Inhibitors of purine nucleoside phosphorylase

Abstract

Purine nucleoside phosphorylase (PNP) catalyses the reversible phosphorolysis of purine ribonucleosides and 2′-deoxyribonu-cleosides to the free base and ribose-1-phosphate or 2′-deoxyribose-1-phosphate. Mammalian PNP is specific for guanosine, inosine and certain analogues, although PNPs from other organisms show varying levels of specificity. Interest in PNP arises primarily from its critical role in purine nucleoside metabolism and in T-cell function, which indicates that inhibitors of this enzyme might be useful in the treatment of T-cell proliferative diseases, such as T-cell leukaemias and lymphomas, in the suppression of host-vs-graft response in organ transplant patients, and in the treatment of T-cell mediated autoimmune diseases. They may also be effective in the treatment of gout and certain parasitic diseases such as malaria. Most potent inhibitors of this enzyme are derivatives of guanine or 8-aminoguanine, including 9-arylmethyl derivatives. Of these, 9-benzylguanines substituted at position 2 or 3 of the phenyl ring by a side chain terminating in a phosphonate moiety are the most potent, with K_i in the 1 nM range, although these compounds have limited potential as drugs because of their very low cell permeability. As a result, inhibitors of this type that have been tested are effective in whole cells only at concentrations of 100 μM or higher. On the other hand, the three-dimensional structure of human PNP, determined by X-ray crystallography, has been used in designing novel inhibitors of this key enzyme, resulting in several families of membrane-permeable inhibitors with IC_50^S in the 6 to 30 nM range. The inhibition is competitive with respect to both inosine and phosphate. 9-(3-pyridylmethyl)-9-deazaguanine (BCX-34) was chosen from this group of inhibitors for further study. Its K_i for the inhibition of human erythrocytic PNP is 31 nM, and its IC₅₀ for inhibition of the proliferation of T-cells (CCRF-CEM) is 0.8 μM in the presence of 2′-deoxyguanosine, which alone has no effect on cells. It did not inhibit the proliferation of B-cells (MGL-8) up to 30 μM. Phase I/II clinical studies of a dermal formulation of BCX-34 have shown this drug to be efficacious and safe in the treatment of cutaneous T-cell lymphoma and psoriasis.