Molecular imaging of tumour-associated pathological biomarkers with smart nanoprobe: From "Seeing" to "Measuring"

Abstract

Although the extraordinary progress has been made in molecular biology, the prevention of cancer remains arduous. Most solid tumours exhibit both spatial and temporal heterogeneity, which is difficult to be mimicked in vitro. Additionally, the complex biochemical and immune features of tumour microenvironment significantly affect the tumour development. Molecular imaging aims at the exploitation of tumour-associated molecules as specific targets of customized molecular probe, thereby generating image contrast of tumour markers, and offering opportunities to non-invasively evaluate the pathological characteristics of tumours in vivo. Particularly, there are no "standard markers" as control in clinical imaging diagnosis of individuals, so the tumour pathological characteristics-responsive nanoprobe-based quantitative molecular imaging, which is able to visualize and determine the accurate content values of heterogeneous distribution of pathological molecules in solid tumours, can provide criteria for cancer diagnosis. In this context, a variety of "smart" quantitative molecular imaging nanoprobes have been designed, in order to provide feasible approaches to quantitatively visualize the tumour-associated pathological molecules in vivo. This review summarizes the recent achievements in the designs of these nanoprobes, and highlights the state-of-the-art technologies in quantitative imaging of tumour-associated pathological molecules.

Keywords: cancer; molecular imaging; nanoprobe; quantitative imaging; tumour microenvironment.

SCHEME 1

Illustration of activatable mechanism of smart quantitative nanoprobes and the quantitative visualization of pathological related biomarkers of solid tumours.

FIGURE 1

The work principle of dual‐ratiometric target‐triggered fluorescent probe and the simultaneous quantitative visualization of protease activity and pH in vivo. (A) The schematic illustration of simultaneous visualization MMP‐9 and pH with dual‐ratiometric target‐triggered fluorescent nanoprobes. (B) Variation in MMP‐9 activity in response to tumour pH adjusted upon intratumoural injection of PBS. (C) Quantified pH and MMP‐9 expression mapping of tumours obtained at D+0, D+2, and D+4. (D) Top: microscopic image of a tissue slide stained with H&E, bottom: immunofluorescence microscopic image stained for E‐cadherin (red) and MMP‐9 expression (green). The scale bars correspond to 200 μm. The orange arrows point to boundaries between healthy tissue and the tumour where MMP‐9 is highly expressed. Reproduced with permission.^{[ 22 ]} Copyright 2018, American Chemical Society.

FIGURE 2

The structure, characterization of ratiometric O₂ probe and the ratiometric mapping of tumour hypoxia in vivo. (A) The schematic illustration of cyclodextrin‐hosted Ir(III) complex. (B) 2D‐ROESY spectra of Ir‐BTPHSA/CD‐NH₂. (C) Luminescence spectra of Ir‐BTPHSA/CD‐Cy7 recorded under various oxygen levels upon excitation at 488 and 747 nm for Ir‐BTPHSA complex and Cy7, respectively (Inset: PL intensities of Ir‐BTPHSA and Cy7 against oxygen levels), (D) a liner fitting of I ₀′/I′ against the oxygen level (I ₀′and I′ refer to PL intensities of Ir‐BTPHSA recorded at 0% oxygen and specific oxygen levels, respectively, after normalized with reference PL intensity of Cy7) (inset: normalized PL intensity of Ir‐BTPHSA against the oxygen level). (E) Oxygen level mapping of the tumour and its control site based on the normalized Ir‐BTPHSA signal and its correlation with oxygen level. Reproduced with permission.^{[ 23 ]} Copyright 2021, Wiley‐VCH.

FIGURE 3

The work principle of DCNP@786s nanoprobe and the quantitatively tracking of adoptive NK cell viability in vivo. (A) Schematic illustration of DCNP@786s with ratiometric NIR‐II fluorescent signal for tracking NK cell viability in vivo. (B) The NIR‐II ratio mapping of DCNP@786s‐labeled NK cells after treatment with different concentrations of DOX for 24 h, together with the correlation of NIR‐II ratio signal and CCK8 for NK cell viability assay after treatment with different DOX doses. (C) Ratiometric NIR‐II fluorescence imaging after NK cell transplantation at different time points in vivo and (D) hepatic tissues ex vivo, together with (E) quantitative analysis of the relative signal intensities and ratios in vivo (upper) and ex vivo (lower). Reproduced with permission.^{[ 28 ]} Copyright 2021, Wiley‐VCH.

