Sympatholysis after neuron-specific, N-type, voltage-sensitive calcium channel blockade: first demonstration of N-channel function in humans

An overview of interventional strategies for the management of oncologic pain

Pain is a ubiquitous part of the cancer experience. Often the presenting symptom of malignancy, pain becomes more prevalent in advanced or metastatic disease and often persists despite curative treatment. Although management of cancer pain improved following publication of the WHO's analgesic ladder, when used in isolation, conservative approaches often fail to control pain and are limited by intolerable side effects. Interventional strategies provide an option for managing cancer pain that remains refractory to pharmacologic therapy. The purpose of this review is to investigate these strategies and discuss the risks and benefits which must be weighed when considering their use. Therapies anticipated to have an increasingly important role in the future of cancer pain management are also discussed.

Perioperative Management of a Patient With an Intrathecal Drug Delivery Device Infusing Ziconotide: A Case Report

Intrathecal ziconotide is used for the treatment of chronic pain and is delivered by an implanted drug delivery device. Anesthesiologists should be familiar with the perioperative management of the pump as well as the potential adverse events related to continued ziconotide infusion during general anesthesia. A case is presented demonstrating the perioperative management of an intrathecal drug delivery device infusing ziconotide in a patient presenting for radical cystectomy with pelvic lymphadenectomy and ileal conduit diversion.

Ziconotide: Can we use it in palliative care?

Ziconotide (PRIALT ® ) is a new nonopioid treatment for chronic pain. It is a peptide that is the synthetic analog of the omega-conotoxin, derived from the marine snail, Conus magus. The therapeutic benefit of ziconotide derives from its potent and selective blockade of neuronal N-type voltage-sensitive calcium channels. Interference with these channels inhibits input from pain-sensing primary nociceptors. A recent clinical trial demonstrated that ziconotide has a significant analgesic effect compared to placebo in patients considered intolerant or refractory to other treatment such as systemic analgesics, adjunctive therapies, or intrathecal (IT) morphine. Thus, ziconotide is the first of a new class of agents—N-type calcium channel blockers, or NCCBs. Ziconotide may represent another option for patients with refractory pain.

The sympathetic nervous system as a target for the treatment of hypertension and cardiometabolic diseases

The regulation of blood pressure by the sympathetic nervous system is reviewed with an emphasis on the role of the sympathetic nervous system in the development and maintenance of hypertension. Evidence from patients and animal models is summarized. Because it is clear that there is a neural contribution to many types of human hypertension and other cardiometabolic diseases, the case is presented for a renewed emphasis on the development of sympatholytic approaches for the treatment of hypertension and other conditions associated with hyperactivity of the sympathetic nervous system.

Intrathecal ziconotide: a review of its use in patients with chronic pain refractory to other systemic or intrathecal analgesics

Ziconotide (Prialt(®)) is a synthetic conopeptide analgesic that acts by selectively antagonizing N-type voltage-gated calcium channels. Intrathecal ziconotide is the only non-opioid intrathecal analgesic that is FDA-approved for use in patients with treatment-refractory, chronic pain. The efficacy of intrathecal ziconotide was demonstrated in randomized, double-blind, placebo-controlled trials in patients with treatment-refractory noncancer-related pain or cancer- or AIDS-related pain. Across trials, ziconotide recipients had significantly greater reductions in pain intensity during ziconotide treatment than those receiving placebo (primary endpoint). At the end of the titration period, approximately one-sixth to one-third of patients with noncancer chronic pain and one-half with cancer- or AIDS-related pain who received ziconotide reached a pain response threshold (≥30 % reduction in the pain intensity score). In ziconotide responders, analgesic effects were enduring, with some patients continuing treatment over extended periods. Across trials, the chief tolerability concerns in ziconotide recipients during the titration phase and during extended treatment were related to CNS adverse events. These were mostly of mild to moderate intensity, although serious adverse events were commonly attributed to ziconotide treatment, especially in trials with rapid ziconotide titration and that permitted higher dosages. In general, clinically important non-CNS adverse events were infrequent, and during the ziconotide titration phase, relatively few patients discontinued treatment because of adverse events. Ongoing research will assess various strategies for selecting patients for ziconotide treatment and for enhancing its efficacy and tolerability. At the present time, intrathecal ziconotide provides a treatment option for patients with severe, unremitting pain who have failed to respond to other intensive analgesic regimens.

