Enhanced delivery of boronophenylalanine for neutron capture therapy by means of intracarotid injection and blood-brain barrier disruption

Evaluation of sodium borocaptate (BSH) and boronophenylalanine (BPA) as boron delivery agents for neutron capture therapy (NCT) of cancer: an update and a guide for the future clinical evaluation of new boron delivery agents for NCT

Boron neutron capture therapy (BNCT) is a cancer treatment modality based on the nuclear capture and fission reactions that occur when boron-10, a stable isotope, is irradiated with neutrons of the appropriate energy to produce boron-11 in an unstable form, which undergoes instantaneous nuclear fission to produce high-energy, tumoricidal alpha particles. The primary purpose of this review is to provide an update on the first drug used clinically, sodium borocaptate (BSH), by the Japanese neurosurgeon Hiroshi Hatanaka to treat patients with brain tumors and the second drug, boronophenylalanine (BPA), which first was used clinically by the Japanese dermatologist Yutaka Mishima to treat patients with cutaneous melanomas. Subsequently, BPA has become the primary drug used as a boron delivery agent to treat patients with several types of cancers, specifically brain tumors and recurrent tumors of the head and neck region. The focus of this review will be on the initial studies that were carried out to define the pharmacokinetics and pharmacodynamics of BSH and BPA and their biodistribution in tumor and normal tissues following administration to patients with high-grade gliomas and their subsequent clinical use to treat patients with high-grade gliomas. First, we will summarize the studies that were carried out in Japan with BSH and subsequently at our own institution, The Ohio State University, and those of several other groups. Second, we will describe studies carried out in Japan with BPA and then in the United States that have led to its use as the primary drug that is being used clinically for BNCT. Third, although there have been intense efforts to develop new and better boron delivery agents for BNCT, none of these have yet been evaluated clinically. The present report will provide a guide to the future clinical evaluation of new boron delivery agents prior to their clinical use for BNCT.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

Strategies for Improved Intra-arterial Treatments Targeting Brain Tumors: a Systematic Review

Conventional treatments for brain tumors relying on surgery, radiation, and systemic chemotherapy are often associated with high recurrence and poor prognosis. In recent decades, intra-arterial administration of anti-cancer drugs has been considered a suitable alternative drug delivery route to intravenous and oral administration. Intra-arterial administration is believed to offer increasing drug responses by primary and metastatic brain tumors, and to be associated with better median overall survival. By directly injecting therapeutic agents into carotid or vertebral artery, intra-arterial administration rapidly increases intra-tumoral drug concentration but lowers systemic exposure. However, unexpected vascular or neural toxicity has questioned the therapeutic safety of intra-arterial drug administration and limits its widespread clinical application. Therefore, improving targeting and accuracy of intra-arterial administration has become a major research focus. This systematic review categorizes strategies for optimizing intra-arterial administration into five categories: (1) transient blood-brain barrier (BBB)/blood-tumor barrier (BTB) disruption, (2) regional cerebral hypoperfusion for peritumoral hemodynamic changes, (3) superselective endovascular intervention, (4) high-resolution imaging techniques, and (5) others such as cell and gene therapy. We summarize and discuss both preclinical and clinical research, focusing on advantages and disadvantages of different treatment strategies for a variety of cerebral tumor types.

Boron delivery agents for neutron capture therapy of cancer

Boron neutron capture therapy (BNCT) is a binary radiotherapeutic modality based on the nuclear capture and fission reactions that occur when the stable isotope, boron-10, is irradiated with neutrons to produce high energy alpha particles. This review will focus on tumor-targeting boron delivery agents that are an essential component of this binary system. Two low molecular weight boron-containing drugs currently are being used clinically, boronophenylalanine (BPA) and sodium borocaptate (BSH). Although they are far from being ideal, their therapeutic efficacy has been demonstrated in patients with high grade gliomas, recurrent tumors of the head and neck region, and a much smaller number with cutaneous and extra-cutaneous melanomas. Because of their limitations, great effort has been expended over the past 40 years to develop new boron delivery agents that have more favorable biodistribution and uptake for clinical use. These include boron-containing porphyrins, amino acids, polyamines, nucleosides, peptides, monoclonal antibodies, liposomes, nanoparticles of various types, boron cluster compounds and co-polymers. Currently, however, none of these have reached the stage where there is enough convincing data to warrant clinical biodistribution studies. Therefore, at present the best way to further improve the clinical efficacy of BNCT would be to optimize the dosing paradigms and delivery of BPA and BSH, either alone or in combination, with the hope that future research will identify new and better boron delivery agents for clinical use.

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.

Effects of l-DOPA pre-loading on the uptake of boronophenylalanine using the F98 glioma and B16 melanoma models

The present study was undertaken to evaluate the effects of l-DOPA pre-loading on the uptake of BPA using the F98 rat glioma and the murine B16 melanoma models. In vitro pretreatments of F98 glioma and B16 melanoma cells with l-DOPA, followed by exposure to BPA increased boron uptake, as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Based on this, in vivo studies were initiated in F98 glioma bearing rats. Initially, the l-DOPA dosing paradigm was evaluated. Maximum tumor boron uptake was observed following i.p. administration of l-DOPA (50mg/kg) followed 24h later by BPA (31.8±8.9 vs. 17.2±6.3µg/g for BPA alone). Next, the effect of l-DOPA pre-loading as a function of the route of administration of BPA was evaluated in F98 glioma bearing rats. The greatest increase in uptake was seen following i.v. administration of BPA, while in contrast no significant increase was seen following intracarotid (i.c.) administration (38.6±12.4 vs. 34.2±10.9). Cellular localization of the F98 glioma, as determined by secondary ion mass spectrometry (SIMS) boron imaging revealed equivalent tumor boron concentrations following l-DOPA pre-loading. In vivo studies in B16 melanoma bearing mice showed equivalent tumor boron values in treated and untreated mice, suggesting that the effects of l-DOPA pre-loading may depend both on the histologic type of tumor and its anatomic site.

Evaluation of unnatural cyclic amino acids as boron delivery agents for treatment of melanomas and gliomas

Unnatural cyclic amino acids (UNAAs) are a new class of boron delivery agents that are in a pre-clinical stage of evaluation. In the present study, the biodistribution of racemic forms of the cis- and trans-isomers of the boronated UNAA 1-amino-3-boronocyclopentanecarboxylic acid (ABCPC) and 1-amino-3-boronocycloheptanecarboxylic acid (ABCHC) were evaluted in B16 melanoma bearing mice and this was compared to l-p-boronophenylalanine (BPA). Boron concentrations were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) at 2.5h following intraperitoneal (i.p.) injection of the test agents at a concentration equivalent to 24mg/B/kg. While all compounds attained comparable tumor boron concentrations, the tumor/blood (T/Bl) boron concentration ratios were far superior for both cis-ABCPC and cis-ABCHC compared to BPA (T/Bl=16.4, and 15.1 vs. 5.4). Secondary ion mass spectrometry (SIMS) imaging revealed that the cis-ABCPC delivered boron to the nuclei, as well as the cytoplasm of B16 cells. Next, a biodistribution study of cis-ABCPC and BPA was carried out in F98 glioma bearing rats following i.p. administration. Both compounds attained comparable tumor boron concentrations but the tumor/brain (T/Br) boron ratio was superior for cis-ABCPC compared to BPA (6 vs. 3.3). Since UNAAs are water soluble and cannot be metabolized by tumor cells, they could be potentially more effective boron delivery agents than BPA. Our data suggest that further studies are warranted to evaluate these compounds prior to the initiation of clinical studies.

Biodistribution and subcellular localization of an unnatural boron-containing amino acid (cis-ABCPC) by imaging secondary ion mass spectrometry for neutron capture therapy of melanomas and gliomas

The development of new boron-delivery agents is a high priority for improving the effectiveness of boron neutron capture therapy. In the present study, 1-amino-3-borono-cyclopentanecarboxylic acid (cis-ABCPC) as a mixture of its L- and D- enantiomers was evaluated in vivo using the B16 melanoma model for the human tumor and the F98 rat glioma as a model for human gliomas. A secondary ion mass spectrometry (SIMS) based imaging instrument, CAMECA IMS 3F SIMS Ion Microscope, was used for quantitative imaging of boron at 500 nm spatial resolution. Both in vivo and in vitro studies in melanoma models demonstrated that boron was localized in the cytoplasm and nuclei with some cell-to-cell variability. Uptake of cis-ABCPC in B16 cells was time dependent with a 7.5:1 partitioning ratio of boron between cell nuclei and the nutrient medium after 4 hrs. incubation. Furthermore, cis-ABCPC delivered boron to cells in all phases of the cell cycle, including S-phase. In vivo SIMS studies using the F98 rat glioma model revealed an 8:1 boron partitioning ratio between the main tumor mass and normal brain tissue with a 5:1 ratio between infiltrating tumor cells and contiguous normal brain. Since cis-ABCPC is water soluble and can cross the blood-brain-barrier via the L-type amino acid transporters (LAT), it may accumulate preferentially in infiltrating tumor cells in normal brain due to up-regulation of LAT in high grade gliomas. Once trapped inside the tumor cell, cis-ABCPC cannot be metabolized and remains either in a free pool or bound to cell matrix components. The significant improvement in boron uptake by both the main tumor mass and infiltrating tumor cells compared to those reported in animal and clinical studies of p-boronophenylalanine strongly suggest that cis-ABCPC has the potential to become a novel new boron delivery agent for neutron capture therapy of gliomas and melanomas.

