This clinical trial is focused on studying a rare genetic condition called Crigler-Najjar syndrome. This condition affects the liver’s ability to process a substance called bilirubin, leading to its buildup in the body. Patients with severe forms of this syndrome often require daily light therapy, known as phototherapy, to help manage bilirubin levels. The study is testing a new treatment called GNT0003, which is a type of gene therapy. This treatment involves using a specially designed virus to deliver a healthy version of the gene responsible for processing bilirubin directly into the liver cells. The goal is to help the liver function more normally and reduce the need for phototherapy.

The purpose of the study is to evaluate the safety and effectiveness of a single dose of GNT0003 given through an intravenous infusion, which means it is delivered directly into the bloodstream. The study is divided into two parts. The first part focuses on determining the safest dose of the treatment, while the second part assesses how well the treatment works at the selected dose. Participants will receive the treatment and be monitored over several weeks to see how their bodies respond. This includes checking for any side effects and measuring changes in bilirubin levels.

Throughout the study, participants will have regular check-ups, including blood tests and other assessments, to ensure their safety and to track the treatment’s impact on their health. The study aims to provide valuable information on whether this gene therapy can offer a new way to manage Crigler-Najjar syndrome and improve the quality of life for those affected by this condition. The trial is expected to continue until 2028, allowing researchers to gather comprehensive data on the long-term effects of the treatment.