Effects of employing a ¹⁰B-carrier and manipulating intratumour hypoxia on local tumour response and lung metastatic potential in boron neutron capture therapy

Importance of radiobiological studies for the advancement of boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is a tumour selective particle radiotherapy, based on the administration of boron carriers incorporated preferentially by tumour cells, followed by irradiation with a thermal or epithermal neutron beam. BNCT clinical results to date show therapeutic efficacy, associated with an improvement in patient quality of life and prolonged survival. Translational research in adequate experimental models is necessary to optimise BNCT for different pathologies. This review recapitulates some examples of BNCT radiobiological studies for different pathologies and clinical scenarios, strategies to optimise boron targeting, enhance BNCT therapeutic effect and minimise radiotoxicity. It also describes the radiobiological mechanisms induced by BNCT, and the importance of the detection of biomarkers to monitor and predict the therapeutic efficacy and toxicity of BNCT alone or combined with other strategies. Besides, there is a brief comment on the introduction of accelerator-based neutron sources in BNCT. These sources would expand the clinical BNCT services to more patients, and would help to make BNCT a standard treatment modality for various types of cancer. Radiobiological BNCT studies have been of utmost importance to make progress in BNCT, being essential to design novel, safe and effective clinical BNCT protocols.

An attempt to improve the therapeutic effect of boron neutron capture therapy using commonly employed 10B-carriers based on analytical studies on the correlation among quiescent tumor cell characteristics, tumor heterogeneity and cancer stemness

Based on our previously published reports concerning the response of quiescent (Q) tumor cell populations to boron neutron capture therapy (BNCT), the heterogeneous microdistribution of 10B in tumors, which is influenced by the tumor microenvironment and the characteristics of the 10B delivery carriers, has been shown to limit the therapeutic effect of BNCT on local tumors. It was also clarified that the characteristics of 10B-carriers for BNCT and the type of combined treatment in BNCT can also affect the potential for distant lung metastases from treated local tumors. We reviewed the findings concerning the response of Q tumor cell populations to BNCT, mainly focusing on reports we have published so far, and we identified the mode of BNCT that currently offers the best therapeutic gain from the viewpoint of both controlling local tumor and suppressing the potential for distant lung metastasis. In addition, based on the finding that oxygenated Q tumor cells showed a large capacity to recover from DNA damage after cancer therapy, the interrelationship among the characteristics in Q tumor cell populations, tumor heterogeneity and cancer stemness was also discussed.

Effect of a change in reactor power on response of murine solid tumors in vivo, referring to impact on quiescent tumor cell population

PURPOSE

To examine the effect of a change in reactor power on the response of solid tumors, referring to impact on quiescent (Q) tumor cell population.

MATERIALS AND METHODS

Tumor-bearing mice received 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) tumor cells, and were treated with boronophenylalanine-¹⁰B (BPA) or sodium mercaptododecaborate-¹⁰B (BSH). After reactor neutron beam irradiation at a power of 1 or 5 MW with an identical beam spectrum, cells from tumors were isolated and incubated with a cytokinesis blocker. The responses of BrdU-unlabeled Q and total (P + Q) tumor cells were assessed based on the frequencies of micronucleation using immunofluorescence staining for BrdU.

RESULTS

After neutron irradiation with or without ¹⁰B-carrier, radio-sensitivity was reduced by decreasing reactor power in both cells, especially in Q cells and after irradiation with BPA. The values of relative and compound biological effectiveness were larger at a power of 5 MW and in Q cells than at a power of 1 MW and in total cells, respectively. The sensitivity difference between total and Q cells was widened when combined with ¹⁰B-carrier, especially with BPA, and through decreasing reactor power.

CONCLUSION

5 MW is more advantageous than 1 MW for boron neutron capture therapy.

Impact of oxygen status on 10B-BPA uptake into human glioblastoma cells, referring to significance in boron neutron capture therapy

Boron neutron capture therapy (BNCT) can potentially deliver high linear energy transfer particles to tumor cells without causing severe damage to surrounding normal tissue, and may thus be beneficial for cases with characteristics of infiltrative growth, which need a wider irradiation field, such as glioblastoma multiforme. Hypoxia is an important factor contributing to resistance to anticancer therapies such as radiotherapy and chemotherapy. In this study, we investigated the impact of oxygen status on 10B uptake in glioblastoma cells in vitro in order to evaluate the potential impact of local hypoxia on BNCT. T98G and A172 glioblastoma cells were used in the present study, and we examined the influence of oxygen concentration on cell viability, mRNA expression of L-amino acid transporter 1 (LAT1), and the uptake amount of 10B-BPA. T98G and A172 glioblastoma cells became quiescent after 72 h under 1% hypoxia but remained viable. Uptake of 10B-BPA, which is one of the agents for BNCT in clinical use, decreased linearly as oxygen levels were reduced from 20% through to 10%, 3% and 1%. Hypoxia with <10% O2 significantly decreased mRNA expression of LAT1 in both cell lines, indicating that reduced uptake of 10B-BPA in glioblastoma in hypoxic conditions may be due to reduced expression of this important transporter protein. Hypoxia inhibits 10B-BPA uptake in glioblastoma cells in a linear fashion, meaning that approaches to overcoming local tumor hypoxia may be an effective method of improving the success of BNCT treatment.

Modeling the detection efficiency of an HP-Ge detector for use in boron neutron capture therapy

The multi-foil method is commonly used to determine upon an energy spectrum of neutrons in boron neutron capture therapy. The method requires to measure the radioactivation of the foils. This study develops a simple modeling procedure of a high-purity Ge detector, which is used to measure the radioactivation, in order to calculate the detection efficiency with GEANT4. By changing four parameters from their manufacturing specifications of the detector, the simulated detection efficiency is able to reproduce the actual detection efficiency.

The dependency of compound biological effectiveness factors on the type and the concentration of administered neutron capture agents in boron neutron capture therapy

Purpose

To examine the effect of the type and the concentration of neutron capture agents on the values of compound biological effectiveness (CBE) in boron neutron capture therapy.

Methods and materials

After the subcutaneous administration of a ¹⁰B-carrier, boronophenylalanine-¹⁰B (BPA) or sodium mercaptododecaborate-¹⁰B (BSH), at 3 separate concentrations, the ¹⁰B concentrations in tumors were measured by γ-ray spectrometry. SCC VII tumor-bearing C3H/He mice received 5-bromo-2′-deoxyuridine (BrdU) continuously to label all intratumor proliferating (P) cells, then treated with BPA or BSH. Immediately after reactor neutron beam irradiation, during which intratumor ¹⁰B concentrations were kept at levels similar to each other, cells from some tumors were isolated and incubated with a cytokinesis blocker. The responses of BrdU-unlabeled quiescent (Q) and total (= P + Q) tumor cells were assessed based on the frequencies of micronucleation using immunofluorescence staining for BrdU.

Results

The CBE values were higher in Q cells and in the use of BPA than total cells and BSH, respectively. In addition, the higher the administered concentrations were, the smaller the CBE values became, with a clearer tendency in the use of BPA than BSH. The values for neutron capture agents that deliver into solid tumors more dependently on uptake capacity of tumor cells became more changeable.

Conclusion

Tumor characteristics, such as micro-environmental heterogeneity, stochastic genetic or epigenetic changes, or hierarchical organization of tumor cells, are thought to partially influence on the value of CBE, meaning that the CBE value itself may be one of the indices showing the degree of tumor heterogeneity.