FIGURE 4

The work principle of SNARF‐PAA nanoparticles and the quantitatively imaging of pH levels and the hemodynamic properties across the entire tumour in vivo. (A) SNARF‐5F encapsulated polyacrylamide‐based nanoparticle (SNARF‐PAA nanoparticle) synthesis and pH‐sensing scheme. (B) Measured PA signal amplitude ratios between the three wavelengths and the isosbestic point (i.e., 576 nm/565 nm, 584 nm/565 nm, and 600 nm/565 nm) from pH 5.8−7.7 with 0.1 pH interval (n  =  3, error bars represent standard deviations). (C) Quantitative pH images of phantoms containing different concentrations (2, 10 and 20 mg mL^‐1) of SNARF‐PAA nanoparticles at different pH levels (pH 6.6, 7.0 and 7.4). PA image showing the spatially distributed (D) haemoglobin oxygen saturation and (E) total haemoglobin concentration in the tumour area at 75 min after injection. (F) PA pH image of a tumour. The pH in the centre area and the peripheral areas are averaged, respectively. (G) Example PA pH image of a normal thigh, showing relatively higher pH. (H) The analysis of the SNARF‐PAA nanoparticles accumulation in the tumours at different time points after systemic injection. (I) The boxplot showing the pH levels in tumours (n  =  4) versus the pH levels in normal thigh (n  =  4), as quantified from PA pH images. Reproduced with permission.^{[ 38 ]} Copyright 2017, Nature Publishing Group.

FIGURE 5

The work principle and structure of the QC probe, together with the quantitative detection of MMP‐2 activity through fluorescence/photoacoustic imaging in vivo. (A) The schematic illustration of QC probe for quantitatively detecting MMP‐2 activity through fluorescence/photoacoustic imaging. (B) PA images of 4T1 tumours of different sizes in vivo, recorded 2 h post injection of the QC probe or its control through 680 and 730 nm channels, together with the photographs of the corresponding tumours harvested right after the PA imaging. Tumour regions are delineated by white dotted circles in the PA images. (C) Ratiometric signal ΔPAS₆₈₀/ΔPAS₇₃₀ against the tumour size. (D, E) MMP‐2 expression levels determined (D) through the conventional tumour‐size‐dependent method and (E) non‐invasively through the ΔPAS₆₈₀/ΔPAS₇₃₀ signal. Reproduced with permission.^{[ 39 ]} Copyright 2019, American Chemical Society.

FIGURE 6

The structure and work principle of TMRET nanoprobe, together with the quantitative MRI of GSH concentration of tumours in vivo. (A) Schematic illustration of the TMRET nanotechnology. Mn²⁺ conjugated to pheophorbide serves as both an ‘enhancer’ in the T ₁ MRI signal and a ‘quencher’ in the T ₂ MRI signal, whereas the SPIO nanoparticle acts as an ‘enhancer’ in the T ₂ MRI signal and a ‘quencher’ in the T ₁ MRI signal. (B, C) ΔR ₁ and ΔR ₂ of in PC‐3 cells treated with various concentrations of the GSH inhibitor (n = 3). ΔR ₁ and ΔR ₂ of the TMRET nanoprobe were measured to be 0.18, 0.31, 0.49, 0.82 and 1.27 s⁻¹ (ΔR ₁) and 1.96, 3.97, 5.89, 11.20 and 18.90 s⁻¹(ΔR ₂), respectively. The GSH concentrations in cells were measured by using thiol tracker violet (GSH detection reagent). Reproduced with permission.^{[ 59 ]} Copyright 2020, Nature Publishing Group.

FIGURE 7

The structure, characterization, and work principle of T ₁/T ₂ interlocked nanoprobe, together with the quantitatively mapping of GSH in intracranial tumour in vivo. (A) Schematic drawings to show the mechanism of the GSH‐induced agglomeration of the nanoprobes and MRI quantitative imaging principle. (B) TEM images of nanoprobes after treatment with GSH (the embedded scale bar corresponds to 30 nm). (C) Temporal hydrodynamic size profiles of nanoprobes in reactions to GSH treatment. (D) GSH concentration dependent ΔR ₁, ΔR ₂, and ΔR ₂/ΔR ₁ for nanoprobe after being incubated with different concentrations of GSH, together with a theoretical fitting (red dashed line). (E) T ₁‐ and T ₂‐weighted MR images of mice bearing orthotopic U87MG glioblastoma xenografts acquired before and at different time points after the intravenous injections of nanoprobes. (F) Quantitative mapping of GSH according to imaging data acquired at 7 h post‐injection through the correlation between ΔR ₂/ΔR ₁ and GSH concentration in vivo. Reproduced with permission.^{[ 74 ]} Copyright 2021, Wiley‐VCH.