The purpose of this study is to assess the safety and tolerability of AMG 139 following single subcutaneous (SC) or intravenous (IV) dose administration in healthy subjects and subjects with moderate to severe psoriasis (PsO).

Inclusion Criteria:

Part A - Healthy Volunteers:

• Healthy male or female of non-reproductive potential subjects between 18 to 45 years-of-age
• Body mass index (BMI) between 18 and 32 kg/m2
• Normal or clinically acceptable physical examination, clinical laboratory values, and ECG
• Additional inclusion criteria apply

Part B - Psoriasis Subjects:

• Male or female of non-reproductive potential subjects with PsO between 18 to 55 years-of-age
• Body mass index (BMI) between 18 and 32 kg/m2
• Normal or clinically acceptable physical examination (PE), clinical laboratory values, and ECG; clinically acceptable PE includes evidence of moderate to severe plaque PsO
• Diagnosis of plaque PsO for at least 6 months
• Moderate to severe plaque PsO defined by:
• A minimum PASI score of ≥ 10
• Psoriasis involving ≥ 10% of the Body Surface Area (BSA)
• Additional inclusion criteria apply

Exclusion Criteria:

Parts A - Healthy Volunteers:

• History or evidence of a clinically significant disorder (including psychiatric), condition or disease that would pose a risk to subject safety or interfere with the study evaluation, procedures or completion;
• Recent or on-going infection(s)
• Underlying condition(s) that predisposes the subject to infections
• History of malignancy of any type, other than in situ cervical cancer or surgically excised non-melanomatous skin cancers in past 5 years
• Additional exclusion criteria apply

Part B - Psoriasis Subjects:

• History or evidence of a clinically significant disorder (including psychiatric), condition or disease that would pose a risk to subject safety or interfere with the study evaluation, procedures or completion;
• Recent or on-going infection(s)
• Underlying condition(s) that predisposes the subject to infections
• Guttate, pustular, or other non-plaque forms of PsO
• Evidence of skin conditions other than PsO (eg, eczema) that would interfere with evaluations of the effect of investigational product on psoriasis
• History of malignancy of any type, other than in situ cervical cancer or surgically excised non-melanomatous skin cancers in past 5 years
• Additional exclusion criteria apply