La(iii)–curcumin-functionalized gold nanocomposite as a red light-activatable mitochondria-targeting PDT agent

Abstract

Red light-activatable transition metal complexes have recently emerged as viable materials for use in photodynamic/photochemotherapeutic applications. Ideal photosensitizers or photochemotherapeutic agents are still an elusive goal. Herein, we report the synthesis, characterization and remarkable photodynamic activity of the [La^III(Cur)₂(L¹)(NO₃)] (1)-functionalized gold nanocomposite, 1-AuNPs, where Cur represents curcumin and L¹ represents 5-(1,2-dithiolan-3-yl)-N-(1,10-phenanthroline-5-yl)pentanamide. Functionalization of complex 1 to gold nanoparticles led to a remarkable red shift (λ_max, 755 nm) in the surface plasmon resonance (SPR) band, making the La(III)–curcumin-functionalized nanocomposite (1-AuNPs) an ideal photo-sensitizer for use in photodynamic applications. Several photophysical assays confirmed its high efficacy in generating singlet oxygen [ϕ(¹O₂), 0.69] through a type-II photo-process. TD-DFT calculations also predicted the presence of a low-lying triplet excited state and efficient intersystem crossing for the nanoconjugate. The log P values along with the high binding affinity of the 1-AuNPs to serum albumin predicted cellular permeability and transportation through blood. Flowcytometric analysis indicated the selective accumulation and retention of 1-AuNPs in adenocarcinomic human alveolar basal epithelial cells (A549) compared with normal alveolar fibroblast cells (WI-38). Confocal microscopy confirmed the localization of the nanocomposite in mitochondria. The nanoconjugate, 1-AuNPs, exhibited remarkable photodynamic activity against adenocarcinomic human alveolar basal epithelial cells (A549) (IC₅₀: 25 μg mL⁻¹ in light; >500 μg mL⁻¹ in dark) and immortalized human keratinocytes (HaCaT) (IC₅₀: 34 μg mL⁻¹ in light; >500 μg mL⁻¹ in dark) under red light (600–720 nm, 30 J cm⁻²), with remarkable photo-indices (PI > 20). Red light-induced generation of ¹O₂ by 1-AuNPsin vitro was found to be primarily responsible for the upsetting of the mitochondrial membrane potential (MMP), which led to caspase-3 activated apoptosis in A549 cells via an intrinsic pathway. During apoptosis, the pro-apoptotic BAX protein was upregulated, while the antiapoptotic Bcl-2 protein was downregulated and responsible for apoptosis after the A549 cells were treated with the red light activated nanoconjugate (1-AuNPs). Overall, the La(III)–curcumin-functionalized gold nanocomposite is an ideal next generation La(III)-based photosensitizer for use in photodynamic applications.