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.

Preparation, biodistribution and neurotoxicity of liposomal cisplatin following convection enhanced delivery in normal and F98 glioma bearing rats

The purpose of this study was to evaluate two novel liposomal formulations of cisplatin as potential therapeutic agents for treatment of the F98 rat glioma. The first was a commercially produced agent, Lipoplatin™, which currently is in a Phase III clinical trial for treatment of non-small cell lung cancer (NSCLC). The second, produced in our laboratory, was based on the ability of cisplatin to form coordination complexes with lipid cholesteryl hemisuccinate (CHEMS). The in vitro tumoricidal activity of the former previously has been described in detail by other investigators. The CHEMS liposomal formulation had a Pt loading efficiency of 25% and showed more potent in vitro cytotoxicity against F98 glioma cells than free cisplatin at 24 h. In vivo CHEMS liposomes showed high retention at 24 h after intracerebral (i.c.) convection enhanced delivery (CED) to F98 glioma bearing rats. Neurotoxicologic studies were carried out in non-tumor bearing Fischer rats following i.c. CED of Lipoplatin™ or CHEMS liposomes or their "hollow" counterparts. Unexpectedly, Lipoplatin™ was highly neurotoxic when given i.c. by CED and resulted in death immediately following or within a few days after administration. Similarly "hollow" Lipoplatin™ liposomes showed similar neurotoxicity indicating that this was due to the liposomes themselves rather than the cisplatin. This was particularly surprising since Lipoplatin™ has been well tolerated when administered intravenously. In contrast, CHEMS liposomes and their "hollow" counterparts were clinically well tolerated. However, a variety of dose dependent neuropathologic changes from none to severe were seen at either 10 or 14 d following their administration. These findings suggest that further refinements in the design and formulation of cisplatin containing liposomes will be required before they can be administered i.c. by CED for the treatment of brain tumors and that a formulation that may be safe when given systemically may be highly neurotoxic when administered directly into the brain.

Blood-brain barrier disruption in the treatment of brain tumors

Standard chemotherapy administered systemically has a limited efficacy in the treatment of brain tumors. One of the major obstacles in the treatment of brain neoplasias is the impediment to delivery across the intact blood-brain barrier (BBB). Many innovative approaches have been developed to circumvent this obstacle. One such strategy is BBB disruption (BBBD), which successfully increases the delivery of antineoplastic agents to the central nervous system (CNS). This chapter describes the application of the BBBD technique in rats. Different methods to evaluate and measure BBB permeability following hyperosmolar mannitol infusion including Evans blue staining, albumin immunohistochemistry, and dynamic magnetic resonance imaging are also described.

Boron neutron capture therapy induces apoptosis of glioma cells through Bcl-2/Bax

BACKGROUND

Boron neutron capture therapy (BNCT) is an alternative treatment modality for patients with glioma. The aim of this study was to determine whether induction of apoptosis contributes to the main therapeutic efficacy of BNCT and to compare the relative biological effect (RBE) of BNCT, γ-ray and reactor neutron irradiation.

METHODS

The neutron beam was obtained from the Xi'an Pulsed Reactor (XAPR) and γ-rays were obtained from [60Co] γ source of the Fourth Military Medical University (FMMU) in China. Human glioma cells (the U87, U251, and SHG44 cell lines) were irradiated by neutron beams at the XAPR or [60Co] γ-rays at the FMMU with different protocols: Group A included control nonirradiated cells; Group B included cells treated with 4 Gy of [60Co] γ-rays; Group C included cells treated with 8 Gy of [60Co] γ-rays; Group D included cells treated with 4 Gy BPA (p-borono-phenylalanine)-BNCT; Group E included cells treated with 8 Gy BPA-BNCT; Group F included cells irradiated in the reactor for the same treatment period as used for Group D; Group G included cells irradiated in the reactor for the same treatment period as used for Group E; Group H included cells irradiated with 4 Gy in the reactor; and Group I included cells irradiated with 8 Gy in the reactor. Cell survival was determined using the 3-(4,5-dimethylthiazol-2-yl-2,5-diphenyltetrazolium (MTT) cytotoxicity assay. The morphology of cells was detected by Hoechst33342 staining and transmission electron microscope (TEM). The apoptosis rate was detected by flow cytometer (FCM). The level of Bcl-2 and Bax protein was measured by western blot analysis.

RESULTS

Proliferation of U87, U251, and SHG44 cells was much more strongly inhibited by BPA-BNCT than by irradiation with [60Co] γ-rays (P < 0.01). Nuclear condensation was determined using both a fluorescence technique and electron microscopy in all cell lines treated with BPA-BNCT. Furthermore, the cellular apoptotic rates in Group D and Group E treated with BPA-BNCT were significantly higher than those in Group B and Group C irradiated by [60Co] γ-rays (P < 0.01). The clonogenicity of glioma cells was reduced by BPA-BNCT compared with cells treated in the reactor (Group F, G, H, I), and with the control cells (P < 0.01). Upon BPA-BNCT treatment, the Bax level increased in glioma cells, whereas Bcl-2 expression decreased.

CONCLUSIONS

Compared with γ-ray and reactor neutron irradiation, a higher RBE can be achieved upon treatment of glioma cells with BNCT. Glioma cell apoptosis induced by BNCT may be related to activation of Bax and downregulation of Bcl-2.

Convection enhanced delivery of carboplatin in combination with radiotherapy for the treatment of brain tumors

The purpose of this study was to further evaluate the therapeutic efficacy of convection enhanced delivery (CED) of carboplatin in combination with radiotherapy for treatment of the F98 rat glioma. Tumor cells were implanted stereotactically into the brains of syngeneic Fischer rats, and 13 or 17 d. later carboplatin (20 μg/10 μl) was administered by either CED over 30 min or by Alzet osmotic pumps (0.5 μg/μl/h for 168 h.) beginning at 7 d after tumor implantation. Rats were irradiated with a 15 Gy fractionated dose (5 Gy × 3) of 6 MV photons to the whole brain beginning on the day after drug administration. Other groups of rats received either carboplatin or X-irradiation alone. The tumor carboplatin concentration following CED of 20 μg in 10 μl was 10.4 μg/g, which was equal to that observed following i.v. administration of 100 mg/kg b.w. Rats bearing small tumors, treated with carboplatin and X-irradiation, had a mean survival time (MST) of 83.4 d following CED and 111.8 d following pump delivery with 40% of the latter surviving >180 d (i.e. cured) compared to 55.2 d for CED and 77.2 d. for pump delivery of carboplatin alone and 31.8 d and 24.2 d, respectively, for X-irradiated and untreated controls. There was no microscopic evidence of residual tumor in the brains of all long-term survivors. Not surprisingly, rats with large tumors had much shorter MSTs. Only modest increases in MSTs were observed in animals that received either oral administration or CED of temozolomide plus X-irradiation (23.2 d and 29.3 d) compared to X-irradiation alone. The present survival data, and those previously reported by us, are among the best ever obtained with the F98 glioma model. Initially, they could provide a platform for a Phase I clinical trial to evaluate the safety and potential therapeutic efficacy of CED of carboplatin in patients with recurrent glioblastomas, and ultimately a Phase II trial of carboplatin in combination with radiation therapy.

Convection enhanced delivery of boronated EGF as a molecular targeting agent for neutron capture therapy of brain tumors

In the present study, we have evaluated a boronated dendrimer-epidermal growth factor (BD-EGF) bioconjugate as a molecular targeting agent for boron neutron capture therapy (BNCT) of the human EGFR gene-transfected F98 rat glioma, designated F98(EGFR). EGF was chemically linked to a heavily boronated polyamidoamine dendrimer (BD) by means of the heterobifunctional reagent, mMBS. Biodistribution studies were carried out at 6 h and 24 h following intratumoral (i.t.) injection or intracerebral (i.c.) convection enhanced delivery (CED) of (125)I-labeled or unlabeled BD-EGF (40 microg (10)B/10 microg EGF) to F98 glioma bearing rats. At 24 h. there was 43% more radioactivity in EGFR(+) tumors following CED compared to i.t. injection, and a doubling of the tumor boron concentration (22.3 microg/g vs. 11.7 microg/g). CED of BD-EGF resulted in a 7.2x increase in the volume of distribution within the infused cerebral hemisphere and a 1.9x increase in tumor uptake of BD-EGF compared with i.t. injection. Based on these favorable biodistribution data, BNCT was carried out at the Massachusetts Institute of Technology nuclear reactor 14 days following i.c. tumor implantation and 24 h. after CED of BD-EGF. These animals had a MST of 54.1 +/- 4.7 days compared to 43.0 +/- 2.8 days following i.t. injection. Rats that received BD-EGF by CED in combination with i.v. boronophenylalanine (BPA), which has been used in both experimental and clinical studies, had a MST of 86.0 +/- 28.1 days compared to 39.8 +/- 1.6 days for i.v. BPA alone (P < 0.01), 30.9 +/- 1.4 days for irradiated controls and 25.1 +/- 1.0 days for untreated controls (overall P < 0.0001). These data have demonstrated that the efficacy of BNCT was significantly increased (P < 0.006), following i.c CED of BD-EGF compared to i.t injection, and that the survival data were equivalent to those previously reported by us using the boronated anti-human-EGF mAb, C225 (cetuximab).

