CAN PRIALT BE IMPROVED UPON? THE CASE FOR IDENTIFYING ORALLY AVAILABLE, STATE-DEPENDENT N-TYPE CHANNEL BLOCKERS Although Prialt has been shown to offer significant therapeutic benefit in a number of chronic, opioid-unresponsive conditions, a case can be made that the development of future N-type channel therapeutics should address several issues. 1) The therapeutic index (ratio of relative toxicity to relative efficacy) of intrathecally administered Prialt is quite narrow in both animals (ratios range from 1.5–2.1) and in humans. 39–41 Clinically, Prialt must be titrated slowly in each individual patient, and typically doses are reached whereby some adverse affects are observed even before the dose is fully efficacious. Initial data with other natural peptides (e.g., ω-conotoxin-CVID and huwentoxin-I; see below) suggest that it may be possible to maintain the potent antinociceptive effects associated with N-type channel blockade and have fewer, less severe adverse effects. 2) The requirement that Prialt (or other peptide N-type channel blockers) be administered intrathecally is extremely limiting. Identifying safe and efficacious orally available N-type channel blockers is relatively straightforward to rationalize from both the patient and marketing perspectives. The invasive surgery and high costs associated with implantable pumps clearly limits the numbers of patients both eligible and willing to use Prialt regardless of its potential as a potent antinociceptive. 3) Voltage-gated ion channels are well known to exist in a number of discrete biophysical states (e.g., open, closed/resting, and inactivated) that presumably reflect distinct time-dependent and voltage-dependent conformations. 50 It has also been well established that the interaction of various drugs (both antagonists and agonists) with voltage-gated ion channels are significantly different depending upon the particular channel state. Certain drugs preferentially interact with channel sites in specific conformations and in some instances drug-channel binding affinity can vary by orders of magnitude from state to state of the channel. This phenomenon is of particular relevance as it relates to both the physiological consequences of drug action but is equally important to the safety profile of most drugs interacting with ion channels. The biophysical states of calcium channels, and hence drug interaction, can be affected by various parameters including both resting membrane potential and the frequency of stimulation. In the case of the widely clinically used L-type channel antagonists, Bean provided the first evidence that the 1,4-dihydropyridine L-type channel blockers likely provide an excellent therapeutic window due to their preferential binding and block of inactivated channels. 51In the case of N-type channels, an initial report found Prialt block to be reversed by strong membrane depolarization suggesting a state-dependent mechanism. 52 However, in a thorough analysis under more relevant physiological conditions, Prialt block was found to occur in all states—the resting, open and inactivated conformations—and there was little evidence of frequency or voltage dependence. 53 In this regard, the development of state-dependent N-type channel blockers would be predicted to significantly improve the therapeutic index over that for Prialt. In consideration of blocking N-type channels for therapeutic intervention, there are some things that we either quite yet don't fully understand or have sufficient information to make definitive conclusions. For example, in the mammalian CNS, whereas N-type channels appear to be widely distributed (as evidenced by mRNA, immunohistochemical, and ω-conotoxin-GVIA binding studies), there seem to be relatively few adverse physiological consequences in knockout mice that completely lack all N-type channel activity. Similarly, there is good evidence for N-type channel