Pharmacokinetics of 10B-p-boronophenylalanine in tumours, skin and blood of melanoma patients: a study of boron neutron capture therapy for malignant melanoma

Fluorinated BPA derivatives enhanced 10B delivery in tumors

Boron neutron capture therapy (BNCT) is an emerging approach for treating malignant tumors with binary targeting. However, its clinical application has been hampered by insufficient 10B accumulation in tumors and low 10B concentration ratios of tumor-to-blood (T/B) and tumor-to-normal tissue (T/N). Herein, we developed fluorinated BPA derivatives with different fluorine groups as boron delivery agents for enabling sufficient 10B accumulation in tumors and enhancing T/B and T/N ratios. Our findings demonstrated that fluorinated BPA derivatives had good biological safety. Furthermore, fluorinated BPA derivatives showed improved 10B accumulation in tumors and enhanced T/B and T/N ratios compared to the clinical boron drug fructose-BPA (f-BPA). In particular, in B16-F10 tumor-bearing mice, fluorinated BPA derivatives met the requirements for clinical BNCT even at half of the clinical dose. Thus, fluorinated BPA derivatives are potentially effective boron delivery agents for clinical BNCT in melanoma.

A transdermal drug delivery system based on dissolving microneedles for boron neutron capture therapy of melanoma

Boron neutron capture therapy (BNCT) is a promising therapy for malignant tumors that requires selective and high concentrations of 10B accumulation in tumor cells. Despite ongoing developments in novel boron agents and delivery carriers, the progress and clinical application of BNCT is still restricted by the low 10B accumulation and tumor-to-normal tissue (T/N) ratio. Herein, a dissolving microneedle-based transdermal drug delivery system was specifically designed for BNCT in a mouse model of melanoma. By incorporating fructose-BPA (F-BPA) into PVA microneedle tips, this system successfully delivered sufficient F-BPA into the melanoma site after the application of only two patches. Notably, the T/N ratio achieved through the treatment combining PVA/F-BPA MNs with BNCT (PVA/F-BPA MNs-BNCT) surpassed 93.16, signifying a great improvement. Furthermore, this treatment approach effectively inhibited tumor growth and significantly enhanced the survival rate of the mice. In brief, our study introduces a novel, simple, and efficient administration strategy for BNCT, opening new possibilities for the design of nanomedicine for BNCT.

Acral cutaneous malignant melanoma treated with linear accelerator-based boron neutron capture therapy system: a case report of first patient

This study reports the first patient treatment for cutaneous malignant melanoma using a linear accelerator-based boron neutron capture therapy (BNCT) system. A single-center open-label phase I clinical trial had been conducted using the system since November 2019. A patient with a localized node-negative acral malignant melanoma and the largest diameter of the tumor ≤ 15 cm who refused primary surgery and chemotherapy was enrolled. After administering boronophenylalanine (BPA), a single treatment of BNCT with the maximum dose of 18 Gy-Eq delivered to the skin was performed. The safety and efficacy of the accelerator-based BNCT system for treating localized cutaneous malignant melanoma were evaluated. The first patient with cutaneous malignant melanoma in situ on the second finger of the left hand did not develop dose-limiting toxicity in the clinical trial. After BNCT, the treatment efficacy was gradually observed, and the patient achieved PR within 6 months and CR within 12 months. Moreover, during the follow-up period of 12 months after BNCT, the patient did not exhibit a recurrence without any treatment-related grade 2 or higher adverse events. Although grade 1 adverse events of dermatitis, dry skin, skin hyperpigmentation, edema, nausea, and aching pain were noted in the patient, those adverse events were relieved without any treatment. This case report shows that the accelerator-based BNCT may become a promising treatment modality for cutaneous malignant melanoma. We expect further clinical trials to reveal the efficacy and safety of the accelerator-based BNCT for cutaneous malignant melanoma.

Boron-peptide conjugates with angiopep-2 for boron neutron capture therapy

Boron neutron capture therapy (BNCT) induces intracellular nuclear reaction to destroy cancer cells during thermal neutron irradiation. To selectively eliminate cancer cells but avoid harmful effects on normal tissues, novel boron-peptide conjugates with angiopep-2, namely ANG-B, were constructed and evaluated in preclinical settings. Boron-peptide conjugates were synthesized using solid-phase peptide synthesis, and the molecular mass was validated by mass spectrometry afterwards. Boron concentrations in 6 cancer cell lines and an intracranial glioma mouse model after treatments were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Phenylalanine (BPA) was tested in parallel for comparison. In vitro treatment with boron delivery peptides significantly increased boron uptake in cancer cells. BNCT with 5 mM ANG-B caused 86.5% ± 5.3% of clonogenic cell death, while BPA at the same concentration caused 73.3% ± 6.0% clonogenic cell death. The in vivo effect of ANG-B in an intracranial glioma mouse model was evaluated by PET/CT imaging at 31 days after BNCT. The mouse glioma tumours in the ANG-B-treated group were shrunk by 62.9% on average, while the BPA-treated tumours shrank by only 23.0%. Therefore, ANG-B is an efficient boron delivery agent, which has low cytotoxicity and high tumour-to-blood ratio. Based on these experimental results, we expected that ANG-B may leverage BNCT performance in clinical applications in future.

