kendall, D. A. The complications of promiscuity: endocannabinoid action and metabolism. Br. J. Pharmacol. 152, 602–623 (2007).
Piomelli, D. The molecular logic of endocannabinoid signalling. Nature Rev. Neurosci. 4, 873–884 (2003).
Ligresti, A., Petrosino, S. & Di Marzo, V. From endocannabinoid profiling to 'endocannabinoid therapeutics'. Curr. Opin. Chem. Biol. 13, 321–331 (2009).
Akopian, A. N., Ruparel, N. B., Jeske, N. A., Patwardhan, A. & Hargreaves, K. M. Role of ionotropic cannabinoid receptors in peripheral antinociception and antihyperalgesia. Trends Pharmacol. Sci. 30, 79–84 (2009).
Di Marzo, V. et al. Levels, metabolism, and pharmacological activity of anandamide in CB1 cannabinoid receptor knockout mice: evidence for non-CB1, non-CB2 receptor-mediated actions of anandamide in mouse brain. J. Neurochem. 75, 2434–2444 (2000). This study shows that knockout of Cb1r does not block the pharmacological action of AEA and Δ9-THC (spontaneous and pain-evoked activity), and suggests the presence of other unknown targets for endogenous cannabinoids; this may be a potential issue associated with the use of FAAH inhibitors.
Tognetto, M. et al. Anandamide excites central terminals of dorsal root ganglion neurons via vanilloid receptor-1 activation. J. Neurosci. 21, 1104–1109 (2001).
O'Sullivan, S. E. Cannabinoids go nuclear: evidence for activation of peroxisome proliferator-activated receptors. Br. J. Pharmacol. 152, 576–582 (2007).
Jhaveri, M. D. et al. Inhibition of fatty acid amide hydrolase and cyclooxygenase-2 increases levels of endocannabinoid related molecules and produces analgesia via peroxisome proliferator-activated receptor-α in a model of inflammatory pain. Neuropharmacology 55, 85–93 (2008).
Kozak, K. R., Prusakiewicz, J. J. & Marnett, L. J. Oxidative metabolism of endocannabinoids by COX-2. Curr. Pharm. Des. 10, 659–667 (2004).
Guindon, J. & Hohmann, A. G. A physiological role for endocannabinoid-derived products of cyclooxygenase-2-mediated oxidative metabolism. Br. J. Pharmacol. 153, 1341–1343 (2008).
Hokfelt, T., Bartfai, T. & Bloom, F. Neuropeptides: opportunities for drug discovery. Lancet Neurol. 2, 463–472 (2003). References 58 and 97 show the differences in synaptic signalling elicited by classical neurotransmitters and endogenous cannabinoids acting at a short distance with micromolar receptor affinities, and neuropeptides (such as enkephalins) acting by volume transmission (extended synaptic area) with nanomolar receptor affinities over long distances. This allows stimulation of opioid receptors away from the site of enkephalin secretion.
Walker, J. M., Huang, S. M., Strangman, N. M. & Sanudo-Pena, M. C. Identification of the role of endogenous cannabinoids in pain modulation: strategies and pitfalls. J. Pain 1, 20–32 (2000).
Rose, C. et al. Characterization and inhibition of a cholecystokinin-inactivating serine peptidase. Nature 380, 403–409 (1996).
Fryer, R. M. et al. Effect of bradykinin metabolism inhibitors on evoked hypotension in rats: rank efficacy of enzymes associated with bradykinin-mediated angioedema. Br. J. Pharmacol. 153, 947–955 (2008). This study shows that aminopeptidase — and not NEP — is the main enzyme involved in bradykinin catabolism, thus reducing the risk of bradykinin-mediated adverse effects caused by DENK inhibitors.
Reed, B., Zhang, Y., Chait, B. T. & Kreek, M. J. Dynorphin A(1–17) biotransformation in striatum of freely moving rats using microdialysis and matrix-assisted laser desorption/ionization mass spectrometry. J. Neurochem. 86, 815–823 (2003).
Reed, B., Bidlack, J. M., Chait, B. T. & Kreek, M. J. Extracellular biotransformation of β-endorphin in rat striatum and cerebrospinal fluid. J. Neuroendocrinol. 20, 606–616 (2008).
Skidgel, R. A. & Erdos, E. G. Angiotensin converting enzyme (ACE) and neprilysin hydrolyze neuropeptides: a brief history, the beginning and follow-ups to early studies. Peptides 25, 521–525 (2004).
Roques, B. P. Cell surface metallopeptidases involved in blood pressure regulation: structure, inhibition and clinical perspectives. Pathol. Biol. (Paris) 46, 191–200 (1998).
Marvizon, J. C., Wang, X., Lao, L. J. & Song, B. Effect of peptidases on the ability of exogenous and endogenous neurokinins to produce neurokinin 1 receptor internalization in the rat spinal cord. Br. J. Pharmacol. 140, 1389–1398 (2003).
Fu, J., Oveisi, F., Gaetani, S., Lin, E. & Piomelli, D. Oleoylethanolamide, an endogenous PPAR-α agonist, lowers body weight and hyperlipidemia in obese rats. Neuropharmacology 48, 1147–1153 (2005).
