Engineered magnetic hybrid nanoparticles with enhanced relaxivity for tumor imaging
Introduction
Nanoparticles have been proposed for the intravascular administration of imaging and therapeutic agents. As compared to freely administered molecules, nanoparticle can provide multiple functionalities simultaneously, such as imaging, therapy, sensing and targeting [1], [2], [3]. In this regard, the design and synthesis of magnetic nanomaterials have attracted the interest of numerous investigators for improving the performance of contrast agents in magnetic resonance imaging (MRI) [4], [5], [6], [7], [8], in vitro cell separation and manipulation [9]; controlled and triggered release of therapeutic agents [10], [11]; and thermal ablation-based therapies via alternating magnetic fields [12], [13]. Super-paramagnetic iron oxide nanoparticles (SPIOs) and gadolinium chelates (Gd-chelates) are the most clinically successful and safe contrast agents for T1- and T2-weigthed MR imaging, respectively [4], [6], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Sufficiently small SPIOs are decomposed by the acidic endo/lisosomal environment and the resulting iron is assimilated by the body for the synthesis of metalloproteins such as hemoglobin [17]; whereas Gd-chelates firmly sequester the metal ions (Gd3+) limiting transmetallation and the consequent possible occurrence of nephrogenic systemic fibrosis [23]. The major limitations of the currently clinically used contrast agents for MR imaging reside in the lack of tissue specificity, inevitably affecting the signal-to-noise ratio; the short circulation time, due to their rapid excretion thought the kidneys and non-specific sequestration by reticulo-endothelial system (RES); and the insufficient relaxivity.
Systemically injected SPIOs are easily recognized and sequestered by macrophages residing within the RES organs, primarily the liver and spleen, so that their circulation half-life is limited to a few minutes and the portion of the injected dose reaching the biological target may be insufficient to induce any significant contrast enhancement. A quite successful and well-known technology to minimize RES uptake consists in coating SPIOs with hydrophilic polymers, such as polyethylene glycol (PEG) and dextran [24], [25], [26], [27]. For instance Feridex® (r2 = 120 (mm s)−1) and Supravist® (r2 = 57 (mm s)−1) have their magnetic cores coated with hydrophilic polymeric chains. This has shown to prolong the half-life in the circulation up to ∼2 h [28], [29], [30], but it also may modulate the relaxation performance [31]. Different nanoparticle formulations engulfing multiple SPIOs have been recently developed in the attempt of extending the circulation half-life and, possibly, enhancing the relaxivity. For instance, 40 nm PEG coated iron oxide nanoparticles induced, at 24 post-injection, a 30% enhancement in contrast as compared to immediately post-injection [32]. This confirms that long circulating and sufficiently small nanoparticles can take advantage of the hyperpermeability of the tumor vessels and accumulate progressively therein [33]. Clusters of SPIOs self-assembled with block copolymers of poly(ethyleneimine) (PEI), poly(caprolactone) (PCL) and PEG have shown in vitro relaxivities up to 300 (mm s)−1 at 1.41 T, for complexes of about 80 nm in diameter [6]. Similarly, magnetic micelles with high transversal relaxivity and long circulation half-life were demonstrated by assembling together SPIOs, molecules of paclitaxel and PEG for a total average size of ∼180 nm [7].
In this work, we demonstrate the synthesis of lipid coated poly(lactic acid-co-glycolic acid) (PLGA) nanoparticles encapsulating hydrophobic ultra-small SPIOs (USPIOs; with a magnetic core of 5 nm) and directly conjugated to Gd-DOTA (Gd-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) through its superficial lipid coating. The resulting magnetic lipid–polymer (hybrid) nanoparticles (MHNPs) are characterized for their physico-chemical properties and stability under physiological conditions. Three different nanoparticles are synthesized including Gd-DOTA only (Gd-MHNPs); USPIOs only (Fe-MHNPs); and the combination thereof (MHNPs). The loading efficiency and in vitro MRI relaxometric properties are quantified for all three different systems. In mice developing a melanoma tumor on their flank, the MHNPs have been systemically injected and their specific organ accumulation has been measured via inductively coupled plasma mass spectroscopy (ICP-MS) on the element Gd. A clinical 3T MRI scanner has been used for imaging the malignant mass.
Section snippets
Materials
DSPE-PEG-COOH (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-carboxy-(polyethyleneglycol)-2000), egg PC (l-α-phosphatidylcholine, egg, chicken), DSPE (1,2-distearoyl-sn-glycero-3-phosphoethanolamine), (l-α-phosphatidyl ethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt), PE-NBD) were purchase from Avanti Polar Lipid Inc. and used as received. PLGA (50:50) was purchased from Lactate Absorbable Polymers – DURECT Corporation. N-Hydroxysuccimidyl ester activated
Synthesis and characterization of Gd–lipid complex
The proposed magnetic hybrid nanoparticles (MHNPs) comprise three distinct compartments with specific functions (Fig. 1A): i) a solid PLGA polymeric core, acting as a cytoskeleton and providing mechanical stability, and encapsulating poorly water-soluble payloads, such as the hydrophobic USPIOs or drug molecules; ii) a lipid shell wrapped around the core, acting as a cell membrane, where the Gd-DOTA molecules are anchored; and iii) a hydrophilic polymer stealth layer outside the lipid shell,
Conclusions
Magnetic lipid–polymer nanoparticles (MHNPs) were synthesized encapsulating 5 nm USPIOs within their hydrophobic PLGA core and decorated with Gd–lipid complexes. Under physiological conditions, they showed high stability and monodispersity with an average hydrodynamic diameter of ∼150 nm. The Gd-MHNPs, hybrid nanoparticles carrying only Gd–lipid complexes, showed longitudinal relaxation about 4 times higher than conventional Gd-based contrast agents. On the other hand, hybrid nanoparticles
Acknowledgment
This work is partially supported by the Cancer Prevention Research Institute of Texas through the grant CPRIT RP110262 and The Methodist Hospital Research Institute.
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