A Study to Learn About the Study Medicine Etrasimod in Adults With Moderate to Severe Atopic Dermatitis (AD) Who Have Already Tried Treatments Taken by Mouth or by Injection

The purpose of this study is to learn about the safety and effects of the study medicine called etrasimod for the possible treatment of atopic dermatitis (AD), also called eczema, in adults who have already tried AD treatments taken by mouth or by injection that work all over the body. These adults can have moderate to severe AD.

This study is seeking participants who:

• have AD for at least 1 year

• have moderate-to-severe AD

• have tried treatments that work all over the body and saw no effects

• are willing to apply a moisturizer at least once daily during the study

This is a 2-part study that is only selecting about 60 participants for Part 1 as of now. In Part 1, half of the participants will receive etrasimod, a pill to be taken by mouth once daily. The other half will receive a placebo, a pill that looks like etrasimod but has no medicine also taken by mouth once daily. No one will know what treatment the participant is taking. The Sponsor will compare participant experiences of those taking etrasimod to those taking placebo for 16 weeks. This will help determine if the study medicine is safe and effective. After the first 16 weeks, some participants may continue the study knowing they are taking etrasimod for an additional 52 weeks.

Those participating for just the first 16-weeks, will need to visit the study clinic at least 6 times during the study (about every 4 weeks), and will have to come for 2 safety follow up visits at 2nd and 4th week after the last dose of study medicine. People who want to and can continue for an additional 52 weeks will need to visit the study clinic for at least 6 more visits making 12 total visits over 68 weeks followed by 2 safety follow up visits at the 2nd and 4th week after the last dose of study medicine.

In Part 2 of the study, around 340 more participants will be participating. Everyone will receive etrasimod pills once daily for 52 weeks. Participants will need to go to the study clinic at least 9 times after which they will have to go for 2 more safety follow up visits at the 2nd and 4th weeks after the last dose of study medicine.

At every study visit in Part 1 and Part 2, the focus will be on signs and symptoms of AD (like lesions, itch, and pain) as well as general health and overall side effects. Blood samples and vital signs will be taken at every visit. Due to the way the study medicine works, the in-study clinic visit will last at least 4 hours on Day 1 (Part 1 and Part 2) and Week 16 (Part 1).