The role of boron MRI in boron neutron capture therapy

Development of an Imaging Technique for Boron Neutron Capture Therapy

The development of 4-¹⁰B-borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸FBPA) for use in positron emission tomography (PET) has contributed to the progress of boron neutron capture therapy (BNCT). ¹⁸FBPA has shown similar pharmacokinetics and distribution to 4-¹⁰B-borono-L-phenylalanine (BPA) under various conditions in many animal studies. ¹⁸FBPA PET is useful for treatment indication. A higher ¹⁸FBPA accumulation ratio of the tumor to the surrounding normal tissue (T/N ratio) indicates that a superior treatment effect is expected. In clinical settings, a T/N ratio of higher than 2.5 or 3 is often used for patient selection. Moreover, ¹⁸FBPA PET is useful for predicting the ¹⁰B concentration delivered to the tumor and surrounding normal tissues, enabling high-precision treatment planning. Precise dose prediction using ¹⁸FBPA PET data has greatly improved the treatment accuracy of BNCT. However, the methodology used for the data analysis of ¹⁸FBPA PET findings varies; thus, data should be evaluated using a consistent methodology so as to be more reliable. In addition to PET applications, the development of ¹⁸FBPA as a contrast agent for magnetic resonance imaging that combines gadolinium and ¹⁰B is also in progress.

4-Borono-2-18F-fluoro-L-phenylalanine PET for boron neutron capture therapy-oriented diagnosis: overview of a quarter century of research

4-¹⁰B-Borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸F-FBPA) was developed for monitoring the pharmacokinetics of 4-¹⁰B-borono-L-phenylalanine (¹⁰B-BPA) used in boron neutron capture therapy (BNCT) with positron emission tomography (PET). The tumor-imaging potential of ¹⁸F-FBPA was demonstrated in various animal models. Accumulation of ¹⁸F-FBPA was higher in melanomas than in non-melanoma tumors in animal models and cell cultures. ¹⁸F-FBPA was incorporated into tumors mediated mainly by L-type amino acid transporters in in vitro and in vivo models. Tumoral distribution of ¹⁸F-FBPA was primarily related to the activity of DNA synthesis. ¹⁸F-FBPA is metabolically stable but is incorporated into melanogenesis non-enzymatically. These in vitro and in vivo characteristics of ¹⁸F-FBPA corresponded well to those of ¹⁰B-BPA. Nuclear magnetic resonance and other studies using non-radioactive ¹⁹F-^10/11B-FBPA also contributed to characterization. The validity and reliability of ^18/19F-FBPA as an in vivo probe of ¹⁰B-BPA were confirmed by comparison of the pharmacokinetics of ¹⁸F-FBPA and ¹⁰B-BPA and direct measurement of both ¹⁸F and ¹⁰B in tumors with various doses of both probes administered by different routes and methods. Clinically, based on the kinetic parameters of dynamic ¹⁸F-FBPA PET, the estimated ¹⁰B-concentrations in tumors with continuous ¹⁰B-BPA infusion were similar to those measured directly in surgical specimens. The significance of ¹⁸F-FBPA PET was verified for the estimation of ¹⁰B-concentration and planning of BNCT. Later ¹⁸F-FBPA PET has been involved in ¹⁰B-BPA BNCT of patients with intractable tumors such as malignant brain tumors, head and neck tumors, and melanoma. Usually a static PET scan is used for screening patients for BNCT, prediction of the distribution and accumulation of ¹⁰B-BPA, and evaluation of treatment after BNCT. In some clinical trials, a tumor-to-normal tissue ratio of ¹⁸F-FBPA > 2.5 was an inclusion criterion for BNCT. Apart from BNCT, ¹⁸F-FBPA was demonstrated to be a useful PET probe for tumor diagnosis in nuclear medicine: better tumor-to-normal brain contrast compared with ¹¹C-methionine, differentiation of recurrent and radiation necrosis after radiotherapy, and melanoma-preferential uptake. Further progress in ¹⁸F-FBPA studies is expected for more elaborate evaluation of ¹⁰B-concentrations in tumors and normal tissues for successful ¹⁰B-BPA BNCT and for radiosynthesis of ¹⁸F-FBPA to enable higher ¹⁸F-activity amounts and higher molar activities.

