Assessing efficacy of non-opioid analgesics in experimental pain models in healthy volunteers: an updated review

Discovery of a novel series of pyridone amides as NaV1.8 inhibitors

The NaV1.8 channel, mainly found in the peripheral nervous system, is recognized as one of the key factors in chronic pain. The molecule VX-150 was initially promising in targeting this channel, but the phase II trials of VX-150 did not show expected pain relief results. By analyzing the interaction mode of VX-150 and NaV1.8, we developed two series with a total of 19 molecules and examined their binding affinity to NaV1.8 in vitro and analgesic effect in vivo. One compound, named 2j, stood out with notable activity against the NaV1.8 channel and showed effective pain relief in models of chronic inflammatory pain and neuropathic pain. Our research points to 2j as a strong contender for developing safer pain-relief treatments.

Discovery of selective NaV1.8 inhibitors based on 5-chloro-2-(4,4-difluoroazepan-1-yl)-6-methyl nicotinamide scaffold for the treatment of pain

The Na_V1.8 channel is a genetically validated target for pain and it is mostly expressed in the peripheral nervous system. Based on the disclosed structures of Na_V1.8-selective inhibitors, we designed and synthesized a series of compounds by introducing bicyclic aromatic fragments based on the nicotinamide scaffold. In this research, a systematic structure-activity relationship study was carried out. While compound 2c possessed moderate inhibitory activity (IC₅₀ = 50.18 ± 0.04 nM) in HEK293 cells stably expressing human Na_V1.8 channels, it showed potent inhibitory activity in DRG neurons and isoform selectivity (>200-fold against human Na_V1.1, Na_V1.5 and Na_V1.7 channels). Moreover, the analgesic potency of compound 2c was identified in a post-surgical mouse model. These data demonstrate that compound 2c can be further evaluated as a non-addictive analgesic agent with reduced cardiac liabilities.

Inflammation differentially controls transport of depolarizing Nav versus hyperpolarizing Kv channels to drive rat nociceptor activity

Inflammation causes pain by shifting the balance of ionic currents in nociceptors toward depolarization, leading to hyperexcitability. The ensemble of ion channels within the plasma membrane is regulated by processes including biogenesis, transport, and degradation. Thus, alterations in ion channel trafficking may influence excitability. Sodium channel NaV1.7 and potassium channel KV7.2 promote and oppose excitability in nociceptors, respectively. We used live-cell imaging to investigate mechanisms by which inflammatory mediators (IM) modulate the abundance of these channels at axonal surfaces through transcription, vesicular loading, axonal transport, exocytosis, and endocytosis. Inflammatory mediators induced a NaV1.7-dependent increase in activity in distal axons. Further, inflammation increased the abundance of NaV1.7, but not of KV7.2, at axonal surfaces by selectively increasing channel loading into anterograde transport vesicles and insertion at the membrane, without affecting retrograde transport. These results uncover a cell biological mechanism for inflammatory pain and suggest NaV1.7 trafficking as a potential therapeutic target.

The fates of internalized NaV1.7 channels in sensory neurons: Retrograde cotransport with other ion channels, axon-specific recycling, and degradation

Neuronal function relies on the maintenance of appropriate levels of various ion channels at the cell membrane, which is accomplished by balancing secretory, degradative, and recycling pathways. Neuronal function further depends on membrane specialization through polarized distribution of specific proteins to distinct neuronal compartments such as axons. Voltage-gated sodium channel NaV1.7, a threshold channel for firing action potentials in nociceptors, plays a major role in human pain, and its abundance in the plasma membrane is tightly regulated. We have recently characterized the anterograde axonal trafficking of NaV1.7 channels in Rab6A-positive vesicles, but the fate of internalized channels is not known. Membrane proteins that have undergone endocytosis can be directed into multiple pathways including those for degradation, recycling to the membrane, and transcytosis. Here, we demonstrate NaV1.7 endocytosis and dynein-dependent retrograde trafficking in Rab7-containing late endosomes together with other axonal membrane proteins using real-time imaging of live neurons. We show that some internalized NaV1.7 channels are delivered to lysosomes within the cell body, and that there is no evidence for NaV1.7 transcytosis. In addition, we show that NaV1.7 is recycled specifically to the axonal membrane as opposed to the soma membrane, suggesting a novel mechanism for the development of neuronal polarity. Together, these results shed light on the mechanisms by which neurons maintain excitable membranes and may inform efforts to target ion channel trafficking for the treatment of disorders of excitability.

Feasibility of quantitative sensory testing in juvenile idiopathic arthritis

OBJECTIVE

Juvenile Idiopathic Arthritis (JIA) is a childhood-rheumatic disease with pain as a major early complaint, and in 10-17% pain remains a major symptom. Very few data exist on sensory threshold changes at the knee in JIA, a location in which inflammation often manifests. We determined whether JIA is associated with sensory threshold changes at the knee by using Quantitative Sensory Testing (QST) and established reference values at the knee of children.

METHODS

Sixteen patients with JIA aged 9-18 years with one affected knee and a patient-reported pain by Visual Analog Scale (VAS) > 10 on a 0-100 scale, and 16 healthy controls completed the study and were included for the analysis. QST was assessed in compliance with the German Research Network on Neuropathic Pain (DFNS) standard. Disease severity was determined using Juvenile Disease Activity Score (JADAS. Perceived pain was assessed with a visual analogue scale(0-100). Feasibility of QST was tested in patients aged 6-9.

RESULTS

Under the age of 9, QST testing showed not to be feasible in 3 out of 5 JIA patients. Patients with JIA aged 9 and older reported an average VAS pain score of 54.3. QST identified a significant reduction in pressure pain threshold (PPT) and increase in cold detection threshold (CDT) compared to healthy controls. PPT is reduced in both the affected and the unaffected knee, CDT is reduced in the unaffected knee, not the affected knee.

CONCLUSION

In a Dutch cohort of Patients with JIA, QST is only feasible from 9 years and up. Also, sensory threshold changes at the knee are restricted to pressure pain and cold detection thresholds in Patients with JIA.

PERSPECTIVE

This article shows that in a Dutch population, the extensive QST protocol is only feasible in the age group from 9 years and older, and a reduced set of QST tests containing at least pressure pain thresholds and cold detection thresholds could prove to be better suited to the pediatric setting with arthritis.

Depolarizing NaV and Hyperpolarizing KV Channels Are Co-Trafficked in Sensory Neurons

Neuronal excitability relies on coordinated action of functionally distinction channels. Voltage-gated sodium (NaV) and potassium (KV) channels have distinct but complementary roles in firing action potentials: NaV channels provide depolarizing current while KV channels provide hyperpolarizing current. Mutations and dysfunction of multiple NaV and KV channels underlie disorders of excitability, including pain and epilepsy. Modulating ion channel trafficking may offer a potential therapeutic strategy for these diseases. A fundamental question, however, is whether these channels with distinct functional roles are transported independently or packaged together in the same vesicles in sensory axons. We have used Optical Pulse-Chase Axonal Long-distance imaging to investigate trafficking of NaV and KV channels and other axonal proteins from distinct functional classes in live rodent sensory neurons (from male and female rats). We show that, similar to NaV1.7 channels, NaV1.8 and KV7.2 channels are transported in Rab6a-positive vesicles, and that each of the NaV channel isoforms expressed in healthy, mature sensory neurons (NaV1.6, NaV1.7, NaV1.8, and NaV1.9) is cotransported in the same vesicles. Further, we show that multiple axonal membrane proteins with different physiological functions (NaV1.7, KV7.2, and TNFR1) are cotransported in the same vesicles. However, vesicular packaging of axonal membrane proteins is not indiscriminate, since another axonal membrane protein (NCX2) is transported in separate vesicles. These results shed new light on the development and organization of sensory neuron membranes, revealing complex sorting of axonal proteins with diverse physiological functions into specific transport vesicles.SIGNIFICANCE STATEMENT Normal neuronal excitability is dependent on precise regulation of membrane proteins, including NaV and KV channels, and imbalance in the level of these channels at the plasma membrane could lead to excitability disorders. Ion channel trafficking could potentially be targeted therapeutically, which would require better understanding of the mechanisms underlying trafficking of functionally diverse channels. Optical Pulse-chase Axonal Long-distance imaging in live neurons permitted examination of the specificity of ion channel trafficking, revealing co-packaging of axonal proteins with opposing physiological functions into the same transport vesicles. This suggests that additional trafficking mechanisms are necessary to regulate levels of surface channels, and reveals an important consideration for therapeutic strategies that target ion channel trafficking for the treatment of excitability disorders.

