Background/Purpose: To assess immunogenicity and safety in patients with active rheumatoid arthritis (RA) transitioning from US-Rituximab (RP) or EU-Rituximab (RMP) to DRL_RI (proposed rituximab biosimilar), in comparison to those continuing RP/RMP.

Methods: This double-blind, Phase 3 study included 140 RA patients having prior exposure to RP/RMP; transitioned to DRL_RI (n=70) or continued with RP/RMP (n=70) for two 1000 mg infusions on Days 1 and 15. Anti-drug antibodies (ADA), neutralizing antibodies (NAb), and ADA titre were assessed over a 12-week period, with a safety follow-up till 26 weeks.

Results: Incidence of ADA after dosing was low in both groups: 1.4% in DRL_RI group on Day 15, Week 8, and Week 12; and 2.9% in RP/RMP group at Week 12. Only 1 patient in DRL_RI group was positive for NAb at Week 8. ADA titre values did not significantly differ between the two groups. The time-matched rituximab concentration was comparable between groups, indicating no interference for immunogenicity assessment.

Treatment-emergent adverse events (TEAEs) were reported by 34.3% and 38.6% patients, respectively, in DRL_RI and RP/RMP groups. Incidences of TEAEs that were drug-related, leading to treatment discontinuation, grade ≥ 3, or serious, were also comparable.

Conclusion: Immunogenicity was low and comparable in RA patients transitioning to DRL_RI or continuing RP/RMP. The overall safety profile in patients transitioning to DRL_RI did not differ in frequency, severity from patients continuing RP/RMP and was consistent with the known safety profile of rituximab. ClinicalTrials.gov identifier: NCT0426877; EudraCT: 2019‐002810‐37

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/comparing-immunogenicity-and-safety-following-transition-from-reference-rituximab-to-biosimilar-rituximab-drl_ri-in-patients-with-rheumatoid-arthritis-a-randomized-double-blind-phase-3-study/