Abstract:
The clinical translation of photodynamic therapy (PDT) is often hindered by the lack of photosensitizers (PSs) that achieve potent cytotoxicity at clinically relevant concentrations. This limitation stems from an incomplete understanding of photochemical mechanisms underlying PDT efficacy. Here, we report RM2, a near-infrared Ir(III)-based PS with a high molar absorption coefficient (17,700 M-1 cm-1 at the excitation wavelength), a key feature for attaining low IC50 values. Upon irradiation, RM2 undergoes two distinct single-electron transfer (SET) pathways: catalytic oxidation of NADH with a turnover frequency (TOF) of 690 h-1, and Type-I ROS generation (O₂•⁻ and H2O2), confirmed by EPR spectroscopy and H2O2 detection assays. In addition, RM2 displays a triplet state energy of 37.68 kcal mol-1 (1.63 eV, 762 nm) with a 5 µs lifetime, enabling efficient energy transfer to 3O2 and a singlet oxygen quantum yield (ϕΔ) of 0.75 in cell-free media. Encapsulation of RM2 in DSPE-mPEG2000 nanoparticles further amplified its activity, enhancing photocytotoxicity by 8.3-fold and achieving an IC50 of 60 nM against ovarian cancer cells. Remarkably, RM2 NPs retained this potency in cisplatin-resistant ID8 cells (IC50 = 60 nM), whereas cisplatin itself showed drastically reduced efficacy (IC50 = 10.46 µM in wild-type and 30.41 µM in resistant cells). Additionally, RM2 NPs exhibited pronounced efficacy against 3D ovarian tumor spheroid models, underscoring their translational potential. These results establish RM2 as a multifunctional photosensitizer, in which the synergy of long-lived triplet energy and favorable redox potentials enables diverse photochemical mechanisms, encompassing NADH oxidation and both Type-I and Type-II ROS generation. This multifaceted mechanism of action offers a powerful strategy to overcome hypoxia and drug resistance, significantly advancing the potential of PDT for effective cancer therapy.