This study evaluates ATI-450 plus MTX versus placebo plus MTX in participants with moderate to severe active RA who have had an inadequate response to MTX alone.

This is a Phase 2b, randomized, multicenter, double-blind, parallel group, placebo controlled, dose ranging study to investigate the efficacy, safety, tolerability, pharmacokinetics, and pharmacodynamics of multiple doses of ATI-450 plus MTX versus placebo plus MTX in participants with moderate to severe active RA who have had an inadequate response to MTX alone.

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.