Regeneron quietly takes SF3B1-mutant therapy into first-in-human trial

Regeneron Pharmaceuticals (Nasdaq: REGN) is preparing to take a previously undisclosed therapy targeting SF3B1-mutant blood cells into first-in-human testing, opening a new approach to treating molecularly defined clonal hematopoietic disorders for which no mutation-targeted therapies are currently approved.

The Phase I study (NCT07753148) will evaluate REGN17235 in adults with either clonal cytopenia of undetermined significance (CCUS) or low-risk myelodysplastic syndrome (LR-MDS) carrying an SF3B1 mutation. The open-label, multicenter trial will enroll an estimated 52 participants across dose-escalation and safety-expansion cohorts, with recruitment expected to begin in September 2026.

The US trial registration follows an earlier European filing for the same study under protocol R17235-HM-24125 (EUCT 2026-525464-18-00). Belgium authorized the Phase I trial on June 22, 2026, with recruitment still pending and an estimated European start date of September 17. The European registry lists an initial 15 participants in Belgium, while the newer global ClinicalTrials.gov record gives a total planned enrollment of 52. citeturn0search0turn0search5

Despite reaching the clinic, little is publicly known about REGN17235. Regeneron has not disclosed its modality, precise molecular target or preclinical profile, and the program does not appear to have been formally introduced through a company pipeline announcement or scientific publication. Public information identified to date is largely confined to clinical trial registries, with the European trial record appearing by June and the US registration following in August.

One clue comes from the study itself. The registry uses the keyword "Selective Elimination Of SF3B1-Mutant Cells," suggesting REGN17235 is intended to preferentially remove clones carrying the SF3B1 mutation rather than simply treating the downstream manifestations of MDS. Regeneron has not confirmed how that selectivity is achieved.

Secondary endpoints include hematologic improvement under International Working Group 2018 criteria, pharmacokinetics and the development of anti-drug antibodies. The inclusion of immunogenicity testing is consistent with REGN17235 being a biologic, although the trial registration does not establish its modality.

SF3B1 encodes a core component of the RNA-splicing machinery and is among the most frequently mutated genes in MDS, particularly in disease characterized by ring sideroblasts. Mutant clones alter RNA splicing across multiple transcripts, creating a genetically distinct disease biology that could potentially be exploited therapeutically. CCUS represents an earlier clonal state in which patients have persistent cytopenias and somatic mutations associated with myeloid malignancy but do not yet meet the morphologic criteria for MDS.

Including both CCUS and LR-MDS makes the program particularly notable. Rather than restricting development to established malignancy, Regeneron appears to be testing whether selective elimination of an SF3B1-mutant clone could intervene earlier in the evolution of myeloid disease. Whether REGN17235 can actually reduce or eradicate mutant clones — and whether doing so translates into hematologic benefit or alters progression toward more advanced disease — remains unknown.

The study is expected to run through May 2031, reflecting the relatively indolent natural history of the populations being enrolled. With its mechanism and modality still undisclosed, the start of dosing and any subsequent Regeneron disclosure will provide the first clearer indication of how the company intends to selectively target SF3B1-mutant hematopoiesis.

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