Cellectar Biosciences plans to initiate a Phase 1b study of CLR 125 in triple-negative breast cancer in the fourth quarter of 2025, anchored at a Mayo Clinic Network site with Pooja Advani as lead investigator. The company has contracted Evestia Clinical for full-service CRO support, leveraging Evestia’s recently expanded oncology platform following its merger with Atlantic Research Group.
The move consolidates two threads: Cellectar’s push to extend its radiopharmaceutical platform from hematologic and CNS settings into solid tumors, and Evestia’s bid to signal scale and specialty depth post-merger. CLR 125 is an iodine-125 Auger-emitting candidate aimed at solid tumors, including TNBC, lung, and colorectal cancers. While the company did not disclose the Phase 1b design, scope, or number of sites, the choice to start at a Mayo Clinic Network center points to an initial emphasis on complex imaging and dosimetry, radiation safety workflows, and early signal generation in a high-expertise environment.
Strategically, this is a focused expansion play for Cellectar at a time when radiopharmaceuticals are gaining momentum, but execution risk remains concentrated in CMC, isotope logistics, and site readiness. The 2025 start suggests a long runway to finalize protocol specifics, secure radiation safety approvals, and align manufacturing and distribution for an iodine-125 construct with a long half-life and tight handling controls. It also gives Evestia-ARG a window to demonstrate that its merged organization can manage radiopharma nuances—central dosimetry, chain-of-custody, nuclear medicine coordination, and cross-functional data flows—without the overhead of a mega-CRO.
For sites, the near-term impact is operational rather than scientific. Activation will hinge on radiation safety committee approvals, availability of nuclear medicine resources, shielding and storage capacity for I‑125, and training across pharmacy, radiation safety, and clinical teams. A Mayo-led start can de-risk first dosing and imaging readouts, but scale will require moving beyond tertiary centers to diversify enrollment and avoid bottlenecks in a TNBC landscape already crowded with trials for ADCs and IO combinations. For sponsors and CROs, the selection underscores a broader trend: early radiopharma trials concentrating at centers of excellence to accelerate first-in-human learning, then contending with slower expansion as community sites grapple with infrastructure and staffing constraints.
Regulators will scrutinize dosimetry methodology, manufacturing controls, and safety given CLR 125’s mechanism, though Cellectar’s prior radiopharmaceutical experience may shorten learning curves on documentation and quality systems. Competitive dynamics in TNBC are unforgiving; with ADCs cementing roles in later lines and moving earlier, the company will need to define CLR 125’s line-of-therapy strategy, rational monotherapy versus combination, and any biomarker or imaging-driven enrichment to avoid diffuse, low-yield expansion cohorts.
What to watch next: the Phase 1b protocol, including dose-escalation schema, dosimetry endpoints, and any expansion cohorts by prior lines or biomarkers; the site list beyond Mayo and the pace of site activations; clarity on isotope sourcing, product release testing, and shipping timelines; and evidence that Evestia-ARG can translate merger rhetoric into execution—particularly on startup speed, data quality, and radiation operations. The risks are typical for radiopharma in solid tumors: competition for nuclear medicine capacity, prolonged site startup under radiation oversight, and enrollment headwinds against higher-profile TNBC studies. If the program can move quickly from a single flagship site to a distributed network while maintaining operational discipline, it will provide a useful read on whether mid-sized CRO platforms can credibly compete for complex radiopharma builds.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
