Science Blog first covered this therapy in February 2024, when an early trial in people with hereditary angioedema, a genetic condition that causes sudden, severe swelling attacks, cut those attacks by a mean of 95 percent after a single infusion. The trial’s principal investigator, Dr Hilary Longhurst of the University of Auckland, told us at the time that the treatment looked “as if” it would provide “a permanent cure” for her patients’ symptoms. That was one clinician’s read of results from exactly ten people, in an open-label study with no placebo arm.

Two years on, the placebo-controlled trial built to test that early promise at scale has reported its results. In a randomised, double-blind Phase 3 trial of 80 patients, the therapy, now named lonvoguran ziclumeran, or lonvo-z, cut attacks by 87 percent compared with placebo. Intellia Therapeutics, the company developing it, has begun filing for approval with the US Food and Drug Administration, and if cleared, lonvo-z would be the first CRISPR-based gene-editing medicine ever approved that edits a patient’s cells inside their own body, rather than editing cells outside the body and infusing them back in.

What the therapy actually does

Hereditary angioedema is caused by a faulty gene that leaves the blood plasma protein plasma kallikrein poorly regulated. Left unchecked, plasma kallikrein triggers the production of bradykinin, a molecule that causes blood vessels to leak fluid into surrounding tissue. The result is swelling that can strike the hands, feet, face, gut or airway with little warning, and airway attacks can be fatal if untreated.

Lonvo-z uses CRISPR-Cas9 gene-editing machinery packaged inside a lipid nanoparticle, delivered by a single intravenous infusion. The nanoparticle is taken up by liver cells, where the CRISPR system cuts and disables the KLKB1 gene, the gene responsible for producing prekallikrein, the precursor to plasma kallikrein. Existing CRISPR-based medicines, such as Casgevy, the first CRISPR therapy approved anywhere, work by removing a patient’s cells, editing them in a laboratory, and infusing them back in. Lonvo-z instead performs the edit directly inside a living person’s liver. That distinction, in vivo editing versus ex vivo editing, is what would make it a first of its kind if approved, not the fact of using CRISPR at all.

What the Phase 3 trial found

The trial, called HAELO, randomised 80 patients with hereditary angioedema, 52 to receive a single 50-milligram infusion of lonvo-z and 28 to receive a placebo infusion. Just under half were enrolled in the United States, and 71 percent were already on long-term preventive medication when the trial began.

Over the six-month primary efficacy period, the treatment group had a mean monthly attack rate of 0.26, against 2.10 in the placebo group, an 87 percent reduction, according to results Intellia reported in an April 2026 announcement and, in fuller form, in a paper published in the New England Journal of Medicine alongside a presentation at the European Academy of Allergy and Clinical Immunology’s annual congress in June 2026. Sixty-two percent of patients on lonvo-z were completely attack-free and off all other angioedema medication for the full six-month window, compared with 11 percent on placebo. Secondary measures reported alongside the primary result included an 89 percent reduction in attacks requiring on-demand treatment, a 91 percent reduction in moderate-to-severe attacks, and a meaningful improvement on a standard quality-of-life questionnaire for angioedema patients.

Safety data through the trial’s 10 February 2026 cutoff reported no serious adverse events in the group that received lonvo-z. The most common side effects were infusion reactions, headache, fatigue, back pain and upper respiratory infections, all described as mild to moderate.

From open-label promise to a controlled result

The gap between the 2024 and 2026 numbers is not simply a story of the drug getting less effective as the trial got bigger, though the 95-to-87 shift is worth sitting with rather than glossing over. The 2024 figure came from an open-label study without a comparison group, which makes it harder to separate the drug’s actual effect from other explanations, including the tendency for a condition with unpredictable, cyclical symptoms to look better than usual around the time patients enrol in a trial that promises relief. The 87 percent figure is a placebo-controlled result, measured against a group that also received an infusion and also knew they were in a trial, which is a more demanding test to pass. That the number held up anywhere near the original open-label result, in a randomised trial nearly ten times the size, is the more meaningful finding here than either number taken alone.

Longhurst’s 2024 comment about a “permanent cure” was framed at the time, and should still be read now, as a clinician’s early impression rather than a claim the trial data itself was making. The mechanism does point toward durability: editing a gene in liver cells, rather than delivering a drug that has to be redosed, is designed to produce a lasting change rather than a temporary one. Plasma kallikrein levels in the Phase 3 trial stayed stable from around week five through the data cutoff, which is consistent with a durable edit. But the longest published follow-up on any patient who has received lonvo-z remains a matter of months to a few years, not decades, and no gene-editing therapy has been tracked in patients for long enough to say with certainty how permanent “permanent” turns out to be.

What the filing does and doesn’t mean

Intellia began a rolling submission of its biologics licence application to the FDA in April 2026 and has said it expects to complete the full submission in the second half of 2026, with a possible US launch in the first half of 2027 if the therapy is approved. A rolling submission lets a company file completed sections of an application as they’re ready rather than waiting for the entire package, a mechanism the FDA reserves for treatments addressing serious conditions with unmet need. It is not itself an approval, and the FDA’s review, once the full application is in, can still raise questions the trial data hasn’t yet answered, including how the therapy performs across a more diverse patient population and over a longer stretch of time than the current data can show.

Hereditary angioedema is rare, affecting an estimated one in 50,000 people worldwide, and existing treatments require either regular injected or infused prophylaxis or on-demand medication during attacks, a burden lonvo-z is designed to remove with a single dose. Whether it gets the chance to do that at scale now depends on a regulatory process that, unlike a company’s own trial results, is not Intellia’s to announce.