Evaluation of the genotoxic effects of the boron neutron capture reaction in human melanoma cells using the cytokinesis block micronucleus assay

DNA damage and biological responses induced by Boron Neutron Capture Therapy (BNCT)

Boron Neutron Capture Therapy (BNCT) is a tumor cell selective high LET (linear energy transfer) particle beam therapy. The patient is administrated a boron (¹⁰B) compound via intravenous injection or infusion, and when ¹⁰B is sufficiently accumulated in the tumor, neutron beams containing epithermal neutrons as the main component are irradiated. Epithermal neutrons lose energy in the body and become thermal neutrons. The captured ¹⁰B undergoes a (n, α) reaction with thermal neutrons, and the resulting α particles and ⁷Li nuclei have short ranges of 9-10μm and 4-5μm, respectively, and do not reach the surrounding cells in normal tissues. Therefore, these high LET-heavy charged particles can selectively kill cancer cells. The cell-killing effect of these heavy charged particles is thought to be triggered by DNA damage. It is known that DNA damage caused by heavy charged particles is more serious and difficult to repair than DNA damage caused by Low LET radiation such as X-rays and γ-rays. This review focuses on DNA damage, e.g., DNA strand breaks and DNA damage repair caused by BNCT and describes the resulting biological response.

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.

A multiple endpoint approach reveals potential in vitro anticancer properties of thymoquinone in human renal carcinoma cells

Thymoquinone (TQ) is a monoterpene isolated from the oil of Nigella sativa seeds. The aim of this work was to evaluate the cytotoxic effects induced by TQ and its impact on the migration and invasion potential of 786-O human renal cancer cells. These cells were exposed to TQ (1-100 μM) for 24 and 48 h and cell viability assessed using the Crystal Violet and MTS assays. TQ treatment clearly decreased cell viability in a concentration- and time-dependent manner. TQ exposure moderately increased intracellular ROS levels and co-incubation with reduced glutathione markedly increased cell viability. Moreover, the effect of TQ in the cell cycle distribution was evaluated using flow cytometry, and an increase in the sub-G1 population was observed, especially at 30 μM, along with an increase in the % of apoptotic cells. TQ did not show genotoxic effects at a non-cytotoxic concentration (1.0 μM). At this concentration level, TQ significantly decreased the collective migration of 786-O cells, whereas it had no effect in chemotactic migration. TQ also decreased the invasiveness potential of 786-O cells, as evaluated by the transwell invasion assay. Overall, these results suggest that TQ presents an anticancer potential in the context of renal cancer, warranting further investigation.

Genotoxic effects of boric acid and borax in zebrafish, Danio rerio using alkaline comet assay

The present study is conducted to determine the potential mechanisms of Boron compounds, boric acid (BA) and borax (BX), on genotoxicity of zebrafish Danio rerio for 24, 48, 72 and 96-hours acute exposure (level:1, 4, 16, 64 mg/l BA and BX) in semi-static bioassay experiment. For that purpose, peripheral erythrocytes were drawn from caudal vein and Comet assay was applied to assess genotoxicity. Acute (96 hours) exposure and high concentrations of boric acid and borax increases % tail DNA and Olive tail moment. Genotoxicity was found for BA as concentration-dependent and BX as concentration and time dependent manner. In general, significant effects (P < 0,05) on both concentrations and exposure times were observed in experimental groups. DNA damage was highest at 96 h and 24 h for all BX and BA concentrations, respectively in peripheral blood of D. rerio. For the first time, our study demonstrates the effect of waterborne BA and BX exposure on genotoxicity at the molecular level, which may contribute to understanding the mechanism of boric acid and borax-induced genotoxicity in fish.

