A randomized, double-blind, placebo-controlled trial evaluating the efficacy and safety of ABT-594 in patients with diabetic peripheral neuropathic pain

Different Drugs for the Treatment of Painful Diabetic Peripheral Neuropathy: A Meta-Analysis

Objective: To systematically evaluate the effects of different drugs for the treatment of painful diabetic peripheral neuropathy.

Methods: All literature from PubMed, Embase, and Cochrane Central Register of Controlled Trials published over the past 12 years (from January 1, 2008 to June 1, 2020) was searched, and two reviewers independently assessed study eligibility, continuous data extraction, independent assessment of bias risk, and graded strength of evidence. The pain score was used as the main result, and 30 and 50% pain reduction and adverse events were used as secondary results.

Results: A total of 37 studies were included. Pregabalin, duloxetine, tapentadol, lacosamide, mirogabalin, and capsaicin were all more effective than placebo in alleviating the pain associated with diabetic peripheral neuropathy, while ABT-894 and gabapentin showed no significant effect. In addition, the efficacy of buprenorphine, tanezumab, fulranumab and others could not be concluded due to insufficient studies.

Conclusion: Pregabalin and duloxetine showed good therapeutic effects on painful DPN, but adverse events were also significant. The analgesic effects of ABT-894 and gabapentin need to be further studied with longer and larger RCTs. As an opioid drug, tapentadol has a good analgesic effect, but due to its addiction, it needs to be very cautious in clinical use. Although lacosamide, mirogabalin, and capsaicin are more effective than placebo, the therapeutic effect is weaker than pregabalin. For the results of our meta-analysis, long-term studies are still needed to verify their efficacy and safety in the future.

Systematic Review Registration: PROSPERO, identifier: CRD42020197397.

Nicotinic acetylcholine receptor redux: Discovery of accessories opens therapeutic vistas

The neurotransmitter acetylcholine (ACh) acts in part through a family of nicotinic ACh receptors (nAChRs), which mediate diverse physiological processes including muscle contraction, neurotransmission, and sensory transduction. Pharmacologically, nAChRs are responsible for tobacco addiction and are targeted by medicines for hypertension and dementia. Nicotinic AChRs were the first ion channels to be isolated. Recent studies have identified molecules that control nAChR biogenesis, trafficking, and function. These nAChR accessories include protein and chemical chaperones as well as auxiliary subunits. Whereas some factors act on many nAChRs, others are receptor specific. Discovery of these regulatory mechanisms is transforming nAChR research in cells and tissues ranging from central neurons to spinal ganglia to cochlear hair cells. Nicotinic AChR-specific accessories also enable drug discovery on high-confidence targets for psychiatric, neurological, and auditory disorders.

Functional α6β4 acetylcholine receptor expression enables pharmacological testing of nicotinic agonists with analgesic properties

The α6β4 nicotinic acetylcholine receptor (nAChR) is enriched in dorsal root ganglia neurons and is an attractive non-opioid therapeutic target for pain. However, difficulty expressing human α6β4 receptors in recombinant systems has precluded drug discovery. Here, genome-wide screening identified accessory proteins that enable reconstitution of human α6β4 nAChRs. BARP, an auxiliary subunit of voltage-dependent calcium channels, promoted α6β4 surface expression while IRE1α, an unfolded protein response sensor, enhanced α6β4 receptor assembly. Effects on α6β4 involve BARP's N-terminal region and IRE1α's splicing of XBP1 mRNA. Furthermore, clinical efficacy of nicotinic agents in relieving neuropathic pain best correlated with their activity on α6β4. Finally, BARP-knockout, but not NACHO-knockout mice lacked nicotine-induced antiallodynia, highlighting the functional importance of α6β4 in pain. These results identify roles for IRE1α and BARP in neurotransmitter receptor assembly and unlock drug discovery for the previously elusive α6β4 receptor.

Countering Opioid-induced Respiratory Depression in Male Rats with Nicotinic Acetylcholine Receptor Partial Agonists Varenicline and ABT 594

BACKGROUND

Opioids can induce significant respiratory depression when administered as analgesics for the treatment of acute, postoperative, and chronic pain. There are currently no pharmacologic means of reversing opioid-induced respiratory depression without interfering with analgesia. Further, there is a growing epidemic of opioid overdose that could benefit from therapeutic advancements. The aim of this study was to test the ability of two partial agonists of α4β2 nicotinic acetylcholine receptors, varenicline (used clinically for smoking cessation) and ABT 594 (tebanicline, developed as an analgesic), to reduce respiratory depression induced by fentanyl, remifentanil, morphine, and a combination of fentanyl and diazepam.

METHODS

Whole body plethysmographic recordings, nociception testing, and righting reflex testing were used to examine ventilation, analgesia, and sedation in adult male Sprague-Dawley rats.

RESULTS

Pre-, co-, or postadministration of varenicline or ABT 594 did not alter baseline breathing but markedly reduced opioid-induced respiratory depression. Varenicline had no effect on fentanyl-induced analgesia and ABT 594 potentiated fentanyl-induced analgesia. Specifically, 10-min administration of fentanyl induced a decrease in respiratory rate to 43 ± 32% of control in vehicle group, which was alleviated by preadministration of varenicline (85 ± 14% of control, n = 8, P < 0.001) or ABT 594 (81 ± 36% of control, n = 8, P = 0.001). ABT 594 or varenicline with a low dose of naloxone (1 µg/kg), but not varenicline alone, partially reversed fentanyl-induced lethal apnea, but neither compound provided the very rapid and complete reversal of apnea achieved with high doses of naloxone (0.03 to 1 mg/kg). Administration of varenicline (n = 4, P = 0.034) or ABT 594 (n = 4, P = 0.034) prevented lethal apneas induced by the combination of fentanyl and diazepam.

CONCLUSIONS

Activation of α4β2 nicotinic acetylcholine receptors by varenicline and ABT 594 counters opioid-induced respiratory depression without interfering with analgesia.

Neuropathic Pain: From Mechanisms to Treatment

Neuropathic pain caused by a lesion or disease of the somatosensory nervous system is a common chronic pain condition with major impact on quality of life. Examples include trigeminal neuralgia, painful polyneuropathy, postherpetic neuralgia, and central poststroke pain. Most patients complain of an ongoing or intermittent spontaneous pain of, for example, burning, pricking, squeezing quality, which may be accompanied by evoked pain, particular to light touch and cold. Ectopic activity in, for example, nerve-end neuroma, compressed nerves or nerve roots, dorsal root ganglia, and the thalamus may in different conditions underlie the spontaneous pain. Evoked pain may spread to neighboring areas, and the underlying pathophysiology involves peripheral and central sensitization. Maladaptive structural changes and a number of cell-cell interactions and molecular signaling underlie the sensitization of nociceptive pathways. These include alteration in ion channels, activation of immune cells, glial-derived mediators, and epigenetic regulation. The major classes of therapeutics include drugs acting on α₂δ subunits of calcium channels, sodium channels, and descending modulatory inhibitory pathways.