Lo Verme, J. et al. The nuclear receptor peroxisome proliferator-activated receptor-α mediates the anti-inflammatory actions of palmitoylethanolamide. Mol. Pharmacol. 67, 15–19 (2005).
Levine, J. D., Gordon, N. C., Jones, R. T. & Fields, H. L. The narcotic antagonist naloxone enhances clinical pain. Nature 272, 826–827 (1978).
Petrovic, P., Kalso, E., Petersson, K. M. & Ingvar, M. Placebo and opioid analgesia — imaging a shared neuronal network. Science 295, 1737–1740 (2002).
Noble, F., Turcaud, S., Fournié-Zaluski, M. C. & Roques, B. P. Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine. Eur. J. Pharmacol. 223, 83–89 (1992).
Noble, F., Coric, P., Turcaud, S., Fournié-Zaluski, M. C. & Roques, B. P. Assessment of physical dependence after continuous perfusion into the rat jugular vein of the mixed inhibitor of enkephalin-degrading enzymes, RB 101. Eur. J. Pharmacol. 253, 283–287 (1994).
Le Guen, S. et al. RB101(S), a dual inhibitor of enkephalinases does not induce antinociceptive tolerance, or cross-tolerance with morphine: a c-Fos study at the spinal level. Eur. J. Pharmacol. 441, 141–150 (2002). This quantitative assessment of the pain-induced reduction in FOS expression at the spinal level shows that repeated administration of the DENK inhibitor RB-101 does not induce tolerance; furthermore, cross-tolerance with morphine was not observed.
Lu, B. et al. Neutral endopeptidase modulation of septic shock. J. Exp. Med. 181, 2271–2275 (1995).
Dempsey, E. C. et al. Neprilysin null mice develop exaggerated pulmonary vascular remodeling in response to chronic hypoxia. Am. J. Pathol. 174, 782–796 (2009).
Rangel, R. et al. Impaired angiogenesis in aminopeptidase N-null mice. Proc. Natl Acad. Sci. USA 104, 4588–4593 (2007).
Salazar-Lindo, E., Santisteban-Ponce, J., Chea-Woo, E. & Gutierrez, M. Racecadotril in the treatment of acute watery diarrhea in children. N. Engl. J. Med. 343, 463–467 (2000).
Tsukamoto, H. et al. Aminopeptidase N (APN)/CD13 inhibitor, ubenimex, enhances radiation sensitivity in human cervical cancer. BMC Cancer 8, 74 (2008).
Campbell, D. J. Vasopeptidase inhibition: a double-edged sword? Hypertension 41, 383–389 (2003). This study demonstrates that the effects of DENK inhibitors on various organs are crucially dependent on the phasic release of enkephalins.
Williams, F. G., Mullet, M. A. & Beitz, A. J. Basal release of Met-enkephalin and neurotensin in the ventrolateral periaqueductal gray matter of the rat: a microdialysis study of antinociceptive circuits. Brain Res. 690, 207–216 (1995).
Dauge, V., Mauborgne, A., Cesselin, F., Fournié-Zaluski, M. C. & Roques, B. P. The dual peptidase inhibitor RB101 induces a long-lasting increase in the extracellular level of Met-enkephalin-like material in the nucleus accumbens of freely moving rats. J. Neurochem. 67, 1301–1308 (1996).
Le Guen, S. et al. Pain management by a new series of dual inhibitors of enkephalin degrading enzymes: long lasting antinociceptive properties and potentiation by CCK2 antagonist or methadone. Pain 104, 139–148 (2003). This study shows that extracellular levels of DENK inhibitor-protected enkephalins (measured by microdialysis in the PAG matter) are parallel to antinociceptive responses.
Valverde, O. et al. Δ9-tetrahydrocannabinol releases and facilitates the effects of endogenous enkephalins: reduction in morphine withdrawal syndrome without change in rewarding effect. Eur. J. Neurosci. 13, 1816–1824 (2001).
Nieto, M. M., Wilson, J., Cupo, A., Roques, B. P. & Noble, F. Chronic morphine treatment modulates the extracellular levels of endogenous enkephalins in rat brain structures involved in opiate dependence: a microdialysis study. J. Neurosci. 22, 1034–1041 (2002).
Llorens-Cortes, C., Gros, C., Schwartz, J. C., Clot, A. M. & Le Bars, D. Changes in levels of the tripeptide Tyr-Gly-Gly as an index of enkephalin release in the spinal cord: effects of noxious stimuli and parenterally-active peptidase inhibitors. Peptides 10, 609–614 (1989).
Chen, H., Noble, F., Coric, P., Fournié-Zaluski, M. C. & Roques, B. P. Aminophosphinic inhibitors as transition state analogues of enkephalin-degrading enzymes: a class of central analgesics. Proc. Natl Acad. Sci. USA 95, 12028–12033 (1998). This study, along with references 52 and 53, provides a description of the first compounds that were able to bind to the catalytic sites of NEP and APN with a similar nanomolar affinity.
