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

Nucleoside Scaffolds and Carborane Clusters for Boron Neutron Capture Therapy: Developments and Future Perspective

Nucleosides containing carboranes are one of the most important boron delivery agents for boron neutron capture therapy, BNCT, which are good substrates of hTK1. The development of several nucleosides containing carboranes at early stages led to the discovery of the first generation of 3CTAs by incorporating a hydrocarbon spacer between the thymidine scaffold and carborane cluster and attaching dihydroxylpropyl group on the second carbon (C2) atom of the carborane cluster (e.g., N5 and N5-2OH). Phosphorylation rate, tumor cellular uptake, and retention have been evaluated in parallel to change the length of the tether arm of spacers in these compounds. Many attempts were reported and discussed to overcome the disadvantage of the first generation of 3CTAs by a) incorporating modified spacers between thymidine and carborane clusters, such as ethyleneoxide, polyhydroxyl, triazole, and tetrazole units, b) attaching hydrophilic groups at C2 of the carborane cluster, c) transforming lipophilic closo-carboranes to hydrophilic nidocarborane. The previous modifications represented the second generation of 3CTAs to improve the hydrogen bond formation with the hTK1 active site. Moreover, amino acid prodrugs were developed to enhance biological and physicochemical properties. The structure-activity relationship (SAR) of carboranyl thymidine analogues led to the roadmap for the development of the 3rd generation of the 3CTAs for BNCT.

Carborane-Containing Matrix Metalloprotease (MMP) Ligands as Candidates for Boron Neutron-Capture Therapy (BNCT)

Based on the previously reported potent and selective sulfone hydroxamate inhibitors SC-76276, SC-78080 (SD-2590), and SC-77964, potent MMP inhibitors have been designed and synthesized to append a boron-rich carborane cluster by employing click chemistry to target tumor cells that are known to upregulate gelatinases. Docking against MMP-2 suggests binding involving the hydroxamate zinc-binding group, key H-bonds by the sulfone moiety with the peptide backbone residues Leu82 and Leu83, and a hydrophobic interaction with the deep P1' pocket. The more potent of the two triazole regioisomers exhibits an IC₅₀ of 3.7 nM versus MMP-2 and IC₅₀ of 46 nM versus MMP-9.

Comparative Study of Carborane- and Phenyl-Modified Adenosine Derivatives as Ligands for the A2A and A3 Adenosine Receptors Based on a Rigid in Silico Docking and Radioligand Replacement Assay

Adenosine receptors are involved in many physiological processes and pathological conditions and are therefore attractive therapeutic targets. To identify new types of effective ligands for these receptors, a library of adenosine derivatives bearing a boron cluster or phenyl group in the same position was designed. The ligands were screened in silico to determine their calculated affinities for the A2A and A3 adenosine receptors. An virtual screening protocol based on the PatchDock web server was developed. In the first screening phase, the effects of the functional group (organic or inorganic modulator) on the adenosine ligand affinity for the receptors were determined. Then, the lead compounds were identified for each receptor in the second virtual screening phase. Two pairs of the most promising ligands, compounds 3 and 4, and two ligands with lower affinity scores (compounds 11 and 12, one with a boron cluster and one with a phenyl group) were synthesized and tested in a radioligand replacement assay for affinity to the A2A and A3 receptors. A reasonable correlation of in silico and biological assay results was observed. In addition, the effects of a phenyl group and boron cluster, which is new adenosine modifiers, on the adenosine ligand binding were compared.

Synthesis and evaluation of thymidine kinase 1-targeting carboranyl pyrimidine nucleoside analogs for boron neutron capture therapy of cancer

A library of sixteen 2nd generation amino- and amido-substituted carboranyl pyrimidine nucleoside analogs, designed as substrates and inhibitors of thymidine kinase 1 (TK1) for potential use in boron neutron capture therapy (BNCT) of cancer, was synthesized and evaluated in enzyme kinetic-, enzyme inhibition-, metabolomic-, and biodistribution studies. One of these 2nd generation carboranyl pyrimidine nucleoside analogs (YB18A [3]), having an amino group directly attached to a meta-carborane cage tethered via ethylene spacer to the 3-position of thymidine, was approximately 3-4 times superior as a substrate and inhibitor of hTK1 than N5-2OH (2), a 1st generation carboranyl pyrimidine nucleoside analog. Both 2 and 3 appeared to be 5'-monophosphorylated in TK1(+) RG2 cells, both in vitro and in vivo. Biodistribution studies in rats bearing intracerebral RG2 glioma resulted in selective tumor uptake of 3 with an intratumoral concentration that was approximately 4 times higher than that of 2. The obtained results significantly advance the understanding of the binding interactions between TK1 and carboranyl pyrimidine nucleoside analogs and will profoundly impact future design strategies for these agents.