Thymidine kinase 1 as a molecular target for boron neutron capture therapy of brain tumors

The purpose of the present study was to evaluate the effectiveness of a 3-carboranyl thymidine analogue (3CTA), 3-[5-{2-(2,3-dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl] thymidine, designated N5-2OH, for boron neutron capture therapy (BNCT) of brain tumors using the RG2 rat glioma model. Target validation was established using the thymidine kinase (TK) 1(+) wild-type, murine L929 cell line and its TK1(-) mutant counterpart, which were implanted s.c. (s.c.) into nude mice. Two intratumoral (i.t.) injections of (10)B-enriched N5-2OH were administered to tumor-bearing mice at 2-hour intervals, after which BNCT was carried out at the Massachusetts Institute of Technology (MIT) Research Reactor. Thirty days after BNCT, mice bearing TK1(+) L929 tumors had a 15x reduction in tumor volume compared with TK1(-) controls. Based on these favorable results, BNCT studies were then initiated in rats bearing intracerebral (i.c.) RG2 gliomas, after i.c. administration of N5-2OH by Alzet osmotic pumps, either alone or in combination with i.v. (i.v.) boronophenylalanine (BPA), a drug that has been used clinically. The mean survival times (MSTs) of RG2 glioma bearing rats were 45.6 +/- 7.2 days, 35.0 +/- 3.3 days, and 52.9 +/- 8.9 days, respectively, for animals that received N5-2OH, BPA, or both. The differences between the survival plots of rats that received N5-2OH and BPA alone were highly significant (P = 0.0003). These data provide proof-of-principle that a 3CTA can function as a boron delivery agent for NCT. Further studies are planned to design and synthesize 3CTAs with enhanced chemical and biological properties, and increased therapeutic efficacy.

Molecular Targeting and Treatment of an Epidermal Growth Factor Receptor-Positive Glioma Using Boronated Cetuximab

PURPOSE

The purpose of the present study was to evaluate the anti-epidermal growth factor monoclonal antibody (mAb) cetuximab (IMC-C225) as a delivery agent for boron neutron capture therapy (BNCT) of a human epidermal growth factor receptor (EGFR) gene-transfected rat glioma, designated as F98(EGFR).

EXPERIMENTAL DESIGN

A heavily boronated polyamidoamine dendrimer was chemically linked to cetuximab by means of the heterobifunctional reagents N-succinimidyl 3-(2-pyridyldithio)-propionate and N-(k-maleimido undecanoic acid)-hydrazide. The bioconjugate, designated as BD-C225, was specifically taken up by F98(EGFR) glioma cells in vitro compared with receptor-negative F98 wild-type cells (41.8 versus 9.1 microg/g). For in vivo biodistribution studies, F98(EGFR) cells were implanted stereotactically into the brains of Fischer rats, and 14 days later, BD-C225 was given intracerebrally by either convection enhanced delivery (CED) or direct intratumoral (i.t.) injection.

RESULTS

The amount of boron retained by F98(EGFR) gliomas 24 h following CED or i.t. injection was 77.2 and 50.8 microg/g, respectively, with normal brain and blood boron values <0.05 mug/g. Boron neutron capture therapy was carried out at the Massachusetts Institute of Technology Research Reactor 24 h after CED of BD-C225, either alone or in combination with i.v. boronophenylalanine (BPA). The corresponding mean survival times (MST) were 54.5 and 70.9 days (P = 0.017), respectively, with one long-term survivor (more than 180 days). In contrast, the MSTs of irradiated and untreated controls, respectively, were 30.3 and 26.3 days. In a second study, the combination of BD-C225 and BPA plus sodium borocaptate, given by either i.v. or intracarotid injection, was evaluated and the MSTs were equivalent to that obtained with BD-C225 plus i.v. BPA.

CONCLUSIONS

The survival data obtained with BD-C225 are comparable with those recently reported by us using boronated mAb L8A4 as the delivery agent. This mAb recognizes the mutant receptor, EGFRvIII. Taken together, these data convincingly show the therapeutic efficacy of molecular targeting of EGFR using a boronated mAb either alone or in combination with BPA and provide a platform for the future development of combinations of high and low molecular weight delivery agents for BNCT of brain tumors.

Molecular targeting and treatment of EGFRvIII-positive gliomas using boronated monoclonal antibody L8A4

PURPOSE

The purpose of the present study was to evaluate a boronated EGFRvIII-specific monoclonal antibody, L8A4, for boron neutron capture therapy (BNCT) of the receptor-positive rat glioma, F98(npEGFRvIII).

EXPERIMENTAL DESIGN

A heavily boronated polyamido amine (PAMAM) dendrimer (BD) was chemically linked to L8A4 by two heterobifunctional reagents, N-succinimidyl 3-(2-pyridyldithio)propionate and N-(k-maleimidoundecanoic acid)hydrazide. For in vivo studies, F98 wild-type receptor-negative or EGFRvIII human gene-transfected receptor-positive F98(npEGFRvIII) glioma cells were implanted i.c. into the brains of Fischer rats. Biodistribution studies were initiated 14 days later. Animals received [(125)I]BD-L8A4 by either convection enhanced delivery (CED) or direct i.t. injection and were euthanized 6, 12, 24, or 48 hours later.

RESULTS

At 6 hours, equivalent amounts of the bioconjugate were detected in receptor-positive and receptor-negative tumors, but by 24 hours the amounts retained by receptor-positive gliomas were 60.1% following CED and 43.7% following i.t. injection compared with 14.6% ID/g by receptor-negative tumors. Boron concentrations in normal brain, blood, liver, kidneys, and spleen all were at nondetectable levels (<0.5 microg/g) at the corresponding times. Based on these favorable biodistribution data, BNCT studies were initiated at the Massachusetts Institute of Technology Research Reactor-II. Rats received BD-L8A4 ( approximately 40 microg (10)B/ approximately 750 mug protein) by CED either alone or in combination with i.v. boronophenylalanine (BPA; 500 mg/kg). BNCT was carried out 24 hours after administration of the bioconjugate and 2.5 hours after i.v. injection of BPA for those animals that received both agents. Rats that received BD-L8A4 by CED in combination with i.v. BPA had a mean +/- SE survival time of 85.5 +/- 15.5 days with 20% long-term survivors (>6 months) and those that received BD-L8A4 alone had a mean +/- SE survival time of 70.4 +/- 11.1 days with 10% long-term survivors compared with 40.1 +/- 2.2 days for i.v. BPA and 30.3 +/- 1.6 and 26.3 +/- 1.1 days for irradiated and untreated controls, respectively.

CONCLUSIONS

These data convincingly show the therapeutic efficacy of molecular targeting of EGFRvIII using either boronated monoclonal antibody L8A4 alone or in combination with BPA and should provide a platform for the future development of combinations of high and low molecular weight delivery agents for BNCT of brain tumors.

Capillary electrophoresis–electrospray mass spectrometry and HR–ICP–MS for the detection and quantification of 10B-boronophenylalanine (10B–BPA) used in boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a bimodal radiotherapeutic treatment based on the irradiation of neoplastic tissues with neutrons after the tissues have selectively accumulated molecules loaded with nuclides with large neutron capture cross-sections (such boron-10). Boron-10 carriers have been tested to a limited extent, and clinical trials have been conducted on sulfhydryl borane (10B-BSH) and boronophenylalanine (10B-BPA). However, precise and accurate measurements of boron-10 concentrations (0.1-100 microg/g) in specimens and samples of limited size (microg scale) are needed in order to be able to biologically characterise new compounds in predictive tissue dosimetry, toxicology and pharmacology studies as well as in clinical investigations. A new approach based on fast separation and detection of 10B-BPA performed by coupling capillary electrophoresis to electrospray mass spectrometry is reported. This method allows the quantitative analysis and characterisation of 10B-BPA in a short time with a high separation efficiency. Detection limits of 3 microM for 10B-BPA and 30 ng/mL for 10B were obtained with CE-ESI-MS. A quantification limit of 10 microM for 10B-BPA (100 ng/mL for 10B) was attained. The total boron-10 concentration was determined by high-resolution inductively coupled mass spectrometry in order to validate the method. Boron-10 isotope measurements were carried out by HR-ICP-MS at medium resolution (R=4000) due to the presence of an isobaric interference at mass 10. Good agreement was obtained between the values from CE-ESI-MS and those from HR-ICP-MS. The method has been successfully used to determine the 10B-BPA in two lines of cultured cells.

Prolonged survival of Fischer rats bearing F98 glioma after iodine-enhanced synchrotron stereotactic radiotherapy

PURPOSE

Heavy-atom-enhanced synchrotron stereotactic radiotherapy (SSR) is a treatment that involves selective accumulation of high-Z elements in tumors followed by stereotactic irradiation with X-rays from a synchrotron source. The purpose of this study was to determine whether the efficacy of iodine-enhanced SSR could be further improved in the F98 rodent glioma model, by using a concomitant injection of an iodinated contrast agent and a transient blood-brain barrier opener (mannitol) during irradiation.

