Inhibition of radiation dose-rate-sparing effects in human tumor cells by 3-aminobenzamide.

The capacity of two recently established human tumor cell lines (a germ cell carcinoma of the testis and a squamous cell cervical carcinoma) to repair radiation damage has been determined by assessing in vitro cell survival over the dose-rate range of 150-1.6 cGy/minute. Survival after irradiation at a high dose rate (150 cGy/min) showed that the cervical carcinoma was more radioresistant. A dose-sparing effect was observed at low dose rates (3.2 and 1.6 cGy/min), compared to the high dose rate, to a similar extent in both cell lines. In addition, 3-aminobenzamide added 2 hours prior to and removed 18 hours after irradiation caused a small but similar increase in cell kill with the high dose rate in both cell lines. It also steepened the survival curves at low dose rates. Since the survival curves obtained at 1.6 and 3.2 cGy/minute help to define a dominant component of the initial slope of the acute curve, we conclude that 3-aminobenzamide can act as a modifier of this component.