Radiation dose heterogeneity in receptor and antigen mediated boron neutron capture therapy

Unlocking the potential: phenylboronic acid as a nuclear-targeting boron agent for neutron capture therapy

It has been proposed that boron neutron capture therapy (BNCT) holds promise as a treatment modality for melanoma. However, the effectiveness of boron agents in delivery remains a critical issue to be addressed for BNCT. To this end, phenylboronic acid, which exhibits good water solubility and low cytotoxicity similar to BPA, has been investigated as a potential nuclear-targeting boron agent. The boron concentration of phenylboronic acid was found to be 74.47 ± 12.17 ng/106 B16F10 cells and 45.77 ± 5.64 ng/106 cells in the nuclei. Molecular docking experiments were conducted to investigate the binding of phenylboronic acid to importin proteins involved in nuclear transport. The potential of phenylboronic acid to serve as a desirable nucleus-delivery boron agent for neutron capture therapy in melanoma warrants further exploration.

New Boron Containing Acridines: Synthesis and Preliminary Biological Study

The synthesis of the first conjugates of acridine with cobalt bis(dicarbollide) are reported. A novel 9-azido derivative of acridine was prepared through the reaction of 9-methoxyacridine with N3CH2CH2NH2, and its solid-state molecular structure was determined via single-crystal X-ray diffraction. The azidoacridine was used in a copper (I)-catalyzed azide-alkyne cycloaddition reaction with cobalt bis(dicarbollide)-based terminal alkynes to give the target 1,2,3-triazoles. DNA interaction studies via absorbance spectroscopy showed the weak binding of the obtained conjugates with DNA. The antiproliferative activity (IC50) of the boronated conjugates against a series of human cell lines was evaluated through an MTT assay. The results suggested that acridine derivatives of cobalt bis(dicarbollide) might serve as a novel scaffold for the future development of new agents for boron neutron capture therapy (BNCT).

Therapeutic nucleus-access BNCT drug combined CD47-targeting gene editing in glioblastoma

Glioblastoma is the most common brain primary malignant tumor with the highest mortality. Boron neutron capture therapy (BNCT) can efficiently kill cancer cells on the cellular scale, with high accuracy, short course and low side-effects, which is regarded as the most promising therapy for malignant brain tumors like glioma. As the keypoint of BNCT, all boron delivery agents currently in clinical use are beset by insufficient tumor uptake, especially in the tumor nucleus, which limits the clinical application of BNCT. In this study, nuclear targeting of boron is achieved by DOX-CB, consisting of doxorubicin (DOX) and carborane (CB) utilizing the nuclear translocation property of DOX. The nucleus of GL261 cells takes up almost three times the concentration of boron required for BNCT. To further kill glioma and inhibit recurrence, a new multifunctional nanoliposome delivery system DOX-CB@lipo-pDNA-iRGD is constructed. It combines DOX-CB with immunotherapy strategy of blocking macrophage immune checkpoint pathway CD47-SIRPα by CRISPR-Cas9 system, coupling BNCT with immunotherapy simultaneously. Compared with clinical drug Borocaptate Sodium (BSH), DOX-CB@lipo-pDNA-iRGD significantly enhances the survival rate of tumor-bearing mice, reduces tumor stemness, and improves the prognosis. The excellent curative effect of this nanoliposome delivery system provides an insight into the combined treatment of BNCT.

Boron rich nanotube drug carrier system is suited for boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a two-step therapeutic process that utilizes Boron-10 in combination with low energy neutrons to effectively eliminate targeted cells. This therapy is primarily used for difficult to treat head and neck carcinomas; recent advances have expanded this method to cover a broader range of carcinomas. However, it still remains an unconventional therapy where one of the barriers for widespread adoption is the adequate delivery of Boron-10 to target cells. In an effort to address this issue, we examined a unique nanoparticle drug delivery system based on a highly stable and modular proteinaceous nanotube. Initially, we confirmed and structurally analyzed ortho-carborane binding into the cavities of the nanotube. The high ratio of Boron to proteinaceous mass and excellent thermal stability suggest the nanotube system as a suitable candidate for drug delivery into cancer cells. The full physicochemical characterization of the nanotube then allowed for further mechanistic molecular dynamic studies of the ortho-carborane uptake and calculations of corresponding energy profiles. Visualization of the binding event highlighted the protein dynamics and the importance of the interhelical channel formation to allow movement of the boron cluster into the nanotube. Additionally, cell assays showed that the nanotube can penetrate outer membranes of cancer cells followed by localization around the cells' nuclei. This work uses an integrative approach combining experimental data from structural, molecular dynamics simulations and biological experiments to thoroughly present an alternative drug delivery device for BNCT which offers additional benefits over current delivery methods.

Design, Synthesis, and Biological Evaluation of Boron-Containing Macrocyclic Polyamines and Their Zinc(II) Complexes for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary therapeutic method for cancer treatment based on the use of a combination of a cancer-specific drug containing boron-10 (¹⁰B) and thermal neutron irradiation. For successful BNCT, ¹⁰B-containing molecules need to accumulate specifically in cancer cells, because destructive effect of the generated heavy particles is limited basically to boron-containing cells. Herein, we report on the design and synthesis of boron compounds that are functionalized with 9-, 12-, and 15-membered macrocyclic polyamines and their Zn²⁺ complexes. Their cytotoxicity, intracellular uptake activity into cancer cells and normal cells, and BNCT effect are also reported. The experimental data suggest that mono- and/or diprotonated forms of metal-free [12]aneN₄- and [15]aneN₅-type ligands are uptaken into cancer cells, and their complexes with intracellular metals such as Zn²⁺ would induce cell death upon thermal neutron irradiation, possibly via interactions with DNA.

