The study has two parts. In Part A, single increasing doses of LY3451838 will be administered intravenously (into a vein). In Part B, a single dose of LY3451838 will be administered subcutaneously (just under the skin).

Inclusion Criteria:

• Male participants must adhere to contraception restrictions

• Female participants must be of non-childbearing potential due to:

  • Menopause: spontaneous amenorrhea for at least 12 months not induced by a medical condition such as anorexia nervosa and not taking medications that induced the amenorrhea (e.g., oral contraceptives, hormones, gonadotropin releasing hormone, anti-estrogens, selective estrogen receptor modulators, or chemotherapy)
  • Surgical sterilization

• Have a body mass index of 18 to 35 kilograms per square meter (kg/m²)

• Have clinical laboratory test results within normal reference range or with acceptable deviations

• Have an estimated glomerular filtration rate greater than or equal to (≥) 60 milliliters per minute per 1.73 meters squared (mL/minute/1.73 m²) of body surface area

• Have venous access sufficient to allow for blood sampling

Exclusion Criteria:

• Are currently enrolled in or discontinued from a clinical trial within the last 30 days, or have previously completed or withdrawn from this study
• Have a history or presence of medical illness including, but not limited to, any cardiovascular, hepatic, respiratory, hematological, renal, endocrine, psychiatric or neurological disease, or any clinically significant laboratory abnormality
• Have history of or presence of uncontrolled asthma, significant atopy, or significant rheumatological or autoimmune diseases
• Have had lymphoma, leukemia, or any malignancy within the past 5 years, or breast cancer within the past 10 years (with some exceptions)
• Have used, or intend to use some prescription or over the counter medications, including herbal medications within 14 days prior to dosing
• Have an abnormality in the 12-lead electrocardiogram (ECG) or Fridericia's corrected QT (QTcF)
• Show evidence of human immunodeficiency virus (HIV) and/or positive human HIV antibodies, hepatitis C and/or positive hepatitis C antibody, or hepatitis B and/or positive hepatitis B surface antigen
• Have donated blood of more than 450 milliliters (mL) within the last 3 months
• Are unwilling to stop alcohol consumption while resident in the Clinical Research Unit (CRU)
• Have an average weekly alcohol intake that exceeds 21 units per week for males and 14 units per week for females (1 unit = 12 ounces (oz) or 360 mL of beer; 5 oz or 150 mL of wine; 1.5 oz or 45 mL of distilled spirits)
• Current smoker of more than 10 cigarettes or equivalent per day and unable to stop smoking while in the CRU
• Have an abnormal blood pressure
• Have clinically significant proteinuria or hematuria
• Positive findings for known drugs of abuse
• Have received treatment with biologic agents within 3 months or 5 half-lives (whichever is longer)
• Have clinically significant allergies, or intolerance to corticosteroids, or severe post treatment hypersensitivity reactions

All randomized participants.

Baseline up to 20 Weeks

All participants who received at least one dose of study drug.