(E-186) Frequency-Dependent Targeting of RGD-Microbubbles In Vitro: A Step Towards Multiple Biomarker Targeting

Introduction:: The use of targeted microbubbles (MBs) for ultrasound molecular imaging (USMI) is a promising technique for assessing endothelial phenotypes. Targeted MBs are decorated with biomolecular ligands that bind specifically to endothelial receptors in order to visualize targets that are usually associated with angiogenesis, thrombosis, or inflammation. However, rather than detecting a single molecular biomarker, a precise diagnosis may require the detection of multiple biomarkers to diagnose disease, follow disease progression and assess therapeutic response. Toward this end, the goal of our work is to selectively displace different sizes of RGD-MBs using acoustic radiation force (F_rad) by matching the ultrasound pulse frequency to the MB resonance frequency. Here, we study this approach in vitro using an α_vβ₃-coated hollow microfiber.

Materials and Methods:: Size-isolated fluorescently labeled MBs with the buried-ligand architecture and diameters of 1 or 5 μm were synthesized and characterized using a Coulter Multisizer. The MBs were pumped through an α_vβ₃-coated microvessel (200 µm ID) at a wall shear stress of 3.5 dyn/cm². MBs were acoustically manipulated using F­_rad at 7 MHz for 47 cycles, or 4 MHz for 27 cycles (6.7 µs pulse length, 0.15 MHz PRF, 133 kPa PNP). The effect of US frequency (4 vs. 7 MHz) on F_rad targeting for the two different monodisperse MB populations was evaluated by counting adherent MBs on the tube surface.

Results, Conclusions, and Discussions:: The mean diameters of our MBs were 1.2 ± 0.3 and 4.4 ± 0.9 µm, where minimal overlap was observed between the two size distributions. Our experimental data showed that at the lower frequency there was an increased binding of the large RGD-MBs compared to the higher frequency. Conversely, the smaller RGD-MBs experienced greater attachment at the higher frequency than at the lower one. In conclusion, frequency-dependent targeting of our RGD-MBs was achieved in vitro, a strategy that has the potential to be used to selectively target multiple biomarkers in applications in vivo.


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