Synthesis of 8-Substituted Analogues of Cyclic ADP-4-Thioribose and Their Unexpected Identification as Ca 2+ -Mobilizing Full Agonists.

A series of 8-substituted analogues of cyclic ADP-4-thioribose (cADPtR, 3), which is a stable equivalent of Ca ²⁺ -mobilizing second messenger cyclic ADP-ribose (cADPR, 1), were designed as potential pharmacological tools for studies on cADPR-modulated Ca ²⁺ signaling pathways. These 8-amino analogue (8-NH ₂ -cADPtR, 4), 8-azido analogue (8-N ₃ -cADPtR, 5), and 8-chloro analogue (8-Cl-cADPtR, 6) were efficiently synthesized, where the stereoselective N1-β-thioribosyladenine ring closure reaction via an α/β-equilibrium of the 1-aminothioribose derivative and construction of the characteristic 18-membered pyrophosphate ring by Ag ⁺ -promoted activation of a phenyl phosphorothioate type substrate were the two key steps. Although 8-NH ₂ -cADPR (2) is a well-known potent antagonist against cADPR-inducing Ca ²⁺ -release, the 4-thioribose congener 8-NH ₂ -cADPtR turned out unexpectedly to be a full agonist in sea urchin egg homogenate evaluation system. This important finding suggested that the ring-oxygen in the N1-ribose of cADPR analogues is essential for the antagonistic activity in the Ca ²⁺ -signaling pathway, which can contribute to clarify the structure-agonist/antagonist activity relationship.