Comparative in vitro and in vivo assessment of genotoxic effects of etoposide and chlorothalonil by the comet assay

The alkaline single cell gel electrophoresis (comet) assay was used to asse ss in vitro and in vivo genotoxicity of etoposide, a topoisomerase II inhib itor known to induce DNA strand breaks, and chlorothalonil, a fungicide wid ely used in agriculture. For in vivo studies, rats were sacrificed at vario us times after treatment and the induction of DNA strand breaks was assesse d in whole blood, bone marrow, thymus, liver, kidney cortex and in the dist al part of the intestine. One hour after injection, etoposide induced DNA d amage in all organs studied except kidney, especially in bone marrow, thymu s (presence of HDC) and whole blood. As observed during in vitro comet assa y on Chinese hamster ovary (CHO) cells, dose- and time-dependent DNA effect s occurred in vivo with a complete disappearance of damage 24 h after admin istration. Even though apoptotic cells were detected in vitro 48 h after ce ll exposure to etoposide, such a result was not found in vivo. After chloro thalonil treatment, no DNA strand breaks were observed in rat organs wherea s a clear dose-related DNA damage was observed in vitro. The discrepancy be tween in vivo and in vitro models could be explained by metabolic and mecha nistic reasons. Our results show that the in vivo comet assay is able to de tect the target organs of etoposide and suggest that chlorothalonil is devo id of appreciable in vivo genotoxic activity under the protocol used. (C) 1 999 Elsevier Science B.V. All rights reserved.