The induction of apoptosis by a positively charged methylene blue derivative

Identifying the cellular responses to photodynamic therapy (PDT) is important if the mechanisms of cell death are to be fully understood. PDT with a methylene blue analog DO15 yielded mitochondrial photodamage whit membrane and lysosomal integrity were maintained. Apoptosis was detected using the DNA stain HO342, by the appearance of 50 kb fragments and by DNA ladder formation. The release of mono-and oligonucleosomes was further quantified using an ELISA protocol. Large DNA fragments were observed immediately following illumination, and nucleosomes were detected at 1-2 h post-treatment. Increasing the dose 4-fold accelerated the apoptotic response to PDT. This is the first report of a thiazine photosensitiser inducing apoptosis and is consistent with recent proposals suggesting that release of mitochondrial components may play an important role in the mechanism of cell death. (C) 1998 Elsevier Science S.A.