Inhibitors of the Enzyme Purine Nucleoside Phosphorylase as Potential Therapy for Psoriasis

Purine nucleoside phosphorylase (PNP) is one of the enzymes comprising the purine salvage pathway, and is responsible for the catalysis of the reversible phosphorolytic cleavage of purine ribonucleosides and 2-deoxyribonucleosides. The pivotal role of PNP in T-cell proliferation has been demonstrated in patients with inherited PNP deficiency, where T-cell levels may be 1-3percent of normal. This observation helped establish the critical role of PNP in T-cells and provided a rationale for developing inhibitors of PNP. Inhibitors of PNP may be useful for treating a variety of T-cell related autoimmune diseases including psoriasis, rheumatoid arthritis and Crohns disease and T-cell cancers. In this manuscript, the x-ray crystal structure of the PNP enzyme is described. Results of a structure-based drug design program aimed at designing small-molecule inhibitors of PNP are also described. Of the many classes of compounds synthesized, studied and reviewed, only one, the 3-pyridinylmethyl-9-deazaguanine (BCX-34, 39) analog has been used in clinical trials. Both topical and oral formulations of BCX-34 were studied in psoriatic patients and the results of these clinical trials are described.