40
D. Li et al.
transmit ultrasonic pulses and record sound echoes from the tissue and reconstruct
the image as different tissues reflect various levels of sound [48–50]. Micron-sized,
gas-containing microbubbles are the most commonly used contrast agent in molecular ultrasound. Intravenously injected ultrasound nanocontrast agents could cross
vascular barriers, bind to specific receptors in the body, and concentrate on target
tissues or organs to provide molecular imaging information [51]. Targeted ultrasound
contrast agents can be prepared by some specific bioconjugation techniques [52].
Due to their high specificity, affinity, targeting property and nonimmunogenicity,
aptamer-based ultrasound imaging probes have a broad application prospect.
Wang et al. proposed a nanobubble covalently conjugated with the sgc8c aptamer
through thiol-maleimide coupling chemistry providing a more specific recognition for ultrasound imaging studies [53]. The sgc8c aptamer was a strong aptamer
with a high binding affinity to CCRF-CEM cells (CCL-119 T-cell, human acute
lymphoblastic leukemia). This nanobubble had low immunogenicity compared with
conventional biotin-avidin complex, and the particle size is about 485 nm, which has
no significant difference from that of the unmodified contrast agent. In contrast to
the resulting images with optical images, ultrasound targeted imaging of conjugated
nanobubbles ex vivo exhibited a higher accumulation and availability in targeted
ultrasound imaging. Conjugation efficiency analysis, flow cytometry assays and
several investigations were used to explore the performance and consistency of the
nanobubbles. However, the three-dimensional imaging effect of the nanobubbles,
binding efficiency to target cells on different blood velocity and quality control of
ultrasound imaging with various particle sizes were still under investigation.
To optimize stability and half-life of the microbubble reported previously, Nakatsuka et al. functionalized a PEG-stabilized thrombin-activated microbubble (PSTAMB). PSTA-MB formulated from interaction with thrombin through incorporation
of aptamer-containing DNA crosslinks in the encapsulating shell (Fig. 2.4a) [54].
Thrombin converts soluble fibrinogen into insoluble fibrinogen and catalyzes other
coagulation-related reactions, acting as a curial role in the blood coagulation pathway.
In the clot region, this microbubble increased acoustic activity about five-fold in the
presence of actively clotting blood after injection of the microbubbles in a rabbit static
thrombosis model (Fig. 2.4b). The microbubble that exhibited stability at in vivo
circulation with no change in biomarker sensitivity could be able to discover clots
not detectable by current contrast agents.
At present, few studies on targeted ultrasound contrast agents use aptamers as
molecular probes. There are several reasons responsible for this phenomenon. First,
the size of conventional ultrasonic contrast agents is sometimes too large (up to
several hundred mm in diameter), and even can make them unable to reach the
vascular tissues or cells. However, microbubbles in nanoscale are complicated to
prepare and need to be highly concentrated in the target site to get the ideal imaging
due to low air carrying capacity of microbubbles. Then, the imaging signal of
ultrasonic contrast agent will be greatly reduced after endocytosis, while many of
aptamers, such as A10 RNA aptamers [55], could promote the endocytosis of the
attached nanoparticles. Finally, the targeted imaging ability and stability of ultrasonic contrast agent still need to be improved compared with probes in other imaging
D. Li et al.
transmit ultrasonic pulses and record sound echoes from the tissue and reconstruct
the image as different tissues reflect various levels of sound [48–50]. Micron-sized,
gas-containing microbubbles are the most commonly used contrast agent in molecular ultrasound. Intravenously injected ultrasound nanocontrast agents could cross
vascular barriers, bind to specific receptors in the body, and concentrate on target
tissues or organs to provide molecular imaging information [51]. Targeted ultrasound
contrast agents can be prepared by some specific bioconjugation techniques [52].
Due to their high specificity, affinity, targeting property and nonimmunogenicity,
aptamer-based ultrasound imaging probes have a broad application prospect.
Wang et al. proposed a nanobubble covalently conjugated with the sgc8c aptamer
through thiol-maleimide coupling chemistry providing a more specific recognition for ultrasound imaging studies [53]. The sgc8c aptamer was a strong aptamer
with a high binding affinity to CCRF-CEM cells (CCL-119 T-cell, human acute
lymphoblastic leukemia). This nanobubble had low immunogenicity compared with
conventional biotin-avidin complex, and the particle size is about 485 nm, which has
no significant difference from that of the unmodified contrast agent. In contrast to
the resulting images with optical images, ultrasound targeted imaging of conjugated
nanobubbles ex vivo exhibited a higher accumulation and availability in targeted
ultrasound imaging. Conjugation efficiency analysis, flow cytometry assays and
several investigations were used to explore the performance and consistency of the
nanobubbles. However, the three-dimensional imaging effect of the nanobubbles,
binding efficiency to target cells on different blood velocity and quality control of
ultrasound imaging with various particle sizes were still under investigation.
To optimize stability and half-life of the microbubble reported previously, Nakatsuka et al. functionalized a PEG-stabilized thrombin-activated microbubble (PSTAMB). PSTA-MB formulated from interaction with thrombin through incorporation
of aptamer-containing DNA crosslinks in the encapsulating shell (Fig. 2.4a) [54].
Thrombin converts soluble fibrinogen into insoluble fibrinogen and catalyzes other
coagulation-related reactions, acting as a curial role in the blood coagulation pathway.
In the clot region, this microbubble increased acoustic activity about five-fold in the
presence of actively clotting blood after injection of the microbubbles in a rabbit static
thrombosis model (Fig. 2.4b). The microbubble that exhibited stability at in vivo
circulation with no change in biomarker sensitivity could be able to discover clots
not detectable by current contrast agents.
At present, few studies on targeted ultrasound contrast agents use aptamers as
molecular probes. There are several reasons responsible for this phenomenon. First,
the size of conventional ultrasonic contrast agents is sometimes too large (up to
several hundred mm in diameter), and even can make them unable to reach the
vascular tissues or cells. However, microbubbles in nanoscale are complicated to
prepare and need to be highly concentrated in the target site to get the ideal imaging
due to low air carrying capacity of microbubbles. Then, the imaging signal of
ultrasonic contrast agent will be greatly reduced after endocytosis, while many of
aptamers, such as A10 RNA aptamers [55], could promote the endocytosis of the
attached nanoparticles. Finally, the targeted imaging ability and stability of ultrasonic contrast agent still need to be improved compared with probes in other imaging