Behavioral and Molecular Basis of Cholinergic Modulation of Pain: Focus on Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) have emerged as a novel therapeutic strategy for pain and inflammatory disorders. In particular, α4β2∗, α7, and α9α10 nAChR subtypes have been investigated as potential targets to treat pain. The nAChRs are distributed on the pain transmission pathways, including central and peripheral nervous systems and immune cells as well. Several agonists for α4β2∗ nAChR subtypes have been investigated in multiple animal pain models with promising results. However, studies in human indicated a narrow therapeutic window for α4β2∗ agonists. Furthermore, animal studies suggest that using agonists for α7 nAChR subtype and antagonists for α9α10 nAChR subtypes are potential novel therapies for chronic pain management, including inflammatory and neuropathic pain. More recently, alternative nAChRs ligands such as positive allosteric modulators and silent agonists have shown potential to develop into new treatments for chronic pain.

RgIA4 Accelerates Recovery from Paclitaxel-Induced Neuropathic Pain in Rats

Chemotherapeutic drugs are widely utilized in the treatment of human cancers. Painful chemotherapy-induced neuropathy is a common, debilitating, and dose-limiting side effect for which there is currently no effective treatment. Previous studies have demonstrated the potential utility of peptides from the marine snail from the genus Conus for the treatment of neuropathic pain. α-Conotoxin RgIA and a potent analog, RgIA4, have previously been shown to prevent the development of neuropathy resulting from the administration of oxaliplatin, a platinum-based antineoplastic drug. Here, we have examined its efficacy against paclitaxel, a chemotherapeutic drug that works by a mechanism of action distinct from that of oxaliplatin. Paclitaxel was administered at 2 mg/kg (intraperitoneally (IP)) every other day for a total of 8 mg/kg. Sprague Dawley rats that were co-administered RgIA4 at 80 µg/kg (subcutaneously (SC)) once daily, five times per week, for three weeks showed significant recovery from mechanical allodynia by day 31. Notably, the therapeutic effects reached significance 12 days after the last administration of RgIA4, which is suggestive of a rescue mechanism. These findings support the effects of RgIA4 in multiple chemotherapeutic models and the investigation of α9α10 nicotinic acetylcholine receptors (nAChRs) as a non-opioid target in the treatment of chronic pain.

Nicotine inhibits rapamycin-induced pain through activating mTORC1/S6K/IRS-1-related feedback inhibition loop

Mammalian target of rapamycin complex 1 (mTORC1) inhibitors increase the incidence of pain in patients, and this finding has been replicated in animal models. However, reports on possible analgesics for this condition are scant. Accumulating evidence finds that nicotinic acetylcholine receptors (nAChRs) are involved in mediating pain. However, whether nicotine, a full agonist of nAChRs, alleviates mTORC1 inhibition-induced pain and its underlying mechanisms remain unknown. In this study, pain was induced in naïve male C57BL/6J mice by intraperitoneally injecting rapamycin acutely or repeatedly. Subsequently, pain thresholds, including mechanical and thermal pain, were measured. The involving signaling pathway was tested using western blot analysis and immunofluorescent assay. Changes in neuronal excitability caused by different treatments were also analyzed using whole-cell recording. Microinjection into the anterior cingulate cortex (ACC) was used to test the role of nAChRs containing the α4β2 or α7 subtype in this brain region in pain modulation. Our results showed that nicotine significantly reduced hyperalgesia in mice that received acute or repeated rapamycin injections, and reversed the effects of rapamycin on the phosphorylation of S6K, 4E-BP1, insulin receptor substrate-1 (IRS-1) at Ser636/639, AKT at Ser473, and ERK at Thr202/Tyr204. Whole-cell recording results showed that nicotine reduced the firing rates of pyramidal neurons in the ACC, and a pharmacological blockade of nAChRs containing the α4β2 or α7 subtype in ACC inhibited the antinociceptive effects of nicotine in mice with rapamycin-induced pain. Our findings indicate that analgesics targeting nAChRs can be developed to help patients with rapamycin-induced pain.

Nicotine Evoked Currents in Human Primary Sensory Neurons

Sensory neuron nicotinic acetylcholine receptors (nAChRs) contribute to pain associated with tissue injury. However, there are marked differences between rats and mice with respect to both the properties and distribution of nAChR currents in sensory neurons. Because both species are used to understand pain signaling in humans, we sought to determine whether the currents present in either species was reflective of those present in human sensory neurons. Neurons from the L4/L5 dorsal root ganglia were obtained from adult male and female organ donors. Nicotine evoked currents were detected in 40 of 47 neurons (85%). In contrast with the naïve mouse, in which almost all nAChR currents are transient, or the rat, in which both mouse-like transient and more slowly activating and inactivating currents are detected, all the currents in human DRG neurons were slow, but slower than those in the rat. Currents were blocked by the nAChR antagonists mecamylamine (30 µmol/L), but not by the TRPA1 selective antagonist HC-030031 (10 µmol/L). Single cell polymerase chain reaction analysis of nicotinic receptor subunit expression in human DRG neurons are consistent with functional data indicating that receptor expression is detected 85 ± 2.1% of neurons assessed (n = 48, from 4 donors). The most prevalent coexpression pattern was α3/β2 (95 ± 4% of neurons with subunits), but α7 subunits were detected in 70 ± 3.4% of neurons. These results suggest that there are not only species differences in the sensory neuron distribution of nAChR currents between rodent and human, but that the subunit composition of the channel underlying human nAChR currents may be different from those in the mouse or rat. PERSPECTIVE: The properties and distribution of nicotine evoked currents in human sensory neurons were markedly different from those previously observed in mice and rats. These observations add additional support to the suggestion that human sensory neurons may be an essential screening tool for those considering moving novel therapeutics targeting primary afferents into clinical trials.

Epibatidine: A Promising Natural Alkaloid in Health

Epibatidine is a natural alkaloid that acts at nicotinic acetylcholine receptors (nAChRs). The present review aims to carefully discuss the affinity of epibatidine and its synthetic derivatives, analogues to nAChRs for α4β2 subtype, pharmacokinetic parameters, and its role in health. Published literature shows a low affinity and lack of binding of epibatidine and its synthetic analogues to plasma proteins, indicating their availability for metabolism. Because of its high toxicity, the therapeutic use of epibatidine is hampered. However, new synthetic analogs endowed from this molecule have been developed, with a better therapeutic window and improved selectivity. All these aspects are also discussed here. On the other hand, many reports are devoted to structure⁻activity relationships to obtain optically active epibatidine and its analogues, and to access its pharmacological effects. Although pharmacological results are obtained from experimental studies and only a few clinical trials, new perspectives are open for the discovery of new drug therapies.

Novel agonist of α4β2* neuronal nicotinic receptor with antinociceptive efficacy in rodent models of acute and chronic pain

Objective

To demonstrate the antinociceptive and antihypersensitivity mechanisms of Cris-104 (1-{2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl]ethyl}piperidine), a novel selective α₄β₂* nicotinic acetylcholine receptor (nAChR) agonist, in rodent acute/inflammatory and chronic pain models.

