161Tb radiopharmaceutical therapy as a treatment for RANKL-positive multiple myeloma

Denosumab, an anti-RANKL monoclonal antibody, is clinically approved for bone protection in multiple myeloma (MM) but lacks direct cytotoxic effects on tumor cells. Herein, we repurpose this well-tolerated antibody as a targeted delivery vehicle by conjugating it with the emerging radionuclide ¹⁶¹Tb, which emits low-energy conversion and Auger electrons with ultra-short tissue range, offering high relative biological effectiveness against single tumor cells and micrometastases. However, ¹⁶¹Tb-based radiopharmaceutical therapy has not been explored in MM. Our purpose is to evaluate the therapeutic potential of [¹⁶¹Tb]Tb-DOTA-Denosumab compared to [¹⁷⁷Lu]Lu-DOTA-Denosumab for RANKL-positive MM.

DOTA-conjugated Denosumab was labeled with ¹⁶¹Tb and ¹⁷⁷Lu. Radiochemical purity, in vitro stability and biodistribution were assessed. Therapeutic efficacy was evaluated in U266 and H929 subcutaneous xenograft models. Tumor volume, survival, body weight, hematologic, and biochemical parameters were monitored. Administered-activity-escalation toxicity studies were performed in tumor-free mice.

Both radiolabeled conjugates showed high radiochemical purity (>95%), excellent in vitro stability and comparable tumor uptake. [¹⁶¹Tb]Tb-DOTA-Denosumab demonstrated significantly enhanced antitumor efficacy compared to [¹⁷⁷Lu]Lu-DOTA-Denosumab at equivalent (1.85 MBq) or lower activities. In the U266 model, median survival was prolonged from 33 days (saline) to 45 days with ¹⁶¹Tb (1.85 MBq) versus 39 days with ¹⁷⁷Lu (1.85 MBq). In the H929 model, median survival extended to 47 days with ¹⁶¹Tb (1.85 MBq) versus 31 days (saline) and 41 days with ¹⁷⁷Lu (1.85 MBq). Toxicity studies confirmed reversible hematologic changes only at administered activities ≥3.70 MBq, with no severe organ damage.

This is the first demonstration of ¹⁶¹Tb-based radiopharmaceutical therapy targeting RANKL in MM. [¹⁶¹Tb]Tb-DOTA-Denosumab exhibits superior antitumor efficacy and prolonged survival compared to its ¹⁷⁷Lu-labeled counterpart. The unique Auger/conversion electron emission of ¹⁶¹Tb enables potent cytotoxicity against MM cells with favorable organ tolerability. These findings support ¹⁶¹Tb as a next-generation theranostic radionuclide for RANKL-positive malignancies and provide a strong rationale for clinical translation, highlighting the value of repurposing a clinically established antibody for radiopharmaceutical therapy.