Alpha2delta-1 splice variants in neuropathic pain

In patients, damage to peripheral nerves results from many different diseases, and can result in chronic pain that is poorly treated by most of the commonly used pain killers. Common conditions such as diabetes and herpes infection, as well as treatment with certain forms of chemotherapy, can all cause nerve damage, which may be irreversible. A drug called gabapentin (initially developed for the treatment of epilepsy) is effective in the alleviation of this type of pain (which is called neuropathic pain). However, gabapentin is not equally effective in all patients with nerve-injury pain. We know how gabapentin works in the treatment of neuropathic pain, as it binds to a protein called alpha2delta-1, and the amount of this protein in pain-sensing nerves is increased several-fold in experimental models of neuropathic pain. Our study aims to examine whether different forms of alpha2delta-1 are produced in neuropathic pain conditions. These forms might show altered properties, such as higher or lower ability to recognise gabapentin. We will also investigate whether genetic variability in the gene for alpha2delta-1 may affect sensitivity to gabapentin. If this were the case it might be highly relevant to the development and treatment of neuropathic pain in patients, as it might explain why gabapentin is not similarly effective in all patients. It is also possible that this research will aid in the development of novel drugs in this class, that might have the potential to be more effective in certain patients or in certain conditions.

In patients, damage to peripheral sensory nerves, in conditions including post-herpetic, trigeminal and diabetic neuralgia, cancers impinging on nerves, and also damage to tissue, may result in long-lasting neuropathic pain. The same is true in animal models of these conditions. The mechanism for establishment and maintenance of neuropathic pain is complex but involves altered expression of several genes in the damaged dorsal root ganglion (DRG) neurons. Of relevance to the present application, there is a very large increase in the expression of the mRNA for the voltage-gated calcium channel auxiliary subunit alpha2delta-1 in DRG neurons in several experimental models of neuropathic pain, for example an 8-10-fold increase in spinal nerve-ligated rats. This results in an increase in alpha2delta-1 protein both in DRG cell bodies and in the central terminals of these neurons in the dorsal horn of the spinal cord. The anticonvulsant gabapentin and the related drug pregabalin are first-line treatments for neuropathic pain and bind to alpha2delta-1 protein. These drugs reduce abnormal activity, but do not inhibit physiological pain in humans or animals, by a mechanism that remains uncertain. However, gabapentin and pregabalin are not equally effective in all patients with nerve injury-induced pain, for reasons that are unclear. The responder rate is ~30-50% for pregabalin in diabetic peripheral neuropathy and post-herpetic neuralgia, and ~30% for gabapentin in post-herpetic neuralgia, taken from prescribing information. This relatively low rate is probably a composite of side effects and environmental and genetic differences and emphasizes the need for novel drugs in this class. We will investigate the hypothesis that there is differential expression of alpha2delta-1 splice variants in DRGs following the development of neuropathic pain, utilising the rat chronic constriction injury model, used routinely at GSK. We will also investigate the biophysical properties of these splice variants, including their sensitivity to gabapentin and pregabalin, and other novel alpha2delta-1 ligands to be supplied by GSK. We will further investigate the properties of two single nucleotide polymorphisms (SNPs) that have been reported in the gabapentin binding motif (RRR) in human alpha2delta-1, in terms of their biophysical properties and 3H-gabapentin binding affinity. Our investigations may shed light on why gabapentinoid drugs are not equally effective in all patients. It is therefore possible that other variants of alpha2delta-1 could be used in drug screening to identify novel drugs in this class that might be more effective in certain patients or certain neuropathic conditions.