Continuous Intrathecal Infusion of Ziconotide for Treatment of Chronic Malignant and Nonmalignant Pain Over 12 Months: A Prospective, Open-label Study

Objectives.  This study aims to assess the safety and efficacy of long-term intrathecal (IT) ziconotide infusion. Materials and Methods.  In this prospective study, 155 patients with severe chronic pain (48 with malignant pain, 107 with nonmalignant pain) who had been responsive to short-term IT ziconotide in a double-blind, placebo-controlled study received long-term, open-label IT ziconotide monotherapy. Efficacy assessments included the mean percentage change on the visual analog scale of pain intensity from baseline in the study of origin; safety was monitored by adverse event (AE) reports, periodic laboratory tests, and vital sign measurements. Results.  At the last available observation, the visual analog scale of pain intensity scores had decreased by a mean of 36.9% from baseline in the short-term trial (N = 144; 95% CI: 30.1-43.7%; p < 0.0001). The mean IT ziconotide dose remained stable over 12 months in the 31 patients who participated in the study for ≥ one year. Ziconotide-related AEs were reported in 147 out of 155 patients (94.8%); 39.4% of patients discontinued treatment because of AEs, the majority of which were considered ziconotide related. Conclusions.  Ziconotide IT monotherapy provided patients with analgesia for 12 months in this open-label study, with an acceptable benefit/risk profile and no evidence of tolerance.

Targeting voltage-gated calcium channels: developments in peptide and small-molecule inhibitors for the treatment of neuropathic pain

Chronic pain affects approximately 20% of people worldwide and places a large economic and social burden on society. Despite the availability of a range of analgesics, this condition is inadequately treated, with complete alleviation of symptoms rarely occurring. In the past 30 years, the voltage-gated calcium channels (VGCCs) have been recognized as potential targets for analgesic development. Although the majority of the research has been focused on Ca(v) 2.2 in particular, other VGCC subtypes such as Ca(v) 3.2 have recently come to the forefront of analgesic research. Venom peptides from marine cone snails have been proven to be a valuable tool in neuroscience, playing a major role in the identification and characterization of VGCC subtypes and producing the first conotoxin-based drug on the market, the ω-conotoxin, ziconotide. This peptide potently and selectively inhibits Ca(v) 2.2, resulting in analgesia in chronic pain states. However, this drug is only available via intrathecal administration, and adverse effects and a narrow therapeutic window have limited its use in the clinic. Other Ca(v) 2.2 inhibitors are currently in development and offer the promise of an improved route of administration and safety profile. This review assesses the potential of targeting VGCCs for analgesic development, with a main focus on conotoxins that block Ca(v) 2.2 and the developments made to transform them into therapeutics.

Venom peptides as a rich source of cav2.2 channel blockers

Ca_v2.2 is a calcium channel subtype localized at nerve terminals, including nociceptive fibers, where it initiates neurotransmitter release. Ca_v2.2 is an important contributor to synaptic transmission in ascending pain pathways, and is up-regulated in the spinal cord in chronic pain states along with the auxiliary α2δ1 subunit. It is therefore not surprising that toxins that inhibit Ca_v2.2 are analgesic. Venomous animals, such as cone snails, spiders, snakes, assassin bugs, centipedes and scorpions are rich sources of remarkably potent and selective Ca_v2.2 inhibitors. However, side effects in humans currently limit their clinical use. Here we review Ca_v2.2 inhibitors from venoms and their potential as drug leads.

Pharmacokinetic analysis of ziconotide (SNX-111), an intrathecal N-type calcium channel blocking analgesic, delivered by bolus and infusion in the dog

BACKGROUND AND PURPOSE

  Ziconotide is a peptide that blocks N-type calcium channels and is antihyperalgesic after intrathecal (IT) delivery. We here characterize the spinal kinetics of IT bolus and infused ziconotide in dog.

EXPERIMENTAL APPROACH

  Male beagle dogs (N= 5) were prepared with chronic IT lumbar injection and cerebrospinal fluid (LCSF) sampling catheters connected to vest-mounted pumps. Each dog received the following: 1) IT bolus ziconotide (10 µg + 1 µCi (3) H-inulin); 2) IT infusion for 48 hours of ziconotide (1 µg/100 µL/hour); 3) IT infusion for 48 hours of ziconotide (5 µg/100 µL/hour); and 4) intravenous injection of ziconotide (0.1 mg/kg). After IT bolus, LCSF ziconotide and inulin showed an initial peak and biphasic (distribution/elimination) clearance (ziconotide T(1/2-α/β) = 0.14 and 1.77 hours, and inulin T(1/2-α/β) = 0.16 and 3.88 hours, respectively). The LCSF : plasma ziconotide concentration ratio was 20,000:1 at 30 min and 30:1 at eight hours. IT infusion of 1 and then 5 µg/hour resulted in LCSF concentrations that peaked by eight hours and remained stable at 343 and 1380 ng/mL, respectively, to the end of the 48-hour infusions. Terminal elimination T(1/2) after termination of continuous infusion was 2.47 hours. Ziconotide LCSF : cisternal CSF : plasma concentration ratios after infusion of 1 and 5 µg/hour were 1:0.017:0.001 and 1:0.015:0.003, respectively. IT infusion of ziconotide at 1 µg/hour inhibited thermal skin twitch by 24 hours and produced modest trembling, ataxia, and decreased arousal. Effects continued through the 48-hour infusion period, increased in magnitude during the subsequent 5 µg/hour infusion periods, and disappeared after drug clearance.

