Boron neutron capture therapy of brain tumors: past history, current status, and future potential

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

A Review of Planned, Ongoing Clinical Studies and Recent Development of BNCT in Mainland of China

Boron neutron capture therapy (BNCT) is a promising cancer treatment modality that combines targeted boron agents and neutron irradiation to selectively destroy tumor cells. In mainland China, the clinical implementation of BNCT has made certain progress, primarily driven by the development of compact neutron source devices. The availability, ease of operation, and cost-effectiveness offered by these compact neutron sources make BNCT more accessible to cancer treatment centers. Two compact neutron sources, one being miniature reactor-based (IHNI-1) and the other one being accelerator-based (NeuPex), have entered the clinical research phase and are planned for medical device registration. Moreover, several accelerator-based neutron source devices employing different technical routes are currently under construction, further expanding the options for BNCT implementation. In addition, the development of compact neutron sources serves as an experimental platform for advancing the development of new boron agents. Several research teams are actively involved in the development of boron agents. Various types of third-generation boron agents have been tested and studied in vitro and in vivo. Compared to other radiotherapy therapies, BNCT in mainland China still faces specific challenges due to its limited clinical trial data and its technical support in a wide range of professional fields. To facilitate the widespread adoption of BNCT, it is crucial to establish relevant technical standards for neutron devices, boron agents, and treatment protocols.

Prompt gamma ray detection and imaging for boron neutron capture therapy using CdTe detector and novel detector shield - Monte Carlo study

BACKGROUND

For boron neutron capture therapy (BNCT), the improvements in patient dosimetry will require information about the spatial variation of ¹⁰ B concentration in the tumor and critical organs. A non-invasive approach, based on the detection of prompt gamma (PG) rays from the BNC reaction, may be well-suited to obtain such information. The detectability of the BNC PG rays has been shown experimentally utilizing energy-resolving cadmium telluride (CdTe) detectors. However, the feasibility of this approach under the clinically relevant conditions of BNCT is currently unknown.

PURPOSE

The present work aimed to investigate the aforementioned feasibility by performing Monte Carlo (MC) simulations under the phantom irradiation geometry relevant to accelerator-based BNCT (a-BNCT). Especially, this investigation focused on demonstrating the enhanced detection of the BNC PG rays using a novel neutron shield for CdTe detectors. Upon demonstrating the efficacy of the proposed detector shield, the BNC PG ray-based quantitative imaging of clinically relevant concentrations of ¹⁰ B was also demonstrated.

METHODS

The Geant4 MC simulation toolkit was used to model the phantom irradiation by an epithermal neutron beam as well as the detection of the BNC PG rays from the phantom by CdTe detectors with and without the proposed gadolinium (Gd)-based detector shield. It was also used to model the BNC PG ray-based quantitative imaging of ¹⁰ B concentrations under a-BNCT scenarios. Each model included a 20 cm-diameter/24 cm-height cylindrical PMMA phantom containing ¹⁰ B inserts at various concentrations. Arrays of CdTe crystals of 5 × 5 × 1 mm³ each (up to 120 in the case of a ring detector) were modeled for acquiring the BNC PG ray signals and quantitative imaging.

RESULTS

According to the MC simulations, thermalized neutrons from the phantom were found to reach the CdTe detector and captured by Cd and Te, resulting in the gamma ray background noise that directly interfered with the BNC PG ray signal. The proposed Gd-based detector shield was found to be highly effective in shielding thermal neutrons from the phantom, thereby reducing the unwanted gamma ray background noise. Owing to this shield, the detection of as low as seven parts-per-million (ppm) of ¹⁰ B within the phantom of clinically relevant size was possible using 20 billion incident neutron histories. Furthermore, quantitative imaging of ¹⁰ B distributed at low concentration (down to 50 ppm) within the phantom was demonstrated using computed tomography (CT) simulations with 16 billion incident neutron histories per angular projection. The ¹⁰ B detection limit (7.5 ppm) was also estimated using the reconstructed CT image. Both ¹⁰ B detection limits determined from this investigation are deemed clinically relevant for BNCT.

CONCLUSIONS

The proposed Gd-based detector shield played an essential role for achieving the currently reported ¹⁰ B detection limits. Overall, the present MC simulation work demonstrated highly sensitive BNC PG ray detection and imaging under a-BNCT scenarios using CdTe detectors coupled with a novel detector shield.

Towards carborane-functionalised structures for the treatment of brain cancer

Boron neutron capture therapy (BNCT) is a promising targeted chemoradiotherapeutic technique for the management of invasive brain tumors, such as glioblastoma multiforme (GBM). A prerequisite for effective BNCT is the selective targeting of tumour cells with ¹⁰B-rich therapeutic moieties. To this end, polyhedral boranes, especially carboranes, have received considerable attention because they combine a high boron content with relative low toxicity and metabolic inertness. Here, we review progress in the molecular design of recently investigated carborane derivatives in light of the widely accepted performance requirements for effective BNCT.

