A Phase 1 Open-label Study to Evaluate Safety in Healthy Participants and Participants With Idiopathic Inflammatory Myopathy (IIM)

The purpose of this study is to determine the safety and tolerability of XmAb657 in healthy participants and participants with IIM. Participants will be given XmAb657 subcutaneously (SC) by injection under the skin.

This is a Phase 1 study to determine the safety, tolerability, pharmacokinetic (PK), and pharmacodynamics (PD), and immunogenicity of XmAb657 in healthy adult participants and participants with IIM.

Inclusion Criteria: Healthy participants - Adult participants in good health Idiopathic inflammatory myopathy participants - Adult participants that meet the 2017 European Alliance of Association Rheumatology (EULAR)/American College of Rheumatology (ACR) classification of Idiopathic inflammatory myopathy Idiopathic inflammatory myopathy participants - Stable doses of Idiopathic inflammatory myopathy medications prior to screening All participants - Use of highly effective methods of contraception Exclusion Criteria: * Major surgery within 12 weeks prior to screening or planned within 12 months after dosing * Recurrent infections or active clinically significant infection * Active or untreated latent tuberculosis * Cancer or history of cancer or lymphoproliferative disease within the previous 5 years * Uncontrolled cardiovascular, pulmonary, renal, hepatic, endocrine, or gastrointestinal disease Note: Additional, more specific inclusion/exclusion criteria are defined in the protocol.