Materials and methods

Hot-plate and formalin tests in mice were used to examine Cris-104-induced antinociceptive effects on thermal/inflammatory pain. Cris-104 effects on hypersensitivity, norepinephrine (NE) release in the spinal dorsal horn, and neuronal activity in the locus coeruleus (LC) were examined in rats with lumbar spinal nerve ligation using behavioral, microdialysis, and extracellular recording methods. Cris-104 effects on spontaneous locomotion were examined in an open-field test.

Results

Cris-104 induced dose-dependent antinociception effects in hot-plate and formalin tests, and these effects were blocked by the general nAChR antagonist mecamylamine, the selective α₄β₂* nAChR antagonist dihydro-beta-erythroidine, and the α₂-adrenoceptor antagonist yohimbine, but not by the α₁-adrenoceptor antagonist prazosin. Systemic and spinally perfused Cris-104 increased NE concentrations in microdialysates from the spinal cord in both normal and SNL rats. Systemic Cris-104 increased neuronal activity in the LC of normal rats. Mecamylamine blocked the effects of Cris-104 on spinal NE release and LC neuronal activity. Systemic Cris-104 did not affect locomotor activity significantly.

Conclusion

The α₄β₂ neuronal nAChR agonist, Cris-104, was effective for treatment of pain via descending noradrenergic inhibition of pain signaling.

Nicotinic modulation of descending pain control circuitry

Along with the well-known rewarding effects, activation of nicotinic acetylcholine receptors (nAChRs) can also relieve pain, and some nicotinic agonists have analgesic efficacy similar to opioids. A major target of analgesic drugs is the descending pain modulatory pathway, including the ventrolateral periaqueductal gray (vlPAG) and the rostral ventromedial medulla (RVM). Although activating nAChRs within this circuitry can be analgesic, little is known about the subunit composition and cellular effects of these receptors, particularly within the vlPAG. Using electrophysiology in brain slices from adult male rats, we examined nAChR effects on vlPAG neurons that project to the RVM. We found that 63% of PAG-RVM projection neurons expressed functional nAChRs, which were exclusively of the α7-subtype. Interestingly, the neurons that express α7 nAChRs were largely nonoverlapping with those expressing μ-opioid receptors (MOR). As nAChRs are excitatory and MORs are inhibitory, these data suggest distinct roles for these neuronal classes in pain modulation. Along with direct excitation, we also found that presynaptic nAChRs enhanced GABAergic release preferentially onto neurons that lacked α7 nAChRs. In addition, presynaptic nAChRs enhanced glutamatergic inputs onto all PAG-RVM projection neuron classes to a similar extent. In behavioral testing, both systemic and intra-vlPAG administration of the α7 nAChR-selective agonist, PHA-543,613, was antinociceptive in the formalin assay. Furthermore, intra-vlPAG α7 antagonist pretreatment blocked PHA-543,613-induced antinociception via either administration method. Systemic administration of submaximal doses of the α7 agonist and morphine produced additive antinociceptive effects. Together, our findings indicate that the vlPAG is a key site of action for α7 nAChR-mediated antinociception.

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Allosteric modulation of α4β2* nicotinic acetylcholine receptors: Desformylflustrabromine potentiates antiallodynic response of nicotine in a mouse model of neuropathic pain

BACKGROUND

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels. The α4β2 subtype of nAChRs plays an important role in the mediation of pain and several nicotine-evoked responses. Agonists and partial agonists of α4β2 nAChRs show efficacy in animal pain models. In addition, the antinociceptive properties of nicotine, a non-selective nAChR agonist with a high affinity for α4β2 nAChRs, is well-known. There is a growing body of evidence pointing to allosteric modulation of nAChRs as an alternative treatment strategy in experimental pain. Desformylflustrabromine (dFBr) is a positive allosteric modulator (PAM) at α4β2 nAChRs that enhances agonist responses without activating receptors. We hypothesized that dFBr may enhance nicotine-induced antinociception.

METHODS

The present study investigated whether dFBr could attenuate mouse chronic constriction injury (CCI)-induced neuropathic pain by increasing endogenous cholinergic tone or potentiating the nicotine-evoked antiallodynic response.

RESULTS

We found that subcutaneous administration of dFBr failed to reduce pain behaviour on its own. However, the combination of dFBr with nicotine significantly reversed neuropathic pain behaviour dose- and time-dependently without motor impairment. Our data revealed that this effect was mediated by the α4β2 nAChRs by using competitive α4β2 antagonist dihydro-β-erythroidine. In addition, dFBr failed to potentiate the antiallodynic effect of morphine, which shows the effect of dFBr is unique to α4β2 nAChRs.

CONCLUSIONS

The present results suggest that allosteric modulation of α4β2 nAChR may provide new strategies in chronic neuropathic pain.

SIGNIFICANCE

α4β2 nAChRs are involved in pain modulation. dFBr, a PAM at α4β2 nAChRs, potentiates the nicotine response dose-dependently in neuropathic pain. Thus, the present results suggest that allosteric modulation of α4β2* nAChR may provide new strategies in chronic neuropathic pain.

Nicotinic acetylcholine receptors in neuropathic and inflammatory pain

Nicotinic acetylcholine receptors (nAChRs) are actively being investigated as therapeutic targets for the treatment of pain and inflammation, but despite more than 30 years of research, there are currently no FDA-approved analgesics that are specific for these receptors. Much of the initial research effort focused on the α4β2 nAChR subtype, but more recently, additional subtypes have been identified as promising new leads and include α6β4, α7, and α9-containing nAChRs. This Review will focus on the distribution of these nAChRs in the cell types involved in neuropathic pain and inflammation and the activity of currently available nicotinic ligands.

Nicotine Prevents and Reverses Paclitaxel-Induced Mechanical Allodynia in a Mouse Model of CIPN

Chemotherapy-induced peripheral neuropathy (CIPN), a consequence of peripheral nerve fiber dysfunction or degeneration, continues to be a dose-limiting and debilitating side effect during and/or after cancer chemotherapy. Paclitaxel, a taxane commonly used to treat breast, lung, and ovarian cancers, causes CIPN in 59-78% of cancer patients. Novel interventions are needed due to the current lack of effective CIPN treatments. Our studies were designed to investigate whether nicotine can prevent and/or reverse paclitaxel-induced peripheral neuropathy in a mouse model of CIPN, while ensuring that nicotine will not stimulate lung tumor cell proliferation or interfere with the antitumor properties of paclitaxel. Male C57BL/6J mice received paclitaxel every other day for a total of four injections (8 mg/kg, i.p.). Acute (0.3-0.9 mg/kg, i.p.) and chronic (24 mg/kg per day, s.c.) administration of nicotine respectively reversed and prevented paclitaxel-induced mechanical allodynia. Blockade of the antinociceptive effect of nicotine with mecamylamine and methyllycaconitine suggests that the reversal of paclitaxel-induced mechanical allodynia is primarily mediated by the α7 nicotinic acetylcholine receptor subtype. Chronic nicotine treatment also prevented paclitaxel-induced intraepidermal nerve fiber loss. Notably, nicotine neither promoted proliferation of A549 and H460 non-small cell lung cancer cells nor interfered with paclitaxel-induced antitumor effects, including apoptosis. Most importantly, chronic nicotine administration did not enhance Lewis lung carcinoma tumor growth in C57BL/6J mice. These data suggest that the nicotinic acetylcholine receptor-mediated pathways may be promising drug targets for the prevention and treatment of CIPN.

Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two α/β subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some α/α and β/α subunit interfaces, such as α4/α4, α5/α4 and β3/α4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (α4β2)₂ α5, (α4β2)₂ β3 and (α6β2)₂ β3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox α4/α4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered α4/α4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at β/α), the C-terminus (e.g. Br-PBTC and 17β-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17β-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Plasticity in Brainstem Mechanisms of Pain Modulation by Nicotinic Acetylcholine Receptors in the Rat

Individuals with chronic pain may be driven to smoke more because the analgesic efficacy of nicotine diminishes. To determine whether persistent pain diminishes the actions of a nicotinic acetylcholine receptor (nAChR) agonist in pain modulatory pathways, we examined the effects of epibatidine in the rostral ventromedial medulla (RVM) of rats with and without inflammatory injury induced by intraplantar injection of complete Freund's adjuvant (CFA). In uninjured rats, epibatidine produced a dose-dependent antinociception that was completely blocked by dihydro-β-erythroidine (DHβE; α4β2 antagonist) and partially blocked by methyllycaconitine (MLA; α7 antagonist). Epibatidine reversed heat hyperalgesia when microinjected in the RVM 4 h, 4 d, or 2 weeks after CFA treatment. Although DHβE completely blocked epibatidine's antihyperalgesic effect at 4 h, at 2 weeks it elicited only partial antagonism. Methyllycaconitine was ineffective at both time points. Epibatidine's antinociceptive efficacy in the uninjured hind paw progressively declined, and it was without effect 2 weeks after CFA. Moreover, as early as 4 h after CFA, the antinociceptive effect of epibatidine was no longer antagonized by DHβE. Neither antagonist alone altered paw withdrawal latency in uninjured or CFA-treated rats, suggesting that neither α4β2 nor α7 nAChRs are tonically active in the RVM. The Bmax and Kd of α4β2 nAChRs in the RVM were unchanged after CFA treatment. These observations provide the first evidence of pharmacological plasticity of the actions of α4β2 nAChR agonists in a critical brainstem pain modulatory pathway and may in part explain why people with chronic pain smoke more than the general population.

Nicotinic receptors modulate the function of presynaptic AMPA receptors on glutamatergic nerve terminals in the trigeminal caudal nucleus

In this study, we demonstrate the existence on trigeminal caudal nucleus (TCN) glutamatergic terminals of α4β2 nicotinic receptors (nAChRs) capable of enhancing the terminals' spontaneous release of [(3)H]d-aspartate ([(3)H]D-Asp). In rat TCN synaptosomes, spontaneous [(3)H]D-Asp release was increased by nicotine and the nicotinic receptor agonists (±)epibatidine and RJR2403. The increase was potentiated by the positive allosteric modulator of nAChRs LY2087101, inhibited by the nicotinic antagonists mecamylamine (MEC) and dihydro-β-erythroidine hydrobromide (DHβE), and unaffected by α-bungarotoxin (α-BgTx) and methyllycaconitine (MLA). Evidence of functional interaction was observed between the α4β2 nAChRs and cyclothiazide-sensitive, alfa-amino-3-hydroxy-5-methyl-4-isoxazolone propionate (AMPA) receptors co-localized on the TCN synaptosomes. Brief pre-exposure of synaptosomes to 30 μM nicotine or 10 μM RJR2403 abolished the AMPA (100 μM) -induced potentiation of [K(+)]e-evoked [(3)H]D-Asp release, an effect that seems to be caused by nicotine-induced increases in the internalization of AMPA receptors. Indeed, the effects of nicotine-pretreatment were not seen in synaptosomes containing pre-entrapped pep2-SVKI, a peptide known to compete for the binding of GluA2 subunit to scaffolding proteins involved in AMPA endocytosis, while entrapment of pep2-SVKE, an inactive peptide used as negative control, was inefficacious. These findings show that nicotine can negatively modulate the function of AMPA receptors present on glutamatergic nerve terminals in the rat TCN. Dynamic control of AMPA receptors by the nicotinic cholinergic system has been observed under other experimental conditions, and it can contribute to the control of synaptic plasticity such as long-term depression and potentiation. Nicotine's ability to reduce the functionality of presynaptic AMPA receptors could contribute to its analgesic effects by diminishing glutamatergic transmission from the primary afferent terminals that convey nociceptive input to TCN.

The role of alpha5 nicotinic acetylcholine receptors in mouse models of chronic inflammatory and neuropathic pain

The aim of the present study was to determine the impact of α5 nicotinic acetylcholine receptor (nAChR) subunit deletion in the mouse on the development and intensity of nociceptive behavior in various chronic pain models. The role of α5-containing nAChRs was explored in mouse models of chronic pain, including peripheral neuropathy (chronic constriction nerve injury, CCI), tonic inflammatory pain (the formalin test) and short and long-term inflammatory pain (complete Freund's adjuvant, CFA and carrageenan tests) in α5 knock-out (KO) and wild-type (WT) mice. The results showed that paw-licking time was decreased in the formalin test, and the hyperalgesic and allodynic responses to carrageenan and CFA injections were also reduced. In addition, paw edema in formalin-, carrageenan- or CFA-treated mice were attenuated in α5-KO mice significantly. Furthermore, tumor necrosis factor-alpha (TNF-α) levels of carrageenan-treated paws were lower in α5-KO mice. The antinociceptive effects of nicotine and sazetidine-A but not varenicline were α5-dependent in the formalin test. Both hyperalgesia and allodynia observed in the CCI test were reduced in α5-KO mice. Nicotine reversal of mechanical allodynia in the CCI test was mediated through α5-nAChRs at spinal and peripheral sites. In summary, our results highlight the involvement of the α5 nAChR subunit in the development of hyperalgesia, allodynia and inflammation associated with chronic neuropathic and inflammatory pain models. They also suggest the importance of α5-nAChRs as a target for the treatment of chronic pain.

Role of α5-containing nicotinic receptors in neuropathic pain and response to nicotine

Nicotinic receptors in the central nervous system (nAChRs) are known to play important roles in pain processing and modulate behavioral responses to analgesic drugs, including nicotine. The presence of the α5-neuronal nicotinic accessory subunit in the nicotinic receptor complex is increasingly understood to modulate reward and aversive states, addiction, and possibly pathological pain. In the current study, using α5-knockout (KO) mice and subunit-specific antibodies, we assess the role of α5-containing neuronal nicotinic receptors in neuropathic pain and in the analgesic response to nicotine. After chronic constriction injury (CCI) or partial sciatic nerve ligation (PSNL), no differences in mechanical, heat, or cold hyperalgesia were found in wild-type (WT) versus α5-KO littermate mice. The number of α5-containing nAChRs was decreased (rather than increased) after CCI in the spinal cord and in the thalamus. Nevertheless, thermal analgesic response to nicotine was marginally reduced in CCI α5-KO mice at 4 days after CCI, but not at later timepoints or after PSNL. Interestingly, upon daily intermittent nicotine injections in unoperated mice, WT animals developed tolerance to nicotine-induced analgesia to a larger extent than α5-KO mice. Our results suggest that α5-containing nAChRs mediate analgesic tolerance to nicotine but do not play a major role in neuropathic pain.

