Targeted drug delivery for boron neutron capture therapy

Boron in cancer therapeutics: An overview

Boron has become a crucial weapon in anticancer research due to its significant intervention in cell proliferation. Being an excellent bio-isosteric replacement of carbon, it has modulated the anticancer efficacy of various molecules in the development pipeline. It has elicited promising results through interactions with various therapeutic targets such as HIF-1α, steroid sulfatase, arginase, proteasome, etc. Since boron liberates alpha particles, it has a wide-scale application in Boron Neutron Capture therapy (BNCT), a radiotherapy that demonstrates selectivity towards cancer cells due to high boron uptake capacity. Significant advances in the medicinal chemistry of boronated compounds, such as boronated sugars, natural/unnatural amino acids, boronated DNA binders, etc., have been reported over the past few years as BNCT agents. In addition, boronated nanoparticles have assisted the field of bio-nano medicines by their usage in radiotherapy. This review exclusively focuses on the medicinal chemistry aspects, radiotherapeutic, and chemotherapeutic aspects of boron in cancer therapeutics. Emphasis is also given on the mechanism of action along with advantages over conventional therapies.

Comparison of Conventional and Radiomic Features between 18F-FBPA PET/CT and PET/MR

Boron-10-containing positron emission tomography (PET) radio-tracer, 18F-FBPA, has been used to evaluate the feasibility and treatment outcomes of Boron neutron capture therapy (BNCT). The clinical use of PET/MR is increasing and reveals its benefit in certain applications. However, the PET/CT is still the most widely used modality for daily PET practice due to its high quantitative accuracy and relatively low cost. Considering the different attenuation correction maps between PET/CT and PET/MR, comparison of derived image features from these two modalities is critical to identify quantitative imaging biomarkers for diagnosis and prognosis. This study aimed to investigate the comparability of image features extracted from 18F-FBPA PET/CT and PET/MR. A total of 15 patients with malignant brain tumor who underwent 18F-FBPA examinations using both PET/CT and PET/MR on the same day were retrospectively analyzed. Overall, four conventional imaging characteristics and 449 radiomic features were calculated from PET/CT and PET/MR, respectively. A linear regression model and intraclass correlation coefficient (ICC) were estimated to evaluate the comparability of derived features between two modalities. Features were classified into strong, moderate, and weak comparability based on coefficient of determination (r2) and ICC. All of the conventional features, 81.2% of histogram, 37.5% of geometry, 51.5% of texture, and 25% of wavelet-based features, showed strong comparability between PET/CT and PET/MR. With regard to the wavelet filtering, radiomic features without filtering (61.2%) or with low-pass filtering (59.2%) along three axes produced strong comparability between the two modalities. However, only 8.2% of the features with high-pass filtering showed strong comparability. The linear regression models were provided for the features with strong and moderate consensus to interchange the quantitative features between the PET/CT and the PET/MR. All of the conventional and 71% of the radiomic (mostly histogram and texture) features were sufficiently stable and could be interchanged between 18F-FBPA PET with different hybrid modalities using the proposed equations. Our findings suggested that the image features high interchangeability may facilitate future studies in comparing PET/CT and PET/MR.

Smart material based on boron crosslinked polymers with potential applications in cancer radiation therapy

Organoboron compounds have been playing an increasingly important role in analytical chemistry, material science, health applications, and particularly as functional polymers like boron carriers for cancer therapy. There are two main applications of boron isotopes in radiation cancer therapy, Boron Neutron Capture Therapy and Proton Boron Fusion Therapy. In this study, a novel and original material consisting of a three-dimensional polymer network crosslinked with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{10}$$\end{document}10B enriched boric acid molecules is proposed and synthesized. The effects of the exposition to thermal neutrons were studied analyzing changes in the mechanical properties of the proposed material. Dedicated Monte Carlo simulations, based on MCNP and FLUKA main codes, were performed to characterize interactions of the proposed material with neutrons, photons, and charged particles typically present in mixed fields in nuclear reactor irradiations. Experimental results and Monte Carlo simulations were in agreement, thus justifying further studies of this promising material.

Similar T/N ratio between 18F-FBPA diagnostic and BPA therapeutic dosages for boron neutron capture therapy in orthotropic tongue cancer model

OBJECTIVE

The tumor-to-normal tissue (T/N) boron ratio is determined in a patient prior to boron neutron capture therapy (BNCT) using 4-borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸F-FBPA) positron emission tomography (PET). The T/N ratio is used as a reference parameter to calculate BNCT dose and to evaluate treatment effects. The boronophenylalanine (BPA) dosage for BNCT treatment is higher than the ¹⁸F-FBPA dosage for PET diagnosis. Therefore, we aimed to determine whether the T/N ratios between diagnosis and treatment were correlated.

