GNT0003 Gene Therapy for Crigler-Najjar Syndrome Maintains Effect for Over 4 Years After Single Dose, Allowing Discontinuation of Phototherapy
Background
Patients with Crigler-Najjar syndrome (CN syndrome), a rare hereditary metabolic disease, must sleep under special lighting for their entire lives. This is due to defects in the UGT1A1 (uridine diphosphate glucuronosyltransferase 1A1) gene, which is essential for regulating bilirubin metabolism in hepatocytes, leading to the abnormal accumulation of unconjugated bilirubin. The accumulated toxic bilirubin can cross the blood-brain barrier, causing permanent neurological damage or kernicterus, which can lead to hearing loss, intellectual disability, or death if not addressed early.
Until now, the only standard treatment available to patients was phototherapy, involving 10 to 12 hours of full-body exposure to special blue light daily. While the principle involves using light energy to convert bilirubin into water-soluble isomers for excretion, patients encounter a limit where the efficiency of light penetration drops noticeably as the stratum corneum thickens with growth. Liver transplantation was the only existing method to fundamentally reverse the disease, but it faced practical barriers such as the lifelong need for immunosuppressants and a shortage of donors. Consequently, there has been an urgent need for in vivo gene therapy to replace the defective single gene with a normal one.
Key Findings
A research team led by Dr. Lorenzo D’Antiga at Papa Giovanni XXIII Hospital in Bergamo, Italy, and the European CureCN consortium have published the four-year long-term follow-up results of the gene therapy candidate GNT0003, based on adeno-associated virus serotype 8 (AAV8) targeting the liver, in the New England Journal of Medicine (NEJM). The research team had been observing the long-term efficacy and safety of patients with UGT1A1 deficiency who received a single intravenous injection of an AAV8 vector carrying a liver-specific promoter and complementary DNA (cDNA) of normal UGT1A1.
In this 4-year follow-up, all patients in the group receiving the therapeutic dose of 5×10¹² viral genomes (vg) per kg of body weight maintained significant therapeutic efficacy. Serum bilirubin levels immediately dropped below the dangerous threshold for neurotoxicity and remained within a safe range near normal (100–150 μmol/L). Even after completely discontinuing the intensive phototherapy that had been mandatory for over 10 hours every night, normal liver metabolic function was maintained. It is considered an improvement that patients can now enjoy a free lifestyle, escaping the daily routine of being tied to phototherapy devices during sleep.
In the early stages of administration, some patients experienced a transient rise in liver enzyme levels due to a cytotoxic T-lymphocyte-mediated immune response to the viral capsid protein, but this was stabilized without significant complications through short-term glucocorticoid immunosuppression. During the 4-year follow-up period, high safety was demonstrated, with no instances of tumor formation or significant chronic immune-related adverse events associated with vector insertion. This proves that functional UGT1A1 expression within liver tissue remains robust for several years after a single vector injection.
Significance and Outlook
This 4-year follow-up data largely resolves doubts regarding the durability of treatment, which has been considered the greatest weakness of non-integrating AAV gene therapy targeting the liver. Because AAV vectors reside in the nucleus as circular episomes without integrating into the host chromosome, concerns had been raised that the genetic material might gradually dilute during hepatocyte division, leading to decreased efficacy. This achievement clinically demonstrates that the effect of a single dose can persist for a long time in adult and adolescent liver tissue, where the cell turnover cycle is relatively slow.
This sets a positive milestone for the development of treatments for single-gene deficiency liver diseases such as Wilson's disease and hemophilia, in addition to Crigler-Najjar syndrome. However, the possibility remains that the duration of effect may be shorter in pediatric or infant patients with active hepatocyte division and growth due to faster vector dilution. Additionally, the technical challenge remains that once high titers of neutralizing antibodies form in the patient's blood after AAV vector administration, it is difficult to re-administer the same vector if efficacy declines. Future follow-up strategies are expected to include technologies for transient antibody depletion, combination with nanoparticle-based non-viral delivery systems, and prime editing techniques to directly correct the hepatocyte genome.
New England Journal of Medicine, Volume 395, Issue 18, Page 1870-1872, November 5, 2026.
These research results present a fundamental treatment alternative without the need for liver transplantation for patients with severe Crigler-Najjar syndrome who previously had to rely on special lighting devices every night for life. In actual clinical practice, the ability to completely discontinue phototherapy, which patients had been performing for over 10 hours daily, has led to a significant expansion of autonomy in their daily lives. Another major achievement is the prevention of cumulative brain damage from childhood to adulthood, thereby reducing the incidence of emergency hospitalizations and neurological sequelae.
From an industrial perspective, it drives the expansion of subsequent pipelines by demonstrating the long-term safety and efficacy of the AAV8 platform targeting liver tissue. Given that a single administration can reduce substantial medical and hospitalization costs over several years, it is expected to serve as a positive benchmark for economic evaluations in discussions regarding health insurance finances and orphan drug pricing.