Hierarchical host–guest assemblies formed on dodecaborate-coated gold nanoparticles

Undecahydro-mercapto-closo-dodecaborate (BSH), a purely inorganic cluster anion, serves as an unconventional stabilizing ligand for the preparation of gold nanoparticles (AuNPs).

Synthesis, characterization and biological evaluation of carboranylmethylbenzo[b]acridones as novel agents for boron neutron capture therapy

Herein we present the synthesis and characterization of benzo[b]acridin-12(7H)-ones bearing carboranyl moieties and test their biological effectiveness as boron neutron capture therapy (BNCT) agents in cancer treatment. The cellular uptake of these novel compounds into the U87 human glioblastoma cells was evaluated by boron analysis (ICP-MS) and by fluorescence imaging (confocal microscopy). The compounds enter the U87 cells exhibiting a similar profile, i.e., preferential accumulation in the cytoskeleton and membranes and a low cytotoxic activity (IC50 values higher than 200 μM). The cytotoxic activity and cellular morphological alterations after neutron irradiation in the Portuguese Research Reactor (6.6 × 10(7) neutrons cm(-2) s(-1), 1 MW) were evaluated by the MTT assay and by electron microscopy (TEM). Post-neutron irradiation revealed that BNCT has a higher cytotoxic effect on the cells. Accumulation of membranous whorls in the cytoplasm of cells treated with one of the compounds correlates well with the cytotoxic effect induced by radiation. Results provide a strong rationale for considering one of these compounds as a lead candidate for a new generation of BNCT agents.

Potential dual imaging nanoparticle: Gd2O3 nanoparticle

Gadolinium (Gd) is a unique and powerful element in chemistry and biomedicine which can be applied simultaneously to magnetic resonance imaging (MRI), X-ray computed tomography (CT), and neutron capture therapy for cancers. This multifunctionality can be maximized using gadolinium oxide (Gd₂O₃) nanoparticles (GNPs) because of the large amount of Gd per GNP, making both diagnosis and therapy (i.e., theragnosis) for cancers possible using only GNPs. In this study, the T₁ MRI and CT dual imaging capability of GNPs is explored by synthesizing various iodine compound (IC) coated GNPs (IC-GNPs). All the IC-GNP samples showed stronger X-ray absorption and larger longitudinal water proton relaxivities (r₁ = 26–38 s⁻¹mM⁻¹ and r₂/r₁ = 1.4–1.9) than the respective commercial contrast agents. In vivo T₁ MR and CT images of mice were also acquired, supporting that the GNP is a potential dual imaging agent.

The study of hyaluronic acid compounds for neutron capture and photon activation therapies

The therapy of radioresistant tumors remains an urgent problem in medicine. To solve this problem neutron capture therapy (NCT) and photon activation therapy (PAT) are used. The essential feature of this such technique is the uptake of tumor chemical elements, which interact with thermal neutrons (NCT) and X-rays (PAT). The aims of the investigation were to study a biodistribution of the complexes of hyaluronic acid with boron (3 mg B mL−1) and gold (20 mg Au mL−1) in mice with melanoma B-16 after intratumoral administration. An optimal time for NCT was 30 minutes after administration when boron concentration in the tumor was more than 30 μg g−1 and exceeded boron content in surrounding tissues. The maximal gold content in tumor (180–260 mg g−1) was obtained in 30 minutes after the preparation introduction. The highest ratios of gold in tumor and surrounding tissues (a necessary condition for forming of the local absorbed dose in tumor) was obtained in 0.5 and 1 h. On the basis of the data obtained, it is possible to assume the perspectives of the non-toxic boron compounds for use in NCT and the gold compounds for use primarily as contrast agents for diagnostic purposes.

Pharmacokinetics of BPA in gliomas with ultrasound induced blood-brain barrier disruption as measured by microdialysis

The blood-brain barrier (BBB) can be transiently disrupted by focused ultrasound (FUS) in the presence of microbubbles for targeted drug delivery. Previous studies have illustrated the pharmacokinetics of drug delivery across the BBB after sonication using indirect visualization techniques. In this study, we investigated the in vivo extracellular kinetics of boronophenylalanine-fructose (BPA-f) in glioma-bearing rats with FUS-induced BBB disruption by microdialysis. After simultaneous intravenous administration of BPA and FUS exposure, the boron concentration in the treated brains was quantified by inductively coupled plasma mass spectroscopy. With FUS, the mean peak concentration of BPA-f in the glioma dialysate was 3.6 times greater than without FUS, and the area under the concentration-time curve was 2.1 times greater. This study demonstrates that intracerebral microdialysis can be used to assess local BBB transport profiles of drugs in a sonicated site. Applying microdialysis to the study of metabolism and pharmacokinetics is useful for obtaining selective information within a specific brain site after FUS-induced BBB disruption.