METHODS

In this study, SAS tongue cancer cells were used to develop an orthotopic nude mouse model. Micro-PET was performed after the mice were injected a dose of 3.7 ± 0.74 MBq of ¹⁸F-FBPA via the tail vein. The ¹⁸F radioactivity in the tumor, muscle, and heart blood pool was calculated using AMIND software. Organs and blood were collected for boron concentration analysis using inductively coupled plasma-atomic emission spectroscopy after the mice were injected with 400 mg/kg BPA at 15, 30, 45, and 60 min.

RESULTS

Pharmacokinetics of the tumor and muscle from 45 to 60 min after ¹⁸F-FBPA and BPA injections were slightly increased, whereas that of blood was slightly decreased. Median T/N ratios at 60 min after ¹⁸F-FBPA and BPA injections were 3.5 and 3.43, respectively. Median value of the T/N ratio between them was 3.49 at 60 min. The T/N ratio at 60 min after ¹⁸F-FBPA injection was similar to that after BPA injection. However, median tumor-to-blood (T/B) boron ratios of ¹⁸F-FBPA and BPA at 60 min were 1.63 and 3.35, respectively. Median value between them was 1.83 at 60 min.

CONCLUSIONS

In this study, the T/B ratios demonstrate the spread of a distribution between ¹⁸F-FBPA and BPA injections. At 60 min, the T/N ratio of the ¹⁸F-FBPA injection was similar to that of the BPA injection. Boron concentration in normal tissue was almost equal to that in blood. Therefore, the representative T/N ratio could be obtained at 60 min after ¹⁸F-FBPA injection, and it was used as a reference parameter for calculating accurate radiation dose.

The Current Status and Perspectives of Delivery Strategy for Boron-based Drugs

Boron-containing compounds are essential micronutrients for animals and plants despite their low-level natural occurrence. They can strengthen the cell walls of the plants and they play important role in supporting bone health. However, surprisingly, boron-containing compounds are seldom found in pharmaceutical drugs. In fact, there are no inherent disadvantages reported so far in terms of the incorporation of boron into medicines. Indeed, drugs based on boron-containing compounds, such as tavaborole (marked name Kerydin) and bortezomib (trade name Velcade) have been investigated and they are used in clinical treatment. In addition, following the advanced development of boron neutron capture therapy and a new emerging proton boron fusion therapy, more boron-containing medicinals are to be expected. This review discusses the current status and perspectives of delivery strategy for boron-containing drugs.

Synthesis, reactivity and biological activity of N(4)-boronated derivatives of 2'-deoxycytidine

By seeking new stable boron-containing nucleoside derivatives, potential BNCT boron delivery agents, a novel synthetic approach was tested, aimed at a boron attachment via a single bond to an aliphatic carbon of sp(3) hybridization. The latter allowed successful modification of deoxycytidine in the reaction with 2-(iodomethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane of the deoxynucleoside amino group. For new compounds, detailed NMR, LDI HRMS (Laser Desorption/Ionization High-Resolution Mass Spectrometry) analyses along with in vivo phosphorylation studies, toxicity assays and DFT modelling are presented.

Low dose of gamma irradiation enhanced boronophenylalanine uptake in head and neck carcinoma cells for boron neutron capture therapy

This study attempted to increase the boron uptake of human head and neck carcinoma SAS cells for BNCT by using a gamma dose of 0.1 Gy for combined treatment. Intracellular boron concentrations in 25 μgB/mL medium of BPA treated and BPA combined gamma-irradiation treated SAS cells were 73.8±1.73 and 95.15±1.36 ppm, respectively. After neutron irradiation, the G2/M-phase cell populations of untreated, BPA treated and BPA combined gamma-irradiation treated SAS cells were 19.31±1.71%, 52.47±2.25% and 59.19±2.63%, respectively. Experimental results indicate that the low dose gamma radiation with combination BPA treatment has the highest killing rate after neutron irradiation. Capable of significantly increasing the G2/M arrest after neutron irradiation, the combined treatment of a low dose of gamma irradiation with 25 μg B/mL medium of BPA also provided a higher killing effect for BNCT.