Differential effects of methoxyamine on doxorubicin cytotoxicity and genotoxicity in MDA-MB-231 human breast cancer cells

Pharmacological inhibition of DNA repair is a promising approach to increase the effectiveness of anticancer drugs. The chemotherapeutic drug doxorubicin (Dox) may act, in part, by causing oxidative DNA damage. The base excision repair (BER) pathway effects the repair of many DNA lesions induced by reactive oxygen species (ROS). Methoxyamine (MX) is an indirect inhibitor of apurinic/apyrimidinic endonuclease 1 (APE1), a multifunctional BER protein. We have evaluated the effects of MX on the cytotoxicity and genotoxicity of Dox in MDA-MB-231 metastatic breast cancer cells. MX has little effects on the viability and proliferation of Dox-treated cells. However, as assessed by the cytokinesis-block micronucleus assay (CBMN), MX caused a significant 1.4-fold increase (P<0.05) in the frequency of micronucleated binucleated cells induced by Dox, and also altered the distribution of the numbers of micronuclei. The fluorescence probe dihydroethidium (DHE) indicated little production of ROS by Dox. Overall, our results suggest differential outcomes for the inhibition of APE1 activity in breast cancer cells exposed to Dox, with a sensitizing effect observed for genotoxicity but not for cytotoxicity.

Boron neutron capture therapy induces cell cycle arrest and cell apoptosis of glioma stem/progenitor cells in vitro

BACKGROUND

Glioma stem cells in the quiescent state are resistant to clinical radiation therapy. An almost inevitable glioma recurrence is due to the persistence of these cells. The high linear energy transfer associated with boron neutron capture therapy (BNCT) could kill quiescent and proliferative cells.

METHODS

The present study aimed to evaluate the effects of BNCT on glioma stem/progenitor cells in vitro. The damage induced by BNCT was assessed using cell cycle progression, apoptotic cell ratio and apoptosis-associated proteins expression.

RESULTS

The surviving fraction and cell viability of glioma stem/progenitor cells were decreased compared with differentiated glioma cells using the same boronophenylalanine pretreatment and the same dose of neutron flux. BNCT induced cell cycle arrest in the G2/M phase and cell apoptosis via the mitochondrial pathway, with changes in the expression of associated proteins.

CONCLUSIONS

Glioma stem/progenitor cells, which are resistant to current clinical radiotherapy, could be effectively killed by BNCT in vitro via cell cycle arrest and apoptosis using a prolonged neutron irradiation, although radiosensitivity of glioma stem/progenitor cells was decreased compared with differentiated glioma cells when using the same dose of thermal neutron exposure and boronophenylalanine pretreatment. Thus, BNCT could offer an appreciable therapeutic advantage to prevent tumor recurrence, and may become a promising treatment in recurrent glioma.

First evaluation of the biologic effectiveness factors of boron neutron capture therapy (BNCT) in a human colon carcinoma cell line

PURPOSE

DNA lesions produced by boron neutron capture therapy (BNCT) and those produced by gamma radiation in a colon carcinoma cell line were analyzed. We have also derived the relative biologic effectiveness factor (RBE) of the neutron beam of the RA-3- Argentine nuclear reactor, and the compound biologic effectiveness (CBE) values for p-boronophenylalanine ((10)BPA) and for 2,4-bis (α,β-dihydroxyethyl)-deutero-porphyrin IX ((10)BOPP).

METHODS AND MATERIALS

Exponentially growing human colon carcinoma cells (ARO81-1) were distributed into the following groups: (1) BPA (10 ppm (10)B) + neutrons, (2) BOPP (10 ppm (10)B) + neutrons, (3) neutrons alone, and (4) gamma rays ((60)Co source at 1 Gy/min dose-rate). Different irradiation times were used to obtain total absorbed doses between 0.3 and 5 Gy (±10%) (thermal neutrons flux = 7.5 10(9) n/cm(2) sec).

RESULTS

The frequency of micronucleated binucleated cells and the number of micronuclei per micronucleated binucleated cells showed a dose-dependent increase until approximately 2 Gy. The response to gamma rays was significantly lower than the response to the other treatments (p < 0.05). The irradiations with neutrons alone and neutrons + BOPP showed curves that did not differ significantly from, and showed less DNA damage than, irradiation with neutrons + BPA. A decrease in the surviving fraction measured by 3-(4,5-dimetiltiazol-2-il)-2,5-difeniltetrazolium bromide (MTT) assay as a function of the absorbed dose was observed for all the treatments. The RBE and CBE factors calculated from cytokinesis block micronucleus (CBMN) and MTT assays were, respectively, the following: beam RBE: 4.4 ± 1.1 and 2.4 ± 0.6; CBE for BOPP: 8.0 ± 2.2 and 2.0 ± 1; CBE for BPA: 19.6 ± 3.7 and 3.5 ± 1.3.

