A Study of TAK-781 in Healthy Volunteers and in Participants With Non-Cirrhotic Primary Sclerosing Cholangitis (PSC)

The main aim of this study is to see if the drug TAK-781 is safe for healthy volunteers and for participants with PSC. The study will also look at how well participants can tolerate TAK-781. In addition, the study will check how the body absorbs, uses, and gets rid of TAK-781 (Pharmacokinetics \[PK\]), how the drug affects the body (Pharmacodynamics \[PD\]), and how the body's immune system reacts to TAK-781 (Immunogenicity). The study consists of two phases (Phase 1a and 1b). Phase 1a includes two parts: Part 1 (Single Ascending Dose \[SAD\]) and Part 2 (Multiple Ascending Dose \[MAD\]). In Part 1, healthy participants will receive either single dose of TAK-781 or a placebo. A placebo looks the same as TAK-781 but has no medicine in it. In Part 2, healthy participants will receive multiple doses of TAK-781 or a placebo. In Phase 1b (Part 3), participants with large duct, non-cirrhotic PSC will receive a single dose of TAK-781. Participants will be in the study for about 36 weeks.