Cationized gelatin-HVJ envelope with sodium borocaptate improved the BNCT efficacy for liver tumors in vivo

BACKGROUND

Boron neutron capture therapy (BNCT) is a cell-selective radiation therapy that uses the alpha particles and lithium nuclei produced by the boron neutron capture reaction. BNCT is a relatively safe tool for treating multiple or diffuse malignant tumors with little injury to normal tissue. The success or failure of BNCT depends upon the 10B compound accumulation within tumor cells and the proximity of the tumor cells to the body surface. To extend the therapeutic use of BNCT from surface tumors to visceral tumors will require 10B compounds that accumulate strongly in tumor cells without significant accumulation in normal cells, and an appropriate delivery method for deeper tissues.Hemagglutinating Virus of Japan Envelope (HVJ-E) is used as a vehicle for gene delivery because of its high ability to fuse with cells. However, its strong hemagglutination activity makes HVJ-E unsuitable for systemic administration.In this study, we developed a novel vector for 10B (sodium borocaptate: BSH) delivery using HVJ-E and cationized gelatin for treating multiple liver tumors with BNCT without severe adverse events.

METHODS

We developed cationized gelatin conjugate HVJ-E combined with BSH (CG-HVJ-E-BSH), and evaluated its characteristics (toxicity, affinity for tumor cells, accumulation and retention in tumor cells, boron-carrying capacity to multiple liver tumors in vivo, and bio-distribution) and effectiveness in BNCT therapy in a murine model of multiple liver tumors.

RESULTS

CG-HVJ-E reduced hemagglutination activity by half and was significantly less toxic in mice than HVJ-E. Higher 10B concentrations in murine osteosarcoma cells (LM8G5) were achieved with CG-HVJ-E-BSH than with BSH. When administered into mice bearing multiple LM8G5 liver tumors, the tumor/normal liver ratios of CG-HVJ-E-BSH were significantly higher than those of BSH for the first 48 hours (p < 0.05). In suppressing the spread of tumor cells in mice, BNCT treatment was as effective with CG-HVJ-E-BSH as with BSH containing a 35-fold higher 10B dose. Furthermore, CG-HVJ-E-BSH significantly increased the survival time of tumor-bearing mice compared to BSH at a comparable dosage of 10B.

CONCLUSION

CG-HVJ-E-BSH is a promising strategy for the BNCT treatment of visceral tumors without severe adverse events to surrounding normal tissues.

Gamma-ray irradiation enhanced boron-10 compound accumulation in murine tumors

Previous studies have demonstrated that X-ray irradiation affects angiogenesis in tumors. Here, we studied the effects of gamma-ray irradiation on boron-10 compound accumulation in a murine tumor model. The mouse squamous cell carcinoma was irradiated with gamma-ray before BSH ((10)B-enriched borocaptate sodium) administration. Then, the boron-10 concentrations in tumor and normal muscle tissues were measured by prompt gamma-ray spectrometry (PGA). A tumor blood flow assay was performed, and cell killing effects of neutron irradiation with various combinations of BSH and gamma-rays were also examined. BSH concentrations of tumor tissues were 16.1 +/- 0.6 microg/g, 16.7 +/- 0.5 microg/g and 17.8 +/- 0.5 microg/g at 72 hours after gamma-ray irradiation at doses of 5, 10, and 20 Gy, compared with 13.1 +/- 0.5 microg/g in unirradiated tumor tissues. The enhancing inhibition of colony formation by neutron irradiation with BSH was also found after gamma-ray irradiation. In addition, increasing Hoechst 33342 perfusion was also observed. In this study, we demonstrated that gamma-ray irradiation enhances BSH accumulation in tumors. The present results suggest that the enhancement of (10)B concentration that occurs after gamma-ray irradiation may be due to the changes in the extracellular microenvironment, including in tumor vessels, induced by gamma-ray irradiation.

Tumor-specific targeting of sodium borocaptate (BSH) to malignant glioma by transferrin-PEG liposomes: a modality for boron neutron capture therapy

OBJECT

Boron neutron capture therapy (BNCT) requires selective delivery of a high concentration of boron-10 ((10)B) to tumor tissue. To improve a drug delivery in BNCT, we devised transferrin-conjugated polyethylene-glycol liposome encapsulating sodium borocaptate (TF-PEG-BSH).

METHODS

(10)B concentrations of U87Delta human glioma cells from three boron delivery systems (BDS) (bare BSH, PEG-BSH, and TF-PEG-BSH) were analyzed in vitro by use of inductively coupled plasma-atomic emission spectrometry (ICP-AES). A colony-forming assay (CFA) was performed using this cell line with the three BDS and neutron irradiation. Subcellular localization of BSH in tumor cells was analyzed in vitro by immunocytochemistry using monoclonal antibodies raised for BSH. Brain tumor models were made and the (10)B concentrations of the tumor, blood, and normal brain from each BDS were analyzed in vivo by use of ICP-AES. The tumor-to-blood and the tumor-to-normal brain ratios were evaluated. BNCT for the brain tumor models was performed and survival was analyzed.

