Emerging trends in the pharmacotherapy of chronic pain

Non-addictive orally-active kappa opioid agonists for the treatment of peripheral pain in rats

Addiction to conventional opioid pain analgesics is a major societal problem that is increasing at an alarming rate. New drugs to combat the effects of opioid abuse are desperately needed. Kappa-opioid agonists are efficacious in peripheral pain models but suffer from centrally-mediated effects. In this article, we discuss our efforts in developing peripheral kappa-based opioid receptor agonists that have the potential analgesic activity of opioids but do not manifest the negative side-effects of opioid use and abuse. Further, derivatives of the tetra-peptide D-Phe-D-Phe-D-Nle-D-Arg-NH2, such as CR665, exhibit high peripheral to central selectivity in analgesic models when administered intravenously (i.v.); however, they are inactive when administered orally. Application of our laboratory's proprietary non-natural amino acid technology to CR665 produced derivatives that exhibit peripheral analgesic activity when dosed orally but do not promote CNS-based effects. Lead compound JT09 activates the kappa-opioid receptor with EC50s in the low nM range, while agonist selectivity for kappa over other peripheral opioid receptors was >33,400 fold. Results indicate that JT09 is approximately as efficacious as morphine in alleviating peripheral pain, while failing to produce undesired CNS-mediated activity. Additionally, JT09 did not promote other CNS-mediated effects associated with morphine (addiction, sedation, dysphoria, tolerance, addiction). Thus, we propose that JT09 has potential for development as a novel analgesic. PERSPECTIVE: This article presents data supporting the analgesic properties of an orally available, peripherally-restricted, kappa-opioid agonist for peripheral pain. A potential out-patient pharmaceutical that acts as efficacious as morphine in alleviating peripheral pain, while failing to produce undesired CNS-mediated effects, could help reduce the current health care burden associated with prescription opioids.

Protein-functionalized PLGA nanoparticles of lamotrigine for neuropathic pain management

Lamotrigine (LTG), a sodium and calcium channel blocker, has demonstrated efficacy for the treatment of neuropathic pain in multiple, randomized, controlled trials. However, its potential clinical applications in neuropathic pain are limited due to the risk of dose-dependent severe rashes associated with high dose and prompt dose escalation. Further, the poor pharmacokinetic profile due to non-selective distribution to organs other than brain reduces the efficacy of dosage regimen. Therefore, the aim of present investigation is to develop surface-engineered LTG nanoparticles (NPs) using transferrin and lactoferrin as ligand to deliver higher amount of drug to brain and improve the biodistribution and pharmacokinetic profile of drug with prolonged duration of action and reduced accumulation in non-target organs. The LTG NPs were prepared by nanoprecipitation and optimized by factorial design for high entrapment and optimized particle size. The optimized NPs were surface functionalized by conjugating with the lactoferrin (Lf) and transferrin (Tf) as ligands. The developed NPs were characterized for different physicochemical parameters and stability. The in vivo biodistribution showed preferential targeting to brain and reduced accumulation in non-target organs over a prolonged duration of time. Finally, partial sciatic nerve injury model was used to demonstrate the increased pharmacodynamic response as antinociceptive effect. Both biodistribution and pharmacodynamic study in mice confirmed that the approach used for LTG can help to increase clinical applications of LTG due to brain targeting and reduced side effects.

Ibudilast (AV-411)

The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.

Pharmacological evaluation of opioid and non-opioid analgesics in a murine bone cancer model of pain

The intramedulary injection of osteosarcoma cells in the mouse femur has served as a laboratory model to study bone cancer pain. However, the efficacy of different classes of analgesics has not fully been analyzed in this model. Therefore, the acute antinociceptive properties of different classes of drugs were evaluated on post-inoculation day 15 when the degrees of spontaneous pain and mechanical hypersensitivity in the ipsilateral inoculated hind paw reached almost their maximal effects. At high doses, the opioids fentanyl, morphine, and tramadol had full efficacies for all pain parameters tested. Antagonism experiments with naloxone (10 mg/kg s.c.) or its peripheral analogue methylnaltrexone (10 mg/kg s.c.), suggest that the analgesic effects of fentanyl were predominantly mediated by centrally located mu-opiate receptors. Acetaminophen, the non-steroidal anti-inflammatory drug indomethacin, and the COX-2-inhibitor celecoxib did not significantly improve pain behavior. The tricyclic antidepressants amitriptyline and desipramine significantly reduced spontaneous pain behavior but this only at sedative doses; the serotonin reuptake inhibitor fluoxetine had limited efficacy. Also with the anticonvulsants lamotrigine, topiramate, and gabapentin limited or no efficacies were found. In conclusion, the present study provided integrated information about the tumor-induced bone pain in mice, and clarified acute efficacies of different categories of analgesics for the spontaneous lifting, limb-use impairment, and mechanical hypersensitivity. Moreover, the finding that bone cancer-pain behaviors are attenuated by various established compounds further supports the validity of the murine bone cancer model for the study of bone cancer pain and its use for the identification of novel treatments.

