The objectives of this clinical trial are to assess, for up to 5 years, the safety, tolerability and pharmacological activity of a single ascending doses of VTX-801, a gene therapy, administered intravenously (IV) to adult patients with Wilson's Disease prior to and following background WD therapy withdrawal.

Main Inclusion Criteria:

• Male or female aged 18 and 65 years inclusive
• Confirmed diagnosis of WD
• Treated for WD according to international recommendations with no current evidence for inadequate treatment
• Stable WD for ≥ 1 year, defined as: (i) No significant change in neurologic examination and in status of mood disorder and (ii) Stable laboratory parameters used to assess copper metabolism

Main Exclusion Criteria:

• ALT level ≥ 2 ULN that is not readily explained by extrinsic factors
• Total bilirubin > 1.5 x ULN in the absence of proven Gilbert's syndrome; in case of Gilbert's syndrome, direct bilirubin > ULN
• INR > 1.2
• Any signs of liver cirrhosis decompensation, including gastrointestinal bleed within 6 months (24 weeks) prior to screening/enrollment visit
• Patient has moderate or severe renal impairment defined as eGFR CKD-EPI < 60 mL/min/1.73 m2, or patient has nephritis or nephrotic syndrome
• Any history or current evidence of HIV-1, HIV-2, HTLV 1, or HTLV-2 infection
• Any history or current evidence of hepatitis B infection
• Any history of hepatitis C infection, unless previous viral RNA assays in two samples, collected at least 6 months apart, are negative
• Positive QuantiFERON®-TB Gold tuberculosis test result
• Any concomitant disorder/condition - including hepatic disorders - or treatment possibly interfering with the conduct or evaluation of the study
• Any history of diabetes
• Pregnancy or breastfeeding
• Body Mass Index ≥ 35 kg/m2

Other protocol defined Inclusion/ Exclusion criteria may apply

From Baseline through primary completion visit (an average of 1 year)

The study was prematurely terminated due to futility and not for safety reasons, because the VTX-801 pharmacodynamics effects observed in the first 4 patients at the first 2 dose levels tested were insufficient to allow withdrawal of the SoC.

Due to the early study termination and limited data, no conclusions can be drawn regarding the predefined objectives. The safety results were consistent with the expected safety profile for VTX-801. No new safety issues were observed.