Nicotinic ACh receptors as therapeutic targets in CNS disorders

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability by acting on the cys-loop cation-conducting ligand-gated nicotinic ACh receptor (nAChR) channels. These receptors are widely distributed throughout the central nervous system (CNS), being expressed on neurons and non-neuronal cells, where they participate in a variety of physiological responses such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and cognitive functions. In the mammalian brain, nine different subunits have been found thus far, which assemble into pentameric complexes with much subunit diversity; however, the α7 and α4β2 subtypes predominate in the CNS. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders. Here we will briefly discuss the functional makeup and expression of the nAChRs in mammalian brain, and their role as targets in neurodegenerative diseases (in particular Alzheimer's disease, AD), neurodevelopmental disorders (in particular autism and schizophrenia), and neuropathic pain.

Engineered α4β2 nicotinic acetylcholine receptors as models for measuring agonist binding and effect at the orthosteric low-affinity α4-α4 interface

The nicotinic acetylcholine receptor α4β2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(β2)3 and (α4)3(β2)2. While these are similar in many aspects, the (α4)3(β2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-β2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(β2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered β2 construct (β2(HQT)), which converts the β(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(β2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of ∼5 nM was observed in sharp contrast to a Kd value of ∼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(β2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-β2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4β2 receptors.

Reporting of sample size calculations in analgesic clinical trials: ACTTION systematic review

Sample size calculations determine the number of participants required to have sufficiently high power to detect a given treatment effect. In this review, we examined the reporting quality of sample size calculations in 172 publications of double-blind randomized controlled trials of noninvasive pharmacologic or interventional (ie, invasive) pain treatments published in European Journal of Pain, Journal of Pain, and Pain from January 2006 through June 2013. Sixty-five percent of publications reported a sample size calculation but only 38% provided all elements required to replicate the calculated sample size. In publications reporting at least 1 element, 54% provided a justification for the treatment effect used to calculate sample size, and 24% of studies with continuous outcome variables justified the variability estimate. Publications of clinical pain condition trials reported a sample size calculation more frequently than experimental pain model trials (77% vs 33%, P < .001) but did not differ in the frequency of reporting all required elements. No significant differences in reporting of any or all elements were detected between publications of trials with industry and nonindustry sponsorship. Twenty-eight percent included a discrepancy between the reported number of planned and randomized participants. This study suggests that sample size calculation reporting in analgesic trial publications is usually incomplete. Investigators should provide detailed accounts of sample size calculations in publications of clinical trials of pain treatments, which is necessary for reporting transparency and communication of pre-trial design decisions.

PERSPECTIVE

In this systematic review of analgesic clinical trials, sample size calculations and the required elements (eg, treatment effect to be detected; power level) were incompletely reported. A lack of transparency regarding sample size calculations may raise questions about the appropriateness of the calculated sample size.

α-conotoxin RgIA protects against the development of nerve injury-induced chronic pain and prevents both neuronal and glial derangement

Neuropathic pain affects millions of people worldwide, causing substantial disability and greatly impairing quality of life. Commonly used analgesics or antihyperalgesic compounds are generally characterized by limited therapeutic outcomes. Thus, there is a compelling need for novel therapeutic strategies able to prevent nervous tissue alterations responsible for chronic pain. The α9α10 nicotinic acetylcholine receptor antagonist α-conotoxin RgIA (RgIA), a peptide isolated from the venom of a carnivorous cone snail, induces relief in both acute and chronic pain models. To evaluate potential disease-modifying effects of RgIA, the compound was given to rats following chronic constriction injury (CCI) of the sciatic nerve. Two or 10 nmol RgIA injected intramuscularly once a day for 14 days reduced the painful response to suprathreshold stimulation, increased pain threshold to nonnoxious stimuli, and normalized alterations in hind limb weight bearing. Histological analysis of the sciatic nerve revealed that RgIA prevented CCI-induced decreases of axonal compactness and diameter, loss of myelin sheath, and decreases in the fiber number. Moreover, RgIA significantly reduced edema and inflammatory infiltrate, including a decrease of CD86(+) macrophages. In L4-L5 dorsal root ganglia, RgIA prevented morphometric changes and reduced the inflammatory infiltrate consistent with a disease-modifying effect. In the dorsal horn of the spinal cord, RgIA prevented CCI-induced activation of microglia and astrocytes. These data suggest that RgIA-like compounds may represent a novel class of therapeutics for neuropathic pain that protects peripheral nervous tissues as well as prevents central maladaptive plasticity by inhibiting glial cell activation.

The therapeutic promise of positive allosteric modulation of nicotinic receptors

In the central nervous system, deficits in cholinergic neurotransmission correlate with decreased attention and cognitive impairment, while stimulation of neuronal nicotinic acetylcholine receptors improves attention, cognitive performance and neuronal resistance to injury as well as produces robust analgesic and anti-inflammatory effects. The rational basis for the therapeutic use of orthosteric agonists and positive allosteric modulators (PAMs) of nicotinic receptors arises from the finding that functional nicotinic receptors are ubiquitously expressed in neuronal and non-neuronal tissues including brain regions highly vulnerable to traumatic and ischemic types of injury (e.g., cortex and hippocampus). Moreover, functional nicotinic receptors do not vanish in age-, disease- and trauma-related neuropathologies, but their expression and/or activation levels decline in a subunit- and brain region-specific manner. Therefore, augmenting the endogenous cholinergic tone by nicotinic agents is possible and may offset neurological impairments associated with cholinergic hypofunction. Importantly, because neuronal damage elevates extracellular levels of choline (a selective agonist of α7 nicotinic acetylcholine receptors) near the site of injury, α7-PAM-based treatments may augment pathology-activated α7-dependent auto-therapies where and when they are most needed (i.e., in the penumbra, post-injury). Thus, nicotinic-PAM-based treatments are expected to augment the endogenous cholinergic tone in a spatially and temporally restricted manner creating the potential for differential efficacy and improved safety as compared to exogenous orthosteric nicotinic agonists that activate nicotinic receptors indiscriminately. In this review, I will summarize the existing trends in therapeutic applications of nicotinic PAMs.

