Smart drug carrier based on polyurethane material for enhanced and controlled DOX release triggered by redox stimulus

Highlights

• Redox-sensitive micelles were obtained by the self-assembly of polyurethane polymers.

• The micelles showed controlled drug release and high cytotoxicity against HepG2 tumor cells.

• DPD simulation indicated that the blank micelle owned hydrophilic PEG shell and hydrophobic PLA core as well as the DOX-loaded micelle.

• The polymers self-assembled micelles are a very promising for hydrophobic anticancer drugs delivery.

Abstract

A series of amphiphilic polymer mPEG-PUSS-mPEG were developed by a combination of ring opening polymerization (ROP) and polyaddition reaction and its self-assembled micelles were developed for hydrophobic anticancer drugs delivery. Polymeric critical micellar concentration (CMC) values in aqueous solution were about 2.8–5.5 mg/L. And the partition equilibrium constant (K_v) of pyrene in micellar solutions ranged from 1.52 × 10⁵ to 2.20 × 10⁵. The average sizes of the self-assembled blank and DOX-loaded micelles were 160–180 nm determined by dynamic light scattering (DLS). The morphology of the DOX-loaded micelles was spherical by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).The in vitro drug release behaviors of DOX and PTX from mPEG-PUSS-mPEG micelles were investigated at different simulated conditions. We found that the PTX release was significantly accelerated by redox stimuli compared with DOX-loaded reduction-sensitive PU micelles. The change of the size of this system under different conditions was further evaluated by DLS. DOX-loaded mPEG-PUSS-mPEG micelles in 50% fetal bovine serum (FBS) were evaluated the hemocompatibility. In addition, the self-assembly process of polymers mPEG-PUSS-mPEG in aqueous solution for micellar formation was also investigated by means of dissipative particle dynamics (DPD) simulations.AndCCK-8 assays revealed that the mPEG-PUSS-mPEG materials had low toxicity, but the DOX-loaded micelles showed a high cytotoxicity against HepG2 tumor cells. All results demonstrate that mPEG-PUSS-mPEG self-assembled micelles are a very promising for hydrophobic anticancer drugs delivery.

Introduction

Health topics have drawn so much attention as the development and improvement of human's life. As the World Health Organization (WHO) reported, cancer have been the second biggest threat of human's health, which killed 8.8 million people just within one year at 2015. Since chemotherapy of tumor was a useful method for highly efficient treatment of cancer, many chemotherapeutics were studied and made use of [1]. While most antitumor drugs were so hydrophobic that they were faced with several bottlenecks in chemotherapy application [2], such as low solubility [3,4], high toxicity [5,6], short circulation time in vivo [7,8] and so on [9,10]. To conquer these problems, researchers have developed series of nanocarriers by means of nanotechnology [[11], [12], [13]], including nanoparticles [14,15], polymersomes [16,17], nanogels [18,19] and micelles [20,21]. Among these carriers, micelle based on hydrophilic polymers has shown many advantages in efficient drug delivery, and some were even taken into clinic trial [[22], [23], [24]]. Micelle consisting of inner core and outer shell was formed by the way of polymer self-assembly in aqueous solution [25,26]. The hydrophobic blocks in polymers formed micellar core, providing a container for drug entrapment, and the hydrophilic blocks formed micellar shell, giving micelle good stability for longer in vivo circulation time [[27], [28], [29], [30], [31]]. In addition, the particle sizes of polymeric micelles were usually lower than 200 nm, and could selectively accumulated in tumor site by EPR effect [[32], [33], [34]]. Moreover, it's convenient to decorate micellar architecture with different targeted group such as folic acid [[35], [36], [37]], aptamer [38,39] and some others for enhancing tumor targeting and cell uptake [[40], [41], [42], [43]]. All these advantages made polymeric micelle a good potential drug delivery system.

However, hydrophobicity is the main driving force for micellar core to entrapment of drug molecules as well as the biggest barrier for drug release from micellar core. As reported, the drug release process was usually a slow and uncontrolled [44], which decreased the antitumor efficiency to a large extent because of tumor's multidrug resistance (MDR) [45,46]. Recently, some researchers have proved it that fast release and high concentration accumulation of drug during a short time was an effective strategy to overcoming the MDR, leading to obvious apoptosis of tumor cells [47,48].Stimuli responsive micelles (SRMs) could easily realize the above goals, which was also called “smart micelles”, and smartly respond to external stimulus, such as pH [49], reductive potential [50], enzyme [51], temperature [52], light [53] and so on [54,55], resulting in obvious change of micellar aggregation morphologies and then following with fast and controlled drug release.

