Boron neutron capture therapy: linkage of a boronated macromolecule to monoclonal antibodies directed against tumor-associated antigens

Derivatization of Undecahalogenated closo-Dodecaborates [B12X11NH3]- (X = F-I): Attaching Isocyanate, Amidinium, and Formamide Functionalities

Halogenated closo-dodecaborates are very robust and versatile weakly coordinating anions for numerous applications. The introduction of additional substituents, e.g., pseudohalides, allows the tuning of their chemical and physical properties. In this report, the synthesis of the isocyanate-substituted closo-dodecaborates [B12X11(NCO)]2- (X = H, F-I) was investigated. In an attempt to synthesize the undecahalogenated derivatives, a selective and halogen-dependent reaction yielding boron clusters carrying the functional groups amidinium (-NHCHNMe2) and formamide (-NHC(O)H) was discovered. The halogenated anions were fully characterized by vibrational and NMR spectroscopy, mass spectrometry, and X-ray diffraction. Salts of the formamide-substituted anion [B12X11(NHC(O)H)]2- are surprisingly thermally stable in the condensed phase. In contrast, collision-induced dissociation in the gas phase reveals that the isolated dianion [B12X11(NHC(O)H)]2- in the gas phase preferentially loses water, while the protonated form, which was generated from decomposition of the tetraalkylammonium counterion [B12X11(NHC(O)H)H]-, tends to lose carbon monoxide. Possible reaction mechanisms are discussed.

Facile Attachment of Halides and Pseudohalides to Dodecaborate(2-) via Pd-catalyzed Cross-Coupling

Cross-coupling reactions with [B12H11I]2− as one partner have been used successfully for Kumada and Buchwald Hartwig couplings with Pd catalysis. Here, we found that the iodide could be substituted easily, and unexpectedly, with other halides such as Br and Cl, and with pseudohalides such as cyanide, azide, and isocyanate. We found that for Cl, Br, N3, and NCO, tetrabutylammonium salts—or sodium salts—were successful halide sources, whereas for cyanide, CuCN was the only halide source that allowed a successful exchange. The azide could be reacted further in a click reaction with triazoles. While no substitution with fluoride occurred, tetrabutylammonium fluoride in the presence of water led to [B12H11OH]2−. Yields were high to very high, and reaction times were short when using a microwave oven as a heating source.

Palladium-Mediated Incorporation of Carboranes into Small Molecules, Peptides, and Proteins

Carboranes represent a class of compounds with increasing therapeutic potential. However, few general approaches to readily embed carboranes into small molecules, peptides, and proteins are available. We report a strategy based on palladium-mediated C-X (X = C, S, and N) bond formation for the installation of carborane-containing moieties onto small molecules and peptides. We demonstrate the ability of Pd-based reagents with appropriate ligands to overcome the high hydrophobicity of the carborane group and enable chemoselective conjugation of cysteine residues at room temperature in aqueous buffer. Accordingly, carboranes can be efficiently installed on proteins by employing a combination of a bis-sulfonated biarylphosphine-ligated Pd reagent in an aqueous histidine buffer. This method is successfully employed on nanobodies, a fully synthetic affibody, and the antibody therapeutics trastuzumab and cetuximab. The conjugates of the affibody Z_HER2 and the trastuzumab antibody retained binding to their target antigens. Conjugated proteins maintain their activity in cell-based functional assays in HER2-positive BT-474 cell lines. This approach enables the rapid incorporation of carborane moieties into small molecules, peptides, and proteins for further exploration in boron neutron capture therapy, which requires the targeted delivery of boron-dense groups.

Synthesis of Derivatives of closo-Dodecaborate Anion Based on Amino Acid Esters

Abstract

This work proposes a new method for the synthesis of N-borylated amino acids based on nucleophilic substitution reactions in the [B12H11IPh]– anion. Esters of glycine and L-phenylalanine were used as nucleophiles. The structure of the products has been determined by multinuclear NMR spectroscopy, IR spectroscopy, and ESI mass spectrometry.

