A pyrimidine analogue and purine nucleoside phosphorylase inhibitor with immunosuppressive and antineoplastic properties. Peldesine inhibits purine nucleoside phosphorylase (PNP) that plays a pivotal role in T-cell proliferation and is responsible for the catalysis of the reversible phosphorolytic cleavage of purine ribonucleosides and 2'-deoxyribonucleosides. Inhibition of PNP results in accumulation of dGTP and the subsequent failure of DNA synthesis. This agent maybe used in T-cell related autoimmune diseases including psoriasis, rheumatoid arthritis and Crohn s disease and T-cell cancers