The Involvement of Caspases in Neuroinflammation and Neuronal Apoptosis in Chronic Pain and Potential Therapeutic Targets

Chronic pain is a common, complex and unpleasant sensation following nerve injury, tissue trauma, inflammatory diseases, infection and cancer. It affects up to 25% of adults and is increasingly recognized as the leading cause of distress, disability and disease burden globally. Chronic pain is often refractory to most current analgesics, thus emphasizing the requirement for improved therapeutic medications. It is of great importance to elucidate the specific pathogenesis of chronic pain with different etiologies. Recent progress has advanced our understanding in the contribution of neuroinflammation and glial cells (microglia and astrocyte) activation in the plasticity of excitatory nociceptive synapses and the development of chronic pain phenotypes. Oxidative stress-associated neuronal apoptosis is also identified to be a pivotal step for central pain sensitization. The family of cysteine aspartate specific proteases (Caspases) has been well known to be key signaling molecules for inflammation and apoptosis in several neurological conditions. Recent studies have highlighted the unconventional and emerging role of caspases in microgliosis, astrocytes morphogenesis, chemokines release, cytokines secretion and neuronal apoptosis in initiating and maintaining synaptogenesis, synaptic strength and signal transduction in persistent pain hypersensitivity, suggesting the possibility of targeting caspases pathway for prevention and treatment of chronic pain. In this review, we will discuss and summarize the advances in the distinctive properties of caspases family in the pathophysiology of chronic pain, especially in neuropathic pain, inflammatory pain, cancer pain and musculoskeletal pain, with the aim to find the promising therapeutic candidates for the resolution of chronic pain to better manage patients undergoing chronic pain in clinics.

Tilidine and dipyrone (metamizole) in cold pressor pain: A pooled analysis of efficacy, tolerability, and safety in healthy volunteers

The cold pressor test (CPT) is widely implemented and offers a simple, experimental acute pain model utilizing cold pain. Previous trials have frequently paired the CPT with opioids in order to investigate the mechanisms underlying pharmacological analgesia, due to their known analgesic efficacy. However, opioid side effects may lead to unblinding and raise concerns about the safety of the experimental setting. Despite the established clinical efficacy of dipyrone (metamizole), its efficacy, tolerability, and safety in cold pressor pain has not been systematically addressed to date. This pooled analysis included data of 260 healthy volunteers from three randomized, placebo‐controlled, double‐blind substudies using the CPT following a pre‐test‐post‐test‐design. These substudies allow for comparing a single dose of 800 mg dipyrone with two different doses of the opioid tilidine/naloxone (50/4 mg and 100/8 mg, respectively). Outcomes included pain intensity ratings, pain tolerance, medication‐attributed side effects, as well as changes of blood pressure and heart rate. We demonstrate that both opioid doses and dipyrone had a comparable, significant analgesic effect on cold pressor pain. However, dipyrone was associated with significantly less self‐reported adverse effects and these were not significantly different from those under placebo. These results indicate that the combination of dipyrone and the CPT provides a safe, tolerable, and effective experimental model for the study of pharmacological analgesia. In combination with a CPT, dipyrone may be useful as a positive control, or baseline medication for the study of analgesic modulation.

Status of peripheral sodium channel blockers for non-addictive pain treatment

The effective and safe treatment of pain is an unmet health-care need. Current medications used for pain management are often only partially effective, carry dose-limiting adverse effects and are potentially addictive, highlighting the need for improved therapeutic agents. Most common pain conditions originate in the periphery, where dorsal root ganglion and trigeminal ganglion neurons feed pain information into the CNS. Voltage-gated sodium (Na_V) channels drive neuronal excitability and three subtypes - Na_V1.7, Na_V1.8 and Na_V1.9 - are preferentially expressed in the peripheral nervous system, suggesting that their inhibition might treat pain while avoiding central and cardiac adverse effects. Genetic and functional studies of human pain disorders have identified Na_V1.7, Na_V1.8 and Na_V1.9 as mediators of pain and validated them as targets for pain treatment. Consequently, multiple Na_V1.7-specific and Na_V1.8-specific blockers have undergone clinical trials, with others in preclinical development, and the targeting of Na_V1.9, although hampered by technical constraints, might also be moving ahead. In this Review, we summarize the clinical and preclinical literature describing compounds that target peripheral Na_V channels and discuss the challenges and future prospects for the field. Although the potential of peripheral Na_V channel inhibition for the treatment of pain has yet to be realized, this remains a promising strategy to achieve non-addictive analgesia for multiple pain conditions.

Reproducibility of a battery of human evoked pain models to detect pharmacological effects of analgesic drugs

BACKGROUND

Although reproducibility is considered essential for any method used in scientific research, it is investigated only rarely; thus, strikingly little has been published regarding the reproducibility of evoked pain models involving human subjects. Here, we studied the reproducibility of a battery of evoked pain models for demonstrating the analgesic effects of two analgesic compounds.

METHODS

A total of 81 healthy subjects participated in four studies involving a battery of evoked pain tests in which mechanical, thermal and electrical stimuli were used to measure pain detection and tolerance thresholds. Pharmacodynamic outcome variables were analysed using a mixed model analysis of variance, and a coefficient of variation was calculated by dividing the standard deviation by the least squares means.

RESULTS

A total of 76 subjects completed the studies. After being administered pregabalin, the subjects' pain tolerance thresholds in the cold pressor and pressure stimulation tests were significantly increased compared to the placebo group. Moreover, the heat pain detection threshold in UVB-irradiated skin was significantly increased in subjects who were administered ibuprofen compared to the placebo group. Variation among all evoked pain tests ranged from 2.2% to 30.6%.

CONCLUSIONS

Four studies using a similar design showed reproducibility with respect to the included evoked pain models. The relatively high consistency and reproducibility of two analgesics at doses known to be effective in treating clinically relevant pain supports the validity of using this pain test battery to investigate the analgesic activity and determine the active dosage of putative analgesic compounds in early clinical development.

SIGNIFICANCE

The consistency and reproducibility of measuring the profile of an analgesic at clinically relevant doses illustrates that this pain test battery is a valid tool for demonstrating the analgesic activity of a test compound and for determining the optimal active dose in early clinical drug development.

Challenges in translational drug research in neuropathic and inflammatory pain: the prerequisites for a new paradigm

AIM

Despite an improved understanding of the molecular mechanisms of nociception, existing analgesic drugs remain limited in terms of efficacy in chronic conditions, such as neuropathic pain. Here, we explore the underlying pathophysiological mechanisms of neuropathic and inflammatory pain and discuss the prerequisites and opportunities to reduce attrition and high-failure rate in the development of analgesic drugs.

METHODS

A literature search was performed on preclinical and clinical publications aimed at the evaluation of analgesic compounds using MESH terms in PubMed. Publications were selected, which focused on (1) disease mechanisms leading to chronic/neuropathic pain and (2) druggable targets which are currently under evaluation in drug development. Attention was also given to the role of biomarkers and pharmacokinetic-pharmacodynamic modelling.

RESULTS

Multiple mechanisms act concurrently to produce pain, which is a non-specific manifestation of underlying nociceptive pathways. Whereas these manifestations can be divided into neuropathic and inflammatory pain, it is now clear that inflammatory mechanisms are a common trigger for both types of pain. This has implications for drug development, as the assessment of drug effects in experimental models of neuropathic and chronic pain is driven by overt behavioural measures. By contrast, the use of mechanistic biomarkers in inflammatory pain has provided the pharmacological basis for dose selection and evaluation of non-steroidal anti-inflammatory drugs (NSAIDs).

CONCLUSION

A different paradigm is required for the identification of relevant targets and candidate molecules whereby pain is coupled to the cause of sensorial signal processing dysfunction rather than clinical symptoms. Biomarkers which enable the characterisation of drug binding and target activity are needed for a more robust dose rationale in early clinical development. Such an approach may be facilitated by quantitative clinical pharmacology and evolving technologies in brain imaging, allowing accurate assessment of target engagement, and prediction of treatment effects before embarking on large clinical trials.

Investigation of the predictive validity of laser-EPs in normal, UVB-inflamed and capsaicin-irritated skin with four analgesic compounds in healthy volunteers

AIMS

The aim of the present study was to assess the predictivity of laser-(radiant-heat)-evoked potentials (LEPs) from the vertex electroencephalogram, using an algesimetric procedure, testing the anti-nociceptive/anti-hyperalgesic effects of single oral doses of four marketed analgesics (of different compound classes) vs. placebo, in healthy volunteers with three skin types.

