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Narrative Review

Cannabinoids, the endocannabinoid system, and pain: a review of preclinical studies

Finn, David P.a,*; Haroutounian, Simonb; Hohmann, Andrea G.c; Krane, Elliotd; Soliman, Nadiae; Rice, Andrew S.C.e

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doi: 10.1097/j.pain.0000000000002268
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Abstract

1. Introduction

Cannabis, cannabis extracts or oils, individual cannabinoid substances, and modulators of the endogenous cannabinoid (endocannabinoid) system have all been suggested as therapeutic agents for pain management.75,94,193 The primary drivers for interest in these agents include (1) major scientific advances in our understanding of the biology of the endocannabinoid system and pharmacology of cannabinoids; (2) development and regulatory approval of cannabis-based (or cannabis-derived) medicines (eg, Nabiximols/Sativex and Epidiolex); and (3) regulatory changes that permit use of cannabis for medical purposes, including pain management, following advocacy by patients and public support. However, the latter development has proven controversial amongst scientists, patients, and healthcare professionals, with uncertainty over whether the current evidence base for efficacy and safety justifies change in legislation of cannabis for medical use, including pain management. In 2018, the International Association for Study of Pain (IASP) established a Presidential Task Force on Cannabis and Cannabinoids Analgesia. The present narrative review represents an output from work package 1 of the IASP task force, which was focused on basic science, including definition of terminology, overview of the endogenous cannabinoid (endocannabinoid) system, compound classification, pharmacology, and assessment of pain-related efficacy and additional pain-relevant effects (adverse and beneficial) in preclinical laboratory animal studies.

The aims of this review are:

  • (1) To provide commentary on the efficacy and side effects of cannabinoids and endocannabinoid system modulators as they pertain to animal models of pathological pain (the term “animal model of pain” is not a universally agreed descriptor, but given that it is common usage, we will use it herein as shorthand. It is also worth noting that there is a distinction between “model,” which reflects the underlying disease or injury, and the pain-associated outcome measures used in evaluating such models), informed by our companion systematic review and meta-analysis of preclinical studies in this area.215
  • (2) To discuss the value of studying cannabinoids and endocannabinoid system modulators in preclinical/animal models of pain, as well as discrepancies between preclinical and clinical studies in this area, and
  • (3) Provide suggestions for future research directions.

In service of the above objectives, we:

  • (1) Clearly define terminology used in this field.
  • (2) Provide an overview of the endocannabinoid system.
  • (3) Review the classification, chemistry, and pharmacology of cannabinoids, cannabis-based medicines (CBM), and endocannabinoid system modulators, including structure–activity relationships and pharmacokinetics in rodents and humans.

2. Terminology and definitions

One factor that has hampered public debate (and sometimes scientific/medical debate) on the topic of cannabis and cannabinoids for pain management is the inappropriate, inconsistent, or unclear use of terminology. For example, the terms “cannabis,” “cannabinoids,” and “cannabis-based medicines (CBM)” are often used interchangeably or conflated, both within public discourse and the media, and within the scientific literature (eg, pooling of data relating to cannabis and individual cannabinoids in meta-analyses and systematic reviews of clinical efficacy). Cannabis refers to the whole plant, or to its parts, and must be clearly distinguished from cannabinoid ligands (cannabinoids) that are either plant-derived natural (phytocannabinoids), synthetic, or semisynthetic, but always chemically defined, single-entity compounds usually having affinity for and activity at cannabinoid receptors. Thus, cannabis, single-entity cannabinoid compounds, and modulators of the endocannabinoid system should not be used synonymously or conflated within the same preclinical or clinical investigation for either efficacy or side effects. Furthermore, when considering or debating the merits, or otherwise, of cannabis, due regard must be given to the fact that cannabis is highly heterogeneous with many different chemical constituents and that a multitude of different strains of the plant exist, all containing different amounts of phytocannabinoids,12,103 most particularly Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). Tetrahydrocannabinol, first isolated in the 1960s, is the primary psychoactive constituent of Cannabis sativa.83,162 Tetrahydrocannabinol has psychoactive properties, and its pharmacological effects are attributable to agonist activity at both cannabinoid type 1 (CB1) and type 2 (CB2) receptors.44,61,148,234 Cannabidiol, by contrast, does not have appreciable agonist activity at CB1 receptors (but may be a negative allosteric modulator of CB1) and lacks the psychoactivity profile of THC.22,68,222 In addition to phytocannabinoids, the cannabis plant also contains a large number of terpenes, flavonoids, and other compounds, which, themselves, may be pharmacologically active but remain understudied.

Cannabis-based medicines are registered medicinal cannabis extracts, approved by regulatory authorities, with well-defined, standardized THC/CBD content, and with no, or only trace levels of, terpenes, flavonoids, and other compounds. Examples of CBMs are nabiximols (Sativex), approved in some countries for adjunctive treatment of spasticity (and in some jurisdictions neuropathic pain) in multiple sclerosis, and CBD extract (Epidiolex), indicated for treatment of childhood epilepsy, which has only minor or trace levels of other phytocannabinoids. These medicinal products with regulatory approval should be distinguished from the so-called cannabis or CBD oils or extracts, which are numerous, typically sold in health food stores, pharmacies, cannabis dispensaries, or over the internet, but often have uncertain and/or unverified THC/CBD content. Synthetic THC (eg, dronabinol) and the synthetic THC analogue nabilone are also approved for indications such as anorexia and weight loss in AIDS, and chemotherapy-induced nausea and vomiting, and available on special prescription in many countries. Table 1 provides a glossary of terminology and definitions.

Table 1 - Terminology and Definitions (Adapted from Soliman et al., 2019, after modification from Hauser et al., 2018).
Term Definition Examples/typical products
(Herbal) cannabis The whole plant or parts or material from the plant (eg, flowers, buds, resin, leaves) Cannabis sativa, hashish
Medical or medicinal cannabis The term “medical/medicinal cannabis” (or “medical/medicinal marijuana”) is used for cannabis plants, plant material, or full plant extracts used for medical purposes. Bedrocan, Bedrobinol, tilray 10THC/10CBD
Cannabis-based (or cannabis-derived) medicines Medicinal cannabis extracts or products with regulatory approval for marketing as a therapeutic with defined and standardized THC and/or CBD content. Nabiximols (Sativex), dronabinol (Marinol), Epidiolex
Cannabinoids Cannabinoids are biologically active constituents of cannabis, or synthetic compounds, usually having affinity for and activity at cannabinoid receptors. THC, CBD, CP55,940, WIN55,212‐2, HU210, nabilone
Phytocannabinoid A cannabinoid found in cannabis plants or purified/extracted from plant material THC, CBD
Endocannabinoid An endogenous ligand found in the body of humans and other animals and which has affinity for, and activity at, cannabinoid receptors Anandamide, 2-AG
Modulators that decrease endocannabinoid system activity Directly block cannabinoid receptors or reduce signalling indirectly via impeding action of endogenous ligand through actions at a distinct site Cannabinoid receptor antagonists (rimonabant [SR141716A], AM251, SR144528, AM630), negative allosteric modulators (PSNCBAM-1), DAGL inhibitors (RHC80267)
Modulators that increase or enhance endocannabinoid system activity In addition to individual phytocannabinoids, cannabis-derived or cannabis-based medicines, and cannabis extracts, other pharmacological approaches under development for manipulation of the endocannabinoid system include selective synthetic cannabinoid receptor agonists, inhibitors of the catabolism (eg, fatty acid amide hydrolase [FAAH] inhibitors), transport (eg, fatty acid-binding protein [FABP] inhibitors) or reuptake of endocannabinoids, or positive allosteric modulators of cannabinoid receptor signalling. FAAH inhibitors (PF-04457845, URB597, URB937), anandamide transport inhibitors (AM404, VDM11), MGL inhibitors (URB602, JZL184, MJN110), positive allosteric modulators of the CB1 receptor (ZCZ011, GAT211)
CBD, cannabidiol; CB1, cannabinoid type 1; DAGL, diacylglycerol lipase; FABP, fatty acid-binding protein; THC, Δ9-tetrahydrocannabinol; 2‐AG, 2‐arachidonoyl glycerol; MGL, monoacylglycerol lipase.

3. The endocannabinoid system

The discovery of THC in the 1960s as the psychoactive constituent of Cannabis inspired research into its pharmacology and mechanism of action. However, it was not until the late 1980s/early 1990s that the cannabinoid receptors were discovered (Fig. 1). The first cannabinoid receptor to be discovered, cloned, and characterised was the CB1 receptor, initially in rat brain57,159 and subsequently localized in human brain.88 In 1993, a second cannabinoid receptor, CB2, was cloned and characterized in a human promyelocytic leukaemic cell line and in rat spleen.167 Both CB1 and CB2 belong to the superfamily of seven-transmembrane domain, G-protein-coupled receptors (Gi/o coupled). The existence of cannabinoid receptors, highly conserved across species, implied the existence of endogenous cannabis-like molecules (endocannabinoids) that bind to and modulate cannabinoid receptors.

F1
Figure 1.:
A historical timeline of key milestones in cannabis and cannabinoid research.