Design and synthesis of boron containing monosaccharides by the hydroboration of d-glucal for use in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is one of the radiotherapies that involves the use of boron-containing compounds for the treatment of cancer. Boron-10 (¹⁰B) containing compounds that can accumulate in tumor tissue are expected to be suitable agents for BNCT. We report herein on the design and synthesis of some new BNCT agents based on a d-glucose scaffold, since glycoconjugation has been recognized as a useful strategy for the specific targeting of tumors. To introduce a boryl group into a d-glucose scaffold, we focused on the hydroboration of d-glucal derivatives, which have a double bond between the C1 and C2 positions. It was hypothesized that a C-B bond could be introduced at the C2 position of d-glucose by the hydroboration of d-glucal derivatives and that the products could be stabilized by conversion to the corresponding boronic acid ester. To test this hypothesis, we prepared some 2-boryl-1,2-dideoxy-d-glucose derivatives as boron carriers and evaluated their cytotoxicity and cellular uptake activity to cancer cells, especially under hypoxic conditions.

Boronated carbohydrate derivatives as potential boron neutron capture therapy reagents

The treatment of cancer remains one of the most challenging problems for humanity. Boron neutron capture therapy is a binary approach for cancer treatment that is particularly attractive in treating high-grade gliomas and metastatic brain tumors. Among the types of boron-containing molecules used as boron neutron capture therapy agents, boronated carbohydrate derivatives have received significant attention because of their preferential uptake by growing tumor cells. This review provides a summary of the recent developments in the chemistry of carborane-containing carbohydrates.

Boron neutron capture therapy (BNCT) in Finland: technological and physical prospects after 20 years of experiences

Boron Neutron Capture Therapy (BNCT) is a binary radiotherapy method developed to treat patients with certain malignant tumours. To date, over 300 treatments have been carried out at the Finnish BNCT facility in various on-going and past clinical trials. In this technical review, we discuss our research work in the field of medical physics to form the groundwork for the Finnish BNCT patient treatments, as well as the possibilities to further develop and optimize the method in the future. Accordingly, the following aspects are described: neutron sources, beam dosimetry, treatment planning, boron imaging and determination, and finally the possibilities to detect the efficacy and effects of BNCT on patients.

11B NMR sensing of d-block metal ions in vitro and in cells based on the carbon-boron bond cleavage of phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane)

Noninvasive magnetic resonance imaging (MRI) including the "chemical shift imaging (CSI)" technique based on (1)H NMR signals is a powerful method for the in vivo imaging of intracellular molecules and for monitoring various biological events. However, it has the drawback of low resolution because of background signals from intrinsic water protons. On the other hand, it is assumed that the (11)B NMR signals which can be applied to a CSI technique have certain advantages, since boron is an ultratrace element in animal cells and tissues. In this manuscript, we report on the sensing of biologically indispensable d-block metal cations such as zinc, copper, iron, cobalt, manganese, and nickel based on (11)B NMR signals of simple phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane), L(6) and L(7), in aqueous solution at physiological pH. The results indicate that the carbon-boron bond of L(6) is cleaved upon the addition of Zn(2+) and the broad (11)B NMR signal of L(6) at 31 ppm is shifted upfield to 19 ppm, which corresponds to the signal of B(OH)(3). (1)H NMR, X-ray single crystal structure analysis, and UV absorption spectra also provide support for the carbon-boron bond cleavage of ZnL(6). Because the cellular uptake of L(6) was very small, a more cell-membrane permeable ligand containing the boronic acid ester L(7) was synthesized and investigated for the sensing of d-block metal ions using (11)B NMR. Data on (11)B NMR sensing of Zn(2+) in Jurkat T cells using L(7) is also presented.

