evaluated as potentialinhibitors of purinenucleosidephosphorylase (PNP) isolated from peripheral blood mononuclear cells (PBMCs) and cell lines of myeloid and lymphoid origin. Two compounds, (S)-3-(guanin-9-yl)pyrrolidin-1-N-ylcarbonylphosphonic acid (2a) and (3S,4R)-4-(guanin-9-yl)-3-hydroxypyrrolidin-1-N-ylcarbonylphosphonic acid (6a), were recognized as nanomolar competitive inhibitors of PNP isolated