Soo-Chang Song

Thermosensitive poly(organophosphazene)-paclitaxel conjugate gels for antitumor applications.

A poly(organophosphazene)-PTX conjugate was synthesized by a covalent ester linkage between PTX and carboxylic acid-terminated poly(organophosphazene), which can be readily modified by various hydrophobic, hydrophilic, and other functional substitutes. The physicochemical properties, hydrolytic degradation and PTX release behaviors of the polymer-PTX conjugate were characterized, in addition to the in vitro and in vivo antitumor activities. The aqueous solutions of these conjugates showed a sol-gel transition behavior that depended on temperature changes. The in vitro antitumor activity of the polymer-PTX conjugate was investigated by an MTT assay against human tumor cell lines. From the in vivo antitumor activity studies with tumor-induced (xenografted) nude mice, the polymer-paclitaxel conjugate hydrogels after local injection at the tumor site were shown to inhibit tumor growth more effectively and longer than paclitaxel and saline alone, indicating that the tumor-active paclitaxel from the polymer-PTX conjugate hydrogel is released slowly over a longer period of time and effectively accumulated locally in the tumor sites. These combined observations suggest that this poly(organophosphazene)-PTX conjugate holds promise for use in clinical studies as single and/or combination therapies.

Doxorubicin-polyphosphazene conjugate hydrogels for locally controlled delivery of cancer therapeutics.

Poly(organophosphazene)-doxorubicin (DOX) conjugate bearing hydrophobic L-isoleucine ethyl ester (IleOEt) and hydrophilic alpha-amino-omega-methoxy-poly(ethylene glycol) with molecular weight of 550 Da (AMPEG 550) along with carboxylic acid as a functional group was synthesized to create a drug delivery system, which is based on locally injectable, biodegradable, and thermosensitive hydrogels. In addition to the evaluation of the in vitro and in vivo antitumor activities, the physicochemical properties, hydrolytic degradation, and DOX release profile of the poly(organophosphazene)-DOX conjugate were determined. The aqueous solution of the polymer-DOX conjugate showed a sol-gel transition behavior depending on temperature changes. Based on the in vivo antitumor activities of the locally injected poly(organophosphazene)-DOX conjugate into the tumor-induced nude mice, the conjugate hydrogel after the local injection at the tumor site was shown to inhibit tumor growth more effectively with less toxicity and much longer than doxorubicin and saline as controls, indicating that tumor active DOX from the conjugate hydrogel is released slowly over a longer period of time and effectively accumulated locally in the tumor sites. These results suggest that the poly(organophosphazene)-doxorubicin conjugates hold great potential for use in preclinical and clinical studies as single and/or combination therapies.

Pharmacokinetics of doxorubicin after intratumoral injection using a thermosensitive hydrogel in tumor-bearing mice

A novel, thermosensitive hydrogel, poly(organophosphazene), is an injectable drug delivery system that transforms from sol to gel at body temperature. Doxorubicin (DOX) is a cytotoxic drug used for the treatment of several solid tumors. Due to its acute cardiac toxicity, DOX is a good candidate for local chemo-drug delivery system. In this study, we evaluated the pharmacokinetics of DOX (30 mg/kg) when given as an intratumoral injection using poly(organophosphazene) hydrogel in mice with human gastric tumor xenografts. DOX was formulated at 0.6% into a 10% hydrogel, and 40% and 90% of the dose was released in a sustained manner over 5 weeks in vitro and in vivo, respectively. The hydrogel mass was well retained over 7 weeks, and T(1/2beta, tumor) was 1.8-fold longer than that of the solution, but the 2.2-fold lower C(max, tumor), produced a similar AUC(tumor) and antitumor effect. However, solution caused a 2-fold higher systemic exposure (AUC(plasma)), which resulted in significant mortality due to acute cardiac toxicity. These data indicate that hydrogel formulation may have similar efficacy but lower systemic exposure than aqueous solution. In conclusion, poly(organophosphazene) showed adequate characteristics for local intratumoral delivery of DOX, including dose capacity, local retention, and minimal systemic spill-over. The safety and biocompatibility of poly(organophosphazene) should be further evaluated and its application should be extended to other anticancer agents.