expression in certain spinal motor neurons and neuroendocrine cells, and yet there is sparse evidence to indicate that animals lacking this channel suffer adverse neuroendocrine or motor affects. While it is possible that compensation by other calcium channel types overcomes any deleterious effects in the knockout animals, studies examining calcium current levels in the knockout mice do not find evidence for compensatory mechanisms. Concerning a major direct involvement in motor transmission, whereas N-type channels have been described in spinal motor neurons and at mammalian neuromuscular junctions, they appear to account for a very small portion of neuromuscular terminals (<5%) compared with that for P/Q-type channels (~95%). 46 Even then, N-type channels are only found at a subset of specific neuromuscular terminals (e.g., tibialis anterior, gastrocnemius, and soleus muscles) and appear not to be localized at presynaptic junctions associated with other muscle such as the diaphragm. Given the relative lack of CNS-related physiological consequences reported in N-type gene knockout mice (which essentially mimics complete blockade of all N-type channels), what then causes the significant cardiovascular and CNS adverse effects found in animals and humans treated with the clinical N-type channel blocker Prialt? This issue remains to be adequately addressed and whether Prialt adverse affects are specifically related to the N-type target itself, the biophysical mechanism of action of Prialt block on N-type channels, or to some as yet to-be-described nonspecific effects of Prialt on other targets remains to be fully elucidated. Interestingly, other natural peptides with high affinity and selectivity for the N-type channel do not appear to have similar deleterious effects in vivo. For example, AM336 (ω-conotoxin-CVID), a 27-amino acid peptide from Conus catus, has been shown to be a potent blocker of N-type currents and to displace radiolabeled ω-conotoxin-GVIA binding at potencies similar to that for both ω-conotoxin-GVIA and ω-conotoxin-MVIIA. 54 In animal models of persistent inflammatory and neuropathic pain, AM336 results in potent dose-dependent antinociception. However, despite AM336's similar high potency block of N-type channels compared to that for ω-conotoxins-GVIA and -MVIIA, it appears to possess a remarkably better CNS safety profile in animals. 55,56 In rodents, intrathecally administered AM336 produces a five-fold better therapeutic index (ratio of relative toxicity to relative potency) concerning the incidence of the serpentine tail movements and whole body shaking observed for similarly administered Prialt. In another example, intrathecal administration of huwentoxin-I, a 33-amino acid peptide from the venom of the Chinese bird spider Ornithoctonus huwena appears as effective as ω-conotoxin-MVIIA in both the early and late phases of the formalin model of inflammatory pain. 57 However, whereas huwentoxin-I blocks N-type channels with high affinity (EC 50 = 100 n m) and selectivity, at high doses in vivo it exhibits a significantly lower degree of the motor dysfunction and ataxia compared with that for Prialt. It is tempting to speculate that the significantly better therapeutic window in animals of AM336 compared with that for Prialt is due to the fact that, in radioligand binding assays, AM336 is approximately 100-fold more selective for N-type channels over P/Q-type channels compared to Prialt. P/Q-type channels are the major calcium channel subtypes implicated in triggering neurotransmission in the CNS in mammals, and even a moderate degree of blockade by intrathecally administered Prialt would be predicted have significant deleterious effects on central and motor neuron functions. It is also tempting to speculate that the fact that the N-type channel gene knockout mice do not exhibit the cerebellar ataxia or whole body shaking characteristic of Prialt