Inhibitors of human purine nucleoside phosphorylase. Synthesis, purine nucleoside phosphorylase inhibition, and T-cell cytotoxicity of 2,5-diaminothiazolo5,4-dpyrimidin-7(6H)-one and 2,5-diaminothiazolo4,5-dpyrimidin-7(6H)-one. Two thioisosteres of 8-aminoguanine