Jones, A. K., Watabe, H., Cunningham, V. J. & Jones, T. Cerebral decreases in opioid receptor binding in patients with central neuropathic pain measured by [11C]diprenorphine binding and PET. Eur. J. Pain 8, 479–485 (2004). This PET neuroimaging study shows that neuropathic pain induces an increase in circulating levels of enkephalins, which displace exogenous opioids bound to opioid receptors.
Boudinot, E. et al. Effects of the potent analgesic enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan on respiration. Pain 90, 7–13 (2001). This study shows that DENK inhibitors are devoid of respiratory depressant effects, and could be a possible therapeutic alternative to morphine.
Maldonado, R., Stinus, L., Gold, L. H. & Koob, G. F. Role of different brain structures in the expression of the physical morphine withdrawal syndrome. J. Pharmacol. Exp. Ther. 261, 669–677 (1992).
Minnis, J. G. et al. Ligand-induced μ opioid receptor endocytosis and recycling in enteric neurons. Neuroscience 119, 33–42 (2003). This study demonstrates that in contrast to morphine, enkephalins induce the rapid internalization and recycling of MORs to the cell surface after a second stimulation.
Di Marzo, V. & Petrosino, S. Endocannabinoids and the regulation of their levels in health and disease. Curr. Opin. Lipidol. 18, 129–140 (2007).
Cravatt, B. F. et al. Supersensitivity to anandamide and enhanced endogenous cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proc. Natl Acad. Sci. USA 98, 9371–9376 (2001). This was the first study on FAAH-knockout mice showing an enhancement of the physiological functions of endogenous cannabinoids.
Jayamanne, A. et al. Actions of the FAAH inhibitor URB597 in neuropathic and inflammatory chronic pain models. Br. J. Pharmacol. 147, 281–288 (2006).
Russo, R. et al. The fatty acid amide hydrolase inhibitor URB597 (cyclohexylcarbamic acid 3′-carbamoylbiphenyl-3-yl ester) reduces neuropathic pain after oral administration in mice. J. Pharmacol. Exp. Ther. 322, 236–242 (2007). This is a detailed presentation of the pharmacology results with oral URB597, the most studied FAAH inhibitor in neuropathic pain models.
Kinsey, S. G. et al. Blockade of endocannabinoid-degrading enzymes attenuates neuropathic pain. J. Pharmacol. Exp. Ther. 330, 902–910 (2009).
Walker, J. M., Huang, S. M., Strangman, N. M., Tsou, K. & Sanudo-Pena, M. C. Pain modulation by release of the endogenous cannabinoid anandamide. Proc. Natl Acad. Sci. USA 96, 12198–12203 (1999). This was the initial demonstration of the relationship between endogenous cannabinoids and antinociception that triggered interest in FAAH as a therapeutic target.
Fegley, D. et al. Characterization of the fatty acid amide hydrolase inhibitor cyclohexyl carbamic acid 3′-carbamoyl-biphenyl-3-yl ester (URB597): effects on anandamide and oleoylethanolamide deactivation. J. Pharmacol. Exp. Ther. 313, 352–358 (2005).
Devault, A. et al. Amino acid sequence of rabbit kidney neutral endopeptidase 24.11 (enkephalinase) deduced from a complementary DNA. EMBO J. 6, 1317–1322 (1987). This study reported the first cloning and sequencing of NEP.
Mina-Osorio, P. The moonlighting enzyme CD13: old and new functions to target. Trends Mol. Med. 14, 361–371 (2008).
Marie-Claire, C. et al. Exploration of the S(′)1 subsite of neprilysin: a joined molecular modeling and site-directed mutagenesis study. Proteins 39, 365–371 (2000).
Tiraboschi, G. et al. A three-dimensional construction of the active site (region 507–749) of human neutral endopeptidase (EC.18.104.22.168). Protein Eng. 12, 141–149 (1999).
Beaumont, A., Le Moual, H., Boileau, G., Crine, P. & Roques, B. P. Evidence that both arginine 102 and arginine 747 are involved in substrate binding to neutral endopeptidase (EC 22.214.171.124). J. Biol. Chem. 266, 214–220 (1991).
Fournié-Zaluski, M. C. & Roques, B. P. in Ectopeptidases: CD13/Aminopeptidase N and CD26/Dipeptidylpeptidase IV in Medicine and Biology (eds Langner, J. & Ansorge, S.) 51–94 (Kluwer Academic/Plenum Publishers, New York, 2002).
Fournié-Zaluski, M. C. et al. New bidentates as full inhibitors of enkephalin-degrading enzymes: synthesis and analgesic properties. J. Med. Chem. 28, 1158–1169 (1985).
Perrot, S., Kayser, V., Fournié-Zaluski, M. C., Roques, B. P. & Guilbaud, G. Antinociceptive effect of systemic PC 12, a prodrug mixed inhibitor of enkephalin-degrading enzymes, in normal and arthritic rats. Eur. J. Pharmacol. 241, 129–133 (1993).
Lee, S. H., Kayser, V. & Guilbaud, G. Antinociceptive effect of systemic kelatorphan, in mononeuropathic rats, involves different opioid receptor types. Eur. J. Pharmacol. 264, 61–67 (1994).
