Electroporation increases the effect of borocaptate (10B-BSH) in neutron capture therapy

Enhancement in the Therapeutic Efficacy of In Vivo BNCT Mediated by GB-10 with Electroporation in a Model of Oral Cancer

Boron neutron capture therapy (BNCT) combines preferential tumor uptake of ¹⁰B compounds and neutron irradiation. Electroporation induces an increase in the permeability of the cell membrane. We previously demonstrated the optimization of boron biodistribution and microdistribution employing electroporation (EP) and decahydrodecaborate (GB-10) as the boron carrier in a hamster cheek pouch oral cancer model. The aim of the present study was to evaluate if EP could improve tumor control without enhancing the radiotoxicity of BNCT in vivo mediated by GB-10 with EP 10 min after GB-10 administration. Following cancerization, tumor-bearing hamster cheek pouches were treated with GB-10/BNCT or GB-10/BNCT + EP. Irradiations were carried out at the RA-3 Reactor. The tumor response and degree of mucositis in precancerous tissue surrounding tumors were evaluated for one month post-BNCT. The overall tumor response (partial remission (PR) + complete remission (CR)) increased significantly for protocol GB-10/BNCT + EP (92%) vs. GB-10/BNCT (48%). A statistically significant increase in the CR was observed for protocol GB-10/BNCT + EP (46%) vs. GB-10/BNCT (6%). For both protocols, the radiotoxicity (mucositis) was reversible and slight/moderate. Based on these results, we concluded that electroporation improved the therapeutic efficacy of GB-10/BNCT in vivo in the hamster cheek pouch oral cancer model without increasing the radiotoxicity.

Electroporation optimizes the uptake of boron-10 by tumor for boron neutron capture therapy (BNCT) mediated by GB-10: a boron biodistribution study in the hamster cheek pouch oral cancer model

Boron neutron capture therapy (BNCT) is a promising cancer binary therapy modality that utilizes the nuclear capture reaction of thermal neutrons by boron-10 resulting in a localized release of high- and low-linear energy transfer (LET) radiation. Electrochemotherapy (ECT) is based on electroporation (EP) that induces opening of pores in cell membranes, allowing the entry of compounds. Because EP is applied locally to a tumor, the compound is incorporated preferentially by tumor cells. Based on the knowledge that the therapeutic success of BNCT depends centrally on the boron content in tumor and normal tissues and that EP has proven to be an excellent facilitator of tumor biodistribution of an anti-tumor agent, the aim of this study was to evaluate if EP can optimize the delivery of boronated compounds. We performed biodistribution studies and qualitative microdistribution analyses of boron employing the boron compound sodium decahydrodecaborate (GB-10) + EP in the hamster cheek pouch oral cancer model. Syrian hamsters with chemically induced exophytic squamous cell carcinomas were used. A typical EP treatment was applied to each tumor, varying the moment of application with respect to the administration of GB-10 (early or late). The results of this study showed a significant increase in the absolute and relative tumor boron concentration and optimization of the qualitative microdistribution of boron by the use of early EP + GB-10 versus GB-10 without EP. This strategy could be a tool to improve the therapeutic efficacy of BNCT/GB-10 in vivo.

An analysis of the structure of the compound biological effectiveness factor

This report is an analysis of the structure of the compound biological effectiveness (CBE) factor. The value of the CBE factor previously reported was revalued for the central nervous system, skin and lung. To describe the structure, the following terms are introduced: the vascular CBE (v-CBE), intraluminal CBE (il-CBE), extraluminal CBE (el-CBE) and non-vascular CBE (nv-CBE) factors and the geometric biological factor (GBF), i.e. the contributions that are derived from the total dose to the vasculature, each dose to vasculature from the intraluminal side and the extraluminal side, the dose to the non-vascular tissue and the factor to calculate el-CBE from il-CBE, respectively. The el-CBE factor element was also introduced to relate il-CBE to el-CBE factors. A CBE factor of 0.36 for disodium mercaptoundecahydrododecaborate (BSH) for the CNS was independent of the (10)B level in the blood; however, that for p-Boron-L-phenylalanine (BPA) increased with the (10)B level ratio of the normal tissue to the blood (N/B). The CBE factor was expressed as follows: factor = 0.32 + N/B × 1.65. The factor of 0.32 at 0 of N/B was close to the CBE factor for BSH. GBFs had similar values, between BSH and BPA, 1.39 and 1.52, respectively. The structure of the CBE factor for BPA to the lung was also elucidated based on this idea. The factor is described as follows: CBE factor = 0.32 + N/B × 1.80. By this elucidation of the structure of the CBE factor, it is expected that basic and clinical research into boron neutron capture therapy will progress.