Multi-nuclear MRS and 19F MRI of 19F-labelled and 10B-enriched p-boronophenylalanine-fructose complex to optimize boron neutron capture therapy: phantom studies at high magnetic fields

Reaction yield optimization for the synthesis and the complexation of a boron neutron capture therapy agent (19)F-labelled, (10)B-enriched p-boronophenylalanine-fructose ((19)F-BPA-fr) complex was obtained. (1)H, (19)F, (13)C and (10)B magnetic resonance spectroscopy (MRS) of the (19)F-BPA-fr complex in aqueous and rat blood solution phantoms and its spatial distribution mapping using (19)F magnetic resonance imaging (MRI) results are reported. 7 T and 9.4 T magnetic fields were used to perform MRI and MRS respectively. Our in vitro results suggest that in vivo studies on (19)F-BPA through (19)F NMR will be feasible.

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.

Synthesis and in vivo biodistribution of BPA-Gd-DTPA complex as a potential MRI contrast carrier for neutron capture therapy

p-boronophenylalanine (BPA) conjugated Gd-DTPA complex (3) was synthesized from the active methyne compound 6, the allylic carbonate 7, and BPA by the palladium-catalyzed allylation reaction followed by the DCC coupling reaction. The in vivo biodistribution of complex 3 was evaluated by prompt gamma-ray analysis and alpha-autoradiography using the tumor-bearing rats. High accumulation of gadolinium was observed in the kidney and the %ID values were 0.17 and 0.088 at 20 and 60 min after injection of 3, respectively. The accumulation was also observed in the tumor and the %ID values were 0.010 and 0.0025 at 20 and 60 min after injection, respectively. The visualization experiment of boron distribution in the tumor-bearing rat by alpha-autoradiography indicates that boron was accumulated in the tumor and the intestines at 20 min after injection.

1H MRS of a boron neutron capture therapy 10B-carrier, L-p-boronophenylalanine-fructose complex, BPA-F: phantom studies at 1.5 and 3.0 T

The quantification of a BNCT 10B-carrier, L-p-boronophenylalanine-fructose complex (BPA-F), was evaluated using 1H magnetic resonance spectroscopy (1H MRS) with phantoms at 1.5 and 3.0 T. For proper quantification, relaxation times T1 and T2 are needed. While T1 is relatively easy to determine, the determination of T2 of a coupled spin system of aromatic protons of BPA is not straightforward with standard MRS sequences. In addition, an uncoupled concentration reference for aromatic protons of BPA must be used with caution. In order to determine T2, the response of an aromatic proton spin system to the MRS sequence PRESS with various echo times was calculated and the product of the response curve with exponential decay was fitted to the measured intensities. Furthermore, the response curve can be used to correct the intensities, when an uncoupled resonance is used as a concentration reference. BPA was quantified using both phantom replacement and internal water referencing methods with accuracies of +/- 5% and +/- 15%. Our phantom results suggest that in vivo studies on BPA concentration determination will be feasible.

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.

Non-linear model for the kinetics of 10B in blood after BPA-fructose complex infusion

A numerical model with a memory effect was created to describe the kinetics of 10B in blood after a single 4-dihydroxyborylphenylalanine-fructose complex (BPA-F) infusion in boron neutron capture therapy (BNCT). The model formulation was based on the averaged data from 10 glioma patients from the Brookhaven National Laboratory (BNL) BNCT-trials. These patients received a 2 h i.v. infusion of a BPA-fructose complex that delivered 290 mg BPA/kg body weight. The model was validated by fitting the original BNL patient data and new patient data from the Finnish BNCT-trials. The new 3-parameter non-linear model provided mean absolute differences between the measured and estimated 10B concentrations in blood that were less than 3.9% when used to simulate actual patient irradiations that comprised two irradiation fields separated by a break to reposition the patient. The flexibility of the model was successfully tested with two different infusion protocols. The patient data were modelled with a two-compartment model and a bi-exponential fit for comparison. The 3-parameter model is better than previously described models in predicting the time course of blood 10B concentration after cessation of intravenous infusion of BPA-fructose.

Optimal timing of neutron irradiation for boron neutron capture therapy after intravenous infusion of sodium borocaptate in patients with glioblastoma

PURPOSE

A cooperative study in Europe and Japan was conducted to determine the pharmacokinetics and boron uptake of sodium borocaptate (BSH: Na(2)B(12)H(11)SH), which has been introduced clinically as a boron carrier for boron neutron capture therapy in patients with glioblastoma.