Boron neutron capture therapy for glioblastoma multiforme: enhanced drug delivery and antitumor effect following blood-brain barrier disruption induced by focused ultrasound

AIM

This study investigated whether the efficacy of boron neutron capture therapy was enhanced by means of intravenous administration of boronophenylalanine (BPA) with blood-brain barrier disruption induced by focused ultrasound (FUS).

MATERIALS & METHODS

BPA was administered, followed by pulsed FUS, and the boron concentration in the treated brains was quantified by inductively coupled plasma mass spectroscopy. Growth of the firefly luciferase-labeled glioma cells was monitored through noninvasive biophotonic imaging. Finally, the brain tissue was histologically examined after sacrifice.

RESULTS

Compared with the nonsonicated tumor group, animals treated with an injection of 500 mg/kg of BPA followed by FUS exhibited not only significantly increased accumulation of the drug at the sonicated tumor site, but also a significantly elevated tumor-to-normal brain drug ratio (p < 0.05).

DISCUSSION

The data demonstrated that FUS significantly enhances the tumor-to-normal brain drug ratio in the sonicated tumor and subsequently the efficacy of boron neutron capture therapy.

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.

Design and synthesis of fluorescence-labeled closo-dodecaborate lipid: its liposome formation and in vivo imaging targeting of tumors for boron neutron capture therapy

The fluorescence-labeled closo-dodecaborane lipid (FL-SBL) was synthesized from (S)-(+)-1,2-isopropylideneglycerol as a chiral starting material. FL-SBL was readily accumulated into the PEGylated DSPC liposomes prepared from DSPC, CH, and DSPE-PEG-OMe by the post insertion protocol. The boron concentrations and the fluorescent intensities of the FL-SBL-labeled DSPC liposomes increased with the increase of the additive FL-SBL, and the maximum emission wavelength of the liposomes appeared at 531 nm. A preliminary in vivo imaging study of tumor-bearing mice revealed that the FL-SBL-labeled DSPC liposomes were delivered to the tumor tissue but not distributed to hypoxic regions.

Synthesis and stereochemistry of highly unsymmetric beta,meso-linked porphyrin arrays

Porphyrin arrays with tailor-made photophysical properties and well-defined three-dimensional geometries constitute attractive synthetic targets in porphyrin chemistry. The paper describes a variable, straightforward synthetic procedure for the construction of beta,meso-linked porphyrin multichromophores in good to excellent yields. In a Suzuki-type coupling reaction beta-borylated 5,10,15,20-tetraarylporphyrins (TAPs) served as versatile building blocks for the preparation of a plethora of directly linked, unsymmetrically substituted di- and triporphyrins. Besides their interesting photophysical properties, especially the trimeric porphyrin arrays show exciting stereochemical features. The established protocols thus open a convenient entry into the synthesis of achiral and chiral, unsymmetrically substituted beta,meso-linked oligoporphyrins, e.g., for applications in biomedicine or nonlinear optics.

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.

Delivery of gadolinium-labeled nanoparticles to the sentinel lymph node: comparison of the sentinel node visualization and estimations of intra-nodal gadolinium concentration by the magnetic resonance imaging

Sentinel node imaging is commonly performed prior to surgery for breast cancer and melanoma. While current methods are based on radio-lymphoscintigraphy, MR lymphangiography (MRL) offers the benefits of better spatial resolution without ionizing radiation. However, the optimal nanoparticle for imaging the sentinel nodes remains unclear. Gadolinium-labeled (Gd) contrast agents ranging in diameter from <1 to 12 nm were evaluated to determine which size provides the most rapid and most concentrated delivery of contrast agent to the lymph nodes in a mouse model of lymphatic metastases. Specifically, PAMAM-G2, -G4, -G6 and -G8, and DAB-G5 Gd-dendrimer agents, as well as Gadomer-17 and Gd-DTPA, were compared. Among these agents, the G6 Gd dendrimer depicted the lymphatics and lymph nodes with the highest peak concentrations and this occurred 24-36 min post-injection (p<0.01; all except G8). Based on ex vivo concentration phantoms, high accumulations of Gd(III) ions occurred within lymph nodes (1.7-4.4 mM Gd/270-680 ppm Gd) with high target to background ratios (>100). These concentrations are sufficient to contemplate the use of Gd-neutron capture therapy of regional lymph nodes. Thus, when injected interstitially, the PAMAM-G6 Gd dendrimer not only provides excellent opacification of sentinel lymph nodes, but also provides the potential for targeted therapy of sentinel lymph nodes.