Polymers and boron neutron capture therapy (BNCT): a potent combination

This review describes the latest polymeric systems used as boron transporters for boron neutron capture therapy.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes

Over the past several years, a number of strategies for the functionalization of dicarba-closo-dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

Photoimmunotherapy of Ovarian Cancer: A Unique Niche in the Management of Advanced Disease

Ovarian cancer (OvCa) is the leading cause of gynecological cancer-related deaths in the United States, with five-year survival rates of 15-20% for stage III cancers and 5% for stage IV cancers. The standard of care for advanced OvCa involves surgical debulking of disseminated disease in the peritoneum followed by chemotherapy. Despite advances in treatment efficacy, the prognosis for advanced stage OvCa patients remains poor and the emergence of chemoresistant disease localized to the peritoneum is the primary cause of death. Therefore, a complementary modality that is agnostic to typical chemo- and radio-resistance mechanisms is urgently needed. Photodynamic therapy (PDT), a photochemistry-based process, is an ideal complement to standard treatments for residual disease. The confinement of the disease in the peritoneal cavity makes it amenable for regionally localized treatment with PDT. PDT involves photochemical generation of cytotoxic reactive molecular species (RMS) by non-toxic photosensitizers (PSs) following exposure to non-harmful visible light, leading to localized cell death. However, due to the complex topology of sensitive organs in the peritoneum, diffuse intra-abdominal PDT induces dose-limiting toxicities due to non-selective accumulation of PSs in both healthy and diseased tissue. In an effort to achieve selective damage to tumorous nodules, targeted PS formulations have shown promise to make PDT a feasible treatment modality in this setting. This targeted strategy involves chemical conjugation of PSs to antibodies, referred to as photoimmunoconjugates (PICs), to target OvCa specific molecular markers leading to enhanced therapeutic outcomes while reducing off-target toxicity. In light of promising results of pilot clinical studies and recent preclinical advances, this review provides the rationale and methodologies for PIC-based PDT, or photo-immunotherapy (PIT), in the context of OvCa management.

Synthesis and Structural Characterization of Amidine, Amide, Urea and Isocyanate Derivatives of the Amino-closo-dodecaborate Anion [B12H11NH₃]. Molecules 2018;23:molecules23123137. [PMID: 30501105 PMCID: PMC6321512 DOI: 10.3390/molecules23123137]

The synthesis and structural characterization of new derivatives of [B₁₂H₁₂]²⁻ is of fundamental interest and is expected to allow for extended applications. Herein we report on the synthesis of a series of amidine, amide, urea and isocyanate derivatives based on the amino-closo-dodecaborate anion [B₁₂H₁₁NH₃]⁻. Their structures have been confirmed by spectroscopic methods, and nine crystal structures are presented.

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

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

Synthesis and Characterization of p-Borono-Phenylalanine-Branched Polypeptide-Monoclonal Antibody Ternary Systems for Potential Use in Boron Neutron Capture Therapy (BNCT)

The application of the 10B ( n,α) 7Li capture reaction to cancer radiotherapy (Boron Neutron Capture Therapy) was studied to avoid the inherent disadvantages of conventional radiation therapy. p-Borono-phenylalanine (Bph) was used as the 10B source and mAb produced against HCMB melanoma cells was applied as targeting device. Since extensive direct boronation of mAb led diminished recognition of antigens, an intermediate carrier was used. Nontoxic, biocompatible, biodegradable and weakly immunogenic branched polypeptides with a polylysine backbone was used to carry a high number of 10B. Protected 10B-Bph was coupled by four different methods to polycationic branched polypeptides. The coupling efficiency varied according to the experimental conditions, with a maximum of 90%. The chiroptical properties of the conjugates indicated an ordered conformation which increased with the number of coupled Bph. The whole body survival (WBS) and tissue distribution profile of mAb (8/6 IgG2a) were markedly altered after conjugation with Bph-branched polypeptide. Decreased WBS and intermediate-carrier-dependent accumulation in the spleen, liver and kidney was observed 24 h after iv. administration. After joining only a few chains of the highly loaded Bph-AK conjugate to mAb, the binding activity of the mAb in the ternary system was preserved compared to control.

