Neuropathic pain arises from nervous system injuries due to trauma, disease, or neurotoxin exposure afflicts 15-20 million people in the U.S is very difficult to treat.1-6 Currently available therapeutics include anticonvulsants, antidepressants and opioids; these have limited efficacy and possess many side effects including high abuse liability.4 Novel non opioid based targets are needed for therapeutic intervention. We believe that we have found such target; the Gi/o protein-coupled receptor (GPCR) Gpr34. Our unbiased transcriptomic approach in a rat model of traumatic nerve injury-induced neuropathic pain7 revealed that GPR34 increases in the dorsal horn of the spinal cord ipsilateral to nerve injury. GPR34 is expressed in both humans and rodents and is highly expressed in microglia.8-10 Its primary endogenous ligand is lysophosphatidylserine (LysoPS).11 Little is known about the roles of LysoPS/GPR34 in pain. One study reported GPR34 deletion in the spinal cord reversed nerve injury-induced neuropathic pain by suppressing pro-inflammatory responses of microglia.12 Chemical probes for GPR34, especially antagonists, are limited with only on reported antagonists (Takeda).13 We synthesized this compound (SLU-PP-2368) and found it antagonized LysoPS-induced β-arrestin recruitment and reversed mechano- and cold allodynia in two rodent models7,14 with no observable side effects or engaging the endogenous opioid system (preliminary data). This compound has poor physicochemical properties limiting its clinical utility. Using the cryoEM structure of GPR34 bound with a synthetic agonist,15 we synthesized three analogs (SLU-PP-2438, -2439 and -2440) with GPR34 antagonistic activity and improved solubility and stability. Systemic and intrathecal administration of these antagonists reversed behavioral hypersensitivities in two models of neuropathic pain7,14 (preliminary data). LysoPS (i.th.) in rodents recapitulated behavioral phenotypes seen in the nerve-injury models and evoked dose- and time-dependent pertussis toxin-sensitive (Gαi/o-linked) behavioral hypersensitivities that were blocked by GPR34 antagonists. Collectively, these data identify the spinal cord as a potential site of GPR34 antagonist action that contributes to neuropathic pain. The mechanisms engaged by LysoPS/GPR34 signaling that contribute to neuropathic pain are unknown, but studies implicate mitogen-activated protein kinase (MAPK) signaling pathways.11,16,17 Our driving hypothesis is that GPR34 in the spinal cord is a non-opioid based target for therapeutic intervention with GPR34 antagonists and the analgesic actions of GPR34 antagonists result from attenuating MAPK in microglia. We also hypothesize that identifying CNS-active GPR34 hits will allow future development of novel GPR34 antagonists with intellectual property potential as drug candidates for neuropathic pain. Results from our studies are anticipated to develop a strong biological rationale and establish a multidisciplinary team and infrastructure to develop CNS-penetrant selective GPR34 antagonists that will be used for a therapeutics development plan to enable a future application for RFA- NS-21-015, thereby directly addressing RFA-NS-21-029.

Neuropathic pain, a debilitating chronic pain condition that affects millions of individuals is difficult to treat and drugs currently used have numerous side effects, high abuse liability and limited efficacy. Here, we will establish a multidisciplinary team with relevant experience in drug development, translational pain research, pain biology and clinical trials and create the required infrastructure for the development of novel non-opioid, GPR34 receptor antagonists for the treatment of neuropathic pain.