Aberrant Cyclization Affords a C-6 Modified Cyclic Adenosine 5′-Diphosphoribose Analogue with Biological Activity in Jurkat T Cells

Two nicotinamide adenine dinucleotide (NAD⁺) analogues modified at the 6 position of the purine ring were synthesized, and their substrate properties toward <i>Aplysia californica</i> ADP-ribosyl cyclase were investigated. 6-<i>N</i>-Methyl NAD⁺ (6-<i>N</i>-methyl nicotinamide adenosine 5′-dinucleotide <b>10</b>) hydrolyzes to give the linear 6-<i>N</i>-methyl ADPR (adenosine 5′-diphosphoribose, <b>11</b>), whereas 6-thio NHD⁺ (nicotinamide 6-mercaptopurine 5′-dinucleotide, <b>17</b>) generates a cyclic dinucleotide. Surprisingly, NMR correlation spectra confirm this compound to be the <i>N</i>1 cyclic product 6-thio <i>N</i>1-cIDPR (6-thio cyclic inosine 5′-diphosphoribose, <b>3</b>), although the corresponding 6-oxo analogue is well-known to cyclize at <i>N</i>7. In Jurkat T cells, unlike the parent cyclic inosine 5′-diphosphoribose <i>N</i>1-cIDPR <b>2</b>, 6-thio <i>N</i>1-cIDPR antagonizes both cADPR- and <i>N</i>1-cIDPR-induced Ca²⁺ release but possesses weak agonist activity at higher concentration. <b>3</b> is thus identified as the first C-6 modified cADPR (cyclic adenosine 5′-diphosphoribose) analogue antagonist; it represents the first example of a fluorescent <i>N</i>1-cyclized cADPR analogue and is a new pharmacological tool for intervention in the cADPR pathway of cellular signaling