Research efforts sought to identify the endogenous ligands that bind to and modulate mammalian cannabinoid receptors. Two endocannabinoids, N-arachidonoylethanolamide (anandamide [AEA])58 and 2-arachidonoyl glycerol (2-AG),161,220 were discovered. The endocannabinoid system is an important physiological system, composed of CB1 and CB2 receptors, their endogenous ligands, AEA and 2-AG, and the enzymes responsible for the synthesis and degradation of the endocannabinoids. Anandamide and 2-AG are the best characterised endocannabinoids. However, there are several other endogenous ligands with affinity and activity at cannabinoid receptors, including 2-AG ether (noladin ether), virodhamine, N-arachidonoyl dopamine, and others.10,59,63,191,192

The CB1 receptor is the most highly expressed G-protein-coupled receptor subtype in the central nervous system (CNS).88,104,191 Within the brain, CB1 is found in high density in the basal ganglia, as well as in key components of the descending pain pathway and the stress/fear/anxiety circuitry. The CB1 receptor is localized to most other tissues and organs of the body. Of relevance to pain, in addition to their supraspinal localisation, CB1 receptors are expressed in the dorsal horn of the spinal cord, synthesized in dorsal root ganglion cells, and transported in peripheral nerves.21,74,108–111,178 Immunohistochemical studies have localized CB1 to dorsal root ganglia and identified CB1 receptors on primary afferent neurons.21,110,111 Cannabinoid type 2 receptors are mainly expressed in the periphery, with particularly high density on cells and tissues of the immune system.11,167 Although localisation of CB2 to otherwise naive CNS remains controversial, CB2 can be induced in the CNS in response to injury or pathophysiological states (for review, see Refs. 99, 134, 205, and 240). Being Gi/o-protein-coupled receptors, CB1 and CB2 are negatively coupled to adenylyl cyclase,116–118 and positively coupled to mitogen-activated protein kinase.18 Within neurons, CB1 activation results in inhibition of N- and P/Q-type voltage-activated Ca2+ channels, and induction of inwardly rectifying K+ currents, with consequent inhibition of neurotransmitter release.55 Cannabinoids, including endocannabinoids, phytocannabinoids, and synthetic cannabinoids, may potentially also act at other non-CB1/non-CB2 receptors, including the transient receptor potential cation channel subfamily V member 1 (TRPV1; also known as the capsaicin or vanilloid receptor VR1), peroxisome proliferator-activated receptors, and G-protein-coupled receptors such as GPR55 and GPR119.6,23,179

Mechanisms underlying endocannabinoid biosynthesis, signaling, and degradation are quite well understood. Biosynthesis of AEA involves its formation from the precursor N-arachidonoylphosphatidylethanolamine (NAPE), catalysed by the hydrolytic activity of the phospholipase D enzyme known as N-acylphosphatidylethanolamine-hydrolyzing phospholipase D (for review, see Refs. 13, 30, 31, and 62). 2-arachidonoyl glycerol is synthesized by conversion of 1,2-diacylglycerol to 2-AG by diacylglycerol lipases (for review, see Refs. 59, 135, 219, and 227). Anandamide is primarily degraded to arachidonic acid and ethanolamine by the enzyme fatty acid amide hydrolase (FAAH), located in the endoplasmic reticulum of postsynaptic neuron46,59 (for review, see Refs. 86 and 182). Fatty acid amide hydrolase also catabolizes other N-acylethanolamines including N-palmitoylethanolamide (PEA) and N-oleoylethanolamide, which themselves do not have appreciable activity at CB1 or CB2 receptors but may elevate levels of AEA through substrate competition at FAAH.133 2-arachidonoyl glycerol is primarily metabolized to arachidonic acid and glycerol by the presynaptic enzyme monoacylglycerol lipase (MGL),65,227 with other enzymes including FAAH, ABHD6, and ABHD12 accounting for a modest (ie, <10%) degree of 2-AG catabolism.15,89 Within the nervous system, newly synthesized endocannabinoids leave the postsynaptic neuron to exert their effects on CB1 receptors expressed on presynaptic nerve terminals in a signaling process known as retrograde neurotransmission. Further work is required to better understand the mechanisms by which endocannabinoids are transported within cells and across cell membranes.

As a lipid-signalling system whose components are expressed widely across the body, the endocannabinoid system plays a key role in the regulation of a wide array of physiological processes including metabolism, mood, motor function, appetite, cardiovascular control, stress response, gastrointestinal tract function, developmental biology, cell fate, immune and inflammatory response, endocrine function, neurotransmission, and pain (for review, see Refs. 60, 183, and 184). In the context of nociception and pain, key components of the endocannabinoid system are expressed throughout nociceptive pathways (Fig. 2): in the periphery on primary afferent neurons, in the dorsal horn of the spinal cord, and in multiple supraspinal regions of the brain associated with pain perception and modulation (for review, see Refs. 96, 99, 196, 207, 217, and 240). As a result, targeting the endocannabinoid system via enhancement of the levels of endogenous cannabinoids (eg, with FAAH or MGL inhibitors) or exogenous cannabinoid ligands (eg, CB1 or CB2 receptor agonists) can reduce nociceptive transmission at all 3 of these neuroanatomical levels. Glial cells, which express components of the endocannabinoid system, represent another substrate through which cannabinoids or endocannabinoid system modulators may regulate pain through neuroimmune interactions29,157 (for review, see Refs. 233, 238, and 242). Preclinical research indicates that endocannabinoids are synthesized on-demand in postsynaptic neurons in response to stress or pain and produce short-term antinociceptive effects via presynaptic inhibitory CB1 receptors.92,113,144,181,218,239 Endocannabinoids are implicated in control of pain initiation32,38 and play an important role in the resolution of tonic pain and in stress-induced and fear-conditioned analgesia in rodents.27,28,77,92,113 In animal models of pathological pain, the endocannabinoid system exhibits adaptive changes or plasticity (eg, altered cannabinoid receptor expression/functionality and endocannabinoid levels) depending on the model and anatomical site under investigation.100,200,206,240,241 These findings support the contention that the endocannabinoid system may represent a viable therapeutic target for chronic pain. This view is supported by numerous pharmacological studies demonstrating efficacy of cannabinoids or modulators of the endocannabinoid system in animal models of pathological or injury-related pain. These latter studies have been reviewed and analysed in our systematic review and meta-analysis of the preclinical literature published in this special issue of PAIN.215 Herein, we include a summary of the key findings of this systematic review and extended commentary on some elements not discussed in detail within the systematic review itself.

F2
Figure 2.:
Distribution of the cannabinoid receptors and enzymes associated with endocannabinoid synthesis and degradation in pain pathways. The endocannabinoid system is widely distributed throughout regions associated with pain processing and modulation in the brain, spinal cord, and periphery, most particularly the CB1 receptor and the enzymes responsible for endocannabinoid synthesis (NAPE-PLD and DAGL) and degradation (FAAHand MGL). CB2 receptors are less abundant in the brain and are primarily located on microglia; however, studies have shown expression of CB2 on neurons in the VTA, and several discrete nuclei of the brainstem. CB1 is also expressed on microglia at a lower level than CB2, and effects of CB1 on microglia may be mediated by neuronal CB1. Therefore, this was not depicted above. In the DRG and periphery, both CB1 and CB2 receptors can be found on neurons (and glia in the dorsal horn), as well as the endocannabinoid enzymes. PFC, prefrontal cortex; VTA, ventral tegmental area; PAG, periaqueductal gray; RVM, rostral ventromedial medulla; PBN, parabrachial nucleus; DMNX, dorsal motor nucleus of the vagus nerve; DRG, dorsal root ganglion; CB, cannabinoid receptor; NAPE-PLD, N-acylphosphatidylethanolamine-hydrolyzing phospholipase D; DAGL, diacylglycerol lipase; FAAH, fatty acid amide hydrolase; MGL, monoacylglycerol lipase.

4. Classification of cannabinoids and endocannabinoid system modulators of relevance to pain

Common pharmacological tools used to manipulate the endocannabinoid system are summarized in Table 2.

Table 2 - Cannabinoid ligands/preparations and pharmacological tools.
Natural cannabinoid ligands/Synthetic analogues CB1-selective agonists
*Δ9-THC (dronabinol)  ACEA
*CBD  Met-F-AEA
 AZ11713908 (peripherally restricted with CB2 inverse agonist properties)
Mixed CB1/CB2 agonists
*Cannabis  BAY59-3074
*eCBD  CP55,940
*Nabilone (Δ9-THC Analogue) *CT-3 (ajulemic acid)
*Nabiximols (oral-mucosal spray, Δ9-THC:CBD, 2.7:2.5 mg)  HU-210
Endocannabinoids  WIN55,212-2
 AEA CB2-selective agonists
 2-AG  AM1241
Endocannabinoid modulators  AM1714
 CB1-Positive Allosteric Modulators  AM1710
  GAT211  A-796260
  GAT229  A-836339
  ZCZ011 *GW842166X
 Uptake Inhibitors *HU308
  AM404  JWH015
  LY2318912  JWH133
  VDM11  LY2828360
  OMDM132  MDA7
  UCM-707  MDA19
 FAAH inhibitors CB1 antagonists
  OL135  AM251
   *PF-00457845  AM281
  URB597  AM6545 (peripherally restricted)
  URB937 (peripherally restricted)  SR141716
 MGL inhibitors CB2 antagonists
  JZL184  AM630
  URB602 (local only)  SR144528
  MJN110 Fatty acids that do not bind CBRs
 Dual FAAH-MGL inhibitors  NaGly
  JZL195  PEA
  SA57  L-29
 FABP5 inhibitors
  SBF126
*Compounds used or tested clinically; Adapted and updated from Hohmann and Rice (2013) Textbook of Pain sixth Edition and Rahn and Hohmann (2009) Neurotherapeutics 6:713-3.
AEA, anandamide; CBD, cannabidiol; CB1, cannabinoid type 1; CB2, cannabinoid type 2; eCBD, high CBD cannabis; Δ9-THC, Δ9-tetrahydrocannabinol; FAAH, fatty acid amide hydrolase; FABP, fatty acid-binding protein; FLAT, FAAH-like anandamide transporter; MGL, monoacylglycerol lipase; N-arachidonoyl glycine, NaGly; PEA, palmitoylethanolamide 2-AG, 2-arachydonoylglycerol.

5. Antinociceptive efficacy of cannabinoids in animal models of pathological or injury-related persistent pain

The antinociceptive efficacy of cannabinoids and endocannabinoid system modulators has been reviewed extensively.99,196,206,217,240,241 Thus, herein, we instead provide a concise commentary on the efficacy of cannabinoids and endocannabinoid system modulators in animal models of pathological or injury-related persistent pain that is uniquely informed by our companion systematic review and meta-analysis of preclinical studies in this area.215 We summarise the key findings and elaborate further on some of their implications for the field.

