Cellectar Biosciences, Inc. (NASDAQ: CLRB) began its Phase 1b study of CLR 125, an iodine‑125 Auger‑emitting radioconjugate, by enrolling the first patient on April 14, 2026. The open‑label, dose‑escalation trial will evaluate three dose regimens—32.75 mCi over four cycles, 62.5 mCi over three cycles, and 95 mCi over two cycles—with approximately 15 patients planned per treatment arm.
CLR 125 is built on Cellectar’s phospholipid drug conjugate (PDC) platform, which has previously been applied to iopofosine I 131 (CLR 131). Preclinical studies show selective tumor uptake and activity in triple‑negative breast cancer (TNBC) models, with no observed end‑organ or hematologic toxicity at the tested doses. The trial will assess tumor‑specific uptake, safety, tolerability, and preliminary efficacy to determine the recommended Phase 2 dose, thereby extending platform validation beyond hematologic malignancies into a high‑need solid‑tumor indication.
James Caruso, Cellectar’s president and CEO, noted that treating the first patient in this Phase 1b trial is a significant milestone for the company and for patients with TNBC, a disease that still lacks targeted therapies. He added that CLR 125 embodies the company’s commitment to optimizing the PDC platform for highly selective radioconjugates that deliver precise cytotoxic radiation while minimizing systemic toxicity. Additional study sites are slated to activate in the second quarter, and the company plans to provide dosimetry, safety, and efficacy updates throughout 2026.
Cellectar’s financial position supports the trial’s early stages. As of December 31, 2025, the company held $13.2 million in cash and cash equivalents, and its 2025 R&D expense of $11.5 million was a significant reduction from $26.1 million in 2024. The cash balance is projected to fund operations through the third quarter of 2026, giving the company runway to advance the CLR 125 program.
Triple‑negative breast cancer accounts for roughly 12 % of all breast cancer diagnoses in the United States and has a relapse rate of about 25 % after initial treatment. The CLR 125 trial therefore targets a substantial unmet medical need. The PDC platform’s prior regulatory designations for CLR 131—including FDA Breakthrough, Orphan Drug, Rare Pediatric Drug, and Fast Track status—provide a foundation for potential regulatory support for CLR 125 if the trial demonstrates safety and efficacy.
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