Fournié-Zaluski, M. C. et al. “Mixed inhibitor-prodrug” as a new approach toward systemically active inhibitors of enkephalin-degrading enzymes. J. Med. Chem. 35, 2473–2481 (1992). Along with reference 143, this study first described disulphide DENK inhibitors.
Noble, F. et al. Pain-suppressive effects on various nociceptive stimuli (thermal, chemical, electrical and inflammatory) of the first orally active enkephalin-metabolizing enzyme inhibitor RB 120. Pain 73, 383–391 (1997). This paper confirms that DENK inhibitors remain active even in morphine-tolerant animals, offering the therapeutic possibility to alternate the drugs.
Ruiz-Gayo, M., Baamonde, A., Turcaud, S., Fournié-Zaluski, M. C. & Roques, B. P. In vivo occupation of mouse brain opioid receptors by endogenous enkephalins: blockade of enkephalin degrading enzymes by RB 101 inhibits [3H]diprenorphine binding. Brain Res. 571, 306–312 (1992). The ceiling effect observed in the displacement of [3H]diprenorphine by RB-101 from opioid receptors in the mouse brain may explain why DENK inhibitors are devoid of the central side effects associated with morphine.
Bruehl, S., Burns, J. W., Chung, O. Y. & Chont, M. Pain-related effects of trait anger expression: neural substrates and the role of endogenous opioid mechanisms. Neurosci. Biobehav. Rev. 33, 475–491 (2009).
Coudore-Civiale, M. A. et al. Enhancement of the effects of a complete inhibitor of enkephalin-catabolizing enzymes, RB 101, by a cholecystokinin-B receptor antagonist in diabetic rats. Br. J. Pharmacol. 133, 179–185 (2001).
Cabanero, D. et al. The pro-nociceptive effects of remifentanil or surgical injury in mice are associated with a decrease in delta-opioid receptor mRNA levels: prevention of the nociceptive response by on-site delivery of enkephalins. Pain 141, 88–96 (2009). This paper describes another method for generating enkephalin-mediated physiological analgesia facilitated by DENK inhibitors.
Rautio, J. et al. Prodrugs: design and clinical applications. Nature Rev. Drug Discov. 7, 255–270 (2008).
Menendez, L., Juarez, L., Garcia, V., Hidalgo, A. & Baamonde, A. Involvement of nitric oxide in the inhibition of bone cancer-induced hyperalgesia through the activation of peripheral opioid receptors in mice. Neuropharmacology 53, 71–80 (2007). This paper provides a possible explanation for the synergy between DENK inhibitors and gabapentin.
Noble, F., Coric, P., Fournié-Zaluski, M. C. & Roques, B. P. Lack of physical dependence in mice after repeated systemic administration of the mixed inhibitor prodrug of enkephalin-degrading enzymes, RB101. Eur. J. Pharmacol. 223, 91–96 (1992).
Joseph, E. K., Reichling, D. B. & Levine, J. D. Shared mechanisms for opioid tolerance and a transition to chronic pain. J. Neurosci. 30, 4660–4666 (2010).
Javelot, H., Messaoudi, M., Garnier, S. & Rougeot, C. Human opiorphin is a naturally occurring antidepressant acting selectively on enkephalin-dependent δ-opioid pathways. J. Physiol. Pharmacol. 61, 355–362 (2010).
Fournié-Zaluski, M. C. et al. Development of [125I]RB104, a potent inhibitor of neutral endopeptidase 24.11, and its use in detecting nanogram quantities of the enzyme by “inhibitor gel electrophoresis”. Proc. Natl Acad. Sci. USA 89, 6388–6392 (1992).
Hutcheson, D. M. et al. Analgesic doses of the enkephalin degrading enzyme inhibitor RB 120 do not have discriminative stimulus properties. Eur. J. Pharmacol. 401, 197–204 (2000).
Song, B. & Marvizon, J. C. Peptidases prevent μ-opioid receptor internalization in dorsal horn neurons by endogenously released opioids. J. Neurosci. 23, 1847–1858 (2003). This study demonstrates that the binding of enkephalins to opioid receptors does not alter the recycling of active receptors (unlike morphine), thus providing an explanation for why no tolerance is observed with DENK inhibitors. See reference 137 as well.
Whistler, J. L., Chuang, H. H., Chu, P., Jan, L. Y. & von Zastrow, M. Functional dissociation of mu opioid receptor signaling and endocytosis: implications for the biology of opiate tolerance and addiction. Neuron 23, 737–746 (1999). This study demonstrates that — unlike morphine — enkephalins induce a recycling of active MORs at the cell surface, thus accounting partly for the lack of DENK inhibitor-evoked tolerance.
Dauge, V., Kalivas, P. W., Duffy, T. & Roques, B. P. Effect of inhibiting enkephalin catabolism in the VTA on motor activity and extracellular dopamine. Brain Res. 599, 209–214 (1992). One explanation for the lack of dependence elicited by the opioid peptides is that the reward system is more strongly stimulated by morphine than by enkephalins protected by DENK inhibitors.
Otrubova, K., Ezzili, C. & Boger, D. L. The discovery and development of inhibitors of fatty acid amide hydrolase (FAAH). Bioorg. Med. Chem. Lett. 21, 4674–4685 (2011).
