A Phase 1, Randomized, Double-blind, Placebo-controlled Study Evaluating AMG 691 in Healthy Participants and Participants With Mild-to-Moderate Asthma

Inclusion Part A and B * Participants must be capable of giving informed consent and have provided informed consent. * Participants must be 18 to 65 inclusive at time of signing of informed consent. * Participants must have a Body Mass Index between 18.0 to 30 kg/m2 and total body weight ≥ 40 kg at screening. * Participants must be overtly healthy as determined by the investigator based on medical evaluation and study screening procedures. * Female participants must be of non-childbearing potential. Inclusion Part C * Participants must be capable of giving informed consent and have provided informed consent. * Participants must be 18 to 65 inclusive at time of signing of informed consent. * Participants must have documentation of physician diagnosed asthma for ≥ 12 months prior to screening. * Participants must have documented bronchodilator responsiveness of forced expiratory volume in 1 second (FEV1) ≥ 12% and ≥200 mL in the 10 years before screening or at the screening visit. * Participants must have a pre-bronchodilator percent predicted FEV1 between 50 and 100% inclusive at screening visit and Day -1. * Participants must have peripheral blood eosinophils ≥ 200 cells/μl at screening visit and Day -1 * Participants must have a fractional exhaled nitric oxide (FeNO) ≥ 25 ppb at screening visit and Day -1. * Participant must not use inhaled corticosteroids (ICS) or must be treated with low-dose or medium-dose ICS and on a stable dose for a minimum of 12 weeks prior to screening. Exclusion (applicable to all study parts) * History of malignancy (except for in situ cervical cancer or surgically excised non-melanoma skin cancer occurring more than 5 years prior to randomization). * History of anaphylaxis or hypersensitivity to biologic therapy or sensitivity to mammalian derived products. * History of immunodeficiency or history of severe infection within the last 3 years requiring IV antibiotics. * History of tuberculosis (TB), TB symptoms, or positive interferon gamma release assay. * History of untreated or unresolved helminthic infection within 24 weeks of day 1. * Positive human immunodeficiency virus (HIV) antibodies, hepatitis B core antigen, hepatitis B core antibody, or hepatitis C virus (HCV) ribonucleic acid (RNA). * Male participants unwilling to follow contraceptive requirements. Additional Exclusion for Part C only * Female of childbearing potential not willing to use 2 methods of contraception with one being a highly effective method of contraception. * History of pulmonary disease that may interfere with interpretation of study results. * History of upper respiratory infection within 6 weeks of screening. * Asthma Control Questionnaire (ACQ-6) \> 3 * Asthma symptoms or exacerbations requiring 2 or more systemic corticosteroid bursts (≥10 mg/day prednisone or equivalent for ≥ 3 days each) in the previous 12 months. * More than one hospitalization or emergency department visit in the last year. * History of life-threatening asthma exacerbation requiring admission to intensive care unit.