Modular Synthetic Approach to Carboranyl‒Biomolecules Conjugates

The development of novel, tumor-selective and boron-rich compounds as potential agents for use in boron neutron capture therapy (BNCT) represents a very important field in cancer treatment by radiation therapy. Here, we report the design and synthesis of two promising compounds that combine meta-carborane, a water-soluble monosaccharide and a linking unit, namely glycine or ethylenediamine, for facile coupling with various tumor-selective biomolecules bearing a free amino or carboxylic acid group. In this work, coupling experiments with two selected biomolecules, a coumarin derivative and folic acid, were included. The task of every component in this approach was carefully chosen: the carborane moiety supplies ten boron atoms, which is a tenfold increase in boron content compared to the l-boronophenylalanine (l-BPA) presently used in BNCT; the sugar moiety compensates for the hydrophobic character of the carborane; the linking unit, depending on the chosen biomolecule, acts as the connection between the tumor-selective component and the boron-rich moiety; and the respective tumor-selective biomolecule provides the necessary selectivity. This approach makes it possible to develop a modular and feasible strategy for the synthesis of readily obtainable boron-rich agents with optimized properties for potential applications in BNCT.

Peptide-Drug Conjugates and Their Targets in Advanced Cancer Therapies

Cancer became recently the leading cause of death in industrialized countries. Even though standard treatments achieve significant effects in growth inhibition and tumor elimination, they cause severe side effects as most of the applied drugs exhibit only minor selectivity for the malignant tissue. Hence, specific addressing of tumor cells without affecting healthy tissue is currently a major desire in cancer therapy. Cell surface receptors, which bind peptides are frequently overexpressed on cancer cells and can therefore be considered as promising targets for selective tumor therapy. In this review, the benefits of peptides as tumor homing agents are presented and an overview of the most commonly addressed peptide receptors is given. A special focus was set on the bombesin receptor family and the neuropeptide Y receptor family. In the second part, the specific requirements of peptide-drug conjugates (PDC) and intelligent linker structures as an essential component of PDC are outlined. Furthermore, different drug cargos are presented including classical and recent toxic agents as well as radionuclides for diagnostic and therapeutic approaches. In the last part, boron neutron capture therapy as advanced targeted cancer therapy is introduced and past and recent developments are reviewed.

A Selective Carborane-Functionalized Gastrin-Releasing Peptide Receptor Agonist as Boron Delivery Agent for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) allows the selective elimination of malignant tumor cells without affecting healthy tissue. Although this binary radiotherapy approach has been known for decades, BNCT failed to reach the daily clinics to date. One of the reasons is the lack of selective boron delivery agents. Using boron loaded peptide conjugates, which address G protein-coupled receptors overexpressed on tumor cells allow the intracellular accumulation of boron. The gastrin-releasing peptide receptor (GRPR) is a well-known target in cancer diagnosis and can potentially be used for BNCT. Here, we present the successful introduction of multiple bis-deoxygalactosyl-carborane building blocks to the GRPR-selective ligand [d-Phe⁶, β-Ala¹¹, Ala¹³, Nle¹⁴]Bn(6-14) (sBB2L) generating peptide conjugates with up to 80 boron atoms per molecule. Receptor activation was retained, metabolic stability was increased, and uptake into PC3 cells was proven without showing any intrinsic cytotoxicity. Furthermore, undesired uptake into liver cells was suppressed by using l-deoxygalactosyl modified carborane building blocks. Due to its high boron loading and excellent GRPR selectivity, this conjugate can be considered as a promising boron delivery agent for BNCT.

Tuning a modular system - synthesis and characterisation of a boron-rich s-triazine-based carboxylic acid and amine bearing a galactopyranosyl moiety

Introduction of a bis(isopropylidene)-protected galactopyranosyl moiety in s-triazine-based boron-rich carboxylic acids and amines results in soluble and suitable coupling partners for tumour-selective biomolecules with applications as selective agents for boron neutron capture therapy (BNCT). Bearing either a carboxylic acid or primary amine as a functional group, these compounds are highly versatile and thus largely extend the possible coupling strategies with suitable biomolecules. Modification of the gastrin-releasing peptide receptor (GRPR) selective agonist [d-Phe⁶, β-Ala¹¹, Ala¹³, Nle¹⁴]Bn(6-14) with the carboxylic acid derivative yielded a bioconjugate with an optimal receptor activation and internalisation profile. This demonstrates the great potential of this approach for the development of novel boron delivery agents.

