Synthesis and Biological Activity of Pyrazolo[3,4-d]Pyrimidine Nucleosides and Nucleotides Related to Tubercidin, Toyocamycin, and Sangivamycin

The 6-aza analogues of toyocamycin and sangivamycin were prepared as potential cytotoxic agents. The toyocamycin analogue (4-amino-1-(β-d-ribofuranosyl)pyrazolo[3,4-d]pyrimidine-3-carbonitrile) could not be obtained directly from its O-acetylated precursor but was accessible via 4-amino-1-(β-d-ribofuranosyl)pyrazolo[3,4- d]pyrimidine-3-thiocarboxamide. The identity of the nitrile was verified by its ultraviolet, infrared, and mass spectra, and by its conversion to the corresponding 3-carboxamide and thiocarboxamide when treated with water or hydrogen sulfide, respectively. Bioassay of the synthetic compounds in comparison with 4-amino-1-(β-d-ribofuranosyl)pyrazolo[3,4- d]pyrimidine (6-azatubercidin) and 4-amino-2-(β-d-ribofuranosyl) pyrazolo [3,4-d] pyrimidine revealed that the 3-thiocarboxamido derivative was more cytotoxic to the growth of mouse fibroblasts than 6-azatubercidin, effecting killing of 3T6 cells at ≤ 1 μg/ml. 4-Amino-1-(β-d-ribofuranosyl)pyrazolo[3,4-d]pyrimidine (but not its 2-ribofuranosyl isomer) was shown to act as a substrate for adenosine deaminase from calf intestinal mucosa with an apparent K_m of 125 (vs. 20 for adenosine) and the corresponding 5′-diphosphate of 6-azatubercidin was polymerized by polynucleotide phosphorylase (Micrococcus luteus) in the presence of Mn²⁺ to afford a homopolymer and copolymers with adenosine. The copolymers directed the binding of [³H]lysyl-tRNA to the A-site of ribosomes from Escherichia coli, but could not be used for the synthesis of polylysine in a cellfree system. The copolymer consisting of adenosine and 6-azatubercidin in a 2:1 ratio was found to form a 1:1 complex with poly(uridylic acid) at 4 °C.