In vivo CRISPR for hereditary angioedema: 87% fewer attacks

In vivo CRISPR for hereditary angioedema takes an unusual approach to treating a genetic disease. Rather than correcting the mutation responsible for the condition, researchers deliberately disabled a different, normally functioning gene. After a single intravenous infusion, patients experienced 87% fewer swelling attacks than those given a placebo.

The findings come from HAELO, an international phase 3 clinical trial published in the New England Journal of Medicine. The study marks an important step in the development of gene editing performed directly inside the human body.

Its central promise is straightforward: one intervention could potentially replace years of repeated preventive treatment.

When swelling becomes life-threatening

Hereditary angioedema is a rare genetic disorder that causes unpredictable episodes of severe swelling.

Attacks can affect the hands, feet, face, intestinal walls, and airways. Abdominal episodes may produce intense pain that resembles a surgical emergency. Swelling of the larynx can obstruct breathing and become life-threatening.

Unlike a typical allergic reaction, hereditary angioedema is primarily driven by bradykinin rather than histamine.

Bradykinin increases the permeability of blood vessels. When too much is produced, fluid escapes into surrounding tissues, causing swelling.

In the most common forms of hereditary angioedema, the underlying problem is a deficiency or dysfunction of C1 inhibitor, a protein that normally helps regulate the molecular system responsible for bradykinin production.

Several treatments can prevent attacks or stop them once they begin. Some patients require preventive medication for many years, and effective therapies have substantially improved disease management.

Nevertheless, unpredictable breakthrough attacks, the burden of ongoing treatment, and the possibility of severe complications remain important concerns.

Why scientists chose to disable a healthy gene

The experimental treatment, lonvoguran ziclumeran, follows a different strategy from correcting the inherited defect.

Instead of repairing the mutation that causes C1 inhibitor deficiency, the therapy targets another component of the biological pathway leading to excessive bradykinin production.

That target is KLKB1.

The gene encodes prekallikrein, the precursor of plasma kallikrein, an enzyme involved in releasing bradykinin.

Reducing kallikrein production can limit the excessive generation of bradykinin and potentially prevent swelling attacks.

The treatment uses CRISPR-Cas9, a molecular system capable of introducing targeted changes into DNA.

Its components are packaged in lipid nanoparticles designed to reach liver cells.

Each nanoparticle carries messenger RNA encoding the Cas9 protein and guide RNA that directs Cas9 toward the KLKB1 gene.

Once inside a liver cell, the editing machinery identifies the target DNA sequence and cuts it. As the cell repairs the break, small insertions or deletions can disrupt the gene.

The result is reduced production of prekallikrein and, consequently, lower levels of plasma kallikrein.

Crucially, the editing takes place inside the patient's body. Cells do not need to be collected, genetically modified in a laboratory, and returned.

Because the treatment changes DNA, the biological effect is intended to persist long after the original editing components have disappeared.

The changes are directed at somatic cells in the liver, not reproductive cells. They are not intended to be inherited by future generations.

One infusion, six months of measured efficacy

HAELO enrolled 80 patients aged 16 years or older with hereditary angioedema associated with C1 inhibitor deficiency.

The study was randomized, double-blind, and placebo-controlled.

Fifty-two participants received a single 50 mg intravenous infusion of lonvoguran ziclumeran. Twenty-eight received placebo.

The primary outcome was the number of investigator-confirmed hereditary angioedema attacks per month between weeks 5 and 28 after treatment.

The difference was substantial.

Patients receiving gene editing had an adjusted mean monthly attack rate of 0.26.

Those receiving placebo had a rate of 2.10.

This represented an 87% relative reduction in attack frequency, with a statistically significant difference between groups.

Another finding made the result especially tangible.

During the six-month efficacy assessment period, 62% of treated patients experienced no attacks and required no hereditary angioedema medication.

Only 11% of participants in the placebo group achieved the same outcome.

For a condition defined by the uncertainty of the next potentially dangerous episode, remaining attack-free without ongoing medication can represent a meaningful change in daily life.

Secondary outcomes reinforced the primary finding.

The trial showed reductions in attacks requiring on-demand treatment and in moderate or severe attacks. Patients receiving the gene-editing therapy also reported greater improvements in disease-related quality of life.

What does safety mean when DNA is permanently changed?

Gene editing presents a different safety challenge from conventional drug treatment.

The effects of most medicines diminish as their active compounds are metabolized or eliminated. A DNA modification, by contrast, is designed to remain in affected cells.

That durability is part of the attraction, but it also makes long-term safety evaluation essential.

Adverse events were reported in 92% of patients receiving lonvoguran ziclumeran and 86% receiving placebo during the observation period.

Events reported more frequently in the active-treatment group included infusion-related reactions, headache, fatigue, back pain, and upper respiratory tract infections.

No serious adverse events or grade 3 or higher adverse events were reported in the active-treatment group during the study's reported assessment period.

However, median follow-up at the data cutoff was 7.5 months.

That is a relatively short observation period for a treatment intended to produce long-lasting genomic changes. The trial cannot yet establish lifelong protection or exclude delayed safety problems.

Another limitation matters when interpreting the results: the comparison was with placebo, not with the most effective preventive treatments already available. The findings therefore do not establish superiority over existing standard therapies.

Following the positive phase 3 results, Intellia Therapeutics submitted a regulatory application in the United States. In September 2026, the FDA accepted the application for priority review, with a decision expected in March 2027. As of October 2026, the treatment had not yet been approved.

The broader significance of HAELO lies in the strategy itself.

Rather than repeatedly suppressing an overactive disease pathway with medication, researchers have demonstrated that editing a gene in liver cells can substantially reduce the production of a protein involved in that pathway.

And they achieved this without repairing the original mutation responsible for hereditary angioedema.

The inherited defect remains, but changing another gene may allow the body to function without many of its dangerous consequences.