CONCLUSIONS AND IMPLICATIONS

  After IT bolus or infusion, ziconotide displays linear kinetics that are consistent with a hydrophilic molecule of approximately 2500 Da that is cleared slightly more rapidly than inulin from the LCSF. Behavioral effects were dose dependent and reversible.

Ziconotide for treatment of severe chronic pain

Pharmacological management of severe chronic pain is difficult to achieve with currently available analgesic drugs, and remains a large unmet therapeutic need. The synthetic peptide ziconotide has been approved by the US Food and Drug Administration and the European Medicines Agency for intrathecal treatment of patients with severe chronic pain that is refractory to other treatment modalities. Ziconotide is the first member in the new drug class of selective N-type voltage-sensitive calcium-channel blockers. The ziconotide-induced blockade of N-type calcium channels in the spinal cord inhibits release of pain-relevant neurotransmitters from central terminals of primary afferent neurons. By this mechanism, ziconotide can effectively reduce pain. However, ziconotide has a narrow therapeutic window because of substantial CNS side-effects, and thus treatment with ziconotide is appropriate for only a small subset of patients with severe chronic pain. We provide an overview of the benefits and limitations of intrathecal ziconotide treatment and review potential future developments in this new drug class.

Drugs from slugs--past, present and future perspectives of omega-conotoxin research

Peptides from the venom of carnivorous cone shells have provided six decades of intense research, which has led to the discovery and development of novel analgesic peptide therapeutics. Our understanding of this unique natural marine resource is however somewhat limited. Given the past pharmacological record, future investigations into the toxinology of these highly venomous tropical marine snails will undoubtedly yield other highly selective ion channel inhibitors and modulators. With over a thousand conotoxin-derived sequences identified to date, those identified as ion channel inhibitors represent only a small fraction of the total. Here we discuss our present understanding of conotoxins, focusing on the omega-conotoxin peptide family, and illustrate how such a seemingly simple snail has yielded a highly effective clinical drug.

Analgesic targets: today and tomorrow

Pain is recognized as a multifactorial sensory experience that is wholly unpleasant. It can vary in intensity from mild to severe and its duration can be anything from transient to persistent. Today we know so much more about the peripheral nociceptor as the primary detection apparatus for painful stimuli. We also understand in far greater detail the neurochemical mechanisms that occur at the level of the spinal cord and the complex interplay that exists between excitatory and inhibitory neural pathways. As a consequence of the assembly of this new body of evidence there are clear pointers that direct our attention to receptors, signaling pathways, enzymes and ion channels that all have the potential to be targets for novel, effective analgesics. The purpose of this review is to highlight some of the knowledge that has been assembled on this subject in recent years.

Ziconotide: an update and review

BACKGROUND

Ziconotide is the only N-type calcium channel blocker approved by the US FDA for the treatment of chronic pain. The approved indication is for the management of severe chronic pain in patients for whom intrathecal therapy is warranted and who are intolerant of or refractory to other treatments such as systemic analgesics, adjunctive therapies or intrathecal morphine.

OBJECTIVE

The purpose of this article was to review the available safety, efficacy and dosing information for ziconotide.

METHODS

The sources searched for literature from 1980 to January 2008 included Pub Med, MEDLINE and PREMEDLINE using the words ziconotide, conotoxins and pain.

RESULTS/CONCLUSION

Ziconotide is administered intrathecally by infusion pump to block nociceptive signal transmission in the spinal cord. It is a synthetic neuroactive peptide equivalent to the omega conotoxin MVIIA, a constituent of the venom of the fish-hunting marine snail Conus magus. It is highly potent, has a steep dose-response curve, a slow onset of action and a narrow margin of safety and responses to dose adjustments are slow. Patients receiving ziconotide should be under the care of physicians experienced in the management of intrathecal infusion therapy for pain control and should have convenient access to medical facilities.

