A COMET ASSAY OF DNA-DAMAGE AND REPAIR IN K562 CELLS AFTER PHOTODYNAMIC THERAPY USING HEMATOPORPHYRIN DERIVATIVE, METHYLENE-BLUE AND MESO-TETRAHYDRO-XYPHENYLCHLORIN

Single-cell electrophoresis (comet assay) has been used to evaluate DN A damage and repair in the human myeloid leukaemia cell line K562 afte r low-dose (predominantly sub-lethal) treatments of hyperthermia and p hotodynamic therapy (PDT). Three different photosensitizers were exami ned: haematoporphyrin derivative (HpD), methylene blue (MB) and meso-t etrahydroxyphenylchlorin (mTHPC). None of the drugs in the absence of light, nor in light alone, resulted in delectable DNA damage. However, a significant amount of DNA damage was detected immediately after tre atment with haematoporphyrin derivative or methylene blue PDT compared with drug-only or light-only treatments; no residual level of DNA dam age was evident for either drug following a 4-h post-treatment incubat ion at 37 degrees C. No significant DNA damage was detected after meso -tetrahydroxyphenylchlorin PDT or hyperthermia either immediately or 4 h after treatment. We conclude that the alkaline comet assay can be a pplied as an effective screening assay for DNA damage induced by a ran ge of laser therapies.