Selective Neuropeptide Y Conjugates with Maximized Carborane Loading as Promising Boron Delivery Agents for Boron Neutron Capture Therapy

G-protein-coupled receptors like the human Y₁ receptor (hY₁R) are promising targets in cancer therapy due to their high overexpression on cancer cells and their ability to internalize together with the bound ligand. This mechanism was exploited to shuttle boron atoms into cancer cells for the application of boron neutron capture therapy (BNCT), a noninvasive approach to eliminate cancer cells. A maximized number of carboranes was introduced to the hY₁R-preferring ligand [F⁷,P³⁴]-NPY by solid phase peptide synthesis. Branched conjugates loaded with up to 80 boron atoms per peptide molecule exhibited a maintained receptor activation profile, and the selective uptake into hY₁R-expressing cells was demonstrated by internalization studies. In order to ensure appropriate solubility in aqueous solution, we proved the need for eight hydroxyl groups per carborane. Thus, we suggest the utilization of bis-deoxygalactosyl-carborane building blocks in solid phase peptide synthesis to produce selective boron delivery agents for BNCT.

Remarkable Boron Delivery Of iRGD-Modified Polymeric Nanoparticles For Boron Neutron Capture Therapy

Purpose

Boron neutron capture therapy (BNCT) is an emerging binary radiotherapy, which is limited for application due to the challenge of targeted delivery into tumor nowadays. Here, we propose the use of iRGD-modified polymeric nanoparticles for active targeted delivery of boron and doxorubicin (DOX) in BNCT.

Methods

¹⁰B-enriched BSH was covalently grafted to PEG-PCCL to prepare ¹⁰B-polymer, then surface-modified with iRGD. And, DOX was physically incorporated into polymers afterwards. Characterization of prepared polymers and in vitro release profile of DOX from polymers were determined by several methods. Cellular uptake of DOX was observed by confocal microscope. Accumulation of boron in cells and tissues was analyzed by ICP-MS. Biodistribution of DOX was studied by ex vivo fluorescence imaging and quantitative measurement. Tumor vascular normalization of Endostar for promoting delivery efficiency of boron on refractory B16F10 tumor was also studied.

Results

The polymers were monodisperse and spheroidal in water with an average diameter of 24.97 nm, which were relatively stable at physiological pH and showed a sustained release of DOX, especially at endolysosomal pH. Enhanced cellular delivery of DOX was found in iRGD-modified polymer group. Cellular boron uptake of iRGD-modified polymers in A549 cells was remarkably raised fivefold (209.83 ng ¹⁰B/10⁶ cells) compared with BSH. The polymers represented prolonged blood circulation, enhanced tumor accumulation of ¹⁰B against BSH, and favorable tumor:normal tissue boron concentration ratios (tumor:blood = 14.11, tumor:muscle = 19.49) in A549 tumor-bearing mice 24 hrs after injection. Both fluorescence imaging and quantitative measurement showed the highest tumor accumulation of DOX at 24 hrs after injecting of iRGD-modified polymers. Improvement of vascular integrity and reduction of vascular mimicries were found after Endostar injection, and raised tumor accumulation of boron as well.

Conclusion

The developed nanoparticle is an inspiring candidate for the safe clinical application for BNCT.

Enlargement of a Modular System-Synthesis and Characterization of an s-Triazine-Based Carboxylic Acid Ester Bearing a Galactopyranosyl Moiety and an Enormous Boron Load

The amount of boron accumulated in tumor tissue plays an important role regarding the success of the boron neutron capture therapy (BNCT). In this article, we report a modular system, combining readily available starting materials, like glycine, 1,3,5-triazine and the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12), as well as α-d-galactopyranose for increased hydrophilicity, with a novel boron-rich tris-meta-carboranyl thiol.

Modular triazine-based carborane-containing carboxylic acids - synthesis and characterisation of potential boron neutron capture therapy agents made of readily accessible building blocks

Based on a modular combination of s-triazine, the well-known 9-mercapto-1,7-dicarba-closo-dodecaborane(12) and commercially available carboxylic acids, namely thioglycolic acid, glycine, and N_α-Boc-l-lysine, several carboxylic acid derivatives were synthesised and fully characterised. The thioglycolic acid derivative was introduced into a peptide hormone by solid phase peptide synthesis. High activity and selective internalisation into peptide receptor-expressing cells was observed. With a very high boron content of twenty boron atoms, these derivatives are interesting as selective Boron Neutron Capture Therapy (BNCT) agents.

Capability of NIPAM polymer gel in recording dose from the interaction of (10)B and thermal neutron in BNCT

The capability of N-isopropylacrylamide (NIPAM) polymer gel to record the dose resulting from boron neutron capture reaction in BNCT was determined. In this regard, three compositions of the gel with different concentrations of (10)B were prepared and exposed to gamma radiation and thermal neutrons. Unlike irradiation with gamma rays, the boron-loaded gels irradiated by neutron exhibited sensitivity enhancement compared with the gels without (10)B. It was also found that the neutron sensitivity of the gel increased by the increase of concentration of (10)B. It can be concluded that NIPAM gel might be suitable for the measurement of the absorbed dose enhancement due to (10)B and thermal neutron reaction in BNCT.

Role of gel dosimeters in boron neutron capture therapy

Gel dosimeters have acquired a unique status in radiotherapy, especially with the advent of the new techniques in which there is a need for three-dimensional dose measurement with high spatial resolution. One of the techniques in which the use of gel dosimeters has drawn the attention of the researchers is the boron neutron capture therapy. Exploring the history of gel dosimeters, this paper sets out to study their role in the boron neutron capture therapy dosimetric process.

