Cellectar Biosciences Announces Publication of Phase 1 Study of Iopofosine I 131 in Peer-Reviewed Journal Cancers

Demonstrated Favorable Safety Profile and Preliminary Clinical Activity in 
Relapsed/Refractory Multiple Myeloma

Publication Supports Continued Development of Iopofosine I 131 as a Novel Targeted
Radiotherapeutic Across Multiple B-Cell Malignancies

FLORHAM PARK, N.J., July 15, 2026 (GLOBE NEWSWIRE) -- Cellectar Biosciences, Inc. (NASDAQ: CLRB), a late-stage clinical biopharmaceutical company focused on the discovery and development of drugs for the treatment of cancer, today announced the publication of results from a Phase 1 dose-escalation study evaluating iopofosine I 131 in combination with low-dose dexamethasone in 31 patients with heavily pretreated relapsed/refractory multiple myeloma (r/r MM). The manuscript, titled “A Phase I Trial of Iopofosine I 131 and Dexamethasone in Patients with Relapsed/Refractory Multiple Myeloma,” was published in the peer-reviewed journal, Cancers.

“Iopofosine I 131 represents a differentiated therapeutic approach for patients with relapsed or refractory multiple myeloma who have limited remaining treatment options,” said Sikander Ailawadhi, M.D., professor of medicine, division of hematology/oncology, Mayo Clinic, Jacksonville, Florida and lead author of the publication. “The study demonstrated manageable toxicity with adverse events being essentially limited to cytopenias, encouraging disease control, and evidence of antitumor activity in a heavily pretreated population, supporting the potential of this novel targeted radiotherapeutic in these most fragile patients. The predictability and recovery of the cytopenias support its potential for repeat dosing strategies in future clinical development.”

Highlights of the Data:

The study evaluated both single-dose and fractionated-dose regimens of iopofosine I 131 in 31 heavily pretreated r/r MM patients, many of whom had exhausted available treatment options.

Among 26 efficacy-evaluable patients:

• Clinical activity appeared more pronounced in patients receiving higher total administered doses
• 84.6% achieved stable disease or better following treatment
• 30% overall response rate observed among evaluable patients (n=10) receiving at least 60 mCi of iopofosine I 131
• The overall response rate was 15.4%, with four patients achieving partial responses
• Investigators reported a favorable and manageable safety profile, with adverse events primarily consisting of predictable and reversible hematologic toxicities
• No new safety signals were identified, and non-hematologic adverse events were generally low grade

“Publication of these data in Cancers further validate the potential of iopofosine as a differentiated targeted radiopharmaceutical and underscore the promise of our phospholipid drug conjugate platform as a novel targeted radiotherapeutic platform,” said Jarrod Longcor, Cellectar’s chief operating officer and co-author of the publication. “Importantly, the mechanism of action is not dependent on a single target or mutation, creating the opportunity to address a broad range of B-cell-mediated malignancies. We believe iopofosine has the potential to serve patients across multiple indications, including Waldenström macroglobulinemia, multiple myeloma, diffuse large B-cell lymphoma and other difficult-to-treat hematologic cancers where new therapeutic options remain urgently needed.”

The authors further noted that iopofosine’s limited impact on renal and hepatic function, coupled with its predictable and manageable safety profile, may make it particularly attractive for older, frail, or heavily pretreated patients who may not be candidates for more intensive therapies. The study also demonstrated recovery of treatment-related cytopenias over time, supporting the potential for repeat dosing strategies in future clinical development.

The full article can be accessed here.

About the Phase 1 Study
The Phase 1 dose escalation and safety study was conducted as an open-label, multi-center trial, which enrolled 31 patients with relapsed or refractory multiple myeloma who had received a median of four prior lines of therapy. Patients received either a single dose or fractionated doses of iopofosine I 131 plus low-dose dexamethasone. The study established 31.25 mCi/m² as the maximum tolerated single dose and identified 20 mCi/m² administered twice, one week apart, as the highest evaluated fractionated dose regimen. The authors recommended a fractionated Phase 2 regimen of 15 mCi/m² administered on days 1 and 7 in combination with weekly low-dose dexamethasone.

About Cellectar Biosciences, Inc.
Cellectar Biosciences is a late-stage clinical biopharmaceutical company focused on the discovery and development of proprietary drugs for the treatment of cancer, independently and through research and development collaborations. The company’s core objective is to leverage its proprietary Phospholipid Drug Conjugate™ (PDC) delivery platform to develop the next-generation of cancer cell-targeting treatments, delivering improved efficacy and better safety as a result of fewer off-target effects.

The company’s product pipeline includes iopofosine I 131, which is a PDC designed to provide targeted delivery of iodine-131 (radioisotope). Iopofosine I 131 has been tested in Phase 2b trials as a treatment for relapsed or refractory Waldenström Macroglobulinemia (WM), in relapsed or refractory multiple myeloma (MM) and central nervous system (CNS) lymphoma. The CLOVER-2 Phase 1b study is evaluating iopofosine I 131 in pediatric patients with high-grade gliomas, for which Cellectar is eligible to receive a Pediatric Review Voucher from the FDA upon approval. The FDA has granted iopofosine I 131 Breakthrough, six Orphan Drug, four Rare Pediatric Drug and two Fast Track Designations for various cancer indications, and the EMA has granted iopofosine I 131 PRIority MEdicines (PRIME) designation.

Cellectar is also developing CLR 121125 (CLR 125), an iodine-125 Auger-emitting program targeted for solid tumors, such as triple negative breast (TNBC), lung, and colorectal cancer, and is currently being evaluated in a Phase 1b study for TNBC, which will determine the recommended dose for the subsequent Phase 2 trial. CLR 125 has been well tolerated in vivo and has demonstrated strong preclinical data showing reduction or inhibition of solid tumor growth.

In addition to these assets, the Cellectar team is developing CLR 121225 (CLR 225), an actinium-225 based program targeting solid tumors in indications with significant unmet need, such as pancreatic cancer, as well as proprietary preclinical PDC chemotherapeutic programs and multiple partnered PDC assets.

For more information, please visit https://www.cellectar.com/or join the conversation by liking and following us on the company’s social media channels: X, LinkedIn, and Facebook.

Forward Looking Statements Disclaimer
This news release contains forward-looking statements. You can identify these statements by our use of words such as "may," "expect," "believe," "anticipate," "intend," "could," "estimate," "continue," "plans," or their negatives or cognates. These statements are only estimates and predictions and are subject to known and unknown risks and uncertainties that may cause actual future experience and results to differ materially from the statements made. These statements are based on our current beliefs and expectations as to such future outcomes. Drug discovery and development involve a high degree of risk. Factors that might cause such a material difference include, among others, uncertainties related to the ability to identify suitable collaborators, partners, licensees or purchasers for our product candidates and, if we are able to do so, to enter into binding agreements with regard to any of the foregoing, or to raise additional capital to support our operations, or our ability to fund our operations if we are unsuccessful with any of the foregoing. A complete description of risks and uncertainties related to our business is contained in our periodic reports filed with the Securities and Exchange Commission including our Form 10-K for the year ended December 31, 2026, and our Form 10-Q for the quarterly period ended March 31, 2026. These forward-looking statements are made only as of the date hereof, and we disclaim any obligation to update any such forward-looking statements.

INVESTORS:
Anne Marie Fields
Precision AQ
212-362-1200
annemarie.fields@precisionaq.com

Released July 15, 2026