Part 1 is a randomized, double-blind, placebo-controlled phase 1b study to evaluate AT193 in approximately 34 patients with nail psoriasis for 16 weeks. Part 2 is an open-label extension phase where all patients will receive active drug for an additional 16 weeks followed by a 2 week nontreatment period.

Inclusion Criteria:

• Diagnosis of nail psoriasis for at least 3 months preceding study entry
• Diagnosis or history of plaque psoriasis or psoriatic arthritis
• A female participant of childbearing potential must use appropriate contraceptive measures during the study period
• A female participant of childbearing potential must have a negative urine pregnancy test result at screening
• Written informed consent must be obtained before any study procedure is performed

Exclusion Criteria:

• Pregnant or breastfeeding
• Diagnosis of other active skin or nail diseases or skin or nail infections (bacterial, fungal, or viral) that may interfere with evaluation of skin or fingernail psoriasis
• History or evidence of a clinically significant disorder, condition, or disease that would pose a risk to patient safety or interfere with the study evaluation, procedures, or completion in the judgment of the investigator