Homogeneous boron targeting of heterogeneous tumors for boron neutron capture therapy (BNCT): chemical analyses in the hamster cheek pouch oral cancer model

Enhanced Resolution of Neutron Autoradiography with UV-C Sensitization to Study Boron Microdistribution in Animal Models

The assessment of boron microdistribution is essential to evaluate the suitability of boron neutron capture therapy (BNCT) in different biological models. In our laboratory, we have reported a methodology to produce cell imprints on polycarbonate through UV-C sensitization. The aim of this work is to extend the technique to tissue samples in order to enhance spatial resolution. As tissue structure largely differs from cultured cells, several aspects must be considered. We studied the influence of the parameters involved in the imprint and nuclear track formation, such as neutron fluence, different NTDs, etching and UV-C exposure times, tissue absorbance, thickness, and staining, among others. Samples from different biological models of interest for BNCT were used, exhibiting homogeneous and heterogeneous histology and boron microdistribution. The optimal conditions will depend on the animal model under study and the resolution requirements. Both the imprint sharpness and the fading effect depend on tissue thickness. While 6 h of UV-C was necessary to yield an imprint in CR-39, only 5 min was enough to observe clear imprints on Lexan. The information related to microdistribution of boron obtained with neutron autoradiography is of great relevance when assessing new boron compounds and administration protocols and also contributes to the study of the radiobiology of BNCT.

Next-Generation Boron Drugs and Rational Translational Studies Driving the Revival of BNCT

BNCT is a high-linear-energy transfer therapy that facilitates tumor-directed radiation delivery while largely sparing adjacent normal tissues through the biological targeting of boron compounds to tumor cells. Tumor-specific accumulation of boron with limited accretion in normal cells is the crux of successful BNCT delivery. Given this, developing novel boronated compounds with high selectivity, ease of delivery, and large boron payloads remains an area of active investigation. Furthermore, there is growing interest in exploring the immunogenic potential of BNCT. In this review, we discuss the basic radiobiological and physical aspects of BNCT, traditional and next-generation boron compounds, as well as translational studies exploring the clinical applicability of BNCT. Additionally, we delve into the immunomodulatory potential of BNCT in the era of novel boron agents and examine innovative avenues for exploiting the immunogenicity of BNCT to improve outcomes in difficult-to-treat malignancies.

Biodistribution of 10B in Glioma Orthotopic Xenograft Mouse Model after Injection of L-para-Boronophenylalanine and Sodium Borocaptate

Boron neutron capture therapy (BNCT) is based on the ability of the boron-10 (¹⁰B) isotope to capture epithermal neutrons, as a result of which the isotope becomes unstable and decays into kinetically active elements that destroy cells where the nuclear reaction has occurred. The boron-carrying compounds—L-para-boronophenylalanine (BPA) and sodium mercaptoundecahydro-closo-dodecaborate (BSH)—have low toxicity and, today, are the only representatives of such compounds approved for clinical trials. For the effectiveness and safety of BNCT, a low boron content in normal tissues and substantially higher content in tumor tissue are required. This study evaluated the boron concentration in intracranial grafts of human glioma U87MG cells and normal tissues of the brain and other organs of mice at 1, 2.5 and 5 h after administration of the boron-carrying compounds. A detailed statistical analysis of the boron biodistribution dynamics was performed to find a ‘window of opportunity’ for BNCT. The data demonstrate variations in boron accumulation in different tissues depending on the compound used, as well as significant inter-animal variation. The protocol of administration of BPA and BSH compounds used did not allow achieving the parameters necessary for the successful course of BNCT in a glioma orthotopic xenograft mouse model.

Effects of Linkers on the Development of Liposomal Formulation of Cholesterol Conjugated Cobalt Bis(dicarbollides)

Boron neutron capture therapy (BNCT) remains an important treatment arm for cancer patients with locally invasive malignant tumors. This therapy needs a significant amount of boron to deposit in cancer tissues selectively, sparing other healthy organs. Most of the liposomes contain water-soluble polyhedral boron salts stay in the core of the liposomes and have low encapsulation efficiency. Thus, modifying the polyhedral boron core to make it hydrophobic and incorporating those into the lipid layer could be one of the ways to increase drug loading and encapsulation efficiency. Additionally, a systematic study about the linker-dependent effect on drug encapsulation and drug-release is lacking, particularly for the liposomal formulation of hydrophobic-drugs. To achieve these goals, liposomal formulations of a series of lipid functionalized cobalt bis(dicarbollide) compounds have been prepared, with the linkers of different hydrophobicity. Hydrophobicity of the linkers have been evaluated through logP calculation and its effect on drug encapsulation and release have been investigated. The liposomes have shown high drug loading, excellent encapsulation efficiency, stability, and non-toxic behavior. Release experiment showed minimal release of drug from liposomes in phosphate buffer, ensuring some amount of drug, associated with liposomes, can be available to tumor tissues for Boron Neutron Capture Therapy.

