Redox-responsive FRET-based polymer dot with BODIPY for fluorescence imaging-guided chemotherapy of tumor

Abstract

Redox-responsive polymer dot (PD) were synthesized from disulfide cross-linked polymers in a carbonized process to allow quenching effects by loading of boron-dipyrromethene (BODIPY) onto the matrix. The disulfide linkage facilitated degradation of the PD system by intracellular glutathione (GSH), leading to fluorescence recovery by BODIPY and intracellular drug release. The paclitaxel release profile showed that approximately 100% of the drug escaped from the matrix in response to 10 mM GSH, whereas less than 10% was released in the absence of GSH. In vitro studies showed that quenching produced by BODIPY loading enabled visual monitoring of cancer cell death, as the quenching disappeared when BODIPY was released by GSH inside of cancer cells. The PD contain disulfide bonds representing a GSH-triggered ligand; thus, nanocarriers presented enhanced in vivo chemotherapeutic inhibition in xenograft tumor-bearing mice localized at the cancer location, guided by fluorescent off-on system tracking and measured by the release of BODIPY. This platform reacts to the redox level in sensitive manner and cancer cell death can be monitored by fluorescence, making this platform useful for bio-applications, particularly in vitro and in vivo therapy and diagnosis, while considering the cell physiological environment. This system may be useful for wider medical applications.

Introduction

Tumors are currently a serious problem causing large numbers of deaths worldwide; thus, many new therapeutic strategies are actively being examined. Among these strategies, multifunctional nanoparticle-based theranostics, including targeting, imaging, and therapy, have provided new opportunities for the diagnosis and treatment of cancer [1], [2]. Abnormal conditions in the cancer microenvironment require more advanced strategies for selective and effective drug delivery systems [3]. Stimuli-responsive moieties have the potential to advance previous methods through functionalization under the different conditions present in normal and tumor cells [4], [5]. However, it remains difficult to ensure that both active and passive targeting agents that are drug-loaded reach the appropriate organelle because conjugation with a labeling agent involves complex techniques. Previously, theranostics techniques have been used with fluorescent dyes for imaging of drug delivery systems, but these approaches are complex. Therefore, methods for effectively killing cancer cells in a safe and accurate manner using guided imaging systems must be improved.

The tumor environment is distinct from that of normal cells in terms of enzyme levels, pH, and reducing agents, which have been intensively studied as parameters for controlling drug release from nanocarriers [6]. Glutathione (GSH), an important reducing agent in cells, is present at approximately 2–5 mM in normal cells, but exhibits up to 4-fold higher concentrations in cancer cells [3], [7]. GSH chemically degrades disulfide bonds via an exchange reaction with dithiol-disulfide groups [8]. Because of their stability are excellent blood circulation, the high levels of GSH in cancer cells enable the design of tunable materials containing a disulfide group as an intracellular redox-responsive drug delivery system in targeted cancer cells [8]. Over the past several decades, various polymers with disulfide linkages have been explored for effective drug delivery, but remain limited to bioimaging and controlled release systems [9], [10], [11].

Numerous studies of polymer dot (PD) have been performed such as in the electrical and optical materials fields. Their properties, such as high brightness, desirable photostability, fast emission rates, high biocompatibility, and low toxicity, suggest that PD can be used as fluorescent biomarkers in living cells [12], [13]. Additionally, PD consist of abundant π-π bonds, making them suitable for aromatic/drug molecule loading through strong π-π stacking or hydrophobic interactions [14]. Thus, PD can be used as transporters without causing structural changes to the drugs. However, it remains challenging to effectively monitor and kill cancer cells under tumor-responsive environment conditions because a PD system with only fluorescent properties cannot be traced. Therefore, stimuli-responsive PD are critical factors for assessing drug delivery and therapeutic efficacy to a specific cancer cell environment without side effects [15], [16].

The application of fluorescent signals alone is not suitable for tracking drug delivery because of their low sensitivity. Many recent studies have reported new models based on “switch-on/off” methods, which show high sensitivity and selectivity detection with quenching effects induced by nanomaterials such as graphene oxide, tungsten oxide, gold nanomaterial, and organic dyes through Förster resonance energy transfer (FRET) [17], [18], [19]. Because GSH is present in cancer cells, we hypothesized that redox-responsive PD with disulfide cross-linked nanoparticles loaded with quenchers and drugs can be used for fluorescence-guided cell death. Here, we synthesized a redox-sensitive nanoparticle from a carbonized disulfide cross-linked polymer (PD) to allow for a FRET mechanism by boron-dipyrromethene (BODIPY) [PD(BODIPY)] and paclitaxel (PTX) carrier [PD (BODIPY/PTX)] through π-π stacking or hydrophobic interactions. BODIPY showed a quenching effect on the PD based on the fluorescent off/on system which was affected by the GSH concentration: after cleavage of the cross-linked disulfide PD, BODIPY was released from the core of PD, switching on fluorescent emission and regulating PTX escape for cancer chemotherapy. After subcutaneous injection in vivo, PD (BODIPY/PTX) passively accumulated at MDA-MB-231 cell sites in mice. The high level GSH promoted cleavage of the disulfide bond for qualified simultaneous PTX transport and BODIPY release to recover the fluorescence. The prepared matrix represents a smart drug nanocarrier for simultaneous diagnosis and chemotherapy to enhance drug efficacy against devastating malignant tumors using a fluorescent off/on system with triggered drug release in response to the cancer cell internal environment.