This study aims to examine the efficacy and safety of obexelimab for the prevention of flare of IgG4-related disease (IgG4-RD)

This study consists of a 1-year randomized control period (RCP), followed by an additional 3-year open label extension (OLE) period.

To enter the Screening Period (Day -28 to Day -1), patients must have active IgG4-RD signs/symptoms (i.e., flare) that require steroid therapy. On Day 1, patients will be randomized in a ratio of 1:1 to receive either obexelimab or placebo administered as subcutaneous (SC) injections. All patients will begin a steroid taper on Day 1 to discontinuation by Week 8.

Patients will be monitored for flares during scheduled in clinic visits and any unscheduled visits. The primary endpoint is time to first IgG4-RD flare that requires initiation of rescue therapy in the opinion of the investigator and the Adjudication Committee (AC).

Following the 52-week RCP, patients will have the opportunity to continue in an OLE period where all patients will receive obexelimab. During the OLE, a subset of patients may be eligible to participate in a sub-study evaluating the immune response to vaccination while receiving obexelimab treatment. Patients who do not wish to enroll into the OLE period will return for an in-clinic safety follow-up visit 8 weeks after the Week 52 visit (i.e., Week 60).

Including screening and follow-up, the maximum duration of participation in this study for an individual patient is 224 weeks (i.e., 28-day screening, 52-week RCP, 156-week OLE, and a 12-week follow-up).

Inclusion Criteria:

1. Males and females, ≥ 18 years of age
2. Clinical diagnosis of IgG4-RD
3. Patients must meet the 2019 ACR/EULAR Classification Criteria for IgG4-RD
4. Patients must have active IgG4-RD signs/symptoms (i.e., flare) that require the initiation of GC therapy or the increase in background long-term GC therapy
5. Other inclusion criteria apply

Exclusion Criteria:

1. Has disease in only 1 organ system whose primary manifestation is fibrosis
2. Has received prednisone equivalent given orally at a dose greater than 60 mg/day within the 4 weeks prior to screening or during screening
3. Has received a non-biologic, disease-modifying anti-rheumatological drug or immunosuppressive agent other than GCs within the 2 weeks prior to screening
4. Has received an investigational treatment or direct medical intervention on another clinical study within 12 weeks or < 5 half-lives of the investigational treatment, whichever is shorter, prior to screening
5. Has received live vaccine or live therapeutic infectious agent within the 2 weeks prior to screening
6. Active tuberculosis or high risk for tuberculosis; hepatitis C infection in absence of curative treatment; evidence of hepatitis B infection
7. Use of B cell depleting or targeting agents within 6 months of randomization
8. Other exclusion criteria apply