Cellectar Biosciences Enrolls First Patient in Clr 125 Auger-Emitting Radioconjugate Phase 1B Clinical Trial Targeting Refractory Triple Negative Breast Cancer

Published: 2026-04-14 15:21:43 ET CLRB

Published on 04/14/2026 at 03:21 pm EDT

Cellectar Biosciences, Inc. announced that the first patient has been enrolled in the Phase 1b trial of CLR 121125 (CLR 125) for the potential treatment of triple negative breast cancer (TNBC). CLR 125 is Cellectar?s proprietary Auger-emitting radioconjugate incorporating iodine-125 to achieve intracellular delivery and direct DNA-level damage in tumor cells. The molecular structure of CLR 125 is identical to that of iopofosine I 131 (CLR 131) and the demonstrated clinical activity, safety, and tumor-targeting characteristics of iopofosine I 131 provide important validation of the platform and support translational relevance.

However, these radioconjugates differ in their radiobiologic behavior at the tumor level, resulting in distinct mechanisms of action and therapeutic profiles. In preclinical studies, CLR 125 showed selective tumor uptake and statistically significant activity in vivo models of TNBC with no observed end-organ or hematologic toxicity at evaluated doses. The Phase 1b clinical trial is an open-label, dose-escalation study in patients with relapsed or refractory TNBC, designed to evaluate three dose levels and dosing regimens of CLR 125 (32.75 mCi administered over 4 cycles, 62.5 mCi over 3 cycles, and 95 mCi over 2 cycles), with approximately 15 patients enrolled per treatment arm.

The study incorporates imaging-based assessments to characterize tumor uptake and biodistribution, supporting prediction of safety and therapeutic activity. Clinical endpoints include safety and tolerability, as well as preliminary efficacy measures, including tumor response per RECIST criteria and progression-free survival.