AnaptysBio shares soar 37% after rheumatoid arthritis candidate rosnilimab delivers promising Phase 2b trial results

AnaptysBio, Inc. experienced a significant boost in its market performance as shares surged 37% in premarket trading following the announcement of promising results from its Phase 2b trial of rosnilimab, an investigational therapy for moderate-to-severe rheumatoid arthritis. The company’s stock rose to approximately $17, although it remains down by about 44% over the past year.

The positive clinical data from the trial mark a potential breakthrough in the rheumatoid arthritis treatment landscape, positioning rosnilimab as a promising candidate in a market that has seen limited innovation in recent years.

What makes AnaptysBio’s rheumatoid arthritis trial results significant?

The clinical trial results come from the Phase 2b RENOIR study, which evaluated the efficacy and safety of rosnilimab in 424 patients with moderate-to-severe rheumatoid arthritis. These patients were already undergoing treatment with conventional disease-modifying antirheumatic drugs (cDMARDs) like methotrexate. The trial spanned multiple regions, including the United States, Canada, and Europe, and included both biologic/targeted synthetic DMARD (b/tsDMARD)-naïve and -experienced patients.

Rosnilimab achieved the trial’s primary endpoint, demonstrating a statistically significant reduction in Disease Activity Score 28 (DAS28-CRP) at Week 12 across all three dosage groups compared to placebo. This reduction reflects an improvement in joint swelling, tenderness, and inflammation markers—key indicators of disease activity in rheumatoid arthritis.

Beyond the primary endpoint, the drug met key secondary endpoints, including the American College of Rheumatology 20%, 50%, and 70% improvement criteria (ACR20, ACR50, ACR70) and the Clinical Disease Activity Index Low Disease Activity (CDAI LDA). At Week 14, 69% of rosnilimab-treated patients achieved CDAI LDA, with sustained responses observed up to Week 28. This represents the highest-ever reported CDAI LDA response rate in rheumatoid arthritis trials, highlighting the drug’s potential to deliver durable benefits to patients.

How does rosnilimab work in treating rheumatoid arthritis?

Rosnilimab is designed to target PD-1, a co-inhibitory receptor expressed on activated T cells that plays a role in immune regulation. By depleting PD-1high T cells and activating the remaining PD-1+ T cells, rosnilimab aims to restore immune balance, reducing the inflammatory processes that drive rheumatoid arthritis treatment resistance in some patients.

The robust pharmacological activity observed in the trial is a key differentiator for rosnilimab. Patients receiving the drug experienced a ~90% reduction in PD-1high T cells, a ~50% reduction in PD-1+ T cells, and an increase in regulatory T cells (Tregs). These changes contribute to a healthier immune profile without significantly impacting total T cell counts, suggesting that rosnilimab may provide effective disease control while maintaining immune function.

Additionally, patients treated with rosnilimab showed a ~50% reduction in C-reactive protein (CRP) levels, an objective marker of systemic inflammation, which was not observed in the placebo group. This reduction in CRP underscores the drug’s potential to manage both the symptoms and underlying inflammation associated with rheumatoid arthritis.

What do experts say about the trial results?

Experts in the field have highlighted the importance of these findings, given the limited progress in rheumatoid arthritis therapeutics over the past decade. Jonathan Graf, M.D., a professor of medicine at the University of California, San Francisco, emphasized the novelty of rosnilimab’s mechanism:

“Patients with rheumatoid arthritis often have an abnormal population of PD-1high T cells. The data support the hypothesis that targeting these cells can offer more durable symptom relief and possibly modify the course of the disease,” he noted.

Paul Emery, M.D., Versus Arthritis Professor of Rheumatology at the University of Leeds, echoed this optimism, stating:

“Despite existing treatments, many patients remain difficult to manage effectively. Rosnilimab’s efficacy, coupled with its favorable safety profile, represents a significant step forward in expanding treatment options.”

AnaptysBio’s Chief Medical Officer, Paul Lizzul, M.D., Ph.D., added that the clinical trial results validate the company’s strategy of targeting the PD-1 pathway, not only for rheumatoid arthritis but also for other autoimmune diseases like ulcerative colitis, where the drug is currently under investigation.

Is rosnilimab safe for long-term use?

Safety remains a critical consideration in any new therapy, especially for chronic conditions like rheumatoid arthritis that require long-term treatment. The Phase 2b trial data revealed that rosnilimab was well-tolerated across all dosage groups, with adverse event rates comparable to placebo.

No new safety concerns emerged during the study. There were no reports of malignancies, major adverse cardiovascular events (MACE), serious infections, or systemic hypersensitivity reactions linked to the drug. Common side effects, such as mild headaches and upper respiratory tract infections, occurred at rates similar to those in the placebo group.

The consistency of the safety profile through Week 28 suggests that rosnilimab may be suitable for extended use, a key advantage in the management of chronic autoimmune conditions.

What’s next for rosnilimab and AnaptysBio?

AnaptysBio plans to release full six-month data from the RENOIR trial in the second quarter of 2025, including additional insights into the drug’s long-term efficacy and safety. The company also expects top-line results from its Phase 2 ulcerative colitis trial in the fourth quarter of 2025, potentially expanding rosnilimab’s therapeutic applications.

With a promising combination of efficacy, robust pharmacological activity, and a favorable safety profile, rosnilimab could emerge as a transformative therapy in the $20 billion U.S. rheumatoid arthritis market. The drug’s novel mechanism of action, targeting immune dysregulation at its core, may offer new hope for patients who have not responded adequately to existing treatments.