Analysis of 10B antitumoral compounds by means of flow-injection into ESI-MS/MS

Boron neutron capture therapy (BNCT) is a promising binary treatment for cancer. BNCT is based on the ability of the nonradioactive isotope (10)B to capture, with a very high probability, thermal neutrons. This nuclear reaction results in two particles (an alpha and a lithium nucleus). The particles have a high biological effectiveness, which is limited in tissue to approximately the diameter of one cell. If the reaction can be limited to a tumor cell, the physical characteristic opens up the possibility to selectively destroy cancer cells, while sparing the surrounding healthy tissue. Quality control of (10)B-containing compounds and their distribution at present are very important, and different analytical methods have been developed, such as time-of-flight secondary ion mass spectrometry (TOF-SIMS), electron energy loss spectrometry (EELS), prompt gamma analysis and inductively coupled plasma-optical emission spectrometry (ICP-OES). These methods allow the analyses of (10)B, but it is not possible to characterize the specific molecular compounds containing (10)B. For this reason, we propose a fast and quantitative method that permits the determination of closo-undecahydro-1-mercaptododecaborate (BSH) and (10)boron-phenylalanine (BPA) and their eventual metabolites. In particular, (10)B-containing compounds are detected by means of flow-injection electrospray tandem mass spectrometry (FI/ESI-MS/MS). This approach allows the identification of Boron compounds, BSH and BPA, using tandem mass spectrometry, and quantitative analysis is also possible (c.v. +/-4.7%; n = 5; linear range 10-10,000 ng/ml). Furthermore, (10)B-containing compounds were detected in actual biological sample (urine and plasma, diluted 10,000- and 1,000-fold, respectively) injecting a small volume (1 microl) of diluted samples.

The effect of dexamethasone on the uptake of p-boronophenylalanine in the rat brain and intracranial 9L gliosarcoma

The steroid dexamethasone sodium phosphate (DEX) is routinely used to treat edema in brain tumor patients. The objective of the present study was to evaluate the effects of DEX on the uptake of boronophenylalanine (BPA) using the rat 9L gliosarcoma tumor model and surrounding brain tissue. Two steroid dosage protocols were used. The high-dose DEX protocol involved five 3mg/kg intraperitoneal injections at 47, 35, 23, 11 and 1 h prior to the administration of the BPA for a total dose of 15 mg DEX/kg rat. The low-dose DEX administration protocol involved two doses of 1.5mg/kg at 17 h and 1h prior to BPA injection for a total dose of 3mg DEX/kg rat. The control animals received no pretreatment, prior to the administration of BPA. Seventeen days after tumor implantation, rats were injected i.p. with 0.014 ml/g body weight BPA solution (1200 mg BPA/kg; approximately 59 mg (10)B/kg). In all groups, rats were euthanized at 3h after BPA injection. Administration of the steroid had an effect on tumor weight, which decreased to approximately 78% (p > 0.05) of the control weight in the low-dose DEX group, and approximately 48% (p < 0.001) of the control weight in the high-dose DEX group. At 3 h after the administration of BPA, the concentration of boron in tumor was comparable (p > 0.1) in the control and high-dose DEX groups. The lowest mean value (73.8+/-1.6 microg/g) was obtained in the low-dose DEX group. This was significantly lower (p > 0.02) than the tumor boron contents in the high-dose DEX and control groups, which were 81.1+/-1.9 and 79.9+/-1.7 microg/g, respectively. Tumor:blood boron partition ratios for the control, low- and high-dose DEX groups were 2.3, 2.3 and 2.5, respectively. Boron concentrations were also measured in the normal brain and in the zone of brain adjacent to the tumor exhibiting edema. Although treatment with DEX had no appreciable effect on boron uptake in the normal brain of the rat, after the administration of BPA, it did impact on the boron levels in the zone of peritumoral edema. After the high-dose DEX administration protocol, boron levels in the zone of edema were reduced by approximately 14% (p < 0.02). This finding suggests that BPA targeting of tumor cells in the peritumoral zone could be compromised by DEX. These cells appear to play a critical role in tumor recurrence after BNCT or conventional radiotherapy.

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

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

Synthesis, preformulation and liposomal formulation of cholesteryl carborane esters with various fatty chains

The elevated expression of LDL receptor on tumor cells provides one attractive approach for targeted drug delivery to tumor cells. Suitable antitumor compounds, however, need to be synthesized and developed which mimic the native cholesteryl esters (as major constituent of LDL) in chemical structure for targeted delivery to tumor cells through the over-expressed LDL receptors. In the present study, new antitumor compounds were designed containing cholesterol, fatty chain and carborane which is used as the antitumor unit. Three new compounds were synthesized with a three-step reaction scheme. Similar to the native cholesteryl esters, these compounds are extremely hydrophobic and, before any further biological studies, suitable liposomal formulations for these new compounds are required. Various liposomal formulations as well as the preformulation characterization of these new compounds were thus examined. The incorporation efficiency of the compounds in liposomes was found to vary significantly depending on the type of fatty chain attached and the ratio of cholesterol:phospholipid used as the excipients of liposomal formulation.

