Introduction of a methyl group(s) into N and/or N′ nitrogen of the guanido group of 2-mercaptoethylguanidine (MEG) significantly increased acute toxicity in mice of the mother compound. When compared on equimolar bases in some narrow ranges, N-methyl, N′-methyl, and N′,N″-dimethyl derivatives showed a same magnitude of prophylactic action against x-irradiation in mice as MEG did. However, the radioprophylactic range of these compounds was obviously narrower than that of MEG. A derivative having three methyl groups at N, N′, and N″ was most toxic and no more radioprotective. Loss of reproductive integrity of x-irradiated HeLa S3 cells was prevented by preincubation of the cells in a medium containing 20-30 mM MEG. On the other hand, proliferation of the cells was markedly inhibited by MEG itself, when the compound was present in the medium for a long period even in a concentration as low as 0.1 mM. The derivatives of MEG also protected HeLa cells against x-rays, but to much less extents than MEG did. In addition, all these MEG derivatives were likewise less potent than MEG with respect to the inhibitory action on proliferation of HeLa cells.

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