Recent Advances in Understanding Pain Mechanisms Provide Future Directions for Pain Management

PURPOSE/OBJECTIVES

To review current knowledge of neurobiologic mechanisms that generate and maintain chronic pain and to explain how they might be applied in targeting treatment of chronic, inflammatory, and neuropathic pain syndromes.

DATA SOURCES

Published research, literature review articles, and abstracts as well as national statistics.

DATA SYNTHESIS

Treatment for chronic pain associated with cancer and other syndromes remains suboptimal and falls significantly short of clinical needs. Data highlight the role that multiple neurobiologic mechanisms play in modulating and maintaining pain at various levels of the central and peripheral nervous systems. Novel agents have been developed that use a more targeted approach to treating chronic pain.

CONCLUSIONS

A growing body of evidence highlights the critical role that neurobiologic mechanisms play in the initiation and maintenance of chronic pain. A thorough understanding of these mechanisms ultimately may result in targeted treatment approaches that focus on the central and peripheral mechanisms involved in mediation of chronic, inflammatory, and neuropathic pain syndromes.

IMPLICATIONS FOR NURSING

A majority of patients undergoing active treatment for cancer experiences unrelieved pain. By gaining a better understanding of the mechanisms that generate and maintain chronic pain, oncology nurses can promote targeted pain management strategies that incorporate novel therapeutic agents.

New Treatments for Neuropathic Pain

Existing treatments for neuropathic pain deliver inadequate pain relief, unacceptable side effects, or both. The unmet medical need for more effective treatment is driving a large volume of research to discover new drugs. Most existing treatments are drugs introduced to treat other pain conditions or other medical conditions, such as antidepressants and anticonvulsants, which were found empirically to be effective for neuropathic pain. Only recently have drug discovery efforts have become mechanistically driven, addressing targets identified by a molecular neurobiological approach to the pathophysiology of neuropathic states.

Use of a von Frey device for evaluation of pharmacokinetics and pharmacodynamics of morphine after intravenous administration as an infusion or multiple doses in dogs

OBJECTIVE

To evaluate the pharmacokinetics and pharmacodynamics of morphine after IV administration as an infusion or multiple doses in dogs by use of a von Frey (vF) device.

ANIMALS

6 dogs.

PROCEDURE

In the first 2 crossover experiments of a 3-way crossover study, morphine or saline (0.9%) solution was administered via IV infusion. Loading doses and infusion rates were administered to attain targeted plasma concentrations of 10, 20, 30, and 40 ng/mL. In the third experiment, morphine (0.5 mg/kg) was administered IV every 2 hours for 3 doses. The vF thresholds were measured hourly for 8 hours. Plasma concentrations of morphine were measured by high-pressure liquid chromatography.

RESULTS

No significant changes in vF thresholds were observed during infusion of saline solution. The vF thresholds were significantly increased from 5 to 8 hours during the infusion phase, corresponding to targeted morphine plasma concentrations > 30 ng/mL and infusion rates > or = 0.15 +/- 0.02 mg/kg/h. The maximal effect (EMAX) was 78 +/- 11% (percentage change from baseline), and the effective concentration to attain a 50% maximal response (EC50) was 29.5 +/- 5.4 ng/mL. The vF thresholds were significantly increased from 1 to 7 hours during the multiple-dose phase; the EC50 and EMAX were 23.9 +/- 4.7 ng/mL and 173 +/- 58%, respectively. No significant differences in half-life, volume of distribution, or clearance between the first and last dose of morphine were detected.

CONCLUSIONS AND CLINICAL RELEVANCE

Morphine administered via IV infusion (0.15 +/- 0.02 mg/kg/h) and multiple doses (0.5 mg/kg, IV, every 2 hours for 3 doses) maintained significant antinociception in dogs.

Nonmedical use of prescription opioids among U.S. college students: prevalence and correlates from a national survey

OBJECTIVES

This study examined the prevalence rates and correlates of nonmedical use of prescription opioid analgesics among U.S. college students in terms of student and college characteristics.

METHODS

This study analyzed data from a nationally representative sample of 10,904 randomly selected students attending 119 four-year colleges in 2001.

RESULTS

The lifetime prevalence of nonmedical prescription opioid use was 12% and the past year prevalence was 7%. Approximately one in every four colleges had a prevalence of 10% or higher for past year nonmedical use of prescription opioids. Multivariate regression analyses indicated nonmedical use was more likely to occur among college students who were white, residents of fraternity and sorority houses, attended more competitive colleges, earned lower grade point averages, and reported higher rates of substance use and other risky behaviors.

CONCLUSIONS

This study provides evidence that the nonmedical use of prescription opioids represents a problem on college campuses. These findings have important implications for developing prevention efforts and therapeutic strategies aimed at reducing the nonmedical use of prescription opioid analgesics among college students while not hindering necessary medication management for pain.

