Preparation and Biological Evaluation of ¹⁰B-Enriched 3-[5-{2-(2,3-Dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl]thymidine (N5-2OH), a New Boron Delivery Agent for Boron Neutron Capture Therapy of Brain Tumors

3-[5-{2-(2,3-Dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl]thymidine (compound 1, N5-2OH) belongs to a novel class of boron delivery agents for neutron capture therapy, which was designated 3-carboranylthymidine analogue (3CTAs). Two shorter and more convenient synthetic routes were developed for the synthesis of 1 in the ¹⁰B-enriched form, which is necessary for its preclinical and clinical evaluation in neutron irradiation studies. For more insight on structure−activity relationships, various stereochemical and geometrical isomers of 1 were synthesized and their specificities as substrate for human thymidine kinase 1 (hTK1) were evaluated. A computational model for the binding of various isomers of 1 to the active site of hTK1 was developed. Preliminary studies carried out in F98 glioma bearing rats that had received a ¹⁰B-enriched form of 1 followed by neutron irradiation demonstrated a significant prolongation in survival times compared to control animals, suggesting that further studies are warranted to evaluate the therapeutic potential of 1.