The purpose of this study is to determine the safety and tolerability of plamotamab in patients with rheumatoid arthritis. Participants will be given XmAb13676 subcutaneously (SC) by injection under the skin.

This is a Phase 1b study to determine the safety, tolerability, pharmacokinetic (PK), and pharmacodynamics (PD), and immunogenicity of plamotamab in adult participants with moderately to severely active RA. This is an open label dose optimization trial with approximately 68 participants assigned to different dose escalation cohorts.

Inclusion Criteria: Adult participants with moderately to severely active RA.

• Documented diagnosis of RA and meeting the 2010 American College of Rheumatology/European Alliance of Associations for Rheumatology classification criteria for RA at least 3 months prior to screening
• Inadequate response to, loss of response to, or intolerance to available RA therapies.
• Stable doses of RA medications prior to screening
• 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.