Radionuclide carriers for targeting of cancer

This review describes strategies for the delivery of therapeutic radionuclides to tumor sites. Therapeutic approaches are summarized in terms of tumor location in the body, and tumor morphology. These determine the radionuclides of choice for suggested targeting ligands, and the type of delivery carriers. This review is not exhaustive in examples of radionuclide carriers for targeted cancer therapy. Our purpose is two-fold: to give an integrated picture of the general strategies and molecular constructs currently explored for the delivery of therapeutic radionuclides, and to identify challenges that need to be addressed. Internal radiotherapies for targeting of cancer are at a very exciting and creative stage. It is expected that the current emphasis on multidisciplinary approaches for exploring such therapeutic directions should enable internal radiotherapy to reach its full potential.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Numerical assessment of radiation binary targeted therapy for HER-2 positive breast cancers: advanced calculations and radiation dosimetry

In our previous publication (Mundy et al 2006 Phys. Med. Biol. 51 1377) we have described the theoretical assessment of our novel approach in radiation binary targeted therapy for HER-2 positive breast cancers and summarized the future directions in this area of research. In this paper we advanced the numerical analysis to show the detailed radiation dose distribution for various neutron sources in combination with the required boron concentration and allowed radiation skin doses. We once again proved the feasibility of the concept and will use these data and conclusions to start with the experimental verifications.

Enhanced cellular uptake with a cobaltacarborane-porphyrin-HIV-1 Tat 48-60 conjugate

A series of four porphyrin-cobaltacarborane conjugates have been synthesized, containing three or four cobaltabisdicarbollide anions linked by O(CH(2)CH(2)O)(2) groups to the porphyrin macrocycle and one of them containing a HIV-1 Tat 48-60 peptide sequence linked via a low molecular weight poly(ethylene glycol) (PEG) spacer. The cellular uptake, cytotoxicity, and preferential sites of intracellular localization of the conjugates were evaluated in human HEp2 cells. All conjugates are nontoxic in the dark at the concentrations studied. Upon exposure to low light dose (1 J cm(-)(2)) only the porphyrin-cobaltacarborane-HIV-1 Tat 48-60 conjugate showed 30% inhibition of cell proliferation at a concentration of 10 microM. The cellular uptake was dependent on the number of carborane cages and was significantly enhanced by the presence of the cell penetrating peptide sequence HIV-1 Tat 48-60. All conjugates preferentially localized in the cell lysosomes.

Hydrophilically enhanced 3-carboranyl thymidine analogues (3CTAs) for boron neutron capture therapy (BNCT) of cancer

Five novel 3-carboranyl thymidine analogues (3CTAs) were designed and synthesized for boron neutron capture therapy (BNCT) of cancer. Phosphorylation of all five 3CTAs was catalyzed by recombinant human thymidine kinase (hTK1) using adenosine triphosphate (ATP) as the phosphate donor. The obtained phosphorylation rates ranged from 4% to 64.5% relative to that of thymidine. The compound with the most favorable hTK1 binding properties had a k(cat)/K(M) value of 57.4% relative to that of thymidine and an IC(50) of inhibition of thymidine phosphorylation by hTK1 of 92 microM. Among the five synthesized 3CTAs, this agent had also the overall most favorable physicochemical properties. Therefore, it may have the potential to replace N5-2OH, the current lead 3CTA, in preclinical studies. An in silico model for the binding of this compound to hTK1 was developed.

Peptide-mediated cell transport of water soluble porphyrin conjugates

Five new porphyrin-peptide conjugates bearing a nuclear localizing sequence SV40 or a fusogenic peptide (HIV-1Tat 40-60 or octa-arginine) linked by low molecular weight poly(ethylene glycol) have been synthesized. In vitro studies using human HEp2 cells show that the cellular uptake of the conjugates depends significantly on the nature and sequence of amino acids in the peptide and on the nature of the substituents on the porphyrin macrocycle. The fusogenic peptide sequences HIV-1Tat 40-60 and octa-arginine were the most effective in delivering the conjugates to the cells. The subcellular distribution of the conjugates was found to be dependent on the nature of substituents on the porphyrin macrocycle. The conjugates bearing a hydrophobic porphyrin localized preferentially in the endoplasmic reticulum and were significantly more phototoxic to HEp2 cells than the carboxylic acid functionalized porphyrin conjugates, which localized mainly in the lysosomes.

Radiation binary targeted therapy for HER-2 positive breast cancers: assumptions, theoretical assessment and future directions

