A Single Shot That Rewrites the Code: What Intellia's CRISPR Breakthrough Means for Rett Families

Last week, a biotech company called Intellia Therapeutics announced positive results from a large clinical trial of a CRISPR-based gene editing therapy. Intellia's therapy, called lonvoguran ziclumeran (lonvo-z), was tested in people with hereditary angioedema (HAE), a rare genetic condition that causes recurrent swelling attacks in the face, airway, abdomen, and limbs.

HAE is caused by overactivity of a protein called kallikrein, which drives swelling attacks. Lonvo-z uses CRISPR gene editing to permanently inactivate the gene (KLKB1) that produces kallikrein, with the goal of stopping attacks at their source. The therapy is delivered as a single intravenous infusion using lipid nanoparticles, which are small fat-based particles, similar to those used in the COVID vaccines. The lipid nanoparticles carry the CRISPR machinery into liver cells, where the editing takes place. The CRISPR enzyme called Cas9 then acts like molecular scissors to cut and inactivate the gene.

The Results

The trial met its primary endpoint and all key secondary endpoints. A single infusion of lonvo-z reduced swelling attacks by 87% compared to placebo over a six-month evaluation period. Most patients remained free from attacks and did not need additional medication during that period. The side effects were temporary and primarily related to the infusion. Intellia has already begun submitting an application to the FDA for approval and is aiming for a potential launch in early 2027.

What this means for gene editing therapies

There are two broad approaches to gene editing in people. The first is ex vivo where cells are removed from a patient, edited in a laboratory, and returned to the body. Casgevy, the approved CRISPR therapy for sickle cell disease, works this way. It is effective but also complex and expensive, requiring intensive preparation and hospitalization. Also, not all cells can be treated with an ex vivo approach, neurons for instance, cannot be removed and put back after editing. The second approach is editing in vivo where the editing tools are packaged and delivered directly into the body, where they travel to target cells and make edits. This lonvo-z study represents the first Phase 3 clinical trial of an in vivo CRISPR gene editing therapy, marking a significant milestone for the field.

The Intellia trial confirms three things that matter for the broader gene editing field. First, in vivo CRISPR can be delivered safely into human cells using lipid nanoparticles with no observed serious off-target effects. Second, the edits are durable even when the gene editing machinery is only temporary. Patients in earlier phases of Intellia's HAE program have been followed for years with no evidence of the edit reversing. Third, if the FDA approves lonvo-z, it will establish the first regulatory framework for an in vivo CRISPR therapy, creating a precedent for how future programs, including those targeting different organs, will be evaluated.

Where Things Stand for Rett

While this breakthrough is significant, applying it to Rett Syndrome presents unique challenges. Intellia's therapy works in liver cells, which are among the easiest tissues to reach with lipid nanoparticles circulating in the bloodstream. The brain is a fundamentally different target. It is protected by the blood-brain barrier, a selective membrane that prevents most large molecules, including standard lipid nanoparticles, from entering the central nervous system.

Ideally, a genome editing therapeutic for Rett will also be delivered using non-viral technology. We want the CRISPR editor to get into a cell, do its edit and then disappear. Editors that are delivered virally, on the other hand, will reside in the cells for many years to come, potentially editing the genome beyond its intended target, and potentially causing complications.

Efforts to develop non-viral delivery approaches for the brain are ongoing in academic labs and biopharma around the world. RSRT is funding a number of these efforts. In the meantime, work is underway to develop the gene editing cargo to be delivered. RSRT has its own internal gene editing program that it's advancing in the labs of Guoping Feng, MIT, Erik Sontheimer and Scot Wolfe, University of Massachusetts, and Adrian Bird, University of Edinburgh.

In Summary

Intellia's Phase 3 trial is a significant milestone for the field of in vivo gene editing. It demonstrates that CRISPR can be delivered into a living person's body, edit a specific gene safely and durably, and produce significant clinical benefit from a single treatment. These are important facts for the entire gene editing field, including researchers working on Rett syndrome.

I look forward to keeping you updated as we continue to progress towards gene editing options for Rett syndrome.