Evaluation of the Key Advantages between Two Modalities of Boronophenylalanine Administration for Clinical Boron Neutron Capture Therapy Using an Animal Model

In clinical boron neutron capture therapy (BNCT), boronophenylalanine (BPA) administrations through one-step infusion (OSI) and two-step infusion (TSI) are the most widely used. This study compared the advantages of OSI and TSI using a human oral squamous cell carcinoma-bearing animal model. OSI was administered at a high-dose rate of 20 mg/kg/min for 20 min (total dose: 400 mg/kg) as the first step infusion. TSI was a prolonged infusion at a low-dose rate of 1.67 mg/kg/min for 15, 30, 45, and 60 min (total dose: 25, 50, 75, and 100 mg/kg) following the first step infusion. The sigmoid Emax model was used to evaluate the boron accumulation effect in the tumor. The advantages of TSI were observed to be greater than those of OSI. The observed advantages of TSI were as follows: a stable level of boron concentration in blood; tumor to blood boron ratio (T/B); tumor to muscle boron ratio (T/M); and skin to blood boron ratio (S/B). The boron accumulation effect in tumors increased to 68.98%. Thus, effective boron concentration in these tumor cells was achieved to enhance the lethal damage in BNCT treatment. Boron concentration in the blood was equal to that in the skin. Therefore, the equivalent dose was accurately estimated for the skin.

Evaluation of Pharmacokinetics of Boronophenylalanine and Its Uptakes in Gastric Cancer

Boron neutron capture therapy (BNCT), a cellular-level particle radiation therapy, combines boron compounds selectively delivered to tumor tissue with neutron irradiation. Boronophenylalanine (BPA) is a boron compound widely used in malignant melanoma, malignant brain tumors, and recurrent head and neck cancer. However, neither basic nor clinical research was reported for the treatment of gastric cancer using BPA. Selective distribution of boron in tumors rather than that in blood or normal tissue prior to neutron irradiation is required for the successful treatment of BNCT. This study evaluated the pharmacokinetics and safety of ¹⁰B-labeled BPA (¹⁰B-BPA, abbreviated as BPA) and its uptakes in gastric cancer. Pharmacokinetics and safety were evaluated in Sprague–Dawley (SD) rats intravenously injected with BPA. The uptakes of boron in gastric cancer cell line MKN45 and in cell-derived xenografts (CDX) and patient-derived xenografts (PDX) animal models were measured. The results showed that the boron concentration in the blood of rats decreased fast in the first 30 min followed by a steady decrease following the observation time, having a half-life of 44.11 ± 8.90 min and an AUC-last of 815.05 ± 62.09 min×μg/ml. The distribution of boron in different tissues (heart, liver, lung, stomach, and small intestine) of rats revealed a similar pattern in blood except for that in the brain, kidney, and bladder. In MKN45 cells, boron concentration increased in a time- and concentration-dependent manner. In both CDX and PDX animal models, the boron is preferentially distributed in tumor tissue rather than in blood or normal tissues. In addition, BPA had no significant adverse effects in rats. Taken together, the results suggested that BPA revealed a fast decrease in boron concentration in rats and is more likely to distribute in tumor cells and tissue.

Key biological mechanisms involved in high-LET radiation therapies with a focus on DNA damage and repair

DNA damage and repair studies are at the core of the radiation biology field and represent also the fundamental principles informing radiation therapy (RT). DNA damage levels are a function of radiation dose, whereas the type of damage and biological effects such as DNA damage complexity, depend on radiation quality that is linear energy transfer (LET). Both levels and types of DNA damage determine cell fate, which can include necrosis, apoptosis, senescence or autophagy. Herein, we present an overview of current RT modalities in the light of DNA damage and repair with emphasis on medium to high-LET radiation. Proton radiation is discussed along with its new adaptation of FLASH RT. RT based on α-particles includes brachytherapy and nuclear-RT, that is proton-boron capture therapy (PBCT) and boron-neutron capture therapy (BNCT). We also discuss carbon ion therapy along with combinatorial immune-based therapies and high-LET RT. For each RT modality, we summarise relevant DNA damage studies. Finally, we provide an update of the role of DNA repair in high-LET RT and we explore the biological responses triggered by differential LET and dose.

Response of Normal Tissues to Boron Neutron Capture Therapy (BNCT) with 10B-Borocaptate Sodium (BSH) and 10B-Paraboronophenylalanine (BPA)

Boron neutron capture therapy (BNCT) is a cancer-selective radiotherapy that utilizes the cancer targeting ¹⁰B-compound. Cancer cells that take up the compound are substantially damaged by the high liner energy transfer (LET) particles emitted mainly from the ¹⁰B(n, α⁷Li reaction. BNCT can minimize the dose to normal tissues, but it must be performed within the tolerable range of normal tissues. Therefore, it is important to evaluate the response of normal tissues to BNCT. Since BNCT yields a mixture of high and low LET radiations that make it difficult to understand the radiobiological basis of BNCT, it is important to evaluate the relative biological effectiveness (RBE) and compound biological effectiveness (CBE) factors for assessing the responses of normal tissues to BNCT. BSH and BPA are the only ¹⁰B-compounds that can be used for clinical BNCT. Their biological behavior and cancer targeting mechanisms are different; therefore, they affect the CBE values differently. In this review, we present the RBE and CBE values of BPA or BSH for normal tissue damage by BNCT irradiation. The skin, brain (spinal cord), mucosa, lung, and liver are included as normal tissues. The CBE values of BPA and BSH for tumor control are also discussed.

