Abstract
INTRODUCTION: Low dose-rate radioimmunotherapy (RIT) using (125)I-labelled monoclonal antibodies ((125)I-mAbs) is associated with unexpected high cytotoxicity per Gy. METHODS: We investigated whether this hypersensitivity was due to lack of detection of DNA damage by the targeted cells. DNA damage was measured with the alkaline comet assay, gamma-H2AX foci and the micronucleus test in p53(-/-) and p53(+/+) HCT116 cells exposed to increasing activities of internalizing anti-HER1 (125)I-mAbs or non-internalizing anti-CEA (125)I-mAbs. The expression of proteins involved in radiation response and progression of cells through the cycle were determined. RESULTS: Cell hypersensitivity to low absorbed doses of anti-CEA (125)I-mAbs was not due to defect in DNA damage detection, since ATM (ataxia telangiectasia mutated gene), gamma-H2AX, p53 and p21 were activated in RIT-treated HCT116 cells and G2/M cell cycle arrest was observed. Moreover, the alkaline comet assay showed that DNA breaks accumulated when cells were placed at 4°C during exposure but were repaired under standard RIT conditions (37°C), suggesting that lesions detected under alkaline conditions (mostly DNA single strand breaks and alkali-labile sites) are efficiently repaired in treated cells. The level of gamma-H2AX protein corroborated by the level of foci measured in nuclei of treated cells was shown to accumulate with time thereby suggesting the continuous presence of DNA double strand breaks. This was accompanied by the formation of micronuclei. CONCLUSION: Hypersensitivity to non-internalizing (125)I-mAbs is not due to lack of detection of DNA damage after low absorbed dose-rates. However, DNA double strand breaks accumulate in cells exposed both to internalizing and non-internalizing (125)I-mAbs and lead to micronuclei formation. These results suggest impairment in DNA double strand breaks repair after low absorbed doses of (125)I-mAbs.