39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 499 



o 



II II 



HN C— F NH C— F 



0=C C— H 0=C C— COOH 



H H 



(XIII) (XIV) 



5-Fluorouracil 5-Fluoroorotic acid 



C 14 of 5-fluoroorotic acid-2-C 14 and 5-fluorouracil-2-C 14 , compounds of great 

 value in metabolic studies with these antimetabolites. Alternatively, 5-flu- 

 orouracil could be obtained by condensing the potassium enolate of the 

 ethyl ester of 2-fluoro-3-oxopropionic acid with S-ethylisothiouronium 

 bromide, followed by hydrolysis of the ethylthiol grouping. 



Several important concepts were involved in the development of the 

 5-fluoropyrimidine derivatives. 367 First, the fluorine atom bears close simi- 

 larity to the H-atom in its physical and certain chemical properties, since it 



o o 



has an atomic radius of 1.35 A., as compared to that of 1.20 A. for H, and 

 its substitution for hydrogen in certain biologically important compounds 

 has given rise to antagonistic activities. Second, a substitution in position 5 

 of the appropriate uracil derivative would block the addition of a "1-C" 

 unit via a folic acid-derived coenzyme and perhaps would provide thereby 

 an inhibitor of the synthesis of thymine. Finally, from the standpoint of 

 potential usefulness as a therapeutic agent in the treatment of cancer, ob- 

 servations relative to the differences in the ability of certain normal and 

 neoplastic tissues to utilize uracil, as discussed earlier, served as a major 

 motivation for the investigation of this compound. 



Preliminary screening of 5-fluoropyrimidines against a number of micro- 

 organisms revealed a wide spectrum of inhibitory activities for fluorouracil, 

 while other fluoro derivatives were considerably less active. 367, 368 The in- 

 hibitory activity of fluorouracil could be reversed best by thymidine in a 

 noncompetitive manner or less well by uracil or cytosine. Both fluorouracil 

 and 5-fluoroorotic caused marked inhibition in the rate of growth of a vari- 

 ety of experimental tumors in mice; however, 5-fluorocytosine, 2-meth- 

 ylthio-5-fluorouracil and 2-thio-5-fluorouracil were without such activ- 

 ity. 367,369 That the inhibition produced by fluorouracil is the result of a 

 specific effect on nucleic acid metabolism was demonstrated by experi- 

 ments in which a profound depression of the incorporation of formate-C 14 



367 q Heidelberger, N. K. Chaudhuri, P. Danneberg, I). Mooren, L. Griesbaeh, R. 

 Duschinsky, R. J. Schnitzer, E. Pleven, and J. Scheiner, Nature 179, 663 (1957). 



368 J. Scheiner, E. Kostelak, and R. Duschinsky, Federation Proc. 16, 242 (1957). 



369 C. Heidelberger, L. Griesbach, B. J. Montag, P. Mooren, O. Cruz, R. J. Schnitzer, 

 and E. Grunberg, Cancer Research 18, 305 (1958). 



