Facile preparation of succinylated-zein-ZIF-8 hybrid for enhanced stability and pH-responsive drug delivery

Highlights

• Enhanced redispersibility and stability of succinylated-zein nanoparticles were achieved.

• A facile approach to prepare zein based core-shell drug carrier was reported.

• The carrier exhibited pH-responsive and sustained drug release ability.

• Release kinetics, cytotoxicity and cellular uptake of the carrier were discussed.

Abstract

Zein nanoparticles have good prospects as a pharmaceutical carrier but limited in further application by its poor stability and re-dispersibility. To obtain high stability and pH-responsive drug delivery, succinylated-zein (Suc-zein) was synthesized by nucleophilic substitution reaction, the material obtained was used to prepare Suc-zein nanoparticles (SZs). The SZs prepared not only had good dispersibility but showed better stability than zein particles on pH, storage and salinity. After loading indomethacin (Indo) into zeolitic imidazolate framework-8 (ZIF-8), the core-shelled Indo-Suc-zein-ZIF-8 (Indo-SZZs) were fabricated by coating Suc-zein onto the surface of ZIF-8 with better stability and high drug loading. Indo-SZZs were pH-responsive and sustained a drug release profile in a simulated environment. Also SZZs presented as being non-toxic, having good biocompatibility and good cellular up-take performance, and the cytotoxicity of Indo-SZZs was significantly higher than that of Indo. This provides a new insight into the preparation of a zein-based oral drug delivery system.

Introduction

Oral administration is the preferred method of administration owing to its safety, convenience, and patient compliance, which makes self-administration possible and provides great flexibility in the dosing regimen (Florek et al., 2017). Despite these advantages, the harsh gastrointestinal environment is still a great challenge for many drugs, such as curcumin, taxane, aminoglycosides and polyene antibiotics (Bapat et al., 2019, De Leo et al., 2010, Malingre et al., 2001, Thornton and Wasan, 2009). After oral administration, drugs must pass through several barriers such as mucus barriers and enzymes, which greatly affect the oral bioavailability of the drugs before they can be absorbed into the gastrointestinal epithelium and the systemic circulation (Fuhrmann et al., 2014). Besides, most drugs are hydrophobic, the absorption and diffusion of drugs in the gastrointestinal tract are impeded, resulting in low solubility and low intestinal permeability (Mitragotri et al., 2014). In order to enable high-efficiency in delivery and treatment, researchers have paid more attention to the development of drug delivery systems (DDS). For instance, a self-nanometer emulsifying DDS for oral paclitaxel based on hydroxypropyl methylcellulose was established, which showed that the bioavailability of anti-cancer drugs was four times higher than that of paclitaxel (Gao et al., 2003). DDS consists of the drug and suitable material which can improve the efficiency of drug delivery, the selection of suitable carrier materials is crucial for the construction of DDS (Allen et al., 2004).

Zein is a natural plant protein macromolecule, with strong hydrophobicity and amphiphilicity, it is renewable and non-toxic (Berardi et al., 2018, Guo and Shi, 2009). Compared to other proteins, such as soy protein, gelatin and albumin, zein possesses some unique advantages: 1) zein has been generally regarded as a safe biomaterial excipient by the US Food and Drug Administration (Zhang et al., 2016); 2) zein is capable of self-assembly into nano- or micro-particles (Labib, 2018), which is beneficial for drug loading; 3) zein resists gastric acid and digestive enzymes (Zhang et al., 2015). Thus, zein has been widely used as the carrier material to encapsulate various active ingredients such as drugs, nutrients and DNA (Huang et al., 2019, Liu et al., 2017, Regier et al., 2012, Wang et al., 2018). However, zein particles are prone to agglomeration and precipitation at near neutral pH or high ionic strength, also the zein particle is hard to re-suspend after centrifugation or lyophilization treatment (Patel et al., 2010). Due to these unavoidable drawbacks, it is vital for further modification and functionalization of zein to meet medicinal needs (Wang et al., 2018).

The aggregation behaviour of particles is usually based on colloidal interactions, i.e. electrostatic interactions (Dickinson, 2010). The ionic strength and the isoelectric point of the particles are key to affect colloidal interactions which initiate aggregation. In order to prevent agglomeration, the repulsive interactions such as the electrostatic repulsion and the spatial repulsion need to be strong enough to offset the van der Waals interactions (Dickinson, 2010). It is reported that the acylation of proteins with succinic anhydride (Suc) is one of the most convenient and commonly used methods for altering the functional properties of proteins (Mirmoghtadaie et al., 2009). Succinylation, one of the acylation methods, had been used to functionalize proteins for performance improvements, such as delayed drug delivery (Caillard et al., 2009), thermal stability (Shilpashree et al., 2015), etc. Inspired by these studies, succinylation of zein seems to be a good way of enhancing the stability of the zein particles for further medical applications.

Furthermore, as a drug carrier, the high stability and dispersion of the particles is far from being enough to satisfy the medical functions. To enhance the therapeutic effect, the controlled release ability of zein particles at corresponding stimuli and specific sites was needed. Recently, a zeolitic imidazolate framework-8 (ZIF-8) has emerged as a new kind of carrier. ZIF-8 is formed by the coordination of metal zinc ions with an organic ligand 2-methylimidazole (Sun et al., 2012). It has combined highly desirable properties of both zeolites and MOFs, such as crystallinity, microporosity, high surface area, exceptional thermal and chemical stability (Venna et al., 2010). Because of the unique property, ZIF-8 is used to construct controlled-release delivery systems such as DNA (Kahn et al., 2017), enzyme (Duan et al., 2018), fluorescein (Zhuang et al., 2014), and drugs (Chowdhuri et al., 2016, Ren et al., 2014, Zheng et al., 2017, Zheng et al., 2015). It is worth noting that ZIF-8 remains stable under neutral conditions and rapidly degrades in an acid environment due to protonation (Liedana et al., 2012). Thus, it would be a promising material for the construction of pH-responsive DDS.

In this study, a pH-responsive DDS was well designed and prepared to solve the shortcomings of a pure zein-based carrier for highly efficient drug delivery. Succinylated-zein (Suc-Zein) was firstly synthesized by nucleophilic substitution reaction, and Suc-zein particles (SZs) were prepared by the Built-in Ultrasonic Dialysis Process (BUDP) method. The physicochemical properties of zein before and after succinylation were investigated. Indomethacin (Indo) is an effective anti-inflammatory drug, which causes gastrointestinal-associated adverse effects via oral routes (Chauhan et al., 2003). In order to solve this problem, in this work, Indo was loaded into ZIF-8 and coated with Suc-zein to prepare Indo-Suc-zein-ZIF-8 (Indo-SZZs). The stability of Indo-SZZs at different salt concentrations and different storage times were investigated. In vitro pH-responsive drug release and its release mechanism was explored. Finally, the in vitro cytotoxicity and cellular up-take are discussed in detail.