METHODS AND MATERIALS

Data from 56 patients with glioblastoma who received BSH intravenous infusion were retrospectively reviewed. The pharmacokinetics were evaluated in 50 patients, and boron uptake was investigated in 47 patients. Patients received BSH doses between 12 and 100 mg/kg of body weight. For the evaluation, the infused boron dose was scaled linearly to 100 mg/kg BSH.

RESULTS

In BSH pharmacokinetics, the average value for total body clearance, distribution volume of steady state, and mean residence time was 3.6 +/- 1.5 L/h, 223.3 +/- 160.7 L, and 68.0 +/- 52.5 h, respectively. The average values of the boron concentration in tumor adjusted to 100 mg/kg BSH, the boron concentration in blood adjusted to 100 mg/kg BSH, and the tumor/blood boron concentration ratio were 37.1 +/- 35.8 ppm, 35.2 +/- 41.8 ppm, and 1.53 +/- 1.43, respectively. A good correlation was found between the logarithmic value of T(adj) and the interval from BSH infusion to tumor tissue sampling. About 12-19 h after infusion, the actual values for T(adj) and tumor/blood boron concentration ratio were 46.2 +/- 36.0 ppm and 1.70 +/- 1.06, respectively. The dose ratio between tumor and healthy tissue peaked in the same interval.

CONCLUSION

For boron neutron capture therapy using BSH administered by intravenous infusion, this work confirms that neutron irradiation is optimal around 12-19 h after the infusion is started.

Models for estimation of the (10)B concentration after BPA-fructose complex infusion in patients during epithermal neutron irradiation in BNCT

PURPOSE

To create simple and reliable models for clinical practice for estimating the blood (10)B time-concentration curve after p-boronophenylalanine fructose complex (BPA-F) infusion in patients during neutron irradiation in boron neutron capture therapy (BNCT).

METHODS AND MATERIALS

BPA-F (290 mg BPA/kg body weight) was infused i.v. during two hours to 10 glioblastoma multiforme patients. Blood samples were collected during and after the infusion. Compartmental models and bi-exponential function fit were constructed based on the (10)B blood time-concentration curve. The constructed models were tested with data from six additional patients who received various amounts of infused BPA-F and data from one patient who received a one-hour infusion of 170 mg BPA/kg body weight.

RESULTS

The resulting open two-compartment model and bi-exponential function estimate the clearance of (10)B after 290 mg BPA/kg body weight infusion from the blood with satisfactory accuracy during the first irradiation field (1 ppm, i.e., 7%). The accuracy of the two models in predicting the clearance of (10)B during the second irradiation field are for two-compartment model 1.0 ppm (8%) and 0.2 ppm (2%) for bi-exponential function. The models predict the average blood (10)B concentration with an increasing accuracy as more data points are available during the treatment.

CONCLUSION

By combining the two models, a robust and practical modeling tool is created for the estimation of the (10)B concentration in blood after BPA-F infusion.

Boron neutron capture therapy of brain tumors: biodistribution, pharmacokinetics, and radiation dosimetry sodium borocaptate in patients with gliomas

OBJECTIVE

The purpose of this study was to obtain tumor and normal brain tissue biodistribution data and pharmacokinetic profiles for sodium borocaptate (Na2B12H11SH) (BSH), a drug that has been used clinically in Europe and Japan for boron neutron capture therapy of brain tumors. The study was performed with a group of 25 patients who had preoperative diagnoses of either glioblastoma multiforme (GBM) or anaplastic astrocytoma (AA) and were candidates for debulking surgery. Nineteen of these patients were subsequently shown to have histopathologically confirmed diagnoses of GBM or AA, and they constituted the study population.

METHODS

BSH (non-10B-enriched) was infused intravenously, in a 1-hour period, at doses of 15, 25, and 50 mg boron/kg body weight (corresponding to 26.5, 44.1, and 88.2 mg BSH/kg body weight, respectively) to groups of 3, 3, and 13 patients, respectively. Multiple samples of tumor tissue, brain tissue around the tumors, and normal brain tissue were obtained at either 3 to 7 or 13 to 15 hours after infusion. Blood samples for pharmacokinetic studies were obtained at times up to 120 hours after termination of the infusion. Sixteen of the patients underwent surgery at the Beijing Neurosurgical Institute and three at The Ohio State University, where all tissue samples were subsequently analyzed for boron content by direct current plasma-atomic emission spectroscopy.

