Methylene blue but not indigo carmine causes DNA damage to colonocytes in vitro and in vivo at concentrations used in clinical chromoendoscopy

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Abstract
Methylene blue, which, unlike indigo carmine, is taken up by cells, induces cellular DNA damage in vitro via the generation of singlet oxygen when photoexcited by white light.4 In contrast, indigo carmine appears to be photostable and to possess little potential to damage genetic material in vitro.5,6 A recent clinical study has shown that the extent of DNA damage (particularly oxidative DNA damage) in human oesophageal cells is increased after methylene blue chromoendoscopy.7 Additional iatrogenic oxidative DNA damage to epithelial cells is of particular concern in such precancerous tissue because of the association between oxidative DNA damage, mutagenesis and the development of malignancy.8 We hypothesised that indigo carmine would induce less DNA damage than methylene blue both in vitro in cultured colon cells during simulated chromoendoscopy conditions and in vivo in colonic biopsy samples collected at chromoendoscopy.