AnaptysBio stock whipsawed Wednesday despite promising results for its experimental rheumatoid arthritis treatment and an accelerated timeline for its ulcerative colitis data.
The biotech company is testing a drug called rosnilimab in patients with rheumatoid arthritis and ulcerative colitis. Both are inflammatory conditions.
In the study called Renoir, patients showed statistically significant improvements across numerous measures. Importantly, after 12 weeks of treatment, 36% to 47% of patients showed at least a 50% improvement in symptoms of rheumatoid arthritis. Nearly seven in 10 patients — 69% — reached a low-disease activity score on another measure after 14 weeks.
These surpass historical benchmarks, Wedbush analyst Martin Fan said in a client report. He also noted the company is now planning to have the results of a study of rosnilimab in patients with ulcerative colitis in the fourth quarter of 2025 vs. earlier forecasts for the first quarter of 2026.
But AnaptysBio stock, which had rocketed near 30% in premarket trades, later dipped into the red before jumping back into the green with a 2.6% gain at 12.70.
AnaptysBio Mets Primary Goal
The primary goal of the study was to show low levels of C-reactive protein after 12 weeks. C-reactive protein in the blood indicates there's inflammation somewhere in the body. Patients who received rosnilimab showed statistically significant improvement in C-reactive protein levels vs. the placebo group.
After 14 weeks, patients showed the best-ever improvements on scales called ACR20, ACR50 and ACR70. This means their symptoms improved by 20%, 50% and 70%. In the first group, 79% to 82% of overall patients showed improvement. More than half — 59% to 67% — reached at least a 50% symptom improvement. And 41% to 48% of patients had at least a 70% improvement in symptoms.
Notably, 68% to 71% of patients reached a low-disease activity score on another scale measuring symptoms.
Maintenance Treatment Results
Up next, AnaptysBio is testing rosnilimab over 28 weeks as a maintenance treatment. Results are expected in the second quarter.
Wedbush's Fan noted more than 60% of patients with rheumatoid arthritis stop responding to their treatments.
"We see substantial upside potential should rosnilimab prove durable, validate deepening of response and provide further positive read-through to rosnilimab development in ulcerative colitis," he said.
He has an outperform rating and 40 price target on AnaptysBio shares.
Follow Allison Gatlin on X/Twitter at @AGatlin_IBD.