RESULTS

In CFA, TF-PEG-BSH showed the most prominent effects by neutron irradiation among the three BDS. TF-PEG-BSH showed highly selective and highly efficient (10)B delivery in tumor tissue. The survival rate in the treatment experiment was best in the TF-PEG-BSH group. Immunocytochemically, TF-PEG-BSH delivered BSH efficiently not only into the cytoplasm but also into the nucleus.

CONCLUSION

TF-PEG-BSH is a potent BDS for BNCT not only in terms of delivering a high concentration of (10)B into tumor tissue, but also the selective delivery of (10)B into the tumor cells.

Pharmacokinetic study of BSH and BPA in simultaneous use for BNCT

In order to improve the effectiveness of boron neutron capture therapy (BNCT) for malignant gliomas, we examined the optimization of the administration of boron compounds in brain tumor animal model. We analyzed the concentration of boron atoms in intracranial C6 glioma -bearing rats using inductively coupled plasma atomic emission spectrometry. Each tumor-bearing rat received one of two different amounts of sodium borocaptate (BSH) and/or 500 mg/kg of boronophenylalanine (BPA) via intraperitoneal injection. We compared the boron concentrations of the tumor, the contralateral normal brain and the blood in rats of 3 different treatment groups (BSH alone, BPA alone and a combination of both BSH and BPA). Our results show that the tumor boron concentration increased much more than 30 microg/g by the coadministration of both compounds. Additionally, the blood boron concentration remained below 30 microg/g and the boron concentration in the normal brain was low (mean 4.7+/-1.1 microg/g). Even in comparison with the administration of BPA alone, coadministration of BPA and BSH shows an improved tumor/normal brain ratio of boron concentrations.

Preparation and characterization of Boron carbide nanoparticles for use as a novel agent in T cell-guided boron neutron capture therapy

Boron carbide nanoparticles are proposed as a system for T cell-guided boron neutron capture therapy. Nanoparticles were produced by ball milling in various atmospheres of commercially available boron carbide. The physical and chemical properties of the particles were investigated using transmission electron microscopy, photon correlation spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, vibrational spectroscopy, gel electrophoresis and chemical assays and reveal profound changes in surface chemistry and structural characteristics. In vitro thermal neutron irradiation of B16 melanoma cells incubated with sub-100 nm nanoparticles (381.5 microg/g (10)B) induces complete cell death. The nanoparticles alone induce no toxicity.

Toxicity, biodistribution, and convection-enhanced delivery of the boronated porphyrin BOPP in the 9L intracerebral rat glioma model

PURPOSE

To investigate the toxicity, biodistribution, and convection-enhanced delivery (CED) of a boronated porphyrin (BOPP) that was designed for boron neutron capture therapy and photodynamic therapy.

METHODS AND MATERIALS

For the toxicity study, Fischer 344 rats were injected with graded concentrations of BOPP (35-100 mg/kg) into the tail vein. For boron biodistribution studies, 9L tumor-bearing rats received BOPP either systematically (intravenously) or locally.

RESULTS

All rats that received 70 mg/kg BOPP and 70% of rats that received < or = 60 mg/kg BOPP i.v. either had to be euthanized or died within 4 days of injection. In the biodistribution study, boron levels were relatively high in liver, kidney, spleen, and adrenal gland tissue, and moderate levels were found in all other organs. The maximum tumor boron concentration was 21.4 mug/g at 48 h after i.v. injection; this concentration of boron in brain tumors is at the low end of the range considered optimal for therapy. In addition, the tumor/blood ratio (approximately 1.2) was not optimal. When BOPP was delivered directly into intracerebral 9L tumors with CED, we obtained tumor boron concentrations much greater than those obtained by i.v. injection. Convection-enhanced delivery of 1.5 mg BOPP produced an average tumor boron level of 519 mug/g and a tumor/blood ratio of approximately 1850:1.

CONCLUSIONS

Our study demonstrates that changing the method of BOPP delivery from i.v. to CED significantly enhances the boron concentration in tumors and produces very favorable tumor/brain and tumor/blood ratios.

