Small Molecule Optoacoustic Contrast Agents: An Unexplored Avenue for Enhancing In Vivo Imaging

Abstract

Almost every variety of medical imaging technique relies heavily on exogenous contrast agents to generate high-resolution images of biological structures. Organic small molecule contrast agents, in particular, are well suited for biomedical imaging applications due to their favorable biocompatibility and amenability to structural modification. PET/SPECT, MRI, and fluorescence imaging all have a large host of small molecule contrast agents developed for them, and there exists an academic understanding of how these compounds can be developed. Optoacoustic imaging is a relatively newer imaging technique and, as such, lacks well-established small molecule contrast agents; many of the contrast agents used are the same ones which have found use in fluorescence imaging applications. Many commonly-used fluorescent dyes have found successful application in optoacoustic imaging, but others generate no detectable signal. Moreover, the structural features that either enable a molecule to generate a detectable optoacoustic signal or prevent it from doing so are poorly understood, so design of new contrast agents lacks direction. This review aims to compile the small molecule optoacoustic contrast agents that have been successfully employed in the literature to bridge the information gap between molecular design and optoacoustic signal generation. The information contained within will help to provide direction for the future synthesis of optoacoustic contrast agents.

Keywords: contrast agents; near-infrared; optoacoustic; small molecule.

Figure 1

(a) Optoacoustic image of naked mouse ear before Evans blue injection at 570 nm (b) optoacoustic image of naked mouse ear before Evans blue injection at 610 nm. (c) optoacoustic image of naked mouse ear after Evans blue injection at 610 nm. Arrows indicated capillaries that become visible and continuous with Evans blue injection. Figure from [48] reproduced with permission from the publisher.

Figure 2

(a) Absorbance spectra of different concentrations of methylene blue in water (b) Absorbance spectra of 80 µM solution of methylene blue in water with 4 different salt concentrations (grey dotted line is absorbance of dimer in water, black dotted line is absorbance of monomer in water) Figure from [54] reproduced with permission from the publisher.

Figure 3

(A) optoacoustic image of control without methylene blue at 635 nm (B) optoacoustic image of SLN 52 min after injection of MB at 635 nm. Figure from [55] reproduced with permission from the publisher.

Figure 4

(a,b): photographs of phantoms with concentrations varying from 0.5 mg/mL (upper left corner of (a)) to 0.01 mg/mL (lower right corner of (b)) (c,d): optoacoustic images of the respective phantoms. Figure from [63] reproduced with permission from the publisher.

Figure 5

The ability to view multiple dyes in a single animal simultaneously is demonstrated. (A) no exogenous contrast agent (B) Image with only ICG (C) Image with only CF-750 probe (D) Image with both ICG and CF-750 Figure from [67] reproduced with permission from the publisher.

Figure 6

The optoacoustic spectra of free ICG in 3 μL/mL saline (■) and in 3 μL/mL plasma (▲). Eight trials were done, and the average of data for the various trials was plotted. Figure from [75] reproduced with permission from the publisher.

Figure 7

Gastric motility imaging using oral delivery of ICG. (a) ICG distribution after administration, (b) Ex vivo fluorescence verification in animals sacrificed after ICG dosing and (c) optoacoustic signal intensity over time compared with ex vivo fluorescence verification (red squares). Figure from [73] reproduced with permission from the publisher.

Figure 8

IR780-based probes have the potential to detect concentrations of calcium and serve as a tracking dye for monitoring microparticle accumulation. (A,B) Response of an IR780 based probe to calcium concentration. As calcium concentration increases, both absorbance and optoacoustic signal decrease. Figure from [82]. (C)—MSOT imaging of IR780 impregnated polystyrene microspheres in mouse liver after ileocolic vein injection. Figure from [83] reproduced with permission from the publisher.

Figure 9

Imaging tumor necrosis with IRDye800 CW and its conjugates (2DG—2-deoxyglucose, EGF—endothelial growth factor, PEG—polyethylene glycol). IRDye 800 CW and its conjugates except 800CW-PEG showed co-localization with TUNEL staining which highlights damaged DNA. Figure from [90] reproduced with permission from the publisher.

Figure 10

Imaging of HER2+ and HER2− cell lines with Alexa Fluor 750-Hercepin conjugates. Figure from [92] reproduced with permission from the publisher.

Figure 11

rQY dependence on degree-of-loading for Alexa Fluor 750-Hercepin conjugate. Figure from [92] reproduced with permission from the publisher.

Figure 12

Relative optoacoustic signals from BHQ3, QXL680, and a variety of NIR dyes with 675 (left) and 750 nm (right) excitation. Figure from [93] reproduced with permission from the publisher.

Figure 13

Quencher-based MMP-2 probe cleavage. (a) Absorbance spectra from before (solid line) and after (dashed line) MMP-2 activation of the probe. Optoacoustic imaging of probe before and after cleavage at 675 (b) and 750 nm (c). Figure from [93] reproduced with permission from the publisher.

Figure 14

Comparison of absorbance (left) and optoacoustic signal (right) for ICG (green) and QSY 21 (red) labeled SWNTs. Figure from [101] reproduced with permission from the publisher.

Figure 15

Animal testing of QSY 21 and ICG labeled SWNTs. Three separate subcutaneous injections were performed on a single mouse as shown in (A). The first contained QSY labeled SWNTs, the second contained ICG labeled SWNTS, and the third contained a mixture. These are shown respectively in inset images (B–D). Figure from [101] reproduced with permission from the publisher.

Figure 16

Optoacoustic spectral shapes of four representative dyes were evaluated at 1 µM (red line) and 10 µM (blue line) in tissue mimicking phantoms. Samples were measured at each 5 nm using an InVision 512-echo. (A) Methylene blue, (B) Indocyanine Green, (C) IR 780 iodide, (D) IRDye QC-1.