A novel radiation targeted therapy is investigated for HER-2 positive breast cancers. The proposed concept combines two known approaches, but never used together for the treatment of advanced, relapsed or metastasized HER-2 positive breast cancers. The proposed radiation binary targeted concept is based on the anti HER-2 monoclonal antibodies (MABs) that would be used as vehicles to transport the nontoxic agent to cancer cells. The anti HER-2 MABs have been successful in targeting HER-2 positive breast cancers with high affinity. The proposed concept would utilize a neutral nontoxic boron-10 predicting that anti HER-2 MABs would assure its selective delivery to cancer cells. MABs against HER-2 have been a widely researched strategy in the clinical setting. The most promising antibody is Trastuzumab (Herceptin). Targeting HER-2 with the MAB Trastuzumab has been proven to be a successful strategy in inducing tumour regression and improving patient survival. Unfortunately, these tumours become resistant and afflicted women succumb to breast cancer. In the proposed concept, when the tumour region is loaded with boron-10 it is irradiated with neutrons (treatment used for head and neck cancers, melanoma and glioblastoma for over 40 years in Japan and Europe). The irradiation process takes less than an hour producing minimal side effects. This paper summarizes our recent theoretical assessments of radiation binary targeted therapy for HER-2 positive breast cancers on: the effective drug delivery mechanism, the numerical model to evaluate the targeted radiation delivery and the survey study to find the neutron facility in the world that might be capable of producing the radiation effect as needed. A novel method of drug delivery utilizing Trastuzumab is described, followed by the description of a computational Monte Carlo based breast model used to determine radiation dose distributions. The total flux and neutron energy spectra of five currently available neutron irradiation treatment facilities are examined for this application. The tumour boron concentrations and tumour to healthy tissue concentration ratios required to deliver 50 Gy-Eq to the tumour without exceeding 18 Gy-Eq in the skin are determined, as well as the associated therapeutic ratios. Discussion is provided to address the future research direction for assessing the feasibility of the proposed concept.

EGF-receptor targeted liposomes with boronated acridine: growth inhibition of cultured glioma cells after neutron irradiation

PURPOSE

To study survival of cultured U-343MGaCl 2:6 glioma cells after incubation with boron-containing liposomes targeting the epidermal growth factor receptor following neutron irradiation.

MATERIALS AND METHODS

Epidermal growth factor-tagged liposomes were loaded with water-soluble boronated acridine developed for boron neutron capture therapy, (BNCT). Cellular uptake and distribution were studied. Further, cells were placed at 3 cm depth in a phantom and exposed to an epithermal neutron beam to study clonogenic cell survival.

RESULTS

The cellular uptake of boron reached 90 ppm and it was determined by subcellular fractionation that most of the cell-associated boron was located outside of the nucleus. For clonogenic survival, the cells were incubated with epidermal growth factor receptor-targeted liposomes for 4 hours resulting in a cellular concentration of 55 ppm boron (11 ppm 10B). At a fluence of 3 x 10(12) neutrons/cm2 the cell killing effect of the boron-containing epidermal growth factor-liposomes was about ten times higher than for neutrons only. Furthermore, theoretical calculation of the survival by enriched compound (55 ppm 10B), using the parameters from non-enriched compound (11 ppm 10B), shows that the killing effect in this case would be approximately five orders of magnitude higher than for neutrons only.

CONCLUSION

The results in this study show that epidermal growth factor-receptor targeted liposomes are suitable as tumor-cell delivery agents of boron for BNCT and support further studies to demonstrate their effectiveness in vivo.

Vascular endothelial growth factor selectively targets boronated dendrimers to tumor vasculature

Tumor neovasculature is a potential but, until very recently, unexplored target for boron neutron capture therapy (BNCT) of cancer. In the present report, we describe the construction of a vascular endothelial growth factor (VEGF)-containing bioconjugate that potentially could be used to target up-regulated VEGF receptors (VEGFR), which are overexpressed on tumor neovasculature. A fifth-generation polyamidoamine dendrimer containing 128 reactive amino groups was reacted with 105 to 110 decaborate molecules to produce a macromolecule with 1,050 to 1,100 boron atoms per dendrimer. This was conjugated to thiol groups of VEGF at a 4:1 molar ratio using the heterobifunctional reagent sulfo-LC-SPDP. In addition, the boronated dendrimer was tagged with a near-IR Cy5 dye to allow for near-IR fluorescent imaging of the bioconjugate in vitro and in vivo. As would be predicted, the resulting VEGF-BD/Cy5 bioconjugate was not cytotoxic to HEK293 cells engineered to express 2.5 x 10(6) VEGFR-2 per cell. Furthermore, it showed binding and activation of VEGFR-2 comparable with that of native VEGF. Internalization of VEGF-BD/Cy5 by PAE cells expressing 2.5 x 10(5) VEGFR-2 per cell was inhibited by excess VEGF, indicating a VEGFR-2-mediated mechanism of uptake. Near-IR fluorescent imaging of 4T1 mouse breast carcinoma revealed selective accumulation of VEGF-BD/Cy5, but not BD/Cy5, particularly at the tumor periphery where angiogenesis was most active. Accumulation of VEGF-BD/Cy5 in 4T1 breast carcinoma was diminished in mice pretreated with a toxin-VEGF fusion protein that selectively killed VEGFR-2-overexpressing endothelial cells. Our data lay the groundwork for future studies using the VEGF-BD/Cy5 bioconjugate as a targeting agent for BNCT of tumor neovasculature.