Our companion systematic review of laboratory animal studies that used models/conditions associated with persistent pain and reported a pain-relevant outcome measure identified 473 published reports of which 374 reported data that could be included in a meta-analysis.215 Data from 6479 rats and 6876 mice, respectively, were included, reflecting 864 and 677 experimental comparisons, respectively. This is a very large data set by preclinical standards. No studies in other species were found. Ninety-nine studies (∼20%) were excluded from the meta-analysis because the methods and/or results were not reported in sufficient detail to permit extractable data to be included in a meta-analysis.

Overall, the data support the hypothesis of cannabinoid-mediated analgesia. The overall effect size (Hedges' G standardised mean difference) was 1.32 [Q = 4101.26, df 1543, P < 0.0001, I2 = 61.58%] (Fig. 3). The models used reflect a range of conventional and diverse inflammatory and neuropathy paradigms, particularly surgically induced nerve injury for the latter. With regard to outcome measures, in common with other preclinical behavioral pain studies, limb withdrawal measures evoked by sensory stimuli were by far the most frequently reported outcome measure, with very few reports of complex behavioral assessments. Lack of consensus on predictive validity of such measures limits the degree to which the presence of pain, and thus pain relief from the intervention, can be inferred.190,202 This contrasts with the clinical trial literature where patient-reported pain intensity is the predominant metric. The formalin test also features strongly in the data; this model of tonic inflammatory pain entails measuring several spontaneous or nonevoked nociceptive behaviors, for example, licking, lifting, flinching, rearing, and guarding, to generate a “combined or composite pain score,” which thereby affords a greater degree of confidence regarding the impact of pain on the animal's behavior.1,69,224

F3
Figure 3.:
The caterpillar plot of 1544 nested comparisons extracted from 374 studies included in the meta-analysis. A Hedges' G standardised mean difference effect size was calculated for each comparison. Overall effect size = 1.32.215

Small-molecule CB1 and CB2 receptor agonists and nonselective cannabinoid receptor agonists (including THC) were the most frequently assessed interventions. Fatty acid amide hydrolase inhibitors and peroxisome proliferator-activated receptor-α agonists (in particular, palmitoylethanolamide; PEA) were also frequently evaluated. In general, studies demonstrated antinociceptive efficacy, as measured predominantly by attenuation of injury-/inflammation-associated hypersensitivity in evoked limb withdrawal (Fig. 4).

F4
Figure 4.:
Forest plot of drug classes assessed for antinociceptive efficacy in rat (A) and mouse (B) models of injury-related or pathological persistent pain. The size of the squares represents the weight (%) and its influence on the pooled result. N denotes the number of animals and K the number of comparisons of each subgroup.215

The differences between the effect sizes of the different interventions may be inherent to the intervention, eg, mechanism of action, route of administration, and dosing regimens, but are also likely to be influenced by other study design characteristics such as the model, choice of species, strain, sex, and behavioral outcome measure.

In rodent inflammatory pain models (eg, formalin, complete Freund's adjuvant [CFA], carrageenan, and osteoarthritis), CB1, CB2 receptor agonists, and PEA consistently attenuated pain-related behaviors across a range of inflammatory pain models (Table 3). The only exception was for carrageenan-induced inflammation in rats, for which CB1 receptor agonists did not significantly attenuate pain-related behaviours. The efficacy of FAAH inhibitors was mixed; pain-related behaviors were significantly attenuated in formalin and CFA but not in osteoarthritis rodent models. In carrageenan models, a species difference was detected in which FAAH inhibitors significantly attenuated pain-related behaviors in mice but not rats. THC also significantly attenuated pain-related behaviors in formalin, CFA, and carrageenan models. Like FAAH inhibitors, the efficacy of CBD was mixed; pain-related behaviors were significantly attenuated in formalin and CFA but not in carrageenan and osteoarthritis rodent models.

Table 3 - Summary of the effects of cannabinoids and endocannabinoid system modulators on pain-related behaviour in the most frequently used rodent models of inflammatory pain.
Drug Class Formalin Complete Freund's Adjuvant Carrageenan Osteoarthritis
K N Effect size 95 % CI K N Effect size 95 % CI K N Effect size 95 % CI K N Effect size 95 % CI
Anandamide transport inhibitor 10 64 1.97 0.608-3.342 NT NT NT NT NT NT NT NT NT NT NT NT
CB1 receptor agonist 59 496 1.12 0.731-1.502 21 144 0.74 0.378-1.107 ns ns ns ns 3 16 1.22 0.378-1.107
CB1 receptor inverse agonist 1 7 1.71 0.427-3.002 NT NT NT NT 5 30 0.44 0.055-0.822 NT NT NT NT
CB1 receptor PAM NT NT NT NT NT NT NT NT 1 8 2.45 1.067-3.837 NT NT NT NT
CB2 receptor agonist 33 265 0.81 0.316-1.307 44 338 1.10 0.829-1.366 10 74 1.39 0.885-1.896 24 173 1.42 0.829-1.366
CB2 receptor inverse agonist 3 22 1.05 0.653-1.444 NT NT NT NT NT NT NT NT NT NT NT NT
CBD 2 16 1.86 1.548-2.169 5 20 1.59 0.71-2.469 ns ns ns ns ns ns ns ns
Dual FAAH/MGL inhibitor NT NT NT NT ns ns ns ns 4 36 1.42 0.25-2.6 NT NT NT NT
FAAH inhibitor 23 184 1.50 1.134-1.862 24 144 1.24 0.702-1.781 ns ns ns ns ns ns ns ns
FABP inhibitor ns ns ns ns 15 98 0.78 0.194-1.373 6 36 1.42 0.947-1.895 NT NT NT NT
Monoacylglycerol lipase inhibitor 8 69 2.28 1.382-3.175 ns ns ns ns 8 60 2.67 0.967-4.378 ns ns ns ns
NAAA inhibitor NT NT NT NT 3 30 1.63 1.235-2.025 12 72 1.91 0.923-2.897 NT NT NT NT
Nonselective cannabinoid receptor agonist 38 253 1.56 1.019-2.11 ns ns ns ns 45 311 0.74 0.202-1.268 ns ns ns ns
PPAR-alpha agonist 21 143 1.73 0.701-2.761 2 16 2.53 0.851-4.205 26 173 3.26 1.51-5.006 9 75 0.98 0.851-4.205
THC 20 118 1.16 0.679-1.643 16 109 2.34 1.687-2.986 5 37 2.13 0.138-4.12 NT NT NT NT
K denotes the number of comparisons and N denotes the number of animals within each subgroup. All entries with an effect size value were statistically significant.
CAB, cannabidiol; CB1, cannabinoid type 1; CB2, cannabinoid type 2; CI, confidence interval; FAAH, fatty acid amide hydrolase; FABP, fatty acid-binding protein; MGL, monoacylglycerol lipase; NAAA, N-acylethanolamine-hydrolyzing acid amidase; NT, not tested; ns, not significant; PAM, positive allosteric modulator; PPAR, peroxisome proliferator-activated receptor; THC, tetrahydrocannabinol.
Adapted from the study by Soliman et al.215

In rodent neuropathic pain models, eg, nerve injury, chemotherapy-induced peripheral neuropathy, and diabetes, CB1 and CB2 receptor agonists, FAAH inhibitors, and CBD consistently demonstrated antinociceptive efficacy (Table 4). PEA also significantly attenuated pain-related behaviors in nerve injury and chemotherapy models. THC significantly attenuated pain-related behaviors in nerve injury models.

Table 4 - Summary of the effects of cannabinoids and endocannabinoid system modulators on pain-related behaviour in the most frequently used rodent models of neuropathic pain.
Drug Class Nerve injury Chemotherapy Diabetes
K N Effect size 95 % CI K N Effect size 95 % CI K N Effect size 95 % CI
Anandamide transport inhibitor 1 5 7.95 3.26-12.643 NT NT NT NT NT NT NT NT
CB1 receptor agonist 102 756 1.17 0.842-1.5 31 173 1.47 1.007-1.937 30 183 1.74 1.234-2.248
CB1 receptor inverse agonist 4 36 1.71 1.171-2.248 NT NT NT NT 2 12 4.53 1.932-7.127
CB2 receptor agonist 87 639 1.15 0.812-1.497 27 192 1.55 1.117-1.983 10 88 1.29 0.772-1.818
CBD 9 72 1.86 1.307-2.406 10 76 1.31 0.594-2.021 7 49 1.08 0.257-1.896
Diacylglycerol lipase inhibitor 1 6 3.69 1.235-6.148 ns ns ns ns NT NT NT NT
Dual FAAH/MGL inhibitor 2 12 2.08 1.348-2.808 NT NT NT NT NT NT NT NT
FAAH inhibitor 59 439 1.75 1.212-2.281 19 148 1.96 1.048-2.868 17 180 2.53 1.455-3.604
Monoacylglycerol lipase inhibitor 16 119 1.31 0.774-1.84 12 80 1.42 0.179-2.653 NT NT NT NT
NAAA inhibitor 5 34 4.54 2.594-6.496 NT NT NT NT NT NT NT NT
Nonselective cannabinoid receptor agonist 75 504 1.18 0.825-1.536 ns ns ns ns 8 48 1.89 0.335-3.437
PPAR-alpha agonist 26 229 1.14 0.493-1.786 11 74 2.54 1.341-3.746 NT NT NT NT
THC 9 86 1.26 0.838-1.677 NT NT NT NT NT NT NT NT
Adapted from the study by Soliman et al.215
K denotes the number of comparisons and N denotes the number of animals within each subgroup. All entries with an effect size value were statistically significant.
CAB, cannabidiol; CB1, cannabinoid type 1; CB2, cannabinoid type 2; CI, confidence interval; FAAH, fatty acid amide hydrolase; FABP, fatty acid-binding protein; MGL, monoacylglycerol lipase; NAAA, N-acylethanolamine-hydrolyzing acid amidase; NT, not tested, ns, not significant; PPAR, peroxisome proliferator-activated receptor; THC, tetrahydrocannabinol.

