Scribe Therapeutics Wins TGA Clearance to Begin First Human Trial of STX-1150, a CRISPR Epigenetic Therapy Targeting PCSK9
Scribe's ELXR platform silences PCSK9 in the liver without cutting DNA; a single low dose sustained LDL reductions for roughly 18 months in primates.
Overview
Scribe Therapeutics, the Alameda, California-based biotech co-founded by Nobel Prize laureate Jennifer Doudna, announced on May 21, 2026 that it has received clearance from Australia’s Therapeutic Goods Administration (TGA) to begin the first-in-human clinical study of STX-1150, an investigational therapy that epigenetically silences the PCSK9 gene to durably reduce LDL cholesterol without permanently altering DNA. The milestone marks the first time a CRISPR-based epigenetic silencing approach targeting a cardiovascular risk factor has entered human testing, according to BioSpace.
What We Know
The drug and its mechanism
STX-1150 is built on Scribe’s proprietary ELXR (Epigenetic Long-Term X-Repressor) platform — described by AllSci as “a nuclease-inactivated, CasX-derived CRISPR protein fused to epigenetic effector domains” — that installs histone and DNA methylation marks at the PCSK9 gene promoter in hepatocytes to achieve transcriptional silencing without permanent DNA sequence alteration. Unlike gene-editing tools that cut the genome, ELXR changes how a gene behaves by layering chemical marks onto the surrounding chromatin.
The therapy is delivered as a liver-tropic lipid nanoparticle encapsulating messenger RNA encoding the ELXR protein along with a single guide RNA targeting PCSK9, according to BioSpace. A built-in allosteric regulatory domain constrains the DNA methyltransferase activity of the ELXR protein, designed to reduce off-target effects while maintaining or enhancing on-target silencing, as described on Scribe’s website. Longevity.Technology reported that the design reduced off-target activity by roughly 10- to 100-fold compared to earlier systems while improving the ability to silence target genes.
Preclinical durability
In non-human primate studies, a single STX-1150 dose below 1.0 mg/kg delivered via lipid nanoparticles achieved greater than 50% LDL-C reduction sustained for approximately 18 months, with the study still ongoing at the time of reporting, according to Scribe’s website. The therapy was generally well tolerated with no significant liver enzyme elevations compared to controls.
The clinical trial
The Phase 1 study is an open-label, single ascending dose trial with a dose expansion phase enrolling up to 64 participants across sites in Australia and New Zealand, according to BioSpace. The lead site is Monash Health’s Victorian Heart Hospital in Clayton, Victoria, Australia, with Stephen Nicholls, MBBS, Ph.D., Director of the Victorian Heart Institute at Monash University, serving as principal investigator. Primary endpoints include safety, tolerability, pharmacokinetics, and pharmacodynamics, and participants will be monitored for one year post-treatment, per AllSci. Human data are anticipated within 12 to 18 months.
The rationale: human genetics as a blueprint
The scientific case for targeting PCSK9 rests on a well-characterized natural experiment: individuals born with loss-of-function variants in the PCSK9 gene maintain meaningfully lower baseline LDL-C and experience up to 88% reduced coronary heart disease risk without adverse effects from lifetime lower cholesterol, as noted by BioSpace and AllSci. That biological precedent underpins the hypothesis that a single epigenetic intervention could confer similar protection without the adherence demands of daily statins or bimonthly injectable PCSK9 inhibitors.
What executives and investigators said
Benjamin Oakes, Ph.D., co-founder and CEO of Scribe, stated: “Advancing STX-1150 into the clinic marks a pivotal moment for Scribe. Years of iterative engineering to enhance the potency and specificity of our CRISPR technologies has culminated in our first clinical program designed to provide access to the effects of known cardioprotective genetics, potentially freeing patients from decades of treatment burden. This milestone reflects our commitment to developing ultra-long-acting therapies that may redefine the standard of care, reduce adherence challenges, and help make earlier intervention and long-term prevention more practical for patients with chronic cardiometabolic disease.”
Principal investigator Dr. Stephen Nicholls added: “The initiation of Scribe’s first-in-human study reflects a critical advance in the development of much-needed, highly durable approaches to LDL-C reduction. Targeting PCSK9 with an innovative epigenetic silencing strategy, STX-1150 offers a differentiated approach to long-term lipid management. I look forward to working with Scribe to evaluate this therapy in patients with elevated LDL-C and increased cardiovascular risk.”
Conference presentations
Ahead of the trial clearance, Scribe presented data from its CRISPR engineering work at the American Society of Gene and Cell Therapy annual meeting in Boston in May and was scheduled to deliver a late-breaking oral presentation titled “Epigenetic PCSK9 silencing with STX-1150 provides durable LDL-C control after a single dose” at the 94th European Atherosclerosis Society Congress in Athens on Monday, May 25, 2026, according to BioSpace.
What We Don’t Know
The trial will enroll its first human participants, and no human efficacy data exist yet. Initial results from the Phase 1 study are expected within 12 to 18 months, covering primarily safety and tolerability. Whether the approximately 18-month durability seen in non-human primates will translate to humans, and at what dose, remains to be established. PCSK9-targeted monoclonal antibodies such as evolocumab and alirocumab have demonstrated large cardiovascular outcome benefits in randomized trials, providing a high bar for any newcomer; Scribe’s approach would also need to demonstrate that epigenetic silencing, unlike permanent DNA editing, can be reversed if unexpected safety issues emerge — though the company has not specified a reversal strategy in publicly available materials.
Analysis
Several features of STX-1150 distinguish it from both existing PCSK9 inhibitors and gene-editing approaches currently in development. Approved injectable PCSK9 monoclonal antibodies require subcutaneous dosing every two to four weeks; inclisiran, a small interfering RNA therapy, reduces dosing to twice yearly; a gene-editing approach such as CRISPR Therapeutics’ CTX310 (targeting ANGPTL3 rather than PCSK9) aims for permanent one-time editing. STX-1150 occupies a different position: it attempts lasting silencing without permanent genomic alteration, which in principle preserves the theoretical option of restoration. The choice of Australia for first-in-human testing — a common strategy among biotech firms seeking faster regulatory review timelines — reflects the TGA’s reputation for efficient clinical clearance. The trial’s design as a single ascending dose study means that safety at multiple dose levels will be established before any dose expansion, and the one-year monitoring period is relatively short for a therapy intended to last years. Scribe’s partnerships with Sanofi and Eli Lilly suggest significant pharmaceutical interest in the platform beyond STX-1150, though the terms and scope of those collaborations have not been publicly detailed.