497. HSV Vector-Mediated Expression of a Novel Drug Activated Pain Signalling Therapeutic Abrogates Pain

Gene therapy approaches to treat chronic pain have been limited by short-term duration, the inability to turn off therapeutic gene expression to avoid unwanted side-effects or tolerance. To overcome this, we employed replication-defective herpes simplex virus (rdHSV) vectors expressing the glycine receptor alpha-1 (GlyRa1) subunit to silence pain-signaling neurons in a variety of pain models upon the addition of glyine. However, the therapeutic window was small since centrally occurring GlyR in the spinal cord were activated at higher glycine doses. While this was encouraging, the application of GlyRa1 as an effective therapeutic requires further modifications to avoid off-target effects. It was clear that the most effective use of this method would require targeted GlyRa1 expression in specific neuronal subpopulations to avoid silencing of non-nociceptive neurons, limiting receptor expression to neurons involved in chronic pain signaling. Treatment specificity may be achieved through the use of a GlyRa1 that is altered in a manner to be activated by a specific drug that does not cause activation of the normal receptor. Others have identified a mutant form of the receptor (GlyRa1F207A/A288G or G2M) that no longer responds to glycine but rather responds to a novel ligand, ivermectin, an FDA-approved anti-helmenthes drug that binds to the mutant receptor in the nM range while mM quantities of ivermectin are needed to activate the wt-GlyRa1.We engineered G2M rdHSV vectors under the control of the CMV promoter and only upon addition of 100μM Ivermectin could we silence thermal pain while the wt-GlyRa1 was only activated by 100mM Glycine. We tested the effective dose range in a rat model of post-herpetic neuralgia (PHN) that elicits both thermal and mechanical pain behaviors that last out to 9 weeks. We showed that 100μM-10nM Ivermectin was able to block both mechanical and thermal pain, while only 100mM Glycine was able to abrogate those same behaviors, over a 10,000-fold difference. To further reduce off target effects, we engineered rdHSV vectors expressing G2M using neuronal fiber-specific promoters whose transgene expression will be limited to either large As-fibers (NF200) or C-fibers (TRPV1). Studies in rats showed that TRPV1p driven G2M dramatically reduced thermal pain as expected with this C-fiber specific promoter only upon Ivermectin addition to a greater magnitude than that observed with CMV. This activity could be reversed specifically using resiniferatoxin (RTx), a capsaicin analogue that caused C-fiber retraction. NF200 driven G2M failed to block thermal pain, though mildly effected mechanical pain only after RTx treatment. We are currently testing the transcriptionally targeted G2M vectors in the rat PHN pain model. Together, these results demonstrate the power of employing a therapeutic product that responds preferentially to Ivermectin rather than glycine, thereby providing the only receptor in vector-infected animals that is activated by relatively low doses of this well-tolerated drug that can be administered systemically, orally or even topically.