214. Additive Effects of HSV Vectors Expressing Proenkephalin, Interleukin-4, and Tumor Necrosis Factor Alpha Soluble Receptor To Treat Bone Cancer Pain

Abstract

Top of pageAbstract In order to determine whether increasing GABA in the spinal cord by glutamic acid decarboxylase 67 (GAD67) gene transfer to the dorsal root ganglion (DRG) would attenuate below-level central neuropathic pain resulting from spinal cord injury (SCI) we engineered a replication defective HSV based vector, deleted for the essential immediate early genes ICP4 and ICP27 and additionally deficient in expression of immediate early genes ICP22 and ICP47, to express GAD 67 driven by the HCMV IE promoter at the UL41 locus (vector QHGAD67). Transduction of DRG neurons in vitro with QHGAD67 resulted in the release of GABA measured by HPLC. Subcutaneous inoculation of the vector in the foot resulted in GAD67 expression and constitutive release of GABA in microdialysate of the dorsal horn. One week after left T13 spinal hemisection in rats, animals were inoculated subcutaneously into the plantar surface of both hindpaws with 30 microliters QHGAD67 (1x109 pfu/ml) or a lacZ-expressing control vector QOZHG. Mechanical allodynia and thermal hyperalgesia in both hindpaws was assessed with a series of calibrated von Frey filaments and Hargreaves heat apparatus, respectively. Animals inoculated with QHGAD67 demonstrated antiallodynic and antihyperalgesic effects that persisted for several weeks and could be effectively reestablished by reinoculation. The vector-mediated analgesic effects were reduced by intrathecal bicuculline or phaclofen administered at doses that produced neither mechanical allodynia nor thermal hyperalgesia. Vector-mediated GABA release attenuated the increase in spinal calcitonin gene related peptide (CGRP) caused by cord hemisection. These results demonstrate that transduction of DRG with a GAD-expressing HSV vector results in the release of GABA from nerve terminals in the spinal cord to provide an analgesic effect through actions at both GABAA and GABAB receptors, and suggests that gene therapy with a GAD67 expressing HSV based vector may prove useful in the treatment of central neuropathic pain after spinal cord injury.