Subcellular localization of boron in cultured melanoma cells by electron energy-loss spectroscopy of freeze-dried cryosections

Boron neutron capture therapy (BNCT) is based on the ability of the non-radioactive isotope 10B to capture thermal neutrons and to disintegrate instantaneously. This reaction opens a way to selectively destroy tumour cells after specific uptake of 10B. In this paper, a method based on electron energy-loss spectroscopy is presented for detecting and quantifying boron in freeze-dried cryosections of human melanoma cells. A practical detection limit of around 6 mmol kg-1 in 0.1- micro m2 areas is estimated using specimens prepared from standard boron solutions. Preliminary results of boron mapping in the spectrum-imaging acquisition mode reveal boron penetration and probably spot-like accumulation within melanoma cells when exposed to culture medium containing sodium borocaptate.

Common challenges and problems in clinical trials of boron neutron capture therapy of brain tumors

Clinical trials for binary therapies, like boron neutron capture therapy (BNCT), pose a number of unique problems and challenges in design, performance, and interpretation of results. In neutron beam development, different groups use different optimization parameters, resulting in beams being considerably different from each other. The design, development, testing, execution of patient pharmacokinetics and the evaluation of results from these studies differ widely. Finally, the clinical trials involving patient treatments vary in many aspects such as their dose escalation strategies, treatment planning methodologies, and the reporting of data. The implications of these differences in the data accrued from these trials are discussed. The BNCT community needs to standardize each aspect of the design, implementation, and reporting of clinical trials so that the data can be used meaningfully.

Ligand liposomes and boron neutron capture therapy

Boron neutron capture therapy (BNCT) has been used both experimentally and clinically for the treatment of gliomas and melanomas, with varying results. However, the therapeutic effects on micro-invasive tumor cells are not clear. The two drugs that have been used clinically, p-boronophenylalanine, (BPA), and the sulfhydryl borane, (BSH), seem to be taken up preferentially in solid tumor areas but it is uncertain whether enough boron is taken up by micro-invasive tumor cells. To increase the selective uptake of boron by such cells, would be to exploit tumor transformation related cellular changes such as over-expression of growth factor receptors. However, the number of receptors varies from small to large and the uptake of large amounts of boron for each receptor interaction is necessary in order to deliver sufficient amounts of boron. Therefore, each targeting moiety must deliver large number of boron atoms. One possible way to meet these requirements would be to use receptor-targeting ligand liposomes, containing large number of boron atoms. This will be the subject of this review and studies of boron containing liposomes, with or without ligand, will be discussed. Two recent examples from the literature are ligand liposomes targeting either folate or epidermal growth factor (EGF) receptors on tumor cells. Other potential receptors on gliomas include PDGFR and EGFRvIII. Besides the appropriate choice of target receptor, it is also important to consider delivery of the ligand liposomes, their pharmacodynamics and pharmacokinetics and cellular processing, subjects that also will be discussed in this review.

Tumor-cell targeted epiderimal growth factor liposomes loaded with boronated acridine: uptake and processing

PURPOSE

The aim of this work was to investigate the cellular binding and processing of polyethylene glycol-stabilized epidermal growth factor (EGF) liposomes. The liposomes were actively loaded with water-soluble boronated acridine (WSA), primarily developed for boron neutron capture therapy.

METHODS

The uptake, internalization, and retention of EGF-liposome conjugates were studied in two cultured monolayer cell-lines, A-431 and U-343, with regard to the nuclide-label on the targeting agent, the carrier, and the load. The subcellular localization of WSA was studied using confocal microscopy.

RESULTS

We found that the liposome complex was internalized after specific binding to the EGF receptor. After internalization in the tumor cells, WSA was distributed mainly in the cytoplasm and was shown to have long cellular retention, with 80% of the boron remaining after 48 h.

CONCLUSIONS

The long retention of the compound and the cellular boron concentration reached makes these targeted liposomes interesting for further development toward boron neutron capture therapy.

Treatment planning and dosimetry for the Harvard-MIT Phase I clinical trial of cranial neutron capture therapy

PURPOSE

A Phase I trial of cranial neutron capture therapy (NCT) was conducted at Harvard-MIT. The trial was designed to determine maximum tolerated NCT radiation dose to normal brain.

