Abstract 4370295: Single dose of a novel CRISPR-CasX editor potently reduces triglycerides in preclinical models and efficiently edits <i>APOC3</i> in non-human primates

Background: Severe hypertriglyceridemia (SHTG) and familial chylomicronemia syndrome (FCS) are marked by extreme triglyceride (TG) elevations, raising the risk for acute pancreatitis and cardiovascular complications. Apolipoprotein C-III (APOC3) impairs TG catabolism and clearance. Loss-of-function APOC3 mutations are linked to lower TGs and reduced atherosclerotic risk, highlighting APOC3 as a promising therapeutic target. CRISPR-CasXE (XE) is an engineered gene editing platform with improved potency and specificity over natural systems. Here, we report the development of STX-1400, the first liver-targeted XE gene editing therapy designed to disrupt APOC3 expression and reduce circulating TGs. Approach: STX-1400 consists of an mRNA encoding an engineered XE editor and a single APOC3-targeting gRNA delivered via lipid nanoparticles. We evaluated the efficacy of a STX-1400 prototype in multiple in vitro and in vivo models. Specificity was assessed using a comprehensive off-target strategy to nominate potential off-target sites, followed by deep sequencing evaluation in primary human hepatocytes (PHHs) treated at 10xEC90. A non-human primate (NHP) surrogate was also tested in cynomolgus monkeys at escalating doses, with APOC3 editing measured in liver biopsies 18 days post-dose. Results: In PHHs, a STX-1400 prototype showed dose-dependent APOC3 editing (up to 90%) with &gt;70% decrease in secreted APOC3 protein. In human APOC3 transgenic mice, hepatic editing at the APOC3 locus exceeded &gt;70%, reducing circulating APOC3 by 80%. In hypertriglyceridemic mice, the STX-1400 prototype reduced APOC3 mRNA by &gt;90% and protein by &gt;95%, which corresponded with decreasing plasma TGs by &gt;95% and total cholesterol by &gt;80%. No off-target editing was observed in PHHs treated at 10xEC90. Notably, a single administration of the NHP analog of the STX-1400 prototype in cynomolgus monkeys achieved saturated editing levels (&gt;70%) in the liver at 1mg/kg, with liver enzyme profiles comparable to controls. Conclusions: Our findings demonstrate that the STX-1400 prototype potently and specifically edits APOC3 across human cells, mouse models, and non-human primates. This study is the first demonstration that a CRISPR-based therapy has achieved saturated in vivo editing of the APOC3 locus in NHP liver. These results provide strong preclinical evidence supporting the potential of STX-1400 as a first-in class genome editing therapy for hypertriglyceridemia in patients with SHTG and FCS.