A Study to Learn How Safe ACT-777991 is and How Well it Works in Adults With Non-segmental Vitiligo

The trial includes three trial periods:

Following a Screening period, during which it will be checked if participants are eligible to take part, eligible participants will be randomized in a 2:1 ratio to receive either ACT-777991 or placebo for 24 weeks (Trial intervention period). On completion of treatment, participants will be followed for 30 (+7) days (Follow-up period).

Trial participation will end with a Follow-up visit (Participant Last Visit) at the end of the Follow-up period.

The maximum trial duration for an individual participant is approximately 33 weeks including a screening period of up to 28 days, a treatment period of 24 weeks, and a follow-up period of up to 37 days.

Inclusion criteria

• Clinical diagnosis of either active or stable non segmental vitiligo for at least 3 months prior to Screening and meet all the following criteria:

• F-VASI score ≥ 0.3 based on BICR at Screening.
• T-VASI score ≥ 5 based on investigator assessment at Screening and Randomization.
• Total body surface area (BSA) involvement, including the face, ≤ 50% based on investigator assessment at Screening and Randomization.
• Participants must agree not to use therapeutic agents and procedures to treat vitiligo from Screening until Participant Last Visit.

Exclusion criteria

• Clinical diagnosis of other forms of vitiligo (e.g., segmental) or other hypo- or depigmentation disorders (e.g., piebaldism, leukoderma, Vogt-Koyanagi-Harada disease, malignancy-induced hypopigmentation).
• Any autoimmune disease, except adequately treated thyroid disease.
• History of systemic immunotherapy treatment, including JAK inhibitors, for any inflammatory disease in the 12 months prior to Randomization.
• History of topical JAK inhibitors for any inflammatory disease in the 6 weeks prior to Screening.
• Use of laser or light-based treatment (phototherapy), including tanning beds, in the 8 weeks prior to Screening.
• eGFR < 90 mL/min/1.73 m2, defined by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation, at Screening.