Methods to produce B-C, B-P, B-N and B-S bonds in boron clusters

Boranes, heteroboranes and metallacarboranes, all named as boron clusters, offer an alternative to typical organic molecules or organic molecular materials. Carbon and boron share the important property of self-catenation thus these elements can produce individually large and sophisticated molecules. Boron clusters and organic molecules display electronic, physical, chemical and geometrical characteristics manifestly different. These differences highlight the complementarity of organic molecules and boron clusters, and therefore the feasibility or necessity to produce hybrid molecules incorporating both types of fragments. To join these two types of fragments, or alternatively these two types of molecular compounds, tools are needed. In this review the current methods of producing boron clusters with carbon, B-C, nitrogen, B-N, phosphorus, B-P and sulphur bonds, B-S, are indicated. As there are many existing borane clusters of different sizes, heteroboranes and metallacarboranes, the revision of methods to generate the B-C, B-P, B-S, and B-N bonds has been restricted to the most widely used boron clusters; [B12H12](2-), dianionic and an example of a borane, 1,2-C2B10H12, neutral and an example of a heteroborane, and [Co(C2B9H11)2](-), monoanionic and an example of a metallacarborane.

Molecular structure investigation and spectroscopic studies on 2,3-difluorophenylboronic acid: a combined experimental and theoretical analysis

This work presents the characterization of 2,3-difluorophenylboronic acid (abbreviated as 2,3-DFPBA, C(6)H(3)B(OH)(2)F(2)) by quantum chemical calculations and spectral techniques. The spectroscopic properties were investigated by FT-IR, FT-Raman UV-Vis, (1)H and (13)C nuclear magnetic resonance (NMR) techniques. The FT-IR spectrum (4000-400 cm(-1)) and the FT-Raman spectrum (3500-10 cm(-1)) in the solid phase were recorded for 2,3-DFPBA. The (1)H and (13)C NMR spectra were recorded in DMSO solution. The UV-Vis absorption spectra of the 2,3-DFPBA that dissolved in water and ethanol were recorded in the range of 200-400 nm. There are four possible conformers for this molecule. The computational results diagnose the most stable conformer of the 2,3-DFPBA as the trans-cis form. The structural and spectroscopic data of the molecule were obtained for all four conformers from DFT (B3LYP) with 6-311++G (d,p) basis set calculations. The theoretical wavenumbers were scaled and compared with experimental FT-IR and FT-Raman spectra. The complete assignments were performed on the basis of the experimental results and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method, interpreted in terms of fundamental modes. We obtained good consistency between experimental and theoretical spectra. (13)C and (1)H NMR chemical shifts of the molecule were calculated by using the gauge-invariant atomic orbital (GIAO) method. The electronic properties, such as excitation energies, absorption wavelengths, HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. Finally the calculation results were analyzed to simulate infrared, Raman, NMR and UV spectra of the 2,3-DFPBA which show good agreement with observed spectra.

The spectroscopic (FTIR, FT-Raman, NMR and UV), first-order hyperpolarizability and HOMO-LUMO analysis of methylboronic acid

The solid phase FTIR and FT-Raman spectra of methylboronic acid (MBA) have been recorded in the regions 400-4000 and 50-4000 cm(-1), respectively. The spectra were interpreted interms of fundamentals modes, combination and overtone bands. The structure of the molecule was optimized and the structural characteristics were determined by density functional theory (B3LYP) and HF method with 6-311++G(d,p) as basis set. The vibrational frequencies were calculated for most stable conformer and were compared with the experimental frequencies, which yield good agreement between observed and calculated frequencies. The infrared and Raman spectra were also predicted from the calculated intensities. (1)H and (13)C NMR spectra were recorded and (1)H and (13)C nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method. UV-visible spectrum of the compound was recorded in the region 200-400 nm and the electronic properties HOMO and LUMO energies were calculated by time-dependent TD-DFT approach. Mulliken charges of the MBA molecule was also calculated and interpreted. The geometric parameters, energies, harmonic vibrational frequencies, IR intensities, Raman intensities and absorption wavelengths were compared with the available experimental data of the molecule.

Fifty years of the closo-decaborate anion chemistry

The review covers the fifty-year period of chemistry of the closo-decaborate anion [B10H10]2– from the first report by Hawthorne and Pitochelli. The main attention is paid to reactions of substitution of various atoms and groups for hydrogen atoms. The general stability of the closo-decaborate cage, including its protonation, cage-opening and cage oxidation reactions, is considered as well. A review with 242 references.