METHODS

This was a randomized, placebo-controlled, single-blind, five-way-crossover trial. Twenty-five healthy male/female Caucasians were included (receiving celecoxib 200 mg, pregabalin 150 mg, duloxetine 60 mg, lacosamide 100 mg or placebo) in a Williams design, with CO₂ laser-induced painful stimuli to normal, ultraviolet (UV) B-inflamed and capsaicin-irritated skin. LEPs and visual analogue scale ratings were taken at baseline and hourly for 6 h postdose from all three skin types.

RESULTS

In normal skin, the averaged postdose LEP peak-to-peak-(PtP)-amplitudes were reduced by pregabalin (-2.68 μV; 95% confidence interval (CI) -4.16, 1.19) and duloxetine (-1.73 μV; 95% CI -3.21, -0.26) but not by lacosamide and celecoxib vs. placebo. On UVB-irradiated skin, reflecting inflammatory pain, celecoxib induced a pronounced reduction in LEP PtP amplitudes vs. placebo (-6.2 μV; 95% CI -7.88, -4.51), with a smaller reduction by duloxetine (-4.54 μV; 95% CI -6.21, -2.87) and pregabalin (-3.72 μV; 95% CI -5.40, -2.04), whereas lacosamide was inactive. LEP PtP amplitudes on capsaicin-irritated skin, reflecting peripheral/spinal sensitization, as in neuropathic pain, were reduced by pregabalin (-3.78 μV; 95% CI -5.31, -2.25) and duloxetine (-2.32 μV; 95% CI -3.82, -0.82) but not by celecoxib or lacosamide vs. placebo, which was in agreement with known clinical profiles. Overall, PtP amplitude reductions were in agreement with subjective ratings.

CONCLUSIONS

LEP algesimetry is sensitive to analgesics with different modes of action and may enable the effects of novel analgesics to be assessed during early clinical development.

Heat pain detection threshold is associated with the area of secondary hyperalgesia following brief thermal sensitization: a study of healthy male volunteers

Introduction

The area of secondary hyperalgesia following brief thermal sensitization (BTS) of the skin and heat pain detection thresholds (HPDT) may both have predictive abilities in regards to pain sensitivity and clinical pain states. The association between HPDT and secondary hyperalgesia, however, remains unsettled, and the dissimilarities in physiologic properties suggest that they may represent 2 distinctively different pain entities. The aim of this study was to investigate the association between HPDT and BTS-induced secondary hyperalgesia.

Methods

A sample of 121 healthy male participants was included and tested on 2 separate study days with BTS (45°C, 3 minutes), HPDT, and pain during thermal stimulation (45°C, 1 minute). Areas of secondary hyperalgesia were quantified after monofilament pinprick stimulation. The pain catastrophizing scale (PCS) and hospital anxiety and depression scale (HADS) were also applied.

Results

A significant association between HPDT and the size of the area of secondary hyperalgesia (p<0.0001) was found. The expected change in area of secondary hyperalgesia due to a 1-degree increase in HPDT was estimated to be −27.38 cm², 95% confidence interval (CI) of −37.77 to −16.98 cm², with an R² of 0.19. Likewise, a significant association between HADS-depression subscore and area of secondary hyperalgesia (p=0.046) was found, with an estimated expected change in secondary hyperalgesia to a 1-point increase in HADS-depression subscore of 11 cm², 95% CI (0.19–21.82), and with R² of 0.03. We found no significant associations between secondary hyperalgesia area and PCS score or pain during thermal stimulation.

Conclusion

HPDT and the area of secondary hyperalgesia after BTS are significantly associated; however, with an R² of only 19%, HPDT only offers a modest explanation of the inter-participant variation in the size of the secondary hyperalgesia area elicited by BTS.

Quantitative sensory testing and pain-evoked cytokine reactivity: comparison of patients with sickle cell disease to healthy matched controls

Sickle cell disease (SCD) is an inherited blood disorder associated with significant morbidity, which includes severe episodic pain, and, often, chronic pain. Compared to healthy individuals, patients with SCD report enhanced sensitivity to thermal detection and pain thresholds and have altered inflammatory profiles, yet no studies to date have examined biomarker reactivity after laboratory-induced pain. We sought to examine this relationship in patients with SCD compared to healthy control participants. We completed quantitative sensory testing in 83 patients with SCD and sequential blood sampling in 27 of them, whom we matched (sex, age, race, body mass index, and education) to 27 healthy controls. Surprisingly, few quantitative sensory testing differences emerged between groups. Heat pain tolerance, pressure pain threshold at the trapezius, thumb, and quadriceps, and thermal temporal summation at 45°C differed between groups in the expected direction, whereas conditioned pain modulation and pain ratings to hot water hand immersion were counterintuitive, possibly because of tailoring the water temperature to a perceptual level; patients with SCD received milder temperatures. In the matched subsample, group differences and group-by-time interactions were observed in biomarkers including tumor necrosis factor alpha, interleukin-1ß, interleukin-4, and neuropeptide Y. These findings highlight the utility of laboratory pain testing methods for understanding individual differences in inflammatory cytokines. Our findings suggest amplified pain-evoked proinflammatory cytokine reactivity among patients with SCD relative to carefully matched controls. Future research is warranted to evaluate the impact of enhanced pain-related cytokine response and whether it is predictive of clinical characteristics and the frequency/severity of pain crises in patients with SCD.

The use of a battery of pain models to detect analgesic properties of compounds: a two-part four-way crossover study

AIM

The aim was to investigate the ability of a battery of pain models to detect analgesic properties of commonly used analgesics in healthy subjects.

METHODS

The battery consisted of tests eliciting electrical, mechanical and thermal (contact heat and cold pressor)-pain and included a UVB model, the thermal grill illusion and a paradigm of conditioned pain modulation. Subjects were administered fentanyl 3 μg kg^-1 , phenytoin 300 mg, (S)-ketamine 10 mg and placebo (part I), or imipramine 100 mg, pregabalin 300 mg, ibuprofen 600 mg and placebo (part II). Pain measurements were performed at baseline and up to 10 h post-dose. Endpoints were analysed using a mixed model analysis of variance.

RESULTS

Sixteen subjects (8 female) completed each part. The pain tolerance threshold (PTT) for electrical stimulation was increased (all P < 0.05) compared to placebo for (S)-ketamine (+10.1%), phenytoin (+8.5%) and pregabalin (+10.8%). The PTT for mechanical pain was increased by pregabalin (+14.1%). The cold pressor PTT was increased by fentanyl (+17.1%) and pregabalin (+46.4%). Normal skin heat pain detection threshold was increased by (S)-ketamine (+3.3%), fentanyl (+2.8%) and pregabalin (+4.1%). UVB treated skin pain detection threshold was increased by fentanyl (+2.6%) and ibuprofen (+4.0%). No differences in conditioned pain modulation were observed.

CONCLUSION

This study shows that these pain models are able to detect changes in pain thresholds after administration of different classes of analgesics in healthy subjects. The analgesic compounds all showed a unique profile in their effects on the pain tasks administered.

Pharmacokinetics and pharmacodynamics of intrathecally administered Xen2174, a synthetic conopeptide with norepinephrine reuptake inhibitor and analgesic properties

AIM

Xen2174 is a synthetic 13-amino acid peptide that binds specifically to the norepinephrine transporter, which results in inhibition of norepinephrine uptake. It is being developed as a possible treatment for moderate to severe pain and is delivered intrathecally. The current study was performed to assess the pharmacodynamics (PD) and the cerebrospinal fluid (CSF) pharmacokinetics (PK) of Xen2174 in healthy subjects.

METHODS

This was a randomized, blinded, placebo-controlled study in healthy subjects. The study was divided into three treatment arms. Each group consisted of eight subjects on active treatment and two or three subjects on placebo. The CSF was sampled for 32 h using an intrathecal catheter. PD assessments were performed using a battery of nociceptive tasks (electrical pain, pressure pain and cold pressor tasks).

