Photoacoustic imaging (PAI) is increasingly employed in (pre‐) clinical research, thus, development of suitable contrast agents, in particular fluorescence‐quenched chromophores, for PAI is of high importance. Small molecule dyes are appropriate due to favorable circulation, excretion properties, and ease of conjugation to targeting moieties. The BODIPY chromophores have been widely used in bioimaging, yet they are not ideal for PAI due to high fluorescence. Hence, here nonfluorescent BODIPY are designed by 1H‐pyrrole conjugation (PyBODIPY) to apply as probes for PAI. The PyBODIPYs exhibit absorption maxima up to 800 nm, and PA signal could be detected in concentrations of 1 nmol mL−1 and 35 pmol mm−3, by tube and tissue phantom, respectively. In addition to nonfluorescent, PyBODIPYs are non‐phototoxic, photostable, and show high molar extinction coefficients, as well as inertness toward nucleophilic addition. PyBODIPYs are modified with PEG‐400, to improve aqueous solubility and to enable in vivo imaging. Thus, PyBODIPY is an attractive small molecule to use as PA contrast agent, which could be coupled to targeting ligands for in vivo use. In addition, 1H‐pyrrole conjugation might be applied to the design of novel near‐infrared ranged quenchers suitable for PAI, and promote the development of probes for clinical translation.
Photoacoustic imaging (PAI) is a rapidly evolving field in molecular imaging that enables imaging in the depths of ultrasound and with the sensitivity of optical modalities. PAI bases on the photoexcitation of a chromophore, which converts the absorbed light into thermal energy, causing an acoustic pressure wave that can be captured with ultrasound transducers, in generating an image. For in vivo imaging, chromophores strongly absorbing in the near-infrared range (NIR; > 680 nm) are required. As tetrapyrroles have a long history in biomedical applications, novel tetrapyrroles and inspired mimics have been pursued as potentially suitable contrast agents for PAI. The goal of this review is to summarize the current state of the art in PAI applications using tetrapyrroles and related macrocycles inspired by it, highlighting those compounds exhibiting strong NIR-absorption. Furthermore, we discuss the current developments of other absorbers for in vivo photoacoustic (PA) applications.
Short-lived reactive intermediates such as reactive oxygen species (ROS) regulate many physiological processes, but overproduction can also lead to severe tissue dysfunction. Thus, there is a high demand for noninvasive detection of reactive molecules, which, however, is challenging. Herein, we report photoacoustic detection of ROS using conjugated BODIPY probes (ROS-BODIPYs). The ROS reaction with conjugated BODIPYs induced a redshift in absorption by ∼100 nm into the near infrared (from ∼700 to ∼800 nm), quenched fluorescence, and generated strong photoacoustic (PA) signals. Thus, the ROS-activated and ROS-nonactivated states of ROS-BODIPYs can be detected in vivo by PA and fluorescence imaging. Interestingly, ROS activation is reversible, in the presence of excess reducing agents, e.g., citric acid, converted back to its original state, suggesting that ROS-BODIPYs can be useful for the detection of over production of ROS but not physiological amounts. This makes the imaging independent of accumulation of the activated probe with the physiological ROS amounts and thus strongly improves applicability and highlights the translational potential of ROS-BODIPYs for detecting overexpression of ROS in vivo by optical and photoacoustic imaging methods.
The superoxide (O 2 •−) ion is a highly reactive oxygen species involved in many diseases, hence its non-invasive detection is desirable to identify the onset of pathological processes. Here, we employed photoacoustic (PA) spectroscopy, which enables imaging at ultrasound resolution with sensitivity of optical modality, for the first time to detect O 2 •− , using stimuliresponsive contrast agents. Meso (3, 5-di-tert-butyl 4-hydroxyphenyl) porphyrins and oxoporphyrinogens were used as PA contrast agents, which trap the O 2 •− and enable its detection. The trapped O 2 •− increased the PA signal amplitude of chromophores up to 9.6fold, and induced a redshift in the PA signal maxima of up to 225 nm. Therefore, these trigger-responsive probes may be highly valuable as smart diagnostic PA probes to investigate pathological events stimulated by O 2 •− species. Reactive oxygen species (ROS) are highly aggressive, biochemically produced molecules that play important roles in physiological and pathological processes. ROS are a set of short-lived species comprised of O 2 •−
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