Boger, D. L. et al. Trifluoromethyl ketone inhibitors of fatty acid amide hydrolase: a probe of structural and conformational features contributing to inhibition. Bioorg. Med. Chem. Lett. 9, 265–270 (1999).
Boger, D. L. et al. Exceptionally potent inhibitors of fatty acid amide hydrolase: the enzyme responsible for degradation of endogenous oleamide and anandamide. Proc. Natl Acad. Sci. USA 97, 5044–5049 (2000).
Boger, D. L. et al. Discovery of a potent, selective, and efficacious class of reversible α-ketoheterocycle inhibitors of fatty acid amide hydrolase effective as analgesics. J. Med. Chem. 48, 1849–1856 (2005).
Mileni, M. et al. Structure-guided inhibitor design for human FAAH by interspecies active site conversion. Proc. Natl Acad. Sci. USA 105, 12820–12824 (2008). This paper reported the first expression and structural analysis of a protein harbouring the sequence of human FAAH. See reference 54 as well.
Jhaveri, M. D., Richardson, D., Kendall, D. A., Barrett, D. A. & Chapman, V. Analgesic effects of fatty acid amide hydrolase inhibition in a rat model of neuropathic pain. J. Neurosci. 26, 13318–13327 (2006).
Holt, S., Comelli, F., Costa, B. & Fowler, C. J. Inhibitors of fatty acid amide hydrolase reduce carrageenan-induced hind paw inflammation in pentobarbital-treated mice: comparison with indomethacin and possible involvement of cannabinoid receptors. Br. J. Pharmacol. 146, 467–476 (2005).
Karbarz, M. J. et al. Biochemical and biological properties of 4-(3-phenyl-[1,2,4] thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide, a mechanism-based inhibitor of fatty acid amide hydrolase. Anesth. Analg. 108, 316–329 (2009).
Chang, L. et al. Inhibition of fatty acid amide hydrolase produces analgesia by multiple mechanisms. Br. J. Pharmacol. 148, 102–113 (2006).
Zhang, D. et al. Fatty acid amide hydrolase inhibitors display broad selectivity and inhibit multiple carboxylesterases as off-targets. Neuropharmacology 52, 1095–1105 (2007).
Ledent, C. et al. Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice. Science 283, 401–404 (1999).
Salio, C. et al. CB1-cannabinoid and mu-opioid receptor co-localization on postsynaptic target in the rat dorsal horn. Neuroreport 12, 3689–3692 (2001).
Welch, S. P. Interaction of the cannabinoid and opioid systems in the modulation of nociception. Int. Rev. Psychiatry 21, 143–151 (2009). This review details the painful situations in which the response to endogenous cannabinoids protected by FAAH inhibitors is due to the subsequent release of enkephalins.
da Fonseca Pacheco, D. et al. The μ-opioid receptor agonist morphine, but not agonists at δ- or κ-opioid receptors, induces peripheral antinociception mediated by cannabinoid receptors. Br. J. Pharmacol. 154, 1143–1149 (2008).
Cichewicz, D. L. Synergistic interactions between cannabinoid and opioid analgesics. Life Sci. 74, 1317–1324 (2004).
Parolaro, D. et al. Cellular mechanisms underlying the interaction between cannabinoid and opioid system. Curr. Drug Targets 11, 393–405 (2010).
Bushlin, I., Rozenfeld, R. & Devi, L. A. Cannabinoid-opioid interactions during neuropathic pain and analgesia. Curr. Opin. Pharmacol. 10, 80–86 (2010).
Ibrahim, M. M. et al. CB2 cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids. Proc. Natl Acad. Sci. USA 102, 3093–3098 (2005).
Maldonado, R. & Valverde, O. Participation of the opioid system in cannabinoid-induced antinociception and emotional-like responses. Eur. Neuropsychopharmacol. 13, 401–410 (2003).
Nieto, M. M. et al. Facilitation of enkephalins catabolism inhibitor-induced antinociception by drugs classically used in pain management. Neuropharmacology 41, 496–506 (2001). This study demonstrates a synergistic analgesic effect of the combination of very low (infra-active) doses of the DENK inhibitor RB-101 and morphine. This opens up the possibility of using low-dose DENK inhibitor–morphine combinations to reduce the unwanted effects associated with morphine.
Finn, A. K. & Whistler, J. L. Endocytosis of the mu opioid receptor reduces tolerance and a cellular hallmark of opiate withdrawal. Neuron 32, 829–839 (2001). This paper shows that, unlike enkephalin, morphine does not induce MOR endocytosis — a process that may be partly a cause of tolerance to morphine.
Gendron, L. et al. Morphine and pain-related stimuli enhance cell surface availability of somatic δ-opioid receptors in rat dorsal root ganglia. J. Neurosci. 26, 953–962 (2006).
Gomes, I. et al. Heterodimerization of μ and δ opioid receptors: a role in opiate synergy. J. Neurosci. 20, RC110 (2000).
Wu, G. et al. A-317491, a selective P2X3/P2X2/3 receptor antagonist, reverses inflammatory mechanical hyperalgesia through action at peripheral receptors in rats. Eur. J. Pharmacol. 504, 45–53 (2004).
