Versatile hollow COF nanospheres via manipulating transferrin corona for precise glioma-targeted drug delivery

Abstract

Covalent organic framework (COF) nanoparticles for interference-free targeted drug delivery to glioma remains in its infancy. Herein, hollow COF nanospheres with high crystallinity and uniformed sizes were facilely prepared via heterogeneous nucleation-growth. The prepared COF had large surface area/pore volume and exhibited appropriate degradation behavior under acid environment, where therefore doxorubicin was effectively encapsulated and exhibited a pH-sensitive release. Most charmingly, T₁₀ peptide that has high affinity with transferrin (Tf), was conjugated to endow the hollow COF interesting properties to specifically absorb Tf in vivo as Tf corona. For the first time, multifunctional hollow COF nanospheres (the better one named DCPT-2) were successfully synthesized to achieve interference-free cascade-targeting glioma drug delivery across the blood-brain barrier. Treatment with DCPT-2 brought an improved therapeutic outcome with significantly prolonged median survival time and low side effects. This work promised not only a potential protein corona-mediated COF-based drug delivery platform with good biocompatibility for effective and precise brain tumor therapy, but also an endogenous protein corona-mediated targeting strategy for general cancer therapy.

Introduction

Glioma is one of the most malignant tumors with high morbidity and mortality [1]. Although various therapies [[2], [3], [4]], including the classical chemotherapy, have been exploited for glioma treatments, the progress remains gloomy [5,6]. One possible reason is the special location of glioma within central nervous system, where the multiple biological barriers, especially the blood-brain barrier (BBB) greatly hinder the drug accumulation within tumor [1,5,6]. Overcoming the BBB and site-specific delivery of therapeutics into tumor cells is still a considerable challenge for current glioma treatment.

Recently, nanoparticles-mediated drug delivery (improved chemotherapy) has shown great prospects for cancer therapy due to their enhanced targeting and sometimes synergistic multifunctionalities [[2], [3], [4]]. Among these nanoparticles, covalent organic framework (COF) is springing up vigorously. As a kind of crystalline porous polymer connected by strong covalent bonds, COF has good chemical stability, plentiful and adjustable micro/mesopores and predictable control over composition, topology and porosity, which are superior to the traditional polymer materials for the drug delivery under complicate in vivo environments [7,8]. Meanwhile, comparing to the burgeoning inorganic nanoparticles (metal, carbon, silica, etc.) [8], COF with polymer frameworks is expected to possess better biocompatibility, more promising biodegradation and much easier surface-functionalization, which are desirable for biomedical applications [9]. However, some drawbacks still exist when exploiting the COF materials for potential drug delivery to tumor, especially to the glioma. First, most COF is synthesized with irregular-flake aggregates and hydrophobic surfaces, which causes the difficulty to deliver drugs [[10], [11], [12]]. Second, the merely micro- or mesopores with small pore volume greatly depresses the drug loading efficiency, resulting in inadequate drug delivery [13,14]. Third, site-specific delivery based on COF materials is still on the way due to the absence of appropriate targeting modification [15,16]. Moreover, to our best knowledge, COF-mediated in vivo drug delivery has not been exploited to overcome the BBB for glioma-targeted therapy. Therefore, regulating the uniformed COF nanoparticles with large drug storage cavity and hydrophilic functionalization for glioma-targeted therapy is in an urgent demand.

As known, ligand-mediated targeting is often adopted to guide the specific accumulation of nanoparticles in tumor [17,18]. However, owing to the high surface free energy, most of these targeted nanocarriers suffer from the serious binding with proteins to form the “protein corona” in the blood stream, leading to the reduction or even loss of their targeting capability [19]. Meanwhile, these targeted nanocarriers are always regarded as exogenous materials that are easy to be cleaned out from the body via mononuclear phagocytic systems, resulting in the rapid invalidity [19,20]. Alternatively, protein corona targeted nano-carriers, where the ligands-modified nanoparticles can specifically bind with the special endogenous protein in the bloodstream to form protein corona (targeting ability) but wouldn't alter their biological function, are proposed as an improved strategy for drug delivery [20,21]. By this way, the targeted protein corona can avoid the nonspecific binding of the nanoparticles with other serum proteins, reduce the immune responses and inflammatory reactions and prevent the uptake from macrophage or other opsonin proteins, improving the drug accumulation [21]. And the endogenous protein corona has much lower immune responses and inflammatory reactions as compared with the exogenous substances. In view of this, transferrin (Tf) and its ligands such as T₁₀ have received great attentions. Tf presents at a high concentration in blood, and its receptor (TfR), a kind of trans-membrane protein, highly expresses in glioma cells, suggesting the potential protein corona for targeted nanoparticles [22,23]. Moreover, TfR plays a principal role in promoting translocation of irons across the BBB, suggesting its ability to mediate the nanoparticles to overcome the BBB [[24], [25], [26]]. T₁₀ belongs to the Tf-binding ligand that can encamp easily on the surface of nanoparticles and bind non-disruptively with Tf to form the Tf corona, in which T₁₀ can selectively recognize a specific nonbinding domain of Tf, leaving its biological function unaltered [23,27]. When conjugated to nanoparticles, T₁₀ mediated Tf corona nanocarriers can improve specificity and targeting capabilities of nanoparticles to tumor, promoting its internalization in tumor cells [27,28]. Therefore, constructing a ligand-conjugated nanoparticle that can specifically bind with the endogenous Tf to form Tf corona in the blood might escape from the interference of nonspecific protein corona and then achieve the selective translocation across the BBB and accumulation in the glioma.

In this study, the hollow COF nanospheres with high crystallinity and large surface area/pore volume were facilely synthesized via heterogeneous nucleation-growth using polyethyleneimine-grafted mesoporous silica nanoparticles (PEI@MSN) as template (Fig. 1). These COF nanospheres were then covalently modified with polyethylene glycol (PEG) and T₁₀ to improve their water-dispersity and realize the specific binding with Tf, respectively. As a result, the Tf corona-transformed COF nanospheres could deliver anticancer drug doxorubicin (DOX) to overcome the BBB for pH-sensitive and interference-free glioma-targeting chemotherapy (Fig. 2). This study paved a way to expand the COF nanomaterials for controllable drug delivery in vitro and in vivo, and also to exploit the protein corona-mediated tumor-targeted therapy.