METHODS AND MATERIALS

Fourteen days after intracerebral inoculations of F98 cells, the rats were irradiated with 50-keV X-rays while receiving an infusion of hyperosmotic mannitol with iodine, either intravenously or via the carotid (9 to 15 rats per group, 117 rats total).

RESULTS

For doses<or=15 Gy, the intracarotid infusion of mannitol and iodine improved the rats' survival compared with intravenous injection or irradiation alone. The percentage-increased life spans (ILS) were 91%, 116%, and 169% without iodine, after infusion of iodine and mannitol intravenously, and intracarotid, respectively (15 Gy). At 25 Gy, the rats irradiated without iodine had the longest survival (ILS=607%), but no additional benefit was obtained with iodine and mannitol.

CONCLUSIONS

Iodine-enhanced SSR is significantly improved with concomitant intracarotid infusion of iodine and mannitol for radiation doses<or=15 Gy.

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.

Enhanced delivery of iodine for synchrotron stereotactic radiotherapy by means of intracarotid injection and blood–brain barrier disruption: Quantitative iodine biodistribution studies and associated dosimetry

PURPOSE

Synchrotron stereotactic radiotherapy (SSR) is a binary cancer treatment modality that involves the selective accumulation of a high Z element, such as iodine, in tumors, followed by stereotactic irradiation with kilovoltage X-rays from a synchrotron source. The success of SSR is directly related to the absolute amount of iodine achievable in the tumor. The purposes of this preclinical study were to determine whether the delivery of iodine to brain tumor models in rats could be enhanced by the means of its intracarotid injection with or without a hyperosmotic solution and to evaluate corresponding absorbed X-ray doses.

METHODS AND MATERIALS

Experiments were performed on four groups of F98 glioma-bearing rats, which received either intracarotid (IC) or intravenous (IV) infusions of a mixture (6 mL in 12 min) of an iodinated contrast agent associated or not with a transient blood-brain barrier opener (mannitol). The mixture volumetric proportions were 8/13 of Iomeron (C = 350 mg/mL) for 5/13 of mannitol or saline, respectively. Absolute iodine concentration kinetic was measured in vivo in the tumor, blood, contralateral and ipsilateral brain, and muscle by monochromatic computed tomography. Associated dosimetry was performed by computing the iodine dose enhancement factor (DEF) in each region and building dose distribution maps by analytical simulations.

RESULTS

Infusion of mannitol significantly enhanced iodine tumor uptake compared with the control values (p < 0.0001 and p = 0.0138, for IC and IV protocols, respectively). The mean iodine concentrations (C) reached 20.5 +/- 0.98 mg/mL (DEF = 4.1) after administration of iodine and mannitol vs. 4.1 +/- 1.2 mg/mL i.c. with serum (DEF = 1.6). The tumor iodine uptakes after jugular injection with mannitol (C = 4.4 +/- 2.1 mg/mL, DEF = 1.7) were not significantly different from IC injection of iodine without mannitol (p = 0.8142). The IV injection of iodine with saline led to an iodine concentration in the tumor of 1.2 +/- 0.98 mg/mL and a DEF of 1.2.

CONCLUSIONS

This study established that optimizing the delivery of iodine by means of IC injection combined with a blood-brain barrier opener (mannitol) significantly increases the iodine uptake of F98 rat gliomas. This infusion protocol could potentially enhance the efficacy of SSR treatment, because the radiation dose is proportional to the iodine amount present in the irradiation bed.

Enhanced chemotherapy delivery by intraarterial infusion and blood-brain barrier disruption in malignant brain tumors

BACKGROUND

The treatment of malignant brain tumors is hampered by the presence of the blood-brain barrier, which limits chemotherapy penetration to the central nervous system (CNS). In recent years, different strategies have been designed to circumvent this physiologic barrier. The osmotic blood-brain barrier disruption (BBBD) procedure is one such strategy, and has been studied extensively in preclinical and clinical studies. The authors detail their experience so far with the procedure in the context of an open Phase II study in the treatment of malignant brain tumors.

METHODS

Patients with histologically proven malignant gliomas, primitive neuroectodermal tumors, primary CNS lymphomas, and metastatic disease to the brain were eligible. Patients enrolled were treated every 4 weeks (1 cycle) for < or = 12 cycles. A methotrexate-based regimen was offered to patients with lymphomas, whereas a carboplatin-based regimen was offered to patients with all other histologies. Before intraarterial chemotherapy infusion, patients were submitted to an osmotic BBBD procedure.

RESULTS

Seventy-two patients were included in the current report. The overall median survival times (MST) from treatment initiation for glioblastoma multiforme (GBM), anaplastic oligodendrogliomas, primary CNS lymphomas, and metastases were, respectively, 9.1, 13.9, not reached, and 9.9 months, whereas time to disease progression was 4.1, 9.2, 12.3, and 3.3 months. The MST from diagnosis was 32.2 months for GBM.

CONCLUSIONS

These encouraging results prompted the authors to further refine their knowledge of the potential contribution of this procedure in the treatment of brain tumors. These authors designed a randomized Phase III study for patients with GBM that is now open.

[The blood-brain barrier should not be underestimated in neuro-oncology]

INTRODUCTION

Limited therapeutic success in the treatment of CNS neoplasia with chemotherapy is generally attributed to two factors: natural or acquired resistance to chemotherapy expressed by tumor cells, and delivery impediment related to the blood-brain barrier.

STATE OF ART

The anatomic and physiological properties of the normal blood-brain barrier prevent passage of ionized water-soluble compounds with a molecular weight greater than 500 Daltons. Although complex, the blood-brain barrier basically functions at the level of the tight junctions of the cerebral vascular endothelial cells. Different approaches have been advocated to improve delivery across the blood-brain barrier. One such approach, transient osmotic permeabilization of the blood-brain barrier, is an invasive procedure offering the potential of global delivery. This strategy involves cerebral intravascular infusion of a hypertonic solution to produce, in a given cerebral distribution (carotid or vertebral), producing a transient increase in blood-brain barrier permeability. Two parameters are paramount in the ability to mediate a hyperosmolar modification of the barrier: the osmolality of the solution, and the infusion time. The procedure has been found to produce a marked increase (10- to 100-fold) in brain and cerebrospinal fluid concentrations of methotrexate and of other markers.

PERSPECTIVES

Pre-clinical studies are underway to assess the use of this procedure to improve delivery of different molecules, including standard chemotherapy, monoclonal antibodies and gene therapy molecules. This approach has been standardized for clinical use. It has been extensively used in patients. Using a standard protocol of osmotic blood-brain barrier disruption to enhance chemotherapy delivery with three different chemotherapy regimens, more than 3000 procedures have been performed in more than 300 patients across the blood-brain barrier distribution consortium, an entity which includes six university centers coordinated by the Oregon Health Sciences University. The procedure has been found to be safe, with very limited toxicity. As part of this consortium, the Sherbrooke University center has been offering this treatment modality since November 1999. We have performed more than 500 procedures in 122 patients with various histologies (malignant gliomas, primary central nervous system lymphoma and metastasis) with low toxicity.

CONCLUSIONS

In our view, the median survival rate of 138 weeks obtained with glioblastoma multiforme patients is promising; further research to improve these results is needed.

Synthesis of copper octabromotetracarboranylphenylporphyrin for boron neutron capture therapy and its toxicity and biodistribution in tumour-bearing mice

Copper tetracarboranyltetraphenylporphyrin (CuTCPH) is a minimally toxic carborane-containing porphyrin that has safely delivered high concentrations of boron for experimental boron neutron capture therapy (BNCT). Copper octabromotetracarboranylphenylporphyrin (CuTCPBr), synthesized by bromination of CuTCPH, is one of several new minimally toxic analogues of CuTCPH being studied in our laboratory, which could possess comparable or better tumour-targeting properties with enhanced tumour cytotoxicity. Its biodistribution, biokinetics and toxicity in mice with subcutaneous EMT-6 (mammary) or SCCVII (squamous cell) carcinomas were compared with those of CuTCPH. The administration of approximately 200 mg kg(-1) of either porphyrin in six intraperitoneal injections over 2 days had no apparent effect, but administration of approximately 400 mg kg(-1) slightly lowered body weights, elevated alanine and aspartate transaminase activities in blood plasma, and depressed blood platelet counts for several days. Enzymes and platelets returned to normal within 5 days after those injections and body weights returned to normal within 2 weeks. High average concentrations of boron from either porphyrin were achieved in the two tumour models from a total dose of approximately 200 mg kg(-1). The high tumour boron concentration decreased slowly while concentrations in blood decreased rapidly. Boron concentrations in brain and skin were consistently lower than in tumour by a factor of 10 or more. Although either CuTCPH or CuTCPBr can be labelled with (64)Cu for imaging by positron emission tomography (PET), CuTCPBr can also be labelled by (76)Br, another PET-imageable nuclide.

