Pluronic nanogel containing calcium carbonate as an ultrasound enhanced contrast agent

摘要

Nanc-sized ultrasound enhanced contrast agents (nano-UECAs) are interesting since they can further enhance the ultrasound (US) imaging of tumor by increasing the local concentration of the contrast agent upon administration due to its nano-size (EPR effect). However, nano-UECAs developed so far have shown limitations for the US imaging of the tumor. First, upon intravenous (i.v.) injection, the accumulation at the tumor was not high enough to show the US imaging. Second, the enhanced US contrast signal by nano-UECA also increased the US signal at other organs, such as liver after i.v. injection, thus, they could not distinguish the tumor from normal tissues by US imaging. To overcome these limitations of nano-UECAs, we synthesized soft nanogel containing US contrast agent, pluronic nanogel containing calcium carbonate (CaCO3), for selective tumor imaging by US. To prepare the pluronic nanogel containing CaCO3, the pluronic nanogel, formed by photo-crosslinking diacrylated pluronic, was used first to load alginate, and then Ca ions were added to induce the alginate-Ca2+ complex formed inside the pluronic nanogel. Then, by adding carbonate ion to the pluronic nanogel containing alginate-Ca2+ gel, CaCO3 nanoparticles were formed in situ inside the pluronic nanogel. This CaCO3 containing nanogel has the size of ～150 nm, and good stability in a serum containing cell culture medium. This nano-UECA also generates CO2 by the dissolution of CaCO3, preferentially at low pHs, thus it could be useful for selective tumor detection by US imaging because tumor usually has a lower pH environment than normal tissues. The nano-UECA showed the enhanced US contrast signal in US imaging and increasing contrast signal with decreasing pH in vitro. Furthermore, after i.v. injection of the nano-UECA into the tumor bearing mice, this UECA showed the selective tumor detection by the enhanced US contrast signal in the tumor site in contrast to no US contrast signal in the liver. Also, the nano-UECA showed a rapid clearance from the mice within 24 h. As the results, the nano-UECA could be used as an effective ultrasound contrast modality for selective tumor diagnosis by using US imaging.