N3-substituted thymidine bioconjugates for cancer therapy and imaging

The compound class of 3-carboranyl thymidine analogues (3CTAs) are boron delivery agents for boron neutron capture therapy (BNCT), a binary treatment modality for cancer. Presumably, these compounds accumulate selectively in tumor cells via intracellular trapping, which is mediated by hTK1. Favorable in vivo biodistribution profiles of 3CTAs led to promising results in preclinical BNCT of rats with intracerebral brain tumors. This review presents an overview on the design, synthesis, and biological evaluation of first- and second-generation 3CTAs. Boronated nucleosides developed prior to 3CTAs for BNCT and non-boronated N3-substituted thymidine conjugates for other areas of cancer therapy and imaging are also described. In addition, basic features of carborane clusters, which are used as boron moieties in the design and synthesis of 3CTAs, and the biological and structural features of TK1-like enzymes, which are the molecular targets of 3CTAs, are discussed.

Synthesis of N3-substituted carboranyl thymidine bioconjugates and their evaluation as substrates of recombinant human thymidine kinase 1

Four different libraries of overall twenty three N3-substituted thymidine (dThd) analogues, including eleven 3-carboranyl thymidine analogues (3CTAs), were synthesized. The latter are potential agents for Boron Neutron Capture Therapy (BNCT) of cancer. Linker between the dThd scaffold and the m-carborane cluster at the N3-position of the 3CTAs contained amidinyl-(3e and 3f), guanidyl-(7e-7g), tetrazolylmethyl-(9b1/2-9d1/2), or tetrazolyl groups (11b1/2-11d1/2) to improve human thymidine kinase 1 (hTK1) substrate characteristics and water solubilities compared with 1st generation 3CTAs, such as N5 and N5-2OH. The amidinyl- and guanidyl-type N3-substitued dThd analogues (3a-3f and 7a-7g) had hTK1 phosphorylation rates of <30% relative to that of dThd, the endogenous hTK1 substrate, whereas the tetrazolyl-type N3-substitued dThd analogues (9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2) had relative phosphorylation rates (rPRs) of >40%. Compounds 9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2 were subjected to in-depth enzyme kinetics studies and the obtained rk(cat)/K(m) (k(cat)/K(m) relative to that of dThd) ranged from 2.5 to 26%. The tetrazolyl-type N3-substitued dThd analogues 9b1/2 and 11d1/2 were the best substrates of hTK1 with rPRs of 52.4% and 42.5% and rk(cat)/K(m) values of 14.9% and 19.7% respectively. In comparison, the rPR and rk(cat)/K(m) values of N5-2OH in this specific study were 41.5% and 10.8%, respectively. Compounds 3e and 3f were >1900 and >1500 times, respectively, better soluble in PBS (pH 7.4) than N5-2OH whereas solubilities for 9b1/2-9d1/2 and 11b1/2-11d1/2 were only 1.3-13 times better.

Synthesis, chemical and enzymatic hydrolysis, and aqueous solubility of amino acid ester prodrugs of 3-carboranyl thymidine analogs for boron neutron capture therapy of brain tumors

Various water-soluble L-valine-, L-glutamate-, and glycine ester prodrugs of two 3-Carboranyl Thymidine Analogs (3-CTAs), designated N5 and N5-2OH, were synthesized for Boron Neutron Capture Therapy (BNCT) of brain tumors since the water solubilities of the parental compounds proved to be insufficient in preclinical studies. The amino acid ester prodrugs were prepared and stored as hydrochloride salts. The water solubilities of these amino acid ester prodrugs, evaluated in phosphate buffered saline (PBS) at pH 5, pH 6 and pH 7.4, improved 48-6600 times compared with parental N5 and N5-2OH. The stability of the amino acid ester prodrugs was evaluated in PBS at pH 7.4, Bovine serum, and Bovine cerebrospinal fluid (CSF). The rate of the hydrolysis in all three incubation media depended primarily on the amino acid promoiety and, to a lesser extend, on the site of esterification at the deoxyribose portion of the 3-CTAs. In general, 3'-amino acid ester prodrugs were less sensitive to chemical and enzymatic hydrolysis than 5'-amino acid ester prodrugs and the stabilities of the latter decreased in the following order: 5'-valine > 5'-glutamate > 5'-glycine. The rate of the hydrolysis of the 5'-amino acid ester prodrugs in Bovine CSF was overall higher than in PBS and somewhat lower than in Bovine serum. Overall, 5'-glutamate ester prodrug of N5 and the 5'-glycine ester prodrugs of N5 and N5-2OH appeared to be the most promising candidates for preclinical BNCT studies.

