New functionalized mercaptoundecahydrododecaborate derivatives for potential application in boron neutron capture therapy: Synthesis, characterization and dynamic visualization in cells

• Published in: European journal of medicinal chemistry 2015-03, Vol.93, p.574-583
• Main Authors: Genady, Afaf R., Ioppolo, Joseph A., Azaam, Mohamed M., El-Zaria, Mohamed E.
• Format: Article
• Language: English
• Subjects: Alkylation; Amidation; Amides - chemistry; Biological Transport - drug effects; BNCT; Borohydrides - chemical synthesis; Borohydrides - chemistry; Borohydrides - pharmacology; Boron - metabolism; Boron Neutron Capture Therapy; Chemistry Techniques, Synthetic; Click Chemistry; Esterification; HeLa Cells; Humans; Imaging; Intracellular Space - drug effects; Intracellular Space - metabolism; Molecular Imaging; NMR spectroscopy; Sulfhydryl Compounds - chemical synthesis; Sulfhydryl Compounds - chemistry; Sulfhydryl Compounds - pharmacology; Triazoles - chemistry
• ISSN: 0223-5234; 1768-3254
• DOI: 10.1016/j.ejmech.2015.02.033

A series of mercaptoundecahydrododecaborate (B12H11SH2−, BSH) bearing mono- and dicarboxyalkyl derivatives was prepared, characterized, and their reactivity towards amidation and esterification in DMF was evaluated. Symmetrical alkylation of BSH was achieved by treatment with primary haloalkyl carboxylic acids in aqueous acetonitrile to produce S,S-bis(carboxyalkyl)sulfonium-undecahydro-closo-dodecaborate tetramethylammonium salts. Unsymmetrically substituted sulfonium salts were obtained through a similar treatment of cyanoethylthioether-undecahydro-closo-dodecaborate tetramethylammonium salt with haloalkyl carboxylic acid. Selective removal of the remaining cyanoethyl group upon treatment with tetramethylammonium hydroxide yielded S-carboxyalkyl-thioether-undecahydro-closo-dodecaborate ditetramethylammonium salts. N,N′-dicyclohexylcarbodiimide (DCC) activated amidation of S,S-bis(carboxyalkyl)sulfonium-undecahydro-closo-dodecaborate or S-carboxyalkyl-thioether-undecahydro-closo-dodecaborate tetramethylammonium salts with propargylamine provided the opportunity to install terminal acetylene groups for further conjugation. These compounds acted as powerful building blocks for the synthesis of a broad range of 1,4-disubstituted 1,2,3-triazole products in high yields, utilizing the Cu(I)-mediated click cycloaddition reaction. The synthesis of BSH-lipid with a two-tailed moiety was also achieved, by esterification of S,S-bis(carboxyethyl)sulfoniumundecahydro-closo-dodecaborate(1-) tetramethylammonium salt with 1,2-O-distearoyl-sn-3-glycerol, which may prove useful in the liposomal boron delivery system. The bio-compatibility of the azide-alkyne click reaction was then utilized by performing this reaction in cell culture. The distribution of BSH in HeLa cells could be visualized by treating the cells first with a BSH-alkyne compound and then with Alexa Fluor 488® azide dye. The BSH-dye conjugate, which did not wash out, revealed the distribution of boron in the HeLa cells. Cytotoxicity assays of these BSH derivatives revealed that the synthesized BSH-conjugated triazoles possessed low cytotoxicity in HeLa cancer cells. Of these compounds, BSH conjugated triazole 15 induced a significant increase in the level of boron accumulation in HeLa cells. New building blocks for preparing BSH-bioconjugates are described. The results of this study demonstrate the broad synthetic applications of these new compounds, which may be used to visualize BSH compounds in cancer ce