Manipulating the Subcellular Localization and Anticancer Effects of Benzophenothiaziniums by Minor Alterations of N-Alkylation.

Cationic, water-soluble benzophenothiaziniums have been recognized as effective type I photosensitizers (PSs) against hypoxic tumor cells. However, the study of the structure-property relationship of this type of PS is still worth further exploration to achieve optimized photodynamic effects and minimize the potential side effects. Herein, we synthesized a series of benzophenothiazine derivatives with minor N-alkyl alteration to study the effects on the structure-property relationships. The cellular uptake, subcellular organelle localization, reactive oxygen species (ROS) generation, and photocytotoxicity performances were systematically investigated. NHNBS and EtNBS specifically localized in lysosomes and exhibited high toxicity under light with a moderate phototoxicity index (PI) due to the undesirable dark toxicity. However, NMeNBS with two methyl substitutions accumulated more in mitochondria and displayed an excellent PI value with moderate light toxicity and negligible dark toxicity. Without light irradiation, NHNBS and EtNBS could induce lysosomal membrane permeabilization (LMP), while NMeNBS showed no obvious damage to lysosomes. After irradiation, NHNBS and EtNBS were released from lysosomes and relocated into mitochondria. All compounds could induce mitochondria membrane potential (MMP) loss and nicotinamide adenine dinucleotide phosphate (NADPH) consumption under light to cause cell death. NMeNBS exhibited remarkable in vivo photodynamic therapy (PDT) efficacy in a xenograft mouse tumor (inhibition rate, 89%) with no obvious side effects. This work provides a valuable methodology to investigate the structure-property relationships of benzophenothiazine dyes, which is of great importance in the practical application of PDT against hypoxia tumor cells.