Abstract: A method of selectively causing drug activation within a living body which comprises administering to a living body a polyamino arylmethylnitrile capable of undergoing controlled cleavage to form a polyaminoaryl methyl ion and/or a free cyanide ion, the drug also being soluble in the body serum so that it is capable of passing via oral, intramuscular or intra-peritoneal administration into a body, and then subjecting a localized area of said body to radiation capable of causing cleavage of the cyanide ion from the drug precursor, the resulting ions acting to sensitize the localized area to radiation.