Synthesis, biological evaluation, and radioiodination of halogenated closo-carboranylthymidine analogues

The synthesis and initial biological evaluation of 3-carboranylthymidine analogues (3CTAs) that are (radio)halogenated at the closo-carborane cluster are described. Radiohalogenated 3CTAs have the potential to be used in the radiotherapy and imaging of cancer because they may be selectively entrapped in tumor cells through monophosphorylation by human thymidine kinase 1 (hTK1). Two strategies for the synthesis of a (127)I-labeled form of a specific 3CTA, previously designated as N5, are described: (1) direct iodination of N5 with iodine monochloride and aluminum chloride to obtain N5-(127)I and (2) initial monoiodination of o-carborane to 9-iodo-o-carborane followed by its functionalization to N5-(127)I. The former strategy produced N5-(127)I in low yields along with di-, tri-, and tetraiodinated N5 as well as decomposition products, whereas the latter method produced only N5-(127)I in high yields. N5-(127)I was subjected to nucleophilic halogen- and isotope-exchange reactions using Na(79/81)Br and Na(125)I, respectively, in the presence of Herrmann's catalyst to obtain N5-(79/81)Br and N5-(125)I, respectively. Two intermediate products formed using the second strategy, 1-(tert-butyldimethylsilyl)-9-iodo-o-carborane and 1-(tert-butyldimethylsilyl)-12-iodo-o-carborane, were subjected to X-ray diffraction studies to confirm that substitution at a single carbon atom of 9-iodo-o-carborane resulted in the formation of two structural isomers. To the best of our knowledge, this is the first report of halogen- and isotope-exchange reactions of B-halocarboranes that have been conjugated to a complex biomolecule. Human TK1 phosphorylation rates of N5, N5-(127)I, and N5-(79/81)Br ranged from 38.0% to 29.6% relative to that of thymidine, the endogenous hTK1 substrate. The in vitro uptake of N5, N5-(127)I, and N5-(79/81)Br in L929 TK1(+) cells was 2.0, 1.8, and 1.4 times greater than that in L929 TK1(-) cells.

Therapeutic potential of atmospheric neutrons

BACKGROUND

Glioblastoma multiform (GBM) is the most common and most aggressive type of primary brain tumour in humans. It has a very poor prognosis despite multi-modality treatments consisting of open craniotomy with surgical resection, followed by chemotherapy and/or radiotherapy. Recently, a new treatment has been proposed - Boron Neutron Capture Therapy (BNCT) - which exploits the interaction between Boron-10 atoms (introduced by vector molecules) and low energy neutrons produced by giant accelerators or nuclear reactors.

METHODS

The objective of the present study is to compute the deposited dose using a natural source of neutrons (atmospheric neutrons). For this purpose, Monte Carlo computer simulations were carried out to estimate the dosimetric effects of a natural source of neutrons in the matter, to establish if atmospheric neutrons interact with vector molecules containing Boron-10.

RESULTS

The doses produced (an average of 1 μGy in a 1 g tumour) are not sufficient for therapeutic treatment of in situ tumours. However, the non-localised yet specific dosimetric properties of 10B vector molecules could prove interesting for the treatment of micro-metastases or as (neo)adjuvant treatment. On a cellular scale, the deposited dose is approximately 0.5 Gy/neutron impact.

CONCLUSION

It has been shown that BNCT may be used with a natural source of neutrons, and may potentially be useful for the treatment of micro-metastases. The atmospheric neutron flux is much lower than that utilized during standard NBCT. However the purpose of the proposed study is not to replace the ordinary NBCT but to test if naturally occurring atmospheric neutrons, considered to be an ionizing pollution at the Earth's surface, can be used in the treatment of a disease such as cancer. To finalize this study, it is necessary to quantify the biological effects of the physically deposited dose, taking into account the characteristics of the incident particles (alpha particle and Lithium atom) and radio-induced effects (by-stander and low dose effect). One of the aims of the presented paper is to propose to experimental teams (which would be interested in studying the phenomena) a simple way to calculate the dose deposition (allometric fit of free path, transmission factor of brain).

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.

Computational molecular biology approaches to ligand-target interactions

Binding of small molecules to their targets triggers complex pathways. Computational approaches are keys for predictions of the molecular events involved in such cascades. Here we review current efforts at characterizing the molecular determinants in the largest membrane-bound receptor family, the G-protein-coupled receptors (GPCRs). We focus on odorant receptors, which constitute more than half GPCRs. The work presented in this review uncovers structural and energetic aspects of components of the cellular cascade. Finally, a computational approach in the context of radioactive boron-based antitumoral therapies is briefly described.

The bioinorganic and medicinal chemistry of carboranes: from new drug discovery to molecular imaging and therapy

The role of carboranes in medicinal chemistry has diversified in recent years and now extends into areas of drug discovery, molecular imaging, and targeted radionuclide therapy. An introduction to carborane chemistry is provided to familiarize the non-expert with some key properties of these molecules, followed by an overview of current medicinally-orientated research involving carboranes. The broad-ranging nature of this research is illustrated, with emphasis placed on recent highlights and advances in this field.