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

Abstract

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 $r_2^{\ast }$ (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 $r_2^{\ast }-r_2$ 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.

Introduction

In recent years, ultrasound (US)-triggered drug vehicles have received considerable attention due to the effectiveness of ultrasound as a diagnostic tool. US is a technique with low electronic interference that allows for easy focus whilst penetrating into deep soft tissue in a non-invasive manner. It is commercially available, and has an established safety record compared to temperature, light and electric and magnetic stimuli [1], [2], [3], [4], [5]. Moreover, US can enhance the intercellular uptake of drugs not only by breaking the tight junction between cells, but also by acoustically enhancing the permeability of the cell membrane to induce endocytosis of drug-loaded micelles [6], [7], [8], [9]. Therefore, an ultrasonically triggered vehicle can potentially be developed into a promising remotely triggered drug delivery system, which can be used in clinical applications.

An US image-guided drug delivery system is one in which ultrasonically triggered vehicles accumulated at a specific site can be simultaneously probed and triggered by US, and in recent years, there has been growing interest in it for the clinical treatment of many diseases [10], [11], [12], [13]. However, whether sufficient drug vehicles travel to the triggering site (i.e. US focus area) must be determined by probing a series of US images, and one limitation of this technique is that the drug carriers might be triggered earlier and/or at the wrong sites during this serial probing with US. Therefore, magnetic resonance imaging (MRI) might be a better imaging tool than US imaging for probing US-triggered drug vehicles since MRI is a powerful non-invasive technique for probing real-time images of living bodies with high resolution at the cellular and molecular level [14], [15], [16]. On this basis, it is necessary to develop a drug vehicle with MRI contrast and ultrasonically triggered release functions, with the expectation that images of vehicles can be probed by MR, and the release of the drug can then be subsequently triggered via US.

In general, drug vehicles exhibiting MR image contrast could be achieved via encapsulating differing amounts of paramagnetic and/or superparamagnetic nanoparticles into polymeric vehicles. Jain et al. developed oleic acid-coated iron oxide nanoparticles to encapsulate doxorubicin and demonstrate MRI contrast [16]. In addition, a magnetic nanovehicle consisting of a magnetite core and a starch shell has been reported to monitor targeting delivery for brain tumors [15]. In these studies, drug released from the polymeric shell was not remotely triggered by US. Hence, in our previous study, a multi-functional drug vehicle exhibiting MRI contrast and ultrasonically triggered behaviors was prepared by encapsulating superparamagnetic nanoparticles inside hydroxyapatite (HA)-coated liposomes [17]. However, encapsulating nanoparticles in the vehicle might decrease the loading volume of the bioactive agent. This was overcome by the novel vehicle developed in the present study. The superparamagnetic iron oxide (SPIO) nanodots were semi-embedded onto the surface of the HA-coated liposome to form a core–shell structure decorated with SPIO nanodots (i.e. HA/SPIO-coated liposome) that will have the ability to exhibit MRI contrast and ultrasonically triggered behaviors. However, it remains unclear whether the SPIO embedded onto the HA coating layer alters the US-triggered release behavior, MRI T₂ contrast and US-induced contrast change of MR images. Few studies have been performed on this issue, which deserves systematic investigation.

The objective of this study was to prepare the HA/SPIO-coated liposome and investigate the influences of SPIO amount and US parameters (i.e. frequency and power density) on background leakage and ultrasonically triggered behaviors of HA/SPIO-coated liposomes. In addition, a preliminary investigation on the MRI contrast of the HA/SPIO-coated liposomes in relation to the SPIO amount and US triggering was also carried out. An understanding of the ultrasonically active behaviors of HA/SPIO-coated liposomes could provide fundamental and valuable information that can be useful in the design and fabrication of MRI-guided drug carriers capable of remotely triggered release.