Our companion systematic review and meta-analysis suggests that the most frequently assessed cannabinoids, CB1 and CB2 receptor agonists and nonselective agonists, consistently attenuated pain-associated behaviors in a broad range of inflammatory and neuropathic pain models.215 Although tested in fewer model types, this was similarly evident for THC. CBD and FAAH inhibitors were not as effective in inflammatory pain models but may be viable candidates for the treatment of neuropathic pain. However, this analysis does not take into account potential side effects such as motor impairment, hypothermia, or anxiolysis that could influence the behavioral outcomes. Prospective preclinical trials are required to better ascertain what factors are influencing the differences in observed efficacy between the different drug classes and model types.

Our companion systematic review and meta-analysis215 highlights several differences between the preclinical and clinical assessment of candidate treatments. The number and profile of the drugs assessed preclinically (171 different interventions) differs markedly from those investigated in clinical trials (11 different interventions)79 where the predominantly evaluated clinical interventions are pharmacologically complex cannabis-based medical extracts, THC, or THC analogues. Indeed, an unusual feature of the cannabinoid field is that the initiation of clinical trials has not generally followed (or has been very slow to follow) the publication of preclinical data providing evidence of benefit for small-molecule drugs, whereas this is the case for many other drug classes. Moreover, certain indications assessed in clinical trials (eg, pain due to third molar extraction) are not represented in the preclinical literature. The routes of administration also differ, and accompanying pharmacokinetic investigation was only evident in 7% of the studies. However, 69% of studies did confirm CB1 or CB2 receptor involvement through the use of antagonists, transgenic mice, or radioligand binding.

Our companion systematic review and meta-analysis215 also highlights several weaknesses in face and construct validity of the animal models and there are several study characteristics that can impact pain-associated behavioral outcome measures and the assessment of novel antinociceptive efficacy. The animal cohorts are genetically very similar, converse to the widely heterogeneous patient population. In rats, most studies used either Sprague-Dawley or Wistar strains, although larger effect sizes were evident in the small number of studies reported with Lewis rats. There was a major bias to the use of male rats (91%). Similar findings were found in mice with 81% of studies reporting the use of males across 29 strains. This does not reflect the clinical situation where women are overrepresented among patients with chronic pain.164 In clinical trials, male and female patients are more equally represented. For example, in a recent systematic review of 36 randomised controlled trials assessing cannabinoids, cannabis, and CBMs, female patients outnumbered (n = 3691) male patients (n = 3613).79

More generally, the animal models do not effectively simulate multidimensional clinical pain conditions including the psychological component. Disease or injury is frequently induced in young, otherwise healthy animals contrasting with the clinical situation in which disease or injury predominantly occurs in older patients with comorbidities. The duration of animal studies is usually brief (up to a few weeks), which results in candidate treatments being tested in the early stages of disease onset, which does not adequately reproduce the impact of prolonged clinical pain or address the clinical need for treatment in the later stages of disease. There is an overreliance on evoked limb withdrawal, a measure of hypersensitivity, which is not appropriate for pain that is characterized by sensory loss or spontaneous pain. Careful consideration should be given to the choice of species, strain, sex, and age in relation to the clinical condition being modelled. To limit threats to external validity, researchers should balance the sexes.39 A broader range of outcome measures that are of clinical relevance and include more complex, ethologically relevant behaviors is required.202 Multicenter testing will increase environmental heterogeneity and study samples thereby improving the generalizability of preclinical findings.231

As with the majority of current preclinical neuroscience and pharmacological research using animal models, the risk of bias is uncertain, due to a generic poverty of reporting sufficient details of the experimental design, conduct, and analysis factors, which govern the veracity of experimental internal validity. There is evidence to suggest that low prevalence of the reporting of measures to mitigate bias tend to give higher estimates of treatment effects.48,105 Our meta-analysis did not show a consistent relationship between the reporting of methodological quality criteria and smaller effect sizes; however, larger effect sizes were reported for studies that did not report allocation concealment and sample size calculations.215 The methods by which biases were mitigated were also infrequently reported. On the rare occasions where they were reported, the methods were often invalid,230 eg, randomization by “picking animals randomly from a cage” rather than computer-generated random sequence, or determining sample size based upon reported sample size norms rather than a power calculation. Thus, there is a critical need for transparency of reporting all experimental details so that the quality of research can be assessed and the validity of the outcomes inferred.

The analysis did not suggest evidence of publication bias. It is also to be expected that animal studies will yield larger effect sizes in comparison to clinical studies due to the more homogenous nature of the laboratory animal study population and better opportunity to control experimental variables, reducing the observed variance and making direct comparison of effect sizes between animal and human studies impossible.

Our accompanying systematic review and meta-analysis215 therefore highlights the need for improvements in experimental design and, perhaps even more importantly, the reporting of experimental design and analysis features in sufficient detail such that primary research can be reproduced and meta-analysed. Notwithstanding the caveats relating to animal studies of analgesia, substantial evidence from animal model experiments supports the hypothesis of cannabinoid-induced analgesia in inflammatory and neuropathy conditions.

6. Side effects relevant to preclinical pain studies

An important consideration in the evaluation of any pain medication is whether analgesic efficacy is accompanied by adverse side effects that limit therapeutic potential, or additional beneficial effects that might enhance therapeutic potential. The present evaluation of on-target “side effects” is restricted to effects of cannabinoids/endocannabinoid system modulators administered to rodents in adulthood. Additional preclinical research is needed to specifically assess potential harms in vulnerable populations (eg, older adults, and the developing fetal and adolescent brain40,41,91). In this section, we consider both potential harms and benefits of pharmacological strategies assessed specifically in animal pain models, particularly as they may emerge with chronic dosing. We also consider pharmacological effects (eg, motor impairment, hypothermia, or anxiolysis) that could influence interpretation of pain-related behavior. Animal models have been used to investigate an array of mechanisms and sites of analgesic action using a much broader array of mechanistically distinct compounds compared with those evaluated in the clinical literature. Most therapeutic interventions that show promise in preclinical studies, nonetheless, fail in clinical trials, albeit for different reasons (ie, efficacy, side-effect profile, lack of adequate target engagement, therapeutic indication, variability in clinical populations, and clinical primary outcome measure); this failure rate is not unique to cannabinoids. Moreover, animal models may not adequately capture adverse side effects that may be problematic in humans, and adverse side effects are often not systematically studied in animal experiments. Nevertheless, preclinical studies have provided evidence of both beneficial (ie, antinociception, antiemetic, antispasticity, antistress, and anti–anxiety-like effects) and adverse (ie, reward or aversion, dependence, tolerance, and motor and memory impairment) effects of cannabinoids and endocannabinoid system modulators in otherwise normal animals (for review, see Refs. 183,184). Here, we primarily consider “on-target” pharmacological effects of cannabinoids in pain models only, including assessments of cannabimimetic effects (ie, in the classic cannabinoid tetrad), tolerance, physical dependence, reward/reinforcement, opioid-sparing effects, antinociceptive synergy, and effects on stress-, anxiety-, and depression-related behavior in animal models of pain.

6.1. Cardinal signs of cannabinoid type 1 receptor activation

The classic cannabinoid tetrad assesses cardinal signs of CB1 activation (ie, hypoactivity, reduction in body temperature, catalepsy, and tail-flick antinociception)156,214 in rodents and is produced by all CNS-penetrant CB1 agonists, including THC. These cannabimimetic effects are consistent with localisation of CB1 to motor and limbic regions in rodent brain.104 Drug-induced motor impairment can mask detection of antinociception in rodents. Consequently, preclinical studies must show that antinociceptive effects of cannabinoids observed in behavioral studies are not artifacts of motor impairment. Although motor effects complicate behavioral evaluation of antinociceptive effects of cannabinoids, they do not preclude the existence of antinociceptive mechanisms. Our accompanying systematic review revealed that a minority (33%) of studies assessed the effects of the drugs on motor activity.215 Direct CB1 agonists and nonselective cannabinoid receptor agonists that penetrate the CNS have potential to produce undesirable CB1-mediated pharmacological effects in humans (eg, psychoactivity, and motor and memory impairment) (for review, see Refs. 183,184). Electrophysiological measures of activity of nociceptive neurons and/or neurochemical measures of noxious stimulus-evoked neuronal activation provide independent lines of evidence that nonselective cannabinoid receptor agonists, CB1 agonists, FAAH inhibitors, and CB2 agonists suppress nociceptive processing in rodents (for review, see Refs. 98, 99, 217, 240). Nonselective cannabinoid receptor agonists (eg, WIN55,212-2) suppress electrophysiological112,114,115,158 and neurochemical markers of pain-evoked neuronal activation226 as well as pain behavior at doses that do not alter body temperature or produce immobility.186,199 Drug-induced reductions in body temperature cannot explain electrophysiological or behavioral indicators of antinociceptive efficacy.158,170,228 WIN55,212-2-induced reduction of evoked hypersensitivity in the chronic constriction injury (CCI) model of neuropathic pain, and side effects (motor incoordination, catalepsy, and sedation), can occur at similar ED50s in mice3; however, antinociceptive effects of WIN55,212-2 correlate with suppression of firing in nociceptive neurons and is reported to outlast these motor effects.158

