On April 14, 2026 Cellectar Biosciences, Inc. (NASDAQ: CLRB), a late-stage clinical biopharmaceutical company focused on the discovery and development of drugs for the treatment of cancer, reported 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).
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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.
"Treating the first patient in this Phase 1b trial is a significant milestone for Cellectar and for those impacted by triple negative breast cancer, a condition still defined by a profound lack of targeted therapies," said James Caruso, president and chief executive officer of Cellectar. "CLR 125 embodies our commitment to optimize our proprietary PDC delivery platform to develop highly selective radioconjugates capable of delivering precise cytotoxic radiation while minimizing systemic toxicity. With additional study sites being activated in Q2, we are poised to rapidly advance this program and plan to provide dosimetry, safety, and efficacy updates throughout 2026."
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.
About Triple Negative Breast Cancer
Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by the absence of estrogen receptors, progesterone receptors, and HER2 protein expression. This lack of common therapeutic targets makes TNBC particularly challenging to treat, with limited options beyond chemotherapy. TNBC tends to grow and spread more quickly than other breast cancer types and disproportionately affects younger women and those of African descent. In the U.S., approximately 12% of breast cancer diagnoses are TNBC. Studies suggest that approximately 25% of TNBC cases relapse after standard treatments like surgery, chemotherapy, and radiation. Due to its high recurrence rate and poor prognosis, there is a critical need for innovative, targeted therapies to improve outcomes for patients facing this difficult diagnosis.
(Press release, Cellectar Biosciences, APR 14, 2026, View Source [SID1234664351])