METHODS AND MATERIALS

Twenty-two patients with brain tumors were treated by infusion of boronophenylalanine-fructose (BPA-f) followed by exposure to epithermal neutrons. The study began with a prescribed biologically weighted dose of 8.8 RBE (relative biologic effectiveness) Gy, escalated in compounding 10% increments, and ended at 14.2 RBE Gy. BPA-f was infused at a dose 250-350 mg/kg body weight. Treatments were planned using MacNCTPlan and MCNP 4B. Irradiations were delivered as one, two, or three fields in one or two fractions.

RESULTS

Peak biologically weighted normal tissue dose ranged from 8.7 to 16.4 RBE Gy. The average dose to brain ranged from 2.7 to 7.4 RBE Gy. Average tumor dose was estimated to range from 14.5 to 43.9 RBE Gy, with a mean of 25.7 RBE Gy.

CONCLUSIONS

We have demonstrated that BPA-f-mediated NCT can be precisely planned and delivered in a carefully controlled manner. Subsequent clinical trials of boron neutron capture therapy at Harvard and MIT will be initiated with a new high-intensity, high-quality epithermal neutron beam.

Selective uptake of p-borophenylalanine by undifferentiated thyroid carcinoma for boron neutron capture therapy

Undifferentiated thyroid carcinoma (UTC) lacks an effective treatment. Boron neutron capture therapy (BNCT) is based on the selective uptake of 10B-boronated compounds by some tumors, followed by irradiation with an appropriate neutron beam. The radioactive boron originated (11B) decays releasing 7Li, gamma rays and alpha particles, and these latter will destroy the tumor. In order to explore the possibility of applying BNCT to UTC we have studied the biodistribution of BPA. In in vitro studies, the uptake of p-10borophenylalanine (BPA) by the UTC cell line ARO, primary cultures of normal bovine thyroid cells (BT), and human follicular adenoma (FA) thyroid was studied. No difference in BPA uptake was observed between proliferating and quiescent ARO cells. The uptake by quiescent ARO, BT, and FA showed that the ARO/BT and ARO/FA ratios were 4 and 5, respectively (p < 0.001). In in vivo studies, ARO cells were transplanted into the scapular region of NIH nude mice, and after 2 weeks BPA (350 or 600 mg/kg body weight) was injected intraperitoneally. The animals were sacrificed between 30 and 150 minutes after the injection. With 350 mg, tumor uptake was highest after 60 minutes and the tumor/normal thyroid and tumor/blood ratios were 3 and 5, respectively. When 600 mg/kg body weight BPA were administered, after 90 minutes the tumor/blood, tumor/normal thyroid, and tumor/distal skin ratios for 10B concentrations per gram of tissue were approximately 3, showing a selective uptake by the tumor. The present experimental results open the possibility of applying BNCT for the treatment of UTC.

Optimization of digital filters for the detection of trace elements in EELS. III--Gaussian, homomorphic and adaptive filters

This paper is the third part of a series of papers dealing with the development of digital filters for the extraction of information from electron energy loss spectra, when the signal-to-background is very weak, as it is the case for elements in weak concentration. Three new filtering approaches are investigated: the use of high-order derivatives (typically, the fourth order derivative of a Gaussian), the use of homomorphic filtering (working on logarithmically-transformed spectra) and the use of adaptive filtering, taking into account the local background shape. These signal processing methods are applied to real spectra to demonstrate and comment their effects in three different cases of signal and background shapes related to calcium, phosphorus and boron detection in definite thickness organic matrix.

Relative biological efficiency for the induction of various gene mutations in normal and enriched with 10B Tradescantia cells by neutrons from 252Cf source

The effectiveness of neutrons from a Californium-252 source in the induction of various abnormalities in the Tradescantia clone 4430 stamen hair cells (Trad-SH assay) were studied. A special attention was paid to check whether any enhancement in effects is visible in the cells enriched with boron ions. Inflorescences, normal or pretreated with chemicals containing boron, were irradiated in the air with neutrons from a 252Cf source at KAERI, Taejon, Korea. To estimate the relative biological effectiveness (RBE) of the beam under the study, numbers of Tradescantia inflorescence without chemical pretreatment were irradiated with various doses of X-rays. The ranges of radiation doses used for neutrons were 0-1.0Gy and for X-rays 0-0.5Gy. Following the culturing according to standard procedures screening of gene and lethal mutations in somatic cells of stamen hairs was done in the extended period, between days 7 and 19 after exposures. Maximal RBE values for the induction of pink, colorless and lethal mutations were evaluated from comparison of the slopes in linear parts of the dose response curves obtained after irradiation with X-rays and californium source. The RBE(max) value or the induction of gene mutation was estimated as 7.2 comparing the value 5.6 in the studies reported earlier. The comparison of dose-response curves and its alteration, due to changes in the cells and plants environment during and after irradiation, explains the observed differences. Inflorescence pretreated with borax responded to neutrons differently depending on the biological end points. Although, for the induction of pink mutations no significant difference was observed, though, in the case of cell lethality, pretreated with boron ion plants have shoved a statistically significant increase of the RBE value from 5.5 to 34.7, and in the case of colorless mutations from 1.6 to 5.6.

