Therapeutic activation of TiO₂-Tf-TC nanoparticles by ⁸⁹Zr-daratumumab in a multiple myeloma tumor model

Background: Multiple myeloma (MM) is a debilitating hematological malignancy caused by abnormal plasma cells in the bone marrow. Despite recent advances in treatment strategies, MM patients frequently relapse and become refractory due to genetic instability and other microenvironmental factors. Photodynamic therapy (PDT) involves activating a photosensitizer by an external light source, leading to the production of reactive oxygen species (ROS), which mediates cell death by apoptosis. However, the limited tissue penetration of light hinders the treatment of deep seated tumors as most of the light is absorbed and/orscattered by the biological tissue. To overcome this limitation, an alternative approach is to use Cerenkov radiation, which is the UV and the visible light emitted during a radioactive decay by charged particles traveling through a dielectric medium at a speed greater than that of light. In this study, ⁸⁹Zr-daratumumab, a CD38 targeted PET imaging agent, 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 ROS in a human MM mouse model.

Methods: TiO₂-Tf-TC nanoparticles were synthesized in-house and characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Daratumumab was conjugated to DFO-Bz-NCS and radiolabeling of the conjugate was optimized with zirconium-89 (⁸⁹Zr). Human MM cells, MM.1S, modified to express click beetle red luciferase (MM.1S-CBR) were injected in NSG mice subcutaneously and tumor progression was monitored by bioluminescence imaging (BLI). Cell viability assay was performed to evaluate the Cerenkov radiation-induced therapy (CRIT) response in vitro. ⁸⁹Zr-daratumumab (1.11 MBq) was injected in mice via tail vein, followed by TiO₂-Tf-TC nanoparticles at 3 d post-injection of the radiotracer. PET imaging with ⁸⁹Zr-daratumumab and BLI was used to evaluate the therapeutic response of CRIT in vivo.

Results: TiO₂ based nanoparticles were synthesized to target the Tf receptors (1:3 TiO₂ to Tf mass ratio) by functionalizing the metal oxide surface with Tf, that enhanced dispersion and enabled the incorporation of TC. The nanoparticles’ size was confirmed by TEM and DLS to be 100nm and 105nm, respectively. Cells treated with ⁸⁹Zr-daratumumab and TiO₂-Tf-TC nanoparticles showed reduced viability of 52.8 ± 10.4% as compared to the untreated control (98.7 ± 1.5%), nanoparticles only (96.3 ± 0.4%), and ⁸⁹Zr-daratumumab only (88 ± 0.4%) treatment groups. Compared to the ⁸⁹Zr-daratumumab, TiO₂-Tf-TC nanoparticles, and untreated controls, CRIT exhibited a significant decrease in the BLI signal of the MM.1S-CBR tumor in mice. The SUV of tumor in mice from the CRIT treatment group decreased from 2.85 to 1.90 post-therapy, whereas the SUV of the tumor increased from 2.58 to 3.02 in the mice injected with ⁸⁹Zr-daratumumab only, showing inhibition of tumor progression by CRIT.

Conclusions: CRIT with a combination of ⁸⁹Zr-daratumumab and TiO₂-Tf-TC nanoparticles showed significant therapeutic effects on MM.1S-CBR cells in vitro and in vivo. The long half-life and high gamma energy of ⁸⁹Zr are suitable for long-term tissue depth- and oxygen-independent photodynamic treatments. This study paves the way to use antibody labeled with positron emitters as an internal radiation source to activate the nanoparticle photosensitizers for the treatment of MM.