Boron Neutron Capture Therapy (BNCT) for Cutaneous Malignant Melanoma Using 10B-p-Boronophenylalanine (BPA) with Special Reference to the Radiobiological Basis and Clinical Results

BNCT is a radiotherapeutic method for cancer treatment that uses tumor-targeting 10B-compounds. BNCT for cutaneous melanoma using BPA, a phenylalanine derivative, was first initiated by Mishima et al. in 1987. This article reviews the radiobiological basis of melanoma control and damage to normal tissues as well as the results of clinical studies. Experimental studies showed that the compound biological effectiveness (CBE) values of the 10B (n, α)7Li reaction for melanoma control ranged from 2.5 to 3.3. The CBE values of the 10B (n, α)7Li reaction for skin damage ranged from 2.4 to 3.7 with moist desquamation as the endpoint. The required single radiation dose for controlling human melanoma was estimated to be 25 Gy-Eq or more by analyzing the 50% tumor control dose data of conventional fractionated radiotherapy. From the literature, the maximum permissible dose to human skin by single irradiation was estimated to be 18 Gy-Eq. With respect to the pharmacokinetics of BPA in patients with melanoma treated with 85-350 mg/kg BPA, the melanoma-to-blood ratio ranged from 2.1-3.8 and the skin-to-blood ratio was 1.31 ± 0.22. Good local tumor control and long-term survival of the patients were achieved in two clinical trials of BNCT conducted in Japan.

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.

Fructose-functionalized polymers to enhance therapeutic potential of p-boronophenylalanine for neutron capture therapy

In boron neutron capture therapy (BNCT), boron drugs should accumulate selectively within a tumor and be quickly cleared from blood and normal organs. However, it is usually challenging to achieve the efficient tumor accumulation and the quick clearance simultaneously. Here we report the complex composed of a fructose-modified poly(ethylene glycol)-poly(l-lysine) block copolymer and p-boronophenylalanine, termed PEG-P[Lys/Lys(fructose)]-BPA, as a boron delivery system permitting selective accumulation within the target tumor with quick clearance from normal organs as well as blood. Our PEG-P[Lys/Lys(fructose)]-BPA could be internalized into tumor cells through LAT1 amino acid transporter-mediated endocytosis and retain in the targeted cells, thereby accomplishing more efficient accumulation and retention in a subcutaneous tumor than clinically used fructose-BPA complexes. Importantly, the moderately cationic property of the polymer facilitated renal clearance and PEG-P[Lys/Lys(fructose)]-BPA exhibited high accumulation contrast between the target tumor and the blood/normal organ. Finally, upon thermal neutron irradiation, PEG-P[Lys/Lys(fructose)]-BPA significantly inhibited the tumor growth in mice. PEG-P[Lys/Lys(fructose)]-BPA may be a promising boron delivery system for BNCT.

Pharmacokinetics of 10B-p-boronophenylalanine (BPA) in the blood and tumors in human patients: A critical review with special reference to tumor-to-blood (T/B) ratios using resected tumor samples

We reviewed ¹⁰B concentration kinetics in the blood and tumors in human patients administered with BPA. The ¹⁰B concentration in the blood peaked at the end of intravenous infusion of BPA, followed by a biphasic-decreasing curve with half-lives for the first and second components of the curve being 0.7-3.7 and 7.2-12.0 h, respectively. The mean tumor-to-blood (T/B) ratio obtained from resected tumor samples was 3.40 ± 0.83 for melanoma and the ratio ranged from 1.4 to 4.7 for glioblastoma.

Long-term outcome of cutaneous melanoma patients treated with boron neutron capture therapy (BNCT)

Our aim was to assess the long-term clinical outcome of boron neutron capture therapy (BNCT) using 10B-para-boronophenylalanine (BPA) as the boron delivery agent for cutaneous melanoma. Eight patients (eight lesions) were treated between October 2003 and April 2014. Their ages ranged from 48 to 86 years at the time of treatment. All of the targets were primary lesions and they were located on the sole or face. No patient had evidence of regional lymph node involvement, distant metastases or an active secondary cancer. The clinical stage was cT1-2N0M0 and performance scores were <2. BNCT was carried out at the Kyoto University Research Reactor (KUR). The patients were irradiated with an epithermal neutron beam between the curative tumor dose and the tolerable skin dose. Eight patients were evaluated and six showed a complete response (CR), while two patients had a partial response (PR). Of the two patients with a PR, one has remained a PR with brown spots persisting for 7.5 years following BNCT. The tumor in the other patient recurred after 6 years at the site of persisting brown macula. The overall control rate (CR + PR without recurrence) for the cohort was 88% (7/8). There have never been any adverse events >Grade 2 for the long follow-up period. Our results suggest that BNCT may be a promising treatment modality in the management of early stage cutaneous melanoma when wide local excision is not feasible.