Tolerance of the normal canine brain to epithermal neutron irradiation in the presence of p-boronophenylalanine

Twelve normal dogs underwent brain irradiation in a mixed-radiation, mainly epithermal neutron field at the Brookhaven Medical Research Reactor following intravenous infusion of 950 mg of 10B-enriched BPA/kg as its fructose complex. The 5 x 10 cm irradiation aperture was centered over the left hemisphere. For a subgroup of dogs reported previously, we now present more detailed analyses including dose-volume relationships, longer follow-ups, MRIs, and histopathological observations. Peak doses (delivered to 1 cm3 of brain at the depth of maximum thermal neutron flux) ranged from 7.6 Gy (photon-equivalent dose: 11.8 Gy-Eq) to 11.6 Gy (17.5 Gy-Eq). The average dose to the brain ranged from 3.0 Gy (4.5 Gy-Eq) to 8.1 Gy (11.9 Gy-Eq) and to the left hemisphere, 6.6 Gy (10.1 Gy-Eq) to 10.0 Gy (15.0 Gy-Eq). Maximum tolerated 'threshold' doses were 6.7 Gy (9.8 Gy-Eq) to the whole brain and 8.2 Gy (12.3 Gy-Eq) to one hemisphere. The threshold peak brain dose was 9.5 Gy (14.3 Gy-Eq). At doses below threshold, some dogs developed subclinical MRI changes. Above threshold, all dogs developed dose-dependent MRI changes, neurological deficits, and focal brain necrosis.

Boron-loaded liposomes in the treatment of hepatic metastases: preliminary investigation by autoradiography analysis

Boronophenylalanine (BPA)-loaded conventional and stabilized liposomes were prepared by the reversed phase evaporation method to treat liver metastases by boron neutron capture therapy. Conventional vesicles were composed of phosphatidylcholine and cholesterol, molar ratio 1:1. To obtain stealth liposomes, GM1 or PEG were included in the lipidic bilayer at a concentration of 6.67 or 5 mol%, respectively. Large unilamellar vesicles were formulated encapsulating BPA in the liposome aqueous compartment as a complex with fructose; BPA free base also was embedded into the lipidic bilayer. In vivo experiments were carried out after intravenous injection of liposome suspensions in BD-IX strain rats in which liver metastases had been induced. Alpha particle spectroscopy associated with histological analysis was performed to visualize boron spatial distribution in liver. Simultaneously, tissue boron concentrations were determined using inductively coupled plasma-mass spectroscopy. Results showed that PEG-modified liposomes accumulated boron in therapeutic concentrations (> 30 micrograms boron/g tissue) in metastatic tissue. The PEG-liposomes could be further explored in enhancing boron delivery to tumor cells.

On the optimal energy of epithermal neutron beams for BNCT

The optimal neutron energy for the treatment of deep-seated tumours using boron neutron capture therapy is studied by analysing various figures of merit. In particular, analysis of the therapeutic gain as a function of the neutron energy indicates that, with the currently available 10B carriers, the most useful neutrons for the treatment of deep-seated tumours, in particular glioblastoma multiforme, are those with an energy of a few keV. Based on the results of the simulations, a method is presented which allows us to evaluate the quality of epithermal neutron beams of known energy spectrum, thus allowing us to compare different neutron-producing reactions and beam-shaping assembly configurations used for accelerator-based neutron sources.

Boronated protoporphyrin (BOPP): localization in lysosomes of the human glioma cell line SF-767 with uptake modulated by lipoprotein levels

PURPOSE

Boronated protoporphyrin (BOPP) is a candidate for use in both boron neutron capture therapy (BNCT) and photodynamic therapy (PDT) of glioblastoma multiforme (GBM). Our objectives are to identify factors that influence the uptake and retention of BOPP in vitro and to determine BOPP distribution in a human glioma cell line in vitro. This information will aid the development of compounds and treatment strategies that increase the effectiveness of BNCT therapy for GBM.

