Closo-dodecaborate(2-) as a linker for iodination of macromolecules. Aspects on conjugation chemistry and biodistribution

RGD Peptide-Conjugated Dodecaborate with the Ga-DOTA Complex: A Preliminary Study for the Development of Theranostic Agents for Boron Neutron Capture Therapy and Its Companion Diagnostics

A boron neutron capture therapy (BNCT) system, using boron-10-introduced agents coupled with companion diagnostics, is anticipated as a promising cancer theranostic. Thus, this study aimed to synthesize and evaluate a probe closo-dodecaborate-(Ga-DOTA)-c(RGDfK) (16) [Ga = gallium, DOTA =1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, and c(RGDfK) = cyclo(arginine-glycine-aspartate-d-phenylalanine-lysine] containing closo-dodecaborate ([B₁₂H₁₂]^2-) as a boron cluster, a [⁶⁷Ga]Ga-DOTA derivative for nuclear medicine imaging, and an RGD peptide for tumor targeting. Moreover, we prepared a radioiodinated probe [¹²⁵I]17 in which I-125 is introduced into a closo-dodecaborate moiety of 16. [⁶⁷Ga]16 and [¹²⁵I]17 showed high stability and high uptake in cancer cells in vitro. Biodistribution experiments in tumor-bearing mice revealed similar biodistribution patterns between [⁶⁷Ga]16 and [¹²⁵I]17, such as a high uptake in the tumor and a low uptake in other non-target tissues. Meanwhile, [¹²⁵I]17 exhibited higher accumulation in most tissues, including the tumor, than [⁶⁷Ga]16, probably because of higher albumin binding. The higher the [¹²⁵I]17 accumulation in the tumor, the more desirable it is for BNCT, with the possibility that the iodo-closo-dodecaborate site may work as an albumin binder.

New Azido Coumarins as Potential Agents for Fluorescent Labeling and Their "Click" Chemistry Reactions for the Conjugation with closo-Dodecaborate Anion

Novel fluorescent 7-methoxy- and 7-(diethylamino)-coumarins modified with azido-group on the side chain have been synthesized. Their photophysical properties and single crystals structure characteristics have been studied. In order to demonstrate the possibilities of fluorescent labeling, obtained coumarins have been tested with closo-dodecaborate derivative bearing terminal alkynyl group. Cu^I catalyzed Huisgen 1,3-dipolar cycloaddition reaction has led to fluorescent conjugates formation. The absorption-emission spectra of the formed conjugates have been presented. The antiproliferative activity and uptake of compounds against several human cell lines were evaluated.

Synthesis of Bis(Carboranyl)amides 1,1'-μ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 (n = 0, 1) and Attempt of Synthesis of Gadolinium Bis(Dicarbollide)

Bis(carboranyl)amides 1,1'-μ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 (n = 0, 1) were prepared by the reactions of the corresponding carboranyl acyl chlorides with ethylenediamine. Crystal molecular structure of 1,1'-μ-(CH2NH(O)C-1,2-C2B10H11)2 was determined by single crystal X-ray diffraction. Treatment of bis(carboranyl)amides 1,1'-μ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 with ammonium or cesium fluoride results in partial deboronation of the ortho-carborane cages to the nido-carborane ones with formation of [7,7'(8')-μ-(CH2NH(O)C(CH2)n-7,8-C2B9H11)2]2-. The attempted reaction of [7,7'(8')-μ-(CH2NH(O)CCH2-7,8-C2B9H11)2]2- with GdCl3 in 1,2-dimethoxy- ethane did not give the expected metallacarborane. The stability of different conformations of Gd-containing metallacarboranes has been estimated by quantum-chemical calculations using [3,3-μ-DME-3,3'-Gd(1,2-C2B9H11)2]- as a model. It was found that in the most stable conformation the CH groups of the dicarbollide ligands are in anti,anti-orientation with respect to the DME ligand, while any rotation of the dicarbollide ligand reduces the stability of the system. This makes it possible to rationalize the design of carborane ligands for the synthesis of gadolinium metallacarboranes on their base.