An experimental and theoretical study of molecular structure and vibrational spectra of 3- and 4-pyridineboronic acid molecules by density functional theory calculations

The experimental and theoretical vibrational spectra of 3- and 4-pyridineboronic acids (abbreviated as p3 and p4) were studied. The Fourier transform Raman and Fourier transform infrared spectra of p3 and p4 molecules were recorded in the solid phase. The structural and spectroscopic analysis of the p3 and p4 acids were made by using density functional harmonic calculations. Both p3 and p4 only one form was most stable using B3LYP level with the 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p) and 6-311++G(d,p) basis sets. Selected experimental bands were assigned based on the scaled theoretical wavenumbers. Finally, geometric parameters, infrared and Raman bands and intensities were compared with experimental data of the molecules.

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.

Avidin self-associates with boric acid gel suspensions: an affinity boron carrier that might be developed for boron neutron-capture therapy

It has been shown in preliminary studies that the antibacterial protein avidin self-associates with the boric acid gel polymer, and avidin-coated gel particles in the micrometer and submicrometer size ranges are of interest for boron neutron-capture therapy (BNCT), which is neutron-induced fission of boron-10 to produce intense alpha radiation for tumor destruction. The gel particles carry large amounts of boron-10 and are theoretically able effect a meaningful tissue dosing through BNCT. A gross precipitation of gel particles occurs within 46 min of mixing when the avidin/colloid ratio is about 0.34 g avidin/g colloid. This is a minimum time if gel and avidin concentrations are in the low microgram/milliliter range, but at higher proportions of avidin the time delay to precipitation increases significantly; i.e., the colloid surface becomes blocked, inhibiting lattice formation. The avidin-coated gel particles eventually cross-link, forming a solid matrix and precipitating on a timescale measured on the order of an hour. At shorter exposure times rapid agglutination-like reactions were observed with biotinylated bovine albumin, suggesting that two-stage pretargeting of specific tissues should be possible with biotinylated antitumor antibodies. However, for BNCT to be practical, avidin's interaction with the gel needs to be strengthened, and all aryl-B(OH)(2) groups on the particle surfaces must be blocked, or else the particles will interact strongly and nonspecifically with each other and with the carbohydrate groups present on most cell surfaces. Glyceric acid delays the precipitation of the particle suspensions while most simple and complex carbohydrates accelerate it.

A critical assessment of boron target compounds for boron neutron capture therapy

Boron neutron capture therapy (BNCT) has undergone dramatic developments since its inception by Locher in 1936 and the development of nuclear energy during World War II. The ensuing Cold War spawned the entirely new field of polyhedral borane chemistry, rapid advances in nuclear reactor technology and a corresponding increase in the number to reactors potentially available for BNCT. This effort has been largely oriented toward the eradication of glioblastoma multiforme (GBM) and melanoma with reduced interest in other types of malignancies. The design and synthesis of boron-10 target compounds needed for BNCT was not channeled to those types of compounds specifically required for GBM or melanoma. Consequently, a number of potentially useful boron agents are known which have not been biologically evaluated beyond a cursory examination and only three boron-10 enriched target species are approved for human use following their Investigational New Drug classification by the US Food and Drug Administration; BSH, BPA and GB-10. All ongoing clinical trials with GBM and melanoma are necessarily conducted with one of these three species and most often with BPA. The further development of BNCT is presently stalled by the absence of strong support for advanced compound evaluation and compound discovery driven by recent advances in biology and chemistry. A rigorous demonstration of BNCT efficacy surpassing that of currently available protocols has yet to be achieved. This article discusses the past history of compound development, contemporary problems such as compound classification and those problems which impede future advances. The latter include means for biological evaluation of new (and existing) boron target candidates at all stages of their development and the large-scale synthesis of boron target species for clinical trials and beyond. The future of BNCT is bright if latitude is given to the choice of clinical disease to be treated and if a recognized study demonstrating improved efficacy is completed. Eventually, BNCT in some form will be commercialized.