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.

Translational boron neutron capture therapy (BNCT) studies for the treatment of tumors in lung

PURPOSE

Boron neutron capture therapy (BNCT) combines selective accumulation of ¹⁰B carriers in tumor tissue with subsequent neutron irradiation. BNCT has been proposed for the treatment of multiple, non-resectable, diffuse tumors in lung. The aim of the present study was to evaluate the therapeutic efficacy and toxicity of BNCT in an experimental model of lung metastases of colon carcinoma in BDIX rats and perform complementary survival studies.

MATERIALS AND METHODS

We evaluated tumor control and toxicity in lung 2 weeks post-BNCT at 2 dose levels, including 5 experimental groups per dose level: T0 (euthanized pre-treatment), Boronophenylalanine-BNCT (BPA-BNCT), BPA + Sodium decahydrodecaborate-BNCT ((BPA + GB-10)-BNCT), Beam only (BO) and Sham (no treatment, same manipulation). Tumor response was assessed employing macroscopic and microscopic end-points. An additional experiment was performed to evaluate survival and oxygen saturation in blood.

RESULTS AND CONCLUSIONS

No dose-limiting signs of short/medium-term toxicity were observed in lung. All end-points revealed statistically significant BNCT-induced tumor control vs Sham at both dose levels. The survival experiment showed a statistically significant 45% increase in post-treatment survival time in the BNCT group (48 days) versus Sham (33 days). These data consistently revealed growth suppression of lung metastases by BNCT with no manifest lung toxicity. Highlights Boron Neutron Capture Therapy suppresses growth of experimental lung metastases No BNCT-induced short/medium-term toxicity in lung is associated with tumor control Boron Neutron Capture Therapy increased post-treatment survival time by 45.

Design and synthesis of novel organometallic complexes using boronated phenylalanine derivatives as potential anticancer agents

Drug design and discovery studies are important because of the prevalence of diseases without available medical cures. New anticancer agents are particularly urgent because of the high mortality rate associated with cancer. A series of mononuclear gold (III) and platinum (II) complexes based on boronated phenylalanine (BPA) were designed and synthesized using 4,4'-dimethyl-2,2'-dipyridyl (L1) or 1,10-phenanthroline-5,6-dion (L2) ligands to obtain promising anticancer drug candidates. Proton nuclear magnetic resonance, infrared, mass spectrometry, and elemental analyses were utilized for chemical characterizations. Cell viability, cancer cell colony formation, endothelial tube formation, and cytoskeleton staining assays were performed using A549 lung adenocarcinoma and human umbilical vein endothelial cells (HUVECs) to investigate preliminary pharmacological activities. L1-based platinum (II) complex (BPA-L1-Pt) was the most promising complex, and has similar activity with the approved chemotherapy drug cis-platinum. Half maximal inhibitory concentration values for BPA-L1-Pt were 9.15 µM on A549s and 16.61 µM on HUVECs; the values for cis-platinum were 5.24 µM on A549s and 23.14 µM on HUVECs. Consequently, further synthesis studies should be performed to boost the cancer cell selectivity feature of BPA by varying metal and ligand types.

Neutron autoradiography to study boron compound microdistribution in an oral cancer model

PURPOSE

We previously reported the therapeutic efficacy of Sequential Boron Neutron Capture Therapy (Seq-BNCT), i.e., BPA (boronophenylalanine) - BNCT followed by GB-10 (decahydrodecaborate) - BNCT 1 or 2 days later, in the hamster cheek pouch oral cancer model. We have utilized the neutron autoradiography methodology to study boron microdistribution in tissue. The aim was to use this method to evaluate if the distribution of GB-10 is altered by prior application of BPA-BNCT in Sequential BNCT protocols.

MATERIALS AND METHODS

Extensive qualitative and quantitative autoradiography analyses were performed in the following groups: G1 (animals without boron); G2 (animals injected with BPA); G3 (animals injected with GB-10); G4 (same as G3, 24 h after BPA-BNCT); and G5 (same protocol as G4, 48 h interval).

