Purine nucleoside phosphorylase (PNP) is a purine-metabolizing enzyme in the purine cascade and has been a target for drug design for sometime. A series of potent human PNP inhibitors, pyrrolo[3,2-d]pyrimidines (9-deazaguanines), has been synthesized and evaluated in the enzyme assay and in the cell line assay using MOLT-4 (T-cell) and MGL-8 (B-cell) lymphoblasts for selectivity. One of the compounds, 2,6-diamino-3,5- dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]pyrimidine-4-one (11c; CI-972), was found to be moderately potent, competitive, and reversible inhibitor of PNP with Ki = 0.83 microM. It was also found to be selectively cytotoxic to MOLT-4 lymphoblasts (IC50 = 3.0 microM) but not to MGL-8 lymphoblasts and was evaluated further. Compound 11c (CI-972) is under development in the clinic.