Evaluation of a Novel Boron-Containing α-D-Mannopyranoside for BNCT

Boron neutron capture therapy (BNCT) is a unique anticancer technology that has demonstrated its efficacy in numerous phase I/II clinical trials with boronophenylalanine (BPA) and sodium borocaptate (BSH) used as 10B delivery agents. However, continuous drug administration at high concentrations is needed to maintain sufficient 10B concentration within tumors. To address the issue of 10B accumulation and retention in tumor tissue, we developed MMT1242, a novel boron-containing α-d-mannopyranoside. We evaluated the uptake, intracellular distribution, and retention of MMT1242 in cultured cells and analyzed biodistribution, tumor-to-normal tissue ratio and toxicity in vivo. Fluorescence imaging using nitrobenzoxadiazole (NBD)-labeled MMT1242 and inductively coupled mass spectrometry (ICP-MS) were performed. The effectiveness of BNCT using MMT1242 was assessed in animal irradiation studies at the Kyoto University Research Reactor. MMT1242 showed a high uptake and broad intracellular distribution in vitro, longer tumor retention compared to BSH and BPA, and adequate tumor-to-normal tissue accumulation ratio and low toxicity in vivo. A neutron irradiation study with MMT1242 in a subcutaneous murine tumor model revealed a significant tumor inhibiting effect if injected 24 h before irradiation. We therefore report that 10B-MMT1242 is a candidate for further clinical BNCT studies.

Biodistribution studies of boronophenylalanine in different types of skin melanoma

A study of the ¹⁰B-enriched Boronophenylalanine-fructose complex(¹⁰BPA-F) infusion procedure in potential BNCT patients, including three skin melanomas of extremities, was performed. ¹⁰B concentration in tumor(T), blood(B), skin(S) were measured to determine tumor/blood(T/B) and skin/blood(S/B) ratios. T/B ratio for three melanoma patients was in the range 1.48-3.82(average 2.56 ± 0.69). S/B ratio was in the range 0.81-1.99(average 1.29 ± 0.35). Results showed that T/B ratio of nodular metastasis melanoma was higher than superficial spreading melanoma. ¹⁰B concentration in skin was higher than blood, which was helpful to avoid over-dose in normal skin.

Poly(vinyl alcohol) boosting therapeutic potential of p-boronophenylalanine in neutron capture therapy by modulating metabolism

In the current clinical boron neutron capture therapy (BNCT), p-boronophenylalanine (BPA) has been the most powerful drug owing to its ability to accumulate selectively within cancers through cancer-related amino acid transporters including LAT1. However, the therapeutic success of BPA has been sometimes compromised by its unfavorable efflux from cytosol due to their antiport mechanism. Here, we report that poly(vinyl alcohol) (PVA) can form complexes with BPA through reversible boronate esters in aqueous solution, and the complex termed PVA-BPA can be internalized into cancer cells through LAT1-mediated endocytosis, thereby enhancing cellular uptake and slowing the untoward efflux. In in vivo study, compared with clinically used fructose-BPA complexes, PVA-BPA exhibited efficient tumor accumulation and prolonged tumor retention with quick clearance from bloodstream and normal organs. Ultimately, PVA-BPA showed critically enhanced antitumor activity in BNCT. The facile technique proposed in this study offers an approach for drug delivery focusing on drug metabolism.

Physical, dosimetric and clinical aspects and delivery systems in neutron capture therapy

Neutron capture therapy (NCT) is a targeted radiotherapy for cancer treatment. In this method, neutrons with a spectra/specific energy (depending on the type of agent used for NCT) are captured with an agent that has a high cross-section with these neutrons. There are some agents that have been proposed in NCT including 10B, 157Gd and 33S. Among these agents, only 10B is used in clinical trials. Application of 157Gd is limited to in-vivo and in-vitro research. In addition, 33S has been applied in the field of Monte Carlo simulation. In BNCT, the only two delivery agents which are presently applied in clinical trials are BPA and BSH, but other delivery systems are being developed for more effective treatment in NCT. Neutron sources used in NCT are fission reactors, accelerators, and 252Cf. Among these, fission reactors have the most application in NCT. So far, BNCT has been applied to treat various cancers including glioblastoma multiforme, malignant glioma, malignant meningioma, liver, head and neck, lung, colon, melanoma, thyroid, hepatic, gastrointestinal cancer, and extra-mammary Paget's disease. This paper aims to review physical, dosimetric and clinical aspects as well as delivery systems in NCT for various agents.

Dynamic changes in 18F-borono-L-phenylalanine uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck and malignant melanoma during boron neutron capture therapy patient selection

Background

We evaluated dynamic changes in ¹⁸F–borono-L-phenylalanine (¹⁸F–BPA) uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck (SCC) and malignant melanoma (MM) during boron neutron capture therapy (BNCT) patient selection.

Methods

Dynamic changes in the maximum standardized uptake value (SUVmax), tumor-to-normal tissue ratio (TNR), and tumor-to-blood pool ratio (TBR) for ¹⁸F–BPA were evaluated in 20 patients with SCC and 8 patients with MM.

Results

SUVmax in SCC tumors decreased significantly from 30 to 120 min. There was a non-statistically significant decrease in SUVmax for SCC tumors from 30 to 60 min and from 60 to 120 min. Patients with MM had nonsignificant SUVmax changes in ¹⁸F–BPA uptake on delayed imaging. Nonsignificant ¹⁸F–BPA TNR and TBR changes were seen in patients with SCC and MM.

Conclusions

Dynamic changes in SUVmax for ¹⁸F–BPA uptake had a washout pattern in SCC and a persistent pattern in MM. Dynamic ¹⁸F–BPA -PET studies should be performed to investigate the pharmacokinetics of ¹⁸F–BPA in humans and select appropriate candidates who may benefit from BNCT.

