The hydrochloride salt of the synthetic high-affinity transition-state analogue forodesine. Forodesine binds preferentially to and inhibits purine nucleotide phosphorylase (PNP), resulting in the accumulation of deoxyguanosine triphosphate and the subsequent inhibition of the enzyme ribonucleoside diphosphate reductase and DNA synthesis. This agent selectively causes apoptosis in stimulated or malignant T-lymphocytes. A transition state analogue is a substrate designed to mimic the properties or the geometry of the transition state of reaction.