Boron neutron capture therapy: cellular targeting of high linear energy transfer radiation

Boron neutron capture therapy (BNCT) is based on the preferential targeting of tumor cells with (10)B and subsequent activation with thermal neutrons to produce a highly localized radiation. In theory, it is possible to selectively irradiate a tumor and the associated infiltrating tumor cells with large single doses of high-LET radiation while sparing the adjacent normal tissues. The mixture of high- and low-LET dose components created in tissue during neutron irradiation complicates the radiobiology of BNCT. Much of the complexity has been unravelled through a combination of preclinical experimentation and clinical dose escalation experience. Over 350 patients have been treated in a number of different facilities worldwide. The accumulated clinical experience has demonstrated that BNCT can be delivered safely but is still defining the limits of normal brain tolerance. Several independent BNCT clinical protocols have demonstrated that BNCT can produce median survivals in patients with glioblastoma that appear to be equivalent to conventional photon therapy. This review describes the individual components and methodologies required for effect BNCT: the boron delivery agents; the analytical techniques; the neutron beams; the dosimetry and radiation biology measurements; and how these components have been integrated into a series of clinical studies. The single greatest weakness of BNCT at the present time is non-uniform delivery of boron into all tumor cells. Future improvements in BNCT effectiveness will come from improved boron delivery agents, improved boron administration protocols, or through combination of BNCT with other modalities.

Combination of boron neutron capture therapy and external beam radiotherapy for brain tumors

PURPOSE

Boron neutron capture therapy (BNCT) has been used clinically as a single modality treatment for high-grade gliomas and melanomas metastatic to the brain. The purpose of the present study was to determine whether its efficacy could be enhanced by an X-ray boost administered after BNCT. Two brain tumor models were used, the F98 glioma as a model for primary brain tumors and the MRA 27 human melanoma as a model for metastatic brain tumors.

METHODS AND MATERIALS

For biodistribution studies, either 10(5) F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats or 10(6) MRA 27 melanoma cells were implanted intracerebrally into National Institutes of Health (NIH)-rnu nude rats. Biodistribution studies were performed 11-13 days after implantation of the F98 glioma and 20-24 days after implantation of the MRA 27 melanoma. Animals bearing the F98 glioma received a combination of two boron-containing drugs, sodium borocaptate at a dose of 30 mg/kg and boron phenylalanine (BPA) at a dose of 250 mg/kg. MRA 27 melanoma-bearing rats received BPA (500 mg/kg) containing an equivalent amount of 10B (27 mg B/kg). The drugs were administered by either intracarotid or i.v. injection.

RESULTS

The tumor boron concentration after intracarotid injection was approximately 50% greater in the F98 glioma and MRA 27 melanoma after intracarotid injection (20.8 and 36.8 microg/g, respectively) compared with i.v. injection (11.2 and 19.5 microg/g, respectively). BNCT was carried out at the Brookhaven National Laboratory Medical Research Reactor approximately 14 days after tumor implantation of either the F98 glioma or the MRA 27 melanoma. Approximately 7-10 days after BNCT, subsets of animals were irradiated with 6-MV photons, produced by a linear accelerator at a total dose of 15 Gy, delivered in 5-Gy daily fractions. F98 glioma-bearing rats that received intracarotid or i.v. sodium borocaptate plus BPA, followed 2.5 h later by BNCT and 7-10 days later by X-rays, had similar mean survival times (61 days and 53 days, respectively, p = 0.25), and the non X-irradiated, BNCT-treated animals had a mean survival time of 52 and 40 days, respectively, for intracarotid vs. i.v. injection; the latter was equivalent to that of the irradiated animals. The corresponding survival time for MRA 27 melanoma-bearing rats that received intracarotid or i.v. BPA, followed by BNCT and then X-irradiation, was 75 and 82 days, respectively (p = 0.5), 54 days without X-irradiation (p = 0.0002), 37 days for X-irradiation alone, and 24 days for untreated controls. In contrast to the data obtained with the F98 glioma, MRA 27 melanoma-bearing rats that received i.v. BPA, followed by BNCT, had a highly significant difference in mean survival time compared with the irradiated controls (54 vs. 37 days, p = 0.008).

CONCLUSION

Our data are the first to suggest that a significant therapeutic gain may be obtained when BNCT is combined with an X-ray boost. Additional experimental studies are required to determine the optimal combination of X-radiation and neutron doses and whether it is more advantageous to administer the photon boost before or after BNCT.

Synchrotron radiation therapy of malignant brain glioma loaded with an iodinated contrast agent: First trial on rats bearing F98 gliomas

PURPOSE

Enhancement of the radiation dose delivered to a brain tumor can be achieved by infusing an iodinated contrast agent to the patient and irradiating with kilovoltage X-rays in computed tomography (CT) mode. Synchrotron sources, providing monochromatic tunable intense beams, are ideal for this treatment. The aim of this study is to assess in vivo the efficiency of this novel radiotherapy modality, Synchrotron Radiation CT-Therapy.

MATERIALS AND METHODS

Intracranial implantations of 10(5) F98 glioma cells were performed on 18 Fisher 344 rats. Six rats were untreated controls, six received radiotherapy alone (10 Gy in the tumor by single fraction tomographic irradiation at 50 keV), and six received the same radiotherapy treatment under a continuous infusion of iodinated contrast agent.

RESULTS

The rats that received contrast agent during radiotherapy survived significantly longer than rats that received radiotherapy alone (p = 0.025). Mean survival times (mean +/- SD) were 12.3 +/- 0.8, 14.8 +/- 2.8, and 18.2 +/- 1.3 days; median survival times were 12.5, 15, and 18 days (untreated controls, irradiated without iodine, irradiated with iodine, respectively). The increase in life spans was 20% and 44% for the rats irradiated without or with iodine, respectively.

CONCLUSIONS

This method of radiation dose enhancement in brain tumors with synchrotron radiation significantly increases survival and appears highly promising.

Rat brain tumor models to assess the efficacy of boron neutron capture therapy: a critical evaluation

Development of any therapeutic modality can be facilitated by the use of the appropriate animal models to assess its efficacy. This report primarily will focus on our studies using the F98 and 9L rat glioma models to evaluate the effectiveness of boron neutron capture therapy (BNCT) of brain tumors. Following intracerebral implantation the biological behavior of each tumor resembles that of human high grade gliomas in a number of ways. In both models, glioma cells were implanted intracerebrally into syngeneic Fischer rats and approximately 10-14 days later BNCT was initiated at the Brookhaven National Laboratory Medical Research Reactor. Two low molecular weight (M(r) < 210Da) 10B-containing drugs, boronophenylalanine (BPA) and/or sodium borocaptate (BSH) were used as capture agents, either alone or in combination with each other. The 9L gliosarcoma, which has been difficult to cure by means of either chemo- or radiotherapy alone, was readily curable by BNCT. The best survival data were obtained using BPA at a dose of 1200 mg/kg (64.8mg 10B), administered intraperitoneally (i.p.), with a 100% survival rate at 8 months. In contrast, the F98 glioma has been refractory to all therapeutic modalities. Tumor bearing animals, which had received 500 mg/kg (27 mg 10B) of BPA, or an equivalent amount of BSH i.v., had mean survival time (MST) of 37 and 33 days, respectively, compared to 29 days for irradiated controls. The best survival data with the F98 glioma model were obtained using BPA + BSH in combination, administered intra-arterially via the internal carotid artery (i.c.) with hyperosmotic mannitol induced blood-brain barrier disruption (BBB-D). The MST was 140 days with a cure rate of 25%, compared to a MST of 73 days with a 5% cure rate without BBB-D, and 41 days following i.v. administration of both drugs. A modest but significant increase in MST also was observed in rats that received intracarotid (i.c.) BPA in combination with Cereport (RMP-7), which produced a pharmacologically mediated opening of the BBB. Studies also have been carried out with the F98 glioma to determine whether an X-ray boost could enhance the efficacy of BNCT, and it was shown that there was a significant therapeutic gain. Finally, molecular targeting of the epidermal growth factor receptor (EGFR) has been investigated using F98 glioma cells, which had been transfected with the gene encoding EGFR and, intratumoral injection of boronated EGF as the delivery agent, followed by BNCT. These studies demonstrated that there was specific targeting of EGFR and provided proof of principle for the use of high molecular weight, receptor targeting-boron delivery agents. Finally, a xenograft model for melanoma metastatic to the brain has been developed using a human melanoma (MRA27), stereotactically implanted into the brains of nude rats, and these studies demonstrated that BNCT either cured or significantly prolonged the survival of tumor-bearing rats. It remains to be determined, which, if any, of these experimental approaches will be translated into clinical studies. Be that as it may, rat brain tumor models already have made a significant contribution to the design of clinical BNCT protocols, and should continue to do so in the future.

Neutron capture therapy of intracerebral melanoma: enhanced survival and cure after blood-brain barrier opening to improve delivery of boronophenylalanine

PURPOSE

Multicentric cerebral metastases of melanoma represent an important clinical problem for which there currently is no satisfactory treatment. We previously developed a model for melanoma metastatic to the brain employing nude rats bearing intracerebral implants of the human MRA27 melanoma. The purpose of the present study was to determine if the efficacy of boron neutron capture therapy (BNCT) could be improved by either Cereport (RMP-7) mediated modulation of blood-brain barrier (BBB) permeability or hyperosmotic mannitol-induced BBB disruption using boronophenylalanine (BPA) as the capture agent.