Lauretti, G. R., Perez, M. V., Reis, M. P. & Pereira, N. L. Double-blind evaluation of transdermal nitroglycerine as adjuvant to oral morphine for cancer pain management. J. Clin. Anesth. 14, 83–86 (2002).
Oliveira, M. C., Pelegrini-da-Silva, A., Tambeli, C. H. & Parada, C. A. Peripheral mechanisms underlying the essential role of P2X3,2/3 receptors in the development of inflammatory hyperalgesia. Pain 141, 127–134 (2009).
Valverde, O., Maldonado, R., Fournié-Zaluski, M. C. & Roques, B. P. Cholecystokinin B antagonists strongly potentiate antinociception mediated by endogenous enkephalins. J. Pharmacol. Exp. Ther. 270, 77–88 (1994).
Xu, X. J. et al. CI 988, an antagonist of the cholecystokinin-B receptor, potentiates endogenous opioid-mediated antinociception at spinal level. Neuropeptides 31, 287–291 (1997).
Noble, F. & Roques, B. P. The role of CCK2 receptors in the homeostasis of the opioid system. Drugs Today (Barc.) 39, 897–908 (2003). This is a review on the counteractive enkephalinergic and cholecystokinergic systems, and their roles in pain and depression.
Buritova, J., Le Guen, S., Fournié-Zaluski, M. C., Roques, B. P. & Besson, J. M. Antinociceptive effects of RB101(S), a complete inhibitor of enkephalin-catabolizing enzymes, are enhanced by (+)-HA966, a functional NMDA receptor antagonist: a c-Fos study in the rat spinal cord. Eur. J. Pain 7, 241–249 (2003).
Naidu, P. S., Booker, L., Cravatt, B. F. & Lichtman, A. H. Synergy between enzyme inhibitors of fatty acid amide hydrolase and cyclooxygenase in visceral nociception. J. Pharmacol. Exp. Ther. 329, 48–56 (2009).
Elphick, M. R. & Egertova, M. The neurobiology and evolution of cannabinoid signalling. Philos. Trans. R. Soc. Lond. B Biol. Sci. 356, 381–408 (2001).
Naidu, P. S., Kinsey, S. G., Guo, T. L., Cravatt, B. F. & Lichtman, A. H. Regulation of inflammatory pain by inhibition of fatty acid amide hydrolase. J. Pharmacol. Exp. Ther. 334, 182–190 (2010).
Corbett, A. D., Henderson, G., McKnight, A. T. & Paterson, S. J. 75 years of opioid research: the exciting but vain quest for the holy grail. Br. J. Pharmacol. 147 (Suppl. 1), S153–S162 (2006).
Tegeder, I. et al. Peripheral opioid analgesia in experimental human pain models. Brain 126, 1092–1102 (2003).
Ballet, S. et al. Expression and G-protein coupling of μ-opioid receptors in the spinal cord and dorsal root ganglia of polyarthritic rats. Neuropeptides 37, 211–219 (2003).
Hassan, A. H., Ableitner, A., Stein, C. & Herz, A. Inflammation of the rat paw enhances axonal transport of opioid receptors in the sciatic nerve and increases their density in the inflamed tissue. Neuroscience 55, 185–195 (1993).
Labuz, D. et al. Immune cell-derived opioids protect against neuropathic pain in mice. J. Clin. Invest. 119, 278–286 (2009). This important study unequivocally demonstrates that immune cells secrete endogenous opioids that accumulate in injured nerves in animal models of neuropathic pain; this supports the mechanism of peripheral neuropathic pain alleviation by DENK inhibitors.
Truong, W., Cheng, C., Xu, Q. G., Li, X. Q. & Zochodne, D. W. Mu opioid receptors and analgesia at the site of a peripheral nerve injury. Ann. Neurol. 53, 366–375 (2003). This is an interesting observation that opioid receptors are overexpressed on both ends of an injured nerve, thus explaining the antinociceptive effects of DENK inhibitors in neuropathic pain.
Zollner, C. et al. Chronic morphine use does not induce peripheral tolerance in a rat model of inflammatory pain. J. Clin. Invest. 118, 1065–1073 (2008). This study shows that there is a continuous availability of endogenous opioids in inflamed tissues, which increases the recycling of MOR and preserves its signalling in sensory neurons, thereby counteracting the development of peripheral opioid tolerance.
Labuz, D. et al. Peripheral antinociceptive effects of exogenous and immune cell-derived endomorphins in prolonged inflammatory pain. J. Neurosci. 26, 4350–4358 (2006). This is the most complete and convincing study on inflammatory pain reduction by endomorphins that are released from immune cells and act on peripheral opioid receptors.
Mousa, S. A., Bopaiah, C. P., Richter, J. F., Yamdeu, R. S. & Schafer, M. Inhibition of inflammatory pain by CRF at peripheral, spinal and supraspinal sites: involvement of areas coexpressing CRF receptors and opioid peptides. Neuropsychopharmacology 32, 2530–2542 (2007).
Schafer, M., Mousa, S. A. & Stein, C. Corticotropin-releasing factor in antinociception and inflammation. Eur. J. Pharmacol. 323, 1–10 (1997).
