Rigel Pharmaceuticals Shares Plunge 11.28% on Mixed Phase 1b Trial Results for R289

Rigel Pharmaceuticals shares plunged 11.28% in pre-market trading on December 9, 2025, amid mixed results from its Phase 1b trial of R289 for lower-risk myelodysplastic syndrome (MDS). The dual IRAK1/4 inhibitor demonstrated 33% red blood cell transfusion independence (RBC-TI) in higher-dose cohorts but reported Grade 3/4 adverse events including anemia and elevated liver enzymes.

The stock decline reflects investor caution as the company faces hurdles in translating early-stage efficacy into broader clinical adoption. While R289’s safety profile appears manageable, long-term safety and effectiveness remain unproven at larger patient scales.

Rigel plans to finalize dose expansion by mid-2026 but has yet to establish a clear path to commercialization in a competitive hematologic disorder market.

Market participants are weighing R289’s potential against risks such as regulatory delays and late-stage setbacks. The therapy’s ability to differentiate from existing treatments and maintain positive trends in Phase 2 trials will be critical for investor sentiment. Until further data confirms its therapeutic value, Rigel’s stock remains vulnerable to volatility tied to clinical outcomes and funding uncertainties.

As the stock continues to be influenced by clinical progress, long-term investors are advised to monitor upcoming trial milestones and potential regulatory updates. These factors will likely serve as key indicators for whether Rigel can regain investor confidence or face prolonged uncertainty in the near term.