In vivo evaluation of the boronated porphyrin TABP-1 in U-87 MG intracerebral human glioblastoma xenografts

Boron neutron capture therapy (BNCT) is an adjuvant therapy that has the potential to control local tumor growth. A selective delivery of sufficient amounts of boron to individual tumor cells, compared to surrounding normal tissues, is the key for successful BNCT. We have designed and synthesized a new highly water-soluble boronated porphyrin, TABP-1, as a possible BNCT agent. When we injected the maximum tolerated dose (MTD: 15 mg/kg) of TABP-1 systemically into the tail vein of athymic rats bearing intracerebral (i.c.) human glioblastoma U-87 MG xenografts, the compound accumulated preferentially in brain tumors compared to normal brain; however, the level of boron in the tumors was less than the 30 microg/g of tissue that is generally considered necessary for BNCT. We next investigated whether convection-enhanced delivery (CED) could improve the boron distribution. The compound was administered directly into i.c. tumors using an osmotic minipump attached to a brain-infusion cannula. TABP-1 doses from 0.25 to 1.0 mg infused locally over 24 h produced tumor boron concentrations greater than those obtained by systemic administration at the MTD. For example, CED administration of 0.5 mg of TABP-1 produced a tumor boron level of 65.4 microg/g of tumor, whereas the serum level was only 0.41 microg/g (tumor to serum ratio of approximately 160:1). CED also produced relatively high tumor to normal brain ratios of approximately 5:1 for ipsilateral brain and approximately 26:1 for contralateral brain tissues at the 0.5 mg dose. Thus, we may be able to achieve therapeutic BNCT efficacy with minimal systemic toxicity or radiation-induced damage to normal tissue by administering TABP-1 using CED.

The Boron–Neutron Capture Agent β-d-5-o-Carboranyl-2′-deoxyuridine Accumulates Preferentially in Dividing Brain Tumor Cells

Boron-neutron capture therapy (BNCT) is based on the preferential targeting of tumor cells with (10)B and subsequent irradiation with epithermal neutrons to produce a highly localized field of lethal alpha particles, while sparing neighboring non-targeted cells. BNCT treatment of 9L brain tumors in a rat model using beta-D-5-o-carboranyl-2'-deoxyuridine (D-CDU) resulted in greater efficacy than predicted based on the assumption of a uniform tumor distribution of (10)B. Thus, the geometric heterogeneity of dividing cells in brain tumors warranted studies on the cell cycle dependency of D-CDU accumulation, metabolism and entrapment in a relevant brain tumor cell system. U-271 human glioma cells were synchronized in G(1) or S-phases of the cell cycle. The cellular accumulation and phosphorylation of D-CDU was measured in the G(1) and S-phase cells using high-performance liquid chromatography (HPLC). Cells synchronized in the S-phase accumulated significantly higher amounts of D-CDU and produced larger amounts of negatively charged D-CDU monophosphate (D-CDU-MP) and nido-CDU metabolites than resting cells. Since brain tumors contain a larger proportion of cycling cells than neighboring tissue, these results support the hypothesis that in addition to breakdown of the blood-brain-barrier (BBB) in tumors, the preferential phosphorylation of D-CDU in cycling cells may further enrich the distribution of (10)B in dividing cells. Therefore, dosimetry calculations that include the spatial distribution of cycling cells may be warranted for D-CDU.

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

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

Synthesis, radioiodination, and biodistribution of some nido- and closo-monocarbon carborane derivatives

Iodination and radioiodination reactions of several anionic nido- and closo-monocarbon carboranes were conducted. Iodinations occurred more rapidly with nido-carboranes than with closo-carboranes. The most rapid iodination and radioiodination reactions occurred with unsubstituted carboranes. C-amino and C-ammonium derivatives did not iodinate under the conditions studied. Both nido- and closo-carboranes with C-NH-acetyl and C-NH-succinyl substituents iodinated, but the nido-carboranes iodinated under milder reaction conditions. Biodistributions of nido-1-succinylamido-[(131)I]carborane and closo-1-succinylamido-[(125)I]carborane were similar in mice, but blood clearance of the nido- compound was slower.