RESULTS

Twenty-five subjects were administered Xen2174. CSF PK analysis showed a higher area under the CSF concentration-time curve of Xen2174 in the highest dose group than allowed by the predefined safety margin based on nonclinical data. The most common adverse event was post-lumbar puncture syndrome, with no difference in incidence between treatment groups. Although no statistically significant differences were observed in the PD assessments between the different dosages of Xen2174 and placebo, pain tolerability in the highest dose group was higher than in the placebo group [contrast least squares mean pressure pain tolerance threshold of Xen2174 2.5 mg-placebo (95% confidence interval), 22.2% (-5.0%, 57.1%); P = 0.1131].

CONCLUSIONS

At the Xen2174 dose level of 2.5 mg, CSF concentrations exceeded the prespecified exposure limit based on the nonclinical safety margin. No statistically significant effects on evoked pain tests were observed.

No evidence of potentiation of buprenorphine by milnacipran in healthy subjects using a nociceptive test battery

BACKGROUND

Serotonin-norepinephrine reuptake inhibitors inhibit the reuptake of serotonin and noradrenalin and are used in the treatment of neuropathic pain. Animal studies suggest that milnacipran co-administered with opioids may potentiate the analgesic effect of μ-opioid receptor agonists. This study hypothesized that co-administration of milnacipran and buprenorphine would have a synergistic effect in evoked pain models in healthy subjects.

METHODS

This was a randomized double-blinded, placebo-controlled, four-way cross-over, multiple dose clinical trial to investigate the analgesic effects of buprenorphine (placebo, 0.5, 1 and 3 μg/kg) in combination with milnacipran (placebo, 25 and 50 mg) in healthy subjects.

RESULTS

11 healthy men were enrolled in the study. Buprenorphine alone showed a dose-response relationship indicative of anti-nociception in the pain tests. Following milnacipran administration, no changes were seen in the pharmacodynamic measurements for pain, psychomotor function, body stability or eye movements. For the electrical tests, cold pressor test and pressure pain test, buprenorphine alone was superior when compared with buprenorphine plus milnacipran. No differences in pharmacodynamic variables, besides an increase in pupil/iris ratio, were observed after repeated administration of milnacipran 50 mg. Single and multiple doses of 25 or 50 mg milnacipran did not further potentiate the anti-nociceptive effects of buprenorphine.

CONCLUSIONS

Buprenorphine showed dose-dependent effects consistent with its pharmacological profile. Milnacipran alone did not affect any of the pain variables. The combination of both buprenorphine and milnacipran did not potentiate or show a synergistic effect on the pain models used in this study.

SIGNIFICANCE

Buprenorphine is known to be a potent opioid agonist. Animal studies suggest that milnacipran co-administered with opioids may potentiate the analgesic effect of μ-opioid receptor agonists. Here, we found that buprenorphine showed a dose-dependent analgesic effect, but that no potentiation or synergy on a battery of evoked pain tasks could be observed after co-administration of both milnacipran and buprenorphine.

Is the Volume of the Caudate Nuclei Associated With Area of Secondary Hyperalgesia? - Protocol for a 3-Tesla MRI Study of Healthy Volunteers

Background

Experience and development of pain may be influenced by a number of physiological, psychological, and psychosocial factors. In a previous study we found differences in neuronal activation to noxious stimulation, and microstructural neuroanatomical differences, when comparing healthy volunteers with differences in size of the area of secondary hyperalgesia following a standardized burn injury.

Objective

We aim to investigate the degree of association between the volume of pain-relevant structures in the brain and the size of the area of secondary hyperalgesia following brief thermal sensitization.

Methods

The study consists of one experimental day, in which whole-brain magnetic resonance imaging (MRI) scans will be conducted including T1-weighed three-dimensional anatomy scan, diffusion tensor imaging, and resting state functional MRI. Before the experimental day, all included participants will undergo experimental pain testing in a parallel study (Clinicaltrials.gov Identifier: NCT02527395). Results from this experimental pain testing, as well as the size of the area of secondary hyperalgesia from the included participants, will be extracted from this parallel study.

Results

The association between the volume of pain-relevant structures in the brain and the area of secondary hyperalgesia will be investigated by linear regression of the estimated best linear unbiased predictors on the individual volumes of the pain relevant brain structures.

Conclusions

We plan to investigate the association between experimental pain testing parameters and the volume, connectivity, and resting state activity of pain-relevant structures in the brain. These results may improve our knowledge of the mechanisms responsible for the development of acute and chronic pain.

ClinicalTrial

Danish Research Ethics Committee (identifier: H-15010473). Danish Data Protection Agency (identifier: RH-2015-149). Clinicaltrials.gov NCT02567318; http://clinicaltrials.gov/ct2/show/NCT02567318 (Archived by WebCite at http://www.webcitation.org/6i4OtP0Oi)

Is heat pain detection threshold associated with the area of secondary hyperalgesia following brief thermal sensitization? A study of healthy volunteers - design and detailed plan of analysis

Background

Several factors are believed to influence the development and experience of pain.

Human clinical pain models are central tools, in the investigation of basic physiologic pain responses, and can be applied in patients as well as in healthy volunteers. Each clinical pain model investigates different aspects of the human pain response.

Brief thermal sensitization induces a mild burn injury, resulting in development of primary hyperalgesia at the site of stimulation, and secondary hyperalgesia surrounding the site of stimulation. Central sensitization is believed to play an important role in the development of secondary hyperalgesia; however, a possible association of secondary hyperalgesia following brief thermal sensitization and other heat pain models remains unknown. Our aim with this study is to investigate how close the heat pain detection threshold is associated with the size of the area of secondary hyperalgesia induced by the clinical heat pain model: Brief thermal sensitization.

Methods and design

We aim to include 120 healthy participants. The participants will be tested on two separate study days with the following procedures: i) Brief thermal sensitization, ii) heat pain detection threshold and iii) pain during thermal stimulation. Additionally, the participants will be tested with the Pain Catastrophizing Scale and Hospital Anxiety and Depression Scale questionnaires.

We conducted statistical simulations based on data from our previous study, to estimate an empirical power of 99.9 % with α of 0.05. We define that an R² < 0.25 and predictive intervals larger than +/−150 cm² are indications of a weak association.

Discussion

The area of secondary hyperalgesia may serve as a quantitative measure of the central sensitization induced by cutaneous heat stimulation, and thus may be a biomarker of an individual’s pain sensitivity. The number of studies investigating secondary hyperalgesia is growing; however basic knowledge of the physiologic aspects of secondary hyperalgesia in humans is still incomplete. We therefore find it interesting to investigate if HPDT, a known quantitative sensory test, is associated with areas of secondary hyperalgesia following brief thermal sensitization

Trial registration

Clinicaltrials.gov (Identifier: NCT02527395). Danish Research Ethics Committee (Identifier: H-8-2014-012). Danish Data Protection Agency (Identifier: 30-1436).

Electronic supplementary material

The online version of this article (doi:10.1186/s12871-016-0193-2) contains supplementary material, which is available to authorized users.

Effect of Intravenous Ethanol on Capsaicin-Induced Hyperalgesia in Human Subjects

BACKGROUND

The objective of this study was to assess ethanol's (EtOH's) effects on capsaicin-induced hyperalgesia in healthy participants. Specifically, we investigated the change in area of capsaicin-induced hyperalgesia following 3 interventions: intravenous EtOH at 2 targeted breath alcohol concentrations (BrAC), or placebo.

METHODS

Eighteen participants participated in 3 test days in a randomized order. Each test day, participants received an intradermal capsaicin injection on the volar surface of the forearm, followed by either infusion of high concentration EtOH (targeted BrAC = 0.100 g/dl), low concentration EtOH (targeted BrAC = 0.040 g/dl), or placebo. The area of hyperalgesia was determined by von Frey technique at 2 time points, prior to EtOH infusion, and again when target BrAC was reached. The primary outcome was the percent change in the area of capsaicin-induced hyperalgesia. Additional outcome measures included the visual analogue scale of mood states (VAS), which was administered at each time point.

RESULTS

There was a marked 30% reduction in the area of capsaicin-induced hyperalgesia with infusion of a high concentration of EtOH (p < 0.05). Low concentration EtOH produced a 10% reduction in hyperalgesia area, although this finding did not reach significance. Further, participants reported significant feelings of euphoria and drowsiness at high concentrations of EtOH (p < 0.05), as measured by the VAS.

CONCLUSIONS

In a human model examining pain phenomena related to central sensitization, this study is the first to demonstrate that capsaicin-induced hyperalgesia is markedly attenuated by EtOH. The capsaicin experimental pain paradigm employed provides a novel approach to evaluate EtOH's effects on pain processing. The antihyperalgesic effects of EtOH observed have important clinical implications for the converging fields of substance abuse and pain medicine and may inform why patients with chronic pain often report alcohol use as a form of self-medication.

