Alkyl-aryl styryl-indole dyes as fluorescent imaging and theranostic probes targeting mitochondria

The development of mitochondria-targeting fluorescent compounds with theranostic potential for tumor cells remains a topic of great interest. Inspired by previously reported delocalized lipophilic cations (DLCs) based on styryl dye framework, we introduce a series of seventeen novel styrene dyes, several of which exhibit enhanced optical properties compared to their parent compounds. Most dyes display strong Stokes shifts (104–112 nm), primarily due to the indolyl chromophore, with minimal influence from other substituents. These dyes bind noncovalently to the grooves of ds-DNA/RNA with moderate affinity, responding by significantly increased fluorescence. The selected derivatives effectively penetrate living human cells, accumulating primarily in mitochondria and becoming highly fluorescent. However, their bioactivity is strongly influenced by subtle structural modifications. Notably, the introduction of a bromo-substituent to the indole ring converts a non-cytotoxic dye into the highly cytotoxic analogues 10a and 10g. Interestingly, while A549 tumor cells treated with 10a and 10g exhibit similar cytotoxic responses, the underlying mechanisms differ between normal and cancerous cells. In WI-38 cells, toxicity appeared to result from mitochondrial hyperactivation and oxidative stress, whereas in A549 cells, it is driven by mitochondrial dysfunction and metabolic collapse. This distinction underscores key differences in mitochondrial metabolism between normal and cancer cells, positioning 10a and 10g as promising lead compounds for further development as theranostic agents targeting mitochondrial vulnerabilities in cancer.