Bispecific Antibodies as Targeting Agents for Boron Neutron Capture Therapy of Brain Tumors

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10, a stable isotope, is irradiated with low energy (≤0.025 eV) or thermal neutrons to yield alpha particles and recoiling lithium-7 nuclei. A major requirement for the success of BNCT is the selective delivery of a sufficient number of boron atoms (∼109) to individual cancer cells to sustain a lethal 10B (n,α)7 Li capture reaction. A panel of BsAb reactive with polyhedral borane anions (PBA) and a tumor-associated chondroitin sulfate proteoglycan has been produced. All of these BsAb showed strong reactivity with a panel of human glioblastoma and melanoma cell lines, as demonstrated by indirect membrane immunofluorescence. Two of them (H6 and B8) also reacted with cells that had been exposed to PBA (Na2B10H10 and Na2B12H11SH) and a boronated starburst dendrimer, which contained ∼250′400 B atoms per molecule. The affinity constant (Ka) of BsAb-B8 was 2.57 x 108 M-1 on M21 human melanoma cell and 3.49 x 108 M-1 on A172 glioblastoma cells, which were almost identical to those of the parental monoclonal antibody (mAb) 9.2.27 on the same cell lines (2.62 x 108 M-1). Since our BsAb recognize both human glioblastoma and melanoma-associated antigens, as well as PBA, they potentially could be used to target 10B to these tumors for BNCT.