Radiation toxicity of indium-111-2-mercaptopyridine-N-oxide on HeLa S-3 cells

As one of a series of systematic studies to determine the cytotoxicity of radiopharmaceuticals, indium-111-2-mercaptopyridine-N-oxide (111In-merc) was studied using a monolayer of HeLa S-3 cells. Exponentially growing cells were incubated with up to 2.96 MBq/ml (80 microCi/ml) of 111In-merc (molecular ratio of In/Cd to merc was 1:1000) in phosphate buffered saline for up to 240 min at room temperature. Surviving fractions were assayed by the colony-forming method. The cell uptake curve was upward concave with increasing activity and was saturated after an incubation time of 60 min. A clonogenic assay resulted in an exponential dose-survival curve with a shoulder (D0 of 0.44 MBq/ml (12 microCi/ml); n = 5), and an upward concave time-survival relationship. Merc alone and decayed In-merc were non-toxic at the equivalent range, substantiating radiation-induced cytotoxicity of 111In-merc.