Toward a clinical application of ex situ boron neutron capture therapy for lung tumors at the RA-3 reactor in Argentina

PURPOSE

Many types of lung tumors have a very poor prognosis due to their spread in the whole organ volume. The fact that boron neutron capture therapy (BNCT) would allow for selective targeting of all the nodules regardless of their position, prompted a preclinical feasibility study of ex situ BNCT at the thermal neutron facility of RA-3 reactor in the province of Buenos Aires, Argentina. (l)-4p-dihydroxy-borylphenylalanine fructose complex (BPA-F) biodistribution studies in an adult sheep model and computational dosimetry for a human explanted lung were performed to evaluate the feasibility and the therapeutic potential of ex situ BNCT.

METHODS

Two kinds of boron biodistribution studies were carried out in the healthy sheep: a set of pharmacokinetic studies without lung excision, and a set that consisted of evaluation of boron concentration in the explanted and perfused lung. In order to assess the feasibility of the clinical application of ex situ BNCT at RA-3, a case of multiple lung metastases was analyzed. A detailed computational representation of the geometry of the lung was built based on a real collapsed human lung. Dosimetric calculations and dose limiting considerations were based on the experimental results from the adult sheep, and on the most suitable information published in the literature. In addition, a workable treatment plan was considered to assess the clinical application in a realistic scenario.

RESULTS

Concentration-time profiles for the normal sheep showed that the boron kinetics in blood, lung, and skin would adequately represent the boron behavior and absolute uptake expected in human tissues. Results strongly suggest that the distribution of the boron compound is spatially homogeneous in the lung. A constant lung-to-blood ratio of 1.3 ± 0.1 was observed from 80 min after the end of BPA-F infusion. The fact that this ratio remains constant during time would allow the blood boron concentration to be used as a surrogate and indirect quantification of the estimated value in the explanted healthy lung. The proposed preclinical animal model allowed for the study of the explanted lung. As expected, the boron concentration values fell as a result of the application of the preservation protocol required to preserve the lung function. The distribution of the boron concentration retention factor was obtained for healthy lung, with a mean value of 0.46 ± 0.14 consistent with that reported for metastatic colon carcinoma model in rat perfused lung. Considering the human lung model and suitable tumor control probability for lung cancer, a promising average fraction of controlled lesions higher than 85% was obtained even for a low tumor-to-normal boron concentration ratio of 2.

CONCLUSIONS

This work reports for the first time data supporting the validity of the ovine model as an adequate human surrogate in terms of boron kinetics and uptake in clinically relevant tissues. Collectively, the results and analysis presented would strongly suggest that ex situ whole lung BNCT irradiation is a feasible and highly promising technique that could greatly contribute to the treatment of metastatic lung disease in those patients without extrapulmonary spread, increasing not only the expected overall survival but also the resulting quality of life.

Reprint of Application of BNCT to the treatment of HER2+ breast cancer recurrences: Research and developments in Argentina

In the frame of the Argentine BNCT Project a new research line has been started to study the application of BNCT to the treatment of locoregional recurrences of HER2+ breast cancer subtype. Based on former studies, the strategy considers the use of immunoliposomes as boron carriers nanovehicles to target HER2 overexpressing cells. The essential concerns of the current stage of this proposal are the development of carriers that can improve the efficiency of delivery of boron compounds and the dosimetric assessment of treatment feasibility. For this purpose, an specific pool of clinical cases that can benefit from this application was determined. In this work, we present the proposal and the advances related to the different stages of current research.

Experimental Studies of Boronophenylalanine ((10)BPA) Biodistribution for the Individual Application of Boron Neutron Capture Therapy (BNCT) for Malignant Melanoma Treatment

PURPOSE

Patients with the same histopathologic diagnosis of cutaneous melanoma treated with identical protocols of boron neutron capture therapy (BNCT) have shown different clinical outcomes. The objective of the present studies was to evaluate the biodistribution of boronophenilalanina ((10)BPA) for the potential application of BNCT for the treatment of melanoma on an individual basis.

METHODS AND MATERIALS

The boronophenilalanine (BPA) uptake was evaluated in 3 human melanoma cell lines: MEL-J, A375, and M8. NIH nude mice were implanted with 4 10(6) MEL-J cells, and biodistribution studies of BPA (350 mg/kg intraperitoneally) were performed. Static infrared imaging using a specially modified infrared camera adapted to measure the body infrared radiance of small animals was used. Proliferation marker, Ki-67, and endothelial marker, CD31, were analyzed in tumor samples.

RESULTS

The in vitro studies demonstrated different patterns of BPA uptake for each analyzed cell line (P<.001 for MEL-J and A375 vs M8 cells). The in vivo studies showed a maximum average boron concentration of 25.9 ± 2.6 μg/g in tumor, with individual values ranging between 11.7 and 52.0 μg/g of (10)B 2 hours after the injection of BPA. Tumor temperature always decreased as the tumors increased in size, with values ranging between 37 °C and 23 °C. A significant correlation between tumor temperature and tumor-to-blood boron concentration ratio was found (R(2) = 0.7, rational function fit). The immunohistochemical studies revealed, in tumors with extensive areas of viability, a high number of positive cells for Ki-67, blood vessels of large diameter evidenced by the marker CD31, and a direct logistic correlation between proliferative status and boron concentration difference between tumor and blood (R(2) = 0.81, logistic function fit).