METHODS AND MATERIALS

Biodistribution studies were carried out at 0.5, 2.5, and 4 h after intracarotid administration of Cereport (1.5 microg/kg) and intracarotid or i.v. administration of BPA (500 mg/kg). Peak tumor boron concentrations (65.4 microg/g) and the best composite tumor:brain (6.1:1) and tumor:blood (6.3:1) ratios were observed at 2.5 h after intracarotid administration. BNCT was initiated at the Brookhaven Medical Research Reactor 13-14 days after intracerebral implantation of 10(6) MRA27 cells.

RESULTS

Untreated control rats had a median survival time (MeST) of 22 days and for irradiated controls, it was 30 days. Rats that received i.v. or intracarotid BPA without Cereport followed by BNCT 2.5 h later had MeSTs of 41 days and 57 days, respectively, with 20% long-term survivors (>180 days) in the latter group. Rats that received intracarotid BPA with Cereport had an MeST of 86 days with 36% long-term survivors, which was very close to that of rats that had hyperosmotic mannitol-induced disruption of the BBB (85 days with 25% long-term survivors). When these two groups were combined, and survival times were compared, using the Wilcoxon rank sum test, to those of rats that received intracarotid BPA without blood-brain barrier disruption, these differences were significant at the level p = 0.01.

CONCLUSIONS

Our data show that optimizing the delivery of BPA by means of intracarotid injection combined with opening the BBB by infusing Cereport or a hyperosmotic solution of mannitol significantly enhanced survival times and produced long-term cures of MRA27 melanoma-bearing rats. These observations are relevant to future clinical studies using BNCT for the treatment of intracerebral melanoma.

Evaluation of systemically administered radiolabeled epidermal growth factor as a brain tumor targeting agent

We have previously reported a method for labeling epidermal growth factor (EGF) with technetium-99m and have shown that 99mTc-EGF localized in EGF receptor (R) positive intracerebral C6EGFR rat gliomas following intratumoral (i.t.) injection of the radioligand. In the present study, we have evaluated the potential use of 99mTc-EGF as a tumor targeting agent after systemic administration to Fischer rats bearing intracerebral implants of C6EGFRgliomas. Radiolocalization was determined following intravenous (i.v.) or intracarotid (i.c.) injection with or without hyperosmotic mannitol induced disruption of the blood-brain barrier (BBB-D). As determined by gamma-scintillation counting, 4 h after i.c. injection of 99mTc-EGF, 0.34% of the injected dose per gram (% ID/g) was localized in C6EGFR tumors. which expressed 10(5)-10(6) EGFR sites per cell, compared to 0.07% ID/g in animals bearing C6 wildtype gliomas, which do not express EGFR. The corresponding tumor to brain ratios were 5.6 and 1.6, respectively. Tumors could be visualized by external gamma-scintigraphy in rats bearing C6EGFR but not C6 wildtype gliomas, thereby establishing that radiolocalization was dependent upon receptor expression. Intracarotid administration of 99mTc-EGF significantly increased tumor uptake compared to i.v. injection (0.34 vs 0.14% ID/g, p < 0.04). BBB-D disruption, followed by i.c. injection of 99mTc-EGF, however, did not significantly enhance tumor uptake compared to i.c. injection without BBB-D (0.45% vs 0.34% ID/g, p > 0.1). The uptake of 99mTc-EGF was approximately 4-9% ID/g in the liver and 12-20% ID/g in the kidneys after i.c. or i.v. administration. External gamma-scintigraphy of regions of interest over the liver and kidneys revealed that approximately 70-80% of the whole body radioactivity accumulated in these organs, and only 0.47-0.83% in the tumor following i.v. or i.c. administration of 99m9Tc-EGF. Our study has demonstrated that EGF can be used as a specific targeting agent for EGFR (+) rat brain tumors. However, it is unlikely that systemic injection of EGF-based bioconjugates can deliver sufficient amounts of the ligand to brain tumors for therapeutic purposes and direct delivery by means of either intratumoral injection or a variant of it such as convection enhanced delivery will be required.

Advances in osmotic opening of the blood-brain barrier to enhance CNS chemotherapy

The blood-brain barrier (BBB) to water-soluble drugs and macromolecules can be opened in vivo by infusing a hypertonic solution of arabinose or mannitol into the carotid artery for 30 sec. Opening involves widening of tight junctions between endothelial cells of the cerebrovasculature and is mediated by endothelial cell shrinkage, vascular dilatation associated with removal of water from brain, and modulation of the contractile state of the endothelial cytoskeleton and junctional proteins by increased intracellular calcium. A 10-fold increase in BBB permeability to intravascular substances, lasting about 10 min following osmotic exposure, reflects both increased diffusion and bulk fluid flow from blood into brain. Furthermore, recent evidence indicates that the duration of peak BBB opening can be extended beyond 30 min, by pre-treatment with a Na(+)/Ca(2+) channel blocker. In experimental animals, the osmotic method has been used to grant wide access to brain of water-soluble drugs, peptides, antibodies, boron compounds for neutron capture therapy, viral vectors for gene therapy and enzymes. Ongoing multi-centre clinical studies suggest that the method, when used with intra-arterially administered anticancer drugs, can prolong survival in patients with malignant brain tumours, with minimal morbidity. However, controlled clinical trials are critical to see if the osmotic procedure with intra-arterial drugs enhances survival in brain tumour patients compared with intra-arterial drug alone.

Determination of boronophenylalanine in biological samples using precolumn o-phthalaldehyde derivatization and reversed-phase high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic method for the detection of boronophenylalanine is described. Determination was obtained by precolumn reaction of o-phthalaldehyde with a mixture of standard amino acids containing boronophenylalanine and separating the corresponding o-phthalaldehyde derivatives, using a Kromasil C-18, 250 x 4.6 mm, 5-microm particle size column, a step gradient with two buffers, a flow rate of 1.2 ml/min, a column temperature of 23 degrees C, and fluorimetric detection (excitation and emission wavelengths of 330 and 430 nm, respectively). The use of such a method for assaying boronophenylalanine in biological samples was tested in neutralized perchloric acid blood and cerebral tissue extracts of rats treated with intracarotid administration of 300 mg/kg of body weight boronophenylalanine. Results of these experiments showed that the present HPLC method represents a valid alternative to currently available analytical techniques for assaying boronophenylalanine based on boron determination in terms of reproducibility, recovery, or sensitivity. Therefore, it is suggested that the present method may routinely be used in all preclinical and clinical studies in which quantification of circulating and tissue concentrations of boronophenylalanine is critical for the application of boron neutron capture therapy.

Boron neutron capture therapy of brain tumors: functional and neuropathologic effects of blood-brain barrier disruption and intracarotid injection of sodium borocaptate and boronophenylalanine

Sodium borocaptate (BSH) and boronophenylalanine (BPA) are two drugs that have been used clinically for boron neutron capture therapy (BNCT) of brain tumors. We previously have reported that hyperosmotic mannitol-induced disruption of the blood-brain barrier (BBB-D), followed by intracarotid (i.c.) administration of BPA or BSH, either individually or in combination, significantly enhanced tumor boron delivery and the efficacy of BNCT in F98 glioma bearing rats. The purpose of the present study was to determine the short-term neuropathologic consequences of this treatment and the long-term effects on motor and cognitive function, as well as the neuropathologic sequelae 1 year following neutron capture irradiation. BBB-D was carried out in non-tumor bearing Fischer rats by infusing a 25% solution of mannitol i.c. followed by i.c. injection of BPA or BSH, either individually or in combination, immediately thereafter. Animals were euthanized 2 days after compound administration, and their brains were processed for neuropathologic examination, which revealed sporadic, mild, focal neuronal degeneration, hemorrhage, and necrosis. To assess the long-term effects of such treatment followed by neutron capture irradiation, non-tumor bearing rats were subjected to BBB-D after which they were injected i.c. with BPA (25 mg B/kg body weight (b.w)) or BSH (30 mg B/kg b.w.) either individually or in combination (BPA 12.5 mg and BSH 14 mg B/kg b.w.). Two and a half hours later they were irradiated at the Medical Research Reactor, Brookhaven National Laboratory, Upton, NY, with the same physical radiation doses (5.79, 8.10 or 10.06 Gy), delivered to the brain, as those that previously had been used for our therapy experiments. The animals tolerated this procedure well, after which they were returned to Columbus, Ohio where their clinical status was monitored weekly. After 1 year, motor function was assessed using a sensitive and reliable locomotor rating scale for open field testing in rats and cognitive function was evaluated by their performance in the Morris water maze, the results of which were similar to those obtained with age matched controls. After functional evaluation, the rats were euthanized, their brains were removed, and then processed for neuropathologic examination. Subtle histopathologic changes were seen in the choroid plexuses of irradiated animals that had received BPA, BSH or saline. Radiation related ocular changes consisting of keratitis, blepharitis, conjunctivitis and cataract formation were seen with similar frequency in most rats in each treatment group. Based on these observations, and the previously reported significant therapeutic gain associated with BBB-D and i.c. injection of BSH and BPA, the present observations establish its safety in rats and suggest that further studies in large animals and humans are warranted.

