Diplatinum(II) Catecholate of Photoactive Boron-Dipyrromethene for Lysosome-Targeted Photodynamic Therapy in Red Light

The binuclear platinum(II) boron-dipyrromethene (BODIPY) complex [{Pt(dach)}2(μ-Dcrb)] (DP), where dach is 1,2-diaminocyclohexane and H4Dcrb is a morpholine-conjugated BODIPY-linked dicatechol bridging ligand, was prepared for lysosome organelle targeting and near-IR (NIR) light (600–720 nm) induced photocytotoxic activity. The platinum complex [Pt(dach)(cat)] (CP), where H2cat is catechol, was synthesized and used as a control complex without bearing the BODIPY unit. The complex DP displayed a band at 660 nm (ε = 2.1 × 104 M–1 cm–1) in the red region of the UV–visible spectrum recorded in 10% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (DMSO/DMEM, pH 7.2). The complex DP and the BODIPY ligand displayed emission in 10% DMSO–DMEM (pH 7.2) giving an λem value of 668 nm (λex = 650 nm) with a ΦF value of 0.02 for DP and 0.16 for H4Dcrb (ΦF, fluorescence quantum yield). Titration experiments using 1,3-diphenylisobenzofuran (DPBF) indicated that the complex DP and H4Dcrb on irradiation with near-IR light of 600–720 nm generated singlet oxygen (1O2) as the ROS (reactive oxygen species). The complex DP showed significant lysosomal localization and remarkable apoptotic photodynamic therapy (PDT) effects, giving half-maximal inhibitory concentration values (IC50) within 0.6–3.4 μM in HeLa cervical cancer, A549 lung cancer, and MDA-MB231 multidrug resistant cancer cells, while being essentially nontoxic in the dark and in the HPL1D immortalized lung epithelial normal cells. The acridine orange assay using A549 cells showed lysosomal membrane permeabilization by the complex DP under near-IR light (600–720 nm). This complex on near-IR light (600–720 nm) activation in A549 cells induced apoptotic cell death, as observed from an Annexin-V FITC assay.