Inclusion Criteria:

Part A:

• Able to provide written informed consent
• Healthy male or female between 18 to 45 years of age, inclusive at the time of screening
• Additional inclusion criteria apply

Part B:

• 18 - 55 years old inclusive at Screening
• Active but clinically stable, plaque psoriasis
• Psoriasis involving ≥ 10% of the body surface area
• A minimum PASI score of ≥ 10 obtained during the screening period
• Additional inclusion criteria apply

Exclusion Criteria:

Part A:

• History or evidence of a clinically significant disorder (including but not limited to cardiopulmonary, oncologic, renal, metabolic, hematologic or psychiatric), condition or disease that, in the opinion of the Investigator and Amgen physician would pose a risk to subject safety or interfere with the study evaluation, procedures or completion
• Underlying condition that predisposes the subject to infections (eg, uncontrolled diabetes - HbA1c > 7%, history of splenectomy)
• Additional exclusion criteria apply

Part B:

• Active guttate, erythrodermic, or pustular psoriasis at the time of the screening visit
• Evidence of skin conditions other than psoriasis (eg, eczema) at the time of the screening visit or between the screening visit and study drug initiation that would interfere with evaluations of the effect of investigational product on psoriasis
• Any condition that, in the judgment of the investigator, might cause this study to be detrimental to the subject
• Additional exclusion criteria apply