Principal Investigator/Program Director (Last, First, Middle: Ridong Chen Project Summary More than 25 million Americans suffer from chronic pain. Due to the lack of other treatments, there has been an overreliance on opioids, contributing to an alarming epidemic of opioid overdose addictions and deaths. Neu- ropathic pain is difficult to treat, with only 30-40% of patients achieving meaningful (>40-50%) pain relief. Current therapies (e.g., duloxeline and gabapentin) mainly address symptoms by focusing on blocking neurotransmis- sion in the pain pathway with limited efficacy, severe side effects, and narrow therapeutic indices. Hence, novel non-opioid therapies are urgently needed to safely manage symptoms and also target the underlying pathophys- iological mechanisms that will improve the functional status and quality of life of affected patients. It has been recently shown that CD4+ Th1 is a major player for neuropathic pain development. Interleukin-2 (IL-2) is the key cytokine for the generation, survival, and function of regulatory T cells by direct binding to its high affinity receptor. Treatment with low-dose rIL-2 increased anti-inflammatory regulatory T cells and M2 type macrophages and inhibited pathogenic interferon- secreting T helper type 1 cells. This rIL-2 treatment was efficacious in human clinical studies for autoimmune diseases without complications or infections. Hence, resto- ration or enhancement of regulatory T cells with low-dose IL2 may offer a novel strategy for prevention and treatment of neuropathic pain. However, several drawbacks exist for current low-dose rIL2 therapy, including a short half-life, propensity for in vitro aggregation causing adverse local reaction at injection sites, and a potential narrow therapeutic window. We have designed proprietary IL2-based variants that will enable selective stimu- lation of Tregs with an extended half-life and a broad therapeutic window. In the Phase I study, we successfully identified the variant, designated APT603, which selectively stimulates regulatory T cells and provides robust analgesic efficacy in the chronic constriction injury model of mononeuropathy in rats with an excellent safety profile. With an experienced drug development team, we propose to determine dose responses in two well es- tablished models of neuropathic pain with distinct pathophysiology and to conduct critical activities necessary to enable IND filing for APT603. Specific Aim 1: Determine the therapeutic index of APT603 for abrogating neuropathic pain in STE- induced (T1D) diabetic rats and in a rat model of sciatic nerve chronic constriction injury (CCI). Specific Aim 2: Manufacture cGMP (Current Good Manufacturing Practice) grade APT603. Specific Aim 3: Evaluate the nonclinical safety of APT603. The long-term goal is to develop the drug candidate as a safe and disease-modifying analgesic therapy. Weekly or bi-weekly dosing will provide sustained neuropathic pain relief for patients ...