Therapy region monitoring based on PET using 478 keV single prompt gamma ray during BNCT: A Monte Carlo simulation study

We confirmed the feasibility of using our proposed system to extract two different kinds of functional images from a positron emission tomography (PET) module by using an insertable collimator during boron neutron capture therapy (BNCT). Coincidence events from a tumor region that included boron particles were identified by a PET scanner before BNCT; subsequently, the prompt gamma ray events from the same tumor region were collected after exposure to an external neutron beam through an insertable collimator on the PET detector. Five tumor regions that contained boron particles and were located in the water phantom and in the BNCT system with the PET module were simulated with Monte Carlo simulation code. The acquired images were quantitatively analyzed. Based on the receiver operating characteristic (ROC) curves in the five boron regions, A, B, C, D, and E, the PET and single-photon images were 10.2%, 11.7%, 8.2% (center region), 12.6%, and 10.5%, respectively. We were able to acquire simultaneously PET and single prompt photon images for tumor regions monitoring by using an insertable collimator without any additional isotopes.

A Bystander Effect Observed in Boron Neutron Capture Therapy: A Study of the Induction of Mutations in the HPRT Locus

PURPOSE

To investigate bystander mutagenic effects induced by alpha-particles during boron neutron capture therapy, we mixed cells that were electroporated with borocaptate sodium (BSH), which led to the accumulation of (10)B inside the cells, and cells that did not contain the boron compound. The BSH-containing cells were irradiated with alpha-particles produced by the 10B(n,alpha)7Li reaction, whereas cells without boron were affected only by the 1H(n,gamma)2H and 14N(n,rho)14C reactions.

METHODS AND MATERIALS

The lethality and mutagenicity measured by the frequency of mutations induced in the hypoxanthine-guanine phosphoribosyltransferase locus were examined in Chinese hamster ovary cells irradiated with neutrons (Kyoto University Research Reactor: 5 MW). Neutron irradiation of 1:1 mixtures of cells with and without BSH resulted in a survival fraction of 0.1, and the cells that did not contain BSH made up 99.4% of the resulting cell population. The molecular structures of the mutations were determined using multiplex polymerase chain reactions.

RESULTS

Because of the bystander effect, the frequency of mutations increased in the cells located nearby the BSH-containing cells compared with control cells. Molecular structural analysis indicated that most of the mutations induced by the bystander effect were point mutations and that the frequencies of total and partial deletions induced by the bystander effect were less than those induced by the original neutron irradiation.

CONCLUSION

These results suggested that in boron neutron capture therapy, the mutations caused by the bystander effect and those caused by the original neutron irradiation are induced by different mechanisms.

A simple model for quantification of the radiobiological effectiveness of the 10B(n,α)7Li capture reaction in BNCT

A simple model has been developed for predicting radiobiological effectiveness of the neutron capture reaction in boron neutron capture therapy. This model was derived from the relationship between the cell survival from the boron capture reaction, the intracellular boron concentration, and the thermal neutron fluence. We found that the cell-killing effect of the boron capture reaction was well described using a power function of the intracellular boron concentration. Hence the relationship between cell survival from the boron capture reaction, intracellular boron concentration, and the thermal neutron fluence could be determined using a simple mathematical equation. We consider that our current approach is more appropriate and realistic than the conventional theoretical mathematical model used to estimate the radiobiological effectiveness of the neutron capture reaction in boron neutron capture therapy.

Tissue disposition of 5-o-carboranyluracil--a novel agent for the boron neutron capture therapy of prostate cancer

The carboranyl nucleotides beta-D-5-o-carboranyl-2'-deoxyuridine (D-CDU), 1-(beta-L-arabinosyl)-5-o-carboranyluracil (D-ribo-CU) and the nucleotide base 5-o-carboranyluracil (CU), were developed as sensitizers for boron neutron capture therapy (BNCT). A structure activity study was initiated to determine the agent most suitable for targeting prostate tumors. Cellular accumulation studies were performed using LNCaP human prostate tumor cells, and the respective tumor disposition profiles were investigated in male nude mice bearing LNCaP and 9479 human prostate tumor xenografts in their flanks. D-CDU achieved high cellular concentrations in LNCaP cells and up to 2.5% of the total cellular compound was recovered in the 5'-monophosphorylated form. In vivo concentrations of D-CDU were similar in LNCaP and 9479 tumor xenografts. Studies in 9479 xenografted bearing mice indicated that increasing the number of hydroxyl groups in the sugar moeity of the carboranyl nucleosides corresponded with an increased rate and extent of renal elimination, shorter serum half-lives and an increased tissue specificity. Tumor/normal prostate ratios were greatest with the nucleoside base CU. These studies indicate that similar nucleoside analogues and bases may have different tissue affinities and retention properties, which should be considered when selecting agents for sensitizing specific tumors for eventual BNCT treatment. CU was found to be the most suitable compound for further development to treat prostate cancer.