Earlier work confirmed it that there was distinct difference of reductive potential between normal tissue and tumor site [56]. In normal extracellular matrices and body fluids, such as plasma, the GSH concentration is approximately 2–20 μM, which is lower than that in human cells (cytosol, about 0.5–10 mM).The microenvironment in the tumor cells are especially hypoxic and reductive compared with normal cells. The GSH concentration in tumor cells is far higher than that in plasma and normal cells (at least 4 fold) [57]. In theory, the difference of GSH concentration between the extracellular and intracellular conditions can be used for GSH-triggered intracellular drug delivery [58,59]. Polymeric micelles containing disulfide bonds in polymer block structure could intelligently respond to high concentration of GSH with quickly breaking up of disulfide bond, and then micelles went through swelling or even disassembly, which was in favor of drug release to a large extent [[60], [61], [62]].

Wang [63]design a redox sensitive micelle from the self-assembly of disulfide bonds containing polymer, PCL-SS-PEEP, and drug loaded micelles was obtained with entrapment of model drug DOX. They found that the micelle maintain good stability without GSH, while disaggregated in the presence of 10 mM GSH. In vitro DOX release profiles, the accumulated drug release during 140 h was up to about 100% with 10 mM GSH, and the DOX-loaded micelles have highly efficient inhibition of proliferation MCF-7/ADR cell. Yang [64] introduced disulfide bonds into the side chain of polymer mPEG-b-PEEP and obtained redox sensitive mPEG-b-(DSSEEP-EEP)by ring-opening polymerization method. In their work, they investigated the responsive behavior in different conditions with or without GSH. It's clear that the disulfide bonds in side-chains of polymer broke up when it comes to GSH, and the hydrophobic blocks changed into hydrophilic ones, and then the micelles disaggregated, following with a relative high drug release (80% during 48 h) in simulated intracellular condition at tumor site.

Polyurethanes have the advantages of easy synthesis, high degree of tailoring, excellent biocompatibility, biological inertness, and easy introduction of target molecules and ligands, making them possible to stimuli in response to polymer structures and a promising drug carrier for next-generation nanotherapy. Therefore, many PU copolymers prepared are now used in hydrophobic drugs and drug delivery system (DDS) [65,66]. It has been observed that PUs synthesized from aliphatic isocyanates are reported as more biocompatible than aromatic isocyanate-based PUs. Therefore, PUs based on aliphatic isocyanates are preferred over aromatics for in vivo applications [67,68]. Besides, PLA as a drug carrier has the disadvantages of leading to burst drug release and degradation rate is slow Polyurethanes not only have excellent biocompatibility, but also provide a storage place and a release channel for drugs. Therefore, polylactic acid polyurethane may exhibit a different drug release behavior from polylactic acid [69,70].

Now in this work, an amphiphilic polyurethane polymer mPEG-PUSS-mPEG was synthesized by means of addition polymerization method, which consisted of mPEG block and PLA block with disulfide bonds. The polymer could self-assemble into micelles with core-shell structure. The micellar inner core was hydrophobic PLA blocks, a container for DOX entrapment, and the hydrophilic mPEG blocks formed outer shell, giving micelle good stability in aqueous solution. As shown in Scheme 1, the particle size and distribution of micelles were investigated by DLS and TEM measurement. Meanwhile, the change of the size of this system under different conditions was further evaluated by DLS. DOX-loaded mPEG-PUSS-mPEG micelles in 50% fetal bovine serum (FBS) were evaluated the hemocompatibility. After that, the work carried out in vitro drug release profiles in different simulated conditions. During the above experiments, the process and mechanism of micellar formation and drug loading were also investigated on mesoscopic level with DPD simulation method. Finally, this work studied and evaluated the cytotoxicity of blank and DOX-loaded micelles against HepG2 cells and HUVEC cells by CCK-8.