Metal Complexes for Two-Photon Photodynamic Therapy: A Cyclometallated Iridium Complex Induces Two-Photon Photosensitization of Cancer Cells under Near-IR Light

Abstract

Photodynamic therapy (PDT) uses photosensitizers (PS) which only become cytotoxic upon light-irradiation. Transition-metal complexes are highly promising PS due to long excited-state lifetimes, and high photo-stabilities. However, these complexes usually absorb higher-energy UV/Vis light, whereas the optimal tissue transparency is in the lower-energy NIR region. Two-photon excitation (TPE) can overcome this dichotomy, with simultaneous absorption of two lower-energy NIR-photons populating the same PS-active excited state as one higher-energy photon. We introduce two low-molecular weight, long-lived and photo-stable iridium complexes of the [Ir(N^C)₂ (N^N)]⁺ family with high TP-absorption, which localise to mitochondria and lysosomal structures in live cells. The compounds are efficient PS under 1-photon irradiation (405 nm) resulting in apoptotic cell death in diverse cancer cell lines at low light doses (3.6 J cm^-2 ), low concentrations, and photo-indexes greater than 555. Remarkably 1 also displays high PS activity killing cancer cells under NIR two-photon excitation (760 nm), which along with its photo-stability indicates potential future clinical application.

Keywords: cancer therapy; iridium; singlet oxygen; transition metals; two-photon photodynamic therapy.

Figure 1

Complexes 1 and 2.

Figure 2

Subcellular localisation of 1. U2OS cells following 4 or 24 hour incubation with 1 (green), co‐localised with: A) Mitotracker orange (red), or B) Lysotracker (red). Zoomed sections are shown as insets. Scale bars=20 μm.

Figure 3

One‐photon‐induced PS activities of complexes 1 and 2. A) Survival of HeLa cells pre‐incubated for 2 or 24 h with complexes 1 or 2 +/− light. B) Anexin V staining in HeLa cells. C) Survival of bladder (EJ), osteosarcoma (U2OS), melanoma (A375) and colorectal (HCT116) cancer cells 1 +/− light treatment.

Figure 4

Two‐photon‐induced PS activity of 1. HeLa cells treated with 1 or DMSO 2 h before irradiation of a central square with 760 nm multiphoton laser. After 24 h cells were stained with markers for apoptosis (Annexin V, red) and cell death (propidium iodide, green). All images are 450×450 μm except those in the 0 mW column which are 900×900 μm.