Effect of O-superfamily conotoxin SO3 on synchronized spontaneous calcium spikes in cultured hippocampal networks

SO3 belongs to the O-superfamily of conotoxins and is known to have analgesic effects in experimental animals. In order to explore the mechanism of its potential pharmacological actions, the effect of SO3 on synchronized spontaneous calcium spikes was examined in cultured hippocampal networks by calcium imaging. Spontaneous oscillations of intracellular concentrations of calcium (Ca(2+)) in the form of waves and spikes are found in cultured hippocampal networks. Exposure to increasing concentrations of SO3 resulted in a progressive decrease in synchronized spontaneous calcium spikes. The higher concentrations (0.1 micromol/L and 1 micromol/L) of SO3 showed the strongest inhibition. The rank order of inhibition was 1 micromol/L > 0.1 micromol/L > 10 micromol/L > 0.01 micromol/L. This action of SO3 in reducing synchronized calcium spikes suggests a possible application for therapeutic treatment of epilepsy.

Ziconotide: a new option for intrathecal analgesia

Ziconotide is the synthetic equivalent of a neuroactive peptide found in the venom of the fish-hunting marine snail Conus magus. Its analgesic effect is mediated by a blockade of the N-type calcium channel in the dorsal horn of the spinal cord. The drug is currently licensed for continuous intrathecal infusion in the treatment of chronic intractable pain, and its analgesic efficacy has been demonstrated in both animal and human studies. Ziconotide-induced analgesia is not associated with the development of tolerance, respiratory depression or endocrine side effects, as is common in opioids. Ziconotide is a potent analgesic with a narrow therapeutic window. A low starting dose with slow upward titration, while monitoring the patient, is the recommended strategy for avoiding the more serious side effects, such as delirium, acute psychotic reactions, suicidal ideation and coma.

Ziconotide

Ziconotide, an intrathecal analgesic for the management of chronic intractable pain, binds with high affinity to N-type calcium channels in neuronal tissue and obstructs neurotransmission. In three pivotal, well designed trials of 5-6 or 21 days' duration, titrated ziconotide was significantly more effective than placebo in treating chronic malignant or nonmalignant pain as assessed by mean percentage improvements from baseline in Visual Analogue Scale Pain Intensity scores. Improvements in secondary endpoints (e.g. proportion of patients who responded or achieved pain relief and the change in opioid use) generally support the efficacy of ziconotide over placebo. Ziconotide maintains its analgesic efficacy in preliminary results from long-term, open-label trials (data available for up to 12 months). Most ziconotide-related adverse events are neurological, mild to moderate in severity, resolve over time and reverse without sequelae on drug discontinuation. Low initial doses of ziconotide and gradual titration to onset of analgesia reduces the incidence and severity of adverse events. No evidence of respiratory depression has been reported with intrathecal ziconotide.

Targeting chronic and neuropathic pain: the N-type calcium channel comes of age

The rapid entry of calcium into cells through activation of voltage-gated calcium channels directly affects membrane potential and contributes to electrical excitability, repetitive firing patterns, excitation-contraction coupling, and gene expression. At presynaptic nerve terminals, calcium entry is the initial trigger mediating the release of neurotransmitters via the calcium-dependent fusion of synaptic vesicles and involves interactions with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex of synaptic release proteins. Physiological factors or drugs that affect either presynaptic calcium channel activity or the efficacy of calcium-dependent vesicle fusion have dramatic consequences on synaptic transmission, including that mediating pain signaling. The N-type calcium channel exhibits a number of characteristics that make it an attractive target for therapeutic intervention concerning chronic and neuropathic pain conditions. Within the past year, both U.S. and European regulatory agencies have approved the use of the cationic peptide Prialt for the treatment of intractable pain. Prialt is the first N-type calcium channel blocker approved for clinical use and represents the first new proven mechanism of action for chronic pain intervention in many years. The present review discusses the rationale behind targeting the N-type calcium channel, some of the limitations confronting the widespread clinical application of Prialt, and outlines possible strategies to improve upon Prialt's relatively narrow therapeutic window.

Ziconotide Infusion for Severe Chronic Pain: Case Series of Patients with Neuropathic Pain