The use of injectable, thermosensitive poly(organophosphazene)–RGD conjugates for the enhancement of mesenchymal stem cell osteogenic differentiation

An injectable and thermosensitive poly(organophosphazene)-RGD conjugate to enhance functionality was synthesized by a covalent amide linkage between a cell adhesion peptide, GRGDS and carboxylic acid-terminated poly(organophosphazene). The aqueous solutions of synthesized poly(organophosphazene)-GRGDS conjugates existed in an injectable fluid state at room temperature and immediately formed a hydrogel at body temperature. The rabbit mesenchymal stem cells (rMSCs) on the polymer-GRGDS conjugate (conjugate 1-2, 0.05 mol fraction as GRGDS) hydrogel constructs using an injection method into a nude mouse were proved to express markers at mRNA level for all stages towards osteogenesis and mainly a sharp increase of osteocalcin (OCN, a typical late osteogenic differentiation marker) levels at 4th week post-induction indicated that the maturation process has started within this period. By histological and immunohistochemical evaluations, significantly high mineralization level by calcium contents was detected qualitatively and collagen type I (Col I), a major characteristic marker protein, was mainly and highly expressed by the rMSCs cultivated in the polymer-GRGDS conjugate hydrogel constructs formed into the nude mouse. The results suggest that the poly(organophosphazene)-GRGDS conjugate to enhance biofunctionality hold a promise for cell delivery material to induce osteogenic differentiation of MSC for enhancing ectopic bone formation.

Sustained delivery of human growth hormone using a polyelectrolyte complex-loaded thermosensitive polyphosphazene hydrogel.

A combined system of polyelectrolyte complex (PEC) and injectable, biodegradable and thermosensitive poly(organophosphazene) hydrogel has been suggested as an injectable depot for a controlled and sustained delivery of human growth hormone (hGH) to improve patient compliance. PEC was prepared by mixing polycations with hGH to suppress diffusion of hGH from the hydrogel through an enlargement of the hydrodynamic size of hGH. Among the polycations, poly-L-arginine (PLA) formed a large complex with hGH and its size increased as the amount of PLA increased. When PLA and/or zinc were added to hGH, the time-dependent stability of hGH increased more than that of native-hGH. The polymer solution containing PECs formed a gel at 37°C. PLA decreased the initial release rate of hGH in proportion to the amount of PLA in vitro and in vivo. Zinc increased the released amount of hGH from the PEC-loaded hydrogel in vitro and in vivo. In a pharmacokinetic study in rats, a single administration of PEC-loaded hydrogel resulted in the sustained release of hGH for 5days. These results suggest that injectable, biodegradable, and thermosensitive PEC-loaded poly(organophosphazene) hydrogel has great potential to be used as an effective delivery system for a sustained release of hGH with improved patient compliance.

Injectable Polyplex Hydrogel for Localized and Long-Term Delivery of siRNA

Here, we describe a concept for localized and long-term delivery of short interfering RNA (siRNA) using an injectable polyplex hydrogel possessing thermosensitivity and biodegradability properties. We prepared a low molecular weight polyethyleneimine-poly(organophosphazene) conjugate as a thermosensitive and cationic polymer that has a cleavable ester linkage. The conjugates formed about 100 nm sized polyplexes with siRNAs, and the polyplex solution turned into a polyplex hydrogel at body temperature via a hydrophobic interaction. We injected the polyplex hydrogel with siRNA of cyclin B1, an essential protein for controlling the cell cycle, into the tumor xenograft model. Polyplexes were slowly released from the polyplex hydrogel by dissolution and degradation, allowing an in vivo antitumor effect via cyclin B1 gene silencing for 4 weeks with only a single injection.

Thermosensitive/magnetic poly(organophosphazene) hydrogel as a long-term magnetic resonance contrast platform

A thermosensitive/magnetic poly(organophosphazene) hydrogel (a magnetic hydrogel) was designed and synthesized for long-term magnetic resonance (MR) imaging. To turn a thermosensitive poly(organophosphazene) hydrogel (an original hydrogel) into a long-term MR contrast platform, cobalt ferrite (CoFe(2)O(4)) nanoparticles, which have hydrophobic surfaces, were bound to the original hydrogel via interactions between the hydrophobic surfaces of the nanoparticles and the (L)-isoleucine ethyl esters of the polymer. The magnetic hydrogel showed extremely low cytotoxicity and adequate magnetic properties for use in long-term MR imaging, in addition to possessing the same properties of the original hydrogel, such as viscosity, thermosensitivity, biodegradability, biocompatibility, a reversible sol-to-gel phase transition near body temperature, and injectability. The magnetic hydrogel was injected into a rat brain using stereotactic surgery. After the injection, the applicable potentiality as a long-term MR contrast platform was successfully estimated over 4-5 weeks. Consequently, it was shown that a magnetic hydrogel as a long-term MR contrast platform has the potential to be applied in a long-term theranostic hydrogel system. Furthermore, it is expected that this platform can be useful in the clinical field of incurable diseases due to either surgical difficulties or lethality, such as with brain tumors, when the platform is combined with therapeutic drugs for long-term MR theragnosis in further studies.