CONCLUSIONS

BNCT and gamma irradiations showed different genotoxic patterns. To our knowledge, these values represent the first experimental ones obtained for the RA-3 in a biologic model and could be useful for future experimental studies for the application of BNCT to colon carcinoma.

Insight into the mechanisms underlying tumor response to boron neutron capture therapy in the hamster cheek pouch oral cancer model

OBJECTIVE

The therapeutic success of different boron neutron capture therapy (BNCT) protocols employing the hamster cheek pouch oral cancer model has been previously reported by our laboratory. The aim of this study was to explore potential mechanisms of BNCT-induced damage to tumor in terms of potential inhibition in DNA synthesis and induction of apoptosis in the tumors that underwent partial remission following application of the different BNCT protocols in this model.

MATERIALS AND METHODS

We evaluated DNA synthesis employing incorporation of 5-bromo-2'-deoxyuridine as an end-point. Apoptosis was evaluated by immunohistochemistry employing the deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling technique and Bax and Bcl-2 labeling. These studies were performed in tumors that underwent partial remission 1-30 days post-BNCT mediated by boronophenylalanine (BPA), GB-10 (Na(2)(10)B(10)H(10)) or (BPA + GB-10).

RESULTS

BNCT exerted a marked inhibitory effect on DNA synthesis in tumors for all the protocols under study. The inhibitory effect of BPA-BNCT occurred as soon as 1 day post-treatment (P < 0.001). Conversely, the effect of GB-10-BNCT became apparent 7-14 days after therapy (P < 0.001) and was sustained until killed at 30 days post-treatment (P < 0.001). (GB-10 + BPA)-BNCT exerted a rapid and persistent effect, conceivably because of the combined effect of BNCT mediated by both boron compounds. The apoptosis studies did not show differences between the pre-treatment group and any of the BNCT groups.

CONCLUSIONS

One of the mechanisms involved in BNCT-induced tumor control in our model would be an inhibitory effect on DNA synthesis. Apoptosis does not seem to have a significant role in BNCT-induced tumor control in our model.

Optimization of boron neutron capture therapy for the treatment of undifferentiated thyroid cancer

PURPOSE

To analyze the possible increase in efficacy of boron neutron capture therapy (BNCT) for undifferentiated thyroid carcinoma (UTC) by using p-boronophenylalanine (BPA) plus 2,4-bis (alpha,beta-dihydroxyethyl)-deutero-porphyrin IX (BOPP) and BPA plus nicotinamide (NA) as a radiosensitizer of the BNCT reaction.

METHODS AND MATERIALS

Nude mice were transplanted with a human UTC cell line (ARO), and after 15 days they were treated as follows: (1) control, (2) NCT (neutrons alone), (3) NCT plus NA (100 mg/kg body weight [bw]/day for 3 days), (4) BPA (350 mg/kg bw) + neutrons, (5) BPA + NA + neutrons, and (6) BPA + BOPP (60 mg/kg bw) + neutrons. The flux of the mixed (thermal + epithermal) neutron beam was 2.8 x 10(8) n/cm(2)/sec for 83.4 min.

RESULTS

Neutrons alone or with NA caused some tumor growth delay, whereas in the BPA, BPA + NA, and BPA + BOPP groups a 100% halt of tumor growth was observed in all mice at 26 days after irradiation. When the initial tumor volume was 50 mm(3) or less, complete remission was found with BPA + NA (2 of 2 mice), BPA (1 of 4), and BPA + BOPP (7 of 7). After 90 days of complete regression, recurrence of the tumor was observed in BPA + NA (2 of 2) and BPA + BOPP (1 of 7). The determination of apoptosis in tumor samples by measurements of caspase-3 activity showed an increase in the BNCT (BPA + NA) group at 24 h (p < 0.05 vs. controls) and after the first week after irradiation in the three BNCT groups. Terminal transferase dUTP nick end labeling analysis confirmed these results.

