1353: Phase 3 Study of the Efficacy, Safety and Immunogenicity of the Proposed Tocilizumab Biosimilar RGB-19, Intravenously Administered to Participants With Active Rheumatoid Arthritis: 52-Week Data

Background/Purpose: RGB-19 is a proposed biosimilar to tocilizumab, a monoclonal antibody that competitively inhibits the binding of interleukin-6 (IL-6) to its receptor. Results of a Phase 3 study (jRCT2031220512) assessing the clinical equivalence in efficacy between RGB-19 and reference tocilizumab at Week 12, and comparing secondary efficacy, safety and immunogenicity outcomes up to Week 24, have been previously reported.¹ Here, secondary outcomes up to Week 52 (safety to Week 54) are reported.

Methods: This Phase 3, randomized, double-blind study enrolled Japanese adults with active rheumatoid arthritis and an inadequate response to methotrexate, a baseline Disease Activity Score based on 28 joints with erythrocyte sedimentation rate (DAS28-ESR) of ≥3.2, and swollen/tender joints (≥6 from 66/68). Participants were randomized 1:1 to intravenous infusions of either RGB-19 8 mg/kg or tocilizumab 8 mg/kg, administered every 4 weeks to Week 52, and participants were followed to Week 54. The primary endpoint was the mean change from baseline (CfB) in DAS28-ESR at Week 12 (previously reported).¹ Key secondary outcomes included efficacy, serum concentration, pharmacodynamics (PD; absolute neutrophil count [ANC], C-reactive protein [CRP], and soluble IL-6 receptor [sIL-6R]), and immunogenicity up to Week 52, and safety up to Week 54.

Results: Overall, 368 participants were randomized. A total of 335 participants completed the secondary evaluation period from Week 12 to Week 52 (RGB-19 n=169; tocilizumab n=166). The most common reason for withdrawal from the secondary evaluation period was adverse events (AEs; RGB-19 n=3; tocilizumab n=8). Baseline demographics were balanced between groups, as previously reported.¹

The CfB in DAS28-ESR at Week 52 was similar between groups (confidence intervals within the primary endpoint equivalence margin of ±0.6; Table 1). Achievement/disease remission rates (American College of Rheumatology [ACR] 20/50/70, Simplified Disease Activity Index [SDAI], Clinical Disease Activity Index [CDAI], and European League Against Rheumatism [EULAR]) were also similar up to Week 52 (Table 1). Mean serum concentration time profiles and ANC, CRP, and sIL-6R PD outcomes were all similar between groups up to Week 52 (Figure).

Safety profiles were similar between groups, with comparable incidences of treatment-emergent AEs, and adverse drug reactions up to Week 54 (Table 2). The incidence of treatment-emergent antidrug antibodies (RGB-19 2.7%, tocilizumab 5.4%) and neutralizing antibodies (RGB-19 2.7%, tocilizumab 4.3%) was similarly low in both groups.

Conclusion: Similar efficacy, serum drug concentration, PD, safety and immunogenicity outcomes were shown for RGB-19 and tocilizumab up to the end of this Phase 3 study. These results support previously reported data that demonstrated the clinical equivalence in efficacy at Week 12 and similarity in secondary outcomes between treatments at Week 24.¹

Reference
1. Choy E, et al. European Alliance of Associations for Rheumatology Congress 2025. Abstract 1398.