Maximal Usage Pharmacokinetics and Safety of ARQ-151 in Children With Plaque Psoriasis (ARQ-151-216)

Inclusion Criteria: * Informed consent of parent(s) or legal guardian. * Males or females, 2 to 5 years old (inclusive). * Clinical diagnosis of psoriasis vulgaris of at least 2 months duration as determined by the Investigator or through subject interview. Stable disease for the past 3 weeks. * Psoriasis vulgaris on the face, extremities, trunk, and/or intertriginous areas involving at least 2% of BSA (excluding the scalp, palms, and soles). * An Investigator Global Assessment of disease severity (IGA) of at least Mild ('2') at Baseline. * Subject has adequate venous access for PK sampling in areas not involved by plaque psoriasis and not being treated with ARQ-151 (e.g., back of the hands). * In good health as judged by the Investigator, based on medical history, physical examination, 12-lead electrocardiogram (ECG), serum chemistry labs and hematology values. * Subjects and parent(s)/legal guardian(s) are considered reliable and capable of adhering to the Protocol and Visit Schedule, according to the judgment of the Investigator. Exclusion Criteria: * Subjects with any serious medical or psychiatric condition or clinically significant laboratory, ECG, vital signs, or physical examination abnormality that would prevent study participation or place the subject at significant risk, as judged by the Investigator. * Planned initiation or changes to concomitant medication that could, in the opinion of the Investigator, affect psoriasis vulgaris * Current diagnosis of non-plaque form of psoriasis (e.g., guttate, erythrodermic/exfoliative, palmoplantar only involvement, or pustular psoriasis). Current diagnosis of drug-induced psoriasis. * Subjects with any condition on the treatment area which, in the opinion of the Investigator, could confound efficacy measurements. * Subjects who cannot discontinue the use of strong cytochrome P-450 CYP3A4 inducers for 2 weeks prior to Baseline/Day 1and during the study period. * Subjects who cannot discontinue the use of strong cytochrome P-450 CYP3A4 inhibitors for 2 weeks prior to Baseline/Day 1 and during the study period. * Subjects who are unwilling to refrain from prolonged sun exposure and from using a tanning bed or other artificial light emitting devices (LEDs) for 4 weeks prior to Baseline/Day 1 and during the study. * Subjects who cannot discontinue specific systemic therapies and/or topical therapies prior to the Baseline/Day 1 and during the study period according to the protocol. * Subjects with any infection requiring oral or intravenous administration of antibiotics, antifungal or antiviral agents within 2 weeks prior to Baseline/Day 1.