Cellectar publishes Phase 1 iopofosine I 131 data in multiple myeloma

Cellectar Biosciences has announced the publication of Phase 1 dose-escalation data for iopofosine I 131 in relapsed or refractory multiple myeloma, with results appearing in the peer-reviewed journal Cancers. The open-label, multi-centre study enrolled 31 patients who had received a median of four prior lines of therapy, and the data suggest both clinical activity and a tolerability profile that the company says could suit older or more fragile patients.

Among 26 efficacy-evaluable patients, 84.6% achieved stable disease or better. An overall response rate of 30% was recorded in the subset of ten patients who received at least 60 mCi of the agent; across the full evaluable population the overall response rate was 15.4%, with four patients achieving partial responses. Adverse events were primarily haematologic, consisting of predictable and reversible cytopenias. The authors reported no new safety signals and described non-haematologic toxicities as generally low grade.

Dosing and mechanism

The study tested both single-dose and fractionated-dose schedules. Investigators established 31.25 mCi/m² as the maximum tolerated single dose and identified a fractionated regimen of 20 mCi/m² administered twice, one week apart, as the highest evaluated fractionated option. The authors' recommended Phase 2 starting point is 15 mCi/m² on days 1 and 7 combined with weekly low-dose dexamethasone.

Iopofosine I 131 uses Cellectar's Phospholipid Drug Conjugate platform to deliver iodine-131 directly to cancer cells. Sikander Ailawadhi, professor of medicine at Mayo Clinic Jacksonville and lead author of the paper, noted that the predictability and recovery of the cytopenias "support its potential for repeat dosing strategies in future clinical development." Jarrod Longcor, Cellectar's chief operating officer and a co-author, highlighted that the mechanism of action is not dependent on a single target or mutation, which the company says opens a path across a range of B-cell malignancies including Waldenström macroglobulinemia, diffuse large B-cell lymphoma and central nervous system lymphoma.

Competitive and regulatory context

Targeted radiopharmaceuticals have attracted growing commercial and regulatory attention following the FDA approval of lutetium-177 DOTATATE (Lutathera) in neuroendocrine tumours and lutetium-177 PSMA-617 (Pluvicto) in prostate cancer. Several large pharmaceutical groups have made sizeable acquisitions in the space, intensifying competition for novel radiopharmaceutical assets. Iopofosine I 131 enters this landscape as a phospholipid-conjugated iodine-131 agent, a chemistry distinct from the PSMA-targeting lutetium agents that have so far defined the commercial wave.

The FDA has granted iopofosine I 131 Breakthrough Therapy designation for Waldenström macroglobulinemia, alongside six Orphan Drug, four Rare Pediatric Drug and two Fast Track designations across various indications. The EMA has granted PRIME designation. Cellectar is also running a Phase 1b study in paediatric high-grade gliomas under the CLOVER-2 protocol, where an FDA approval would make it eligible for a Rare Pediatric Disease priority review voucher. A Phase 2b readout in Waldenström macroglobulinemia would represent the next major catalyst for the programme. Investors will be watching that dataset closely, as a single-indication approval would materially de-risk the broader B-cell platform thesis the company is promoting.