Enzyme regulatory site-directed drugs. MOdulation of thymidine triphosphate inhibition of thymidine kinase by 5'-amino-5'-deoxythymidine.

5'-Amino-5'-deoxythymidine (5'-AdThd) was found to antagonize the feedback inhibition exerted by dTTP on dThd kinase (EC 2.7.1.21). This effect was demonstrable in intact cells, cellular extracts, and purified enzyme preparations. Thus, 5'-AdThd markedly stimulated the uptake of dThd in Hela and Vero cells and reduced the inhibitory effects of dTTP on the dThd kinase activity measured in extracts from both cell types. dThd kinase was purified by affinity column chromatography from Hela and Vero cells, and 5'-AdThd was again shown to reduce the inhibition caused by dTTP. The ability of 5'-AdThd to disrupt the homeostatic mechanisms normally regulating the uptake of dThd was sufficient to convert a noncytotoxic concentration of dThd (30 microM) to one that inhibited Hela cell growth by 40%. Since the activity of regulatory enzymes critically influences the pharmacological response produced by many agents, we propose the design of compounds specifically targeted at enzyme regulatory sites as an approach to drug development.