Metabolism and disposition of ABT-894, a novel α4β2 neuronal acetylcholine receptor agonist, in mice and monkeys

1.  Metabolism and disposition of ABT-894 was investigated in hepatocytes, in mice and monkeys receiving [(14)C]ABT-894. 2.  In hepatocytes, turnover rate of ABT-894 was slow in all species with more than 90% of parent remaining. M3 (carbamoyl glucuronide) and M6 (mono-oxidation) were detected across species. 3.  ABT-894 showed species-specific disposition profiles. ABT-894 was primarily eliminated by renal secretion in mice. Whereas, monkey mainly cleared ABT-894 metabolically. 4.  ABT-894 underwent two primary routes of metabolism in monkeys: N-carbamoyl glucuronidation to form M3 and oxidation product M1. M3 was the major metabolite in monkey excreta. M3 was observed in mice urine. Circulating levels of M3 in terms of M3/ABT-894 ratios were essentially absent in mice, but were high in monkeys. 5.  Understanding the species difference in the clearance mechanism is the key to the accurate projection of the human clearance and preclinical safety assessment. Lack of species difference in the metabolism of ABT-894 in hepatocytes certainly creates a challenge in predicting its metabolism and pharmacokinetics in human. Based on available metabolism and pharmacokinetic data of ABT-894 in human, monkey is the preferred species in predicting human clearance since it presents a similar clearance mechanism from that observed in human.

Nicotinic receptors in neurodegeneration

Many studies have focused on expanding our knowledge of the structure and diversity of peripheral and central nicotinic receptors. Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily of pentameric ligand-gated ion channels, which include GABA (A and C), serotonin, and glycine receptors. Currently, 9 alpha (α2-α10) and 3 beta (β2-β4) subunits have been identified in the central nervous system (CNS), and these subunits assemble to form a variety of functional nAChRs. The pentameric combination of several alpha and beta subunits leads to a great number of nicotinic receptors that vary in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitize.

In the CNS, nAChRs play crucial roles in modulating presynaptic, postsynaptic, and extrasynaptic signaling, and have been found to be involved in a complex range of CNS disorders including Alzheimer’s disease (AD), Parkinson’s disease (PD), schizophrenia, Tourette´s syndrome, anxiety, depression and epilepsy. Therefore, there is growing interest in the development of drugs that modulate nAChR functions with optimal benefits and minimal adverse effects. The present review describes the main characteristics of nAChRs in the CNS and focuses on the various compounds that have been tested and are currently in phase I and phase II trials for the treatment of neurodegenerative diseases including PD, AD and age-associated memory and mild cognitive impairment.

Neuronal nicotinic receptors as analgesic targets: it's a winding road

Along with their well known role in nicotine addiction and autonomic physiology, neuronal nicotinic receptors (nAChRs) also have profound analgesic effects in animal models and humans. This is not a new idea, even in the early 1500s, soon after tobacco was introduced to the new world, its proponents listed pain relief among the beneficial properties of smoking. In recent years, analgesics that target specific nAChR subtypes have shown highly efficacious antinociceptive properties in acute and chronic pain models. To date, the side effects of these drugs have precluded their advancement to the clinic. This review summarizes the recent efforts to identify novel analgesics that target nAChRs, and outlines some of the key neural substrates that contribute to these physiological effects. There remain many unanswered mechanistic questions in this field, and there are still compelling reasons to explore neuronal nAChRs as targets for the relief of pain.

Covalent trapping of methyllycaconitine at the α4-α4 interface of the α4β2 nicotinic acetylcholine receptor: antagonist binding site and mode of receptor inhibition revealed

The α4β2 nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain and are implicated in a variety of physiological processes. There are two stoichiometries of the α4β2 nAChR, (α4)2(β2)3 and (α4)3(β2)2, with different sensitivities to acetylcholine (ACh), but their pharmacological profiles are not fully understood. Methyllycaconitine (MLA) is known to be an antagonist of nAChRs. Using the two-electrode voltage clamp technique and α4β2 nAChRs in the Xenopus oocyte expression system, we demonstrate that inhibition by MLA occurs via two different mechanisms; that is, a direct competitive antagonism and an apparently insurmountable mechanism that only occurs after preincubation with MLA. We hypothesized an additional MLA binding site in the α4-α4 interface that is unique to this stoichiometry. To prove this, we covalently trapped a cysteine-reactive MLA analog at an α4β2 receptor containing an α4(D204C) mutation predicted by homology modeling to be within reach of the reactive probe. We demonstrate that covalent trapping results in irreversible reduction of ACh-elicited currents in the (α4)3(β2)2 stoichiometry, indicating that MLA binds to the α4-α4 interface of the (α4)3(β2)2 and providing direct evidence of ligand binding to the α4-α4 interface. Consistent with other studies, we propose that the α4-α4 interface is a structural target for potential therapeutics that modulate (α4)3(β2)2 nAChRs.

α4β2* neuronal nicotinic receptor ligands (agonist, partial agonist and positive allosteric modulators) as therapeutic prospects for pain

α4β2* neuronal nicotinic acetylcholine receptor are ligand-gated ion channels and widely expressed throughout the central and peripheral nervous system. α4β2* neuronal nicotinic acetylcholine receptor play crucial role in pain signaling via modulation of multiple neurotransmitters like acetylcholine, dopamine, γ-amino butyric acid (GABA) and norepinephrine. Both spinal and supraspinal pathways are involved in the mechanisms by which α4β2* neuronal nicotinic acetylcholine receptor ligands modulate the neuropathic and inflammatory pain. Selective α4β2* neuronal nicotinic acetylcholine receptor ligands are being developed for the treatment of neuropathic and inflammatory pain as they show considerable efficacy in a wide range of preclinical pain models. Agonists/partial agonists of α4β2* neuronal nicotinic acetylcholine receptor show efficacy in animal models of pain and their anti-nociceptive properties are blocked by nicotinic antagonists. Positive allosteric modulators are being developed with the aim to increase the potency or therapeutic window of agonists/partial agonists. Accumulating evidences suggest that anti-nociceptive effects of nicotinic acetylcholine receptor ligands may not be mediated solely by α4β2* neuronal nicotinic acetylcholine receptor. We have also reviewed the stage of clinical development of various α4β2* neuronal nicotinic acetylcholine receptor ligands.

Allosteric modulators of α4β2 nicotinic acetylcholine receptors: a new direction for antidepressant drug discovery

Allosteric modulation of ligand-gated ion channels has been intensively studied in the past three decades and is now an established strategy to control receptor function in numerous disease states. Allosteric sites on the GABA(A) receptor are targets for widely prescribed drugs that are used for a variety of pathophysiological states including insomnia and epilepsy. Modulators might be especially valuable to control receptors for which the design of selective orthosteric drugs has proven difficult due to safety issues (e.g., α4β2 nicotinic acetylcholine receptors and might have several advantages over orthosteric ligands. Modulators influence the action of the endogenous agonist but generally have no effect of their own on the unoccupied receptor. Moreover, the higher subtype selectivity exerted by modulators and that the effects of modulators depend on the simultaneous presence of agonist help to overcome safety problems by preventing over-dosage compared with the administration of orthosteric drugs.

Systematic review and meta-analysis of pharmacological therapies for painful diabetic peripheral neuropathy

BACKGROUND

Painful diabetic peripheral neuropathy (pDPN) is prevalent among persons with diabetes and increases over time. Published guidelines recommend a number of medications to treat this condition providing clinicians with a variety of treatment options. This study provides a comprehensive systematic review and meta-analysis of published pharmacologic therapies for pDPN.