administration reflects its action on central targets other than the N-type calcium channel. N-type currents (along with other calcium channels subtypes) are described in both peripheral sensory neurons and neuroendocrine cells, and it remains unclear as to the specific consequences of N-type channel blockade on peripheral transmission and endocrine function. The ability to predict potential deleterious affects concerning N-type channel blockade in sympathetic neurons is far from clear. In some sympathetic neurons, neurotransmitter release mediate by N-type channels appears to be frequency dependent although not necessarily in the expected manner. In both carotid body and postganglionic sympathetic nerve terminals, for example, release triggered through N-type channel activation is actually higher at lower stimulation frequencies (0.4–2 Hz) than at higher frequencies (30 Hz). 58 In other sympathetic neurons (e.g., noradrenergic constrictions of the inferior vena cava and uterine artery), there is no affect of stimulation frequency on N-type channel-dependent release, whereas in still other cells sympathetic-mediated vasoconstriction does not appear to involve N-type channel-mediated release. 59In one homozygous N-type channel gene knockout strain, it was found that heart rate and blood pressure were unaffected by the complete lack of N-type channels, whereas in another independent strain both heart rate and blood pressure were elevated. 35,37 Affects on the carotid baroreflex were more definitive in that N-type gene knockout mice appeared to lack the normal response to carotid artery occlusion. Additionally, a contribution to sympathetic neurotransmission was suggested in that N-type-deficient mice showed an impaired cardiac inotropic response, although contradictorily it was noted that circulating norepinephrine levels were actually normal. 35In support of the notion for developing state-dependent N-type blockers for peripheral administration, N-type currents in adrenal chromaffin cells exhibit variable inactivation characteristics with a large fraction of the channels being resistant to inactivation even during prolonged depolarization. 6 Under such circumstances it could be envisioned that N-type blockers with significantly higher affinity for the inactivated states would have minimal affects on N-type channel-mediated endocrine functions. Additional factors likely relevant to the discussion include the fact that it is well known that all neurotransmission is not created equal: different size synaptic vesicles exist, the different vesicles often contain distinct types of neurotransmitters and neuropeptides, and release can be differentially dependent upon the nature of stimulation (e.g., in some cases release from small vesicles can occur in response to single or low frequency stimuli, whereas large vesicle release can require stronger stimuli and/or higher frequency stimulation). Release dependent upon these various factors has been predicted to involve both differential vesicle interactions with SNARE proteins and also the involvement distinct subtypes of presynaptic calcium channels. Overall, the data suggest that some portion of sympathetic function would likely not be affected by N-type channel blockade, whereas in other instances state-dependent N-type channel blockers aimed at selectively targeting channel states associated with higher frequency stimulation might have minimal affect on normal sympathetic functions such as those associated with carotid bodies. |
