Rational chemical design of the next generation of molecular imaging probes based on physics and biology: mixing modalities, colors and signals

Abstract

In recent years, numerous in vivo molecular imaging probes have been developed. As a consequence, much has been published on the design and synthesis of molecular imaging probes focusing on each modality, each type of material, or each target disease. More recently, second generation molecular imaging probes with unique, multi-functional, or multiplexed characteristics have been designed. This critical review focuses on (i) molecular imaging using combinations of modalities and signals that employ the full range of the electromagnetic spectra, (ii) optimized chemical design of molecular imaging probes for in vivo kinetics based on biology and physiology across a range of physical sizes, (iii) practical examples of second generation molecular imaging probes designed to extract complementary data from targets using multiple modalities, color, and comprehensive signals (277 references).

Fig. 1

Schematic explanation of multiplexed imaging technologies based on the electromagnetic waves of different wavelengths.

Fig. 2

A schema of the rational strategies for target specific imaging in all physical levels.

Fig. 3

A schema of the in vivo NIR window and the extinction coefficient value of water, oxy- and deoxy-hemoglobin are plotted ranging from visible to near infrared wavelength.

Fig. 4

Multi-color and quantitative lymphatic images using a nuclear and NIR optical dual labeled imaging agent are shown. As depicted in this schema, generation-6 PAMAM dendrimer-based dual labeled contrast agents of approximately 10 nm in diameter are employed. A gamma scintigram camera shows quantitative lymphatic drainage and a 5-color optical image shows distinct drainages from different injection sites of agents.

Fig. 5

MR and NIR optical sentinel lymph node imaging of breast cancer in a mouse are shown. As depicted in this schema, generation-6 PAMAM dendrimer-based dual labeled contrast agents of approximately 10 nm in diameter containing 172 Gd ions and 2 Cy5.5 fluorophores are employed. Both MRI and NIR optical images define neck (yellow arrow) and axillary (red arrow) lymph nodes as sentinel lymph node of this breast cancer.

Fig. 6

Fluorescence intensity and lifetime multi-signal images of a HER2-positive (3T3/HER2) and negative (Blab/3T3) tumors’ bearing mouse 2 days after injection with trastuzumab–Alexa680 fluorescent antibody are shown. The HER2-negative tumor still shows high fluorescence intensity due to superior EPR effect. In contrast, the HER2-positive tumor shows elongation of fluorescence lifetime due to specific binding of antibodies to antigens on the cell surface.

Fig. 7

A schema of the molecular design for target-specific activatable imaging probes.

Fig. 8

Schemas explanation of blood clearance and tumor accumulation of antibody and genetically modified antibody fragments of various sizes are shown.

Fig. 9

Schematic explanation of activation mechanisms based on the photo-chemical reactions; a. FRET, b. H-dimer formation, c. photon-induced electron transfer (PeT), d. caged fluorophore.

Fig. 10

Two practical examples for cancer-cell specific imaging using combinations of proposed technologies; multi-color imaging (a, 2-color; b, 3-color), and reversible pH-activatable imaging using a target-cell activatable imaging probe. (a and b) Multi-color imaging targeting HER1, HER2, and CD25 using Cy5-, Cy7-, and Alexa700-labeled respective specific antibodies; cetuximab (anti-HER1), trastuzumab (anti-HER2), and daclizumab (anti-CD25) clearly shows different specific target molecules, which were expressed by distinct cancers, in different colors shown in pink, yellow, and blue, respectively. (c) A reversible activatable imaging probe of pH-sensitive trastuzumab–dimethyl-phenyl-BODIPY conjugate cannot only depict receptor-positive tiny target tumor metastasis (green) but also monitor the therapeutic effect of anti-cancer therapy in real time.

Fig. 11

Two practical examples for visualizing target-binding of imaging probes using combinations of proposed technologies; dual-modality imaging using an always on ¹¹¹In and activatable ICG optical dual-labeling trastuzumab antibody (a), and Dual-signaling tumor imaging of trastuzumab–Alexa750 conjugate employing fluorescence intensity and lifetime technologies (b). Both methods simultaneously show distribution of imaging probes with always on gamma-ray (¹¹¹In; a) or Alexa750 signal (b) as well as binding specific-image with activatable ICG (a) or elongated fluorescence lifetime of Alexa750 (b).