Determining Pain Detection and Tolerance Thresholds Using an Integrated, Multi-Modal Pain Task Battery

Human pain models are useful in the assessing the analgesic effect of drugs, providing information about a drug's pharmacology and identify potentially suitable therapeutic populations. The need to use a comprehensive battery of pain models is highlighted by studies whereby only a single pain model, thought to relate to the clinical situation, demonstrates lack of efficacy. No single experimental model can mimic the complex nature of clinical pain. The integrated, multi-modal pain task battery presented here encompasses the electrical stimulation task, pressure stimulation task, cold pressor task, the UVB inflammatory model which includes a thermal task and a paradigm for inhibitory conditioned pain modulation. These human pain models have been tested for predicative validity and reliability both in their own right and in combination, and can be used repeatedly, quickly, in short succession, with minimum burden for the subject and with a modest quantity of equipment. This allows a drug to be fully characterized and profiled for analgesic effect which is especially useful for drugs with a novel or untested mechanism of action.

Effect of a high-dose target-controlled naloxone infusion on pain and hyperalgesia in patients following groin hernia repair: study protocol for a randomized controlled trial

Background

Central sensitization is modulated by the endogenous opioid system and plays a major role in the development and maintenance of pain. Recent animal studies performed following resolution of inflammatory pain showed reinstatement of tactile hypersensitivity induced by administration of a mu-opioid-antagonist, suggesting latent sensitization is mediated by endogenous opioids. In a recent crossover study in healthy volunteers, following resolution of a first-degree burn, 4 out of 12 volunteers developed large secondary areas of hyperalgesia areas after a naloxone infusion, while no volunteer developed significant secondary hyperalgesia after the placebo infusion. In order to consistently demonstrate latent sensitization in humans, a pain model inducing deep tissue inflammation, as used in animal studies, might be necessary. The aim of the present study is to examine whether a high-dose target-controlled naloxone infusion can reinstate pain and hyperalgesia following recovery from open groin hernia repair and thus consistently demonstrate opioid-mediated latent sensitization in humans.

Methods/Design

Patients submitted to unilateral, primary, open groin hernia repair will be included in this randomized, placebo-controlled, double-blind, crossover study. The experimental days take place 6–8 weeks after surgery, time-points at which patients are expected to be almost pain- free. Prior to administration of naloxone or placebo, the primary outcome (a summated measure of pain: at rest, during transition from supine to standing position, and evoked by pressure algometry) and the secondary outcomes (secondary hyperalgesia/allodynia, pressure pain thresholds, assessed at the surgical site and at the mirror-site in the contralateral groin, and, opioid withdrawal symptoms) will be assessed. These assessments will be repeated at each step of the target-controlled infusion of placebo or naloxone at estimated median (95 % CI) plasma concentrations of 344 ng/ml (130;567), 1059 ng/ml (400;1752) and 3196 ng/ml (1205;5276).

Discussion

We aim to demonstrate opioid-mediated latent sensitization in a post-surgical setting, using pain as a clinical relevant variable. Impairment of the protective endogenous opioid system may play an important role in the transition from acute to chronic pain. In order to sufficiently block the endogenous opioid system, a high-dose target-controlled naloxone-infusion is used, in accordance with recent findings in animal studies.

Trial registration number

EUDRACT: 2015-000793-36 (Registration date: 16 February 2015)

Clinicaltrials.gov: NCT01992146 (Registration date: 12 December 2014)

Dynamic Pain Phenotypes are Associated with Spinal Cord Stimulation-Induced Reduction in Pain: A Repeated Measures Observational Pilot Study

OBJECTIVE

Spinal cord stimulation (SCS) has become a widely used treatment option for a variety of pain conditions. Substantial variability exists in the degree of benefit obtained from SCS and patient selection is a topic of expanding interest and importance. However, few studies have examined the potential benefits of dynamic quantitative sensory testing (QST) to develop objective measures of SCS outcomes or as a predictive tool to help patient selection. Psychological characteristics have been shown to play an important role in shaping individual differences in the pain experience and may aid in predicting responses to SCS. Static laboratory pain-induction measures have also been examined in their capacity for predicting SCS outcomes.

METHODS

The current study evaluated clinical, psychological and laboratory pain measures at baseline, during trial SCS lead placement, as well as 1 month and 3 months following permanent SCS implantation in chronic pain patients who received SCS treatment. Several QST measures were conducted, with specific focus on examination of dynamic models (central sensitization and conditioned pain modulation [CPM]) and their association with pain outcomes 3 months post SCS implantation.

RESULTS

Results suggest few changes in QST over time. However, central sensitization and CPM at baseline were significantly associated with clinical pain at 3 months following SCS implantation, controlling for psycho/behavioral factors and pain at baseline. Specifically, enhanced central sensitization and reduced CPM were associated with less self-reported pain 3 months following SCS implantation.

CONCLUSIONS

These findings suggest a potentially important role for dynamic pain assessment in individuals undergoing SCS, and hint at potential mechanisms through which SCS may impart its benefit.

Individualized pharmacological treatment of neuropathic pain

Patients with the same disease may suffer from completely different pain symptoms yet receive the same drug treatment. Several studies elucidate neuropathic pain and treatment response in human surrogate pain models. They show promising results toward a patient stratification according to the mechanisms underlying the pain, as reflected in their symptoms. Several promising new drugs produced negative study results in clinical phase III trials. However, retrospective analysis of treatment response based on baseline pain phenotyping could demonstrate positive results for certain subgroups of patients. Thus, a prospective classification of patients according to pain phenotype may play an increasingly important role in personalized treatment of neuropathic pain states. A recent prospective study using stratification based on pain-related sensory abnormalities confirmed the concept of personalized pharmacological treatment of neuropathic pain.

Pain hypersensitivity in juvenile idiopathic arthritis: a quantitative sensory testing study

BACKGROUND

Juvenile Idiopathic Arthritis (JIA) is the most common cause of non-infectious joint inflammation in children. Synovial inflammation results in pain, swelling and stiffness. Animal and adult human studies indicate that localized joint-associated inflammation may produce generalized changes in pain sensitivity. The aim was to characterize pain sensitivity in children with JIA to mechanical and thermal stimulus modalities using quantitative sensory testing (QST) at an affected inflamed joint, and compare to children in clinical remission. Generalized hypersensitivity was evaluated by comparing QST measures at the thenar eminence between JIA and healthy control children.

METHODS

60 children aged 7-17 years with JIA participated. QST assessed sensory detection threshold and pain threshold at two sites: (1) affected joint (clinically active or inactive), (2) contralateral thenar eminence. Joint site included finger, wrist, knee and ankle. Clinical status was measured using objective and subjective markers of disease severity. Questionnaires assessed pain intensity and frequency, functional disability, anxiety, pain catastrophization and fatigue. QST data collected from joints were compared within JIA patients: active vs. inactive inflammation; and data from the contralateral thenar eminence were compared between JIA and healthy control cohorts in Europe [EU, (n = 151)] and the US (n = 92). Statistical analyses were performed using Kruskal-Wallis with Dunn's post-hoc comparison, Mann-Whitney or Fisher's exact test, where appropriate.

RESULTS

Overall, children with JIA reported low pain scores and low degrees of functional disability. Sensory detection thresholds and pain thresholds were similar in "active" compared to "inactive" joints. Despite this, children with JIA had generalized hypersensitivity at the thenar eminence when compared to healthy children for pressure (vs. EU p < 0.001), light touch (vs. EU p < 0.001), cold (vs EU, p < 0.01; vs US, p < 0.001) and heat pain (vs EU, p < 0.05; vs US p < 0.001).

CONCLUSIONS

JIA is associated with increased sensitivity to painful mechanical and thermal stimuli, even in absence of pain reports, or markers of disease activity. Future research investigating mechanisms underlying pain hypersensitivity in JIA is warranted; this will in turn guide pharmacologic and non-pharmacologic interventions to prevent or reverse these processes.

Assessment of deep tissue hyperalgesia in the groin - a method comparison of electrical vs. pressure stimulation

BACKGROUND

Deep pain complaints are more frequent than cutaneous in post-surgical patients, and a prevalent finding in quantitative sensory testing studies. However, the preferred assessment method - pressure algometry - is indirect and tissue unspecific, hindering advances in treatment and preventive strategies. Thus, there is a need for development of methods with direct stimulation of suspected hyperalgesic tissues to identify the peripheral origin of nociceptive input.