ESR as a valuable tool for the investigation of the dynamics of EPC and EPC/cholesterol liposomes containing a carboranyl-nucleoside intended for BNCT

Electron Spin Resonance (ESR) spectroscopy of long-chain nitroxides (5-, 7-, and 16-doxyl stearic acid) has been used to evaluate the perturbations induced by beta-5-o-carboranyl-2'-deoxyuridine (CDU) on the structure and dynamics of egg phosphatidylcholine (EPC) and EPC/cholesterol liposomes. Loaded liposomes are intended for the use in Boron Neutron Capture Therapy (BNCT). From a detailed analysis of the motional and order parameters determining the ESR line shape as a function of temperature and of CDU content in liposomes, an increased order and a hindered motion of the phospholipid membranes resulted in the presence of increasing CDU concentration. This occurred particularly at the liposome surface level. Both higher ordering and increased rigidity of the membrane lipids were the result of the constraints exerted by the embedded carboranyl-nucleoside.

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.

Multinuclear platinum(ii)–amine complexes containing bis(aminopropyl)dicarba-closo-dodecaborane(12) ligands

Treatment of the bridging bidentate 1,Z-bis(aminopropyl)-1,Z-dicarba-closo-dodecaborane(12)(1,Z-bis(aminopropyl)-1,Z-carborane) ligands of the type 1,Z-[H(2)N(CH(2))(3)](2)-1,Z-C(2)B(10)H(10)(L(1), Z= 7, 5) or (L(2), Z= 12, 6) with two equivalents of trans-[PtClI(2)(NH(3))](-), followed by halogen ligand metathesis with AgOTf and HCl((aq)) afforded the novel diplatinum(II)-amine species cis-[[PtCl(2)(NH(3))](2)L(n)](7(n= 1) or 8(n= 2), respectively). Similarly, the reaction of L(1) or L(2) with the labile trans-[PtCl(dmf)(NH(3))(2)](+) afforded trans-[[PtCl(NH(3))(2)](2)L(n)](OTf)(2)(9(n= 1) or 10(n= 2), respectively) in good yield and purity. However, isolation of the analogous 1,2-carborane complexes was not possible owing to decomposition reactions that led to extensive degradation of the carborane cage and reduction of the metal centre. The mixed dinuclear complex [cis-[PtCl(2)(NH(3))]-L(1)-trans-[PtCl(NH(3))(2)]]OTf (19) was prepared by treatment of the Boc-protected amine ligand 1-[(Boc)(2)N(CH(2))(3)]-7-[H(2)N(CH(2))(3)]-1,7-C(2)B(10)H(10)(L(3), 15) with trans-[PtCl(dmf)(NH(3))(2)](+) to yield trans-[PtCl(NH(3))(2)L(3)]OTf (16), followed by acid deprotection of the pendant amine group, complexation with trans-[PtClI(2)(NH(3))](-), and halogen ligand metathesis using AgOTf and HCl((aq)). A novel trinuclear species containing 5 was prepared by the addition of two equivalents of 15 to the labile precursor cis-[Pt(dmf)(2)(NH(3))(2)](2+) followed by acid deprotection of the pendant amine groups. Further complexation with two equivalents of trans-[PtClI(2)(NH(3))](-) followed by halogen ligand metathesis using AgOTf and HCl((aq)) afforded the triplatinum(II)-amine species [cis-[Pt(NH(3))(2)(L(1))(2)]-cis-[PtCl(2)(NH(3))](2)](OTf)(2)(23). Complexes 7-10, 19 and 23 represent the first examples of multinuclear platinum(ii)-amine derivatives containing carborane cages. Preliminary in vitro cytotoxicity studies for selected complexes are also reported.

Boronic acid compounds as potential pharmaceutical agents

Boronic acid compounds have been used, because of their unique structural features, for the development of potent enzyme inhibitors, boron neutron capture agents for cancer therapy, and as antibody mimics that recognize biologically important saccharides. Consequently, there has been a surge of interests in boronic acid compounds. This study reviews the recent development in this area during the last six years.

Biodistribution of a carborane-containing porphyrin as a targeting agent for Boron Neutron Capture Therapy of oral cancer in the hamster cheek pouch

Boron Neutron Capture Therapy (BNCT) is a bimodal cancer treatment based on the selective accumulation of 10B in tumors and concurrent irradiation with thermalized neutrons. The short-range, high-LET radiation produced by the capture of neutrons by 10B could potentially control tumor while sparing normal tissue if the boron compound targets tumor selectively within the treatment volume. In previous studies, we proposed and validated the hamster cheek pouch model of oral cancer for BNCT studies, proved that absolute and relative uptake of the clinically employed boron compound boronophenylalanine (BPA) would be potentially therapeutic in this model and provided evidence of the efficacy of in vivo BPA-mediated BNCT to control hamster oral mucosa tumors with virtually no damage to normal tissue. We herein present the biodistribution and pharmacokinetics of a lipophilic, carborane-containing tetraphenylporphyrin (CuTCPH) in the hamster oral cancer model. CuTCPH is a novel, non-toxic compound that may be advantageous in terms of selective and absolute delivery of boron to tumor tissues. For potentially effective BNCT, tumor boron concentrations from a new agent should be greater than 30 ppm and tumor/blood and tumor/normal tissue boron concentration ratios should be greater than 5/1 without causing significant toxicity. We administered CuTCPH intraperitoneally (i.p.) as a single dose of 32 microg/g body weight (b.w.) (10 microg B/g b.w.) or as four doses of 32 microg/g b.w. over 2 days. Blood (Bl) and tissues were sampled at 3, 6, 12, 24, 48, and 72 h in the single-dose protocol and at 1-4 days after the last injection in the multidose protocol. The tissues sampled were tumor (T), precancerous tissue surrounding tumor, normal pouch (N), skin, tongue, cheek and palate mucosa, liver, spleen, parotid gland and brain. The maximum mean B ratios for the single-dose protocol were T/N: 9.2/1 (12h) and T/Bl: 18.1/1 (72 h). The B value peaked to 20.7+/-18.5 ppm in tumor at 24h. The multidose protocol maximum mean ratios were T/N: 11.9/1 (3 days) and T/Bl: 235/1 (4 days). Absolute boron concentration in tumor reached a maximum value of 116 ppm and a mean value of 71.5+/-48.3 ppm at 3 days. The fact that absolute and relative B values markedly exceeded the BNCT therapeutic threshold with no apparent toxicity may confer on this compound a therapeutic advantage. CuTCPH-mediated BNCT would be potentially useful for the treatment of oral cancer in an experimental model.