Drug development efforts have focused on elucidating therapeutic potential of small molecules that engage targets within the endocannabinoid system that lack unwanted cannabimimetic effects associated with direct CB1 activation (for review, see Refs. 50, 113, and 139). Cannabinoid type 2 agonists, FAAH inhibitors, MGL inhibitors (at appropriate doses), CB1-positive allosteric modulators (PAMs), peripherally restricted nonselective cannabinoid receptor agonists (PrNM1166) have all been shown to suppress pain behavior without unwanted motor effects of CB1 agonists (for review, see Refs. 98, 99, 217, and 240). High-dose MGL inhibitors can elicit tetrad effects, whereas a dual FAAH/MGL inhibitor JZL195 suppressed neuropathic nociception in the mouse CCI model with an ED50 4 times lower than that which produced side effects (motor incoordination, catalepsy, and sedation),3 although brain-permeant and -impermeant FAAH inhibitors lack such effects.5,73,197,223 Dual FAAH/MGL inhibitors represent a pharmacological strategy to elevate both AEA and 2-AG and produce antinociceptive efficacy without producing unwanted cannabimimetic effects in the tetrad.66 Cannabinoid type 1 knockout mice have also been used to assess the impact of CB2 activation without the confound of CB1-mediated side effects.56,66,126,208 Such studies show that mixed cannabinoid agonists and CB2 agonists can engage CB2 receptors independently of CB1 to alleviate neuropathic pain-related behavior induced by paclitaxel administration or spinal nerve ligation.56,126

The abundance of CB1 in brain regions controlling motor activity and memory accounts for adverse side effects of mixed and CB1-preferring strategies.104 Very few studies evaluate possible memory impairment induced by pharmacological treatments in laboratory animal pain models. However, studies that involve learning approaches document that rodents in pathological pain states can show conditioned place preferences50,84 or perform operant responses102,166 to chambers/tasks associated with pain relief; memory impairment would preclude demonstrations of efficacy in such studies. Peripherally restricted CB1 agonists, CB2 agonists, MGL inhibitors, and CBD represent cannabinoid modulators that have shown efficacy in such assays in animal pain models.50,84,166

6.2. Tolerance

Tolerance, the loss of therapeutic efficacy with repeated administration, is undesirable in an analgesic and can lead to dose escalation and potential for misuse and abuse. In animal models of pathological pain, tolerance to therapeutic efficacy develops to direct-acting CB1 agonists, nonselective cannabinoid receptor agonists, as well as high, but not low, doses of MGL inhibitors,56,142,212 presumably via downregulation and desensitization of CB1 receptors. In a mouse model of chemotherapy-induced neuropathic pain induced by paclitaxel, tolerance developed to the antinociceptive effects of THC and other nonselective cannabinoid receptor agonists (CP55,940 and WIN55,212-2), as well as to other classic cannabimimetic effects.56,212 Low-dose chronic infusion of WIN55,212-2 and AM1241 (CB2 agonist) were associated with sustained antinociceptive efficacy in the paclitaxel model without motor impairment.199 Tolerance develops more quickly to high compared with low doses of the centrally acting CB1-preferring ligands.56,212 By contrast, antinociceptive tolerance is typically absent in neuropathic as well as inflammatory pain models after repeated administration of CB2 agonists,56,199,212 brain-permeant and -impermeant FAAH inhibitors,38,212,213 low-dose (but not high-dose) MGL inhibitors,142 CB1 PAMs,128,212 and a peripherally restricted CB1 agonist.166 The FAAH/MGL dual inhibitor JZL195 exhibited greater efficacy than FAAH or MGL inhibitor alone and did not produce tolerance, exhibiting an improved therapeutic window compared with direct cannabinoid CB1 agonists.3 Further preclinical studies are necessary to determine whether different therapeutic strategies (ie, biased CB1 agonism, peripherally restricted nonselective cannabinoid receptor agonists) can separate therapeutic effects from unwanted side effects.

6.3. Physical dependence

Receptor antagonists have been used to precipitate a withdrawal syndrome, a sign of physical dependence, in mice subjected to pathological pain states. In mice rendered neuropathic with paclitaxel, the CB1 antagonist rimonabant precipitates signs of physical dependence (ie, paw tremors) in mice treated chronically with orthosteric cannabinoid agonists (ie, THC, CP55940, and WIN55,212-2); severity of withdrawal symptoms was dose-related.56,212 In the same neuropathic pain model, both tolerance and rimonabant-precipitated withdrawal signs were produced by the MGL inhibitor JZL184 but not by a CB2 agonist,56,212 brain-permeant or -impermeant inhibitors of FAAH,213 or a CB1 PAM.212 Where present, physical dependence was induced by challenge with CB1, but not CB2, antagonists and in animals receiving direct-acting agonists (or high-dose MGL inhibitor) that penetrated the CNS. By contrast, neither CB1 PAMs nor brain-permeant or -impermeant inhibitors of FAAH were associated with signs of physical dependence after 3 weeks of once daily administration in paclitaxel-treated mice.212,213 Notably, CB1 antagonist-precipitated withdrawal syndromes (ie, paw tremors) lacked the more striking somatic (ie, jumping) and autonomic (ie, diarrhea) signs associated with naloxone-precipitated opioid withdrawal.9,212

6.4. Reward/reinforcement

Tetrahydrocannabinol can produce both rewarding and aversive effects in laboratory animals.35,151,216 Rewarding properties of cannabinoids have historically been demonstrated in otherwise healthy, young, male rodents, which may not necessarily mimic the situation in people with chronic pain. In pathological pain states, interpretation of positive reinforcing effects of cannabinoids (ie, reward in the typical drug abuse sense) should be differentiated from negative reinforcing effects (ie, removal of an aversive pain state). Preclinical research involving rodent pain models has investigated therapeutic strategies targeting the endocannabinoid system that hold promise for suppressing pain without producing abuse liability. Using a classic drug self-administration approach, rats rendered neuropathic by a spared nerve injury self-administered a CB2 agonist in a CB2 receptor-dependent manner; naive animals did not reliably self-administer the drug.102 Moreover, CB2 agonists did not produce CPP.93,130 These observations suggest that the CB2 agonists were not inherently reinforcing in the absence of the pathological pain state. In the absence of pathological pain, CB1 PAMs ZCZ011 and GAT211 do not produce CPP when administered alone.129,212 Moreover, ZCZ011 did not substitute for CB1 agonists CP55940 or AEA in a drug discrimination assay,129 suggesting that it does not produce cannabimimetic side effects. Inhibition of MGL with MJN110 also reversed a negative affective state associated with paclitaxel treatment using a CPP approach.50 Similarly, a nonrewarding dose of CBD produced reward-related effects in the presence of pain due to incisional injury.84Thus, in the absence of a pathological pain state, CB1 PAMs and endocannabinoid deactivation inhibitors did not produce rewarding or aversive effects when administered alone in rodents. These studies raise the possibility that components of the endocannabinoid signaling system may be targeted for therapeutic benefit without the rewarding properties of direct CB1 activation in the CNS.

6.5. Opioid-sparing effects and antinociceptive synergy

Opioids remain a mainstay of pain management but also produce tolerance, physical dependence, reward, constipation, and respiratory depression, among other effects. Efficacy of adjunctive therapies for suppressing pathological pain and producing opioid-sparing effects has, consequently, been evaluated. Tetrahydrocannabinol produced synergistic antinociceptive effects with morphine in both arthritic (Freund's adjuvant-induced) and naive rats45; additive, rather than synergistic, interactions were reported for unwanted side effects (eg, tetrad and motor ataxia).45 WIN55,212-2 produced synergistic antinociceptive interactions with morphine in mice with CCI of the sciatic nerve but only additive effects on motor coordination.3 Brain-permeant and -impermeant inhibitors of FAAH,212 MGL inhibitors,237 dual FAAH-MGL inhibitor,66 CB2 agonists,93,130 and CB1 PAMs211 produce synergistic antinociceptive effects with morphine in neuropathic pain models. The CB2 agonist JWH133 produced additive antinociceptive effects with morphine in the formalin test.245 Notably, in a mouse neuropathic pain model, brain-permeant (URB597) and -impermeant (URB937) FAAH inhibitors,213 a CB1 PAM (GAT211;211), and CB2 agonists (AM1710, LY2828360;9,153,212) suppressed development of morphine tolerance without enhancing naloxone-precipitated opioid withdrawal. Multiple CB2 agonists attenuated opioid tolerance and naloxone-precipitated opioid withdrawal in neuropathic mice.9,130,153,155,212 Cannabinoid type 2 agonists produce synergistic antinociceptive effects with opioids and also attenuate opioid-induced respiratory depression.236,246 Neither FAAH inhibitors (URB597 and URB937) nor a CB1 PAM (GAT211) enhanced naloxone-precipitated opioid withdrawal in paclitaxel-treated mice.212,213 Evaluations of other opioid side effects in the same studies (ie, slowing of GI motility and reward) have typically used normal animals not subjected to pathological pain states. Nonetheless, lowering opioid doses required to elicit therapeutic effects could enhance therapeutic ratios. Most studies have combined opioids with endocannabinoid modulators that themselves lack observable cannabimimetic side effects (JWH015, MJN110, URB597, and URB937); consequently, additivity of adverse side effects would not be expected. Interestingly, CB2 agonists (JWH015 and LY2828360) produced synergistic antiallodynic effects with morphine in models of inflammatory (formalin), postoperative (paw incision), and neuropathic (SNI) nociception but did not produce synergy for nociceptive pain.93,130 Synergy between CBD and morphine has also been reported in the acetic acid-induced writhing model,176 and coadministration of THC and morphine reduced the second phase of formalin-evoked nociceptive behaviour in rats to a greater extent than either drug alone.78 More work is necessary to examine whether cannabinoids alter other unwanted side effects of opioids (eg, slowing of GI motility) in pain models and evaluate the clinical relevance of these findings

