Catch and Release Photosensitizers: Combining Dual-Action Ruthenium Complexes with Protease Inactivation for Targeting Invasive Cancers

Dual action agents containing a cysteine protease inhibitor and Ru-based photosensitizer for photodynamic therapy (PDT) were designed, synthesized, and validated in 2D culture and 3D functional imaging assays of triple-negative human breast cancer (TNBC). These combination agents deliver and release Ru-based PDT agents to tumor cells and cause cancer cell death upon irradiation with visible light, while at the same time inactivating cathespin B (CTSB), a cysteine protease strongly associated with invasive and metastatic behavior. In total five Ru-based complexes were synthesized with the formula [Ru­(bpy)₂(1)]­(O₂CCF₃)₂ (3), where bpy = 2,2′-bipyridine and 1 = a bipyridine-based epoxysuccinyl inhibitor; [Ru­(tpy)­(NN)­(2)]­(PF₆)₂, where tpy = terpiridine, 2 = a pyridine-based epoxysuccinyl inhibitor and NN = 2,2′-bipyridine (4); 6,6′-dimethyl-2,2′-bipyridine (5); benzo­[i]­dipyrido­[3,2-a:2′,3′-c]­phenazine (6); and 3,6-dimethylbenzo­[i]­dipyrido­[3,2-a:2′,3′-c]­phenazine (7). Compound 3 contains a [Ru­(bpy)₃]²⁺ fluorophore and was designed to track the subcellular localization of the conjugates, whereas compounds 4–7 were designed to undergo either photoactivated ligand dissociation and/or singlet oxygen generation. Photochemical studies confirmed that complexes 5 and 7 undergo photoactivated ligand dissociation, whereas 6 and 7 generate singlet oxygen. Inhibitors 1–7 all potently and irreversibly inhibit CTSB. Compounds 4–7 were evaluated against MDA-MB-231 TNBC and MCF-10A breast epithelial cells in 2D and 3D culture for effects on proteolysis and cell viability under dark and light conditions. Collectively, these data reveal that 4–7 potently inhibit dye-quenched (DQ) collagen degradation, whereas only compound 7 causes efficient cell death under light conditions, consistent with its ability to release a Ru­(II)-based photosensitizer and to also generate ¹O₂.