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.

Synthesis, dark toxicity and induction of in vitro DNA photodamage by a tetra(4-nido-carboranylphenyl)porphyrin

The total synthesis of tetra(4-carboranylphenyl)porphyrins 4 and 6 and their zinc(II) complexes 5 and 7 are described. These compounds were characterized by analytical and spectroscopic methods and, in the case of 5, by X-ray crystallography. The water-soluble nido-carboranylporphyrins 6 and 7 were found to have low dark toxicity towards V79 hamster lung fibroblast cells, using a clonogenic assay (50% colony survival, CS(50)>300 microM). Upon light activation nido-carboranylporphyrin 6 effectively induced DNA damage in vitro. Two different methods were used to assess the extent of DNA damage: the super-coiled to nicked DNA and the alkaline Comet assay using human leukemia K562 cells. Significant PDT-induced DNA damage was observed for porphyrin 6 using both assays, compared to light-only and porphyrin-only experiments. It is concluded that this type of nido-carboranylporphyrin is a promising sensitizer for both the boron neutron capture therapy and the photodynamic therapy of tumors.

Synthesis of a small library of 3-(carboranylalkyl)thymidines and their biological evaluation as substrates for human thymidine kinases 1 and 2

A small library consisting of two series of thymidine derivatives containing o-carboranylalkyl groups at the N-3 position was prepared. In both series, alkyl spacers of 2-7 methylene units were placed between the o-carborane cage and the thymidine scaffold. In one series, an additional dihydroxypropyl substituent was introduced at the second carbon atom of the carborane cage. In the series of N-3-substituted carboranyl thymidines without additional dihydroxypropyl substituent, three steps were required to obtain the target compounds in overall yields as high as 75%, while in the series of N-3-substituted carboranyl thymidines with additional dihydroxypropyl substituent, 9-10 steps were necessary with significantly lower overall yield. All target compounds were good substrates of human cytosolic thymidine kinase 1 while they were, if at all, poor substrates of the mitochondrial thymidine kinase 2. There was only a minor difference in phosphorylation rates between N-3-substituted carboranyl thymidines with additional dihydroxypropyl substituents with thymidine kinase 1 (range: 13-49% relative to thymidine) and their counterparts lacking this group (range: 11-57% relative to thymidine). Tether lengths of two and five methylene groups in both series gave the highest enzyme activities in the present study. A hypothesis for this result is presented.

Boron-containing folate receptor-targeted liposomes as potential delivery agents for neutron capture therapy

Boron neutron capture therapy (BNCT) depends on the selective delivery of a sufficient number of (10)B atoms to tumor cells to sustain a lethal (10)B(n,alpha)(7)Li reaction. Expression of FR frequently is amplified among human tumors. The goal of the present study was to investigate folate receptor (FR)-targeted liposomes as potential carriers for a series of boron-containing agents. Two highly ionized boron compounds, Na(2)[B(12)H(11)SH] and Na(3) (B(20)H(17)NH(3)), were incorporated into liposomes by passive loading with encapsulation efficiencies of 6% and 15%, respectively. In addition, five weakly basic boronated polyamines were investigated. Two were the spermidine derivatives: N(5)-(4-carboranylbutyl)spermidine.3HCl (SPD-5), N(5)-[4-(2-aminoethyl-o-carboranyl)butyl]spermidine.4HCl (ASPD-5). Three were the spermine derivatives: N(5)-(4-carboranylbutyl)spermine.4HCl (SPM-5), N(5)-[4-(2-aminoethyl-o-carboranyl)butyl]spermine.5HCl (ASPM-5), and N(5),N(10)-bis(4-carboranylbutyl)spermine.4 HCl (SPM-5,10). These were incorporated into liposomes by a pH-gradient-driven remote-loading method with varying loading efficiencies, which were influenced by the specific trapping agent and the structure of the boron compound. Greater loading efficiencies were obtained with lower molecular weight boron derivatives, using ammonium sulfate as the trapping agent, compared to those obtained with sodium citrate. The in vitro uptake of folate-derivatized, boronated liposomes was investigated using human KB squamous epithelial cancer cells, which have amplified FR expression. Higher cellular boron uptake (up to 1584 microg per 10(9) cells) was observed with FR-targeted liposomes than with nontargeted control liposomes (up to 154 microg per 10(9) cells), irrespective of the chemical form of the boron and the method used for liposomal preparation. KB cell binding of the FR-targeted liposomes was saturable and could be blocked by 1 mM free folic acid. Our findings suggest that further evaluation of FR-targeted liposomes is warranted to assess their potential as boron carriers for neutron capture therapy.

