Functional aza-boron dipyrromethenes for subcellular imaging and organelle-specific photodynamic therapy

Two hydrophilic aza-boron dipyrromethene (aza-BODIPY) derivatives have first been designed and synthesised for subcellular imaging. With a triphenylphosphonium or morpholine substituent, these probes show high affinity towards the mitochondria or lysosomes of a range of cancer cell lines, including human cervical cancer HeLa cells, hepatocellular carcinoma HepG2 cells, breast cancer MCF-7 cells and colon adenocarcinoma HT29 cells as revealed by confocal microscopy. By introducing two bromo groups into the aza-BODIPY skeleton, which can promote intersystem crossing by the heavy-atom effect, the resulting compounds become efficient singlet-oxygen generators and can serve as organelle-specific photosensitisers for photodynamic therapy (PDT). Upon irradiation, both compounds are photocytotoxic. The lysosome-targeted derivative exhibits higher cellular uptake and more efficient reactive oxygen species generation inside HeLa cells, resulting in higher PDT efficacy with an IC₅₀ value of 0.48 μM and cell death mainly through apoptosis.