Evaluation of AMG 714 for Vitiligo

Individuals must meet all of the following criteria to be eligible for enrollment as study participants: 1. Adults 18-75 years of age. 2. Clinical diagnosis of active or stable vitiligo made by a dermatologist, as defined in Protocol Section 3.4.2. 3. F-VASI ≥ 0.25 (Appendix 2 of Protocol). 4. T-VASI ≥ 3 (Appendix 2 of Protocol). 5. Willingness to: 1. Undergo nbUVB phototherapy, as outlined in Protocol Section 7.3. 2. Stop all other treatments for vitiligo from screening through the final follow up visit as outlined in Protocol Section 7.2. Exclusion Criteria: Individuals who meet any of the following criteria are not eligible for enrollment as study participants: 1. Inability or unwillingness of a participant to give written informed consent or comply with the study protocol. 2. Segmental vitiligo. 3. Contraindication to nbUVB phototherapy. 4. More than 33% leukotrichia on the face or on the total body. 5. Use of biologic immunosuppressive or immunomodulatory agents, or investigational therapy or procedure within 12 weeks or 5 half-lives prior to Visit 0 (whichever is longer), except agents authorized for prevention and treatment of SARS-CoV-2 infection according to FDA Emergency Use Authorization (EUA). 6. Use of laser or light-based treatment (phototherapy) including tanning beds within 8 weeks prior to Visit 0. 7. Use of non-biologic systemic or topical immunosuppressive or immunomodulatory agents within 4 weeks prior to Visit 0. 8. History of melanocyte-keratinocyte transplantation procedure (MKTP) or other surgical treatment for vitiligo. 9. Current or past use of the depigmenting agent monobenzyl ether of hydroquinone, including Benoquin® (Monobenzone). 10. Presence of skin conditions or lesions that would confound the vitiligo assessments. 11. Spontaneous repigmentation within 6 months prior to Visit 0 (repigmentation without any treatment and significant in amount as determined by the investigator). 12. Uncontrolled thyroid function at screening as determined by the investigator. If the participant has a history of thyroid disease and is on treatment, the participant must be on a stable thyroid regimen for at least three months prior to Visit 0. 13. Greater than 3 adequately treated nonmetastatic basal cell carcinomas (BCC) or squamous cell carcinomas (SCC) within 12 months prior to Visit 0; or previous history of multiple BCC or SCC which may pose additional risks from participation in the study in the opinion of the investigator. 14. Previous or current diagnosis of other cancer, except adequately treated cervical carcinoma in situ. 15. Acute or chronic infection, including current use of suppressive therapy for chronic infection, hospitalization for treatment of infection within 90 days prior to Visit 0, or parenteral anti-microbial (including anti-bacterial, anti-viral, or anti-fungal agents) use within 90 days prior to Visit 0. 16. Evidence of infection, including: 1. Human immunodeficiency virus (HIV) 2. Current or prior infection with hepatitis B (HBV), as indicated by positive HBsAg or positive HBcAb 3. Current or prior hepatitis C (HCV), unless treated with anti-viral therapy with achievement of a sustained virologic response (undetectable viral load 12 weeks after cessation of therapy) 4. Positive Quantiferon-TB Gold or Quantiferon-TB Gold Plus test. PPD or T-SPOT.TB test may be substituted for Quantiferon-TB Gold or Quantiferon-TB Gold Plus test 17. Any of the following laboratory abnormalities: 1. White blood count (WBC) \< 3.5 x 10\^3/μL 2. Hemoglobin \< 10 g/dL 3. Platelets (Plt) \< 125,000/mm\^3 4. Alanine aminotransferase (ALT) ≥ 2x ULN 5. Aspartate aminotransferase (AST) ≥ 2x ULN 18. Women of child-bearing potential who are unwilling to use a medically acceptable form of contraception or be sexually inactive by abstinence until study Week 48 (Protocol Section 7.4). Contraception or abstinence is required for 2 weeks prior to Visit 0. 19. Women who are pregnant or lactating. 20. Vaccination with a live attenuated vaccine within 30 days prior to Visit 0. 21. Known drug allergy or reaction to any component of AMG 714 (Protocol Section 6.1.1) or proteins derived from mammalian cell lines. 22. Past or current medical problems or findings from physical examination or laboratory testing, which, in the opinion of the investigator, may pose additional risks from participation in the study, may interfere with the participant's ability to comply with study requirements or that may impact the quality or interpretation of the data obtained from the study. 23. Current, diagnosed mental illness (e.g. severe depression) or current, diagnosed or self-reported drug or alcohol abuse that, in the opinion of the investigator, would interfere with the participant's ability to comply with study requirements.