Pharmacological strategies in relieving neuropathic pain

Opioid use trends in Spain: the case of the island of La Gomera (2016-2019)

This study is an evaluation of prescription opioid use on the island of La Gomera, a mainly rural area, during the period 2016-2019 at various levels. Data were extracted from the wholesalers who supply the community pharmacies at the population level. Prescription opioid use was measured as defined daily doses per 1,000 inhabitants/day (DID) and by the number of units sold per 1,000 inhabitants and year (units sold). This provided an island total of La Gomera's overall prescription of opioids and its rate of change, as well as differences in prescribing at the municipal and health area level. Tramadol with acetaminophen and tramadol in monotherapy were the most consumed by "units sold" parameter, which accounted for 69.48% and 18.59% of the total. The situation was similar for DID, although with lower percentages, but a significant increase was observed in the use of fentanyl and buprenorphine, around 15% in each case. The balance between the uses of weak or strong opioids was different in La Gomera compared to that of Spain as a whole. In Spain, almost 70% of the prescriptions were for weak opioids compared to 58.67% in La Gomera. Fentanyl was the most used strong opioid (16.10%) followed by tapentadol and buprenorphine, around 5% each, whereas in La Gomera, buprenorphine was the most consumed (15.75%) followed by fentanyl (14.87%) and tapentadol (5.82%). These differences in prescription opioid use are most likely explained by prescriber characteristics, whereas the population age, socioeconomic status, or living in rural/urban area are not decisive determinants.

Hypothalamic-pituitary-adrenal axis function in patients with complex regional pain syndrome type 1

An exaggerated inflammatory process is considered an important pathophysiological feature of complex regional pain syndrome type 1 (CRPS-1). The hypothalamic-pituitary-adrenal (HPA) axis serves as a negative feedback mechanism for inflammatory processes. The present study examined the HPA axis function in patients with CRPS-1 by a determination of cortisol concentrations in saliva. Three sets of saliva samples were sequentially collected from 24 patients with CRPS-1 during medication (on-Med), 72 h after stopping medication (off-Med) and 8h after the oral administration of 1mg dexamethasone. One set of saliva samples was collected from healthy controls. The cortisol awakening response (CAR) and diurnal cortisol decline (DCD) were used as indices for HPA axis function. Cortisol levels during the post-awakening period in patients were increased following withdrawal of medications. The CAR during the off-Med condition was disappeared after administration of dexamethasone. Among the examined CRPS-related numerical variables, the frequency of spontaneous pain attacks showed relationships with the indices of HPA axis function. After classifying the patients into two subgroups, we observed that the CAR and DCD in patient who had a relatively high frequency of spontaneous pain attacks (subgroup 5 ≤) were lower and less steep than those in patient who had a relatively low frequency of spontaneous pain attacks (subgroup 0-4) for the on- and off-Med conditions. The CAR and DCD in subgroup 5 ≤ during their off-Med condition were comparable to those in controls. These results suggest that the increase in frequency of spontaneous pain attacks is associated with a reduced CAR and flattened DCD in patients CRPS-1.

Assessing carrageenan-induced locomotor activity impairment in rats: comparison with evoked endpoint of acute inflammatory pain

BACKGROUND

Most animal models currently used to evaluate antinociceptive efficacy of analgesics rely on the assessment of evoked pain behaviours as primary endpoints.

METHODS

Here, we have developed and characterized the carrageenan-induced locomotor activity impairment (CLAIM) model to objectively assess non-evoked inflammatory pain behaviour in rats. In this model, 100 µL of 1% carrageenan was subcutaneously injected into the plantar aspect of the right hind paw and exploratory behaviour in the novel testing chamber was recorded using an automated locomotor activity system.

RESULTS

Carrageenan-injected animals exhibited an exploratory behavioural deficit 2-7 h following injection compared to saline-injected animals. The severity of impairment was carrageenan dose related, and sensitive to the light intensity in the testing room. The effects of standard analgesics on CLAIM were examined 2 or 3 h following carrageenan injection. Diclofenac and ibuprofen, in a dose range exerting no effect on locomotor activity in naïve rats, exhibited dose-related reversal of CLAIM (ED(50) = 1.5 and 5.0 mg/kg, respectively), with comparable efficacy on carrageenan-induced thermal hyperalgesia (ED(50) = 2.0 and 6.0 mg/kg, respectively). Gabapentin and duloxetine produced no reversal of CLAIM, or attenuation of thermal hyperalgesia. Efficacy discrepancy was noted for morphine on thermal hyperalgesia and CLAIM. Additionally, amphetamine dose dependently reversed CLAIM, and similarly increased locomotor activity in normal animals.