REFERENCES 1. Snutch TP, Peloquin J, Mathews E, McRory J. Molecular properties of voltage-gated calcium channels. In: Voltage-gated calcium (Zamponi G, ed), pp 61–94. New York: Landes Bioscience, 2005. 2. Catterall WA. Biochemical studies of Ca2+ channels. In: Voltage-gated calcium (Zamponi G, ed), pp 48–60. New York: Landes Bioscience, 2005. 3. Westenbroek RE, Hell JW, Warner C, Dubel SJ, Snutch TP, Catterall WA. Biochemical properties and subcellular distribution of an N-type calcium channel α1 subunit. Neuron 9: 1099–1115, 1992. [PubMed]4. Dunlap K, Luebke JI, Turner TJ. Exocytotic Ca2+ channels in mammalian central neurons. Trends Neurosci 18: 89–98, 1995. [PubMed]5. Janis RJ, Triggle DJ. In: Calcium channels: their properties, functions, regulation and clinical relevance. London: CRC, 1991. 6. Cahill AL, Hurley JH, Fox AP. Coexpression of cloned α(1B), β(2a), and α(2)/δ subunits produces non-inactivating calcium currents similar to those found in bovine chromaffin cells. J Neurosci 20: 1685–1693, 2000. [PubMed]7. Stea A, Soong TW, Snutch TP. Determinants of PKC-dependent modulation of a family of neuronal calcium channels. Neuron 15: 929–940, 1995. [PubMed]8. Bourinet E, Soong TW, Stea A, Snutch TP. Determinants of the G-protein-dependent opioid modulation of neuronal calcium channels. Proc Natl Acad Sci USA 93: 1486–1491, 1996. [PubMed]9. Artalejo CR, Perlman RL, Fox AP. ω-Conotoxin GVIA blocks a Ca2+ current in bovine chromaffin cells that is not the “classic” N-type. Neuron 8: 85–95, 1992. [PubMed]10. Dubel SJ, Starr TVB, Hell J, Ahlijanian MK, Enyeart JJ, Catterall WA, Snutch TP. Molecular cloning of the α-1 subunit of an ω-conotoxin-sensitive calcium channel. Proc Natl Acad Sci USA 89: 5058–5062, 1992. [PubMed]11. Lin Z, Haus S, Edgerton J, Lipscombe D. Identification of functionally distinct isoforms of the N-type Ca2+ channel in rat sympathetic ganglia and brain. Neuron 18: 153–166, 1997. [PubMed]12. Lin Z, Lin T, Schorge S, Pan JQ, Beierlein M, Lipscombe D. Alternative splicing of a short cassette exon in α1B generates functionally distinct N-type calcium channels in central and peripheral neurons. J Neurosci 19: 5322–3531, 1999. [PubMed]13. Bourinet E, Soong TW, Sutton K, Slaymaker S, Mathews E, Monteil A, Zamponi GW, Nargeot J, Snutch TP. Phenotypic variants of P- and Q-type calcium channels are generated by alternative splicing of the α1A subunit gene. Nat Neurosci 2: 407–415, 1999. [PubMed]14. Kerr LM, Filloux F, Olivera BM, Jackson H, Wamsley JK. Autoradiographic localization of calcium channels with [125I] ω-conotoxin in rat brain. Eur J Pharmacol 146: 181–183, 1988. [PubMed]15. Beedle AM, Zamponi GW. Modulation of high voltage-activated calcium channels by G protein coupled receptors. In: Calcium channel pharmacology (McDonough SI, ed), pp 331–367. New York: Kluwer Academic/Plenum Publishers, 2004. 16. Soldo BL, Moises HC. μ-Opioid receptor activation inhibits N- and P-type Ca2+ channel currents in magnocellular neurones of the rat supraoptic nucleus. J Physiol (Lond) 513: 787–804, 1998. [PubMed]17. Pan X, Ikeda SR, Lewis DL. Rat brain cannabinoid receptor modulates N-type Ca2+ channels in a neuronal expression system. Mol Pharmacol 49: 707–714, 1996. [PubMed]18. Sun L, Miller RJ. Multiple neuropeptides Y receptors regulate K+ and Ca2+ channels in acutely isolated neurons from the rat arcuate nucleus. J Neurophysiol 81: 1391–1403, 1999. [PubMed]19. Shapiro MS, Hille B. Substance P and somatostatin inhibit calcium currents in rat sympathetic neurons via different G protein pathways. Neuron 10: 11–20, 1993. [PubMed]20. Zamponi GW, Bourinet E, Nelson D, Nargeot J, Snutch TP. Crosstalk between G-proteins and protein kinase C mediated by the calcium channel α1 subunit. Nature 385: 442–446, 1997. [PubMed]21. Zamponi GW, Snutch TP. Decay of prepulse facilitation during G-protein inhibition of N-type calcium channels involves binding of a single G βγ subunit. Proc Natl Acad Sci USA 95: 4035–4039, 1998. [PubMed]22. Patil PG, de Leon M, Reed RR, Dubel SJ, Snutch TP, Yue DT. Elementary events underlying voltage-dependent G-protein inhibition of N-type calcium channels. Biophys J 71: 2509–2521, 1996. [PubMed]23. Sheng ZH, Rettig J, Takahashi M, Catterall WA. Identification of a syntaxin-binding site on N-type calcium channels. Neuron 13: 