CONCLUSION

We propose that these methods could be suitable for designing new screening protocols applied before melanoma BNCT treatment for each individual patient and lesion.

Effect of diameter of nanoparticles and capture cross-section library on macroscopic dose enhancement in boron neutron capture therapy

Purpose

The aim of this study is evaluation of the effect of diameter of ¹⁰B nanoparticles and various neutron capture cross-section libraries on macroscopic dose enhancement in boron neutron capture therapy (BNCT).

Material and methods

MCNPX Monte Carlo code was used for simulation of a ²⁵²Cf source, a soft tissue phantom and a tumor containing ¹⁰B nanoparticles. Using ²⁵²Cf as a neutron source, macroscopic dose enhancement factor (MDEF) and total dose rate in tumor in the presence of 100, 200, and 500 ppm of ¹⁰B nanoparticles with 25 nm, 50 nm, and 100 nm diameters were calculated. Additionally, the effect of ENDF, JEFF, JENDL, and CENDL neutron capture cross-section libraries on MDEF was evaluated.

Results

There is not a linear relationship between the average MDEF value and nanoparticles’ diameter but the average MDEF grows with increased concentration of ¹⁰B nanoparticles. There is an increasing trend for average MDEF with the tumor distance. The average MDEF values were obtained the same for various neutron capture cross-section libraries. The maximum and minimum doses that effect on the total dose in tumor were neutron and secondary photon doses, respectively. Furthermore, the boron capture related dose component reduced in some extent with increase of diameter of ¹⁰B nanoparticles.

Conclusions

Based on the results of this study, it can be concluded that from physical point of view, various nanoparticle diameters have no dominant effect on average MDEF value in tumor. Furthermore, it is concluded that various neutron capture cross-section libraries are resulted to the same macroscopic dose enhancements. However, it is predicted that taking into account the biological effects for various nanoparticle diameters will result in different dose enhancements.

Boron concentration measurement in biological tissues by charged particle spectrometry

Measurement of boron concentration in biological tissues is a fundamental aspect of boron neutron capture therapy, because the outcome of the therapy depends on the distribution of boron at a cellular level, besides on its overall concentration. This work describes a measurement technique based on the spectroscopy of the charged particles emitted in the reaction (10)B(n,α)(7)Li induced by thermal neutrons, allowing for a quantitative determination of the boron concentration in the different components that may be simultaneously present in a tissue sample, such as healthy cells, tumor cells and necrotic cells. Thin sections of tissue containing (10)B are cut at low temperatures and irradiated under vacuum in a thermal neutron field. The charged particles arising from the sample during the irradiation are collected by a thin silicon detector, and their spectrum is used to determine boron concentration through relatively easy calculations. The advantages and disadvantages of this technique are here described, and validation of the method using tissue standards with known boron concentrations is presented.

Evaluation of performance of an accelerator-based BNCT facility for the treatment of different tumor targets

PURPOSE

Encouraging Boron Neutron Capture Therapy (BNCT) clinical results obtained in recent years have stimulated intense research to develop accelerator-based neutron sources to be installed in clinical facilities. In this work an assessment of an accelerator-based BNCT facility for the treatment of different tumor targets was performed, comparing the accelerator-derived results with reported reactor-based trials under similar conditions and subjected to the same clinical protocols.

MATERIALS AND METHODS

A set of real image studies was used to cover clinical-like cases of brain and head-and-neck tumors. In addition, two clinical cases of malignant nodular melanoma treated at the RA-6 BNCT facility in Argentina were used to thoroughly compare the clinical dosimetry with the accelerator-derived results.

RESULTS

The minimum weighted dose delivered to the clinical target volume was higher than 30 Gy and 14 Gy for the brain tumor and head-and-neck cases, respectively, in agreement with those achieved in clinical applications. For the melanoma cases, the minimum tumor doses were equal or higher than those achieved with the RA-6 reactor for identical field orientation and protocol. The whole-body dose assessment showed that the maximum photon-equivalent doses for those normal organs close to the beam direction were below the upper limits considered in the protocols used in the present work.

CONCLUSIONS

The obtained results indicate not only the good performance of the proposed beam shaping assembly design associated to the facility but also the potential applicability of accelerator-based BNCT in the treatment of both superficial and deep-seated tumors.

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.

BNCT for skin melanoma in extremities: updated Argentine clinical results

As part of phase I/II melanoma BNCT clinical trial conducted in Argentina in a cooperative effort of the Argentine Atomic Energy Commission (CNEA) and the Oncology Institute Angel H. Roffo (IOAHR), 7 patients (6 female-1 male) received eight treatment sessions covering ten anatomical areas located in extremities. Mean age of the patients was 64 years (51-74). The treatments were performed between October 2003 and June 2007. All patients presented multiple subcutaneous skin metastases of melanoma and received an infusion containing approximately 14 gr/m(2) of (10)borophenyl-alanine (BPA) followed by the exposition of the area to a mixed thermal-epithermal neutron beam at the RA-6 reactor. The maximum prescribed dose to normal skin ranged from 16.5 to 24 Gy-Eq and normal tissue administered dose varied from 15.8 to 27.5 Gy-Eq. Considering evaluable nodules, 69.3% of overall response and 30.7% of no changes were seen. The toxicity was acceptable, with 3 out of 10 evaluable areas showing ulceration (30% toxicity grade 3).