RESULTS

A detailed study of boron localization in the different tissue structures of tumor, premalignant and normal tissue in the hamster cheek pouch was performed. GB-10 accumulated preferentially in non-neoplastic connective tissue, whereas for BPA neoplastic cells showed the highest boron concentration. Boron distribution was less heterogeneous for GB-10 than for BPA. In premalignant and normal tissue, GB-10 and BPA accumulated mostly in connective tissue and epithelium, respectively.

CONCLUSIONS

BPA-BNCT could alter boron microlocalization of GB-10 administered subsequently. Boron targeting homogeneity is essential for therapeutic success.

Therapeutic efficacy of boron neutron capture therapy mediated by boron-rich liposomes for oral cancer in the hamster cheek pouch model

The application of boron neutron capture therapy (BNCT) mediated by liposomes containing (10)B-enriched polyhedral borane and carborane derivatives for the treatment of head and neck cancer in the hamster cheek pouch oral cancer model is presented. These liposomes are composed of an equimolar ratio of cholesterol and 1,2-distearoyl-sn-glycero-3-phosphocholine, incorporating K[nido-7-CH3(CH2)15-7,8-C2B9H11] (MAC) in the bilayer membrane while encapsulating the hydrophilic species Na3[ae-B20H17NH3] (TAC) in the aqueous core. Unilamellar liposomes with a mean diameter of 83 nm were administered i.v. in hamsters. After 48 h, the boron concentration in tumors was 67 ± 16 ppm whereas the precancerous tissue contained 11 ± 6 ppm, and the tumor/normal pouch tissue boron concentration ratio was 10:1. Neutron irradiation giving a 5-Gy dose to precancerous tissue (corresponding to 21 Gy in tumor) resulted in an overall tumor response (OR) of 70% after a 4-wk posttreatment period. In contrast, the beam-only protocol gave an OR rate of only 28%. Once-repeated BNCT treatment with readministration of liposomes at an interval of 4, 6, or 8 wk resulted in OR rates of 70-88%, of which the complete response ranged from 37% to 52%. Because of the good therapeutic outcome, it was possible to extend the follow-up of BNCT treatment groups to 16 wk after the first treatment. No radiotoxicity to normal tissue was observed. A salient advantage of these liposomes was that only mild mucositis was observed in dose-limiting precancerous tissue with a sustained tumor response of 70-88%.

Boron biodistribution for BNCT in the hamster cheek pouch oral cancer model: combined administration of BSH and BPA

Sodium mercaptoundecahydro-closo-dodecaborate (BSH) is being investigated clinically for BNCT. We examined the biodistribution of BSH and BPA administered jointly in different proportions in the hamster cheek pouch oral cancer model. The 3 assayed protocols were non-toxic, and showed preferential tumor boron uptake versus precancerous and normal tissue and therapeutic tumor boron concentration values (70-85ppm). All 3 protocols warrant assessment in BNCT studies to contribute to the knowledge of (BSH+BPA)-BNCT radiobiology for head and neck cancer and optimize therapeutic efficacy.

Biodistribution of sodium borocaptate (BSH) for boron neutron capture therapy (BNCT) in an oral cancer model

Boron neutron capture therapy (BNCT) is based on selective accumulation of ¹⁰B carriers in tumor followed by neutron irradiation. We previously proved the therapeutic success of BNCT mediated by the boron compounds boronophenylalanine and sodium decahydrodecaborate (GB-10) in the hamster cheek pouch oral cancer model. Based on the clinical relevance of the boron carrier sodium borocaptate (BSH) and the knowledge that the most effective way to optimize BNCT is to improve tumor boron targeting, the specific aim of this study was to perform biodistribution studies of BSH in the hamster cheek pouch oral cancer model and evaluate the feasibility of BNCT mediated by BSH at nuclear reactor RA-3. The general aim of these studies is to contribute to the knowledge of BNCT radiobiology and optimize BNCT for head and neck cancer. Sodium borocaptate (50 mg ¹⁰B/kg) was administered to tumor-bearing hamsters. Groups of 3-5 animals were killed humanely at nine time-points, 3-12 h post-administration. Samples of blood, tumor, precancerous pouch tissue, normal pouch tissue and other clinically relevant normal tissues were processed for boron measurement by optic emission spectroscopy. Tumor boron concentration peaked to therapeutically useful boron concentration values of 24-35 ppm. The boron concentration ratio tumor/normal pouch tissue ranged from 1.1 to 1.8. Pharmacokinetic curves showed that the optimum interval between BSH administration and neutron irradiation was 7-11 h. It is concluded that BNCT mediated by BSH at nuclear reactor RA-3 would be feasible.