1303–1313, 1994. [PubMed]24. Jarvis SE, Zamponi GW. Distinct molecular determinants govern syntaxin 1A-mediated inactivation and G-protein inhibition of N-type calcium channels. J Neurosci 21: 2939–2948, 2001. [PubMed]25. Jarvis SE, Barr W, Feng Z-P, Hamid J, Zamponi GW. Molecular determinants of syntaxin 1 modulation of N-type calcium channels. J Biol Chem 277: 44399–44407, 2002. [PubMed]26. Cruz LJ, Olivera BM. Calcium channel antagonists. ω-Conotoxin defines a new high affinity site. J Biol Chem 261: 6230–6233, 1986. [PubMed]27. Olivera BM, Cruz LJ, de Santos V, LeCheminant GW, Griffin D, Zeikus R, McIntosh JM, Galyean R, Varga J, Gray WR. Neuronal calcium channel antagonists. Discrimination between calcium channel subtypes using ω-conotoxin from Conus magus venom. Biochemistry 26: 2086–2090, 1987. [PubMed]28. Gohil K, Bell JR, Ramachandran J, Miljanich GP. Neuroanatomical distribution of receptors for a novel voltage-sensitive calcium channel antagonist, SNX-230 (ω-conopeptide MVIIC). Brain Res 653: 258–266, 1994. [PubMed]29. Maggi CA, Giuliani S, Santicioli P, Tramontana M, Meli A. Effects of ω conotoxin on reflex responses mediated by activation of capsaicin-sensitive nerves of the rat urinary bladder and peptide release from rat spinal cord. Neurosci 34: 243–250, 1990. 30. Santicioli P, Del Biaanco E, Tramontana M, Geppetti P, Maggi CA. Release of calcitonin gene-related peptide-like immunoreactivity induced by electrical field stimulation from rat spinal afferents is mediated by conotoxin sensitive calcium channels. Neurosci Lett 136: 161–164, 1992. [PubMed]31. Evans AR, Nicol GD, Vasko MR. Differential regulation of evoked peptide release by voltage-sensitive calcium channels in rat sensory neurons. Brain Res 712: 265–273, 1996. [PubMed]32. Vanegas H, Schaible H-G. Effects of antagonists to high-threshold Ca channels upon spinal mechanisms of pain, hyperalgesia and allodynia. Pain 85: 9–18, 2000. [PubMed]33. WangY-X, Pettus M, Gao D, Phillips C, Bowersox SS. Effects of intrathecal administration of ziconotide, a selective neuronal N-type Ca channel blocker, on mechanical allodynia and heat hyperalgesia in a rat model of postoperative pain. Pain 84: 151–158, 2000. [PubMed]34. Wang Y-X, Gao D, Pettus M, Phillips C, Bowersox SS. Interactions of intrathecally administered ziconotide, a selective blocker of neuronal N-type voltage-sensitive Ca channels, with morphine on nociception in rats. Pain 84: 271–281, 2000. [PubMed]35. Ino M, Yoshinaga T, Wakamori M, Miyamoto N, Takahashi E, Sonoda J, Kagaya T, Oki T, Nagasu T, Nishizawa Y, Tanaka I, Imoto K, Aizawa S, Koch S, Schwartz A, Niidome T, Sawada K, Mori Y. Functional disorders of the sympathetic nervous system in mice lacking the α 1B subunit (Cav 2.2) of N-type calcium channels. Proc Natl Acad Sci USA 98: 5323–5328, 2001. [PubMed]36. Kim C, Jun K, Lee T, Kim S-S, McEnery MW, Chin H, Kim H-L, Park JM, Kim DW, Jung SJ, Kim J, Shin H-S. Altered nociceptive response in mice deficient in the α1B subunit of the voltage-dependent Ca channel. Mol Cell Neurosci 18: 235–245, 2001. [PubMed]37. Saegusa H, Kurihara T, Zong S, Kazuno A, Matsuda Y, Nonaka T, Han W, Toriyama H, Tanabe T. Suppression of inflammatory and neuropathic pain symptoms in mice lacking the N-type Ca channel. EMBO J 20: 2349–2356, 2001. [PubMed]38. Saegusa H, Matsuda Y, Tanabe T. Effects of ablation of N- and R-type Ca(2+) channels on pain transmission. Neurosci Res 43: 1–7, 2002. [PubMed]39. Brose WG, Gutlove DP, Luther RR, Bowersox SS, McGuire D. Use of intrathecal SNX-111, a novel, N-type, voltage-sensitive, calcium channel blocker, in the management of intractable brachial plexus avulsion pain. Clin J Pain 13: 256–259, 1997. [PubMed]40. Mathur VS. Ziconotide: a new pharmacological class of drug for the management of pain. Semin Anesthesia Perioperative Med Pain 19: 67–75, 2000. 