A novel drug vehicle capable of ultrasound-triggered release with MRI functions

A novel remotely triggered drug vehicle, magnetic hydroxyapatite (HA)-coated liposome (i.e. HA-coated liposome decorated with superparamagnetic iron oxide (SPIO) nanodots; HA/SPIO-coated liposome), was developed to exhibit ultrasound-triggered release behavior, magnetic resonance imaging (MRI) contrast and ultrasound-induced MRI contrast change. In this study, the effects of the HA/SPIO coating layer on the background leakage, response to ultrasound and MR signal were investigated. The background leakage of the liposome was significantly reduced due to HA/SPIO coating of the liposome. This coating layer also enhanced the sensitivity of the drug vehicle to ultrasound under sonication conditions at high frequencies (1 and 3 MHz) and low power densities (0.2-0.4 W cm^-2). Moreover, the ultrasound-triggered vehicle exhibited a concentration-dependent T₂ (spin-spin relaxation time) contrast in MR images due to their decoration with SPIO nanodots. In addition, r₂ and r₂* (transverse relaxivity) values increased with increasing amounts of SPIO decoration, suggesting that the MR images of HA/SPIO-coated liposomes could be probed by the T₂ signal. Most importantly, the r₂*-r₂ value of HA/SPIO-coated liposomes decreased after sonication, which was more prominent for the sample with lower SPIO amounts. This suggests that this novel drug vehicle can be used not only as an MR image-guided drug vehicle capable of ultrasound-triggered release, but also as an MR reporter to probe the status of vehicles after ultrasonic triggering.