Tae Gwan Park

Surface-functionalized electrospun nanofibers for tissue engineering and drug delivery

Electrospun nanofibers with a high surface area to volume ratio have received much attention because of their potential applications for biomedical devices, tissue engineering scaffolds, and drug delivery carriers. In order to develop electrospun nanofibers as useful nanobiomaterials, surfaces of electrospun nanofibers have been chemically functionalized for achieving sustained delivery through physical adsorption of diverse bioactive molecules. Surface modification of nanofibers includes plasma treatment, wet chemical method, surface graft polymerization, and co-electrospinning of surface active agents and polymers. A variety of bioactive molecules including anti-cancer drugs, enzymes, cytokines, and polysaccharides were entrapped within the interior or physically immobilized on the surface for controlled drug delivery. Surfaces of electrospun nanofibers were also chemically modified with immobilizing cell specific bioactive ligands to enhance cell adhesion, proliferation, and differentiation by mimicking morphology and biological functions of extracellular matrix. This review summarizes surface modification strategies of electrospun polymeric nanofibers for controlled drug delivery and tissue engineering.

Porous biodegradable polymeric scaffolds prepared by thermally induced phase separation

Poly(L-lactic acid) and its copolymers with D-lactic and glycolic acid were used to fabricate various porous biodegradable scaffolds suitable for tissue engineering and drug delivery based on a thermally induced phase separation (TIPS) technique. A variety of parameters involved in TIPS process, such as types of polymers, polymer concentration, solvent/nonsolvent ratio, and quenching temperature, were examined in detail to produce a wide array of micro- and macroporous structures. A mixture of dioxane and water was used for a binary composition of solvent and nonsolvent, respectively. In particular, the coarsening effect of pore enlargement affected by controlling the quenching temperature was used for the generation of a macroporous open cellular structure with pore diameters above 100 microm. The use of amorphous polymers with a slow cooling rate resulted in a macroporous open cellular structure, whereas that of semicrystalline polymers with a fast cooling rate generated a microporous closed cellular structure. The fabricated porous devices loaded with recombinant human growth hormone (rhGH) were tested for the controlled delivery of rhGH, as a potential additional means to cell delivery.

A novel fabrication method of macroporous biodegradable polymer scaffolds using gas foaming salt as a porogen additive

Highly open porous biodegradable poly(L-lactic acid) ¿PLLA scaffolds for tissue regeneration were fabricated by using ammonium bicarbonate as an efficient gas foaming agent as well as a particulate porogen salt. A binary mixture of PLLA-solvent gel containing dispersed ammonium bicarbonate salt particles, which became a paste state, was cast in a mold and subsequently immersed in a hot water solution to permit the evolution of ammonia and carbon dioxide within the solidifying polymer matrix. This resulted in the expansion of pores within the polymer matrix to a great extent, leading to well interconnected macroporous scaffolds having mean pore diameters of around 300-400 microm, ideal for high-density cell seeding. Rat hepatocytes seeded into the scaffolds exhibited about 95% seeding efficiency and up to 40% viability at 1 day after the seeding. The novelty of this new method is that the PLLA paste containing ammonium bicarbonate salt particles can be easily handled and molded into any shape, allowing for fabricating a wide range of temporal tissue scaffolds requiring a specific shape and geometry.

siRNA delivery systems for cancer treatment

With increasing knowledge on the molecular mechanisms of endogenous RNA interference, small interfering RNAs (siRNAs) have been emerging as innovative nucleic acid medicines for treatment of incurable diseases such as cancers. Although several siRNA candidates for the treatment of ocular and respiratory diseases are undergoing clinical trials, there are challenges inherent in the further development of siRNAs for anti-cancer therapeutics, because systemic administration will be required in most cases. In addition to nonspecific off-target and immune stimulation problems, appropriate delivery remains a major hurdle. The technologies developed for delivery of nucleic acid medicines such as plasmid DNA and antisense oligonucleotides have paved the way to rapid progress for in vivo delivery of siRNAs. Here, we review various in vivo delivery strategies including chemical modification, conjugation, lipid-based techniques, polymer-based nanosystems, and physical methods. Moreover, the current progress in siRNA delivery systems for gynecologic, liver, lung, and prostate cancers is discussed.

Biodegradable polymeric micelles composed of doxorubicin conjugated PLGA-PEG block copolymer

Biodegradable polymeric micelles containing doxorubicin in the core region were prepared from a di-block copolymer composed of doxorubicin-conjugated poly(DL-lactic-co-glycolic acid) (PLGA) and polyethyleneglycol (PEG). The di-block copolymer of PLGA-PEG was first synthesized and the primary amino group of doxorubicin was then conjugated to the terminal hydroxyl group of PLGA, which had been pre-activated using p-nitrophenyl chloroformate. The resulting polymeric micelles in aqueous solution were characterized by measurement of size, drug loading, and critical micelle concentration. The micelles containing chemically-conjugated doxorubicin exhibited a more sustained release profile than PEG-PLGA micelles containing physically-entrapped doxorubicin. The cytotoxic activity of the micelles against HepG2 cells was greater than free doxorubicin, suggesting that the micelles containing conjugated doxorubicin were more effectively taken up cellularly, by an endocytosis mechanism rather than by passive diffusion. Confocal microscopic observation and flow cytometry analysis supported the enhanced cellular uptake of the micelles.