Feasibility of boron neutron capture therapy (BNCT) for malignant pleural mesothelioma from a viewpoint of dose distribution analysis

PURPOSE

To investigate the feasibility of boron neutron capture therapy (BNCT) for malignant pleural mesothelioma (MPM) from a viewpoint of dose distribution analysis using Simulation Environment for Radiotherapy Applications (SERA), a currently available BNCT treatment planning system.

METHODS AND MATERIALS

The BNCT treatment plans were constructed for 3 patients with MPM using the SERA system, with 2 opposed anterior-posterior beams. The (10)B concentrations in the tumor and normal lung in this study were assumed to be 84 and 24 ppm, respectively, and were derived from data observed in clinical trials. The maximum, mean, and minimum doses to the tumors and the normal lung were assessed for each plan. The doses delivered to 5% and 95% of the tumor volume, D(05) and D(95), were adopted as the representative dose for the maximum and minimum dose, respectively.

RESULTS

When the D(05) to the normal ipsilateral lung was 5 Gy-Eq, the D(95) and mean doses delivered to the normal lung were 2.2-3.6 and 3.5-4.2 Gy-Eq, respectively. The mean doses delivered to the tumors were 22.4-27.2 Gy-Eq. The D(05) and D(95) doses to the tumors were 9.6-15.0 and 31.5-39.5 Gy-Eq, respectively.

CONCLUSIONS

From a viewpoint of the dose-distribution analysis, BNCT has the possibility to be a promising treatment for MPM patients who are inoperable because of age and other medical illnesses.

1H MRS studies in the Finnish boron neutron capture therapy project: detection of 10B-carrier, L-p-boronophenylalanine-fructose

This article summarizes the current status of 1H MRS in detecting and quantifying a boron neutron capture therapy (BNCT) boron carrier, L-p-boronophenylalanine-fructose (BPA-F) in vivo in the Finnish BNCT project. The applicability of 1H MRS to detect BPA-F is evaluated and discussed in a typical situation with a blood containing resection cavity within the gross tumour volume (GTV). 1H MRS is not an ideal method to study BPA concentration in GTV with blood in recent resection cavity. For an optimal identification of BPA signals in the in vivo 1H MR spectrum, both pre- and post-infusion 1H MRS should be performed. The post-infusion spectroscopy studies should be scheduled either prior to or, less optimally, immediately after the BNCT. The pre-BNCT MRS is necessary in order to utilise the MRS results in the actual dose planning.

Biodistribution studies of boronophenylalanine-fructose in melanoma and brain tumor patients in Argentina

A study of the (10)B-enriched p-boronophenylalanine-fructose complex ((10)BPA-F) infusion procedure in potential BNCT patients, including four melanoma of extremities and two high-grade gliomas (glioblastoma and ganglioglioma) was performed. T/B and S/B ratios for (10)B concentrations in tumor (T), blood (B) and skin (S) were determined. The T/B ratio for the glioblastoma was in the 1.8-3.4 range. The ganglioglioma did not show any significant boron uptake. For the nodular metastasic melanoma T/B values were between 1.5 and 2.6 (average 2.1+/-0.4), corresponding to the lower limit of the mean values reported for different melanoma categories. This result might suggest a lower boron uptake for nodular metastasic melanomas. S/B was 1.5+/-0.4. An open two-compartment pharmacokinetic model was applied to predict the boron concentration during the course and at the end of a BNCT irradiation.

Boron neutron capture therapy for glioblastoma multiforme

AIM

Glioblastoma multiforme (GBM) is an incurable disease that can only be managed in a palliative way. The GBM accounts for approximately half of all newly diagnosed primary brain tumors with an incidence of 2-3 cases per 100,000 people each year. Surgery and radiation are the standard options for palliation, and whether there is a place for chemotherapy is still discussed. Boron neutron capture therapy (BNCT) is a promising and possibly curative method of treating GBM. The purpose of this article is to provide an updated review on the current management and future possibilities of treating GBM with BNCT.

METHOD

Use was made of computerized searches and of checking cross-references of articles and book chapters.

RESULTS

The principle of BNCT uses the high ability of 10B to capture thermal neutrons and to disintegrate immediately into a He nucleus (alpha-particle) and a Li nucleus. To reach a sufficient concentration of 10B in the malignant cells compared to the surrounding healthy tissue, 10B-carriers must be highly tumor-selective. At present, the 10B carriers boronophenylalanine (BPA) and sodium borocaptate (BSH) are used in clinical trials to perform BNCT.

CONCLUSION

The BNCT is a promising and possibly curative method of treating GBM, but at present this procedure is far from perfect. Because of the lack of selectivity of the boron carriers, it appears so far that radiation toxicity limits the radiation dose, so that tumor damage is modest. Current investigations and developments are aimed at targeting the boron carriers to the tumor, in order to limit the damage to the healthy, surrounding tissue.

