Radiotoxicity of Intracellular ⁶⁷Ga, ¹²⁵I and ³H

Summary

L1210 leukaemia cells were labelled with various doses of ⁶⁷Ga-citrate, ³H-thymidine, or ¹²⁵I-iododeoxyuridine to evaluate the cytocidal effects of the intracellular decay of the three radionuclides. Based on radioisotope incorporation data, cellular dimensions, and intracellular radioisotope distributions (³H and ¹²⁵I intranuclear, ⁶⁷Ga cytoplasmic) the rates of deposition of cellular, nuclear, and cytoplasmic energy were calculated.

In terms of energy absorption/cell, ⁶⁷Ga (LD₅₀: 2250 keV/hr; 69 rad/hr) was much less toxic than either ³H (LD₅₀: 325 keV/hr; 10 rad/hr) or ¹²⁵I (LD₅₀: 50 keV/hr; 1·5 rad/hr). In terms of energy absorption/nucleus, ⁶⁷Ga and ³H produced almost identical effects (LD₅₀: 230 versus 255 keV/hr; 22·2 versus 24·6 rad/hr), but ¹²⁵I remained much more toxic (LD₅₀: 40 keV/hr; 3·9 rad/hr).

These findings indicate that, although decay by electron capture in the cell nucleus (¹²⁵I) is highly destructive, the same type of decay occurring in the cytoplasm (⁶⁷Ga) is ineffective in killing L1210 cells. An analysis of the data suggests that the cytotoxic effects of the three radioisotopes result exclusively from nuclear damage. Cytoplasmic absorption of radiation energy appears to contribute little, if anything, to the lethal effects of ionizing radiations.