Intracellular targeting of sodium mercaptoundecahydrododecaborate (BSH) to solid tumors by transferrin-PEG liposomes, for boron neutron-capture therapy (BNCT)

The successful treatment of cancer by boron neutron-capture therapy (BNCT) requires the selective delivery of relatively high concentration of 10B compounds to malignant tumor tissue. This study focuses on a new tumor-targeting drug delivery system for BNCT that uses small (less than 200 nm in diameter), unilamellar mercaptoundecahydrododecaborate (BSH)-encapsulating, transferrin (TF)-conjugated polyethyleneglycol liposomes (TF-PEG liposomes). When TF-PEG liposomes were injected at a dose of 35 mg 10B/kg, we observed a prolonged residence time in the circulation and low uptake by the reticuloendothelial system (RES) in Colon 26 tumor-bearing mice, resulting in enhanced accumulation of 10B into the solid tumor tissue (e.g., 35.5 microg/g). TF-PEG liposomes maintained a high 10B level in the tumor, with concentrations over 30 microg/g for at least 72 h after injection. This high retention of 10B in tumor tissue indicates that binding and concomitant cellular uptake of the extravasated TF-PEG liposomes occurs by TF receptor and receptor-mediated endocytosis, respectively. On the other hand, the plasma level of 10B decreased, resulting in a tumor/plasma ratio of 6.0 at 72 h after injection. Therefore, 72 h after injection of TF-PEG liposomes was selected as the time point of BNCT treatment. Administration of BSH encapsulated in TF-PEG liposomes at a dose of 5 or 20 mg 10B/kg and irradiation with 2 x 10(12) neutrons/cm2 for 37 min produced tumor growth suppression and improved long-term survival compared with PEG liposomes, bare liposomes and free BSH. Thus, intravenous injection of TF-PEG liposomes can increase the tumor retention of 10B atoms, which were introduced by receptor-mediated endocytosis of liposomes after binding, causing tumor growth suppression in vivo upon thermal neutron irradiation. These results suggest that BSH-encapsulating TF-PEG liposomes may be useful as a new intracellular targeting carrier in BNCT therapy for cancer.

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.

Influence of formulation parameters on gadolinium entrapment and tumor cell uptake using folate-coated nanoparticles

Emulsifying wax and polyoxyl 2 stearyl ether (Brij 72) nanoparticles (2 mg/ml) containing high concentrations of gadolinium hexanedione (GdH) (0-3 mg) have been engineered from oil-in-water microemulsion templates. Solid nanoparticles were cured by cooling warm microemulsion templates (prepared at 55 degrees C) to room temperature in one vessel. Nanoparticles were characterized by transmission electron microscopy (TEM), photon correlation spectroscopy (PCS) and gel permeation chromatography (GPC). To obtain folate-coated nanoparticles, a folate ligand was added to either the microemulsion templates or nanoparticle suspensions at 25 degrees C. Since the concentration of Gd in the tumor is critical to the success of Gd-neutron capture therapy (NCT), the effects of various formulation factors on GdH entrapment in nanoparticles as well as tumor-targeting were studied. GdH entrapment in nanoparticles was affected mostly by the method of GdH incorporation and surfactant concentration used in preparing the microemulsion templates. Cell uptake studies were carried out in KB cells (human nasopharyngeal epidermal carcinoma cell line). The method of adding folate ligand to the formulations did not significantly affect nanoparticle cell uptake (P>0.11; t-test). However, the concentration of folate ligand added to nanoparticles had the greatest influence on nanoparticle uptake (P<0.01; t-test). The results showed that GdH entrapment and cell uptake were optimized and suggested that engineered folate-coated nanoparticles may serve as effective carrier systems for Gd-NCT of tumors.

Folate receptor-mediated liposomal delivery of a lipophilic boron agent to tumor cells in vitro for neutron capture therapy

PURPOSE

This study was aimed at the in vitro evaluations of folate receptor (FR)-targeted liposomes as carriers for a lipophilic boron agent, K[nido-7-CH3(CH2)15-7,8-C2B9H11, in FR-overexpressing tumor cells for neutron capture therapy.

METHODS

Large unilamellar vesicles (-200 nm in diameter) were prepared with the composition of egg PC/chol/K[nido-7-CH3(CH2)15-7,8-C2B9H11] (2:2:1, mol/mol), with an additional 0.5 mol % of folate-PEG-DSPE or PEG-DSPE added for the FR-targeted or nontargeted liposomal formulations, respectively.

RESULTS

Boron-containing, FR-targeted liposomes readily bound to KB cells, an FR-overexpressing cell line, and were internalized via FR-mediated endocytosis. The boron uptake in cells treated with these liposomes was approximately 10 times greater compared with those treated with control liposomes. In contrast, FR-targeted and nontargeted liposomes showed no difference in boron delivery efficiency in F98 cells, which do not express the FR. The subcellular distribution of the boron compound in KB cells treated with the FR-targeted liposomes was investigated by cellular fractionation experiments, which showed that most of the boron compound was found in either the cytosol/endosomal or cell membrane fractions, indicating efficient internalization of the liposomal boron.

