ATI-450 Plus Methotrexate (MTX) Versus Placebo Plus MTX in Participants With Moderate to Severe Active Rheumatoid Arthritis (RA)

Inclusion Criteria: * Able to comprehend and be willing to sign the Institutional Review Board (IRB)/Independent Ethics Committee (IEC) approved participant ICF prior to administration of any study-related procedures. * Diagnosis of adult-onset RA as defined by the 2010 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) classification criteria. * Have active moderate to severe RA at Screening. * A minimum of 12 weeks on MTX with a stable MTX dose. Exclusion Criteria: * Current acute or chronic immunoinflammatory disease other than RA which may impact the course or assessment of RA. * Uncontrolled non-immunoinflammatory disease that may place the participant at increased risk during the study or impact the interpretation of results (eg, previous malignancy, recurrent infection, previous venous thromboembolism). * Participant has experience with \> 2 biologics, \> 1 JAK inhibitor, or a combination of 1 biologic experience and 1 JAK inhibitor. * Currently receiving corticosteroids at doses \> 10 mg/day of prednisone (or equivalent) or have been receiving an unstable dosing regimen of corticosteroids within 2 weeks of screening.