Rittner, H. L. et al. Opioid peptide-expressing leukocytes: identification, recruitment, and simultaneously increasing inhibition of inflammatory pain. Anesthesiology 95, 500–508 (2001).
Olerud, J. E. et al. Neutral endopeptidase expression and distribution in human skin and wounds. J. Invest. Dermatol. 112, 873–881 (1999).
Zurita, A., Martijena, I., Cuadra, G., Brandao, M. L. & Molina, V. Early exposure to chronic variable stress facilitates the occurrence of anhedonia and enhanced emotional reactions to novel stressors: reversal by naltrexone pretreatment. Behav. Brain Res. 117, 163–171 (2000).
Jutkiewicz, E. M. & Roques, B. P. Endogenous opioids as physiological antidepressants: complementary role of delta receptors and dopamine. Neuropsychopharmacology 37, 303–304 (2012).
Saitoh, A. et al. Potential anxiolytic and antidepressant-like activities of SNC80, a selective δ-opioid agonist, in behavioral models in rodents. J. Pharmacol. Sci. 95, 374–380 (2004).
Baamonde, A. et al. Antidepressant-type effects of endogenous enkephalins protected by systemic RB 101 are mediated by opioid δ and dopamine D1 receptor stimulation. Eur. J. Pharmacol. 216, 157–166 (1992).
Tejedor-Real, P. et al. Involvement of δ-opioid receptors in the effects induced by endogenous enkephalins on learned helplessness model. Eur. J. Pharmacol. 354, 1–7 (1998).
Cordonnier, L., Sanchez, M., Roques, B. P. & Noble, F. Facilitation of enkephalins-induced delta-opioid behavioral responses by chronic amisulpride treatment. Neuroscience 135, 1–10 (2005).
Jutkiewicz, E. M. et al. Behavioral and neurobiological effects of the enkephalinase inhibitor RB101 relative to its antidepressant effects. Eur. J. Pharmacol. 531, 151–159 (2006).
Roques, B. P., Dauge, V., Gacel, G. & Fournié-Zaluski, M. C. in Biological Psychiatry, Developments in Psychiatry (eds Shagass, C. et al.) 287–289 (Elsevier, New York, 1985).
Calenco-Choukroun, G., Dauge, V., Gacel, G., Feger, J. & Roques, B. P. Opioid δ agonists and endogenous enkephalins induce different emotional reactivity than mu agonists after injection in the rat ventral tegmental area. Psychopharmacology (Berl.) 103, 493–502 (1991). This study provided an early demonstration — using kelatorphan — that DORs are involved in mood regulation via activation of the dopaminergic mesocorticolimbic pathways, whereas MORs govern fear, anxiety and drug dependence.
Forbes, E. E. et al. Altered striatal activation predicting real-world positive affect in adolescent major depressive disorder. Am. J. Psychiatry 166, 64–73 (2009).
Maldonado, R. et al. Absence of opiate rewarding effects in mice lacking dopamine D2 receptors. Nature 388, 586–589 (1997).
Berrocoso, E., Sanchez-Blazquez, P., Garzon, J. & Mico, J. A. Opiates as antidepressants. Curr. Pharm. Des. 15, 1612–1622 (2009). References 216–218 are important studies underlying the role of endogenous opioids and dopamine in mood control.
Jardinaud, F. et al. CB1 receptor knockout mice show similar behavioral modifications to wild-type mice when enkephalin catabolism is inhibited. Brain Res. 1063, 77–83 (2005).
McNally, G. P. Facilitation of fear extinction by midbrain periaqueductal gray infusions of RB101(S), an inhibitor of enkephalin-degrading enzymes. Behav. Neurosci. 119, 1672–1677 (2005). This was the first demonstration that DENK inhibitor-induced neuromodulation of endogenous opioids increases resistance to fear, suggesting a possible use for DENK inhibitors in treating anxiety disorders.
Ragnauth, A. et al. Female preproenkephalin-knockout mice display altered emotional responses. Proc. Natl Acad. Sci. USA 98, 1958–1963 (2001).
Kung, J. C., Chen, T. C., Shyu, B. C., Hsiao, S. & Huang, A. C. Anxiety- and depressive-like responses and c-fos activity in preproenkephalin knockout mice: oversensitivity hypothesis of enkephalin deficit-induced posttraumatic stress disorder. J. Biomed. Sci. 17, 29 (2010).
Stander, S., Schmelz, M., Metze, D., Luger, T. & Rukwied, R. Distribution of cannabinoid receptor 1 (CB1) and 2 (CB2) on sensory nerve fibers and adnexal structures in human skin. J. Dermatol. Sci. 38, 177–188 (2005).
Tsou, K. et al. Fatty acid amide hydrolase is located preferentially in large neurons in the rat central nervous system as revealed by immunohistochemistry. Neurosci. Lett. 254, 137–140 (1998).
Simonini, G. et al. Neprilysin levels in plasma and synovial fluid of juvenile idiopathic arthritis patients. Rheumatol. Int. 25, 336–340 (2005).