6.6. Synergism of antinociceptive efficacy: nonopioid analgesics

Tetrahydrocannabinol has been shown to produce synergistic antinociceptive interactions with gabapentin in the mouse CCI model of neuropathic pain.9 Synergy between CBD and THC, and between PEA and gabapentin, is also reported in chemotherapy-induced neuropathic pain models,66,67,141 between CBD and THC in the mouse CCI model of neuropathic pain,34 and between PEA and acetaminophen (paracetamol) in the rat streptozotocin-induced model of diabetic neuropathy.53 The COX-2 inhibitor celecoxib increased the antihypersensitivity activity of the CB1 agonist Met-F-AEA and the CB2 agonist AM1241 in the streptozotocin model.26 Low doses of the MGL inhibitor JZL184 and the nonselective COX inhibitor diclofenac synergistically attenuated mechanical allodynia and additively reduced cold allodynia in the mouse CCI model of neuropathic pain.49 Furthermore, coadministration of the FAAH inhibitor URB597 and diclofenac yielded synergistic antinociceptive effects in the acetic acid-induced abdominal stretching model of visceral nociception in mice.174 In other work, AEA coadministered with either ibuprofen (nonselective COX inhibitor) or rofecoxib (selective COX-2 inhibitor) resulted in synergistic antinociceptive effects in the rat formalin test.97,101 Mechanistically, it is important to note that there are a number of interactions between COX/NSAIDs/acetaminophen and cannabinoids/endocannabinoid system, including inhibitory effects of NSAIDs or an acetaminophen metabolite on endocannabinoid catabolism or transport (for review, see Ref. 189). Thus, adjunctive therapies could enhance therapeutic ratios of existing treatments for both inflammatory and neuropathic pain. Cannabinoid type 1 PAMs enhance antinociceptive and unwanted side effects of orthosteric CB1 agonists128,212 and produce synergistic antiallodynic effects with FAAH and MGL inhibitors without enhancing other tetrad parameters.212

6.7. Beneficial on-target pharmacological effects

Beneficial on-target pharmacological effects (ie, in suppressing stress, anxiety, nausea, and producing improvements in sleep) of cannabinoids (for review, see Refs. 183, and 184) could contribute to perceived therapeutic benefits in relevant pathological pain states. However, only small numbers of studies have evaluated such features specifically in animal pain models. Indeed, our accompanying systematic review revealed that only 3% of studies assessed the anxiolytic or antidepressant-like effects of the drugs in the animal models of injury-related or pathological persistent pain.215 Systematic preclinical studies are necessary to determine whether, rather than purely modulating sensory thresholds, cannabinoid-based modulation of affective behavior could contribute to therapeutic efficacy of analgesics by attenuating symptoms that exacerbate pain.

6.8. Stress, anxiety, depression, and pain-depressed behavior

The therapeutic potential of cannabinoids/endocannabinoid system modulators has been assessed in animal models of anxiety, stress, and depression (for review, seeRefs. 76, 90, 165, 183, 184, and 187). However, fewer studies have evaluated such therapeutic effects within the context of pathological pain states (for review, see Refs. 42, 80, 217, and 240). The CB2 agonist GW405833 reduced immobility (ie, a measure of depression-like behavior) in the forced swim test and allodynia (although mediation by CB2 was not assessed), whereas the antidepressant desipramine primarily attenuated immobility time.120 In the mouse monosodium iodoacetate model of osteoarthritis pain, the CB1 agonist ACEA and the CB2 agonist JWH133 ameliorated nociceptive and affective alterations, and ACEA also improved associated memory impairment.147 In rats with CCI, the brain-permeant FAAH inhibitor URB597 but not the brain-impermeant FAAH inhibitor URB937 suppressed immobility in the forced swim test, and attenuated novelty-induced suppression of feeding and CCI-induced reductions in hippocampal neurogenesis, all indicative of antidepressant-like effects, whereas both URB597 and URB937 suppressed allodynia.132 Thus, the CNS-penetrant FAAH inhibitor produced beneficial effects on both evoked pain and pain-induced depression-like behavior. Rats with spared nerve injury exhibited an anxiety-like phenotype relative to shams; repeated CBD prevented anxiety-like behavior in the open field test, elevated plus maze and novelty-induced suppression of feeding test, in addition to suppressing allodynia via a 5-HT1A receptor mechanism.52 Thus, effects of CBD revealed in that study are likely to be independent of cannabinoid receptor activation.

In addition to evaluations of responses to evoked pain (pain-stimulated behaviors), an emerging preclinical literature has evaluated pain-depressed behavior (eg, marble burying, nestlet shredding, and intracranial self-stimulation thresholds). Although the extent to which assays of pain-depressed behavior assess stress-, anxiety-, or depression-related behavior is uncertain, they represent an attempt to more completely model the multifaceted complexities associated with pathological pain states.177 Assessing the effects of drugs on pain-depressed behavior may also help to distinguish between antinociception and motoric side effects of putative analgesics. Tetrahydrocannabinol and CP55,940 produced antinociception for pain-stimulated responding in an acid-induced writhing assay but exacerbated pain-depressed behavior (ie, noxious stimulus-induced suppression of intracranial self-stimulation thresholds).146,150 By contrast, the FAAH inhibitor URB597 suppressed both pain-stimulated as well as pain-depressed behavior.145 An MGL inhibitor (MJN110), CB2 agonist (LEI101), nonselective cannabinoid receptor agonist (CP55940), and reference analgesics (morphine, gabapentin, and valdecoxib) all reversed pain-stimulated behavior (ie, mechanical hypersensitivity) as well as pain-depressed behavior (ie, deficits in marble burying) in mice with CCI.50 By contrast, the benzodiazepine diazepam reversed neither dependent measure, whereas a kappa-opioid receptor agonist (U69593) and an FAAH inhibitor (PF3845) reversed deficits in mechanical hypersensitivity only.50 Deficits in marble burying resolved within a week of surgery,50 and not all neuropathic pain models are associated with deficits in marble burying/nestlet shredding.212 Thus, the translational relevance of these behaviors to pain and/or anxiety, consequently, remain unclear but may they provide insights of some relevance to general health/quality of life.

6.9. Sleep

Effects of cannabinoids on sleep have not been assessed in animal pain models. Clinical data on the FAAH inhibitor PF-04457845 in cannabis use disorder have suggested improvements in both withdrawal and sleep.51 More work is necessary to ascertain presence of sleep disruptions in laboratory animal models of pathological pain states and ascertain whether endocannabinoid modulators could restore such deficits.

7. Pharmacokinetics of cannabinoids

In addition to the mechanisms of drug action that affect the endocannabinoid system, and cannabinoid structure-activity relationships (ie, pharmacodynamics), the assessment of drug pharmacokinetics is of critical importance both for better understanding the preclinical pharmacology, and for improving animal-to-human translation.235 The pharmacokinetic processes of drug absorption to the systemic circulation, its distribution to tissues of interest, as well as its metabolism and excretion will determine how much drug, after a particular dose delivered via a particular route, will reach its site of action. In addition, drug pharmacokinetics will determine the duration of a drug's effect, and additional parameters related to drug–drug interactions and dose adjustments required in cases when organs such as liver or kidneys do not function properly.

Animal models provide advantages for investigating drug pharmacology because endogenous systems often cannot be manipulated safely in humans. However, because drug pharmacokinetics can be investigated directly in humans, this section of the article will also discuss clinically relevant pharmacokinetic data, where possible. Pharmacokinetic properties of cannabinoids, particularly their absorption, demonstrate substantial variability as a function of both route of administration and the specific formulation in which the cannabinoids are delivered. Human studies repeatedly demonstrate large intersubject variability in the pharmacokinetics of identical cannabinoid products, the basis of which is currently not well understood. Therefore, comparisons of nonhuman and human pharmacokinetics can have important translational relevance.

7.1. Absorption

Cannabinoid absorption is generally faster via the inhalational route than oral route, resulting in a more rapid onset of pharmacological effects, and shorter time to peak effect. In addition, cannabinoid delivery via the inhalational route circumvents the variability in oral absorption processes that are due to first pass metabolism. The limitations of inhalational routes include the variability in interindividual efficiency that is caused by differences in inhalation techniques, respiratory tract irritation during inhalation, and the inconvenience or lack of adherence associated with smoked, vaporised, or nebulised cannabinoid products. Although most preclinical experiments are performed with injected or oral/gastric administration of cannabinoids, most clinical therapeutic studies have used either inhaled products, oromucosal products such as nabiximols/Sativex, or oral THC and its analogues.

The reported oral bioavailability of cannabinoids, and THC in particular, varies as a function of drug vehicle and the coingested food. For example, orally ingested cannabis-containing cookies demonstrate only 6% THC bioavailability, whereas THC dissolved in sesame oil delivered in soft gelatin capsules is ∼20% bioavailable (for review, see Ref. 122). A similar pattern of improved oral bioavailability approaching 30%, when dissolved in sesame oil, has been reported in rat studies with both THC and CBD.131

The bioavailability of THC delivered via smoked cannabis products reportedly varies between 18% and 50%.180 In a small study comparing absorption of a physiologically compatible, nebulised inhalation solution of THC with that of intravenous THC in 8 healthy volunteers, the average bioavailability of THC delivered via the inhalational route was approximately 28%,172 somewhat comparable to average bioavailability with smoking. However, the measured area under the THC concentration–time curve (AUC) among study participants varied more than 5-fold after intravenous administration, and more than 15-fold after inhalation. Adverse effects, as expected, were more prominent with intravenous dosing.172

After oral THC administration, the time to reach maximum plasma concentration (Tmax) has been reported between 30 and 120 minutes (but up to 6 hours in some studies) and is comparable between young and older adults.4 Similarly, a wide range of Tmax values, between 1 and 8 hours, has been reported with rat studies of THC, depending on the fasted/fed status and the formulation in which THC is administered.131,198 However, peak plasma concentrations (Cmax) of THC seem to be highly variable, with 4- to 7-fold differences among individuals, particularly in older adults.4 With single oral doses of THC, the average Cmax ranged between 1.5 and 3 ng/mL after 3 to 6.5 mg doses,4 and approximately 7 ng/mL after 20 mg oral THC (dronabinol) administration.171 With inhaled doses, 0.053 mg/kg nebulised THC (mean dose 3.5 mg)172 resulted in average Cmax of 20 ng/mL, and with smoked cannabis, THC doses of 0.25 mg/kg body weight (14-22 mg per cigarette) have resulted in mean Cmax of 48 ng/mL. The plasma concentration profiles of key THC metabolites, namely 11-COOH-THC- and 11-OH-THC, also vary substantially among people. With such variability in absorption and plasma concentrations of some cannabinoids, it is not surprising that clinical trial results are inconsistent.79

