Reoxygenation in quiescent and total intratumor cells following thermal neutron irradiation with or without (10)B-compound-compared with that after gamma-ray irradiation

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.

Recent advances in the biology of heavy-ion cancer therapy

Superb biological effectiveness and dose conformity represent a rationale for heavy-ion therapy, which has thus far achieved good cancer controllability while sparing critical normal organs. Immediately after irradiation, heavy ions produce dense ionization along their trajectories, cause irreparable clustered DNA damage, and alter cellular ultrastructure. These ions, as a consequence, inactivate cells more effectively with less cell-cycle and oxygen dependence than conventional photons. The modes of heavy ion-induced cell death/inactivation include apoptosis, necrosis, autophagy, premature senescence, accelerated differentiation, delayed reproductive death of progeny cells, and bystander cell death. This paper briefly reviews the current knowledge of the biological aspects of heavy-ion therapy, with emphasis on the authors' recent findings. The topics include (i) repair mechanisms of heavy ion-induced DNA damage, (ii) superior effects of heavy ions on radioresistant tumor cells (intratumor quiescent cell population, TP53-mutated and BCL2-overexpressing tumors), (iii) novel capacity of heavy ions in suppressing cancer metastasis and neoangiogenesis, and (iv) potential of heavy ions to induce secondary (especially breast) cancer.

Significance of the response of quiescent cell populations within solid tumors in cancer therapy

In analyzing the response of quiescent (Q) cells in solid tumors, we have developed a combined method with a micronucleus (MN) assay and the identification of proliferating (P) cells by 5-bromo-2'-deoxyuridine (BrdU) and an anti-BrdU monoclonal antibody. Using this method, the responses of Q tumor cells as well as total tumor (P + Q) cells within murine solid tumors to various DNA-damaging treatments were evaluated. Based on this evaluation, combining with tirapazamine, a well-known bioreductive agent, and/or heat treatment at mild temperatures was thought to be a promising modality for cancer therapy in terms of conventional anticancer treatment-resistant Q cell control. Recently, our method for detecting the Q-cell response using P cell labeling with BrdU and the MN frequency assay was also shown to be applicable to an apoptosis detection assay. Meanwhile, our method for detecting the intratumor Q-cell response was also applicable toward high linear energy transfer radiation, including reactor neutrons. Thus, using our method, a new neutron capture compound that has the potential to be distributed in neutron capture therapy-resistant intratumor Q cell populations is now under development.

Alteration of sensitivity of intratumor quiescent and total cells to gamma-rays following thermal neutron irradiation with or without 10B-compound

PURPOSE

Changes in the sensitivity of intratumor quiescent (Q) and total cells to gamma-rays following thermal neutron irradiation with or without 10B-compound were examined.

METHODS AND MATERIALS

5-Bromo-2'-deoxyuridine (BrdU) was injected to SCC VII tumor-bearing mice intraperitoneally 10 times to label all the proliferating (P) tumor cells. As priming irradiation, thermal neutrons alone or thermal neutrons with 10B-labeled sodium borocaptate (BSH) or dl-p-boronophenylalanine (BPA) were administered. The tumor-bearing mice then received a series of gamma-ray radiation doses, 0 through 24 h after the priming irradiation. During this period, no BrdU was administered. Immediately after the second irradiation, the tumors were excised, minced, and trypsinized. Following incubation of tumor cells with cytokinesis blocker, the micronucleus (MN) frequency in cells without BrdU labeling (= Q cells at the time of priming irradiation) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU before the priming irradiation. To determine the BrdU-labeled cell ratios in the tumors at the time of the second irradiation, each group also included mice that were continuously administered BrdU until just before the second irradiation using mini-osmotic pumps which had been implanted subcutaneously 5 days before the priming irradiation.

RESULTS

In total cells, during the interval between the two irradiations, the tumor sensitivity to gamma-rays relative to that immediately after priming irradiation decreased with the priming irradiation ranking in the following order: thermal neutrons only > thermal neutrons with BSH > thermal neutrons with BPA. In contrast, in Q cells, during that time the sensitivity increased in the following order: thermal neutrons only < thermal neutrons with BSH < thermal neutrons with BPA. The longer the interval between the two irradiations, the higher was the BrdU-labeled cell ratio at the second irradiation. The labeled cell ratio at the same time point after each priming irradiation increased in the following order: thermal neutrons only < thermal neutrons with BSH < thermal neutrons with BPA.

CONCLUSION

These findings indicated that the use of 10B-compound, especially BPA, in thermal neutron irradiation causes the recruitment from the Q to P population.