METHODS

We compared the reliability of an ultrasound-guided needle stimulation protocol of electrical detection and pain thresholds to pressure algometry, by performing identical test-retest sequences 10 days apart, in deep tissues in the groin region. Electrical stimulation was performed by five up-and-down staircase series of single impulses of 0.04 ms duration, starting from 0 mA in increments of 0.2 mA until a threshold was reached and descending until sensation was lost. Method reliability was assessed by Bland-Altman plots, descriptive statistics, coefficients of variance and intraclass correlation coefficients.

RESULTS

The electrical stimulation method was comparable to pressure algometry regarding 10 days test-retest repeatability, but with superior same-day reliability for electrical stimulation (P < 0.05). Between-subject variance rather than within-subject variance was the main source for test variation. There were no systematic differences in electrical thresholds across tissues and locations (P > 0.05).

CONCLUSION

The presented tissue-specific direct deep tissue electrical stimulation technique has equal or superior reliability compared with the indirect tissue-unspecific stimulation by pressure algometry. This method may facilitate advances in mechanism based preventive and treatment strategies in acute and chronic post-surgical pain states.

Human models of pain for the prediction of clinical analgesia

Human experimental pain models are widely used to study drug effects under controlled conditions. However, efforts to improve both animal and human experimental model selection, on the basis of increased understanding of the underlying pathophysiological pain mechanisms, have been disappointing, with poor translation of results to clinical analgesia. We have developed an alternative approach to the selection of suitable pain models that can correctly predict drug efficacy in particular clinical settings. This is based on the analysis of successful or unsuccessful empirical prediction of clinical analgesia using experimental pain models. We analyzed statistically the distribution of published mutual agreements or disagreements between drug efficacy in experimental and clinical pain settings. Significance limits were derived by random permutations of agreements. We found that a limited subset of pain models predicts a large number of clinically relevant pain settings, including efficacy against neuropathic pain for which novel analgesics are particularly needed. Thus, based on empirical evidence of agreement between drugs for their efficacy in experimental and clinical pain settings, it is possible to identify pain models that reliably predict clinical analgesic drug efficacy in cost-effective experimental settings.

Psychiatric agents and implications for perioperative analgesia

The use of antidepressants, anxiolytics, mood stabilizers, anticonvulsants, and major tranquilizers introduces neurochemical, behavioral, cognitive, and emotional factors that increase the complexity of medical and surgical tasks. Increasingly, various classes of psychotropic medications are being prescribed in the perioperative setting for their analgesic properties in patients with or without a psychiatric diagnosis. In many cases, the precise mechanisms of action and dose-response relationships by which these agents mediate analgesia are largely unclear. An appreciation of the side effects and adverse-effect profiles of such medications and familiarity with the clinically relevant drug interactions that may occur in the perioperative setting are imperative to ensure the best possible outcome in dealing with patients on these medications. This review focuses on various classes of psychotropic agents, which are addressed individually, with particular focus on their analgesic properties. The latest published research is summarized, deficiencies in our current collective knowledge are discussed, and evidence-based recommendations are made for clinical practice.

Behavioral economics and empirical public policy

The application of economics principles to the analysis of behavior has yielded novel insights on value and choice across contexts ranging from laboratory animal research to clinical populations to national trends of global impact. Recent innovations in demand curve methods provide a credible means of quantitatively comparing qualitatively different reinforcers as well as quantifying the choice relations between concurrently available reinforcers. The potential of the behavioral economic approach to inform public policy is illustrated with examples from basic research, pre-clinical behavioral pharmacology, and clinical drug abuse research as well as emerging applications to public transportation and social behavior. Behavioral Economics can serve as a broadly applicable conceptual, methodological, and analytical framework for the development and evaluation of empirical public policy.

Reproducibility of the heat/capsaicin skin sensitization model in healthy volunteers

Introduction

Heat/capsaicin skin sensitization is a well-characterized human experimental model to induce hyperalgesia and allodynia. Using this model, gabapentin, among other drugs, was shown to significantly reduce cutaneous hyperalgesia compared to placebo. Since the larger thermal probes used in the original studies to produce heat sensitization are now commercially unavailable, we decided to assess whether previous findings could be replicated with a currently available smaller probe (heated area 9 cm² versus 12.5–15.7 cm²).

Study design and methods

After Institutional Review Board approval, 15 adult healthy volunteers participated in two study sessions, scheduled 1 week apart (Part A). In both sessions, subjects were exposed to the heat/capsaicin cutaneous sensitization model. Areas of hypersensitivity to brush stroke and von Frey (VF) filament stimulation were measured at baseline and after rekindling of skin sensitization. Another group of 15 volunteers was exposed to an identical schedule and set of sensitization procedures, but, in each session, received either gabapentin or placebo (Part B).

Results

Unlike previous reports, a similar reduction of areas of hyperalgesia was observed in all groups/sessions. Fading of areas of hyperalgesia over time was observed in Part A. In Part B, there was no difference in area reduction after gabapentin compared to placebo.

Conclusion

When using smaller thermal probes than originally proposed, modifications of other parameters of sensitization and/or rekindling process may be needed to allow the heat/capsaicin sensitization protocol to be used as initially intended. Standardization and validation of experimental pain models is critical to the advancement of translational pain research.

Mechanistic, translational, quantitative pain assessment tools in profiling of pain patients and for development of new analgesic compounds

Background

Mechanistic, translational, human experimental pain assessment technologies (pain bio markers) can be used for: (1) profiling the responsiveness of various pain mechanisms and pathways in healthy volunteers and pain patients, and (2) profiling the effect of new or existing analgesic drugs or pain management procedures. Translational models, which may link mechanisms in animals to humans, are important to understand pain mechanisms involved in pain patients and as tools for drug development. This is urgently needed as many drugs which are effective in animal models fail to be efficient in patients as neither the mechanisms involved in patients nor the drugs’ mechanistic actions are known.

Aim

The aim of the present topical review is to provide the basis for how to use mechanistic human experimental pain assessment tools (pain bio markers) in the development of new analgesics and to characterise and diagnose pain patients. The future aim will be to develop such approaches into individualised pain management regimes.

Method

Experimental pain bio markers can tease out mechanistically which pain pathways and mechanisms are modulated in a given patient, and how a given compound modulates them. In addition, pain bio markers may be used to assess pain from different structures (skin, muscle and viscera) and provoke semi-pathophysiological conditions (e.g. hyperalgesia, allodynia and after-sensation) in healthy volunteers using surrogate pain models.

Results

With this multi-modal, multi-tissue, multi-mechanism pain assessment regime approach, new opportunities have emerged for profiling pain patients and optimising drug development. In this context these technologies may help to validate targets (proof-of-concept), provide dose-response relationships, predicting which patient population/characteristics will respond to a given treatment (individualised pain management), and hence provide better understanding of the underlying cause for responders versus non-responders to a given treatment.

Conclusion

In recent years, pain bio markers have been substantially developed to have now a role to play in early drug development, providing valuable mechanistic understanding of the drug action and used to characterise/profile pain patients. In drug development phase I safety volunteer studies, pain bio marker scan provide indication of efficacy and later if feasible be included in clinical phase II, III, and IV studies to substantiate mode-of-action.

Implications

Refining and optimizing the drug development process ensures a higher success rate, i.e. not discarding drugs that may be efficient and not push non-efficient drugs too far in the costly development process. Mechanism-based pain bio markers can help to qualify the development programmes and at the same time help qualifying them by pain profiling (phenotyping) and recognising the right patients for specific trials. The success rate from preclinical data to clinical outcome may be further facilitated by using specific translational pain bio-markers. As human pain bio markers are getting more and more advanced it could be expected that FDA and EMA in the future will pay more attention to such mechanism-related measures in the approval phase as proof-of-action.