Biro, T., Toth, B. I., Hasko, G., Paus, R. & Pacher, P. The endocannabinoid system of the skin in health and disease: novel perspectives and therapeutic opportunities. Trends Pharmacol. Sci. 30, 411–420 (2009). This is a review on the strategies for targeting peripheral cannabinoid receptors to treat inflammatory pain. See reference 230 as well.
Kioussi, C. & Matsas, R. Endopeptidase-24.11, a cell-surface peptidase of central nervous system neurons, is expressed by Schwann cells in the pig peripheral nervous system. J. Neurochem. 57, 431–440 (1991).
Hohmann, A. G. & Herkenham, M. Cannabinoid receptors undergo axonal flow in sensory nerves. Neuroscience 92, 1171–1175 (1999).
Croxford, J. L. & Yamamura, T. Cannabinoids and the immune system: potential for the treatment of inflammatory diseases? J. Neuroimmunol. 166, 3–18 (2005).
Calignano, A., La Rana, G., Giuffrida, A. & Piomelli, D. Control of pain initiation by endogenous cannabinoids. Nature 394, 277–281 (1998).
Besse, D., Lombard, M. C., Perrot, S. & Besson, J. M. Regulation of opioid binding sites in the superficial dorsal horn of the rat spinal cord following loose ligation of the sciatic nerve: comparison with sciatic nerve section and lumbar dorsal rhizotomy. Neuroscience 50, 921–933 (1992).
Dickenson, A. H., Sullivan, A. F., Knox, R., Zajac, J. M. & Roques, B. P. Opioid receptor subtypes in the rat spinal cord: electrophysiological studies with μ- and δ-opioid receptor agonists in the control of nociception. Brain Res. 413, 36–44 (1987).
Le Guen, S. et al. The effects of RB101, a mixed inhibitor of enkephalin-catabolizing enzymes, on carrageenin-induced spinal c-Fos expression are completely blocked by β-funaltrexamine, a selective μ-opioid receptor antagonist. Brain Res. 834, 200–206 (1999).
Richardson, J. D., Aanonsen, L. & Hargreaves, K. M. Hypoactivity of the spinal cannabinoid system results in NMDA-dependent hyperalgesia. J. Neurosci. 18, 451–457 (1998).
Matthews, B. M. Structural basis of the action of thermolysin and related zinc peptidases. Acc. Chem. Res. 21, 333–340 (1988).
Jayaram, A., Singh, P., Noreuil, T., Fournié-Zaluski, M. C. & Carp, H. M. RB 101, a purported pro drug inhibitor of enkephalin metabolism, is antinociceptive in pregnant mice. Anesth. Analg. 84, 355–358 (1997).
Benoist, J. M. et al. Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes. Eur. J. Pharmacol. 445, 201–210 (2002).
Lichtman, A. H. et al. Reversible inhibitors of fatty acid amide hydrolase that promote analgesia: evidence for an unprecedented combination of potency and selectivity. J. Pharmacol. Exp. Ther. 311, 441–448 (2004).
Schuelert, N. et al. Local application of the endocannabinoid hydrolysis inhibitor URB597 reduces nociception in spontaneous and chemically induced models of osteoarthritis. Pain 152, 975–981 (2011).
The authors wish to thank A. Bouju for her help in preparing the manuscript.
B.P.R is Chief Scientific Officer at Pharmaleads SAS.
M.C.F.Z. is Director of the Chemistry Department at Pharmaleads SAS.
M.W. is Director of Corporate Development at Pharmaleads SAS.
A disorder of unknown aetiology that is characterized by widespread pain, abnormal pain processing, sleep disturbance, fatigue and often psychological distress.
- Neuropathic pain
Pain caused by a lesion or a disease of the somatosensory nervous system.
- Chronic constrictive injury model
An animal model of mononeuropathic pain in rodents resulting from ligation of the sciatic nerve, which induces a painful syndrome analogous to that observed in humans. Chronic constrictive injury models may differ according to the location and the tightness of the ligation along the sciatic nerve.
- Mononeuropathic rats
Rats that mimic the symptoms induced by nerve injury in humans. Symptoms are restricted to the area innervated by the injured nerve.
- Isobolographic plot
A method of determining drug synergy. The theoretical additive ED50 value (the half-maximal effective dose) is estimated from the dose–response curves of each drug administered individually. This theoretical ED50 value is compared with the experimental ED50 value. If a statistically significant difference is observed, synergy is present.
About this article
Cite this article
Roques, B., Fournié-Zaluski, MC. & Wurm, M. Inhibiting the breakdown of endogenous opioids and cannabinoids to alleviate pain. Nat Rev Drug Discov 11, 292–310 (2012). https://doi.org/10.1038/nrd3673
Quantitative analysis and expression of salivary opiorphin in painful oral soft-tissue conditions: A descriptive study
Journal of Global Oral Health (2020)
Pharmacology & Therapeutics (2020)
Behavioural Pharmacology (2020)
Positive Allosteric Modulation of CB1 Cannabinoid Receptor Signaling Enhances Morphine Antinociception and Attenuates Morphine Tolerance Without Enhancing Morphine- Induced Dependence or Reward
Frontiers in Molecular Neuroscience (2020)