Nanoparticle-Based Activatable Probes for Bioimaging

Abstract

Molecular imaging can provide functional and molecular information at the cellular or subcellular level in vivo in a noninvasive manner. Activatable nanoprobes that can react to the surrounding physiological environment or biomarkers are appealing agents to improve the efficacy, specificity, and sensitivity of molecular imaging. The physiological parameters, including redox status, pH, presence of enzymes, and hypoxia, can be designed as the stimuli of the activatable probes. However, the success rate of imaging nanoprobes for clinical translation is low. Herein, the recent advances in nanoparticle-based activatable imaging probes are critically reviewed. In addition, the challenges for clinical translation of these nanoprobes are also discussed in this review.

Keywords: activatable nanoprobes; clinical translation; molecular imaging; nanomaterials; theranostics.

Figure 1.

The illustration of activatable nanoprobes for bioimaging in this review.

Figure 2.

a) Illustration to demonstrate the aggregation of ^99mTc-labeled Fe₃O₄ nanoparticles in the tumor microenvironment through GSH induced interparticle crosslinking reaction. b) T2-weighted MR images of tumor-bearing mice acquired 2 h after the intravenous injections of the responsive probe (top left) and the nonresponsive probes (bottom left), together with T2 values extracted from the tumor sites before and at different time points after the intravenous injections (right). b) SPECT/CT images of tumor-bearing mice acquired 2 h after the intravenous injections with the responsive probe (top left) and the control probe (bottom left), respectively, together with temporal γ-signals of the tumorous areas (right). Reproduced with permission.^[29] Copyright 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3.

a) Illustration to demonstrate the activatable probe for MRI and its therapeutic function. The probe accumulates in the tumor through the impaired blood vessel after intravenous injection. Transferrin receptor-induced tumor ingestion of probes. Low pH could activate the probe and release FeIII, enhancing the T1 imaging. Released FeIII accelerates tumor cell death through upregulated ROS, while the remained GA-FeIII irradiation for PTT. b) T1-weighted MR images of tumor-bearing mice acquired at different time points pre- and post-injection of the probe and UCNP-GA control, respectively. c) The upconversion luminescence imaging based on the I₄₇₅/I₈₀₀ ratio revealed the release of Fe³⁺ ions. Reproduced with permission.^[59] Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 4.

a) Scheme and structure of a non-invasively fluorescence/photoacoustic probe for MMP-2 activity imaging. b) The UV-vis absorption spectra of the probe (8 μM) after incubation in different MMP-2 concentrations (ng mL⁻¹). c) After incubation in different MMP-2 concentrations for 2 h, the ratio of ΔPAS680/ ΔPAS730 (0.25 μM) and the concentration of MMP2 recorded at 37 °C (inset: PA images of the probe solutions under different amounts of MMP-2). d) The PA images of tumor-bearing mice acquired after injection with the probe (60 μM, 200 μL) at different time points under the illumination of 680 and 730 nm, respectively. The tumor regions are delineated by white dotted circles. e) Temporal PA signal (ΔPAS) of the tumor site after subtracting the pre-contrasted signal recorded under illumination at 680 and 730 nm, respectively. f) Temporal ratiometric signal ΔPAS680/ΔPAS730 at different time points after injection of the probe. Reproduced with permission.^[65] Copyright 2019, American Chemical Society

Figure 5.

a) By introducing two electron-withdrawing pentafluorophenyl groups, the EM 1²⁺ was optimized into EM F1²⁺, which can be reduced by H₂S into EM F2. b) Scheme of the Preparation and H₂S induced NIR afterglow of F1²⁺-ANP. c) The photograph (bright field), afterglow, and FL images of the liver specimen resected from an HCC patient. The specimen was incubated with F1²⁺-ANP-Gal in PBS buffer (1×, pH 7.4) at 37 °C for 3 h, and then washed three times with PBS buffer. After irradiated with the 808-nm laser (1Wcm⁻², 1 min), the afterglow image was acquired under an open filter (exposure: 60 s). The fluorescence image was collected with λex/em =740/790 nm. d) The fluorescence imaging of liver tissue slices from c), which was incubated with F1²⁺-ANP-Gal (green) for 3 h and stained with DAPI (blue). e) H&E staining of the liver tissue slice. Black dash boxes indicate the enlarged areas, in which box ROI 1 shows the tumor tissue, and box ROI 2 indicates the normal liver tissue, respectively. f) Quantitative analysis of the average SBRs for afterglow and fluorescence imaging of liver specimens resected from HCC patients. Data denote mean ± sd. (n = 4). Reproduced with permission.^[92] Copyright 2020, Springer Nature.

Figure 6.

a) Scheme of the nanoprobe. Upon cleavage of the peptide linker, the fluorescence of ANNA would be activated. Cy5.5 is always in an “on” state. Folic acid was modified to enhance tumor target ability. b) Mapping of MMP-9 activity after intratumoral injection of low pH PBS (pH 6.2). c) Quantified pH and MMP-9 expression mapping of tumors obtained at D+0 day, D+2, and D+4 (in color bar shading from black to yellow for reading MMP-9 expression ranging from 4.3−6.8 ng/mL; each step thus corresponds to 0.25 units). d) Photographs of tumors showing their growth in four days. Reproduced with permission.^[109] Copyright 2018, American Chemical Society

Figure 7.

Schematic showing the mechanisms of metals and metal oxide nanoparticle toxicity, as determined by the toxicological profiling of dozens of these materials in macrophages and the intact lung. Soluble MOxs and redox-active TMOs induce ROS production and oxidative stress due to the release of toxic metal ions and overlap of conduction band energy with the cellular redox potential, respectively. In contrast, the lysosomal dissolution of REOs in an acidic environment, except for CeO2 nanoparticles, leads to the release of rare-earth ions that, upon complexation to biological phosphates precipitate on the particle surface, leading to biotransformation into urchin-shaped structures. This triggers lysosomal damage, NLRP3 inflammasome activation, and IL-1β production. High aspect ratio materials trigger lysosomal damage and NLRP3 inflammasome activation. Fumed silica induces membrane lysis, potassium efflux, and NLRP3 inflammasome activation. Cationic nanoparticles induce lysosomal damage and cell death.