Ziconotide intrathecal infusion was recently approved by the United States Food and Drug Administration for the treatment of intractable severe chronic pain. Patients with neuropathic pain make up a significant population among those who experience chronic pain for which there are less than optimal pharmacotherapeutic options. Published clinical trials provide a global view of ziconotide efficacy and safety. A subset of patients in clinical trials obtained complete pain relief, a remarkable finding given the history of drug treatment for neuropathic pain. To provide more information regarding those who respond to ziconotide therapy, we discuss three patients with neuropathic pain who received ziconotide infusion. Two patients with longstanding neuropathic pain, one with complex regional pain syndrome (formerly known as reflex sympathetic dystrophy) of the leg and one with lumbar radiculitis, achieved temporary but complete pain relief from single 5- and 10-microg epidural test doses. In the third case, a patient with longstanding bilateral leg and foot neuropathic pain from acquired immunodeficiency syndrome and antiretroviral drug therapy achieved considerable pain relief from a long-term continuous intrathecal infusion. The patients who received a single dose had mild central nervous system adverse effects such as sedation, somnolence, nausea, headache, and lightheadedness. The patient who received the intrathecal infusion experienced mild-to-severe adverse effects depending on the rate of infusion; these effects included sedation, confusion, memory impairment, slurred speech, and double vision. This patient could sense impending adverse effects and made rate adjustments or suspended infusion to avert untoward symptoms. In all three cases, patients achieved considerable pain relief that was long-lasting and persisted well after dose administration or suspension of infusion.

Ziconotide, an intrathecally administered N-type calcium channel antagonist for the treatment of chronic pain

Ziconotide is a novel peptide that blocks the entry of calcium into neuronal N-type voltage-sensitive calcium channels, preventing the conduction of nerve signals. N-type calcium channels are present in the superficial laminae of the dorsal horn of the spinal cord. In various animal models of pain, intrathecal administration of ziconotide blocked nerve transmission and nociception. The United States Food and Drug Administration recently approved ziconotide intrathecal infusion for the management of severe chronic pain in patients who require intrathecal therapy and who are intolerant of or refractory to other treatment, such as systemic analgesics, adjunctive therapies, or intrathecal morphine. The drug has a narrow therapeutic window and a lag time for the onset and offset of analgesia and adverse events. In early clinical trials, frequent and severe psychiatric and central nervous system adverse effects were associated with rapid intrathecal infusion (0.4 microg/hr) and frequent up-titration (every 12 hrs). Therefore, patients with psychiatric symptoms are not candidates for this drug. Drug trials of external intrathecal catheters and microinfusion devices demonstrated a 3% risk of meningitis. A low initial infusion rate of 0.1 microg/hour and limiting infusion rate increases to 2-3 times/week are now recommended. Patients responsive to intrathecal ziconotide require an implanted infusion system to receive long-term therapy.

Use-dependent blockade of Cav2.2 voltage-gated calcium channels for neuropathic pain

The translocation of extracellular calcium (Ca(2+)) via voltage-gated Ca(2+) channels (VGCCs) in neurons is involved in triggering multiple physiological cell functions but also the abnormal, pathophysiological responses that develop as a consequence of injury. In conditions of neuropathic pain, VGCCs are involved in supplying the signal Ca(2+) important for the sustained neuronal firing and neurotransmitter release characteristic of these syndromes. Preclinical data have identified N-type VGCCs (Ca(v)2.2) as key participants in contributing to these Ca(2+) signaling events and clinical data with the peptide blocker Prialt have now validated Ca(v)2.2 as a bona fide target for future drug discovery efforts to identify new and novel therapeutics for neuropathic pain. Imperative for the success of such an endeavor will be the ability to identify compounds selective for Ca(v)2.2, versus other VGCCs, but also compounds which demonstrate effective blockade during the pathophysiological states of neuropathic pain without compromising channel activity associated with sustaining normal housekeeping cellular functions. An approach to obtain this research target profile is to identify compounds, which are more potent in blocking Ca(v)2.2 during higher frequencies of firing as compared to the slower more physiologically-relevant frequencies. This may be achieved by identifying compounds with enhanced potency for the inactivated state of Ca(v)2.2. This commentary explores the rationale and options for engineering a use-dependent blocker of Ca(v)2.2. It is anticipated that this use-dependent profile of channel blockade will result in new chemical entities with an improved therapeutic ratio for neuropathic pain.

Neuroprotection in the rat lateral fluid percussion model of traumatic brain injury by SNX-185, an N-type voltage-gated calcium channel blocker

Overload of intracellular calcium ([Ca(2+)](i)) following traumatic brain injury (TBI) has been implicated in the pathogenesis of neuronal injury and death. Voltage-gated calcium channels (VGCCs) provide one of the major sources of Ca(2+) entry into cells. Therefore, the potential neuroprotective activity of SNX-185, a specific N-type VGCC blocker, was tested in rats using the lateral fluid percussion (LFP) model of TBI. SNX-185 (50, 100, or 200 pmol) or vehicle was injected 5 min after injury into the CA2-3 subregion of the hippocampus ipsilateral to TBI. Acute neuronal degeneration was visualized in brain sections 24 h postinjury using the histofluorescent marker Fluoro-Jade (FJ), and the number of surviving neurons in the CA2-3 subregion of the hippocampus 42 days after injury was determined stereologically. Behavioral outcome after TBI and drug treatment was assessed in the beam walk test and Morris water maze. Direct injection of SNX-185 into the CA2-3 region of the hippocampus reduced neuronal injury 24 h after TBI and increased neuronal survival at 42 days at each of the three drug concentrations. Behavioral outcome in both the beam walk and Morris water maze were also improved by SNX-185, with 100 and 200 pmol, but not 50 pmol SNX-185 providing neuroprotection. These data support previous studies demonstrating substantial neuroprotection after TBI by treatment with N-type VGCC blockers.

