J. S. MITCHELL 



and 60 per cent inhibition at the sixteenth passage. It appears from these 

 experiments that initial treatment of the cultures leads to a persistence of 

 radio-sensitivity in the subcultures. To assess the part played by retained 

 Synkavit in the cultures, Mrs. Simon-Reuss has carried out independent 

 experiments in which it has been found that radio-sensitization persists 

 down to a concentration between 10"^ and 10" i« M Synkavit. Some experi- 

 ments are in progress with ^*C labelled Synkavit. 



Possible mechanism of radio-sensitization 



Compound I appears to be selectively concentrated by the tumour cells with 

 a high differential absorption ratio. The phosphate groups are probably 

 essential for its passage through the cell membranes. Inside the tumour cell 

 it appears to be concentrated in the perinuclear region perhaps in the 

 mitochondria. The parent 2-methyl-l : 4-naphthoquinone is likely to be 

 formed in situ and react specifically with sulph)dryl compounds. In this 

 way, selective radio-sensitization of the tumour cells by Compound I is 

 envisaged as the converse of — SH protection against ionizing radiations. 

 Then increase in the oxygen tension in the tumour cells is likely to be 

 associated with optimum use of the available oxygen for increasing the 

 effects of the radiation. It is of interest that in the measurements by Drs. 

 Cater and Phillips of oxidation-reduction potentials in vivo in the Walker 

 rat carcinoma it was found when the animal breathed oxygen the potential 

 taken up by the platinum electrode relative to the silver-silver chloride 

 electrode increased by 10-50mV, while intravenous injection of Synkavit 

 (5-lOmg per 200g rat) caused an immediate fall of 30 to 200 mV followed 

 by gradual recovery. 



The observation that the mitotic inhibition produced in chick fibroblast 

 cultures by Compound I is abolished by an equimolecular concentration of 

 guanosine, but not by isoguanine riboside, guanylic acid, guanine or any 

 other related compound suggests a chemical specificity in the action of Com- 

 pound I. It is not impossible that like some 2-OH-3-alkyl-naphthoquinones 

 (Ball, Anfinsen and Cooper^^, 1947, see Potter and Reif'o, 1952), it pre- 

 vents the reduction of cytochrome C and interferes with some process 

 of phosphorylation. A problem is raised by the observation that tetra- 

 sodium 2 : 3-dimethyl-l : 4-naphthohydroquinone diphosphate, Compound 

 XXVIII (Mitchell and Simon-ReusV- ^o, 1952, and Mitchell^, 1953) is 

 a good, radio-sensitizer for both chick fibroblast cultures and the Walker 

 rat carcinoma and does not react in vitro with sulphydryl compounds. 



Animal experiments 



The remarkably low acute and chronic toxicity of Compound I has been 

 confirmed in experiments on rats, mice and rabbits. In rats, the repeated 

 intramuscular administration of the compound in large doses increased the 

 mortality following exposure of the whole body to X-radiation. Experi- 

 ments by Jolles^* (1952) in the rabbit and guinea-pig suggests species 

 differences. Further experiments are in progress. It has recently been found 

 that using similar methods 2-ethylenimino-l : 4-naphthoquinone shows no 

 acute toxic effects after intravenous administration in doses in the region of 

 2-5 mg per kilo in rats, mice and rabbits. 



173 



