Framing litifilimab's Phase 3 data in systemic lupus erythematosus expected in 4Q

BIIB is expected to report Phase 3 TOPAZ 1/2 data in 4Q (primary completion estimated for late September) via two studies, evaluating the BDCA2-targeting antibody litifilimab in systemic lupus erythematosus (SLE), a multi-system autoimmune disease affecting ~235K patients in the US (~3mn globally). Recall, SLE has historically been a challenging indication due to heterogeneous disease manifestations, fluctuating disease activity, background therapy use, and the stringent SLE Responder Index-4 (SRI-4) endpoint, which may not fully capture partial clinical improvements. In our view, the key question is whether litifilimab's supportive Phase 2 activity in skin and joint manifestations, combined with BIIB's patient enrichment and trial execution strategy (including stable background therapy and steroid tapering), can translate into a clinically meaningful SRI-4 benefit (15-25%+ placebo-adjusted response per AZN's Saphnelo and Roche's Gazyva, albeit GSK's Benlysta was approved on ~10-14%).