FDA Lifts Clinical Hold on Intellia's CRISPR Gene-Editing Trial for Heart Disease After Patient Death and Safety Overhaul
The FDA has cleared Intellia Therapeutics to resume its MAGNITUDE Phase 3 trial of nexiguran ziclumeran for ATTR cardiomyopathy, five months after a patient death and subsequent clinical hold prompted a safety overhaul.
Overview
The U.S. Food and Drug Administration has lifted its clinical hold on Intellia Therapeutics’ MAGNITUDE Phase 3 trial, allowing the company to resume testing nexiguran ziclumeran (nex-z) in patients with transthyretin amyloidosis with cardiomyopathy (ATTR-CM), according to an Intellia press release issued on March 2, 2026. The decision comes roughly five months after a Grade 4 liver adverse event and subsequent patient death led to a clinical hold on the trial.
Nex-z is built on Nobel Prize-winning CRISPR/Cas9 gene-editing technology and is designed to permanently inactivate the TTR gene with a single intravenous infusion. If successful, it would become the first one-time CRISPR-based treatment for a common form of cardiac amyloidosis, a progressive and often fatal condition caused by misfolded transthyretin protein accumulating in heart tissue.
What We Know
A patient dosed with nex-z on September 30, 2025, developed Grade 4 liver transaminase elevations and increased total bilirubin, according to Intellia’s initial disclosure on October 27, 2025. The adverse event was reported on October 24, and the patient was hospitalized, with the company stating the patient was “being closely monitored and receiving medical intervention.” Intellia voluntarily paused enrollment in both the MAGNITUDE and MAGNITUDE-2 trials in response.
The patient died on the evening of November 5, 2025. Intellia stated it had “been advised by the treating physician that this is a case with complicating comorbidities,” though the company did not disclose the specific conditions involved. The FDA formally imposed a clinical hold on both trials on October 29, 2025, following Intellia’s voluntary pause two days earlier.
To resume the trial, Intellia has aligned with the FDA on several mitigation measures. These include enhanced monitoring of liver laboratory tests in the initial period following dosing, guidance for short-term steroid treatment if elevated liver transaminases are observed, and the exclusion of patients with certain pre-existing liver abnormalities. For MAGNITUDE specifically, new criteria will exclude patients with recent cardiovascular instability or an ejection fraction below 25 percent at screening.
The company noted that the severe liver event occurred in fewer than one percent of the more than 450 patients who had received nex-z across the MAGNITUDE and MAGNITUDE-2 trials combined. No Grade 4 liver events were reported among the 47 patients dosed in the separate MAGNITUDE-2 trial, which tests nex-z in hereditary ATTR with polyneuropathy. The FDA lifted the hold on MAGNITUDE-2 in January 2026.
What We Don’t Know
Intellia has not disclosed the full demographic profile or detailed medical history of the patient who died, beyond referencing complicating comorbidities. It remains unclear whether the liver toxicity was directly caused by the CRISPR-based mechanism of nex-z, by the lipid nanoparticle delivery system, or by an interaction with the patient’s underlying cardiac condition.
The long-term safety profile of in vivo CRISPR gene editing in patients with advanced heart failure also remains an open question. MAGNITUDE is designed to enroll approximately 1,200 patients with ATTR-CM, making it by far the largest clinical trial of an in vivo CRISPR therapy to date. Whether the new exclusion criteria will meaningfully reduce the risk of similar events, or significantly narrow the eligible patient population, will only become clear as enrollment resumes.
The trial’s primary endpoint is a composite measure of cardiovascular-related events including mortality, and no timeline for interim or final data readouts has been updated since the hold was imposed.
Analysis
The lifting of the MAGNITUDE hold is a pivotal moment for the CRISPR medicine field. While earlier CRISPR therapies such as Vertex’s Casgevy treat blood disorders by editing cells outside the body, nex-z represents a fundamentally different approach: delivering gene-editing machinery directly into a patient’s liver via a single infusion. The stakes are correspondingly higher, as any safety signal in this setting carries implications for the entire class of in vivo gene-editing therapeutics.
The FDA’s willingness to allow the trial to proceed, with enhanced safeguards rather than a permanent halt, suggests regulators view the risk-benefit calculus as still favorable for a disease with limited treatment options. ATTR-CM is estimated to affect tens of thousands of patients in the United States alone, and current standard-of-care treatments such as tafamidis stabilize the TTR protein but do not eliminate its production. A one-time gene-editing cure, if proven safe and effective, would represent a fundamentally different therapeutic paradigm.
CEO John Leonard stated that the company is “very pleased to have aligned with the FDA on the path forward for our MAGNITUDE clinical trial, with measures designed to further enhance patient safety.” The market will be watching closely as enrollment resumes and additional safety data accumulate.