39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 485 



The ribonucleoside, thioguanosine, has been synthesized 220 and found in 

 most respects to resemble thioguanine as an inhibitor of the growth of 

 sarcoma-180 and other mammalian cells in tissue culture. However, pre- 

 liminary clinical reports 221 have suggested that the ribonucleoside, at dose 

 levels considerably lower than those required for thioguanine, has some 

 value in the treatment of leukemia, a circumstance which implies that the 

 derivative either has greater potency or is more favorably absorbed, dis- 

 tributed, and excreted than is the free base. 



Recently, more detailed studies of the mechanism of action of thio- 

 guanine in mammalian systems have been described. 222 These studies were 

 facilitated by the use of thioguanine-C 14 and by the fact that this analog 

 and its various derivatives have absorption maxima in the range of 320 to 

 360 m,u. Following intraperitoneal injection of the analog into mice bearing 

 Ehrlich ascites tumor cells, and sacrifice of the animals after 45 minutes, 

 the majority of the compound in the tumor tissue could be recovered as 

 thioguanosine-5'-phosphate. The ribonucleotide was also prepared by con- 

 densation of thioguanine with pyrophosphorylribose-5-phosphate; this re- 

 action was catalyzed by a dialyzed extract (obtained from an acetone pow- 

 der prepared from Ehrlich ascites cells) in the presence of Tris buffer, Mg ++ , 

 and inorganic pyrophosphatase. Small amounts of what appeared to be di- 

 and triphosphates of thioguanosine also were present in acid-soluble ex- 

 tracts of tumors, but their identity has not been established unequivocally. 

 Other metabolites isolated from tumor cells exposed to thioguanine in vivo 

 included small amounts of thioguanine ribonucleoside and thiouric acid 

 (6-mercapto-2,8-dihydroxypurine). When thioguanine or thioxanthine 

 (2-hydroxy-6-mercaptopurine) were incubated with a partially purified 

 preparation of xanthine oxidase, thioxanthine was readily converted to 

 thiouric acid; however, thioguanine was not affected by the enzyme, a find- 

 ing which indicates the necessity for deamination (of position 2) before 

 oxidation in position 8 will occur. In contrast to tumor cells, extracts ob- 

 tained at the same time from the intestine contained considerably less thio- 

 guanylic acid and more thiouric acid. It is of interest that pretreatment of 

 the animals with azaserine, which has been shown to deplete the pools of 

 acid-soluble purine ribonucleotides, 151 resulted in an enhanced conversion 

 of thioguanine to the ribonucleotides in tumor cells; however, in the intes- 

 tine, in which azaserine at this particular dosage level had little effect on 

 the pools of purine-containing metabolites, ribose-containing derivatives of 

 thioguanine did not accumulate. This finding resembles the compensatory 



220 J. J. Fox, I. Wempen, A. Hampton, and I. L. Doerr, J. Am. ('hem. Soc. 80, 1669 

 (1958). • 



221 I. H. Krakoff, R. R. Ellison, and C. T. Tan, Proc. Am. Assoc. Cancer Research 3, 

 34 (1959). 



222 E. C. Moore and G. A. LePage, Cancer Research 18, 1075 (1958). 



