Inhibitors of poly(adenosine diphosphoribose) synthetase, examination of metabolic perturbations, and enhancement of radiation response in Chinese hamster cells.
3-Aminobenzamide, a specific inhibitor of poly(adenosine diphosphoribose) synthesis, has been shown to enhance the response of mammalian cells to ionizing radiation and alkylating agents. Observations such as these usually have been taken to be an indication of the involvement of poly(adenosine diphosphoribose) in the repair of DNA damage. It has been reported that some inhibitors of adenosine diphosphoribosyl transferase (ADPRT) affect cell viability, glucose metabolism, and DNA synthesis when present at low concentrations in the growth medium for extended periods (e.g., lymphoid cells exposed to a few millimolar for 24 h [Milam, K. M., and Cleaver, J. E. Science (Wash. DC), 223: 589, 1984]). The latter report questioned previous interpretations of radiation results based on the use of ADPRT inhibitors which enhance cell killing. We have studied the enhanced radiation lethality of Chinese hamster cells using higher concentrations of these inhibitors, but for shorter periods, in an effort to determine if metabolic perturbations are produced and if they are relatable to enhanced cell killing. The compounds used were 2-aminobenzamide, 3-aminobenzamide, 4-aminobenzamide, benzamide, and nicotinamide, compounds which show a large variation in their potency as inhibitors of ADPRT. It was found that none of the compounds was toxic at the highest doses used (20 mM for 2 h) and that, during a 2-h period, the potent inhibitor 3-aminobenzamide had little or no effect on DNA synthesis. Two h is long enough to yield a near-maximum radiosensitization with 3-aminobenzamide. Although glucose metabolism was affected to varying degrees (up to a 50% inhibition by 4-aminobenzamide in 2 h), there was no correlation between effectiveness in inhibiting ADPRT and effectiveness in inhibiting glucose metabolism. A correlation was observed, however, between the inhibitory potential of ADPRT and the enhancement of radiation response. When used for sufficiently short times, we conclude that the effects at even high concentrations of a potent inhibitor of ADPRT (e.g., 3-aminobenzamide) are consistent with an involvement of poly(adenosine diphosphoribose) synthesis in the expression of a radiation-induced end point like cell killing.