Improved survival after boron neutron capture therapy of brain tumors by Cereport-mediated blood-brain barrier modulation to enhance delivery of boronophenylalanine

OBJECTIVE

Cereport (Alkermes, Inc., Cambridge, MA), or, as it has been previously called, RMP-7 (receptor-mediated permeabilizer-7), is a bradykinin analog that has been shown to produce a transient, pharmacologically mediated opening of the blood-brain barrier. The purpose of the present study was to determine whether the efficacy of boron neutron capture therapy (BNCT) could be enhanced by means of intracarotid (i.c.) infusion of Cereport, in combination with intravenous (i.v.) injection or i.c. infusion of boronophenylalanine (BPA) in the F98 rat glioma model.

METHODS

For biodistribution studies, Fischer rats bearing intracerebral implants of the F98 glioma received i.v. or i.c. injections of 300 or 500 mg/kg body weight (b.w.) of BPA with or without i.c. infusion of 1.5 microg/kg b.w. of Cereport. For therapy studies, BNCT was initiated 14 days after intracerebral implantation of 10(3) F98 cells. The i.v. or i.c. injection of BPA (500 mg/kg b.w.) was given with or without Cereport, and the animals were irradiated 2.5 hours later at the Brookhaven Medical Research Reactor with a collimated beam of thermal neutrons delivered to the head.

RESULTS

At a BPA dose of 500 mg/kg b.w., tumor boron concentrations (mean +/- standard deviation) were 55.7 +/- 9.6 microg/g with Cereport versus 33.6 +/- 3.9 microg/g without Cereport at 2.5 hours after i.c. infusion of BPA, and concentrations were 29.4 +/- 9.9 microg/g with Cereport versus 15.4 +/- 3.5 microg/g without Cereport (P < 0.05) after i.v. injection of BPA. After i.c. administration of BPA and Cereport, the tumor-to-blood ratio was 5.4 +/- 0.6, and the tumor-to-brain ratio was 5.2 +/- 2.4. After BNCT with BPA at a dose of 500 mg/kg, the survival time was 50 +/- 16 days for i.c. administration of BPA with Cereport versus 40 +/- 6 days without Cereport (P = 0.05), 38 +/- 4 days for i.v. administration of BPA with Cereport versus 34 +/- 3 days without Cereport (P = 0.02), 28 +/- 5 days for irradiated controls, and 23 +/- 3 days for untreated controls. Compared with untreated controls, there was a 117% increase in lifespan in rats that received an i.c. infusion of Cereport and then BPA, and an 86% increase in lifespan in rats that received i.c. administration of BPA without Cereport.

CONCLUSION

These studies have established that i.c. administration of Cereport can not only increase tumor uptake of BPA, but also enhance the efficacy of BNCT.

Osmotic opening of the blood-brain barrier: principles, mechanism, and therapeutic applications

1. Osmotic opening of the blood-brain barrier by intracarotid infusion of a hypertonic arabinose or mannitol solution is mediated by vasodilatation and shrinkage of cerebrovascular endothelial cells, with widening of the interendothelial tight junctions to an estimated radius of 200 A. The effect may be facilitated by calcium-mediated contraction of the endothelial cytoskeleton. 2. The marked increase in apparent blood-brain barrier permeability to intravascular substances (10-fold for small molecules) following the osmotic procedure is due to both increased diffusion and bulk fluid flow across the tight junctions. The permeability effect is largely reversed within 10 min. 3. In experimental animals, the osmotic method has been used to grant wide access to the brain of water-soluble drugs, peptides, antibodies, boron compounds for neutron capture therapy, and viral vectors for gene therapy. The method also has been used together with anticancer drugs to treat patients with metastatic or primary brain tumors, with some success and minimal morbidity.

Boron neutron capture therapy of brain tumors: enhanced survival and cure following blood-brain barrier disruption and intracarotid injection of sodium borocaptate and boronophenylalanine

PURPOSE

Boronophenylalanine (BPA) and sodium borocaptate (Na(2)B(12)H(11)SH or BSH) have been used clinically for boron neutron capture therapy (BNCT) of high-grade gliomas. These drugs appear to concentrate in tumors by different mechanisms and may target different subpopulations of glioma cells. The purpose of the present study was to determine if the efficacy of BNCT could be further improved in F98-glioma-bearing rats by administering both boron compounds together and by improving their delivery by means of intracarotid (i.c.) injection with or without blood-brain barrier disruption (BBB-D).

METHODS AND MATERIALS

For biodistribution studies, 10(5) F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats. Eleven to 13 days later animals were injected intravenously (i.v.) with BPA at doses of either 250 or 500 mg/kg body weight (b.w.) in combination with BSH at doses of either 30 or 60 mg/kg b.w. or i.c. with or without BBB-D, which was accomplished by i.c. infusion of a hyperosmotic (25%) solution of mannitol. For BNCT studies, 10(3) F98 glioma cells were implanted intracerebrally, and 14 days later animals were transported to the Brookhaven National Laboratory (BNL). They received BPA (250 mg/kg b.w.) in combination with BSH (30 mg/kg b.w. ) by i.v. or i.c. injection with or without BBB-D, and 2.5 hours later they were irradiated with a collimated beam of thermal neutrons at the BNL Medical Research Reactor.

RESULTS

The mean tumor boron concentration +/- standard deviation (SD) at 2.5 hours after i. c. injection of BPA (250 mg/kg b.w.) and BSH (30 mg/kg b.w.) was 56. 3 +/- 37.8 microgram/g with BBB-D compared to 20.8 +/- 3.9 microgram/g without BBB-D and 11.2 +/- 1.8 microgram/g after i.v. injection. Doubling the dose of BPA and BSH produced a twofold increase in tumor boron concentrations, but also concomitant increases in normal brain and blood levels, which could have adverse effects. For this reason, the lower boron dose was selected for BNCT studies. The median survival time was 25 days for untreated control rats, 29 days for irradiated controls, 42 days for rats that received BPA and BSH i.v., 53 days following i.c. injection, and 72 days following i.c. injection + BBB-D with subsets of long-term survivors and/or cured animals in the latter two groups. No histopathologic evidence of residual tumor was seen in the brains of cured animals.

CONCLUSIONS

The combination of BPA and BSH, administered i.c. with BBB-D, yielded a 25% cure rate for the heretofore incurable F98 rat glioma with minimal late radiation-induced brain damage. These results demonstrate that using a combination of boron agents and optimizing their delivery can dramatically improve the efficacy of BNCT in glioma-bearing rats.

Posttreatment with the immunosuppressant cyclosporin A in transient focal ischemia

Cyclosporin A (CsA) reduces ischemic brain damage when administered in such a way that its penetration across the blood-brain barrier is enhanced. Since only pretreatment has previously been used in focal ischemia, the objective of the present study was to establish whether posttreatment is efficacious and to assess the window of therapeutic opportunity for CsA. To that end, CsA was given 5 min to 6 h after the start of reperfusion following 2 h transient ischemia, and infarct volume was assessed after 48 h by triphenyltetrazolium chloride staining. Attempts were made to circumvent the BBB to CsA by an intracerebral needle lesion, by an increase in the intravenous CsA dose, or by osmotic opening with intracarotid mannitol. The results were compared to those obtained with FK506. Intravenous CsA in a dose of 10 mg/kg failed to reduce infarct volume, unless preceded by a needle lesion. That procedure, and an increase in CsA dose to 50 mg/kg, reduced infarct volume to about 50% of control, but the higher dose had toxic side effects. The coupled intracarotid infusion of mannitol and CsA (10 mg/kg) was more efficacious, without overt side effects. However, mannitol proved dispensable since CsA alone reduced infarct volume to 30% of control, with a therapeutic window of 3-6 h. When given after 5 min of reflow, CsA reduced infarct volume to 10% of control and was clearly more neuroprotective than FK506. Possibly, this is because CsA blocks the mitochondrial permeability transition pore which is opened under adverse conditions.

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.

Enhanced delivery of boronophenylalanine for neutron capture therapy of brain tumors using the bradykinin analog Cereport (Receptor-Mediated Permeabilizer-7)

OBJECTIVE

Using the well-characterized F98 rat glioma model, the purpose of the present study was to determine whether the delivery of boronophenylalanine (BPA) could be enhanced by prior administration of the bradykinin analog Cereport (Alkermes, Inc., Cambridge, MA) (previously known as Receptor-Mediated Permeabilizer-7), which produces a transient, pharmacologically mediated opening of the blood-brain barrier.

METHODS

Two series of experiments were performed in F98 glioma-bearing rats that had received either intracarotid (i.c.) or intravenous infusions of Cereport (at doses ranging from 1.5 to 7.5 microg/kg of body weight), followed by i.c. (or intravenous) injection of BPA (300 mg/kg of body weight). Animals were killed 0.5, 2.5, or 4 hours later, samples of blood, skin, muscle, and eye were obtained, brains were removed, and tumors were excised for boron determination by direct current plasma-atomic emission spectroscopy.

