No abstract

A series of 8-substituted analogues of cyclic ADP-4-thioribose (cADPtR, 3), which is a stable equivalent of Ca-mobilizing second messenger cyclic ADP-ribose (cADPR, 1), were designed as potential pharmacological tools for studies on cADPR-modulated Ca signaling pathways. These 8-amino analogue (8-NH-cADPtR, 4), 8-azido analogue (8-N-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-cADPR (2) is a well-known potent antagonist against cADPR-inducing Ca-release, the 4-thioribose congener 8-NH-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-signaling pathway, which can contribute to clarify the structure-agonist/antagonist activity relationship.

Cyclic ADP-carbocyclic-ribose (cADPcR, 3) is a biologically and chemically stable equivalent of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. We became interested in the biological activity of the 7-deaza analogues of cADPcR, i.e., 7-deaza-cADPcR (7) and its 7-bromo derivative, i.e., 7-deaza-7-Br-cADPcR (8), because 7-deazaadenosine is an efficient bioisostere of adenosine. The synthesis of 7 and 8 required us to construct the key N1-carbocyclic-ribosyl-7-deazaadenosine structure. Therefore, we developed a general method for preparing N1-substituted 7-deazaadenosines by condensing a 2,3-disubstituted pyrrole nucleoside with amines. Using this method, we prepared the N1-carbocyclic ribosyl 7-deazaadenosine derivative 10a, from which we then synthesized the target 7-deaza-cADPcR (7) via an Ag(+)-promoted intramolecular condensation to construct the 18-membered pyrophosphate ring structure. The corresponding 7-bromo derivative 8, which was the first analogue of cADPR with a substitution at the 7-position, was similarly synthesized. Biological evaluation for Ca(2+)-mobilizing activity in the sea urchin egg homogenate system indicated that 7-deaza-cADPcR (7) and 7-deaza-7-Br-cADPcR (8) acted as a full agonist and a partial agonist, respectively.

Cyclic adenosine diphosphate-carbocyclic-ribose (cADPcR, 2) is a stable equivalent of cyclic adenosine diphosphate-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. On the basis of the structure-activity relationship of cADPR-related compounds and three-dimensional structural modeling of cADPcR, we designed and synthesized cyclic-ADP-4″α-azidoethyl carbocyclic-ribose (N3-cADPcR, 3) to demonstrate that it has a highly potent Ca(2+)-mobilizing activity (EC50 = 24 nM). N3-cADPcR will be a useful precursor for the preparation of biological tools effective to investigate cADPR-mediated signaling pathways.

No abstract