Boron delivery with liposomes for boron neutron capture therapy (BNCT): biodistribution studies in an experimental model of oral cancer demonstrating therapeutic potential

Boron neutron capture therapy (BNCT) combines selective accumulation of (10)B carriers in tumor tissue with subsequent neutron irradiation. We previously demonstrated the therapeutic efficacy of BNCT in the hamster cheek pouch oral cancer model. Optimization of BNCT depends largely on improving boron targeting to tumor cells. Seeking to maximize the potential of BNCT for the treatment for head and neck cancer, the aim of the present study was to perform boron biodistribution studies in the oral cancer model employing two different liposome formulations that were previously tested for a different pathology, i.e., in experimental mammary carcinoma in BALB/c mice: (1) MAC: liposomes incorporating K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)] in the bilayer membrane and encapsulating a hypertonic buffer, administered intravenously at 6 mg B per kg body weight, and (2) MAC-TAC: liposomes incorporating K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)] in the bilayer membrane and encapsulating a concentrated aqueous solution of the hydrophilic species Na(3) [ae-B(20)H(17)NH(3)], administered intravenously at 18 mg B per kg body weight. Samples of tumor, precancerous and normal pouch tissue, spleen, liver, kidney, and blood were taken at different times post-administration and processed to measure boron content by inductively coupled plasma mass spectrometry. No ostensible clinical toxic effects were observed with the selected formulations. Both MAC and MAC-TAC delivered boron selectively to tumor tissue. Absolute tumor values for MAC-TAC peaked to 66.6 ± 16.1 ppm at 48 h and to 43.9 ± 17.6 ppm at 54 h with very favorable ratios of tumor boron relative to precancerous and normal tissue, making these protocols particularly worthy of radiobiological assessment. Boron concentration values obtained would result in therapeutic BNCT doses in tumor without exceeding radiotolerance in precancerous/normal tissue at the thermal neutron facility at RA-3.

Boron Neutron Capture Therapy (BNCT) in an oral precancer model: therapeutic benefits and potential toxicity of a double application of BNCT with a six-week interval

Given the clinical relevance of locoregional recurrences in head and neck cancer, we developed a novel experimental model of premalignant tissue in the hamster cheek pouch for long-term studies and demonstrated the partial inhibitory effect of a single application of Boron Neutron Capture Therapy (BNCT) on tumor development from premalignant tissue. The aim of the present study was to evaluate the effect of a double application of BNCT with a 6 week interval in terms of inhibitory effect on tumor development, toxicity and DNA synthesis. We performed a double application, 6 weeks apart, of (1) BNCT mediated by boronophenylalanine (BPA-BNCT); (2) BNCT mediated by the combined application of decahydrodecaborate (GB-10) and BPA [(GB-10+BPA)-BNCT] or (3) beam-only, at RA-3 nuclear reactor and followed the animals for 8 months. The control group was cancerized and sham-irradiated. BPA-BNCT, (GB-10+BPA)-BNCT and beam-only induced a reduction in tumor development from premalignant tissue that persisted until 8, 3, and 2 months respectively. An early maximum inhibition of 100% was observed for all 3 protocols. No normal tissue radiotoxicity was detected. Reversible mucositis was observed in premalignant tissue, peaking at 1 week and resolving by the third week after each irradiation. Mucositis after the second application was not exacerbated by the first application. DNA synthesis was significantly reduced in premalignant tissue 8 months post-BNCT. A double application of BPA-BNCT and (GB-10+BPA)-BNCT, 6 weeks apart, could be used therapeutically at no additional cost in terms of radiotoxicity in normal and dose-limiting tissues.

