Activatable photothermal agents with target-initiated large spectral separation for highly effective reduction of side effects

Abstract

Photothermal agents (PTAs) with minimized side effects are critical for transforming cancer photothermal therapy (PTT) into clinical applications. However, most currently available PTAs lack true selective activation to reduce side effects because of heavy spectral overlap between photothermal agents and their corresponding products. This study reports the construction of activatable PTAs with target-initiated large spectral separation for highly effective reduction of side effects. Such designed probes involve two H₂O₂-activatable PTAs, aza-BOD-B1 (single activatable site) and aza-BOD-B2 (multiple activatable site). After interacting with H₂O₂, aza-BOD-B1 only displays a mild absorption redshift (60 nm) from 750 nm to 810 nm with serious spectral overlap, resulting in a mild photothermal effect on normal tissues upon 808 nm light irradiation. In contrast, aza-BOD-B2 displays a large absorption spectral separation (150 nm) from 660 nm to 810 nm, achieving true selective activation to minimize side effects during PTT of cancer. Besides, in vitro and in vivo investigations demonstrated that aza-BOD-B2 can specifically induce photothermal ablation of cancer cells and tumors while leaving normal sites undamaged, whereas aza-BOD-B1 exhibits undesirable side effects on normal cells. Our study provides a practical solution to the problem of undesired side effects of phototherapy, an advance in precision medicine.

Scheme 1. Rational design of NIR photothermal reagents with H₂O₂-activated large spectral separation for highly effective reduction of side effects during PTT of cancer.

Fig. 1. Time-dependent absorption changes of (a) aza-BOD-B1 (10 μM) and (b) aza-BOD-B2 (10 μM) in the presence of 300 μM H₂O₂. (c) Under the 808 nm laser irradiation (3.0 W cm⁻²), the photothermal temperature curves of PBS, aza-BOD-B1 (20 μM), aza-BOD-B2 (20 μM), and aza-BOD-B2 (20 μM) containing 300 μM H₂O₂. (d) The photothermal temperature change curves of aza-BOD-B2 (20 μM) activated by H₂O₂ under different power irradiations. (e) Temperature elevation for aza-BOD-B2 at different concentrations in the presence of H₂O₂ (300 μM) under 808 nm laser irradiation (1.2 W cm⁻²). (f) Representative IR thermal images of aza-BOD-B2 (20 μM) in the presence of H₂O₂ (300 μM) under 808 nm laser irradiation (3 W cm⁻²). Error bars represent the standard error of the mean (n = 3).

Fig. 2. (a) Viability of A549 cells incubated with aza-BOD-B1 and aza-BOD-B2 in the presence or absence of 808 nm laser irradiation. Viability of HEK-293T cells incubated with aza-BOD-B1 and aza-BOD-B2 in the absence (b) or presence (c) of 808 nm laser irradiation. (d) Confocal fluorescence images of calcein AM/PI stained A549 cells with different treatments (control, laser only, aza-BOD-B2 only, and laser + aza-BOD-B2). Scale bars: 50 μm. The laser power was 1.2 W cm⁻². Error bars represent the standard error of the mean (n = 5).

Fig. 3. (a) Infrared thermal images of A549 tumor-bearing mice under continuous NIR laser irradiation with different treatments. Laser irradiation was performed at 4 h post-administration of aza-BOD-B2. (b) Mean temperature as a function of irradiation time. Laser irradiation was performed at 4 h post-administration of aza-BOD-B2. (c) Representative photos of mice in different groups during the aza-BOD-B2-mediated PTT process. (d) Tumor growth curves of mice in different groups by administration of various treatments. (e) Bodyweight changes in mice with various treatments. The laser power was 1.2 W cm⁻². Error bars represent the standard error of the mean (n = 5).