A Study of CLN-978, a Subcutaneously Administered CD19-directed T Cell Engager, in Subjects With Rheumatoid Arthritis

Inclusion Criteria: * Documented diagnosis of RA which fulfills the 2010 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) Classification Criteria ≥12 weeks prior to screening. * Evidence of B-cell-driven disease at Screening or previous medical documentation, defined as either (i) seropositivity for RF and/or any AMPA and/or (ii) a synovial biopsy showing B-cell infiltration * Active RA disease, as demonstrated by a DAS28 - ESR ≥3.2 and at least one swollen joint at screening. * Inadequate response to at least 2 targeted treatments with different mechanisms. These can include tsDMARD and/or bDMARD, after failing conventional synthetic DMARD (unless contraindicated). * Local laboratory assessments must meet the following criteria: * Absolute lymphocyte count (ALC) ≥0.5 x 10\^9/L * Peripheral CD19+ B cell count ≥25 cells/μL * Absolute neutrophil count (ANC) ≥1.0 x 10\^9/L * Hemoglobin (Hgb) ≥8 g/dL * Platelet count ≥75 x 10\^9/L * Estimated glomerular filtration rate (eGFR) (based on the Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] formula) ≥30 mL/min/1.73m\^2 * Total bilirubin ≤1.5 × ULN, except patients with confirmed Gilbert's Syndrome * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 × ULN * Concurrent corticosteroids is permitted if the dose is ≤10 mg/day prednisone equivalent in the 2 weeks prior to initiation of study treatment. Hydroxychloroquine ≤400 mg/d or chloroquine ≤ 250 m…