Radiobromination of closo-dodecaborate anion. Aspects of labelling chemistry in aqueous solution using Chloramine-T

Closo-dodecaborate dianion is a three-dimensional aromatic inorganic molecule, which can be easily halogenated forming a stable halogen-boron bond. Derivatives of closo-dodecaborate were considered as a convenient chemical form of delivery of enriched 10B to malignant tumors for boron neutron capture therapy (BNCT). Some properties of closo-dodecaborate (hydrophilicity, strength of halogen-boron bond, charge at lysosomal pH) make it attractive as a potential prosthetic group for attachment of radioactive halogens to tumor-targeting proteins. Bromine radioisotopes possess a variety of useful nuclear characteristics, and can be used in different areas of clinical diagnostics and therapy. In this work, a basic chemistry of closo-dodecaborate radiobromination was studied. It was found, that di(triethylamonium) dodecahydro-closo-dodecaborate can be labelled in high yield, more then 90%, in a wide pH range. By decreasing the pH, the bromination can be directed to closo-dodecaborate in the presence of phenolic compounds. The results of the study indicate a possibility of using the radioactive bromine label for investigation of pharmacokinetics of boronated compounds for BNCT.

Carborane clusters in computational drug design: a comparative docking evaluation using AutoDock, FlexX, Glide, and Surflex

Compounds containing boron atoms play increasingly important roles in the therapy and diagnosis of various diseases, particularly cancer. However, computational drug design of boron-containing therapeutics and diagnostics is hampered by the fact that many software packages used for this purpose lack parameters for all or part of the various types of boron atoms. In the present paper, we describe simple and efficient strategies to overcome this problem, which are based on the replacement of boron atom types with carbon atom types. The developed methods were validated by docking closo- and nido-carboranyl antifolates into the active site of a human dihydrofolate reductase (hDHFR) using AutoDock, Glide, FlexX, and Surflex and comparing the obtained docking poses with the poses of their counterparts in the original hDHFR-carboranyl antifolate crystal structures. Under optimized conditions, AutoDock and Glide were equally good in docking of the closo-carboranyl antifolates followed by Surflex and FlexX, whereas Autodock, Glide, and Surflex proved to be comparably efficient in the docking of nido-carboranyl antifolates followed by FlexX. Differences in geometries and partial atom charges in the structures of the carboranyl antifolates resulting from different data sources and/or optimization methods did not impact the docking performances of AutoDock or Glide significantly. Binding energies predicted by all four programs were in accordance with experimental data.

Reagents for astatination of biomolecules. 3. Comparison of closo-decaborate(2-) and closo-dodecaborate(2-) moieties as reactive groups for labeling with astatine-211

In vivo deastatination has been a major problem in the development of reagents for therapeutic applications of the alpha-particle emitting radionuclide (211)At. Our prior studies demonstrated that the use of a closo-decaborate(2-) ([closo-B(10)H(9)R](2-)) moiety for (211)At labeling of biomolecules provides conjugates that are stable to in vivo deastatination. In this investigation, the closo-decaborate(2-) moiety was compared with the structurally similar closo-dodecaborate(2-) ([closo-B(12)H(11)R](2-)) to determine if one has more favorable properties than the other for use in pendant groups as (211)At labeling molecules. To determine the differences, two sets of structurally identical molecules, with the exception that they contained either a closo-decaborate(2-) or a closo-dodecaborate(2-) moiety, were compared with regard to their synthesis, radiohalogenation, stability to in vivo deastatination and tissue distribution. Quite different rates of reaction were noted in the synthetic steps for the two closo-borate(2-) moieties, but ultimately the yields were similar, making these differences of little importance. Differences in radiohalogenation rates were also noted between the two closo-borate(2-) moieties, with the more electrophilic closo-decaborate(2-) reacting more rapidly. This resulted in somewhat higher yields of astatinated closo-decaborate(2-) derivatives (84% vs 53%), but both cage moieties gave good radioiodination yields (e.g., 79-96%). Importantly, both closo-borate(2-) cage moieties were shown to have high stability to in vivo deastatination. The largest differences between pairs of compounds containing the structurally similar boron cage moieties were in their in vivo tissue distributions. For example, [Et(3)NH](2)B(12)H(10)I-CONHpropyl, [(125)I]2b had high concentrations in kidney (1 h, 19.8%ID/g; 4 h, 26.5%ID/g), whereas [Et(3)NH](2)B(10)H(8)I-CONHpropyl, [(125)I]1e had much lower concentrations in kidney (1 h, 6.6%ID/g; 4 h, 0.27%ID/g). Interestingly, when another salt of the closo-decaborate(2-), [nBu(4)N](2)B(10)H(8)I-CONHpropyl, [(125)I]1b, was evaluated, the route of excretion appeared to be hepatobiliary rather than renal. Identical biotin derivatives containing the two closo-borate(2-) cage moieties had similar tissue distributions, except the closo-decaborate(2-) derivative had lower concentrations in kidney (1 h, 19.9%ID/g; 4 h, 24.4%ID/g vs 1 h, 38.9%ID/g; 4 h, 40.6%ID/g). In summary, the higher reactivity, faster tissue clearance, and lower kidney concentrations make the closo-decaborate(2-) more favorable for further studies using them in reactive groups for (211)At labeling of biomolecules.