Novel carboranes with a DNA binding unit for the treatment of cancer by boron neutron capture therapy

The synthesis and biological evaluation of two ortho-carborane derivatives which contain a 5,6,7-trimethoxyindole (TMI) unit for use in boron neutron capture therapy is described. The TMI moiety is known to be the DNA-binding part of the highly potent anticancer agent duocarmycin A. The ortho-carborane derivatives were prepared from amino alkynes which were bound to a protected TMI carboxylic acid. Addition of decaborane(14) to the alkyne triple bond with subsequent removal of the tert-butoxycarbonyl (Boc) and benzyl protecting groups gave the desired product. Boron uptake from the ortho-carborane derivatives into B-16 melanoma cells was higher and faster than that observed with L-p-boronophenylalanine (BPA), which is in use in the clinic.

Syntheses and preliminary biological studies of four meso-Tetra[(nido-carboranylmethyl)phenyl]porphyrins

Two meso-tetra[(nido-carboranylmethyl)phenyl]porphyrins (para- and meta-regioisomers) and their corresponding Zn(II) complexes have been synthesized with the aim of studying the effect of carborane distribution and metalation on the biological properties of this series of compounds. In vitro cell toxicity, uptake/efflux, and subcellular localization using rat 9L, mouse B16 and/or human U-373MG cells were evaluated. All four amphiphilic porphyrins display very low cytotoxicities and time- and concentration-dependent uptake by cells, which is influenced by serum proteins. Preliminary subcellular localization studies suggest that one of these compounds localizes in close proximity to the cell nucleus. All four nido-carboranylporphyrins show promise as boron-carriers for the boron neutron capture therapy of cancers, particularly the metal-free nido-carboranylporphyrins 5 and 12, which are able to deliver higher amount of boron to cells in vitro than the corresponding zinc complexes.

Synthesis of boroxifen, a nido-carborane analogue of tamoxifen

A nido-carborane analogue of tamoxifen, the widely employed breast cancer therapy agent, was prepared as an archetype of a potential new class of antiestrogen and boron neutron capture therapy agent in which the carborane is incorporated within the framework of the parent compound. The carborane was introduced through the reaction of 6,9-bis(acetonitrile)decaborane with a unique and highly conjugated ene-yne, which was prepared stereoselectively. NMR spectroscopy and a crystal structure of a key intermediate, the carborane analogue of chloro-tamoxifen, demonstrated the structural similarities between the tamoxifen carboranes and their corresponding phenyl analogues.

Radiation doses to the cell nucleus in single cells and cells in micrometastases in targeted therapy with (131)I labeled ligands or antibodies

PURPOSE

The aim of this study was to theoretically investigate how the radiation dose to cell nuclei depends on the subcellular position of (131)I. The influence of the size of the cells and crossfire irradiation in clusters of cells was also studied.

METHODS AND MATERIAL

Using data describing the dose rate around a point source of (131)I, we calculated the dose distributions inside and around cell models of different sizes. The assumed positions of (131)I were on the cellular or nuclear membrane, in the cytoplasm, in the nucleus, or spread in the whole cell. The mean doses to the nucleus of the targeted cell and to the nuclei of its neighbors were calculated using the dose distributions.

RESULTS

The dose distributions inside a single targeted cell showed very different distribution profiles depending on the subcellular position of the (131)I. Targeting the nucleus instead of the cellular membrane could increase the dose to the nucleus 10-fold. Crossfire irradiation can be the major contributor to the nuclear dose in clusters of more than six cells.

CONCLUSIONS

Dosimetry without microscopic considerations is inadequate for targeted radionuclide therapy of disseminated or clustering tumor cells exposed to (131)I. Therapeutic doses could be achieved, even in single cells, when (131)I was positioned near, or inside the cell nucleus, or when the clusters were large enough.

Computational validation of the stereology principle applied to the microdosimetry of boron neutron capture therapy

High resolution quantitative autoradiography (HRQAR) is a novel technique that has been developed in our laboratory and applied to the microdosimetry of boron neutron capture therapy (BNCT). High resolution quantitative autoradiography is employed to define the microdistribution of boron-10 atoms within a 1-2 microm frozen tissue section. This microdistribution is used as input to a novel two-dimensional Monte Carlo charged particle transport calculation that computes various microdosimetric parameters, such as the number of nuclear "hits," energy absorbed in the nuclei, etc., within the environment of actual tissue morphology (i.e., cell nuclei, cytoplasm, and intracellular space). Stereological transformation is then implemented to transform the two-dimensional calculations into effectively three-dimensional results. In the present study no seek to demonstrate the validity of the surrogate two-dimensional 2-D computation as being quantitatively equivalent to a hypothetical full 3-D calculation. The results show that within the limitations of the test parameters used the surrogate 2-D and 3-D results are completely equivalent within the statistical constraints of the Monte Carlo calculations. Limitations of this approach also are evaluated, including a Monte Carlo calculation of the influence of the thickness of the histological tissue section and the track detector and the influence of 4He and 7Li particle lateral and range straggling.