Synthesis and biological evaluation of folate receptor-targeted boronated PAMAM dendrimers as potential agents for neutron capture therapy

Successful treatment of cancer by boron neutron capture therapy (BNCT) requires the selective delivery of (10)B to constituent cells within a tumor. The expression of the folate receptor is amplified in a variety of human tumors and potentially might serve as a molecular target for BNCT. In the present study we have investigated the possibility of targeting the folate receptor on cancer cells using folic acid conjugates of boronated poly(ethylene glycol) (PEG) containing 3rd generation polyamidoamine dendrimers to obtain (10)B concentrations necessary for BNCT by reducing the uptake of these conjugates by the reticuloendothelial system. First we covalently attached 12-15 decaborate clusters to 3rd generation polyamidoamine dendrimers. Varying quantities of PEG units with varying chain lengths were then linked to these boronated dendrimers to reduce hepatic uptake. Among all prepared combinations, boronated dendrimers with 1-1.5 PEG(2000) units exhibited the lowest hepatic uptake in C57BL/6 mice (7.2-7.7% injected dose (ID)/g liver). Thus, two folate receptor-targeted boronated 3rd generation polyamidoamine dendrimers were prepared, one containing approximately 15 decaborate clusters and approximately 1 PEG(2000) unit with folic acid attached to the distal end, the other containing approximately 13 decaborate clusters, approximately 1 PEG(2000) unit, and approximately 1 PEG(800) unit with folic acid attached to the distal end. In vitro studies using folate receptor (+) KB cells demonstrated receptor-dependent uptake of the latter conjugate. Biodistribution studies with this conjugate in C57BL/6 mice bearing folate receptor (+) murine 24JK-FBP sarcomas resulted in selective tumor uptake (6.0% ID/g tumor), but also high hepatic (38.8% ID/g) and renal (62.8% ID/g) uptake, indicating that attachment of a second PEG unit and/or folic acid may adversely affect the pharmacodynamics of this conjugate.

Azanonaboranes [(RNH(2))B(8)H(11)NHR] as possible new compounds for use in boron neutron capture therapy

The synthesis and biological in vitro and in vivo activities of possible new compounds for Boron Neutron Capture Therapy (BNCT) are reported. The azanonaboranes of the type [(RNH(2))B(8)H(11)NHR] are water-soluble when hydrophilic groups are introduced. The reaction of B(9)H(13)SMe(2) with primary amines yields azanonaboranes. Five compounds with different numbers of hydroxypropyl groups have been isolated: [(HO(CH(2))(3)NH(2))B(8)H(11)NHCH(3)] (4), [(HO(CH(2))(3)NH(2))B(8)H(11)NH(CH(2))(3)OH] (2), [((HO(CH(2))(3))(2)NH)B(8)H(11)NHCH(3)] (6), [((HO(CH(2))(3))(2)NH)B(8)H(11)NH(CH(2))(3)OCH(3)] (11) and [((HO(CH(2))(3))(2)NH)B(8)H(11)NH(CH(2))(3)OH] (8). In vitro experiments as judged by cloning survival tests showed that two of the synthesised compounds are not toxic. The in vivo experiments were carried out with C3H/He mice bearing SCCVII tumours and C57 mice bearing B16 tumours. Compounds 2 and 6 have no particular affinity to any tissue, but are excluded from the brain.

Linkage of boronated polylysine to glycoside moieties of polyclonal antibody; boronated antibodies as potential delivery agents for neutron capture therapy

Among the ways to deliver comparatively large amounts of boron to cells in vitro for boron neutron capture studies is the linkage of a boronated macromolecule such as polylysine to an antibody. In order to reduce interference with immunoreactivity, boronated polylysine (BPL) was linked to oligosaccharide moieties on the IgG molecule distant from the antibody combining sites. The resultant bioconjugate was chromatographically separated from free BPL and unconjugated antibody using a Sephacryl S300 column. The total measured boron per BPL-IgG conjugate, determined by direct current plasma atomic emission spectroscopy, was estimated to be approximately 6 x 10(3) atoms. This, together with molecular weight estimations, indicated conjugation of about 3 polylysines to each IgG molecule. Immunoreactivity of the conjugate was found to be the same as that of the unconjugated polyclonal antibody. This was based on its concentration dependent interference with immunometric reactions for an antigen (TSH), whereas heat inactivated or non-specific antibody had no such inhibitory effects. The results support the hypothesis that the binding affinity of the conjugate for antigen was preserved after its linkage to BPL under the conditions described. The methodology described in this report may have applicability for the preparation of boronated antibodies as delivery agents for BNCT.

Chemistry of closo-Dodecaborate Anion [B12H12]2-: A Review

Synthesis and chemical properties of the closo-dodecaborate anion [B12H12]2- and its derivatives are reviewed. Attention is also paid to potential applications of the closo-dodecaborate derivatives with emphasis on medical applications. A review with 325 references.