Psychological screening/phenotyping as predictors for spinal cord stimulation

Spinal cord stimulation (SCS) is becoming a widely used treatment for a number of pain conditions and is frequently considered as a pain management option when conservative or less invasive techniques have proven to be ineffective. Potential indications for SCS include complex regional pain syndrome (CRPS), postherpetic neuralgia, traumatic nerve injury, failed back surgery syndrome, refractory angina pectoris, peripheral vascular disease, neuropathic pain, and visceral pain (Guttman et al. Pain Pract. 9:308-11, 2009). While research on SCS is in its infancy, it is clear that substantial variation exists in the degree of benefit obtained from SCS, and the procedure does not come without risks; thus focused patient selection is becoming very important. Psychological characteristics play an important role in shaping individual differences in the pain experience and may influence responses to SCS, as well as a variety of other pain treatments (Doleys Neurosurg Focus 21:E1, 2006). In addition to psychological assessment, quantitative sensory testing (QST) procedures offer another valuable resource in forecasting who may benefit most from SCS and may also shed light on mechanisms underlying the individual characteristics promoting the effectiveness of such procedures (Eisenberg et al. Pain Pract. 6:161-165, 2006). Here, we present a brief overview of recent studies examining these factors in their relationship with SCS outcomes.

Exploration of the effects of gender and mild esophagitis on esophageal pain thresholds in the normal and sensitized state of asymptomatic young volunteers

BACKGROUND

Clinical data suggest gender differences in gastrointestinal pain, but very little experimental data exist. Esophageal painful thresholds to mechanical, thermal, electric, and chemical stimuli can be measured with the esophageal multimodal pain model. The aim was to measure the effect of gender and mild esophagitis on esophageal pain perception.

METHODS

Thirty-five healthy asymptomatic volunteers [19 men, median age 29 (22-56 years)] underwent upper GI endoscopy, 24 h pH/impedance measurement, and multimodal esophageal pain stimulation before and after sensitization with acid. Stimulus intensities at painful thresholds were recorded.

KEY RESULTS

Men had higher pain thresholds (PT) to mechanical stimulation (mean volume: men 20.9 ± 10 mL vs women 15.2 ± 6.8 mL, P = 0.02) and more men tolerated the maximum acid challenge (58% vs 20%, P = 0.03). There were no differences between genders for PT to (1) thermal stimulation [mean stimulation time (men, women): heat; 20 ± 5 s vs 21 ± 6 s or cold; 33.3 ± 20.1 s vs 20.7 ± 21.4 s, P > 0.2], (2) electrical current (mean current: men 17.6 ± 9.2 mA vs women 12.9 ± 3.7 mA, P = 0.11), or (3) acid volume [median volume: men 200 (20;200) mL vs women 133 (40;200) mL, P = 0.2]. Fifteen asymptomatic subjects had mild esophagitis (10 men, all Los Angeles A). There were no differences in esophageal PT between subjects with normal endoscopy or mild esophagitis (all P > 0.3).

CONCLUSIONS & INFERENCES

The effects of gender and mild esophagitis on esophageal multimodal pain perception have been measured in asymptomatic volunteers. The study suggests that gender, not mild esophagitis, tends to influence mechanical and chemical esophageal pain.

Clinical pharmacology of analgesics assessed with human experimental pain models: bridging basic and clinical research

The medical impact of pain is such that much effort is being applied to develop novel analgesic drugs directed towards new targets and to investigate the analgesic efficacy of known drugs. Ongoing research requires cost-saving tools to translate basic science knowledge into clinically effective analgesic compounds. In this review we have re-examined the prediction of clinical analgesia by human experimental pain models as a basis for model selection in phase I studies. The overall prediction of analgesic efficacy or failure of a drug correlated well between experimental and clinical settings. However, correct model selection requires more detailed information about which model predicts a particular clinical pain condition. We hypothesized that if an analgesic drug was effective in an experimental pain model and also a specific clinical pain condition, then that model might be predictive for that particular condition and should be selected for development as an analgesic for that condition. The validity of the prediction increases with an increase in the numbers of analgesic drug classes for which this agreement was shown. From available evidence, only five clinical pain conditions were correctly predicted by seven different pain models for at least three different drugs. Most of these models combine a sensitization method. The analysis also identified several models with low impact with respect to their clinical translation. Thus, the presently identified agreements and non-agreements between analgesic effects on experimental and on clinical pain may serve as a solid basis to identify complex sets of human pain models that bridge basic science with clinical pain research.

Evaluation of anti-hyperalgesic and analgesic effects of two benzodiazepines in human experimental pain: a randomized placebo-controlled study

Background and Aims

Compounds that act on GABA-receptors produce anti-hyperalgesia in animal models, but little is known on their effects in humans. The aim of this study was to explore the potential usefulness of GABA-agonism for the control of pain in humans. Two agonists at the benzodiazepine-binding site of GABA_A-receptors (clobazam and clonazepam) were studied using multiple experimental pain tests. Positive results would support further investigation of GABA agonism for the control of clinical pain.

Methods

In a randomized double-blind crossover design, 16 healthy male volunteers received clobazam 20 mg, clonazepam 1 mg and tolterodine 1 mg (active placebo). The area of static hyperalgesia after intradermal capsaicin injection was the primary endpoint. Secondary endpoints were: area of dynamic hyperalgesia, response to von Frey hair stimulation, pressure pain thresholds, conditioned pain modulation, cutaneous and intramuscular electrical pain thresholds (1, 5 and 20 repeated stimulation), and pain during cuff algometry.

Results

For the primary endpoint, an increase in the area of static hyperalgesia was observed after administration of placebo (p<0.001), but not after clobazam and clonazepam. Results suggestive for an anti-hyperalgesic effect of the benzodiazepines were obtained with all three intramuscular pain models and with cuff algometry. No effect could be detected with the other pain models employed.

Conclusions

Collectively, the results are suggestive for a possible anti-hyperalgesic effect of drugs acting at the GABA_A-receptors in humans, particularly in models of secondary hyperalgesia and deep pain. The findings are not conclusive, but support further clinical research on pain modulation by GABAergic drugs. Because of the partial results, future research should focus on compounds acting selectively on subunits of the GABA complex, which may allow the achievement of higher receptor occupancy than unselective drugs. Our data also provide information on the most suitable experimental models for future investigation of GABAergic compounds.

Trial Registration

ClinicalTrials.gov NCT01011036

Randomized clinical trial: inhibition of the TRPV1 system in patients with nonerosive gastroesophageal reflux disease and a partial response to PPI treatment is not associated with analgesia to esophageal experimental pain

OBJECTIVE

Many patients with nonerosive reflux disease (NERD) have insufficient relief on proton pump inhibitors (PPIs). Some patients have a hypersensitive esophagus and may respond to transient receptor potential vanilloid 1 (TRPV1) antagonists. Aim. To investigate the effect of the TRPV1 antagonist AZD1386 on experimental esophageal pain in NERD patients.

MATERIAL AND METHODS

Enrolled patients had NERD and a partial PPI response (moderate-to-severe heartburn or regurgitation ≥3 days/week before enrolment despite ≥6 weeks' PPI therapy). Fourteen patients (21-69 years, 9 women) were block-randomized into this placebo-controlled, double-blinded, crossover study examining efficacy of a single dose (95 mg) of AZD1386. On treatment days, each participant's esophagus was stimulated with heat, distension, and electrical current at teaching hospitals in Denmark and Sweden. Heat and pressure pain served as somatic control stimuli. Per protocol results were analyzed.

RESULTS

Of 14 randomized patients, 12 were treated with AZD1386. In the esophagus AZD1386 did not significantly change the moderate pain threshold for heat [-3%, 95% confidence interval (CI), -22;20%], distension (-11%, 95% CI, -28;10%), or electrical current (6%, 95% CI, -10;25%). Mean cutaneous heat tolerance increased by 4.9°C (95% CI, 3.7;6.2°C). AZD1386 increased the maximum body temperature by a mean of 0.59°C (95% CI, 0.40-0.79°C), normalizing within 4 h.

CONCLUSIONS

AZD1386 had no analgesic effect on experimental esophageal pain in patients with NERD and a partial PPI response, whereas it increased cutaneous heat tolerance. TRPV1 does not play a major role in heat-, mechanically and electrically evoked esophageal pain in these patients. ClinicalTrials.gov identifier: D9127C00002.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Monoacylglycerol lipase - a target for drug development?

The endocannabinoid (eCB) system is involved in processes as diverse as control of appetite, perception of pain and the limitation of cancer cell growth and invasion. The enzymes responsible for eCB breakdown are attractive pharmacological targets, and fatty acid amide hydrolase inhibitors, which potentiate the levels of the eCB anandamide, are now undergoing pharmaceutical development. 'Drugable' selective inhibitors of monoacylglycerol lipase, a key enzyme regulating the levels of the other main eCB, 2-arachidonoylglycerol, were however not identified until very recently. Their availability has resulted in a large expansion of our knowledge concerning the pharmacological consequences of monoacylglycerol lipase inhibition and hence the role(s) played by the enzyme in the body. In this review, the pharmacology of monoacylglycerol lipase will be discussed, together with an analysis of the therapeutic potential of monoacylglycerol lipase inhibitors as analgesics and anticancer agents.

