39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 405 



than with amethopterin, 64 have not indicated that the derivatives offer any 

 clinical advantages over the parent compound. 65 



Although much research effort has been expended in search of a defini- 

 tive explanation of the mechanisms of amethopterin-resistance, this goal 

 has not been attained. However, many studies with both bacteria and neo- 

 plastic cells in culture, as well as in mice with transplanted neoplasms, in- 

 dicate that spontaneously appearing, genetically stable, drug-resistant 

 mutant cells are selected by amethopterin, and that the development of a 

 high degree of resistance in a cell population is a stepwise process of selec- 

 tion of the products of successive mutational events. 66 ' 67 Therapy with sub- 

 curative doses of amethopterin in successive mice, inoculated with cells 

 obtained progressively from the drug-treated animals, leads to the gradual 

 selection of amethopterin-resistant strains of leukemia cells. 66 ' 68 ' 69 



Studies with Streptococcus faecalis have indicated that cells which are 

 profoundly resistant to amethopterin (e.g., inhibited only by a 1,000-fold 

 to 3,000,000-fold higher concentration than that required for the parent 

 sensitive strain 70 ) convert folic acid to functional forms (measured as folinic 

 acid) more efficiently (about 100-fold) than do the drug-sensitive cells. 71-73 

 However, this phenomenon per se does not appear to be sufficient to account 

 for the tremendous changes in drug-sensitivity. Indeed, results obtained 

 with several strains of mouse leukemic cells have not indicated that greater 

 efficiency in the utilization of folic acid, or in its conversion to derivatives 

 measured as folinic acid, is a concomitant event in the development of ame- 

 thopterin-resistance. 57, 72, 74 ' 75 In S. faecalis the enzyme system which is 



64 D. P. Rail, A. J. Pallotta, and J. R. Elsea, Proc. Am. Assoc. Cancer Research 3, 

 54 (1959). 



65 E. J. Freireich, M. Lane, and R. K. Shaw, Proc. Am. Assoc. Cancer Research 3, 20 

 (1959). 



66 L. \V. Law, Cancer Research 16, 698 (1956). 



67 A. D. Welch, Cancer Research 19, 359 (1959). 



68 J. H. Burchenal, E. Robinson, S. F. Johnston, and M. N. Kushida, Science 111, 

 116 (1950). 



69 L. W. Law and P. J. Boyle, Proc. Soc. Exptl. Biol. Med. 74, 599 (1950). 



70 J. H. Burchenal, G. B. Waring, and D. J. Hutchison, Proc. Soc. Exptl. Biol. Med. 

 78, 311 (1951). 



71 H. P. Broquist, A. R. Kohler, D. J. Hutchison, and J. H. Burchenal, J . Biol. Chem. 

 202, 59 (1953). 



72 C. A. Nichol and A. D. Welch, in "Antimetabolites and Cancer" (C. P. Rhoads, 

 ed.), p. 63. Am. Assoc. Advancement Sci., Washington, D. C, 1955. 



73 A. Anton and C. A. Nichol, Proc. Am. Assoc. Cancer Research 2, 91 (1956); Bio- 

 chem. Pharmacol. 3, 1 (1959). 



74 C. A. Nichol, in "The Leukemias: Etiology, Pathophysiology, and Treatment" 

 (J. W. Rebuck, F. H. Bethell, and R. W. Monto, eds.), p. 583. Academic Press, 

 New York, 1957. 



75 G. A. Fischer, Ann. N. Y. Acad. Sci. 76, 673 (1958); Cancer Research 19, 372 (1959). 