RESULTS

Averaged over all time points, i.c. infusion of Cereport significantly enhanced tumor boron uptake (P = 0.0001), compared with the excipient (saline) control values. Tumor boron values were equivalent at 0.5 (36.0 microg/g) and 2.5 hours (38.5 microg/g) after i.c. administration of Cereport and BPA and then decreased by 33% (to 25.7 microg/g) at 4 hours. These tumor boron uptake values were significantly different (alpha = 0.05), compared with values measured at the corresponding times after i.c. administration of BPA without Cereport (22.6, 21.8, and 15.3 microg/g, respectively). Although no time-related effects were observed, i.c. administration of Cereport followed by intravenous administration of BPA also significantly enhanced (alpha = 0.05) tumor boron uptake at 0.5, 2.5, and 4 hours (27.4, 30.3, and 28.0 microg/g, respectively), compared with values obtained without Cereport (11.3, 13.4, and 15.2 microg/g, respectively). Boron levels in normal brain tissue from tumor-bearing and non-tumor-bearing cerebral hemispheres and in blood were not significantly different from those measured in saline-treated control animals.

CONCLUSION

This study established that i.c. infusion of Cereport significantly increased delivery of BPA to F98 rat gliomas, and this could enhance the efficacy of boron neutron capture therapy of this tumor.

Effect of dose and infusion time on the delivery of p-boronophenylalanine for neutron capture therapy

Clinical trials of boron neutron capture therapy (BNCT) for glioblastoma multiforme are currently in progress using p-boronophenylalanine (BPA) as the 10B delivery agent. Enhancement of tumor boron uptake and/or the tumor-to-blood (T:B) boron concentration ratio would have the potential of significantly improving the therapeutic gain of BNCT. The effects of total dose, infusion time, and route of administration of BPA on tumor and blood boron concentrations were studied in rats bearing the 9L gliosarcoma. Increasing the total dose of BPA from 250 to 1000 mg/kg, administered intravenously over a 2-h infusion period, resulted in an increase in tumor boron concentration from approximately 30 to approximately 70 microg 10B/g, with a constant T:B boron concentration ratio of about 3.7:1. Similarly, extension of the infusion time from 2 to 6 h, at a constant dose-rate of 125 mg BPA/kg/h, resulted in an increase in tumor boron concentration from approximately 30 to approximately 80 microg 10B/g, while, again, maintaining a constant T:B ratio of about 3.7:1. In contrast, intracarotid infusion of BPA for 1 h at a dose rate of 125 mg BPA/kg resulted in an increase in the tumor boron concentration from approximately 26 to approximately 38 microg 10B/g with a corresponding increase in the T:B ratio from 3.5:1 to 5.0:1. The effects of these results on the therapeutic gain potentially achievable with BNCT are discussed.

Outwitting the blood-brain barrier for therapeutic purposes: osmotic opening and other means

OBJECTIVE

This article reviews historical aspects of the blood-brain barrier (BBB) and recent advances in mechanisms to deliver therapeutic agents across the BBB for the treatment of intracerebral tumors and other neurological diseases.

METHODS

The development of the osmotic BBB disruption procedure as a clinically useful technique is described. Osmotic BBB disruption is contrasted with alternative methods for opening or bypassing the BBB, including pharmacological modification of the BBB with bradykinin and direct intracerebral infusion.

RESULTS

Laboratory studies have played a fundamental role in advancing our understanding of the BBB and delivery of agents to brain. Preclinical animal studies will continue to serve an integral function in our efforts to improve the diagnosis and treatment of a number of neurological disorders. Techniques involving the modification of the BBB and/or blood-tumor barrier to increase delivery of therapeutic agents have been advanced to clinical trials in patients with brain tumors with very favorable results.

CONCLUSION

Improving delivery of agents to the brain will play a major role in the therapeutic outcome of brain neoplasms. As techniques for gene therapy are advanced, manipulation of the BBB also may be important in the treatment of central nervous system genetic disorders.

Boron neutron capture therapy: principles and potential

This book on the therapeutic applications of neutrons and high-LET radiations in cancer therapy would not have been complete without a review of the present situation of boron neutron capture therapy (BNCT) and a discussion of its future perspectives. BNCT is a special type of high-LET radiation therapy that attempts to achieve a selectivity at the cellular level. The rationale is to incorporate boron atoms selectively in the cancer cells and then bombard those atoms with thermal neutrons to produce a neutron capture reaction and subsequent decay that emits alpha and lithium particles. The efficiency of the technique depends upon achieving selective incorporation of the boron atoms in the cancer cells and not (or to a lesser extent) in the normal cells. The present status and future directions are described, with emphasis on boron carriers (drugs) and their delivery, as well as physical and treatment planning aspects.

Selective boron drug delivery to brain tumors for boron neutron capture therapy

Malignant glioma is one of the most deadly forms of cancer in humans and remains refractory to presently available treatments. Boron neutron capture therapy (BNCT) is a promising therapeutic modality for the treatment of malignant brain tumors. For successful BNCT, a sufficient quantity of boron atoms must be selectively delivered to individual brain tumor cells while at the same time the boron concentration in the normal brain tissue should be kept low to minimize the damage to normal brain tissue. However, the brain entry of drugs is restricted by the blood-brain barrier (BBB), even though the permeability of the pathological area of this barrier may be partially increased due to the present of brain tumors. Therefore, selective delivery of boron to tumor cells across the BBB is a major challenge to the BNCT of brain tumors. This review briefly discusses four main mechanisms responsible for drug transport across the BBB. Brain tumor-localizing boron compounds are described, such as borocaptate sodium, p-boronophenylalanine, boronated porphyrins and boronated nucleosides. Strategies employed to selectively deliver boron drug into brain tumors are reviewed including hyperosmotic BBB modification, biochemical opening of BBB, electropermeabilization and direct intracerebral delivery of boron drugs. Conjugation of boron drugs to macromolecules like monoclonal antibodies and epidermal growth factor are discussed for active tumor targeting. Boron delivery via microparticles such as liposomes, high density lipoproteins and nanoparticles is also covered for their potential utilization in BNCT of brain tumors.

The rationale and requirements for the development of boron neutron capture therapy of brain tumors

The dismal clinical results in the treatment of glioblastoma multiforme despite aggressive surgery, conventional radiotherapy, and chemotherapy, either alone or in combination has led to the development of alternative therapeutic modalities. Among these is boron neutron capture therapy (BNCT). This binary system is based upon two key requirements: (1) the development and use of neutron beams from nuclear reactors or other sources with the capability for delivering high fluxes of thermal neutrons at depths sufficient to reach all tumor foci, and (2) the development and synthesis of boron compounds that can penetrate the normal bloodbrain barrier, selectively target neoplastic cells, and persist therein for suitable periods of time prior to irradiation. The earlier clinical failures with BNCT related directly to the lack of tissue penetration by neutron beams and to boron compounds that showed little specificity for and low retention by tumor cells, while attaining high concentrations in blood. Progress has been made both in neutron beam and compound development, but it remains to be determined whether these are sufficient to improve therapeutic outcomes by BNCT in comparison with current therapeutic regimens for the treatment of malignant gliomas.

Enhanced survival of glioma bearing rats following boron neutron capture therapy with blood-brain barrier disruption and intracarotid injection of boronophenylalanine

Boronophenylalanine (BPA) has been used for boron neutron capture therapy (BNCT) of brain tumors in both experimental animals and humans. The purpose of the present study was to determine if the efficacy of BNCT could be enhanced by means of intracarotid (i.c.) injection of BPA with or without blood-brain barrier disruption (BBB-D) and neutron irradiation using a rat brain tumor model. For biodistribution studies, F98 glioma cells were implanted stereotactically into the brains of Fischer rats, and 12 days later BBB-D was carried out by i.c. infusion of 25% mannitol (1.373 mOsmol/ml), followed immediately by i.c. administration of 300, 500 or 800 mg of BPA/kg body weight (b.w.). At the 500 mg dose a fourfold increase in tumor boron concentration (94.5 micrograms/g) was seen at 2.5 hours after BBB-D. compared to 20.8 micrograms/g in i.v. injected animals. The best composite tumor to normal tissue ratios were observed at 2.5 hours after BBB-D, at which time the tumor: blood (T: Bl) ratio was 10.9, and the tumor: brain (T:Br) ratio was 7.5, compared to 3.2 and 5.0 respectively for i.v. injected rats. In contrast, animals that had received i.c. BPA without BBB-D had T : Bl and T:Br ratios of 8.5 and 5.9, respectively, and the tumor boron concentration was 42.7 micrograms/g. For therapy experiments, initiated 14 days after intracerebral implantation of F98 glioma cells, 500 mg/kg b.w. of BPA were administered i.v. or i.c. with or without BBB-D, and the animals were irradiated 2.5 hours later at the Brook-haven Medical Research Reactor with a collimated beam of thermal neutrons delivered to the head. The mean survival time for untreated control rats was 24 +/- 3 days, 30 +/- 2 days for irradiate controls, 37 +/- 3 days for those receiving i.v. BPA, 52 +/- 15 days for rats receiving i.c. BPA without BBB-D, and 95 +/- 95 days for BBB-D followed by i.c. BPA and BNCT. The latter group had a 246% increase in life span (ILS) compared to untreated controls and a 124% ILS compared to that of i.v. injected animals. These survival data are the best ever obtained with the F98 glioma model and suggest that i.c. administration of BPA with or without BBB-D may be useful as a means to increase the efficacy of BNCT.