Open Label Two-Arm Study to Evaluate Rilzabrutinib in IgG4-Related Disease Participants

Key Inclusion Criteria: * Be male or female with age ≥18 years. * Have a clinical diagnosis of IgG4-RD. * Be willing to taper off an equivalent prednisone dose of between 20-40 mg/day in 2 weeks. Key Exclusion Criteria: * Currently or within 6 months of screening taking rituximab, other B-cell depleting agents, or alkylating agents unless B cell concentrations have been demonstrated by flow cytometry to return to normal values (defined as 5 cells per cubic mm). * History of solid organ transplant * Positive at Screening for HIV, hepatitis B, hepatitis C, or TB * Female patients who are pregnant or nursing. * NOTE: Other Inclusion/Exclusion criteria may apply.