Rigel Pharmaceuticals, Inc. (NASDAQ: RIGL), a US-based commercial stage biotechnology company, on Wednesday announced the enrolment of the first patient in the dose expansion phase of its ongoing Phase 1b study of R289 in relapsed or refractory lower-risk myelodysplastic syndrome (MDS).

R289 is a selective dual inhibitor of interleukin receptor-associated kinases 1 and 4 (IRAK1/4). The study aims to evaluate its safety, tolerability, pharmacokinetics and preliminary activity in patients with transfusion dependent lower-risk MDS.

In the dose expansion phase, up to 40 patients will be randomised to receive either 500 mg once daily or 500 mg twice daily, in order to determine the recommended Phase 2 dose for future clinical development.

Rigel completed enrolment in the dose escalation phase in July 2025 and expects to share updated data later this year. An exploratory cohort of erythropoiesis-stimulating agent relapsed or refractory, or ineligible, lower-risk MDS patients will also be evaluated once the Phase 2 dose is established.

R289 has previously received Orphan Drug designation for myelodysplastic syndromes and Fast Track designation from the US Food and Drug Administration for transfusion dependent lower-risk MDS.