DISCUSSION AND CONCLUSION

The results presented here demonstrate that CLAIM provides an objective assessment of non-evoked pain behaviours for acute inflammatory pain. The pharmacological profile of standard analgesics supports that CLAIM model can be used to identify agents to treat acute inflammatory pain in the clinic.

[Treatment of neuropathic pain in HIV-infected patients]

Neuropathic pain of various etiologies is a frequent symptom in HIV-infected patients that is underdiagnosed and inadequately treated. It requires a multidisciplinary pain approach based on psychosocial factors, diet and exercise, etiologic treatment whenever possible, symptomatic medical treatment, and sometimes, interventional techniques. Medical treatment should be individualized and introduced gradually, with a mind to potential drug interactions. Neuropathic pain responds poorly to conventional analgesics, such as nonsteroidal antiinflammatory drugs and opiates; tricyclic antidepressants and anticonvulsants are the drugs of choice. Before establishing an analgesic treatment, possible drug interactions should be ruled out, mainly those occurring with antiretroviral agents.

Mirtazapine decreases the pain feeling in healthy participants

OBJECTIVES

The treatment of neuropathic pain is mainly based on antiepileptics, tricyclic antidepressants, and opiates. These drugs have important side effects disturbing the patient's quality of life. Mirtazapine (MTZ) is a new and well-tolerated tricyclic antidepressants with both monoaminergic and opioid properties that might favorably influence pain. The aim of this study was to assess whether MTZ can reduce the pain induced by a standardized stimulus presented to healthy human participants. The nociceptive flexion reflex (NFR) to an electric stimulus was chosen to determine the pain threshold.

METHODS

The effect of MTZ compared to placebo was assessed on 10 healthy participants in a double-blinded cross-over design. The NFR was measured the day after a single oral dose of drug (30 mg) or placebo.

RESULTS

A significant increase in upper limb (+29%, P=0.006) NFR threshold was observed.

DISCUSSION

MTZ increases the pain tolerance in healthy participants. The potential benefit of this effect on pain should be investigated more thoroughly in chronic neuropathic pain patients. The NFR might serve as an additional tool for the monitoring of these patients.

Comparison of antinociceptive actions of standard analgesics in attenuating capsaicin and nerve-injury-induced mechanical hypersensitivity

Intradermal capsaicin injection produces immediate spontaneous pain behaviors, and a secondary mechanical hypersensitivity (SMH) that is employed in the clinic as a model potentially predictive of human neuropathic pain. Presently, we have characterized capsaicin-induced SMH in rats, and compared pharmacological actions of standard analgesics in this and two nerve injury models, the L5/L6 spinal nerve ligation (SNL) and sciatic nerve chronic constriction injury (CCI) models. Intraplantar capsaicin produced dose-related SMH (enhanced paw withdrawal response to von Frey monofilament stimulation at an area away from injection site) that lasted for over 4 h. While pretreatment with a potent selective transient receptor potential vanilloid receptor-1 (TRPV1) antagonist A-425619 (1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea) prevented development of acute nocifensive (flinching) behavior immediately following capsaicin injection (ED(50)=4.9 mg/kg), the compound failed to attenuate the SMH when administered 2 h following capsaicin (10 microg/10 microl). Additional standard analgesics were also tested 3 h following intraplantar capsaicin in the SMH model. Comparison of their potencies in attenuating mechanical hypersensitivity in capsaicin, SNL and CCI models revealed similar ED(50)s for morphine (2.3 mg/kg, 1.6 mg/kg and 3.2 mg/kg, respectively), gabapentin (33.1 mg/kg, 33.9 mg/kg and 26.3 mg/kg, respectively) and lamotrigine (9.1 mg/kg, 8.9 mg/kg and 15.5 mg/kg, respectively). Duloxetine produced 50-65% effect at the highest tested dose (50 mg/kg), whereas the highest tested doses of morphine (10 mg/kg), gabapentin (85.5 mg/kg) and lamotrigine (30 mg/kg) all produced >70% efficacy in capsaicin SMH, SNL and CCI models. In contrast, celecoxib and ibuprofen showed weak effects in all three models. All standard analgesics generally had weak efficacy in attenuating capsaicin-induced immediate acute flinching behavior when administered before capsaicin. These results provide further support to the suggestions that distinct pharmacological mechanisms underlie capsaicin-induced acute nocifensive and SMH behaviors, and certain neuronal mechanisms underlying neuropathic pain states are also contributory to capsaicin-induced SMH.

