Chronic or intractable pain, such as may occur in conditions such as bone degenerative diseases and cancer, is a debilitating condition which is treated with a variety of analgesic agents, and often opioid compounds, such as morphine.
In general, brain pathways governing the perception of pain are still incompletely understood, sensory afferent synaptic connections to the spinal cord, termed "nociceptive pathways" have been documented in some detail. In the first leg of such pathways, C- and A-fibers which project from peripheral sites to the spinal cord carry nociceptive signals. Polysynaptic junctions in the dorsal horn of the spinal cord are involved in the relay and modulation of sensations of pain to various regions of the brain, including the periaqueductal grey region (McGeer). Analgesia, or the reduction of pain perception, can be effected directly by decreasing transmission along such nociceptive pathways. Analgesic opiates are thought to act by mimicking the effects of endorphin or enkephalin peptide-containing neurons, which synapse presynaptically at the C- or A-fiber terminal and which, when they fire, inhibit release of neurotransmitters, including substance P. Descending pathways from the brain are also inhibitory on C- and A-fiber firing.
Certain types of pain have complex etiologies. For example, neuropathic pain is generally a chronic condition attributable to injury or partial transection of a peripheral nerve. This type of pain is characterized by hyperesthesia, or enhanced sensitivity to external noxious stimuli. The hyperesthetic component of neuropathic pain does not respond to the same pharmaceutical interventions as does more generalized and acute forms of pain.
Opioid compounds (opiates) such as morphine, while effective in producing analgesia for many types of pain, are not always effective, and may induce tolerance in patients. When a subject is tolerant to opioid narcotics, increased doses are required to achieve a satisfactory analgesic effect. At high doses, these compounds produce side effects, such as respiratory depression, which can be life threatening. In addition, opioids frequently produce physical dependence in patients. Dependence appears to be related to the dose of opioid taken and the period of time over which it is taken by the subject. For this reason, alternate therapies for the management of chronic pain are widely sought after. In addition, compounds which serve as either a replacement for or as an adjunct to opioid treatment in order to decrease the dosage of analgesic compound required, have utility in the treatment of pain, particularly pain of the chronic, intractable type.
Although calcium blocking agents, including a number of L-type calcium channel antagonists, have been tested as adjunct therapy to morphine analgesia, positive results are attributed to direct effects on calcium availability, since calcium itself is known to attenuate the analgesic effects of certain opioid compounds (Ben-Sreti). EGTA, a calcium chelating agent, is effective in increasing the analgesic effects of opioids. However, results from tests of calcium antagonists as adjunct. therapy to opioids have been contradictory; some L-type calcium channel antagonists have been shown to increase the effects of opioids, while others of these compounds have been shown to decrease opioid effects (Contreras).
U.S. Pat. No. 5,051,403 describes the use of omega-conopeptides having defined binding/inhibitory properties in the treatment of ischemia-related neuronal damage. Co-pending parent U.S. Patent application Ser. No. 07/814,759 demonstrates the effectiveness of omega-conopeptide compositions in certain animal models of pain. Specifically, omega-conopeptides MVIIA and TVIA and derivatives thereof having related inhibitory and binding activities were demonstrated to produce analgesia in animal models of analgesia in which morphine is the standard positive control. The present invention is directed to the discovery that such omega conopeptides also exhibit analgesic properties in certain models of analgesia, such as neuropathic pain models of analgesia, in which morphine is not expected to produce positive results.