Hydrophilically enhanced 3-carboranyl thymidine analogues (3CTAs) for boron neutron capture therapy (BNCT) of cancer q Sureshbabu Narayanasamy, a,  B. T. S. Thirumamagal, a,  Jayaseharan Johnsamuel, a,  Youngjoo Byun, a Ashraf S. Al-Madhoun, b Elena Usova, c Guirec Y. Cosquer, a Junhua Yan, a Achintya K. Bandyopadhyaya, a Rohit Tiwari, a Staffan Eriksson c and Werner Tjarks a, * a College of Pharmacy, The Ohio State University, Columbus, OH 43210, USA b Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ont., Canada c Department of Molecular Biosciences, Division of Veterinary Medical Biochemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden Received 4 May 2006; revised 16 June 2006; accepted 19 June 2006 Available online 10 July 2006 Abstract—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 lM. 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. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Boron neutron capture therapy (BNCT) is a binary can- cer treatment modality, which relies both on a chemical and a radiation component. 1 In BNCT, a neutron cap- ture reaction occurs when a stable 10 B isotope is irradi- ated with low energy thermal neutrons to produce highly cytotoxic particle radiation in the form of helium (a-particle) and lithium nuclei, both of which mediate cell death primarily by the induction of DNA double strand breaks. 1,2 Thermal neutrons and 10 B per se are harmless for human tissue and the helium and lithium nuclei have limited destructive ranges of approximately one cell diameter (<10 lm) in biological tissue, which restricts damage to cells that have been targeted with 10 B. 1 Criteria for successful BNCT agents have been discussed in a recent review. 1 They are: (1) selective targeting of tumor versus normal cells, preferably with intracellular localization; (2) attaining cellular concentrations of 10 9 10 B atoms/cell or 15 lg 10 B/g tumor; (3) persist- ing at a relatively constant concentration in the tumor during neutron radiation in order to deliver a constant radiation dose to the tumor; and (4) being sufficiently non-toxic to attain adequate in vivo tumor boron concentrations. Boron-containing nucleosides, in particular those that target human thymidine kinase 1 (hTK1), have the potential to fulfill these criteria. 3–6 Human TK1 is a cytosolic deoxynucleoside kinase of the salvage path- ways of nucleic acid synthesis and its function is 5 0 -monophosphorylation of thymidine (Thd) and 2 0 -de- oxyuridine (dUrd). 7 The activity of this enzyme is very low in G 1 and G 0 cells, increases at G 1 /early S boundary, Bioorganic & Medicinal Chemistry 14 (2006) 6886–6899 0968-0896/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmc.2006.06.039 Keywords: 3-Carboranyl thymidine analogues (3CTAs); Boron neutron capture therapy (BNCT); Thymidine kinase 1 (TK1). q The results described in this paper were partially presented at the XVI International Roundtable (IRT) on Nucleosides, Nucleotides, and Nucleic Acids, September 12–16, 2004, Minneapolis, MN, USA. * Corresponding author. Tel.: +1 614 292 7624; fax: +1 614 292 2435; e-mail: tjarks.1@osu.edu   S.N., B.T.S.T., and J.J. made equal contributions to this paper.