Cannabidiol, considered by many as devoid of the psychotropic side effects of other cannabinoids, has garnered attention in recent years. After oral administration, CBD follows close-to-linear absorption (eg, Cmax of 530 ng/mL with 3000 mg/d, and 780 ng/mL with 6000 mg/d oral dosing), but high-fat meals can increase CBD plasma exposure and peak plasma concentrations by 4- to 5-fold.82,221 Following doses of 10 mg CBD with 10 mg THC either in an oral capsule or oromucosal nabiximol spray, mean reported Cmax was 2.5 to 3 ng/mL for CBD and 6.1 to 6.4 ng/mL for THC.138

Cannabinoids are highly lipophilic molecules with low aqueous solubility and are susceptible to degradation and oxidation, especially in a solution. Drug formulation can thus play a crucial role in increasing the solubility and physicochemical stability of cannabinoid drugs, thus improving their pharmacokinetic properties.25 Commonly used strategies in marketed products include pH adjustment, use of cosolvents, the use of micelles and nanoemulsions, complexation with cyclodextrins, or encapsulation in lipid-based formulations such as liposomes and nanoparticles. Even simple approaches such as administration of THC and CBD in lipid-rich oral formulations can increase plasma bioavailability of cannabinoids 2- to 3-fold, compared with lipid-free formulations.248

Formulations that are based on self-(nano)emulsifying drug delivery technology (SEDDS) have been proposed as a means of improving the oral bioavailability of drugs that show poor aqueous solubility, including cannabinoids. This approach results in a consistent increase in oral bioavailability of both THC (9-fold) and CBD (6-fold) in rats,37 and a 1.5-fold and 2.2-fold increase, respectively, in humans.36

Despite structural similarities, oral bioavailability differs between different cannabinoids. Nabilone, a synthetic cannabinoid derivative, is only slightly structurally different from THC (and dronabinol); however, it has substantially higher bioavailability than dronabinol (95% vs 10%-20%). One proposed difference relates to relative affinity to chylomicrons, and therefore the extent of lymphatic absorption of a particular cannabinoid.85 Drug formulations containing long-chain triglycerides may substantially improve the lymphatic absorption of selected cannabinoids.85 Cannabinoids have demonstrated immunomodulatory effects in vitro; however, plasma concentrations achieved with clinically relevant dosing are well below those shown to affect lymphocyte function. By choosing the appropriate lipid-based formulation, lymphatic absorption of orally administered cannabinoids can be improved to achieve 100-fold and 250-fold higher lymph (vs plasma) concentrations of THC and CBD, respectively, which could help in the targeting of conditions where neuroimmunological effects of cannabinoids are of interest.247,248

Although there is high variability with the oral routes, consideration of variability related to smoked or vaporised delivery is also important. In pulmonary delivery routes, the depth of inhalation, how long breath is held for, and vaporiser temperature all affect cannabinoid absorption.25 Inhalational devices that are temperature-regulated to control the inhaled doses may allow less variable pharmacokinetic profile of inhaled cannabinoids.70

The transdermal route of administration provides an alternative approach for systemic delivery of lipophilic compounds with highly variable oral absorption, but more research is required to optimise these delivery systems and assess their efficacy and safety profiles.188,229

7.2. Distribution

Most naturally occurring cannabinoid receptor ligands are lipophilic, readily penetrate the blood–brain barrier, and permeate well to lipid-rich tissues such as the brain and peripheral fat. Cannabinoids display slower elimination rates from these tissues compared with plasma.24 Tetrahydrocannabinol achieves higher concentration in rodent brain than in plasma, and CBD achieves approximately a 1:1 ratio.106 Many of the synthetic ligands behave similarly. For example, WIN 55,212-2 and MK-9470, cannabinoid receptor ligands commonly used in preclinical studies, demonstrate similarly high brain:plasma penetration ratio in rodents.163,194

In rhesus monkeys exposed to either intermittent (days) or long-term (years) cannabis consumption, the pharmacokinetics of single-dose subcutaneous THC were not substantially different.87 However, physiological responses of change in core temperature, or behavioral response time were different, and less profound, in monkeys with chronic exposure, likely suggesting physiological/behavioral tolerance to THC. Although blood THC levels were highest at 30-minute postinjection, maximal effects on temperature and response rate did not occur until 120-minute after injection, after blood THC levels had substantially decreased, suggesting a nonlinear relationship between blood concentration of THC and its pharmacodynamic effects.

The existence of peripheral antinociceptive mechanisms32,81 (for review, see Refs. 107, 152, 169, 196, 204, 240, and 250) prompted development of peripherally restricted cannabinoid receptor ligands. However, developing such compounds that are not lipophilic but have high binding affinity to CB receptors has been challenging. Several compounds meeting the desired criteria of limited brain penetration with high affinity to CB1 and CB2 receptors have been synthesized.2 Some achieve 0.16 to 0.18 brain:plasma ratio and produce antinociceptive effects in a rat model of neuropathy without producing catalepsy, but are yet to be evaluated clinically. Brain-impermeant inhibitors of endocannabinoid metabolism have also been developed and show promise in preclinical studies of inflammatory and neuropathic pain.38,100,212,213

There may be sex-specific differences in cannabinoid pharmacokinetics. Plasma levels of THC after intraperitoneal injection are comparable between male and female rats but levels of THC metabolites in brain tissue, including 11-OH-THC, the major active metabolite, were higher in female than in male rats. When behavioral effects such as response to heat nociception and catalepsy were tested, SKF525A, a cytochrome P450 inhibitor, attenuated THC-induced antinociception and catalepsy in female but not in male rats. Greater levels of active THC metabolites produced by females could potentially contribute to greater behavioral effects of THC in female compared with male rats.225 Human studies have found somewhat similar tendencies, with11-OH-THC levels higher in females after THC administration, along with higher subjective ratings of drug effects.210

7.3. Metabolism and excretion

Cannabinoids undergo a variety of metabolic processes in the gut, the liver, and various other tissues. Many metabolites have been identified, but the relative activity and toxicity of each is unknown. Once absorbed, THC is primarily oxidized by the cytochrome P450 hepatic mixed-function oxidase system to equipotent 11-hydroxy-THC (11-OH-THC), and further metabolized to inactive 11-COOH-THC. Most studies assessing the metabolic profile of THC have measured the plasma concentrations of these 2 metabolites, 11-OH-THC and 11-COOH-THC. The area under the concentration–time curve (AUC) of the psychoactive metabolite 11-OH-THC is 4 to 6 times lower compared with the AUC of THC itself after inhalational or IV administration, but overall, 11-OH-THC has a similar pharmacokinetic profile and elimination half-life.171,172

11-COOH-THC, the main cannabinoid metabolite detected upon urine drug screens, has average terminal elimination half-life of 3 to 5 days. However, due to high interpatient variability, both in adults and adolescents, 11-COOH-THC can be detected in plasma or urine a month or more after biochemically verified abstinence.209

Cannabidiol undergoes multiphasic elimination. Its effective half-life estimates ranged from 10 to 17 hours in humans, and the terminal elimination half-life is approximately 2 to 3 days.221 The effective half-life of some of its metabolites, including 7-carboxy-cannabidiol (COOH-CBD), is around 24 hours, with plasma concentrations in hundreds of ng/mL detectable many days after single-dose administration. In rats and mice, the half-life of CBD has been reported in the range of 1 to 4 hours, with some unexplained variability among administration by different routes and in different formulations.20,54,137,244

When oral CBD (Epidiolex) was administered in people with variable extent of liver impairment, the total exposure (AUC) to CBD was increased by 50% in subjects with mild hepatic impairment, 2.5-fold in subjects with moderate, and 5-fold in severe hepatic impairment.221

There is debate in the literature on whether CBD potentiates or antagonises analgesic effects produced by THC. One hypothesis is that CBD is an inhibitor of CYP450 enzyme systems and may affect the pharmacokinetic profile of THC. Cannabidiol does inhibit microsomal CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 enzymes, but seems to do so at much higher concentrations than typically achieved in plasma with clinically relevant doses.138 In addition, in a human study of oromucosal THC and CBD administration, no major differences in THC pharmacokinetics were observed in the presence or absence of coadministered CBD.138

In summary, cannabinoids demonstrate substantial interindividual variability in pharmacokinetics, particularly in absorption processes, whether via oral or inhaled routes. This can result in differences in drug concentrations at the desired sites of action, leading to inconsistent clinical effects. Only a minority of preclinical studies of cannabinoids and endocannabinoid system modulators in animal pain models have assessed drug pharmacokinetics. Particular attention is required in the future studies to understand the pharmacokinetic–pharmacodynamic relationships of drugs that interact with the endocannabinoid system, to improve translational success.

8. Discussion, conclusions, and future perspectives

Preclinical laboratory animal models provide an opportunity to investigate molecular and cellular changes associated with cannabinoid-based interventions. Pharmacological, electrophysiological, genetic, and optogenetic methodologies can facilitate understanding of the involvement of the endocannabinoid system in neural circuits and the opportunities to modulate the system to interfere with nociception and pain behaviour.