Effect of new O-superfamily conotoxin SO3 on sodium and potassium currents of cultured hippocampal neurons

The effects of a new O-superfamily conotoxin SO3 on sodium and potassium currents were examined in cultured rat hippocampal neurons using the whole-cell patch clamp technique. SO3 caused a concentration-dependent, rapidly developing and reversible inhibition of sodium currents (I(Na)). The IC(50) value for the blockage of I(Na) was calculated to be 0.49 and the Hill coefficient was 1.7. Using electrophysiological and pharmacological protocols, transient A-type potassium currents (I(A)) and delayed rectifiers potassium currents (I(K)) were isolated. SO3 caused a concentration-dependent, and reversible inhibition of I(K). The IC(50) value for the blockage of I(K) was calculated to be 1.6 and the Hill coefficient was 0.6, with no significant effect on I(A). These results indicate that SO3 can selectively inhibit neuronal sodium and potassium currents.

Antisympathetic and hemodynamic property of a dual L/N-type Ca(2+) channel blocker cilnidipine in rats

The in vivo antisympathetic property of a dual L/N-type Ca(2+) channel blocker cilnidipine compared with that of typical N-type Ca(2+) channel blockers has never been clarified. We investigated the effects of the drug on a sympathetic nerve-mediated vascular response and vasodilating action in rats in comparison with those of an N-type Ca(2+) channel blocker omega-conotoxin MVIIA. In pithed rats, omega-conotoxin MVIIA preferentially suppressed the sympathetic nerve stimulation-induced pressor response, whereas cilnidipine suppressed the pressor response induced by sympathetic nerve stimulation and angiotensin II. In anesthetized rats, cilnidipine or omega-conotoxin MVIIA decreased mean blood pressure, while heart rate was decreased by omega-conotoxin MVIIA, but slightly increased by cilnidipine. These results suggest that cilnidipine can affect sympathetic N-type Ca(2+) channels in addition to vascular L-type Ca(2+) channels in antihypertensive doses in the rat in vivo. The antisympathetic activity of cilnidipine is not excessive for an antihypertensive drug in comparison with that of omega-conotoxin MVIIA.

Neuraxial infusion in patients with chronic intractable cancer and noncancer pain

Ever since the application in 1980 of morphine for spinal analgesia in patients with refractory cancer pain, spinal infusion therapy has become one of the cornerstones for the management of chronic, medically intractable pain. Initially, spinal infusion therapy was indicated only for patients with cancer pain that could not be adequately controlled with systemic narcotics. However, over the past decade, there has been a significant increase in the number of pumps implanted for the treatment of nonmalignant pain. Indeed, "benign" pain syndromes, particularly failed back surgery syndrome, are the most common indication for intrathecal opiates. As we have gained more experience with this therapy, it has become apparent that even intrathecal opiates, when administered in the long term, can be associated with problems such as tolerance, hyperalgesia, and other side effects. Consequently, long-term efficacy has not been as significant as had been hoped. Because of the difficulties associated with long-term intrathecal opiate therapy, much of the research, both basic and clinical, has focused on developing alternative nonopioid agents to be used either alone or in combination with opiates. Clinical trials have been and continue to be conducted to evaluate drugs such as clonidine, SNX-111, local anesthetics, baclofen, and many other less common agents to determine their efficacy and potential toxicity for intrathecal therapy. This article reviews the agents developed as alternatives to intrathecal opiates.

Pharmacologic treatment of neuropathic pain

Neuropathic pain, or pain after nervous system injury, can be very refractory to pharmacologic interventions. Through a better understanding of the pathophysiology of neuropathic pain, it has been suggested that nonopioid agents, such as antidepressants and anticonvulsants, may be more efficacious in the treatment of neuropathic pain than common analgesics, such as opioids or nonsteroidal anti-inflammatory drugs. However, this has not been consistently demonstrated in clinical studies. Conversely, many confounding factors of neuropathic pain make it difficult to interpret clinical studies. Therefore, we must develop a better understanding of the preclinical models of neuropathic pain to better understand the application of new and old drugs to the human neuropathic pain state. This article provides an overview of the commonly used preclinical neuropathic pain models, followed by a summary of the efficacy of currently available agents in preclinical pain models and human correlates.

