Evaluation of Radionuclide Stimulated Therapy response in a preclinical multiple myeloma model

Introduction: Background: Multiple myeloma (MM) is a hematological malignancy caused by abnormal plasma cells in the bone marrow. MM patients frequently relapse and become refractory to standard of care MM therapies, necessitating new therapeutic breakthroughs. Radionuclide stimulated therapy (RaSt) is a promising treatment paradigm that involves activation of light sensitive drugs (photosensitizers) by Cerenkov radiation from radionuclides. The activated photosensitizers result in the production of reactive oxygen species (ROS), which mediates cell death by mechanisms such as apoptosis. In this study, we evaluated the treatment effect of RaSt in disseminated myeloma model. The CD38 targeted PET radiopharmaceutical, ⁸⁹Zr-daratumumab, was employed as an in vivo depth-independent Cerenkov radiation source to excite titanium dioxide (TiO₂) nanoparticles coated with transferrin (Tf) and titanocene (TC) (TiO₂-Tf-TC) for generating cytotoxicity in human MM mouse models.

Methods: Methods: TiO₂-Tf-TC nanoparticles used as photosensitizers in this study were synthesized in house to target the transferrin receptors on myeloma cells. The transferrin receptor expression on human myeloma, MM.1S cells was evaluated by flow cytometry. The in vitro TiO₂-Tf nanoparticles uptake on these myeloma cells was confirmed by fluorescent and bright-field imaging using a confocal microscope. Zirconium-89 (⁸⁹Zr), a positron emitting radionuclide with a long half-life of 3.3 d was used to radiolabel CD38 targeted monoclonal antibody, daratumumab. Human myeloma cells, MM.1S-luc (luc; luciferase), were injected in FCSB mice via tail vein route to develop disseminated myeloma model and tumor progression in vivo was monitored by bioluminescence imaging (BLI). These tumor bearing mice from RaSt treatment group were injected with ⁸⁹Zr-daratumumab (1.11 MBq), followed by TiO₂-Tf-TC nanoparticles (1mg/mL) at 3d post-injection of the radiotracer. BLI was used to evaluate the therapeutic response of RaST, and other control groups in vivo.

Results: Results: The in vitro flow data determined that >99% of MM.1S cells showed high transferrin receptor expression. The in vitro cell imaging of TiO₂-Tf nanoparticles showed strong fluorescence signal in the cell membrane suggesting that these nanoparticles were membrane bound. The in vivo quantitative BLI data demonstrated that there was a significant increase in tumor burden in tumor bearing mice with no treatment as compared to the mice treated with RaSt. Post-initiation of respective therapies, no significant differences were observed in the BLI signal among other control groups like ⁸⁹Zr –daratumumab treated only, and TiO₂-Tf-TC nanoparticles only. Mice treated with RaSt showed better survival rate as compared to other control groups.

Conclusions: Conclusion: ⁸⁹Zr-daratumumab can be successfully used as a Cerenkov radiation source targeted to myeloma cells and can efficiently activate TiO₂-Tf-TC nanoparticles to induce cytotoxic effects. Radionuclide stimulated therapy showed significant therapeutic response in preclinical myeloma mouse models and paves the way to using this approach for treatment of MM.