CONCLUSIONS

Although NA combined with BPA showed an increase of apoptosis at early times, only the group irradiated after the combined administration of BPA and BOPP showed a significantly improved therapeutic response.

Therapeutic effect of boron neutron capture therapy (BNCT) on field cancerized tissue: Inhibition of DNA synthesis and lag in the development of second primary tumors in precancerous tissue around treated tumors in DMBA-induced carcinogenesis in the hamster cheek pouch oral cancer model

OBJECTIVE

We previously reported the therapeutic success of different BNCT protocols in the treatment of oral cancer, employing the hamster cheek pouch model. The aim of the present study was to evaluate the effect of these BNCT protocols on DNA synthesis in precancerous and normal tissue in this model and assess the potential lag in the development of second primary tumors in precancerous tissue. The data are relevant to potential control of field cancerized tissue and tolerance of normal tissue.

MATERIALS AND METHODS

We evaluated DNA synthesis in precancerous and normal pouch tissue 1-30 days post-BNCT mediated by boronophenylalanine (BPA), GB-10 (Na(2)(10)B(10)H(10)) or (BPA+GB-10) employing incorporation of 5-bromo-2'-deoxyuridine as an end-point. The BNCT-induced potential lag in the development of second primary tumors from precancerous tissue was monitored.

RESULTS

A drastic, statistically significant reduction in DNA synthesis occurred in precancerous tissue as early as 1 day post-BNCT and was sustained at virtually all time-points until 30 days post-BNCT for all the protocols. The histological categories evaluated individually within precancerous tissue (dysplasia, hyperplasia and NUMF [no unusual microscopic features]) responded similarly. DNA synthesis in normal tissue treated with BNCT oscillated around the very low pre-treatment values. A BNCT-induced lag in the development of second primary tumors was observed.

CONCLUSIONS

BNCT induced a drastic fall in DNA synthesis in precancerous tissue that would be associated to the observed lag in the development of second primary tumors. The minimum variations in DNA synthesis in BNCT-treated normal tissue would correlate with the absence of normal tissue radiotoxicity. The present data would support the control of field-cancerized areas by BNCT.

Effect of poly(ADP-ribosyl)ation inhibitors on the genotoxic effects of the boron neutron capture reaction

The boron neutron capture (BNC) reaction results from the interaction of 10B with low-energy thermal neutrons and gives rise to highly damaging lithium and alpha-particles. In this work the genotoxicity caused by the BNC reaction in V79 Chinese hamster cells was evaluated in the presence of poly(ADP-ribosyl)ation inhibitors. Poly(ADP-ribose) polymerase-1 (PARP-1), the most important member of the PARP enzyme family, is considered to be a constitutive factor of the DNA damage surveillance network present in eukaryotic cells, acting through a DNA break sensor function. Inhibition of poly(ADP-ribosyl)ation was achieved with the classical compound 3-aminobenzamide (3-AB), and with two novel and very potent inhibitors, 5-aminoisoquinolinone (5-AIQ) and PJ-34. Dose-response increases in the frequencies of aberrant cells excluding gaps (%ACEG) and chromosomal aberrations excluding gaps per cell (CAEG/cell) were observed for increasing exposures to the BNC reaction. The presence of 3-AB did not increase the %ACEG or CAEG/cell, nor did it change the pattern of the induced chromosomal aberrations. Results with 5-AIQ and PJ-34 were in agreement with the results obtained with 3-AB. We further studied the combined effect of a PARP inhibitor and a DNA-dependent protein kinase (DNA-PK) inhibitors (3-AB and wortmannin, respectively) on the genotoxicity of the BNC reaction, by use of the cytokinesis-block micronucleus assay. DNA-PK is also activated by DNA breaks and binds DNA ends, playing a role of utmost importance in the repair of double-strand breaks. Our results show that the inhibition of poly(ADP-ribosyl)ation does not particularly modify the genotoxicity of the BNC reaction, and that PARP inhibition together with a concomitant inhibition of DNA-PK revealed barely the same sensitizing effect as DNA-PK inhibition per se.