Delivery of 10boron to oral squamous cell carcinoma using boronophenylalanine and borocaptate sodium for boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a unique radiation therapy in which boron compounds are trapped into tumor cells. To determine the biodistribution of boronophenylalanine (BPA) in nude mice carrying oral squamous cell carcinoma (SCC), BPA was administered at a dose of 250 mg/kg body weight intraperitoneally. Two hours later, (10)B concentration in the tumor was 15.96 ppm and tumor/blood, tumor/tongue, tumor/skin and tumor/bone (10)B concentration ratios were 6.44, 4.19, 4.68 and 4.56, respectively. Two hours after the administration of borocaptate sodium (BSH) at a dose of 75 mg/kg body weight, (10)B concentration in the tumor was 3.61 ppm, and tumor/blood, tumor/tongue, tumor/skin and tumor/bone (10)B concentration ratios were 0.77, 1.05, 0.60 and 0.59, respectively. When cultured oral SCC cells were incubated with BPA or BSH for 2 h and then exposed to thermal neutrons, the proportion of survival cells that were capable of forming cell colonies decreased exponentially, depending on (10)B concentration. BPA-mediated BNCT was more efficient than BSH-mediated BNCT. Addition of boron compounds in the cell suspension during neutron irradiation enhanced the cell-killing effect of the neutrons. These results indicate that BPA is more selectively incorporated into human oral SCC as compared with normal oral tissues, and that both extra- and intra-cellular BPA contribute to the cell-killing effect of BNCT. BPA may be a useful boron carrier for BNCT in the treatment of advanced oral SCC.

Synthesis and application of functional peptides as cell nucleus-directed molecules in the treatment of malignant diseases

The unique functions of biomolecules, including transport across biological membranes (e.g. the cell membrane, the nuclear envelope), modulation of protein function, gene transcription, reconstitution of the malignant transformation, and viral, bacterial and fungal activities underlie a high pharmaceutical potential. The development of combinatorial functional peptide modules in this important area has been slow, in contrast to the rapid development in the synthesis of small biopolymers. The conjugation of a short transmembrane transport peptide module with a cell nucleus address peptide module and with any substance is attractive for preparation of BioShuttle-based peptides because of the well-established automated synthesis of peptides. Variation of the different functional modules for drug targeting and the choice of substances can be combined to create novel bioconjugates with unique properties. This article provides an overview of previous work on the BioShuttle technology and outlines the promising use of this approach in combinatorial peptide synthesis and drug discovery.

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.

The combined effect of electroporation and borocaptate in boron neutron capture therapy for murine solid tumors

10 B-Enriched borocaptate (BSH) was administered intraperitoneally to SCCVII tumor-bearing C3H / He mice. Electroporation (EP) was conducted by using a tweezers-type electrode. The (10) B contents in tumors were measured by prompt gamma-ray spectrometry. The colony formation assay was applied to investigate the antitumor effects of boron neutron capture therapy (BNCT) and thereby to estimate the intratumor localization of BSH. The (10) B concentrations in tumors decreased with time following BSH administration, falling to 5.4(0. 1) ppm at 3 h, whereas EP treatment (3 repetitions) 15 min after BSH injection delayed the clearance of BSH from tumors, and the (10) B level remained at 19.4(0.9) ppm at 3 h. The effect of BNCT increased with the (10) B concentration in tumors, and the combination with EP showed a remarkably large cell killing effect even at 3 h after BSH injection. The effect of BNCT, i.e., slope coefficient of the cell survival curve of tumors, without EP was proportional to tumor (10) B level (r = 0.982), and that of BSH-BNCT combined with EP lay close to the same correlation line. However, tumors subjected to EP after BSH injection did not show high radiosensitivity when irradiated after conversion to a single cell suspension by enzymatic digestion. This indicates that the increase of the BNCT effect by EP was a consequence of enclosure of BSH in the interstitial space of tumor tissue and not within tumor cells. This is different from a previous in vitro study. The combination of EP and BNCT may be clinically useful, if a procedure to limit EP to the tumor region becomes available or if an alternative similar method is employed.