Efficacy of Boron Neutron Capture Therapy in Primary Central Nervous System Lymphoma: In Vitro and In Vivo Evaluation

Abstract

Background: Boron neutron capture therapy (BNCT) is a nuclear reaction-based tumor cell-selective particle irradiation method. High-dose methotrexate and whole-brain radiation therapy (WBRT) are the recommended treatments for primary central nervous system lymphoma (PCNSL). This tumor responds well to initial treatment but relapses even after successful treatment, and the prognosis is poor as there is no safe and effective treatment for relapse. In this study, we aimed to conduct basic research to explore the possibility of using BNCT as a treatment for PCNSL.

Methods: The boron concentration in human lymphoma cells was measured. Subsequently, neutron irradiation experiments on lymphoma cells were conducted. A mouse central nervous system (CNS) lymphoma model was created to evaluate the biodistribution of boron after the administration of borono-phenylalanine as a capture agent. In the neutron irradiation study of a mouse PCNSL model, the therapeutic effect of BNCT on PCNSL was evaluated in terms of survival.

Results: The boron uptake capability of human lymphoma cells was sufficiently high both in vitro and in vivo. In the neutron irradiation study, the BNCT group showed a higher cell killing effect and prolonged survival compared with the control group.

Conclusions: A new therapeutic approach for PCNSL is urgently required, and BNCT may be a promising treatment for PCNSL. The results of this study, including those of neutron irradiation, suggest success in the conduct of future clinical trials to explore the possibility of BNCT as a new treatment option for PCNSL.

Keywords: boron neutron capture therapy (BNCT); malignant brain tumor; primary central nervous system lymphoma (PCNSL); radiation therapy.

Figure A1

Representative aspect of CNS lymphoma sections stained with hematoxylin-Eosin (HE). (a): Photomicrographs of brain parenchyma after Raji lymphoma cells implantation. The mice were sacrificed when the body weight was reduced by 25%. The brain was removed and sectioned serially. The character T in the figure represents tumor formation (×4). (b): Perivascular cuffing and diffuse proliferation of microblasts typical of lymphomas is seen (×100).

Figure A2

Cell viability ratio after neutron irradiation and photon irradiation fitted to the linear-quadratic (LQ) model for Raji and RL lymphoma cells. The physical doses required to achieve the same biological effects as the BNCT group 3 h after exposure to 5 μg B/mL of Raji, RL were 4.6 and 2.3 Gy (cell viability ratio = 0.5). The physical doses for Raji and RL were 8.5 and 4.1 Gy when the cell viability ratio was 0.1. The CBE that could be calculated from the LQ model when cell viability ratio = 0.006 was equal to the CBE value (Raji = 10.6, RL = 5.6) calculated in this study (Raji = 9.5, RL = 3.4).

Figure 1

Cellular uptake of boron in human lymphoma cells. Boron concentration of Raji (blue) and RL (green) lymphoma in RPMI 1640, using 5, 10, and 20 µg B/mL of BPA for 3 h. The boron concentration of lymphoma cells was high for both Raji and RL.

Figure 2

Relationship between cell viability ratio and photon radiation dose. The cell viability ratio of lymphoma cells after 96 h of irradiation from 0 to 32 Gy. Cell viability ratio of Raji cells are shown in blue, and that of RL are shown in green. There was a significantly decreased cell viability ratio by more than 2 Gy irradiation compared to 0 Gy in Raji and RL (p < 0.05).

Figure 3

Relationship between boron concentration and cell viability ratio after neutron irradiation. Cell viability of lymphoma cells after exposure to BPA at concentrations of 5, 10, and 20 μg B/mL for 3 h followed by irradiation with 1 MW neutrons for 30 min and incubation for 96 h. Blue is Raji lymphoma cells, and green is RL lymphoma cells. In Raji lymphoma cells. BNCT significantly reduced the cell viability of lymphoma cells (p < 0.05).

Figure 4

Boron concentrations in various organs of mouse CNS lymphoma models treated with BPA. Tumors had significantly higher boron concentrations (9.9 ± 1.6 µg/g) compared to the brain (3.5 ± 1.8 µg/g: p < 0.05). The tumor to brain ratio (: T/Br) was 2.85. The tumor to blood ratio (: T/Bl) was 0.72. Boron accumulation was also observed in other organs.

Figure 5

Kaplan–Meier survival curves of mouse CNS lymphoma models after neutron irradiation. Survival times in days after tumor implantation were plotted for the following groups: untreated control (black line), BPA only (green line), neutron irradiation only (orange line), BNCT 15-min: BPA exposure followed by 15 min of neutron irradiation (blue line), and BNCT 30-min: BPA exposure followed by 30 min of neutron irradiation (brown line). Median survival times of untreated control group, BPA only group, Neutron irradiation only group, BNCT 15-min group, and BNCT 30-min group were 28.2 ± 3.8, 28.6 ± 2.3, 31.8 ± 1.7, 42.3 ± 3.4, and 41.3 ± 1.8 days. There were statistically significant differences between the BNCT 30-min group and untreated control group (p = 0.007) and between the BNCT 15-min group and untreated control group (p = 0.02).