Folate receptor-targeted ultrasonic PFOB nanoparticles: Synthesis, characterization and application in tumor-targeted imaging

In this study, we aimed to determine an effective strategy for the synthesis of folate receptor (FR) targeted-nanoparticles (FRNPs). The nanoparticles used as ultrasound contrast agents (UCAs) were composed of a liquid core of perfluorooctyl bromide (PFOB) liposome and a targeted shell chemically conjugated with folic acid (FA) and polyethylene glycol (PEG). This was done in order to avoid recognition and clearance by the mononuclear phagocyte system [also known as the reticuloendothelial system (RES)] and enhance the targeting capability of the nanoparticles to tumors overexpressing folate receptor (FR). The FRNPs exhibited an average particle size of 301±10.8 nm and surface potential of 39.1±0.43 mV. Subsequently, in vitro, FRNPs labeled with FITC fluorescence dye were visibly uptaken into the cytoplasm of FR-overexpressing cancer cells (Bel7402 and SW620 cells), whereas the A549 cells expressing relatively low levels of FR just bound with few FRNPs. These results demonstrated that FRNPs have a high affinity to FR-overexpressing cancer cells. Additionally, in in vivo experiments, FRNPs achieved a greater enhancement of tumor ultrasound imaging and a longer enhancement time in FR-overexpressing tumors and the Cy7-labeled FRNPs exhibited a relatively high tumor-targeted distribution in FR‑overexpressing tumors. Targeted ultrasound and fluorescence imaging revealed that FRNPs have the ability to target FR-overexpressing tumors and ex vivo fluorescence imaging was then used to further verify and confirm the presence of FRNPs in tumor tissues with histological analysis of the tumor slices. On the whole, our data demonstrate that the FRNPs may prove to be a promising candidate for the early diagnosis for FR-overexpressing tumors at the molecular and cellular levels.