Boron neutron capture therapy (BNCT) for the treatment of liver metastases: biodistribution studies of boron compounds in an experimental model

We previously demonstrated the therapeutic efficacy of different boron neutron capture therapy (BNCT) protocols in an experimental model of oral cancer. BNCT is based on the selective accumulation of (10)B carriers in a tumor followed by neutron irradiation. Within the context of exploring the potential therapeutic efficacy of BNCT for the treatment of liver metastases, the aim of the present study was to perform boron biodistribution studies in an experimental model of liver metastases in rats. Different boron compounds and administration conditions were assayed to determine which administration protocols would potentially be therapeutically useful in in vivo BNCT studies at the RA-3 nuclear reactor. A total of 70 BDIX rats were inoculated in the liver with syngeneic colon cancer cells DHD/K12/TRb to induce the development of subcapsular tumor nodules. Fourteen days post-inoculation, the animals were used for biodistribution studies. We evaluated a total of 11 administration protocols for the boron compounds boronophenylalanine (BPA) and GB-10 (Na(2)(10)B(10)H(10)), alone or combined at different dose levels and employing different administration routes. Tumor, normal tissue, and blood samples were processed for boron measurement by atomic emission spectroscopy. Six protocols proved potentially useful for BNCT studies in terms of absolute boron concentration in tumor and preferential uptake of boron by tumor tissue. Boron concentration values in tumor and normal tissues in the liver metastases model show it would be feasible to reach therapeutic BNCT doses in tumor without exceeding radiotolerance in normal tissue at the thermal neutron facility at RA-3.

Biodistribution of (10)B for Boron Neutron Capture Therapy (BNCT) in a mouse model after injection of sodium mercaptoundecahydro-closo-dodecaborate and l-para-boronophenylalanine

In boron neutron capture therapy, the absorbed dose from the (10)B(n,alpha)(7)Li reaction depends on the (10)B concentration and (10)B distribution in the irradiated volume. Thus compounds used in BNCT should have tumor-specific uptake and low accumulation in normal tissues. This study compares in a mouse model the (10)B uptake in different organs as delivered by l-para-boronophenylalanine (BPA, 700 mg/kg body weight, i.p.) and/or sodium mercaptoundecahydro-closo-dodecaborate (BSH, 200 mg/kg body weight, i.p). After BSH injection, the (10)B concentration was high in kidneys (20 +/- 12 microg/g) and liver (20 +/- 12 microg/g) but was low in brain (1.0 +/- 0.8 microg/g) and muscle (1.9 +/- 1.2 microg/g). After BPA injection, the (10)B concentration was high in kidneys (38 +/- 25 microg/g) and spleen (17 +/- 8 microg/g) but low in brain (5 +/- 3 microg/g). After combined BPA and BSH injection, the effect on the absolute (10)B concentration was additive in all organs. The ratio of the (10)B concentrations in tissues and blood differed significantly for the two compounds depending on the compound combination, which implies a different uptake profile for normal organs.

EORTC trial 11001: distribution of two 10B-compounds in patients with squamous cell carcinoma of head and neck, a translational research/phase 1 trial

Boron neutron capture therapy (BNCT) provides highly targeted delivery of radiation through the limited spatial distribution of its effects. This translational research/phase I clinical trial investigates whether BNCT might be developed as a treatment option for squamous cell carcinoma of head and neck (SCCHN) relying upon preferential uptake of the two compounds, sodium mercaptoundecahydro-closo-dodecaborate (BSH) or L-para-boronophenylalanine (BPA) in the tumour. Before planned tumour resection, three patients received BSH and three patients received BPA. The (10)B-concentration in tissues and blood was measured with prompt gamma ray spectroscopy. Adverse effects from compounds did not occur. After BPA infusion the (10)B-concentration ratio of tumour/blood was 4.0 +/- 1.7. (10)B-concentration ratios of tumour/normal tissue were 1.3 +/- 0.5 for skin, 2.1 +/- 1.2 for muscle and 1.4 +/- 0.01 for mucosa. After BSH infusion the (10)B-concentration ratio of tumour/blood was 1.2 +/- 0.4. (10)B-concentration ratios of tumour/normal tissue were 3.6 +/- 0.6 for muscle, 2.5 +/- 1.0 for lymph nodes, 1.4 +/- 0.5 for skin and 1.0 +/- 0.3 for mucosa. BPA and BSH deliver (10)B to SCCHN to an extent that might allow effective BNCT treatment. Mucosa and skin are the most relevant organs at risk.

Effect of Boron Neutron Capture Therapy (BNCT) on normal liver regeneration: Towards a novel therapy for liver metastases

PURPOSE

The effect of Boron Neutron Capture Therapy (BNCT) on normal liver regeneration was examined in the Wistar rat. The model used is clinically relevant to a novel technique proposed for the treatment of multifocal non-resectable liver metastases in man. The success of the technique also requires that BNCT should not significantly impair regeneration of normal hepatocytes.

