Platinum(II) Complexes of Curcumin Showing Photocytotoxicity in Visible Light

Three platinum(II) complexes of curcumin, [Pt(NH3)2(cur)](NO3) (1), [Pt(en)(cur)](NO3) (2) and [Pt(dach)(cur)](NO3) (3), where Hcur is curcumin, en is ethylenediamine and dach is 1R,2R‐(–)‐1,2‐diaminocyclohexane, were synthesized, characterized and their photocytotoxic properties were explored. The acetylacetone (Hacac) analogue of 1, [Pt(NH3)2(acac)]NO3 (4), was prepared to investigate the role of photoactive curcumin. The crystal structure of [Pt(NH3)2(acac)]CF3SO3 (4a) revealed square‐planar geometry of the platinum(II) centre. Complexes 1–3 displayed an intense absorption band at 450 nm (ε = 22500 m–1 cm–1). The curcumin in metal‐bound form showed improved stability in 10 % DMSO/buffer solution at 37 °C. The complexes released curcumin upon irradiation with visible light (400–700 nm) as evident from the increment in emission intensity. The complexes formed Pt–DNA covalent adducts when treated with calf thymus (ct)‐DNA and exposed to light. The platinum–curcumin conjugates demonstrated photocytotoxicity in cancer cells giving IC50 values of 15–45 µm (400–700 nm, 10 J cm–2) with no apparent activity in the dark (IC50 > 200 µm). Cytoplasmic localization and reactive oxygen species (ROS)‐mediated apoptotic cell death in visible light was observed for the curcumin complexes. Platinum(II)–curcumin complexes having diammine or chelating amines as ligands show photocytotoxicity: in visible light the curcumin ligand dissociates, thereby generating platinum(II)–DNA crosslinking agents and curcumin as a phototoxin.