Accumulation of boron in human malignant glioma cells in vitro is cell type dependent

It has been shown that human malignant glioma tumours consist of several subpopulations of tumour cells. Due to heterogeneity and different degrees of vascularisation cell subpopulations possess varying resistance to chemo- or radiation therapy. Therefore, therapy is dependent on the ability to specifically target a tumour cell. Boron neutron capture therapy (BNCT) is a bimodal method, in radiation therapy, taking advantage of the ability of the stable isotope boron-10 to capture neutrons. It results in disintegration products depositing large amounts of energy within a short length, approximately one cell diameter. Thereby, selective irradiation of a target cell may be accomplished if a sufficient amount of boron has been accumulated and hence the cell-associated boron concentration is of critical importance. The accumulation of boron, boronophenylalanine (BPA), was investigated in two human glioma cell subpopulations and a human fibroblast cell line in vitro. The cells were incubated at low boron concentrations (0-5 microg B/ml). Oil filtration was then used for separation of extracellular and cell-associated boron. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used for boron determination. Significant (P < 0.05) differences in accumulation ratio (relation between cell-associated and extracellular boron concentration) between human malignant glioma cell lines were found. Human fibroblasts, used to represent normal cells, showed a growth-dependent uptake and a lower accumulation ratio than the glioma cells. Our findings indicate that BPA concentration, incubation time and differences in boron uptake between cell subpopulations should be considered in BNCT.

Boron neutron capture therapy (BNCT) for malignant melanoma with special reference to absorbed doses to the normal skin and tumor

Twenty-two patients with malignant melanoma were treated with boron neutron capture therapy (BNCT) using 10B-p-boronophenylalanine (BPA). The estimation of absorbed dose and optimization of treatment dose based on the pharmacokinetics of BPA in melanoma patients is described. The doses of gamma-rays were measured using small TLDs of Mg2SiO4 (Tb) and thermal neutron fluence was measured using gold foil and wire. The total absorbed dose to the tissue from BNCT was obtained by summing the primary and capture gamma-ray doses and the high LET radiation doses from 10B(n, alpha)7Li and 14N(n,p)14C reactions. The key point of the dose optimization is that the skin surrounding the tumour is always irradiated to 18 Gy-Eq, which is the maximum tolerable dose to the skin, regardless of the 10B-concentration in the tumor. The neutron fluence was optimized as follows. (1) The 10B concentration in the blood was measured 15-40 min after the start of neutron irradiation. (2) The 10B-concentration in the skin was estimated by multiplying the blood 10B value by a factor of 1.3. (3) The neutron fluence was calculated. Absorbed doses to the skin ranged from 15.7 to 37.1 Gy-Eq. Among the patients, 16 out of 22 patients exhibited tolerable skin damage. Although six patients showed skin damage that exceeded the tolerance level, three of them could be cured within a few months after BNCT and the remaining three developed severe skin damage requiring skin grafts. The absorbed doses to the tumor ranged from 15.7 to 68.5 Gy-Eq and the percentage of complete response was 73% (16/22). When BNCT is used in the treatment of malignant melanoma, based on the pharmacokinetics of BPA and radiobiological considerations, promising clinical results have been obtained, although many problems and issues remain to be solved.

Improved treatment planning for boron neutron capture therapy for glioblastoma multiforme using fluorine-18 labeled boronophenylalanine and positron emission tomography

Boron neutron capture therapy (BNCT) is a cancer brachytherapy based upon the thermal neutron reaction: 10B(n,alpha)7Li. The efficacy of the treatment depends primarily upon two conditions being met: (a) the preferential concentration of a boronated compound in the neoplasm and (b) an adequate fluence of thermal neutrons delivered to the neoplasm. The boronated amino acid, para-boronophenylalanine (BPA), is the agent widely used in clinical trials to deliver 10B to the malignancy. Positron emission tomography (PET) can be used to generate in vivo boron distribution maps by labeling BPA with the positron emitting nuclide fluorine-18. The incorporation of the PET-derived boron distribution maps into current treatment planning protocols is shown to provide improved treatment plans. Using previously established protocols, six patients with glioblastoma had 18BPA PET scans. The PET distribution maps obtained were used in the conventional BNCT treatment codes. The isodose curves derived from the PET data are shown to differ both qualitatively and quantitatively from the conventional isodose curves that were derived from calculations based upon the assumption of uniform uptake of the pharmaceutical in tumor and normal brain regions. The clinical course of each of the patients who eventually received BNCT (five of the six patients) was compared using both sets of isodose calculations. The isodose contours based upon PET derived distribution data appear to be more consistent with the patients' clinical course.

Ortho-carboranyl glycosides for the treatment of cancer by boron neutron capture therapy

Distinct biological properties of the ortho-carboranyl (1,2-dicarba-closo-dodecaboranyl) glycosides 1, 2 and 3 were evaluated to estimate the suitability of these compounds for cancer treatment by boron neutron capture therapy. The boron uptake into B16-Melanoma cells was significantly higher by incubating the cells with aqueous solutions of carboranyl glucoside 1 (11.2 ppm after 3h), lactoside 2 (13.2 ppm after 12h) and maltoside 3 (20.0 ppm after 24h) compared with solutions of clinically used p-boronophenylalanine (BPA) 5 (3.1 ppm after 24h). Carboranyl maltoside 3 was more effective than boron-10 enriched 5 in killing C-6 rat glioma cells by incubating the cells with the compound and subsequent treatment with thermal neutrons. 3 was also administrated iv, in concentrations of 25 mg boron/kg body weight to rats bearing brain tumors. After a period of 4h after administration the concentration of boron in the tumor tissue was 3.0 ppm.