Inhibition of hypoxanthine-guanine phosphoribosyl transferase

The affinities of eighteen purines or purine analogs for human erythrocytic hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8; HGPRTase) were compared to assess the feasibility of obtaining active inhibitors of the enzyme. Three compounds appeared to inhibit the utilization of hypoxanthine by L5178Y cells in vitro due to inhibition of the enzyme rather than depletion of the intracellular 5-phosphoribosyl-1-pyrophosphate pool. The three competitive inhibitors and their affinity constants ( K i ) using 6-mercaptopurine as substrate were: 6-mercapto-9-(tetrahydro-2-furyl)-purine, 37 μM; 2,6-bis-(hydroxyamino)-9-β- d-ribofuranosyl-purine, 12 μM; and 6-iodo-9-(tetrahydro-2-furyl)-purine, 108 μM. The KIn m for 6-mercaptopurine was 9 μM. Thus, the enzyme tolerates bulky substitution at N 9. 6-Mercapto-9-(tetrahydro-2-furyl)-purine also potentiated the chemotherapeutic effect of azaserine, an inhibitor of de novo purine biosynthesis, in L5178Y ascites tumor-bearing mice. Four 2-substituted, oxazolo-[5, 4- d]-pyrimidine-7-ones and 2-methylthiazolo-[5, 4- d]-pyrimidine-7-one had K i values in the range of 84–173 μM. Consequently, isosteric substitution at N 9 may also be a fruitful and logical course to pursue in the design and synthesis of more potent inhibitors of this important enzyme.