(370d) Award Submission: On-Demand Drug Release of Polymeric Core-Shell Nanoparticles Developed through Aqueous Surface-Initiated Sara-ATRP

Chemotherapeutic delivery vehicles that can be externally addressed to release drugs "on-demand" offer significant potential to dynamically adjust treatment according to the patient's response (i.e. personalized medicine) without requiring repeated invasive and often painful drug administrations. Even though new drug delivery methods that lower secondary risks have been developed via nanomedicine, they have not significantly improved patients' survival rates [1]. New therapeutic paradigms are required to achieve the desired improvements in patient outcomes.

One key group of materials with potential in this regard is nanoscale stimuli-responsive systems that react to either exogenous stimuli such as temperature variations, magnetic fields, ultrasound, light, or electric fields [2] or endogenous changes in local pH and enzyme concentration associated with a disease [3]. Magnetic triggering is especially attractive as an external trigger since magnetic fields are highly penetrative (>1 m in all tissues [4], as opposed to light and, in specific scenarios, ultrasound, that have penetration depths as short as the millimetre scale) without adversely affecting biological tissues at biologically-relevant field strengths/frequencies. The most common implementation of magnetic triggering applies superparamagnetic iron oxide nanoparticles (SPIONs), whose single-crystal nanostructure enables the transformation of energy from an alternating magnetic field (AMF) into heat through Neel and Brownian relaxation mechanisms. SPIONs can be used alone for cancer therapy by leveraging this heat generation capacity to induce localized hyperthermia to kill cancer cells, with thermotherapy noted to have fewer side effects than chemo or radiotherapy [5]. However, increasing recent interest has focused on creating hybrid nanosystems that merge inorganic nanoparticles with drug delivery systems to synergistically induce apoptosis in cancer cells, with the drug acting together with the higher local temperature to create a more efficient cancer therapy [6], [7].

Introducing the potential for pulsatile, rather than simple diffusion-controlled, release would add another therapeutic option to trigger release of drug “on-demand” as per a patient’s need, addressing the lack of control over release kinetics currently achievable once a drug delivery vehicle is administered into the body. Thermoresponsive hydrogels have been previously used for this purpose; however, their high porosity allows loaded drug to be released rapidly coupled with relatively high “off” state drug release by diffusion through even the collapsed gel phase. As an alternative, the use of the glass transition (Tg) properties of solid polymer-based particles would significantly reduce “off” state release while still allowing for temperature-induced “on” state release enhancements if the Tg is tuned to be just above physiological temperature. While current research has successfully led to the development of a range of injectable smart materials on many length scales and with many geometries, the capacity to safely and non-invasively control the smart response(s) locally and repeatedly without biological side-effects over the long term, remains challenging. By combining the glass transition with core-shell nanoparticles that can enable stabilization/targeting of the nanotherapeutic, we see the potential to develop an injectable drug delivery vehicle with the capacity for repeated pulsing while avoiding substantial drug delivery in the "off" state that currently hinders the translation of on-demand drug delivery devices.

Objective

We aim to develop a nanoscale injectable on-demand drug delivery system that can be activated by heat and/or magnetic fields to selectively deliver drug only when triggered. Such a system will decrease the secondary effects of systemic cancer therapy drugs, increasing drug concentration at the target site, and reduce total drug dose compared to the systemic delivery approach. The gel shell around the glass transition-switchable nanoparticle promotes colloidal stability and reduces biological responses. The heat-induced trigger will allow for on-demand release of only the needed amount of drug only when heat or an alternating magnetic field is applied, the latter enabled by the incorporation of SPIONs in the shell of the particles.

Methodology

The core nanoparticles were fabricated by free radical polymerization of methyl methacrylate (MMA), butyl methacrylate (BMA), and 2-(2bromoisobutyryloxy)ethyl methacrylate BIEM) using an oil-in-water miniemulsion technique, using rhodamine B as the model drug loaded by directly incorporating it during the miniemulsion process. The particle size was determined via dynamic light scattering. Following, the bromine groups in BIEM were used as an initiator to grow the OEGMA shell via aqueous surface-initiated SARA-ATRP. SPION groups were incorporated in the shell. Zwitterionic polymers functionalized with either ketone or hydrazide polymers (enabling the formation of a hydrazone bond upon contact) were used to form an in situ-gelling hydrogel to immobilize the core-shell particles at a specific injection site, with the polymer concentration adjusted to obtain a desirable gelation time (1-2 min). The core-shell particles were dispersed in one or both precursor polymer solutions and then co-injected through a double barrel syringe fitted with a static mixer to form the nanocomposite hydrogels for drug release testing. Rhodamine B release was assessed by immersing the nanocomposite gels in PBS and exposing them to heat (45°C) or a 200 kHz alternating magnetic field for activation.

Results

Core nanoparticles were formulated using different ratios of monomers and macroinitiators to determine the effect the macroinitiator would have on the final glass transition temperature. Increasing the concentration of the macroinitiator from 0.25% to 1.4% only decreased the Tg by 2°C (58.5°C to 55.7°C), too high for safe triggering under physiological conditions. As such, to obtain a desirable Tg (45°C). butyl methacrylate was added to the MMA-BIEM mix, decreasing the Tg by 10°C and obtaining a final Tg of 45.6°C. The particle sizes were not significantly affected by the comonomer composition, yielding core particles of ~120 nm in diameter across a range of monomer contents.

The SARA-ATRP reaction was subsequently optimized to grow the passivating polymeric shell around the core nanoparticle. Different parameters such as the polymerization temperature and the amount of macroinitiator/BIEM were changed to determine the best conditions for shell growth. Using sub-ambient temperatures or higher concentrations of macroinitiator promoted interactions between the particles, creating a particle network rather than individual core-shell nanoparticles. In contrast, performing the reaction at room temperature and decreasing macroinitiator concentration resulted in stable core-shell nanoparticles. Shell growth was analyzed using dynamic light scattering after different polymerization times to control the shell thickness, allowing for the creation of core-shell nanoparticles of ~300 nm diameter. SPIONs were then precipitated into the shell using the xxx groups to nucleate the particle growth, with the successful incorporation of SPIONs into the shell confirmed by transmission electron microscopy.

Rhodamine B release was then tracked following encapsulation of the core-shell nanoparticles in the in situ-gelling zwitterionic hydrogel to immobilize the core-shell nanoparticles at a target therapeutic site. Release was tracked (via fluorescence analysis) into a PBS bath by: (1) heating the nanocomposite hydrogel from 37°C and 45°C; or (2) maintaining the bulk temperature at 37°C but applying alternating magnetic field (AMF) pulses. A 30-minute AMF pulse at 37°C achieved the same release as heating the nanocomposite to 45°C, with the 37°C/No-AMF condition facilitating significantly lower drug release. As such, AMF and temperature can both trigger a significant pulse of drug release from the core-shell nanoparticles.

Conclusion

Core-shell nanoparticles containing a core with a super-physiological glass transition temperature (Tg ~ 45°C), a stabilizing shell, and precipitated superparamagnetic iron nanoparticles in the shell can promote triggered release of drug upon direct heating or an alternating magnetic field stimulus.

Significance

We developed the first polymer-polymer core-shell nanoparticles via aqueous SI-SARA-ATRP and the first polymeric core-shell nanoparticle system used as a drug delivery vehicle, triggerable by either heating or magnetic field activation. We anticipate this system offers potential for facilitating improved pulsatile drug delivery with an improved “off” state release (taking advantage of the very slow drug diffusion through the core particle under glassy conditions), facilitating more specific on-off drug release and thus longer functionality and higher specificity of drug release at a target site.

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