Abstract 4369344: STX-1200: A Novel CRISPR-CasX based Gene Editing Approach for Potent and Selective Reduction of Lipoprotein(a) in Multiple Preclinical Models

Background: Elevated lipoprotein(a) [Lp(a)] is a prevalent, causal risk factor for cardiovascular disease, with no approved therapies to lower its levels. Circulating Lp(a) concentrations are largely dictated by genetic variation at the LPA locus, positioning gene editing as a promising strategy for long-term control. CRISPR-CasXE (XE) is a gene editing platform highly engineered for greater potency and specificity than natural CRISPR systems. We have developed STX-1200, the first investigational XE gene editing therapy designed to knock out LPA expression in the liver. Methods: STX-1200 consists of an LPA -targeting gRNA and an mRNA encoding a highly engineered XE gene editor encapsulated into lipid nanoparticles (LNPs). We created a diverse panel of XE variants using an integrated molecular engineering strategy and screened in primary human (PHHs) and cynomolgus hepatocytes (PCHs) for editing potency and Lp(a) reduction. In vivo efficacy was tested in human liver chimeric mice (PXB) and human (h LPA ) transgenic mice. Potential off-target sites were nominated using a comprehensive approach (combining in silico prediction, in vitro assays, and cellular analysis) and validated by deep sequencing in PHHs treated at 10xEC90. Given its homology to LPA , plasminogen ( PLG ) was also evaluated for unintended editing. Results: A lead STX-1200 candidate achieved >90% editing of LPA in PHHs and >90% reduction in secreted Lp(a). In PXB mice, a low dose of the STX-1200 lead molecule led to 70% liver editing and >85% reduction in serum Lp(a). In h LPA mice, a single dose achieved saturated liver editing and >90% reduction in secreted Apo(a) at therapeutic levels. Specificity analysis showed no off-target effects, including no impact on PLG . Deep sequencing of >125 predicted off-target sites confirmed the high specificity of the STX-1200 lead molecule, with no detectable editing even at supersaturating doses. Further engineering improved editing efficiency in both PHHs and PCHs, enabling advancement to non-human primates. Conclusions: STX-1200 is a novel CRISPR-CasXE therapeutic designed for superior potency and specificity, showing strong potential to significantly lower Lp(a). Its potent activity at low doses and clean off-target profile positions it as a transformative, one-time therapy for individuals with elevated Lp(a) and high cardiovascular risk.