Co-delivery of siRNA and paclitaxel into cancer cells by biodegradable cationic micelles based on PDMAEMA-PCL-PDMAEMA triblock copolymers

Biodegradable cationic micelles were prepared from PDMAEMA-PCL-PDMAEMA triblock copolymers and applied for the delivery of siRNA and paclitaxel into cancer cells. PDMAEMA-PCL-PDMAEMA copolymers were readily obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization of dimethylaminoethyl methacrylate (DMAEMA) using CPADN-PCL-CPADN (CPADN: 4-cyanopentanoic acid dithionaphthalenoate; PCL: 3600 Da) as a macro-RAFT agent. The molecular weights of PDMAEMA blocks, controlled by monomer/CPADN-PCL-CPADN mole ratios, varied from 2700, 4800 to 9100 (denoted as polymer 1, 2 and 3, respectively). These triblock copolymers formed nano-sized micelles in water with positive surface charges ranging from +29.3 to +35.5 mV. Both micelles 1 and 2 revealed a low cytotoxicity. Gel retardation assay showed that micelles 1 and 2 could effectively complex with siRNA at and above N/P ratios of 4/1 and 2/1, respectively. Notably, GFP siRNA complexed with micelle 1 exhibited significantly enhanced gene silencing efficiency as compared to that formulated with 20 kDa PDMAEMA or 25kDa branched PEI in GFP-expressed MDA-MB-435-GFP cells. Moreover, micelle 1 loaded with paclitaxel displayed higher drug efficacy than free paclitaxel in PC3 cells, due to most likely improved cellular uptake. The combinatorial delivery of VEGF siRNA and paclitaxel showed an efficient knockdown of VEGF expression. Confocal laser scanning microscope studies on GFP siRNA complexed with nile red-loaded micelle revealed that nile red was delivered into GFP-expressed MDA-MB-435-GFP cells and that GFP expression was significantly inhibited. These results demonstrated that cationic biodegradable micelles are highly promising for the combinatorial delivery of siRNA and lipophilic anti-cancer drugs.

Biodegradable polymeric microcellular foams by modified thermally induced phase separation method

Thermally induced phase separation (TIPS) for the fabrication of porous foams based on various biodegradable polymers of poly(L-lactic acid) and its copolymers with D-lactic acid and/or glycolic acid is presented. Diverse foam morphologies were obtained by systematically changing several parameters involved in the TIPS process, such as polymer type and concentration, coarsening conditions, solvent/nonsolvent composition, and the presence of an additive. The produced foams had microcellular structures with average pore diameters ranging from 1 to 30 microns depending on the process parameters, which were characterized by scanning electron microscopy (SEM) and mercury intrusion porosimetry. Additionally, Pluronic F127 was used as an additive porogen to control the pore geometry and size.

Local and systemic delivery of VEGF siRNA using polyelectrolyte complex micelles for effective treatment of cancer

For efficient cancer therapy, small interfering RNA (siRNA) should be stably and efficiently delivered into the target tissue and readily taken up by cancer cells. To address these needs, a polyelectrolyte complex (PEC) micelle-based siRNA delivery system was developed for anti-angiogenic gene therapy. The interaction between poly(ethylene glycol) (PEG)-conjugated vascular endothelial growth factor siRNA (VEGF siRNA-PEG) and polyethylenimine (PEI) led to the spontaneous formation of nanoscale polyelectrolyte complex micelles (VEGF siRNA-PEG/PEI PEC micelles), having a characteristic siRNA/PEI PEC inner core with a surrounding PEG shell layer. Intravenous as well as intratumoral administration of the PEC micelles significantly inhibited VEGF expression at the tumor tissue and suppressed tumor growth in an animal tumor model without showing any detectable inflammatory responses in mice. Upon examination of the PEC micelle distribution and in vivo optical imaging following intravenously injection, enhanced accumulation of the PEC micelles was also observed in the tumor region. This study demonstrates the feasibility of using PEC micelles as a potential carrier for therapeutic siRNAs in local and systemic treatment of cancer.

In vitro and in vivo anti-tumor activities of nanoparticles based on doxorubicin–PLGA conjugates

Doxorubicin was chemically conjugated to a terminal end group of poly(D,L-lactic-co-glycolic acid) [PLGA] by an ester linkage and the doxorubicin-PLGA conjugate was formulated into nanoparticles. A carboxylic acid end group of PLGA was conjugated to a primary hydroxyl group of doxorubicin. The primary amine group of doxorubicin was protected during the conjugation process and then deprotected. The nanoparticles containing the conjugate exhibited sustained doxorubicin release profiles over a 1-month period, whereas those containing unconjugated free doxorubicin showed a rapid doxorubicin release within 5 days. Doxorubicin release patterns could be controlled by conjugating doxorubicin to PLGA having different molecular weights. The conjugated doxorubicin nanoparticles showed increased uptake within a HepG2 cell line, which was quantitated by a flow cytometry and visualized by confocal microscopy. The nanoparticles exhibited slightly lower IC(50) value against the HepG2 cell line compared to that of free doxorubicin. In vivo anti-tumor activity assay also showed that a single injection of the nanoparticles had comparable activity to that of free doxorubicin administered by daily injection. The conjugation approach of doxorubicin to PLGA was potentially useful for the formulation of nanoparticles that requires targeting for cancer cells as well as sustained release at the site.

siRNA Conjugate Delivery Systems

Small interfering RNA (siRNA) has been chemically conjugated to a variety of bioactive molecules, lipids, polymers, peptides, and inorganic nanostructured materials to enhance their pharmacokinetic behavior, cellular uptake, target specificity, and safety. To efficiently deliver siRNAs to the target cells and tissues, many different siRNA bioconjugates were synthesized and characterized, and their gene silencing efficiencies were tested in vitro and in vivo. In this review, recent developments of siRNA bioconjugates are summarized.