4GG R. E. HANDSCHUMACHER AND A. D. WELCH 



concerned with the conversion of folic acid to materials measured as folinic 

 acid, and which is highly susceptible to inhibition by amethopterin in the 

 sensitive but not in the resistant bacterial cells, nevertheless, can be ex- 

 tracted from the latter in an active form which is only slightly less suscepti- 

 ble to inhibition by the drug than the enzyme system extracted from drug- 

 sensitive cells. 72-74 Unfortunately, however, cellfree extracts of leukemic 

 cells have not yet been described which have exhibited suitable activity in 

 the formation of FH 4 from folic acid 72 ■ 74 ; since aqueous extracts of acetone 

 powders and high-speed supernatant fractions of sucrose homogenates of 

 liver have exhibited high folic acid-reductase activity, similar results with 

 neoplastic cells may soon be forthcoming. 



The successful progressive growth in culture of L-5178-Y lymphoblasts 

 of mice and the progressive selection from them of amethopterin-resistant 

 stable mutant strains, the growth of which is resistant to inhibition by up 

 to 100,000 times the concentration of drug which inhibits the parent 

 strain, 75 afford some degree of optimism concerning comparative studies of 

 the possible mechanisms of drug-resistance in mammalian cells using both 

 intact cells and cell-free extracts. 



The data now available are compatible with the concept that amethop- 

 terin-resistant bacterial (and probably mammalian) cells (1) are dependent 

 on a source of folic acid -derived coenzymes, and (2) do not metabolize either 

 folic acid or the antagonist in a manner very significantly different from 

 that of drug-sensitive cells; rather, in some manner not yet clear, the drug- 

 sensitive enzyme system of drug-resistant cells appears to be relatively less 

 accessible to the antagonist, while remaining available to folic acid in the 

 extracellular environment. The nature and locus of the "barrier" between 

 the enzyme and the drug is the subject of much investigation. 



Mention should be made of studies of a condition referred to as amethop- 

 terin-dependence which occurred in a strain of L-1210 leukemia in mice. 66 ' 76 

 Animals inoculated with these cells died of leukemia in a signifi- 

 cantly shorter period of time when given folic acid-antagonists in doses of 

 the usual magnitude, an effect partially prevented by folinic acid. The in- 

 corporation of formate-C 14 into the ribonucleotide of aminoimidazolecar- 

 boxamide, purines, nucleic acids, and other components of tumors which 

 was observed in these cells was unequivocally increased by treatment of 

 the mice with amethopterin. However, formate incorporation was sharply 

 diminished in the analogous components of the normal tissues of the host 

 and in tumors derived from the parent, drug-sensitive L-1210 cells. 77 In 

 studies by another group of a similarly dependent line of L-1210, thepres- 



76 L. W. Law, Proc. Soc. Exptl. Biol. Med. 77, 340 (1951); J. Natl. Cancer Inst. 11, 849 

 (1951;. 



77 H. E. Skipper, L. L. Bennett, Jr., and L. W. Law, Cancer Research 12, 677 (1952). 