RESULTS

Blood boron values peaked at the end of the infusion and then decreased triexponentially during the 120-hour sampling period. At 6 hours after termination of the infusion, these values had decreased to 20.8, 29.1, and 62.6 microg/ml for boron doses of 15, 25, and 50 mg/kg body weight, respectively. For a boron dose of 50 mg/kg body weight, the maximum (mean +/- standard deviation) solid tumor boron values at 3 to 7 hours after infusion were 17.1+/-5.8 and 17.3+/-10.1 microg/g for GBMs and AAs, respectively, and the mean tumor value averaged across all samples was 11.9 microg/g for both GBMs and AAs. In contrast, the mean normal brain tissue values, averaged across all samples, were 4.6+/-5.1 and 5.5+/-3.9 microg/g and the tumor/normal brain tissue ratios were3.8 and 3.2 for patients with GBMs and AAs, respectively. The large standard deviations indicated significant heterogeneity in uptake in both tumor and normal brain tissue. Regions histopathologically classified either as a mixture of tumor and normal brain tissue or as infiltrating tumor exhibited slightly lower boron concentrations than those designated as solid tumor. After a dose of 50 mg/kg body weight, boron concentrations in blood decreased from 104 microg/ml at 2 hours to 63 microg/ml at 6 hours and concentrations in skin and muscle were 43.1 and 39.2 microg/g, respectively, during the 3- to 7-hour sampling period.

CONCLUSION

When tumor, blood, and normal tissue boron concentrations were taken into account, the most favorable tumor uptake data were obtained with a boron dose of 25 mg/kg body weight, 3 to 7 hours after termination of the infusion. Although blood boron levels were high, normal brain tissue boron levels were almost always lower than tumor levels. However, tumor boron concentrations were less than those necessary for boron neutron capture therapy, and there was significant intratumoral and interpatient variability in the uptake of BSH, which would make estimation of the radiation dose delivered to the tumor very difficult. It is unlikely that intravenous administration of a single dose of BSH would result in therapeutically useful levels of boron. However, combining BSH with boronophenylalanine, the other compound that has been used clinically, and optimizing their delivery could increase tumor boron uptake and potentially improve the efficacy of boron neutron capture therapy.

Boron neutron capture therapy of brain tumors: an emerging therapeutic modality

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10, a stable isotope, is irradiated with low-energy thermal neutrons to yield alpha particles and recoiling lithium-7 nuclei. For BNCT to be successful, a large number of 10B atoms must be localized on or preferably within neoplastic cells, and a sufficient number of thermal neutrons must be absorbed by the 10B atoms to sustain a lethal 10B (n, alpha) lithium-7 reaction. There is a growing interest in using BNCT in combination with surgery to treat patients with high-grade gliomas and possibly metastatic brain tumors. The present review covers the biological and radiobiological considerations on which BNCT is based, boron-containing low- and high-molecular weight delivery agents, neutron sources, clinical studies, and future areas of research. Two boron compounds currently are being used clinically, sodium borocaptate and boronophenylalanine, and a number of new delivery agents are under investigation, including boronated porphyrins, nucleosides, amino acids, polyamines, monoclonal and bispecific antibodies, liposomes, and epidermal growth factor. These are discussed, as is optimization of their delivery. Nuclear reactors currently are the only source of neutrons for BNCT, and the fission reaction within the core produces a mixture of lower energy thermal and epithermal neutrons, fast or high-energy neutrons, and gamma-rays. Although thermal neutron beams have been used clinically in Japan to treat patients with brain tumors and cutaneous melanomas, epithermal neutron beams now are being used in the United States and Europe because of their superior tissue-penetrating properties. Currently, there are clinical trials in progress in the United States, Europe, and Japan using a combination of debulking surgery and then BNCT to treat patients with glioblastomas. The American and European studies are Phase I trials using boronophenylalanine and sodium borocaptate, respectively, as capture agents, and the Japanese trial is a Phase II study. Boron compound and neutron dose escalation studies are planned, and these could lead to Phase II and possibly to randomized Phase III clinical trials that should provide data regarding therapeutic efficacy.