41. Staats PS, Yearwood T, Charapata SG, Presley RW, Wallace MS, Byas-Smith M, Fisher R, Bryce D, Mangieri EA, Luther RR, Mayo M, McGuire D, Ellis D. Intrathecal ziconotide in the treatment of refractory pain in patients with cancer or AIDS: a randomized controlled trial. JAMA 291: 63–70, 2004. [PubMed]42. Ridgeway B, Wallace M, Gerayli A. Ziconotide for the treatment of severe spasticity after spinal cord injury. Pain 85: 287–289, 2000. [PubMed]43. McGuire D, Bowersox S, Fellmann JD, Luther RR. Sympatholysis after neuron-specific, N-type, voltage-sensitive calcium channel blockade: first demonstration of N-channel function in humans. J Cardiovasc Pharmacol 30: 400–403, 1997. [PubMed]44. Malmberg AB, Yaksh TL. Voltage-sensitive calcium channels in spinal nociceptive processing: blockade of N- and P-type channels inhibits formalin-induced nociception. J Neurosci 14: 4882–4890, 1994. [PubMed]45. Schroeder JE, McCleskey EW. Inhibition of Ca2+ currents by a μ-opiod in a defined subset of rat sensory neurons. J Neurosci 13: 867–873, 1993. [PubMed]46. Westenbroek RE, Hoskins L, Catterall WA. Localization of Ca2+ channel subtypes on rat spinal motor neurons, interneurons, and nerve terminals. J Neurosci 18: 6319–6330, 1998. [PubMed]47. Murakami M, Nakagawasai O, Suzuki T, Mobarakeh II, Sakurada Y, Murata A, Yamadera F, Miyoshi I, Yanai K, Tan-No K, Sasano H, Tadano T, Iijima T. Antinociceptive effect of different types of Ca channel inhibitors and the distribution of various Ca channel α1 subunits in the dorsal horn of spinal cord in mice. Brain Res 1024: 122–129, 2004. [PubMed]48. Porreca F, Ossipov MH, Gebhart GF. Chronic pain and medullary descending facilitation. Trends Neurosci 25: 319–325, 2002. [PubMed]49. Urban MO, Ren K, Sablad M, Park KT. Medullary N-type and P/Q-type Ca channels contribute to neuropathy-induced allodynia. Neuroreport 16: 563–566, 2005. [PubMed]50. Hille B. Ionic channels of excitable membranes. Sunderland, MA: Sinauer Associates, 2001. 51. Bean BP. Nitrendipine block of cardiac calcium channels: high-affinity binding to the inactivated state. Proc Natl Acad Sci USA 81: 6388–6392, 1984. [PubMed]52. Stocker JW, Nadasdi L, Aldrich RW, Tsien RW. Preferential interaction of ω-conotoxins with inactivated N-type Ca2+ channels. J Neurosci 17: 3002–3013, 1997. [PubMed]53. Feng Z-P, Doering CJ, Winkfein RJ, Beedle AM, Spafford JD, Zamponi GW. Determinants of inhibition of transiently expressed voltage-gated calcium channels by ω-conotoxins GVIA and MVIIA. J Biol Chem 278: 20171–20178, 2003. [PubMed]54. Lewis RJ, Nielsen KJ, Craik DJ, Loughnan ML, Adams DA, Sharpe IA, Luchian T, Adams DJ, Bond T, Thomas L, Jones A, Matheson JL, Drinkwater R, Andrews PR, Alewood PF. Novel ω-conotoxins from Conus catus discriminate among neuronal calcium channel subtypes. J Biol Chem 275: 35335–35344, 2000. [PubMed]55. Scott DA, Wright CE, Angus JA. Actions of intrathecal ω-conotoxins CVID, GVIA, MVIIA, and morphine in acute and neuropathic pain in the rat. Eur J Pharmacol 451: 279–286, 2002. [PubMed]56. Smith MT, Cabot PJ, Ross FB, Robertson AD, Lewis RJ. The novel N-type calcium channel blocker, AM336, produces potent dose-dependent antinociception after intrathecal dosing in rats and inhibits substance P release in rat spinal cord slices. Pain 96: 119–127, 2002. [PubMed]57. Chen J-Q, Zhang Y-Q, Dai J, Luo Z-M, Liang S-P. Antinociceptive effects of intrathecally administered huwentoxin-I, a selective N-type Ca channel blocker, in the formalin test in conscious rats. Toxicon 45: 15–20, 2005. [PubMed]58. Uhrenholt TR, Nedergaard OV. Involvement of different calcium channels in the depolarization-evoked release of noradrenaline from sympathetic neurones in rabbit carotid artery. Basic Clin Pharm Tox 97: 109–114, 2005. 59. Morris, JL, Ozols DI, Lewis RJ, Gibbins IL, Jobling P. Differential involvement of N-type Ca channels in transmitter release from vasoconstrictor and vasodilator neurons. Br J Pharmacol 141: 961–970, 2004. [PubMed] |