Remotely Triggered Release from Magnetic Nanoparticles

Abstract

Multivalent nanoparticles have tremendous potential in the diagnosis and treatment of human disease. Their multivalency allows simultaneous conjugation of targeting ligands to improve nanoparticle homing, polymers (e.g., polyethylene glycol (PEG)) to improve nanoparticle pharmacokinetics, as well as therapeutic drug cargo. Drug release from a nanoparticle surface has been accomplished by bonds that are sensitive to hydrolytic degradation or pH; however, complex release profiles that can be controlled from large distances (>10 cm) have not been achieved. Here, we describe a multifunctional nanoparticle that is: (1) multivalent, (2) remotelyactuated, and (3) imaged non-invasively by magnetic resonance imaging. Superparamagnetic nanoparticles act as transducers to capture external electromagnetic energy at 350– 400 kHz, which is not significantly absorbed by tissue, to disrupt hydrogen bonding between complementary oligonucleotides on demand. With a nucleic acid strand covalently linked to the nanoparticle, dye-labeled single stranded DNA (a model antisense therapeutic) self-assembles on the particle’s surface, forming a tunable, heat-labile linker. The multifunctional nanoparticles are used to demonstrate remote, pulsatile release of a single species and multistage release of two species in vitro, as well as noninvasive imaging and remote actuation upon implantation in vivo. Release from surfaces or polymers triggered by an external stimulus (electric current, magnetic fields, temperature, light, ultrasound) has been extensively studied (reviewed in). These strategies, however, have been principally applied to macroand micro-scale materials and drug reservoirs. For focal diseases, such as cancer, these devices must be implanted at the tumor site (e.g., Gliadel). Another approach is to replace these larger depots with drug-carrying nanoparticles that can be individually targeted to the tumor. Heat and light-sensitive liposomes, for example, can be delivered systemically and their contents released in response to an external stimulus. Our strategy has the added advantage of radiofrequency electromagnetic field (EMF) activation, which improves penetration depth over heat or light (at 400 kHz, field penetration into 15 cm of tissue is > 99 %). Similarly, energy absorption, and thus background heating, of water and tissue is insignificant in the 350–400 kHz frequency regime. In contrast, when applied to magnetic materials, these fields produce heat as the magnetic dipole of the material aligns with the external field. We conjugated a 30 bp DNA to dextran-coated iron oxide nanoparticles and added a complement of 12, 18, or 24 bp linked to a model drug, a fluorophore. Excess fluorescent DNA was removed by trapping the particle on a magnetic column and washing with buffer. Particles were trapped in a matrigel plug as an in vitro model of tumor tissue, allowing fluorescent DNA to diffuse out into the surrounding buffer only when liberated from the particles. In Figure 1B we demonstrate pulsatile release of a fluorophore initiated by EMF pulses (400 kHz, 1.25 kW) of 5 min duration every 40 min. The fluorescence of the surrounding buffer increased markedly in the sampling immediately after EMF application, followed by a fluorescence decrease in subsequent samplings. Because much of the fluorescent DNA rehybridized to the particles upon cooling of the plug to room temperature, subsequent EMF application allowed further release. Such a profile would be useful for metronomic dosing of a cytotoxic drug. The use of a nucleic acid duplex as a heat-labile linker adds the additional feature of temperature tunability through changes in chain length and variations in G/C content. Using a variable-gain RF amplifier to control particle heating, biomolecules tethered to these oligonucleotides can be released in multiple stages. In Figure 1C we use oligonucleotides of two different lengths and corresponding fluorescent species (12 bp, FAM; 24 bp, HEX) to demonstrate the potential for complex release profiles. Low power EMF pulses (0.55 kW) triggered release predominantly of FAM by melting of the C O M M U N IC A TI O N