CONCLUSION

FR-targeted liposomes incorporating the lipophilic boron agent, K[nido-7-CH3(CH2)15-7,8-C2B9H11], into its bilayer were capable of specific receptor binding and receptor-mediated endocytosis in cultured KB cells. Such liposomes warrant further investigations for use in neutron capture therapy.

VLDL-resembling phospholipid-submicron emulsion for cholesterol-based drug targeting

The objective of the current study was to develop and evaluate VLDL-resembling phospholipid-submicron emulsion (PSME) as a carrier system for new cholesterol-based compounds for targeted delivery to cancer cells. BCH, a boronated cholesterol compound, was originally developed in our laboratory to mimic the cholesterol esters present in the LDL and to follow a similar pathway of cholesterol transport into the rapidly dividing cancer cells. The VLDL-resembling system was then designed to solubilize BCH, facilitate the interaction with LDL, and thus assist the BCH delivery to cancer cells. BCH-containing PSME was prepared by sonication. Chemical compositions and particle sizes of different PSME fractions were determined. The lipid structure of PSME and location of BCH in the formulation were assessed based on experimental results. Density gradient ultracentrifugation fractionated the emulsion into three particle-size populations with structures and compositions resembling native VLDL. In vitro interaction between PSME and LDL was evident by agarose electrophoresis, as both formed a single band with an intermediate mobility. The transfer of BCH from PSME to LDL was also observed in the presence of other serum components including serum proteins. Cell culture data showed sufficient uptake of BCH in rat 9L glioma cells (> 50 microg boron/g cells). In conclusion, this system has the capability to incorporate the cholesterol-based compound, interact with native LDL, and assist the delivery of this compound into cancer cells in vitro.

Cell culture and animal studies for intracerebral delivery of borocaptate in liposomal formulation

The efficacy of a liposomal formulation for intracerebral delivery of borocaptate (BSH) to brain tumor cells has been investigated using cell culture to study BSH uptake and persistence and using tumor-bearing rats to determine BSH distribution in the brain. During a 16-hr incubation, cellular uptake of BSH solution or BSH liposomal formulation was similar. However, the cellular persistence of BSH greatly increased when BSH was present in liposome. The differences in cellular persistence for BSH solution and BSH-loaded liposomes were significant both in 12-hr and 24-hr incubation experiments (p < 0.05 and p < 0.01, respectively). For the studies involving tumor-bearing rats, BSH level in tumor tissue was significantly higher than that in normal brain tissue at 2 hr and 6 hr after intracerebral injection of BSH-loaded liposomes (p < 0.01). Our study indicated that the liposomal formulation enhanced cellular persistence of BSH in tumor cells and therefore favored the boron accumulation in the cells. With the prolonged physical retention of liposomes at the local injection site and the cellular retention of BSH enhanced by the liposomes, the intracerebral delivery of BSH using liposomal formulation may provide an effective boron delivery approach for boron neutron capture therapy of brain tumors.

Biodistribution of boron concentration on melanoma-bearing hamsters after administration of p-, m-, o-boronophenylalanine

Although p-boronophenylalanine (p-BPA), a boronate analogue of tyrosine, has proven to be one of the most successful compounds for boron neutron capture therapy (BNCT) of malignant melanoma, the selective uptake mechanism of this compound into melanoma cells is not well understood. Therefore, the relationship between the structure of BPA and its specific affinity to melanoma cells appears worthy of investigation. In the present study, m- and o-boronophenylalanine (m- and o-BPA) were administered to melanoma-bearing hamsters and their uptake was measured. The time courses (0.5, 2.0, 4.0 and 48.0 h) of boron concentrations in melanoma, normal skin, and blood were determined in male Syrian (golden) hamsters bearing Greene's melanomas following a single intraperitoneal injection of either p-, m- or o-BPA (100 mg/kg of BPA fructose in 1.0 ml of saline). The boron concentrations in these tissues were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES). In melanoma, the order of boron uptake was p- > m- > o-BPA at all time points, and the boron concentrations obtained with p-BPA and m-BPA resembled each other in that they had a peak at 2 h after administration and decreased with time. The melanoma/skin boron concentration ratio of p-BPA had a peak at 4 h after administration and the ratio ranged between 7/1 and 8/1. On the other hand, m-BPA and o-BPA had a peak at 2 h and their ratios ranged between 4/1 and 5/1. The difference in the accumulations of p-BPA and m-BPA could be due to a difference in the property of p-BPA as a tyrosine analogue for melanin synthesis. The accumulation of m-BPA into melanoma might indicate the baseline level of metabolism-related amino acid transport. Our experimental findings indicate that this melanin synthesis, or the structural analogy between the boron compound and tyrosine as a precursor of melanin, is an important factor in the increased accumulation of p-BPA in melanoma cells.

