INHIBITION OF XANTHINE OXIDASE 283 



A new xanthine oxidase competitive inhibitor, 4-hydroxypyTazolo(3,4-d) 

 pyrimidine (allopurinol, Zyloprim), is now being clinically tested in hyper- 

 uricemia and for the potentiation of the antitumor activity of the 6-substi- 

 tuted purines (e.g. 6-mercaptopurine) (Elion et al., 1963; Information for 

 Investigators report from the Burroughs Wellcome Company). It is a 



N 



I 

 H 



4-Hydroxypyrazolo(3, 4-d)pyrimidine 



very potent inhibitor, with K^ = 0.000032 mM, being bound around 100 

 times more tightly to the enzyme than is xanthine, but it is also a substrate 

 for xanthine oxidase and its oxidation product is likewise a potent inhib- 

 itor, with Ki = 0.000054 mM. Mice and dogs given 100 mg/kg intraperi- 

 toneally show an increased urinary excretion of xanthine and hypoxanthine, 

 with a decrease in aUantoin, and in man a similar action has been demon- 

 strated, serum and urinary urate being depressed. It is well tolerated by 

 man at 200-1000 mg/day orally up to several weeks. It apparently has no 

 antitumor activity itself but is able to potentiate the action of 6-mercapto- 

 purine by interfering with its metabolism. Mice given 20 mg/kg intraperi- 

 toneally along with 6-mercaptopurine exhibit a reduced urinary excretion 

 of thiourate. The value of this analog in neoplastic disease and gout is 

 not yet known. 



Inhibition of Uricase by Purine Analogs 



It is appropriate at this time to refer to certain studies on uricase (urate 

 oxidase) before proceeding with the inhibition of xanthine oxidase by the 

 pteridines. Uricase catalyzes the oxidative opening of the pyrimidine ring 

 to form aUantoin. Many methyl and ethyl derivatives of urate were tested 

 by Keilin and Hartree (1936) on an enzyme from pig liver; they found that 

 none is a substrate but that several inhibit quite potently. They considered 

 the mechanism to be competitive and stated, "The fact that the methyl 

 compounds of uric acid, although not oxidizable by the enzyme, inhibit 

 the oxidation of uric acid shows that these methyl compounds react with 

 the same active grouping of the enzyme molecule as uric acid itself." It 

 is very interesting that the 1,3,7-derivative is so much more potent than 

 the 1,3,9-derivative, particularly in view of the greater potency of the 

 monomethyl compounds compared to the latter derivative (the urate was 

 10 mM in all cases) (see tabulation). 2,6,8-Trisubstituted purines were 



