Phase 2 Study on Efficacy and Safety of GNT0003 with Imlifidase Pre-treatment in Adults with Severe Crigler-Najjar Syndrome and Anti-AAV8 Antibodies

The primary objective of this study is to assess the **efficacy** of a single intravenous administration of GNT0003 following imlifidase pre-treatment in participants with severe Crigler-Najjar syndrome requiring phototherapy and presenting pre-existing anti-AAV8 antibodies. This is clinically relevant as it aims to address the unmet medical need in patients with this rare genetic disorder, potentially reducing the dependency on phototherapy and improving patient outcomes.

Secondary objectives include:

• To evaluate the safety and tolerability profile of GNT0003.
• To evaluate the safety and tolerability profile of imlifidase.
• To assess the efficacy of imlifidase in the cleavage of anti-AAV8 IgG.
• To assess the pharmacokinetic profile of GNT0003.
• To assess the pharmacodynamic profile of GNT0003.
• To evaluate the impact of GNT0003 on participants' quality of life.

The clinical trial involves participants diagnosed with **Crigler-Najjar syndrome**, a rare genetic disorder. The study population includes both male and female subjects who are **18 years or older**, or of legal consenting age in their respective jurisdictions. Participants must have severe Crigler-Najjar syndrome, necessitating at least six hours of phototherapy daily, and must have a confirmed mutation in the UGT1A1 gene. Additionally, participants are required to have detectable serum neutralizing antibodies against AAV8. The trial population was selected based on specific inclusion criteria, including laboratory parameters that are not clinically significant, as assessed by the investigator. Participants must also agree to use a highly effective method of contraception from the screening visit to at least 48 weeks after the start of the investigational medicinal product administration. The sponsor has not provided information regarding the total number of participants. The trial includes a vulnerable population, and all participants must be capable of providing informed consent, complying with the study's requirements and restrictions. Lifestyle considerations such as diet and physical activity are not specified in the available data.

The clinical trial is designed to evaluate the efficacy and safety of a single intravenous administration of **GNT0003**, an **adeno-associated viral vector serotype 2/8 containing the human UGT1A1 gene**, following pre-treatment with **imlifidase** in adult participants with severe **Crigler-Najjar syndrome**. This is a phase II, open-label trial, which is not randomized or blinded. The trial aims to assess the restoration of the UGT1A1 enzyme expression in hepatocytes, potentially allowing for the glucuronidation of bilirubin. The trial is expected to commence on October 15, 2024, and conclude by August 15, 2030.

Participants will undergo a series of study visits, beginning with an inclusion (screening) visit to confirm eligibility based on criteria such as age, severity of the syndrome, and presence of specific antibodies. The primary endpoint is the proportion of participants achieving serum total bilirubin levels ≤ 300 µmol/L, 48 weeks post-GNT0003 infusion, without phototherapy from Week 16. Secondary endpoints include monitoring vital signs, laboratory assessments, and the incidence of adverse events up to 60 months post-administration.

The trial involves multiple follow-up visits to monitor the participants' health status, including assessments of bilirubin levels, liver function, and the presence of anti-AAV8 antibodies. The end-of-study visit will evaluate the long-term effects of the treatment, including any adverse drug reactions or malignancies. Participants are expected to be involved in the study for a minimum of 48 weeks, with the possibility of extended follow-up for up to 60 months to monitor long-term outcomes.

Participant involvement may be terminated early if they experience significant adverse events, fail to comply with study protocols, or withdraw consent. The trial will ensure that all participants provide informed consent and agree to use effective contraception throughout the study period. The study's design and procedures are structured to ensure the collection of comprehensive data on the safety and efficacy of the investigational treatment in this rare disease population.

The clinical trial involves the administration of **rAAV8-hUGT1A1**, an experimental medication formulated as a **solution for infusion**. This investigational product is an **adeno-associated viral vector serotype 2/8** containing the human **UGT1A1** gene. The primary objective is to restore the expression of the UGT1A1 enzyme in hepatocytes, facilitating the glucuronidation of bilirubin. The maximum total dose is 5 trillion vector genomes, administered intravenously as a single infusion. This treatment is specifically designed for adult participants with severe Crigler-Najjar syndrome who require daily phototherapy and have pre-existing anti-AAV8 antibodies. Compliance with the dosing schedule is monitored through clinical assessments and laboratory evaluations.

**Idefirix**, containing the active substance **imlifidase**, is used as a pre-treatment in this trial. It is provided as a **powder for concentrate for solution for infusion** and is administered intravenously. The role of imlifidase is to reduce pre-existing antibodies against the AAV8 vector, thereby enhancing the efficacy of the gene therapy. The dosing is based on the participant's body weight, with the specific dosage regimen determined by the clinical team.

In addition to the experimental treatments, the trial includes several non-experimental medications. **Sirolimus**, an immunosuppressant, is administered orally with a maximum daily dose of 4 mg. It is used to prevent immune responses that could interfere with the gene therapy. **Prednisolone**, a corticosteroid, is also administered orally with a maximum daily dose of 40 mg for up to 7 days, to manage inflammation and immune reactions.

Other auxiliary medications include **betamethasone sodium phosphate**, administered orally with a maximum daily dose of 40 mg for up to 7 days, and **pseudoephedrine hydrochloride** combined with **cetirizine dihydrochloride**, administered orally with a maximum daily dose of 10 mg. These medications are used to manage symptoms and potential side effects associated with the primary treatment.

Additionally, **amoxicillin sodium**, an antibiotic, is available for oral administration as needed, although specific dosing is not predetermined. **Lidocaine hydrochloride monohydrate** combined with **methylprednisolone acetate** is administered via intravenous infusion with a maximum daily dose of 100 mg, primarily for managing pain and inflammation. **Loratadine**, an antihistamine, is administered orally with a maximum daily dose of 10 mg to manage allergic reactions.

Participant compliance with the treatment regimen is closely monitored through regular clinical visits, laboratory tests, and adherence assessments. The trial protocol ensures that all medications are administered according to the specified dosing schedules to optimize therapeutic outcomes and minimize adverse effects.

The efficacy of the clinical trial will be assessed primarily by evaluating the proportion of participants with serum total **bilirubin** levels ≤ 300 µmol/L, 48 weeks after the administration of GNT0003, without the need for phototherapy from Week 16. This primary endpoint is crucial in determining the effectiveness of the treatment in reducing bilirubin levels in participants with severe Crigler-Najjar syndrome.

Secondary endpoints will include a variety of measures to provide a comprehensive assessment of the treatment's efficacy and safety. These will involve monitoring vital signs, conducting physical examinations, and identifying any clinically significant abnormalities in safety laboratory assessments. Electrocardiograms (ECG) will also be performed. The incidence of treatment-emergent adverse events (TEAE), serious adverse events (SAE), and adverse events of special interest (AESI) will be recorded from the time of imlifidase administration to 48 weeks post-GNT0003 infusion. Additionally, the incidence of adverse drug reactions (ADR) and malignancies, including liver carcinogenicity, will be tracked up to 60 months following GNT0003 administration.

Further assessments will include the comparison of total anti-AAV8 IgG levels in serum before and after imlifidase infusion, as well as prior to GNT0003 administration. The time to clearance of the GNT0003 vector from blood, urine, saliva, feces, and semen (for male participants) will be measured. Changes in serum total bilirubin and the serum bilirubin/albumin ratio from baseline to Week 48 and up to 60 months will be evaluated. The mean time to restart phototherapy and changes in health-related quality of life, as measured by the SF-36 questionnaire after GNT0003 administration, will also be analyzed.