Rigel Pharmaceuticals Inc. has announced updated data from its ongoing Phase 1b clinical study evaluating R289, an oral prodrug of R835 and a dual IRAK1/4 inhibitor, in patients with relapsed or refractory lower-risk myelodysplastic syndrome (MDS). The data were presented by Dr. Guillermo Garcia-Manero during an oral session at the 67th American Society of Hematology $(ASH)$ Annual Meeting and Exposition held December 6-9, 2025. According to the company, R289 continues to be generally well tolerated. Preliminary efficacy was observed at doses of 500 mg or greater once daily, with 33% of evaluable transfusion dependent patients achieving red blood cell transfusion independence (RBC-TI). The ongoing study is assessing safety, tolerability, pharmacokinetics, and preliminary efficacy, with the dose expansion phase currently enrolling patients to determine the recommended Phase 2 dose. Rigel anticipates selecting this dose for future clinical studies in the second half of 2026. R289 has received Orphan Drug and Fast Track designations from the U.S. FDA for the treatment of myelodysplastic syndromes.