Targeted drug delivery for brain cancer treatment

The blood brain barrier (BBB) and the systemic toxicity of conventional chemotherapy present obstacles to the success of future blood-borne drug therapies of brain tumors. The work with polymer-encapsulated cancer drugs suggests an alternative and more focused treatment approach. Our experimental strategy integrates direct intracerebral drug delivery, sustained drug release from liposomes or polymer implants, and increased targeting of the drug either by chemically modifying the drug or by using tumor-specific carriers. This review will present some of the recent work on targeted drug delivery for brain cancer treatment.

Cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine

The cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine (CDU), compounds designed for boron neutron capture therapy (BNCT), were studied using human CEM lymphoblast and U-251 glioblastoma cells, at a physiologically achievable concentration (1 microM). Accumulation of the enantiomers was rapid and indistinguishable, reaching cellular concentrations > 40-fold higher than extracellular levels, with approximately 5% persisting in cells after incubation in fresh medium for more than 2 hr. Uptake was not affected by nucleoside uptake inhibitors, but was inhibited by the purine base uptake inhibitor papaverine.

Cytotoxic and antitumor effect of fibrinogen-methotrexate conjugate

In this paper we describe the chemical procedure of fibrinogen-methotrexate (F-MTX) conjugate preparation and its in vitro and in vivo antitumor activity. F-MTX conjugates were synthesized in reaction of fibrinogen with MTX N-hydroxysuccynimide ester. The conjugates were not cross-linked and were soluble in water. The results of the in vitro and in vivo studies have shown: (1) a lower in vitro cytotoxicity of the F-MTX conjugate as compared with MTX alone; (2) a significantly higher in vivo antitumor activity of the F-MTX conjugate in mice with P388 leukemia as compared with MTX alone; (3) a significantly increased in vivo lethal toxicity of F-MTX as compared with MTX. The results suggest the therapeutic utility of the fibrinogen-methotrexate conjugate and the usefulness of fibrinogen as a chemotherapeutic drug carrier. However, a new effort in the preparation of F-MTX conjugate should be made to decrease its in vivo toxicity.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

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.

Liposomes containing boronophenylalanine for boron neutron capture therapy

In the present work, liposomes loaded with Boronophenylalanine (BPA), with or without stabilization, were formulated for the application in boron neutron capture therapy. BPA was encapsulated into liposomes as a complex with fructose, but also as a free drug in two different pH buffers. The influence of critical variables (cholesterol content, drug:lipid molar ratio, osmotic stress of liposomes containing hyperosmotic drug solution) on liposome morphology and drug content was evaluated. The drug content and dissolution profile of different BPA loaded liposomes were also studied. The physical stability of liposomes in terms of changes in the size distribution in different osmotic pressure buffers and the chemical oxidation of phospholipids during storing conditions were investigated. The encapsulation efficiencies of all formulations were always satisfactory, being between 20-48%; even when the liposomes were exposed to high osmotic stress, the particle size was below 200 nm. The BPA-fructose complex loaded liposomes showed a slower drug release profile.

In vivo and in vitro effects of boron and boronated compounds

Boron is ubiquitously present in soils and water. Associated with pectin it is essential for vascular plants as a component of cell walls, and it stabilizes cell membranes. It is required for the growth of pollen tubes and is involved in membrane transport, stimulating H(+)-pumping ATPase activity and K+ uptake. However, a high boron concentration in the soils is toxic to plants and some boronated derivatives are used as herbicides. An absolute requirement for boron has not been definitively demonstrated in animals and humans. However, experiments with boron supplementation or deprivation show that boron is involved in calcium and bone metabolism, and its effects are more marked when other nutrients (cholecalciferol, magnesium) are deficient. Boron supplementation increases the serum concentration of 17 beta-estradiol and testosterone but boron excess has toxic effects on reproductive function. Boron may be involved in cerebral function via its effects on the transport across membranes. It affects the synthesis of the extracellular matrix and is beneficial in wound healing. Usual dietary boron consumption in humans is 1-2 mg/day for adults. As boron has been shown to have biological activity, research into the chemistry of boronated compounds has increased. Boronated compounds have been shown to be potent anti-osteoporotic, anti-inflammatory, hypolipemic, anti-coagulant and anti-neoplastic agents both in vitro and in vivo in animals.

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.