8- (2- Aminoethylthio)guanosine- 3', 5'- cyclic monophosphorothioate, Sp- isomer, immobilized on agarose gel (Sp-8-AET-cGMPS-Agarose)[8- (2- Aminoethylthio)guanosine- 3', 5'- cyclic monophosphorothioate, Sp- isomer, immobilized on agarose gel (Sp-8-AET-cGMPS-Agarose)]