Cardiovascular and autonomic effects of omega-conotoxins MVIIA and CVID in conscious rabbits and isolated tissue assays

1. The effects of a novel N-type voltage-operated calcium channel antagonist, omega-conotoxin CVID, were compared with omega-conotoxin MVIIA on sympathetic-evoked activation of right atria (RA), small mesenteric arteries (MA) and vasa deferentia (VD) isolated from the rat. Their effects were also compared on blood pressure and cardiovascular reflexes in conscious rabbits. 2. The pIC(50) values for MVIIA and CVID, respectively, for inhibiting sympathetic-evoked responses were equivalent in RA (8.7 and 8.7) and VD (9.0 and 8.7); however, in MA the values were 8.4 and 7.7. The cardiac to vascular (RA/MA) potency ratios, antilog (plog RA - plog MA), for MVIIA and CVID were 2 and 10. The offset rates for CVID and MVIIA were rapid, and peptide reapplication caused rapid onset of blockade, suggesting limited desensitization. 3. In the conscious rabbit, CVID and MVIIA (100 microg kg(-1) i.v.) caused a similar fall in blood pressure and a tachycardia that rapidly reached maximum. Both peptides decreased the vagal- and sympathetic-mediated components of the baroreflex, but had no effect on the vagal nasopharyngeal reflex. The orthostatic reflex to 90 degrees tilt was blocked by MVIIA with sustained postural hypotension for > or = 90 min after administration. In contrast, CVID caused postural hypotension at 30 min which recovered rapidly. 4. Neither CVID nor MVIIA (3 microg kg(-1) i.t.) significantly altered cardiovascular variables or autonomic reflexes. 5. In conclusion, CVID appears to be relatively weak at inhibiting the reflex response to tilt consistent with its weaker inhibition of rat mesenteric artery constriction to perivascular nerve stimulation. This may point to subtype N-type calcium channel selectivity.

Structure-activity relationships of omega-conotoxins at N-type voltage-sensitive calcium channels

Due to their selectivity towards voltage-sensitive calcium channels (VSCCs) omega-conotoxins are being exploited as a new class of therapeutics in pain management and may also have potential application in ischaemic brain injury. Here, the structure-activity relationships (SARs) of several omega-conotoxins including GVIA, MVIIA, CVID and MVIIC are explored. In addition, the three-dimensional structures of these omega-conotoxins and some structurally related peptides that form the cysteine knot are compared, and the effects of the solution environment on structure discussed. The diversity of binding and functional assays used to measure omega-conotoxin potencies at the N-type VSCC warranted a re-evaluation of the relationship between these assays. With one exception, [A22]-GVIA, this analysis revealed a linear correlation between functional (peripheral N-type VSCCs) and radioligand binding assays (central N-type VSCCs) for the omega-conotoxins and analogues that were tested over three studies. The binding and functional results of several studies are compared in an attempt to identify and distinguish those residues that are important in omega-conotoxin function as opposed to those that form part of the structural scaffold. Further to determining what omega-conotoxin residues are important for VSCC binding, the range of possible interactions between the ligand and channel are considered and the factors that influence the selectivity of MVIIA, GVIA and CVID towards N-type VSCCs examined.

Ion channel toxins and therapeutics: from cone snail venoms to ciguatera

Ion channels are intimately linked to all neurotransmission and neurotransmitter release processes, but in disease states often contribute adversely to disease pathology. The diversity and distribution of ion channel types and subtypes being uncovered through the use of molecular biology and toxin probes present an exciting opportunity for the discovery of new, more selective drugs. Among ion channels targeted by cone shell venom peptides (conotoxins) are the voltage-sensitive sodium, calcium, and potassium channels which open and then close (inactivate) in response to membrane depolarization, and thus regulate neurotransmission and the neurotransmitter release process. Conotoxins also target ligand-gated ion channels, including the NMDA-glutamate channel and the nicotinic acetylcholine receptor channel. The diversity of subtypes, especially those subtypes upregulated in disease states, makes ion channels a rapidly expanding therapeutic area. Conotoxins represent some of the most selective inhibitors of ion channel subtypes and have often been used as the defining ligand. In this overview, the structures and therapeutic potential of conotoxins active at ion channels are highlighted. The activity and structures are then contrasted with ciguatoxins, which are responsible for the food poisoning known as ciguatera. A universal liquid chromatography/mass spectrometry approach to the detection of these classes of toxins is briefly discussed.