METHODS

The published literature was systematically searched to identify randomized, controlled trials of all available pharmacologic treatments for pDPN (recommended or nonrecommended) reporting predefined efficacy and safety outcomes. Bayesian fixed-effect mixed treatment comparison methods were used to assess relative therapeutic efficacy and harms.

RESULTS

Data from 58 studies including 29 interventions and 11,883 patients were analyzed. Pain reduction over that of placebo on the 11-point numeric rating scale ranged from -3.29 for sodium valproate (95% credible interval [CrI] = [-4.21, -2.36]) to 1.67 for Sativex (-0.47, 0.60). Estimates for most treatments were clustered between 0 and -1.5 and were associated with more study data and smaller CrIs. Pregabalin (≥ 300 mg/day) was the most effective on the 100-point visual analog scale (-21.88; [-27.06, -16.68]); topiramate was the least (-3.09; [-3.99, -2.18]). Relative risks (RRs) of 30% pain reduction ranged from 0.78 (Sativex) to 1.84 (lidocaine 5% plaster). Analysis of the RR ratio of these 2 treatments reveals marginal significance for Sativex (3.27; [1.07, 9.81]), indicating the best treatment is only slightly better than the worst. Relative risks of 50% pain reduction ranged from 0.98 (0.56, 1.52) (amitriptyline) to 2.25 (1.51, 3.00) (alpha-lipoic acid). RR ratio for these treatments was not statistically different (3.39; [0.88, 3.34]). Fluoxetine had the lowest risk of adverse events (0.94; [0.62, 1.23]); oxycodone had the highest (1.55; [1.45, 1.64]). Discontinuation RRs were clustered around 0.8 to 1.5, with those on the extreme having greater uncertainty.

CONCLUSIONS

Selecting an appropriate pDPN therapy is key given the large number of available treatments. Comparative results revealed relative equivalence among many of the studied interventions having the largest overall sample sizes and highlight the importance of standardization of methods to effectively assess pain.

The α7 nicotinic ACh receptor agonist compound B and positive allosteric modulator PNU-120596 both alleviate inflammatory hyperalgesia and cytokine release in the rat

BACKGROUND AND PURPOSE

Agonists selective for the α7 nicotinic acetylcholine (nACh) receptor produce anti-hyperalgesic effects in rodent models of inflammatory pain, via direct actions on spinal pain circuits and possibly via attenuated release of peripheral pro-inflammatory mediators. Increasingly, allosteric modulation of ligand-gated receptors is recognized as a potential strategy to obtain desired efficacy in the absence of the putative adverse effects associated with agonist activation.

EXPERIMENTAL APPROACH

We compared the anti-hyperalgesic and anti-inflammatory effects of the α7 nACh receptor agonist compound B with the positive allosteric modulator (PAM) PNU-120596 and the standard non-steroidal anti-inflammatory drug (NSAID), diclofenac, in rats with hind paw inflammation induced by either formalin, carrageenan or complete Freund's adjuvant (CFA).

KEY RESULTS

When administered before carrageenan, both diclofenac (30 mg·kg(-1) ) and PNU-120596 (30 mg·kg(-1) ) significantly reduced mechanical hyperalgesia and weight-bearing deficits for up to 4 h. Compound B (30 mg·kg(-1) ) also attenuated both measures of pain-like behaviour, albeit less robustly. Whereas compound B and PNU-120596 attenuated the carrageenan-induced increase in levels of TNF-α and IL-6 within the hind paw oedema, diclofenac only attenuated IL-6 levels. Established mechanical hyperalgesia induced by carrageenan or CFA was also partially reversed by compound B and PNU-120596. However, diclofenac was considerably more efficacious. Formalin-induced nocifensive behaviours were only reversed by compound B, albeit at doses which disrupted motor performance.

CONCLUSIONS AND IMPLICATIONS

α7 nACh receptor PAMs could prove to be useful in the treatment of inflammatory pain conditions, which respond poorly to NSAIDs or in situations where NSAIDs are contra-indicated.

Augmentation of cognitive function by NS9283, a stoichiometry-dependent positive allosteric modulator of α2- and α4-containing nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Positive allosteric modulation of α4β2 nicotinic acetylcholine (nACh) receptors could add a new dimension to the pharmacology and therapeutic approach to these receptors. The novel modulator NS9283 was therefore tested extensively.

EXPERIMENTAL APPROACH

Effects of NS9283 were evaluated in vitro using fluorescence-based Ca(2+) imaging and electrophysiological voltage clamp experiments in Xenopus oocytes, mammalian cells and thalamocortical neurons. In vivo the compound was tested in models covering a range of cognitive domains in mice and rats.

KEY RESULTS

NS9283 was shown to increase agonist-evoked response amplitude of (α4)(3) (β2)(2) nACh receptors in electrophysiology paradigms. (α2)(3) (β2)(2) , (α2)(3) (β4)(2) and (α4)(3) (β4)(2) were modulated to comparable extents, but no effects were detected at α3-containing or any 2α : 3β stoichiometry nACh receptors. Native nACh receptors in thalamocortical neurons similarly displayed DHβE-sensitive currents that were receptive to modulation. NS9283 had favourable effects on sensory information processing, as shown by reversal of PCP-disrupted pre-pulse inhibition. NS9283 further improved performance in a rat model of episodic memory (social recognition), a rat model of sustained attention (five-choice serial reaction time task) and a rat model of reference memory (Morris water maze). Importantly, the effects in the Morris water maze could be fully reversed with mecamylamine, a blocker of nACh receptors.

CONCLUSIONS AND IMPLICATIONS

These results provide compelling evidence that positive allosteric modulators acting at the (α4)(3) (β2)(2) nACh receptors can augment activity across a broad range of cognitive domains, and that α4β2 nACh receptor allosteric modulation therefore constitutes a promising therapeutic approach to symptomatic treatment of cognitive impairment.

Management of painful neuropathies

Neuropathic pain is the most common type of pain in neuropathy. In painful polyneuropathies the pain usually has a "glove and stocking" distribution. The pain may be predominantly spontaneous, e.g., with a burning, pricking, or shooting character or characterized by evoked pain such as mechanical or cold allodynia. In the clinical setting, the prevention of painful neuropathies and treatment of underlying neuropathy remains inadequate and thus symptomatic treatment of the pain and related disability needs to be offered. Most randomized, double-blind, placebo-controlled trials (RCTs) published in painful neuropathy have been conducted in patients with diabetes and to what extent a treatment which is found effective in painful diabetic polyneuropathy can be expected to relieve other conditions like chemotherapy- or HIV-induced neuropathy is unknown. Tricyclic antidepressants (TCAs), gabapentin, pregabalin, and serotonin noradrenaline reuptake inhibitors (SNRIs) are first drug choices. In patients with localized neuropathic pain, a topical lidocaine patch may also be considered. Second-line treatments are tramadol and other opioids. New types of treatment include botulinum toxin type A (BTX-A), high-dose capsaicin patches, and cannabinoids. Other types of anticonvulsant drugs such as lamotrigine, oxcarbazepine, and lacosamide have a more questionable efficacy in painful polyneuropathy but may have an effect in a subgroup of patients. Combination therapy may be considered in patients with insufficient effect from one drug. Treatment is usually a trial-and-error process and has to be individualized to the single patient, taking into account all comorbidities such as possible concomitant depression, anxiety, diseases, and drug interactions. Side-effects to antidepressants include dry mouth, nausea, constipation, orthostatic hypotension, and sedation. ECG should always be obtained prior to treatment with TCAs, which also should not be used in patients with cardiac incompensation and epilepsy. The most common side-effects of gabapentin and pregabalin are CNS-related side-effects with dizziness and somnolence. Peripheral edema, weight gain, nausea, vertigo, asthenia, dry mouth, and ataxia may also occur. Topical treatments are better tolerated due to lack of systemic side-effects but there is still limited evidence for the long-term efficacy of these drugs. With available drugs, the average pain reduction is about 20-30%, and only 20-35% of the patients will achieve at least 50% pain reduction, which stresses the need of a multidisciplinary approach to pain treatment.