MATERIALS AND METHODS

The effect of therapeutic doses of boronophenylalanine (BPA), GB-10 (Na(2)(10)B(10)H(10)) and (GB-10 + BPA) and of BNCT mediated by these boron delivery agents on normal liver regeneration and liver function in the Wistar rat was examined using partial hepatectomy as the regenerative stimulus. The end-points evaluated were body weight, liver weight/body weight ratio, DNA synthesis in terms of 5-bromo-2'-deoxyuridine incorporation, hemogram, kidney function in terms of blood urea nitrogen and creatinine levels, liver function in terms of serum albumin, total and direct bilirubin and liver enzymes (alanine transaminase and aspartate transaminase) and liver histology/architecture.

RESULTS

BNCT mediated by BPA, GB-10 or (GB-10 + BPA) did not cause alterations in the outcome of normal liver regeneration, regenerated liver function/proliferation or histology/architecture.

CONCLUSION

The BNCT protocols, at the physical doses selected, did not impair the capacity of normal liver hepatocytes to regenerate.

Dosimetry and radiobiology at the new RA-3 reactor boron neutron capture therapy (BNCT) facility: application to the treatment of experimental oral cancer

The National Atomic Energy Commission of Argentina (CNEA) constructed a novel thermal neutron source for use in boron neutron capture therapy (BNCT) applications at the RA-3 research reactor facility located in Buenos Aires. The aim of the present study was to perform a dosimetric characterization of the facility and undertake radiobiological studies of BNCT in an experimental model of oral cancer in the hamster cheek pouch. The free-field thermal flux was 7.1 x 10(9) n cm(-2)s(-1) and the fast neutron flux was 2.5 x 10(6) n cm(-2)s(-1), indicating a very well-thermalized neutron field with negligible fast neutron dose. For radiobiological studies it was necessary to shield the body of the hamster from the neutron flux while exposing the everted cheek pouch bearing the tumors. To that end we developed a lithium (enriched to 95% in (6)Li) carbonate enclosure. Groups of tumor-bearing hamsters were submitted to BPA-BNCT, GB-10-BNCT, (GB-10+BPA)-BNCT or beam only treatments. Normal (non-cancerized) hamsters were treated similarly to evaluate normal tissue radiotoxicity. The total physical dose delivered to tumor with the BNCT treatments ranged from 6 to 8.5 Gy. Tumor control at 30 days ranged from 73% to 85%, with no normal tissue radiotoxicity. Significant but reversible mucositis in precancerous tissue surrounding tumors was associated to BPA-BNCT. The therapeutic success of different BNCT protocols in treating experimental oral cancer at this novel facility was unequivocally demonstrated.

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.

Boron neutron capture therapy (BNCT) for the treatment of spontaneous nasal planum squamous cell carcinoma in felines

Recently, Boron neutron capture therapy (BNCT) was successfully applied to treat experimental squamous cell carcinomas (SCC) of the hamster cheek pouch mucosa, with no damage to normal tissue. It was also shown that treating spontaneous nasal planum SCC in terminal feline patients with low dose BNCT is safe and feasible. In an extension of this work, the present study aimed at evaluation of the response of tumor and dose-limiting normal tissues to potentially therapeutic BNCT doses. Biodistribution studies with (10)B-boronophenylalanine (BPA enriched in (10)B) as a (10)B carrier were performed on three felines that showed advanced nasal planum SCC without any standard therapeutic option. Following the biodistribution studies, BNCT mediated by (10)BPA was done using the thermalized epithermal neutron beam at the RA-6 Nuclear Reactor. Follow-up included clinical evaluation, assessment of macroscopic tumor and normal tissue response and biopsies for histopathological analysis. The treated animals did not show any apparent radiation-induced toxicity. All three animals exhibited partial tumor control and an improvement in clinical condition. Enhanced therapeutic efficacy was associated with a high (10)B content of the tumor and a small tumor size. BNCT is therefore believed to be potentially effective in the treatment of spontaneous SCC. However, improvement in targeting (10)B into all tumor cells and delivering a sufficient dose at a greater depth are still required for the treatment of deep-seated, large tumors. Future studies are needed to evaluate the potential efficacy of the dual mode cellular (e.g. BPA-BNCT) and vascular (e.g. GB-10-BNCT) targeting protocol in a preclinical scenario, employing combinations of (10)B compounds with different properties and complementary uptake mechanisms.

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.