Pharmacological correlation between the formalin test and the neuropathic pain behavior in different species with chronic constriction injury

Research on mechanisms of drug action, and preclinical screening of molecules with a potential activity on neuropathic pain requires extensive animal work. The chronic constriction injury model is one of the best-characterized models of neuropathic pain behavior in rats, but requires extensive time consuming operations and animal handling. The formalin test is easier to perform, and a well validated model. The latter may serve as an effective prescreening test of molecules and may facilitate drug targeting. In the present study the activity of different pharmacological reference compounds was tested in rats and gerbils on the cold plate for animals that had undergone chronic constriction injury and in the second phase of the formalin test. In rats, a comparable outcome in both test conditions was observed for morphine, fentanyl, MK-801 and flunarizine. Clonidine had more activity in the second phase of the formalin test, whereas baclofen, tramadol, amitryptiline, ketamine and topiramate showed more activity in the cold plate. In gerbils, both test conditions yielded comparable results for fentanyl and ketoprofen. Tramadol and CP-96345 tended to have more activity in the second phase of the formalin test, whereas morphine, SR-48968, SR-142801 and R116301 demonstrated more activity in the cold plate test. This study demonstrates a good correlation between the second phase of the formalin test and the cold allodynia in the CCI model for, both for rats and gerbils. Drugs with a proven activity in humans, used as reference compounds, also showed good pharmacological activity in this animal study.

Recent developments in the pharmacological management of pain in children

PURPOSE OF REVIEW

This review explores progress in developmental pharmacokinetics, pharmacogenomics and formulations of analgesic agents, and discusses potential implications for pain therapy.

RECENT FINDINGS

Characterization of the developmental pharmacokinetics of morphine, tramadol, paracetamol and nonsteroidal anti-inflammatory drugs has improved dosing in children. Oral sugar solutions have replaced the brandy/sugar pacifier and are effective for single painful events in neonates. Intravenous paracetamol offers increased dosing accuracy, and avoids absorption and bioavailability variability. New nitric-oxide-releasing versions of paracetamol and nonsteroidal anti-inflammatory drugs offer safer alternatives to their parent drugs with enhanced potency. Ketamine has come under a cloud for its possible effects on the neonatal developing brain, but it is being used increasingly in children to supplement opioids for pain after major surgery. Hopes that morphine analgesia may improve neurological outcome in premature babies have not materialized. Reports concerning chronic pain are generally case series and controlled trials are rare and nearly nonexistent in children.

SUMMARY

Unlicensed drug use in the very young will increase as familiarity increases. Pharmacogenomic studies have the potential to tailor drug therapy to the individual and decrease between-patient variability. Unfortunately, the pharmacodynamic knowledge in children of analgesic agents remains neglected and is usually extrapolated from adult data.

Targeting N-type and T-type calcium channels for the treatment of pain

Severe chronic pain afflicts a large number of people worldwide but satisfactory relief from such pain is difficult to achieve with drugs that are currently available, and so there is a great need for the development of new, efficacious and safe analgesics. Voltage-gated calcium-permeable ion channels are multi-subunit complexes that regulate neuronal excitability, action-potential firing patterns and neurotransmission in nociceptive pathways. Although multiple subtypes of voltage-gated calcium channels exist, pharmacological and ion-channel gene knockdown approaches in animals have revealed N-type and T-type calcium channels to be particularly attractive molecular targets for the discovery and development of new analgesic drugs. The recent approval of Prialt (Elan Pharmaceuticals) provides the ultimate target validation for N-type calcium channels, namely proof that they are key regulators of nociceptive signaling in humans.

Amitriptyline prevents thermal hyperalgesia and modifications in rat spinal cord GABAB receptor expression and function in an animal model of neuropathic pain

Using an animal model of neuropathic pain, behavioral and biochemical experiments were performed to assess the effects of this condition on pain threshold and GABA(B) receptor sensitivity and subunit gene expression in the rat lumbar spinal cord. The results indicate that partial sciatic nerve ligation decreases thermal and mechanical pain withdrawal latencies, and increases baclofen-stimulated [35S]GTPgammaS binding and GABA(B) receptor subunit gene expression in the rat lumbar spinal cord, suggesting that neuropathic pain may be due, in part, to a deficiency in GABAergic transmission. The experiments also demonstrate that daily administration (10 mg/kg, i.p.) of amitriptyline, a tricyclic antidepressant used for the treatment of neuropathic pain, for 1 week after surgery prevents the decline in thermal pain threshold, the increase in GABA(B2) gene expression, and development of increased GABA(B) receptor function in spinal cord resulting from nerve damage. These findings indicate that the efficacy of amitriptyline as a treatment for neuropathic pain may be related to an ability to maintain spinal cord GABA(B) receptor activity.