The purpose of the study is to evaluate the safety and efficacy of BCMAxCD3 T-cell engager (cizutamig) in patients with refractory seropositive RA.

B cells mature into plasmablasts and plasma cells that are prolific antibody producers and the predominant source of pathogenic autoantibodies, a hallmark of RA. Autoantibodies contribute to the pathogenesis of RA in several ways, including formation of immune complexes, activation of complement and downstream cell lysis. Clinical trials of cizutamig (BCMAxCD3 T-cell engager) demonstrated safety and efficacy in RRMM. Cizutamig offers a promising mechanism of action for refractory seropositive RA. This study aims to assess the safety, tolerability, PK, pharmacodynamics, immunogenicity, and preliminary clinical activity of cizutamig administered in patients with refractory seropositive RA. Patients will be invited to participate in the study, to receive cizutamig and monitored after dosing with cizutamig through Week 52.

Inclusion Criteria:

1. 18 to 75 years old at the time of signing the informed consent form
2. Diagnosis of adult-onset RA as defined by the 2010 ACR/EULAR classification criteria
3. Moderately to severely active RA.
4. Positive test results for RF and/or ACPA at Screening.

Inadequate treatment response defined as either lack of clinical benefit or intolerability to treatment with tsDMARD and/or bDMARD Exclusion Criteria:

1. Inadequate clinical laboratory parameters at Screening
2. Patients with active infection
3. Receipt of live vaccine within 4 weeks prior to Screening
4. Presence of any concomitant autoimmune disease
5. History of progressive multifocal leukoencephalopathy
6. History of primary immunodeficiency or a hereditary deficiency of the complement system
7. Central nervous system disease
8. Presence of 1 or more significant concurrent medical conditions per investigator judgment
9. Have a diagnosis or history of malignant disease within 5 years
10. Serious mental illness, alcohol or drug abuse, dementia, or any other condition that would impair the patient's ability to receive the planned treatment or to understand informed consent at the study site as determined by local practice