A biocompatible phosphorescence probe based on iridium(III) solvent complex for monitoring integrin expression in living cells.

Visualizing and monitoring of integrin expression on or within living cells is essential for cancer diagnosis and treatment as they correlate well with tumor invasion and metastasis. Here, a phosphorescence probe, named as Ir-[HRGDH], was designed and chemically synthesized for selectively monitoring integrin expression in living cells by simply functionalizing histidine-arginine-glycine-aspartate-histidine (HRGDH) motifs with an iridium(III) solvent complex ([(pbz)₂Ir(DMSO)Cl], pbz = 3-(2-pyridyl)benzoic acid, Ir-DMSO) via coordination reaction. The synthesized Ir-[HRGDH] shows strong photoluminescence (PL) emission with maximum wavelength at 483 nm and 506 nm. Absolute fluorescence quantum yield was determined to be 6.5 % and lifetime was determined to be 453 ns (67 ns, 1 %; 569 ns, 12 %; 442 ns, 87 %) for Ir-[HRGDH]. The synthesized Ir-[HRGDH], confirmed by mass spectrometry and optical technique, displayed high specificity to RGD-binding integrins and excellent biocompatibility. Good cell permeability enabled Ir-[HRGDH] to be distributed throughout the entire cell region. Cellular uptake pathways of Ir-[HRGDH] were further investigated for A549 cells and an energy-dependent endocytosis was revealed, in which the PL intensity of Ir-[HRGDH] within the cells decreased significantly by incubation at 4 °C, or uptake pathway inhibitor pretreatments. Importantly, time-dependent integrin expression within living cells stimulated by phorbol myristate acetate was successfully monitored by Ir-[HRGDH]. Particularly, Ir-[HRGDH] staining allowed for distinguishing cancer cells from normal cells based on their different intracellular levels of RGD-binding integrins. This work demonstrates that the synthesized Ir-[HRGDH] is a promising PL probe for targeting RGD-binding integrins within living cells, which can be further used for cancer diagnosis and treatment.