Long-term Safety and Efficacy of ESK-001 in Moderate to Severe Plaque Psoriasis

The objective of the ESK-001-018 long term extension is to evaluate the safety and efficacy of ESK-001 over time. The scientific questions it aims to answer are: * How safe is taking ESK-001 long-term in people with moderate to severe plaque psoriasis? * Does taking ESK-001 long-term reduce the severity of people's plaque psoriasis? Patients will enter the long-term extension study following completion of one of the parent studies (ESK-001-016 or ESK-001-017) and will receive open-label ESK-001 twice daily for 24 weeks. After 24 weeks, the first 200 patients meeting at least PASI-75 clinical response will be randomly assigned to receive ESK-001 or placebo. At any point during this time, the patients losing the initial clinical response may return to the open-label ESK-001 treatment. Patients who complete Week 48 will return to open-label ESK-001 treatment and they will receive ESK-001 until the end of the study or discontinuation. All the remaining patients not meeting the entry criteria for the randomized withdrawal phase will continue to receive open-label ESK-001 for the remainder of the study. Patients taking part in the study must be men or women aged at least 18 years old and have completed a previous (parent) study of ESK-001 in moderate to severe plaque psoriasis. Patients must consent and agree to: * ensure drug daily compliance until end of study or discontinuation. * visit the clinic for checkups and assessments. * provide blood and urine samples.

Inclusion Criteria: 1. Males or females, age ≥18 years 2. Completed either of the two previous (parent) studies of ESK-001 (ESK-001-016 or ESK-001-017) 3. ESK-001 safety and tolerability were acceptable in the parent study 4. Women of childbearing potential (WOCBP) and males who are sexually active with WOCBP must agree to adhere to highly effective methods of contraception for the entirety of the study Exclusion Criteria: 1. Pregnant, lactating, or planning to get pregnant during the study period 2. Any acute or chronic illness/condition or evidence of an unstable clinical condition that in the Investigator's judgment will substantially increase the risk to the patient if they continue to receive ESK-001 treatment in this extension study 3. Deemed by the Investigator to be inappropriate for the study or unable to comply with the protocol