Abstract/Summary Systemic sclerosis (SSc) is an autoimmune disorder driven by inflammation, fibrosis, and microvascular disease that effects 75,000 Americans and over 2.5M people word wide. The common histopathological features of SSc are endothelial and fibroblast dysfunction resulting in excessive extra cellular matrix deposits that disrupts the normal tissue architecture. Interstitial lung (ILD) disease is very common in SSc (SSc ILD) and is the main driver of mortality with a 10-year survival of 60%. Nintedanib and Tocilizumab are FDA-approved treatments for SSc ILD, however, their overall impact for prolonging life is limited and patients either succumb to the disease or undergo lung transplantation. Therefore, new therapeutic interventions that slow down or reverse fibrosis are in great need. Data from several reports have demonstrated significant activation of the master transcriptional regulator STAT3 (pSTAT3-Y705) in SSc patient samples. Significant STAT3 activation was observed in skin SSc compared to healthy subjects. In follow-up studies, pharmacological inhibition of STAT3 or STAT3 knockdown ameliorated skin fibrosis in experimental mouse models demonstrating its role as a key driver of SSc. Our group has demonstrated significant STAT3 activation in SSc ILD patient tissues. Based on these findings, we hypothesize that targeting STAT3 will block profibrogenic pathways and reduce inflammation and ECM accumulation in SSC ILD. Although TFs are attractive therapeutic targets, they are challenging to target with small molecules due to lack of clear small molecule binding pockets; domains important for protein-protein interactions; and large stretches of intrinsically disordered domains (IDDs). At Altay, we developed a platform that enables identification of small molecule binding pockets within IDDs, allowing a novel approach for specific targeting of STAT3 and development of potent STAT3 inhibitors (STAT3is). Using our platform, we identified next generation inhibitors that reduced STAT3 DNA binding. Importantly, these STAT3i had minimal STAT1 inhibitory activity, low cytotoxicity and demonstrated potent inhibition of STAT3 targets and fibrosis genes. We propose three aims to characterize the most promising lead to treat SSc ILD based on inhibiting STAT3. In Aim 1, we will measure cytotoxicity and STAT3 target gene inhibition in a panel of normal fibroblasts and SSc-ILD fibroblasts isolated from patients through our collaboration with Dr. Paul Wolters at UCSF. In Aim 2, we will determine the anti-fibrotic efficacy of STAT3is in SSC ILD patient tissues and determine target engagement by measuring STAT3 target gene expression. In Aim 3 we will demonstrate in vivo efficacy of lead STAT3i using a bleomycin-induced fibrosis model where we will determine the therapeutic and prophylactic effect of our lead STAT3i. The proposed studies will establish the potential for targeting STAT3 in treating SSc ILD and guide new therapeutic strategies in thi...