26 



NATURE, FORMATION, AND ACTIVITIES 



mycetes from f^oil, growing- them in li(iuid 

 media, and testing the fihrates for activity 

 against experimental tumors in mice. The 

 test systems can be soUd tumors, ascitic 

 forms, or blood tumors. The results can be 

 evaluated in terms of reduction of tumor 

 sizes, weights of animals, number of tumor 

 cells in the ascitic fluid, or even the pro- 

 longation of the life of the treated animals 

 in comparison with the respective conditions 

 of the untreated controls. 



To save time and animals, efforts have 

 been made to elaborate preliminary screen- 

 ing programs in n'tro. \'ari()us lines of reason- 

 ing have been followed: 



1. Cancer cells are rapidly dividing cells. 

 In the search for antimitotic agents, the in- 

 hibition of mitosis in onion root tips has been 

 investigated. H<)we^'er, this method gives no 

 information about the selective action of the 

 antimitotic agents found. 



2. Cancer cells ha\'e active dehydrogenase 

 systems. One can put such cells in ritro in 

 presence of an oxidation-reduction indicator 

 such as methylene blue. If the cells are killed 

 in vitro they will be unable to reduce meth- 

 ylene l)lue to the leuco form and they will 

 be stained blue. Here again, the method is 

 not selective, but it can be most useful for 

 assay purposes. 



8. Cancer cells ha\-e an impaired respira- 

 tion mechanism. One can thus select for 

 further studies substances which would ha\'e 

 a selective antifermentative action or sub- 

 stances that would be selectively active 

 against microbial mutants which would also 

 ha^'e an impaired respiration. These micro- 

 bial mutants can be considered as the 

 e(|uivalent of cancer cells in the microbial 

 work, (lood correlation has been shown be- 

 tween antitumor activity and activity 

 against such mutants. This subject was re- 

 viewed Iw Clause in 1959 (Udintzev ct a/., 

 1959). Specific studies on different actino- 

 mycins were reported by Hackmann and 

 Schmidt-Kastner (1957). 



The screening procedui'c imoh'ing tests 



for antitumor activity can be carried out by 

 the cylinder plate method with carcinoma 

 cells and a dye such as 2,()-dichlorophenol- 

 indophenol. This method showed, for ex- 

 ample, that Ehrlich carcinoma cells were 

 about 10 times more sensitive to sarkomycin 

 than were cells of Yoshida sarcoma, upon 

 which sarkomycin was almost inactive. 

 Testing of the toxicity on HeLa cells in 

 tissue culture was also found to be useful for 

 the screening and extraction of the active 

 agent. There were substances which were 

 destructive to HeLa cells but not to Ehrlich 

 carcinoma and chicken em})ryo cells, as 

 shown in Tables 3 to 5 (Umezawa, 1956). 



Perlman et al. (1959) made a study of 

 effects of antibiotics on multiplication of L 

 cells of mouse fibroblasts in suspension 

 culture. They reported positive inhibition of 

 multiplication with less than 1 /xg per ml of 

 actinomycins B, By , or Diy ; of cyclo- 

 heximide, gliotoxin, hygromycin, and mito- 

 mycin C; of clavacin, thiostrepton, and 

 xanthomycin. Actinomycin By was most 

 active, with 1.5 Mg PPI" n^l causing a 50 

 per cent reduction in growth of culture. It 

 was suggested that this method can be used 

 in determining the presence of cytotoxic 

 microbial products. 



Once an organism with desirable prop- 

 erties, as shown by the screening test used, 

 has been isolated, a series of investigations 

 must be initiated to e\'aluate the true po- 

 tentialities of the antitumor substance. 

 Only a few compounds have found practical 

 application. The others either were never 

 described or were described purely because 

 of their academic interest. Some compounds, 

 such as mitomycin, off'ered great promise 

 but were found to be of little practical value 

 because of their high toxicity. Others, like 

 actinomycin, seemed to be stillborn but were 

 only ahead of their time. Still others, like 

 puromvcin, sarkomycin, carzinophilin, aza- 

 serine, and DON, proxcd to l)e of limited 

 value. 



In discussing the problem of anticancer 