Chemically crosslinkable thermosensitive polyphosphazene gels as injectable materials for biomedical applications

Chemically crosslinkable and thermosensitive poly(organophosphazenes) containing multiple thiol (-SH) groups along with hydrophobic isoleucine ethyl ester and hydrophilic alpha-amino-omega-methoxy-poly(ethylene glycol) of the molecular weight 550 have been synthesized and characterized as an injectable biomaterial. The aqueous solutions of these polymers were transformed into hydrogel with desired gel strength at body temperature via hydrophobic interactions, and the gel strength was further improved by the cross-linking of thiol groups with crosslinkers, divinyl sulfone (VS) and PEG divinyl sulfone (PEGVS) under physiological conditions. The kinetics of cross-linking behavior of polymer thiol groups with crosslinkers was studied in both in vitro and in vivo conditions. Field Emission-Scanning Electron Microscopy (FE-SEM), swelling experiments, and rheology study of present polymers revealed that the inner three-dimensional hydrogel networks depended on the degree of thiol units in the polymer network. From the in vivo (in mice) degradation studies, the dual cross-linked gels showed to have a controlled degradation. These results demonstrate that the inner network of the hydrogels can be tuned, gel strength and degradation rate can be controlled, and the chemically crosslinkable and thermosensitive poly(organophosphazenes) hold promises for uses as injectable systems for biomedical applications including tissue engineering and protein delivery.

Dual ionic interaction system based on polyelectrolyte complex and ionic, injectable, and thermosensitive hydrogel for sustained release of human growth hormone.

A dual ionic interaction system composed of a positively charged polyelectrolyte complex (PEC) containing human growth hormone (hGH) and anionic thermosensitive hydrogel has been suggested for sustained delivery of bioactive hGH. The PEC was prepared by ionic interaction between negatively charged hGH and positively charged protamine sulfate (PS) to suppress diffusion of hGH. Moreover, we loaded the positively charged PEC into an anionic, injectable, and thermosensitive poly(organophosphazene) hydrogel to enhance sustained release of hGH by dual ionic interactions. PS formed a spherical complex with hGH, and their ionic interaction grew stronger with increasing amounts of PS. From a weight ratio of 0.5, the PS/hGH complex had a size and zeta-potential that were constantly maintained around 500 nm and +8 mV, respectively, in 0.9% NaCl. The PEC-loaded hydrogels suppressed the initial burst release of hGH and extended the release period in vitro and in vivo. In a pharmacokinetic study in rats, the PEC-loaded anionic hydrogel extended half-life 13-fold with similar area under the curve (AUC) compared to hGH solution. Furthermore, single injection of PEC-loaded anionic hydrogel showed a more increased growth rate than daily injection of hGH solution for 7 days in hypophysectomized rats, demonstrating its potential as an injectable, sustained delivery system that can release bioactive hGH.

Long-term theranostic hydrogel system for solid tumors.

The long-term theranostic hydrogel system for solid tumors was prepared via simple physical mixing, which consisted of three major parts: the thermosensitive/biodegradable poly(organophosphazene) hydrogel, PEGylated cobalt ferrite nanoparticles, and paclitaxel (PTX). The PEGylated cobalt ferrite nanoparticles showed extremely low cytotoxicity due to the surface modification using PEG chains. The long-term theranostic hydrogel system showed adequate properties to be used for long-term MR theragnosis. In particular, the theranostic hydrogel gradually degraded over 28 days, and the PTX was sustainedly released out from the theranostic hydrogel over the same period in vitro. Furthermore, the in vivo efficacy of long-term MR theragnosis using the theranostic hydrogel system was estimated successfully over 3 weeks by using high field (4.7 T) animal MRI and solid tumor-bearing mice. Based on our results, we expect that this system can supply multiple data regarding a) the progress of therapy and b) the treatment processes via one- or two-time i.t. administration for cases in which surgical approaches are difficult to apply. Meanwhile, cancer patients can be free from the pain of multiple surgical treatments and have the advantage of therapy through a simple i.t. administration.