Postural hypotension following N-type Ca2+ channel blockade is amplified in experimental hypertension

OBJECTIVE

To determine the relative importance of the cardiac and vascular sympathetic components of the orthostatic response to 90 degrees head-up tilt after N-type calcium-channel blockade in normotensive (sham renal cellophane wrap) and hypertensive (renal wrap) conscious rabbits.

METHODS

The effects of N-type calcium-channel blockade with omega-conotoxin GVIA (omega-CTX, 10 microg/kg i.v. bolus) were assessed in the absence or presence of cardiac block by propranolol and methscopolamine. These were contrasted with the effects of alpha1-adrenoceptor antagonism (prazosin 0.5 mg/kg i.v. bolus, in the presence of cardiac block) or ganglion blockade (mecamylamine 4 mg/kg i.v. bolus).

RESULTS

In vehicle (0.9% saline) treatment groups, the response to tilt consisted of a small pressor effect (4 +/- 2 and 7 +/- 1 mmHg) and tachycardia (29 +/- 6 and 17 +/- 6 beats/min) in sham (n = 6) and wrap (n = 5) rabbits, respectively. After prazosin administration (with cardiac block), there were significant falls in MAP of 3 +/- 1 and 7 +/- 2 mmHg in sham (n = 7) and wrap (n = 6) rabbits, respectively, in response to tilt omega-CTX caused postural hypotensive responses of 8 +/- 2 and 13 +/- 2 mmHg in sham (n = 6) and wrap (n = 7) rabbits, respectively, and 7 +/- 1 and 14 +/- 2 mmHg in sham (n = 7) and wrap (n = 7) rabbits with prior cardiac block. Similarly, mecamylamine caused falls in MAP of 8 +/- 1 and 10 +/- 2 mmHg in response to tilt in sham (n = 6) and wrap (n = 9) animals, respectively.

CONCLUSION

Sympathetic vasoconstrictor effectors are primarily responsible for maintaining blood pressure during tilt in conscious rabbits. The postural hypotension caused by sympatholytic agents is about double in hypertensive rabbits, and N-type calcium-channel blockade is as effective as ganglion blockade at inducing this syndrome.

An assessment of the present and future roles of non-ligand gated ion channel modulators as CNS therapeutics

Few approved drugs have, as their primary known mechanism of action, modulation of non-ligand gated ion channels. However, these proteins are important regulators of neuronal function through their control of sodium, potassium, calcium and chloride flux, and are ideal candidates as drug discovery targets. Recent progress in the molecular biology and pharmacology of ion channels suggests that many will be associated with specific pharmacological profiles that will include both activators and inhibitors. Ion channels, through their regulation by G-proteins, are a major component of the final common pathway of many drugs acting at classical neuronal receptors. Thus, targeting of the ion channels themselves may confer different profiles of efficacy and specificity to drug action in the brain and spinal cord. Three areas for drug discovery are profiled that the authors consider prime targets for ion channel based therapies, anticonvulsant drugs, cognition enhancing drugs and drugs for improving neurone survival following ischaemia.

Selective peptide antagonist of the class E calcium channel from the venom of the tarantula Hysterocrates gigas

We describe the first potent and selective blocker of the class E Ca2+channel. SNX-482, a novel 41 amino acid peptide present in the venom of the African tarantula, Hysterocrates gigas, was identified through its ability to inhibit human class E Ca2+ channels stably expressed in a mammalian cell line. An IC50 of 15-30 nM was obtained for block of the class E Ca2+ channel, using either patch clamp electrophysiology or K+-evoked Ca2+ flux. At low nanomolar concentrations, SNX-482 also blocked a native resistant or R-type Ca2+ current in rat neurohypophyseal nerve terminals, but concentrations of 200-500 nM had no effect on R-type Ca2+ currents in several types of rat central neurons. The peptide has the sequence GVDKAGCRYMFGGCSVNDDCCPRLGCHSLFSYCAWDLTFSD-OH and is homologous to the spider peptides grammatoxin S1A and hanatoxin, both peptides with very different ion channel blocking selectivities. No effect of SNX-482 was observed on the following ion channel activities: Na+ or K+ currents in several cultured cell types (up to 500 nM); K+ current through cloned potassium channels Kv1.1 and Kv1. 4 expressed in Xenopus oocytes (up to 140 nM); Ca2+ flux through L- and T-type Ca2+ channels in an anterior pituitary cell line (GH3, up to 500 nM); and Ba2+ current through class A Ca2+ channels expressed in Xenopus oocytes (up to 280 nM). A weak effect was noted on Ca2+ current through cloned and stably expressed class B Ca2+ channels (IC50 > 500 nM). The unique selectivity of SNX-482 suggests its usefulness in studying the diversity, function, and pharmacology of class E and/or R-type Ca2+ channels.