Key differences exist between preclinical and clinical evaluations of cannabinoids, CBMs, and endocannabinoid system modulators. Substantial evidence from the preclinical literature supports the hypothesis of cannabinoid-induced analgesia at multiple levels of analysis,215 whereas there is less evidence of efficacy in human patients with pain and evidence is identified to be of low or very low quality.79 Foremost amongst these differences is that the clinical and preclinical literatures generally evaluate different compounds. For example, preclinical animal studies rarely test cannabis itself, whereas most human studies use whole cannabis or cannabis extracts containing THC and CBD in various ratios or doses and a limited number of other cannabinoids (eg, THC or synthetic THC). Indeed, very few preclinical animal studies have evaluated vaporised/inhaled THC; none of the preclinical studies included in our meta-analysis assessed the effects of THC or cannabis administered in vaporised/inhaled form, and only 38 preclinical studies have administered cannabis extracts via other routes.215 A recent preclinical brain imaging study showed that inhaled vaporised cannabis plant enriched in THC (10.3% THC; 0.05% CBD) uncoupled brain resting-state functional connectivity in the raphe nuclei in paclitaxel-treated rats, normalizing paclitaxel-induced hyperconnectivity to levels observed in vehicle-treated rats, and also produced antinociception in the cold plate test.7 The paucity of such studies may reflect difficulty in administering cannabis plant material to rats and mice in a manner where the dose of THC and other phytocannabinoids administered would be precise. Preclinical animal studies have performed mechanistic evaluations using individual cannabinoids and endocannabinoid system modulators (or sometimes combinations of 2 or more of these) at very well-defined doses and via routes of administration (mostly parenteral) that yield predictable pharmacokinetics.

Second, a much wider variety of cannabinoids and endocannabinoid system modulators have been tested in animal models of pain than have been assessed in human pain patients. This point is underscored by comparing our IASP Task Force clinical and preclinical systemic reviews, which reviewed 1179 and 171215 pharmacological interventions, respectively. Ethical, safety, and methodological standpoints require that novel/experimental drugs are administered in rodent models before human subjects and where mechanism of action can also be identified. Significant barriers to performing clinical research with cannabis and cannabinoids also remain. There are, collectively, over 150 preclinical studies assessing the efficacy of CB2 receptor agonists, endocannabinoid reuptake inhibitors, MGL inhibitors, and FAAH inhibitors in animal models of pain, but not a single randomised controlled clinical trial of the first 3 drug classes, and only 3 randomised controlled clinical trials assessing the efficacy of FAAH inhibitors in patients with pain.19,123,232 This phenomenon, wherein the clinical studies lag significantly behind the preclinical studies, is of course to be expected and is certainly not unique to the cannabinoid field. However, cannabis and cannabinoid use in humans has bypassed the usual preclinical and clinical studies typical of conventional drug development because of the widespread illicit, or more recently licit, use of cannabis by the public. The disparity that exists between the variety of cannabinoids and endocannabinoid system modulators that has been tested in animal models of pain vs what has been assessed in human pain patients should be kept in mind when comparing or contrasting efficacy of specific cannabinoids in animal models vs cannabis in human patients (in addition to translational relevance of the animal model and appropriateness of the clinical indication).

Another key difference between preclinical and clinical studies is that the latter very often administer the cannabinoid or cannabis as adjunctive/add-on treatment, alongside other analgesics that the patient is using. By contrast, preclinical studies in animals almost always investigate the cannabinoid drug against a “clean” background, in the absence of any other analgesics (unless drug–drug interactions are specifically investigated). When administered adjunctively, the “window” or opportunity to see therapeutic efficacy of the cannabinoid/cannabis may well be smaller than if it were investigated against a “clean” background; if the other analgesics are already having partial effects in lowering pain, it may be more difficult for an add-on cannabinoid/cannabis to induce a further measurable reduction superimposed upon the preexisting reduction.

Our accompanying systematic review and meta-analysis provides a comprehensive summary of studies in which cannabinoids, CBMs, and endocannabinoid system modulators were assessed for antinociceptive efficacy in animal models of injury-related or pathological persistent pain.215 The overall behavioural data effect sizes are significant, and it is clear that there is a spectrum of efficacy dependent on drug and pain model type, and also on species, strain, and other experimental factors.

The risk of internal and external validity-related biases confounding these data and exaggerating effect sizes is uncertain, but not insignificant. This could be better ascertained if future preclinical studies were reported with sufficient transparency to ascertain the methods that were used to mitigate against such biases.8,121,140,143,201,203 There is a need for preclinical living systematic reviews that incorporate new evidence as it becomes available,71 and closer, multidisciplinary, cross-sector collaboration to ensure that laboratory animal studies have sufficient rigor to identify promising candidate drugs and more accurately inform clinical trial design.

Several intervention strategies offer the potential for analgesia while also circumventing adverse side effects of direct-acting CB1 agonists. These include cannabinoid CB2 agonists, peripherally restricted CB1 agonists, inhibitors of endocannabinoid deactivation (especially FAAH inhibitors), synergism between cannabinoids and other analgesics, mode of drug delivery (eg, local, targeted delivery), biased agonism, CB1 PAMs, and CBD. Fatty acid amide hydrolase represents a promising target whose engagement is likely to show a reduced pattern of unwanted CB1-mediated effects, and produce anxiolytic effects and opioid-sparing effects without being inherently reinforcing. Moreover, the recent case report of a woman with a mutation in the FAAH gene who does not experience pain or anxiety provides further support that FAAH inhibitors may not only exhibit reduced adverse side effects compared with conventional analgesic but may also exhibit additional desirable therapeutic properties (ie, reduction of stress, fear, and anxiety).64,160 With regards to reduction of abuse liability, CB2 receptor agonists deserve special mention for their potential to produce antiaddiction effects.9,17,130,175,212,249 Cannabidiol represents another promising therapeutic agent that does not produce THC-like effects in humans or animals.95,127,173 The mechanism underlying therapeutic effects of CBD remains incompletely understood.52,95 Preclinical studies, nonetheless, suggest that CBD may attenuate opioid addiction,124,125 THC withdrawal,168 reduce nausea,154,185 suppress inflammation and nociceptive behavior,9,17,43,195,243 and attenuate stress responding.14,16,33,47,72,119,136,149 Further preclinical studies of underlying mechanisms as well as clinical trials of CBD are warranted to better ascertain the therapeutic effects that may be exploited as adjunct or unitary therapy. There is also a pressing requirement to better understand the pharmacology and therapeutic potential of the other phytocannabinoids in the cannabis plant beyond THC and CBD, and whether/how these may interact with THC and CBD in the context of analgesia. Given the substantial interindividual differences in cannabinoid pharmacokinetics among humans, further research is needed to improve the consistency of drug delivery methods, for better translation of preclinical developments to human studies. Preclinical laboratory animal models are evolving to capture aspects associated with pathological pain states observed clinically, eg, spontaneous pain, anxiety, stress, and depression comorbidities, conditions under which dynamic changes in endocannabinoid tone, enzyme activity, or receptors may be observed (for review, see Refs. 42, 76, 80, 90, 165, and 187). Consequently, future preclinical studies in laboratory animals, which evaluate novel cannabinoid interventions, reference analgesics, as well as drug development candidates that previously failed for efficacy, are required to validate treatments, understand mechanisms of action, and enhance clinical translation.

Conflict of interest statement

D. P. Finn reports an Industry-Academia research grant from Alkermes Inc and Science Foundation Ireland outside of the submitted work. He also reports research grants in the area of cannabinoids or the endocannabinoid system from Shionogi Ltd (Shionogi Science Programme), from B. Braun Ltd jointly with Science Foundation Ireland, and from the Irish Research Council, CNPq Brazil and EU INTERREG Programmes. S. Haroutounian reports grants from Disarm Therapeutics, and advisory board/consultancy fees from Vertex Pharmaceuticals, Medoc Ltd, and Rafa laboratories, outside the submitted work. A.G. Hohmann reports grants from the National Institutes of Health (National Institute on Drug Abuse, National Cancer Institute, National Institute of Neurological Diseases and Stroke), Indiana Addiction Grand Challenge, and is a coinventor on a provisional patent related to CB2-opioid interactions. A. S.C. Rice is a Council Member of IASP and Chair of the Presidential Task Force of the IASP, undertook consultancy and advisory board work for Imperial College Consultants in the last 24 months; this has included personally remunerated work outside of the submitted work for Abide, Confo, Vertex, Pharmanovo, Lateral, Novartis, Mundipharma, Orion, Asahi Kasei, Toray & Theranexis. He was the owner of share options in Spinifex Pharmaceuticals from which personal benefit accrued between 2015 and 2019 upon the acquisition of Spinifex by Novartis. A.S.C. Rice is a named inventor on the patents—A. S.C. Rice, S. Vandevoorde, and D.M. Lambert. Methods using N-(2propenyl)hexadecanamide and related amides to relieve pain. WO2005/079771 pending, and Okuse K. et al. Methods of treating pain by inhibition of vgf activity EP13702262.0/WO2013110945. During the conduct of the study Imperial College received grants funding to support Prof Rice's programme of research from Biotechnology and Biological Sciences Research Council (BBSRC), Medical Research Council (MRC), Wellcome Trust, Alan and Sheila Diamond Charitable Trust, British Pain Society, Royal British Legion and the European Commission (IMI2 [EQIPD]; FP7 [Neuropain] and H2020 [Dolorisk]). The remaining authors have no conflicts of interest to declare.

Acknowledgements

This work is part of the effort of the IASP Presidential Task Force on cannabis and cannabinoid analgesia. The authors would like to thank Dr Emer Power and Darragh Mattimoe for assistance with preparation of figures, and Dr Christopher Eccleston and Dr Emma Fisher for comments on the manuscript.

The International Association for the Study of Pain commissioned this work in the form of a Presidential Task Force and funded attendance for the authors at a working meeting in Washington DC, November 2019.

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Keywords:

Cannabinoid1 (CB1) receptor; Cannabinoid2 (CB2) receptor; Endocannabinoid; Chronic pain; Neuropathic pain; Inflammatory pain; Nociception; Rats; Mice; Behavior

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