The purpose of this study is to explore the efficacy and safety of multiple, escalating doses of ASP015K when compared to placebo in subjects with moderate to severe plaque psoriasis.

Following completion of at least 1 week of study drug for all subjects in group 1, the Sponsor will review the safety information of these subjects prior to enrolling off-treatment subjects into the next dose group. Dose escalation will continue in this manner (unless stopping criteria are met) until all groups have completed the study.

Inclusion Criteria:

• Subject has moderate to severe plaque psoriasis for 6 months or longer with at least 10% Body Surface Area (BSA) affected
• Subject must be a candidate for phototherapy and/or systemic therapy

Exclusion Criteria:

• Subject has non-plaque psoriasis or drug-induced psoriasis
• Subject is on systemic psoriasis therapy within 56 days or 5 half-lives, whichever is longer, prior to first dose of study drug
• Subject has a positive TB skin test within 3 months of screening or at screening
• Subject has an abnormal chest x-ray

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.