Current challenges in translational pain research

The current gap between basic science research and the development of new analgesics presents a serious challenge for the future of pain medicine. This challenge is particularly difficult in the search for better treatment for comorbid chronic pain conditions because: (i) animal 'pain' models do not simulate multidimensional clinical pain conditions; (ii) animal behavioral testing does not assess subjective pain experience; (iii) preclinical data provide little assurance regarding the direction of new analgesic development; and (iv) clinical trials routinely use over-sanitized study populations and fail to capture the multidisciplinary consequences of comorbid chronic pain. Therefore, a paradigm shift in translational pain research is necessary to transform the current strategy from focusing on molecular switches of nociception to studying pain as a system-based integral response that includes psychosocial comorbidities. Several key issues of translational pain research are discussed in this review.

Structure-activity studies of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes: a novel class of highly potent nicotinic receptor ligands

The potential for nicotinic ligands with affinity for the α4β2 or α7 subtypes to treat such diverse diseases as nicotine addiction, neuropathic pain, and neurodegenerative and cognitive disorders has been exhibited clinically for several compounds while preclinical activity in relevant in vivo models has been demonstrated for many more. For several therapeutic programs, we sought nicotinic ligands with various combinations of affinity and function across both subtypes, with an emphasis on dual α4β2-α7 ligands, to explore the possibility of synergistic effects. We report here the structure-activity relationships (SAR) for a novel series of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes and characterize many of the analogues for activity at multiple nicotinic subtypes.

Analgesic effects mediated by neuronal nicotinic acetylcholine receptor agonists: correlation with desensitization of α4β2* receptors

Nicotinic α4β2* agonists are known to be effective in a variety of preclinical pain models, but the underlying mechanisms of analgesic action are not well-understood. In the present study, we characterized activation and desensitization properties for a set of seventeen novel α4β2*-selective agonists that display druggable physical and pharmacokinetic attributes, and correlated the in vitro pharmacology results to efficacies observed in a mouse formalin model of analgesia. ABT-894 and Sazetidine-A, two compounds known to be effective in the formalin assay, were included for comparison. The set of compounds displayed a range of activities at human (α4β2)(2)β2 (HS-α4β2), (α4β2)(2)α5 (α4β2α5) and (α4β2)(2)α4 (LS-α4β2) receptors. We report the novel finding that desensitization of α4β2* receptors may drive part of the antinociceptive outcome. Our molecular modeling approaches revealed that when receptor desensitization rather than activation activitiesat α4β2* receptors are considered, there is a better correlation between analgesia scores and combined in vitro properties. Our results suggest that although all three α4β2 subtypes assessed are involved, it is desensitization of α4β2α5 receptors that plays a more prominent role in the antinociceptive action of nicotinic compounds. For modulation of Phase I responses, correlations are significantly improved from an r(2) value of 0.53 to 0.67 and 0.66 when HS- and LS-α4β2 DC(50) values are considered, respectively. More profoundly, considering the DC(50) at α4β2α5 takes the r(2) from 0.53 to 0.70. For Phase II analgesia scores, adding HS- or LS-α4β2 desensitization potencies did not improve the correlations significantly. Considering the α4β2α5 DC(50) value significantly increased the r(2) from 0.70 to 0.79 for Phase II, and strongly suggested a more prominent role for α4β2α5 nAChRs in the modulation of pain in the formalin assay. The present studies demonstrate that compounds which are more potent at desensitization of α4β2* receptors display better analgesia scores in the formalin test. Consideration of desensitization propertiesat α4β2* receptors, especially at α4β2α5, in multiple linear regression analyses significantly improves correlations with efficacies of analgesia. Thus, α4β2* nicotinic acetylcholine receptor desensitization may contribute to efficacy in the mediation of pain, and represent a mechanism for analgesic effects mediated by nicotinic agonists.

Diabetic neuropathy: clinical manifestations and current treatments

Diabetic peripheral neuropathy is a prevalent, disabling disorder. The most common manifestation is distal symmetrical polyneuropathy (DSP), but many patterns of nerve injury can occur. Currently, the only effective treatments are glucose control and pain management. While glucose control substantially decreases the development of neuropathy in those with type 1 diabetes, the effect is probably much smaller in those with type 2 diabetes. Evidence supports the use of specific anticonvulsants and antidepressants for pain management in patients with diabetic peripheral neuropathy. However, the lack of disease-modifying therapies for diabetic DSP makes the identification of new modifiable risk factors essential. Growing evidence supports an association between components of the metabolic syndrome, including prediabetes, and neuropathy. Studies are needed to further explore this association, which has implications for the development of new treatments for this common disorder.

Population analyses of efficacy and safety of ABT-594 in subjects with diabetic peripheral neuropathic pain

ABT-594, a neuronal nicotinic acetylcholine receptor ligand, is 30- to 100-fold more potent than morphine in animal models of nociceptive and neuropathic pain. Efficacy and safety of ABT-594 in subjects with painful diabetic polyneuropathy was evaluated in a phase 2 study. The objective of this work was to use a nonlinear mixed effects model-based approach for characterizing the relationship between dose and response (efficacy and safety) of ABT-594. Subjects (N = 266) were randomized into four groups in a double-blind, placebo-controlled, 7-week study to receive twice daily regimens of placebo or 150, 225, and 300 μg of ABT-594. The primary efficacy variable, pain score (11-point Likert scale), was assessed on five occasions. The probability of change from baseline pain score of ≥1, ≥2, and ≥3 was modeled using cumulative logistic regression with dose and days of treatment as explanatory variables. The incidence of five most frequently occurring adverse events (AEs) was modeled using linear logistic regression. ABT-594 ED(50) values (improvement in 50% of subjects) for improvement in pain scores of ≥1, ≥2, and ≥3 were 50, 215, and 340 μg, respectively, for the average number of days (33) on treatment. The rank order of ED(50) values for AEs was nausea, vomiting, dizziness, headache, and abnormal dreams; nicotine users were less sensitive to AEs. Population pharmacodynamic models developed to characterize the improvement in pain score and incidence of adverse events indicate an approximately twofold separation between the ED(50) values for efficacy and AEs.