Potential applications of siRNA for pain therapy

Chronic pain is a major socio-economic burden which remains poorly treated owing to either the lack of efficacy or the dose-limiting, adverse effects of current clinical therapies. It is currently a subject of intense pharmaceutical activity, focused on developing more efficacious therapies. A recent development involving RNA interference (RNAi) in a pathophysiological model of chronic pain highlighted a novel approach to potential treatment in humans. Delivery of oligonucleotides led to a potent and highly selective degradation of a target mRNA and produced a measurable abrogation in pain-related behaviour. This review will discuss the potential of small interfering RNA (siRNA) in the treatment of chronic pain by evaluating points of intervention in chronic pain pathways that may be amenable to siRNA application. Selected studies using RNAi in vivo in various model systems which allow insight into its application in humans are reviewed. Chemical formats and routes of administration that may suit chronic pain siRNA therapies are considered, and some potential clinical hurdles to siRNA delivery in humans are discussed.

Illicit use of prescription pain medication among college students

This study identifies the prevalence, correlates, and sources associated with the illicit use of prescription pain medication among undergraduate college students. A cross-sectional, web-based survey was self-administered by a large random sample of 9,161 undergraduate students attending a large Midwestern university in the United States. Although undergraduate women were more likely to be prescribed pain medication, men were more likely to be approached to divert their prescription pain medication and report illicit use of prescription pain medication. Multivariate analyses indicated past year illicit use of prescription pain medication was higher among undergraduate students who were: previously prescribed pain medication, living in a house or apartment, and earning lower grade point averages. The majority of students obtained prescription pain medication for illicit use from peers and the second leading source was family members. There were several gender differences in the risk factors and sources of illicit use of prescription pain medication. Based on qualitative data, illicit use included recreational use for the purposes of intoxication and self-medication for pain episodes. Illicit users, who obtained prescription pain medication from peers, reported significantly higher rates of other substance use while those who obtained prescription medications from family members did not. These findings suggest that the illicit use of prescription pain medications may represent a problem among undergraduate students and effective prevention efforts are needed that account for gender differences.

Deletion of vanilloid receptor 1-expressing primary afferent neurons for pain control

Control of cancer, neuropathic, and postoperative pain is frequently inadequate or compromised by debilitating side effects. Inhibition or removal of certain nociceptive neurons, while retaining all other sensory modalities and motor function, would represent a new therapeutic approach to control severe pain. The enriched expression of transient receptor potential cation channel, subfamily V, member 1 (TRPV1; also known as the vanilloid receptor, VR1) in nociceptive neurons of the dorsal root and trigeminal ganglia allowed us to test this concept. Administration of the potent TRPV1 agonist resiniferatoxin (RTX) to neuronal perikarya induces calcium cytotoxicity by opening the TRPV1 ion channel and selectively ablates nociceptive neurons. This treatment blocks experimental inflammatory hyperalgesia and neurogenic inflammation in rats and naturally occurring cancer and debilitating arthritic pain in dogs. Sensations of touch, proprioception, and high-threshold mechanosensitive nociception, as well as locomotor function, remained intact in both species. In separate experiments directed at postoperative pain control, subcutaneous administration of RTX transiently disrupted nociceptive nerve endings, yielding reversible analgesia. In human dorsal root ganglion cultures, RTX induced a prolonged increase in intracellular calcium in vanilloid-sensitive neurons, while leaving other, adjacent neurons unaffected. The results suggest that nociceptive neuronal or nerve terminal deletion will be effective and broadly applicable as strategies for pain management.

The synthesis and pharmacological evaluation of (±)-2, 3- seco-fentanyl analogues

An efficient, five-step synthetic approach to various acyclic 1,3-diamines has been developed and applied to the preparation of a novel class of open-chained fentanyl analogues. The acyclic derivatives 5.1?5.5 (all new compounds) were synthesized with the aim of estimating the significance of the piperidine ring for the opioid analgesic activity of anilido-piperidines. The starting ?-keto-amide 1.1, prepared by the aminolysis of methyl acetoacetate with methylphenethylamine, (93 % yield), was successively reacted with NaH and BuLi, to form the highly reactive ?,?-dienolate anion 1.1a. Regio and chemoselective ?-alkylation of the dienolate with various primary and secondary alkyl halides furnished the ?-keto-amides 1.2?1.5 (76?91 %). Reductive amination of the keto-amides 1.1?1.5 with aniline and Zn powder in acetic acid, via the enamine intermediates 2.1?2.5, afforded the ?-anilino amides 3.1?3.5 (74?85 %). After reductive deoxygenation of the tertiary amide group, using in situ generated diborane, the corresponding 1,3-diamines 4.1?4.5 were obtained (87?97 %). The synthesis of (?)-2,3-seco-fentanyls 5.1?5.5 was completed by N-acylation of the diamines 4.1?4.5 with propionyl chloride, followed by precipitation of the monooxalate salts (86?95 %). The parent compound, 2,3-seco-fentanyl 5.1, was found to be a 40 times less potent narcotic analgesic than fentanyl but still 5?6 times more active than morphine in rats, while i-Pr derivative 5.3 was inactive. Apart from the pharmacological significance, the general procedure described herein may afford various functionalized, 1,3-diamines as potential complexing agents and building blocks for the synthesis of aza-crown ethers.