The effect of paracetamol and tropisetron on pain: experimental studies and a review of published data

Experimental studies suggest that paracetamol-induced analgesia is mediated via central serotonergic pathways and attenuated by 5-HT3-antagonists. However, clinical studies do not support this, and 5-HT3-antagonists are expected to reduce pain by blocking the descending pronociceptive pathway. The current project tested whether tropisetron attenuates analgesia by paracetamol. Two randomized, double-blind, crossover studies with 18 healthy male volunteers in each were performed. Pain stimuli were cold water immersion (cold pressor test), contact heat pain (study 1) and electrical stimulation (study 2). In both studies, tropisetron 5 mg i.v. or saline was administered, followed by paracetamol 2 g i.v. 30 min. later. Individual changes in heat and cold pain intensity, cold pain tolerance and unpleasantness were recorded. The same thresholds were also expressed as scores (% of the individual score at baseline). Additionally, previously published findings on the effects of paracetamol and its interaction with 5HT3-antagonists in human experimental pain models were reviewed. After calculation of the sensory and pain scores (%), tropisetron seemed to amplify the analgesic action of paracetamol. Paracetamol 2 g i.v. did not show any statistically significant analgesia in thermal tests (study 1), or differences in sensory, pain detection or moderate pain thresholds of the electrical stimulus (study 2). As paracetamol did not have a measurable analgesic effect in these tests, no conclusions can be drawn about the interaction between paracetamol and tropisetron. However, tropisetron may have an analgesic effect of its own. Clinicians should not avoid using these drugs together, unless larger clinical studies indicate otherwise.

Development of a simple radiant heat induced experimental pain model for evaluation of analgesics in normal healthy human volunteers

Objective:

Human experimental pain models help to understand the mechanism of the painful conditions and can also be adopted to test analgesic efficacy of drugs. In early phases, the clinical development of new analgesics is hindered due to the lack of reliable tests for the experimental pain models. In the present study, we have developed and validated a simple radiant heat pain model which can be used for future screening of various analgesic agents.

Materials and Methods:

We have standardized the thermal pain model by recording pain threshold and pain tolerance time in seconds at three different intensities and levels in 24 healthy subjects. Reproducibility of the test procedure was evaluated by recording the pain parameters by two observers on three consecutive days. Validity of model was further tested by evaluating the analgesic effect of tramadol.

Results and Conclusions:

Use of radiant heat pain model with high intensity and short level was found to produce low variability with coefficient of variation less than 5%. Interobserver and interperiod reproducibility was very good as shown by Bland - Altman plot; with most of the values within ± 2SD. Tramadol produced statistically significant increase in pain threshold time. The newly developed pain model produces a type of experimental pain which is responsive to analgesic effects of tramadol at clinically relevant doses.

The fatty acid amide hydrolase (FAAH) inhibitor PF-3845 acts in the nervous system to reverse LPS-induced tactile allodynia in mice

BACKGROUND AND PURPOSE

Inflammatory pain presents a problem of clinical relevance and often elicits allodynia, a condition in which non-noxious stimuli are perceived as painful. One potential target to treat inflammatory pain is the endogenous cannabinoid (endocannabinoid) system, which is comprised of CB1 and CB2 cannabinoid receptors and several endogenous ligands, including anandamide (AEA). Blockade of the catabolic enzyme fatty acid amide hydrolase (FAAH) elevates AEA levels and elicits antinociceptive effects, without the psychomimetic side effects associated with Δ(9) -tetrahydrocannabinol (THC).

EXPERIMENTAL APPROACH

Allodynia was induced by intraplantar injection of LPS. Complementary genetic and pharmacological approaches were used to determine the strategy of blocking FAAH to reverse LPS-induced allodynia. Endocannabinoid levels were quantified using mass spectroscopy analyses.

KEY RESULTS

FAAH (-/-) mice or wild-type mice treated with FAAH inhibitors (URB597, OL-135 and PF-3845) displayed an anti-allodynic phenotype. Furthermore, i.p. PF-3845 increased AEA levels in the brain and spinal cord. Additionally, intraplantar PF-3845 produced a partial reduction in allodynia. However, the anti-allodynic phenotype was absent in mice expressing FAAH exclusively in the nervous system under a neural specific enolase promoter, implicating the involvement of neuronal fatty acid amides (FAAs). The anti-allodynic effects of FAAH-compromised mice required activation of both CB1 and CB2 receptors, but other potential targets of FAA substrates (i.e. µ-opioid, TRPV1 and PPARα receptors) had no apparent role.

CONCLUSIONS AND IMPLICATIONS

AEA is the primary FAAH substrate reducing LPS-induced tactile allodynia. Blockade of neuronal FAAH reverses allodynia through the activation of both cannabinoid receptors and represents a promising target to treat inflammatory pain.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

The analgesic effect of pregabalin in patients with chronic pain is reflected by changes in pharmaco-EEG spectral indices

AIM

To identify electroencephalographic (EEG) biomarkers for the analgesic effect of pregabalin in patients with chronic visceral pain.

METHODS

This was a double-blind, placebo-controlled study in 31 patients suffering from visceral pain due to chronic pancreatitis. Patients received increasing doses of pregabalin (75mg-300mg twice a day) or matching placebo during 3 weeks of treatment. Pain scores were documented in a diary based on a visual analogue scale. In addition, brief pain inventory-short form (BPI) and quality of life questionnaires were collected prior to and after the study period. Multi-channel resting EEG was recorded before treatment onset and at the end of the study. Changes in EEG spectral indices were extracted, and individual changes were classified by a support vector machine (SVM) to discriminate the pregabalin and placebo responses. Changes in individual spectral indices and pain scores were correlated.

RESULTS

Pregabalin increased normalized intensity in low spectral indices, most prominent in the theta band (3.5-7.5Hz), difference of -3.18, 95% CI -3.57, -2.80; P= 0.03. No changes in spectral indices were seen for placebo. The maximum difference between pregabalin and placebo treated patients was seen in the parietal region, with a classification accuracy of 85.7% (P= 0.009). Individual changes in EEG indices were correlated with changes in pain diary (P= 0.04) and BPI pain composite scores (P= 0.02).

CONCLUSIONS

Changes in spectral indices caused by slowing of brain oscillations were identified as a biomarker for the central analgesic effect of pregabalin. The developed methodology may provide perspectives to assess individual responses to treatment in personalized medicine.

Evoked pain analgesia in chronic pelvic pain patients using respiratory-gated auricular vagal afferent nerve stimulation

OBJECTIVE

Previous vagus nerve stimulation (VNS) studies have demonstrated antinociceptive effects, and recent noninvasive approaches, termed transcutaneous-vagus nerve stimulation (t-VNS), have utilized stimulation of the auricular branch of the vagus nerve in the ear. The dorsal medullary vagal system operates in tune with respiration, and we propose that supplying vagal afferent stimulation gated to the exhalation phase of respiration can optimize t-VNS.

DESIGN

Counterbalanced, crossover study.

PATIENTS

Patients with chronic pelvic pain (CPP) due to endometriosis in a specialty pain clinic. INTERVENTIONS/OUTCOMES: We evaluated evoked pain analgesia for respiratory-gated auricular vagal afferent nerve stimulation (RAVANS) compared with nonvagal auricular stimulation (NVAS). RAVANS and NVAS were evaluated in separate sessions spaced at least 1 week apart. Outcome measures included deep-tissue pain intensity, temporal summation of pain, and anxiety ratings, which were assessed at baseline, during active stimulation, immediately following stimulation, and 15 minutes after stimulus cessation.

RESULTS

RAVANS demonstrated a trend for reduced evoked pain intensity and temporal summation of mechanical pain, and significantly reduced anxiety in N = 15 CPP patients, compared with NVAS, with moderate to large effect sizes (η(2) > 0.2).

CONCLUSION

Chronic pain disorders such as CPP are in great need of effective, nonpharmacological options for treatment. RAVANS produced promising antinociceptive effects for quantitative sensory testing (QST) outcomes reflective of the noted hyperalgesia and central sensitization in this patient population. Future studies should evaluate longer-term application of RAVANS to examine its effects on both QST outcomes and clinical pain.