Drug delivery with nanostructured porous silicon nanoparticles

Nanomedicines have great potential to address some of the big problems in cancer therapy, such as how to get enough of the right drug to the right place without causing side effects or inducing drug resistance. Traditional chemotherapies can be toxic, but the current research advances in nanotechnology enable the design and manufacture of nanoparticles that carry drugs to tumor sites and release them in situ in a controlled manner. Multifunctional nanoparticles with carefully controlled chemistry, size, surface charge, and other properties can carry drugs and give them new functions, providing a safer and more effective therapy than conventional chemotherapy. Despite intensive research in the development of efficient nanomedicines, there are still several hurdles that need to be overcome before the efficient clinical application of nanodrugs.

Nanostructured mesoporous silicon (PSi) has received considerable attention in the past few years in the field of biomedical nanotechnology. PSi-based materials are fabricated by electrochemical etching. This top-down approach enables the pore sizes to be tailored within the nanometer range, with different surface chemistries (hydrophilic or hydrophobic) and a wide range of porosities (up to 90%) to enable high drug loading levels. Furthermore, the nanoparticles can be engineered to be highly biodegradable and biocompatible. Thus, nanostructured PSi-based nanomaterials can be strictly designed for specific applications (see Figure 1).^1–6 For example, the surface chemistry provides a suitable platform for covalent conjugation and electrostatic attachment of fluorophores—fluorescent and radioactive molecules^{2, 3}—that have considerable promise as the next generation of nanomedicines for the early detection, simultaneous monitoring, and treatment of diseases with minimal toxicity.

Figure 1. Schematic representation of (i) spherical-shaped nano- structured mesoporous silicon (PSi) drug carriers (ii) that can be biofunctionalized with different biological ligands and polymers (iii) to allow travel through the bloodstream and release the therapeutic compounds in the vicinity of tumor sites, (iv) as well as enabling simultaneous, real-time monitoring of its actions both in vitro and in vivo.

Figure 2. The self-assembly of hydrophobin protein onto the surface of radiolabelled thermally hydrocarbonized PSi (THCPSi) nanoparticles modulates the biodistribution and clearance of the fraction of PSi nanoparticles found in the liver and spleen compared with the uncoated PSi nanoparticles.

Figure 3. Transmission electron microscope images of (a) THCPSi nanoparticles and (b) THCPSi solid-lipid nanocomposites (THCPSi-SLNCs) prepared using a solid-in-oil-in-water (S/O/W) emulsion evaporation method. (c) Dispersions of the THCPSi nanoparticles and THCPSi-SLNCs in aqueous solution, demonstrating the higher stability of the latter. (d) Impact of the human plasma on the particle size for both nanoparticles. (e) Confocal fluorescence microscopy images of macrophages cells (orange) after a 3h incubation with THCPSi nanoparticles and THCPSi-SLNCs (green), showing a reduction in cellular association to the latter particles. (f) Controlled furosemide (FUR) release from the THCPSi-SLNCs at pH 5.5 and 37°C.

To develop a bioimaging platform based on PSi nanoparticles for biomedical applications, we first successfully developed a method to radiolabel thermally hydrocarbonized PSi (THCPSi) nanoparticles of size ∼142nm with a fluorine-18 (¹⁸F) radioisotopic label.^5–7 The biodistribution of the nanosized ¹⁸F-nanoparticles after oral, subcutaneous, and intravenous administration in rats confirmed that fluorine-18 was adequate for determining the blood circulation life-time of the labelled particles and for tracking them through the gastrointestinal tract.^{6, 7} The radiolabelled nanosystem exhibited excellent in vivo stability and was even able to trace the fate of the PSi nanoparticles after biofunctionalization with a hydrophobin protein,^{5, 6} which renders the PSi-based material a flexible platform for in vivo imaging purposes.

Then, to study the ‘biofate’ (distribution) of the particles after intravenous administration to rats, we coated ¹⁸F-labeled THCPSi nanoparticles with self-assembling hydrophobin protein from the fungus Trichoderma reesei and studied the intravenous biodistribution in rats. Protein coating altered the hydrophobicity of the THCPSi nanoparticles, resulting in a pronounced change in the degree of plasma protein adsorption to the nanoparticle surface in vitro. This also changed the biofate of the nanoparticles between the liver and spleen (see Figure 2),⁵ which can be used to modulate the immune recognition and subsequent elimination of the nanoparticles from the circulation.

Recently, we created a novel nanocomposite for controlled and targeted drug delivery based on the encapsulation of THCPSi nanoparticles—see Figure 3(a)—with solid-lipid nanoparticles in a 1 :1 ratio using a solid-in-oil-in-water emulsion solvent evaporation method.⁸ This approach enables us to form a nanocomposite with rather different surface smoothness and hydrophobicity compared with the uncoated PSi nanoparticles: see Figure 3(b) and (c). In addition, the method greatly improved the stability of the nanocomposite in human plasma and the cytocompatibility, and reduced cellular association: see Figure 3(d) and (e). A clear prolonged release for a model drug (furosemide) further demonstrated the capability of the nanoparticles to sustain the release of the compound: see Figure 3(f).

Overall, we anticipate the emergence of PSi-based nanodrugs to result in substantial benefits to the field of theranostics (a combination of therapy and diagnostics). PSi is an acceptable biomaterial with several advantageous features that render it an efficient drug carrier and imaging agent. The great advantages of the nanostructured PSi materials are the good biocompatibility, biodegradability, high pore volume necessary for hosting large amounts of therapeutics, tunable pore sizes for fine control of drug loads and release kinetics, high surface area for drug adsorption, easy surface chemistry modification for further biofunctionalization, and control of drug loading and release. We are now developing advanced nanostructured PSi-based theranostic platforms to improve the intracellular transport of drugs to the desired unhealthy cells or tissues with simultaneous real-time imaging, without damaging the healthy cells.

This research is supported by grants from the Academy of Finland (decision numbers 252215 and 256394), University of Helsinki and European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013) grant number 310892. A special acknowledgment goes to Jouni Hirvonen (University of Helsinki), Jarno Salonen (University of Turku), and Anu Airaksinen (University of Helsinki) for their faithful collaborations throughout the years.