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

Abstract 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.