Gadolinium-Chelated Conjugated Polymer-Based Nanotheranostics for Photoacoustic/Magnetic Resonance/NIR-II Fluorescence Imaging-Guided Cancer Photothermal Therapy

Abstract

Our exploiting versatile multimodal theranostic agent aims to integrate the complementary superiorities of photoacoustic imaging (PAI), second near-infrared (NIR-II, 1000-1700) fluorescence and T₁-weighted magnetic resonance imaging (MRI) with an ultimate objective of perfecting cancer diagnosis, thus improving cancer therapy efficacy. Herein, we engineered and prepared a water-soluble gadolinium-chelated conjugated polymer-based theranostic nanomedicine (PFTQ-PEG-Gd NPs) for in vivo tri-mode PA/MR/NIR-II imaging-guided tumor photothermal therapy (PTT). Methods: We firstly constructed a semiconducting polymer composed of low-bandgap donor-acceptor (D-A) which afforded the strong NIR absorption for PAI/PTT and long fluorescence emission to NIR-II region for in vivo imaging. Then, the remaining carboxyl groups of the polymeric NPs could effectively chelate with Gd³⁺ ions for MRI. The in vitro characteristics of the PFTQ-PEG-Gd NPs were studied and the in vivo multimode imaging as well as anti-tumor efficacy of the NPs was evaluated using 4T1 tumor-bearing mice. Results: The obtained theranostic agent showed excellent chemical and optical stability as well as low biotoxicity. After 24 h of systemic administration using PQTF-PEG-Gd NPs, the tumor sites of living mice exhibited obvious enhancement in PA, NIR-II fluorescence and positive MR signal intensities. Better still, a conspicuous tumor growth restraint was detected under NIR light irradiation after administration of PQTF-PEG-Gd NPs, indicating the efficient photothermal potency of the nano-agent. Conclusion: we triumphantly designed and synthesized a novel and omnipotent semiconducting polymer nanoparticles-based theranostic platform for PAI, NIR-II fluorescence imaging as well as positive MRI-guided tumor PTT in living mice. We expect that such a novel organic nano-platform manifests a great promise for high spatial resolution and deep penetration cancer theranostics.

Keywords: conjugated polymer; magnetic resonance imaging; photoacoustic imaging; photothermal therapy; second near-infrared fluorescence imaging.

Scheme 1

Schematic Description of the Deep-Tissue and High-Resolution Multimodality Imaging-Guided Cancer Photothermal Therapy in Vivo Using PFTQ-PEG-Gd NPs.

Figure 1

Basic characterization of PFTQ-PEG-Gd NPs and PFTQ-PEG NPs. Representative TEM images of (A) PFTQ-PEG NPs and (B) PFTQ-PEG-Gd NPs. (C) Zeta potentials of PFTQ-PEG-Gd NPs and PFTQ-PEG NPs. Hydrodynamic size distribution graphs of (D) PFTQ-PEG NPs, (E) PFTQ-PEG-Gd NPs and photographs of them in PBS (100 μg mL^-1, pH 7.4). (F) The Gd-chelated stability evaluation of PFTQ-PEG-Gd NPs cultivating in different PBS (pH 6.5 and 7.4).

Figure 2

Optical properties of PFTQ-PEG-Gd NPs. (A) Optical spectra of the NPs, manifesting the absorption crest at 760 nm as well as a peak emission at 1056 nm. (B) NIR-II fluorescence images of the NPs (100 μg mL^-1) and PBS (pH = 7.4). (C) Photostability assays of the NPs in serum, saline and water via collecting their NIR-II fluorescence signals. (D) Photostability of the NPs (50 μg mL^-1) and commercial ICG NPs (50 μg mL^-1) via recording their maximum absorption peak (laser exposure: 808 nm, 1 W cm^-2).

Figure 3

Extracorporeal imaging capacity and photothermal experiments of PFTQ-PEG-Gd NPs. (A) In vitro PA images of the NPs with varying contents ranging from 31.25 to 500 µg mL^-1. (B) Linear dependence between the PA signals and concentrations of the NPs (R² = 0.996). (C) In vitro positive magnetic contrast images of the NPs (15.6, 31.3, 62.5, 125, 250 and 500 µg mL^-1) at varying gadolinium contents (from 6.8 to 218.8 µM), the commercial Gd-DTPA contrast agent (with the equivalent Gd content) served as the control group. (D) Plot of relaxation rates (1/T₁) as a function of Gd content of PFTQ-PEG-Gd NPs and Gd-DTPA in saline. (E) Photothermal conversion behavior of PFTQ-PEG-Gd NPs at varying contents (0 - 0.5 mg mL^-1) exposed an 808 nm light irradiation. (F) IR thermal images of PFTQ-PEG-Gd NPs at varying contents (0 - 0.5 mg mL^-1) under an 808 nm light irradiation.

Figure 4

Intravital imaging of 4T1 neoplastic mice. (A) In vivo MRI (below) and PAI (top) of tumorous mice imaged at varying point-in-time after intravenous administration with PFTQ-PEG-Gd NPs. (B) Whole-body MRI images of living mice at 24 h post-injection and pre-injection. (C) PA and MR relative signal values of the tumor regions from neoplastic mice systemically treated with PFTQ-PEG-Gd NPs at different post-injection time points.

Figure 5

Intravital imaging of 4T1 tumor mice. (A) The NIR-II image of the vascular tissues of the mouse treated with PFTQ-PEG-Gd NPs at 2 min post-injection under 808 nm excitation. The red arrows show the blood vessels. (B) The NIR-II fluorescence of tumor/normal tissue ratio from neoplastic mice systemically treated with PFTQ-PEG-Gd NPs at different post-injection time points. (C) The intravital NIR-II fluorescence contrasts of 4T1 neoplastic mice at varying time points (2, 4, 10, 24, and 48 h) after systemic injection of PFTQ-PEG-Gd NPs. (D) The ex-biodistribution of the agent in neoplastic mice at 24 h post-injection. From left to right and from top to below: tumor, heart, liver, spleen, lung and kidney. (E) Ex-vivo NIR-II fluorescence signal values of some organs from neoplastic mice after 24 h injection with the agent.

Figure 6

Intravital photothermal tumor therapy. (A) IR thermal images of 4T1 neoplastic mice; (B) Temperature rise on tumor areas of the neoplastic mice injected without or with the agent under an 808 nm laser excitation (1 W cm^-2); (C) Tumor volume growth curves of each treatment group of mice; (D) Body weight changes of mice from three treatment groups; (E) Representative photographs of the 4T1 tumors collected from these mice at the end of PTT.

Figure 7

(A) Histological H&E staining of tumor and various organs from the treatment mice. Scale bar: 100 µm. (B) H&E-stained tumors from (left) group “Saline + laser”, (middle) group “Agent” and (right) group “Agent + laser”. Scale bar: 20 µm.