Abstract 4146303: A Single-Dose of a Novel CasX-Editor Lowers APOC3 Levels In Vivo

Background: Apolipoprotein C-III (APOC3) is a key regulator of lipid metabolism that inhibits the catabolism and clearance of triglyceride (TG)-rich lipoproteins in the blood. Loss-of-function APOC3 variants are associated with significantly lower TG levels and a reduced risk of heart disease, underscoring the potential of APOC3 inactivation as a therapeutic strategy for patients with familial chylomicronemia syndrome (FCS) and severe hypertriglyceridemia (SHTG). Here, we developed STX-1400 lead molecules as a novel investigational gene editing treatment using a highly engineered CRISPR-CasX mRNA and a guide RNA (gRNA) encapsulated into lipid nanoparticles (LNPs) to edit the APOC3 gene and knock out its hepatic expression. Methods: STX-1400 lead molecules were evaluated for editing potency and resulting functional effects on APOC3 reduction in vitro in primary human hepatocytes (PHHs) and primary cynomolgus hepatocytes (PCHs). In vivo studies were conducted in PXB mice, a humanized liver mouse model. Liver biopsies and serum samples were collected at two weeks post-dose. Human APOC3 mRNA and APOC3 protein levels were measured in liver biopsies and serum samples, respectively. Lipid profile evaluation, which included measuring TGs and total cholesterol (TC) levels, was also performed. Results: Treatment of PHHs with LNPs containing STX-1400 lead molecules showed a dose-dependent increase in editing activity; up to 90% editing at the APOC3 locus was associated with >70% reduction in secreted APOC3 protein levels. Potency was also observed in PCHs, with up to 70% editing. A single dose of LNPs encapsulating STX-1400 lead molecules delivered into PXB humanized mice resulted in >60% editing at the APOC3 locus in the liver, which was associated with >90% reduction of serum hAPOC3 protein levels, accompanied by reduction in TG and TC levels. Conclusions: These data demonstrate that the STX-1400 lead molecules efficiently knock down hepatic expression of APOC3 in preclinical models. This work suggests that the CRISPR-CasX-based editor approach could be developed as a single dose therapy for the treatment of hypertriglyceridemia.