Doxorubicin-conjugated <sup>10</sup>B<sub>4</sub>C nanoparticles: Preparation and application in combined boron neutron capturetherapy/chemotherapy

Boron neutron capture therapy (BNCT), which is capable of killing cancer cells precisely with α particles and recoiling ⁷Li nuclei produced by the nuclear fission reaction of thermal neutrons and ¹⁰B in the cells, has attracted tremendous attentions in recent years. Delivering sufficient ¹⁰B into the cancer cells (> 10⁹/cell) is crucial to successful BNCT. Although considerable effort has been devoted to develop cancer-targeted ¹⁰B delivery agents, currently only two boron-containing compounds, i.e., borylphenylalanine (BPA) and sodium mercaptoundecahydro-closo-dodecaborate (BSH) are clinically used in BNCT. On the other hand, combining BNCT with other cancer treatments such as chemotherapy, photodynamic therapy, and immunotherapy can enhance the therapeutic efficacy. Thus, it is highly desired to develop multifunctional ¹⁰B delivery agents that enable combination cancer therapy. Boron-containing inorganic nanoparticles such as ¹⁰B, ¹⁰B₄C, and ¹⁰BN nanoparticles, containing a large number of boron atoms in a single nanoparticle are promising candidates as ¹⁰B delivery agents for BNCT. In addition, these nanoparticles possess low toxicity and good biocompatibility, and are rich in surface chemistry that facilitates further tailoring. For biomedical applications of inorganic nanoparticles, surface modification is essential to increase their aqueous dispersibility and colloidal stability. We have recently developed polyglycerol (PG) grafting as a convenient and effective method for surface modification of inorganic nanoparticles. The PG layer of hyperbranched topology contains many hydroxyl groups on the periphery, which not only largely increase the hydrophilicity of the nanoparticles, but also provide reactive groups for further functionalization through chemical reactions. Through this approach, a wide range of functional moieties including targeting ligands, fluorescent tags, and therapeutic drugs and DNAs have been immobilized on the surface of nanoparticles. Herein, we developed DOX-conjugated ¹⁰B₄C nanoparticles (¹⁰B₄C-PG-DOX) for combined BNCT/chemotherapy. ¹⁰B₄C nanoparticles prepared by wet ball milling of ¹⁰B₄C powder possessed many oxygen-containing groups (e.g., B-OH and C-OH) on the surface, which were used as starting points to graft PG through ring-opening polymerization of glycidol. The obtained ¹⁰B₄C-PG was surface engineered to conjugate DOX through pH-sensitive hydrazone bond, yielding ¹⁰B₄C-PG-DOX. The PG layer provides ¹⁰B₄C-PG-DOX with sufficient dispersibility and stability in aqueous media. On the other hand, the conjugated DOX endows the nanoparticles with positive charge and lipophilicity that facilitate cell uptake. When the concentration of ¹⁰B₄C-PG-DOX is 10 μg ¹⁰B/mL, the number of ¹⁰B atoms in a single treated 4T1 murine breast cancer cell can reach as high as (4.92±0.36)×10¹¹. Following the cell internalization, ¹⁰B₄C-PG-DOX nanoparticles mainly distributed in the lysosomes of 4T1 cells, where the mild acidic environment is capable of cleaving the hydrazone bond to release chemotherapeutic DOX. Upon exposure to thermal neutrons generated by an accelerator, ¹⁰B₄C-PG-DOX exerted excellent therapeutic efficacy of combined BNCT/chemotherapy towards 4T1 cells. In conclusion, multifunctional ¹⁰B₄C-PG-DOX was fabricated by rational surface engineering of ¹⁰B₄C nanoparticles, showing a great promise in combined BNCT/chemotherapy of cancer. Future work will focus on in vivo studies of ¹⁰B₄C-PG-DOX, including pharmacokinetics and combination therapy on mouse tumor models.