METHODS AND MATERIALS

The amount, distribution pattern, and site of internalization of BOPP were assessed using fluorescence microscopy. Living human glioma (SF-767) cells were imaged after a 24-h exposure to BOPP (20-135.6 microg/ml, normal serum). Dose-dependent uptake of BOPP was determined using both fluorescence microscopy of individual living cells and inductively-coupled plasma-atomic emission spectroscopy (ICP-AES) analysis of cell pellets. Lysosome- or mitochondria-specific fluorescent probes were used to identify the cellular compartment containing BOPP. Two human fibroblast cell lines, AG-1522 (LDL receptor-positive) and GM019-15C (LDL receptor-deficient), were used to investigate LDL receptor-dependent BOPP uptake. The dependence of BOPP uptake on lipoproteins in the media was determined by exposing each of the three cell types to BOPP in medium containing either normal (NS) or lipoprotein deficient serum (LPDS).

RESULTS

BOPP accumulated in the lysosomes of human glioma cells in vitro, and not in the mitochondria, as reported for C6 rat glioma cells in vitro. BOPP uptake was concentration-dependent and was also dependent on the amount of lipoproteins in the medium. Over the range of incubation concentrations studied and at the single exposure duration time point investigated (24 h), all cells retained a similar amount of BOPP. At the lowest incubation concentration (20 microg/ml, NS), the amount of boron retained was near 10(9) atoms per cell (15 microg B/g cells). Lysosomes containing high concentrations of BOPP were randomly distributed throughout the cytoplasm; however, larger lysosomes containing BOPP were concentrated around the cell nucleus. Little or no BOPP accumulated in the cell nucleus. At incubation concentrations of 20 and 40 microg/ml (24-h time point), BOPP uptake in SF-767 cells was reduced in LPDS compared with NS (66% reduction). A similar result was observed for normal human fibroblasts (AG-1522 cells, 40 microg/ml, 24 h). At 40 microg/ml, in both NS and LPDS at 24 h, BOPP accumulation in LDL receptor-deficient human fibroblasts (GM019-15C cells) was reduced relative to AG-1522 cells. BOPP accumulation in GM019-15C cells (40 microg/ml, 24 h) was not affected by serum lipoprotein levels.

CONCLUSION

In cell culture, BOPP is taken up by human glioma cells via the LDL pathway and is compartmentalized into cellular lysosomes. Knowledge of this mechanism of BOPP uptake and retention will be important in attempts to modify toxicity and efficacy of this drug.

Measurements of low-energy (d,n) reactions for BNCT. Boron Neutron Capture Therapy

Neutron yields and energy spectra have been measured for various deuteron-induced reactions at low energy. Neutrons of energy > 100 keV emitted in the 9Be(d,n)10B, 12C(d,n)13N, and 13C(d,n)14N reactions at Ed= 1.5 MeV were detected at five angles by means of liquid scintillator detectors. While low-energy neutrons were observed in all studied reactions, only 13C(d,n)14N is characterized by a relatively large yield with spectral features potentially interesting for an accelerator-based neutron source for BNCT.

Reoxygenation in quiescent and total intratumor cells following thermal neutron irradiation with or without (10)B-compound-compared with that after gamma-ray irradiation

PURPOSE

Reoxygenation in quiescent (Q) and total tumor cells within solid tumors after thermal neutron irradiation with or without (10)B-compound was examined, comparing with that following gamma-ray irradiation.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty minutes after intraperitoneal injection of sodium borocaptate-(10)B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-(10)B (BPA), the tumors were irradiated with thermal neutrons, or those without (10)B-compounds were irradiated with thermal neutrons alone or gamma-rays. At various time points after each treatment, a series of test doses of gamma-rays were given to tumor-bearing mice while alive or after being killed to obtain hypoxic fractions in the tumors. Immediately after irradiation, the tumors were excised, minced, and trypsinized. Following incubation of tumor cells with cytokinesis blocker, the micronucleus (MN) frequency in cells without BrdU labeling ( = Q cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU. The MN frequency of BrdU-unlabeled cells was then used to calculate the surviving fraction of the unlabeled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total tumor cells.

RESULTS

In both total and Q tumor cells, the hypoxic fractions immediately after each treatment went up suddenly. Reoxygenation after each treatment occurred more rapidly in total cells than in Q cells. In both cell populations, reoxygenation appeared to be rapidly induced in the following order: neutron irradiation without (10) gamma-ray irradiation.

CONCLUSION

Based on our previous report that total and Q cell fractions within these tumors have larger acutely and chronically hypoxic fractions, respectively, acute hypoxic cells appeared to play a larger role in reoxygenation. BSH was thought to have a potential to distribute more homogeneously in solid tumors than BPA, because BSH induced the nearer reoxygenation pattern to that following neutron irradiation alone than BPA.

Pharmacological applications of inorganic complexes

Inorganic complexes have long been utilized for many therapeutic purposes. They were used or tried, perhaps because of the general notion that inorganic compounds (e.g., metal complexes) are toxic and a controlled use of such a compound may suppress some biological process. In this review, we briefly outline the properties of several selected groups of inorganic complexes and how they can affect biological systems and contribute to human pathologies.