Use of cytogenetic indicators in radiobiology

The study of ionising radiation has systematically relied on cytogenetic indicators to evaluate the biological effects and has led to theoretical approaches to explain observations associated with radiation exposure. In many of the early studies on radiobiology, the induction of chromosomal aberrations was the method of choice to evaluate dose-response relationships. But progressively, this and other cytogenetic biomarkers were used to obtain mechanistic insight on the biological effects induced by radiation. This paper attempts to give a view on the use of cytogenetic indicators in the study of various radiation-related phenomena, including radiation dosimetry, mechanisms involved in the various cellular responses to radiation, such as bystander effects, chromosomal instability and adaptive response, as well as DNA repair pathways. One future direction may involve the use of cytogenetic indicators to evaluate various molecular determinants in individuals' susceptibility to radiation, using other techniques such as fluorescence in situ hybridisation (FISH) and linking them to specific gene functions and single nucleotide polymorphisms.

DNA-PK inhibitor wortmannin enhances DNA damage induced by bleomycin in V79 Chinese hamster cells

The fungal metabolite wortmannin (WM) is a potent and irreversible inhibitor of the enzyme DNA-dependent protein kinase (DNA-PK), a nuclear serine-threonine kinase, member of the phosphaditylinositol-3 kinase related kinase family. WM has been used in the last few years as a promising radiosensitizer mainly throughout cell survival experiments. However, few studies have addressed the role of DNA-PK inhibition in the repair of DNA lesions generated by antitumor agents. Bleomycin (BLM) is an antitumor agent used in the treatment of various neoplasia with a unique genotoxicity profile that mimics the ionizing radiation effects. In this study, we evaluated the effect of different concentrations of WM on the DNA damage induced by BLM. The cytokinesis-block micronucleus assay (CBMN) in V79 Chinese hamster cells was used as the end-point. WM significantly increased the frequency of micronucleated cells (%MNBN) by about 2.2-fold, the number of micronuclei per binucleated cell (MN/BN) by about 2.4-fold, and also changed the pattern of the distribution of micronuclei induced by BLM. The frequency of micronucleated cells with 2 MN per cell and with > or = 3 MN per cell increased, whereas the frequency of micronucleated cells with 1 MN per cell decreased. WM was not genotoxic but decreased cell proliferation as assessed by the frequency of binucleated cells. Our results show that WM clearly enhances the efficacy of BLM in terms of DNA damage inflicted and therefore reinforces its use as a chemosensitizer.

Mutagenic effect of borocaptate sodium and boronophenylalanine in neutron capture therapy

PURPOSE

To investigate the mutagenic effect in boron neutron capture therapy (BNCT), Chinese hamster ovary cells were incubated with 10 B-enriched borocaptate sodium (BSH) or para-boronophenylalanine (BPA) before exposure to thermal neutrons, and the occurrence of mutations at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus was measured.

METHODS AND MATERIALS

BSH or BPA was added to cells 20 h or 2 h before irradiation and removed before irradiation. Cells were irradiated with thermal neutrons. The biologic end point of cell survival was measured by colony formation assay. The mutagenicity was calculated from the mutation frequency at the HPRT locus.

RESULTS

The mutagenicity of BSH and BPA was similar to that of 10B boric acid when the cells were irradiated with neutrons at an isosurvival dose after 2-h preincubation. Preincubation with BSH for 20 h, compared with preincubation for just 2 h, had no effect on either cytotoxicity or mutagenicity in BNCT. However, with BPA, 20-h preincubation, compared with 2-h preincubation, caused an increase in the cell killing effect, but a decrease in the mutagenic effect of the BNCT.

CONCLUSION

After 20-h incubation, BPA was less mutagenic than BSH. The mutagenic study of electroporated BPA or BSH revealed a reduced mutagenicity. These results suggest that the retention of these boron compounds in the cells causes a more accurate assault on the cell and lessens the chance of misrepair after neutron irradiation.