Radiobromination of monoclonal antibody using potassium [76Br] (4 isothiocyanatobenzyl-ammonio)-bromo-decahydro-closo-dodecaborate (Bromo-DABI)

The use of charged linkers in attaching radiohalogens to tumor-seeking biomolecules may improve intracellular retention of the radioactive label after internalization and degradation of targeting proteins. Derivatives of polyhedral boron clusters, such as closo-dodecaborate (2-) anion, might be possible charged linkers. In this study, a bifunctional derivative of closo-dodecaborate, (4-isothiocyanatobenzyl-ammonio)-undecahydro-closo-dodecaborate (DABI) was labeled with positron-emitting nuclide (76)Br (T 1/2 = 16.2 h) and coupled to anti-HER2/neu humanized antibody Trastuzumab. The overall labeling yield at optimized conditions was 80.7 +/- 0.6%. The label was proven to be stable in vitro in physiological and a set of denaturing conditions. The labeled antibody retained its capacity to bind to HER-2/neu antigen expressing cells. The results of the study demonstrated feasibility for using derivatives of closo-dodecaborate in indirect labeling of antibodies for radioimmunoPET.

Targeting of a head and neck squamous cell carcinoma xenograft model using the chimeric monoclonal antibody U36 radioiodinated with a closo-dodecaborate-containing linker

OBJECTIVE

High rates of local recurrence and distant metastases following surgery of high-grade head and neck squamous cell carcinoma (HNSCC) necessitate the use of adjuvant systemic treatment. Radioimmunotargeting might be a possible treatment modality in this case. The nuclear properties of 131I make it a suitable isotope for treatment of minimal residual disease and small metastases, but the conventional radioiodine label has poor cellular retention and its radiocatabolites accumulate in the thyroid. We attempted to overcome these problems by using closo-dodecaborate derivatives for attachment of radioiodine.

MATERIAL AND METHODS

We investigated the feasibility of targeting an SCC25 HNSCC xenograft in vivo using a benzylisothiocyanate derivative of closo-dodecaborate (DABI) as radioiodine linker and the chimeric anti-CD44v6 antibody U36. 125I was used in biodistribution studies.

RESULTS

The use of DABI enabled tumor targeting and decreased the radioactivity uptake of the thyroid.

CONCLUSION

Tumor localization of DABI-labeled U36 was similar to its para-iodobenzoate-labeled counterpart, presumably due to the strong dependence of targeting efficiency on tumor size.

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.

Polyhedral Boron Compounds as Potential Linkers for Attachment of Radiohalogens to Targeting Proteins and Peptides. A Review

Polyhedral boron clusters (PBC) are three-dimensional inorganic aromatic systems. Some of them can easily be halogenated, and the halogen-boron bond in such systems is very strong. We consider the use of PBC as linkers for attachment of radioactive halogen isotopes to tumor-targeting proteins and peptides. In this review the major preconditions for such applications, such as biological considerations, knowledge concerning coupling chemistry and radiolabeling of PBC, are described. A review with 90 references.

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.