pH-Sensitive drug delivery system based on hydrophobic modified konjac glucomannan

Highlights

• KGM-g-AHs were developed as novel intracellular pH-sensitive micelles.

• Curcumin loaded KGM-g-AH₈ micelles exhibited good stability, high LC and EE.

• KGM-g-AH₈ micelles prevented curcumin from degradation and enhanced its solubility.

• The in vitro release showed distinct intracellular pH-sensitivity of the micelles.

• The in vitro cell study showed low cytotoxicity and high cell uptake capability.

Abstract

Amphiphilic aliphatic amines grafted konjac glucomannan (KGM-g-AH₈, KGM-g-AH₁₂ and KGM-g-AH₁₈) micelles were prepared via a simple two-step synthesis with Schiff’s base as the “switch” to achieve intracellular acid-triggered curcumin release. The KGM-g-AH₈ self-assembled into spherical nano-micelles (107.6 ± 11.6 nm) in an aqueous medium, and presented high curcumin loading capacity as well as good physical stability in 28 days. The in vitro curcumin release behaviors proved the controlled release property and the endosomal/lysosomal pH response of KGM-g-AH₈ micelles. The cytotoxicity and cellular uptake studies were also investigated to exhibit the intracellular pH-sensitivity, safety and biocompatibility of KGM-g-AH₈ micelles. This research focuses on the feasibility of KGM-based micelles to be extrapolated as promising strategies for cancer therapy and offers new potential options for intracellular drug delivery.

Introduction

The low solubility, poor targeting property, multidrug resistance (MDR) and toxicity to the normal tissues are major factors accounting for the low therapeutic efficiency of conventional chemotherapeutic agents (Tian & Mao, 2012). To overcome these deficiencies, various drug delivery systems (DDSs) such as liposomes (Kuang, Liu, Liu & Zhuo, 2011), micelles (Wei, Chen, Zhang, & Zhuo, 2009) and microspheres (Zhang et al., 2012) have been intensively investigated. Among these systems, micelles have currently raised considerable interests due to their high drug loading capacity, excellent physical stability, relatively long circulation time with a slow clearance rate and the passive tumor targeted property via enhanced permeability and retention (EPR) effect (Kedar, Phutane, Shidhaye, & Kadam, 2010; Letchford & Burt, 2007). However, they also face a problem about the slow drug release, which significantly lead to the insufficient intracellular drug availability for killing cancer cells (Dai, Lin, Cheng, Zou, & Shuai, 2011). One of the most promising strategies to overcome this problem is the development of micelles in response to environmental stimuli (Stewart et al., 2016). The differences in pH between extracellular normal tissues (7.4) and intracellular endosomes/lysosomes (4.5–6.0) provide an important stimulus (Wei, Zhuo, & Zhang, 2013), which arouse the interest of researchers to design pH-sensitive micelles to achieve acid-triggered intracellular drug release.

Considering the poor biocompatibility and biodegradability of synthetic micelles (Mendiondo, Suit, & Fixler, 2015), lots of efforts have been paid to find suitable natural polymers for pH-sensitive micelles preparation in recent years (Mendiondo et al., 2015; Wu, Wang, Zhuo, & Cheng, 2014; Zhang et al., 2016). However, the intrinsic deficiencies of natural polymers have limited their biomedical application, e.g. chitosan has low solubility in neutral pH (Kadajji & Betageri, 2011), alginate shows poor cellular adhesion and low in vivo degradation (Ulery, Nair, & Laurencin, 2011), and proper modifications are always needed.

Konjac glucomannan (KGM) is a water-soluble natural polysaccharide that is widely used in nutraceutical, biological and pharmaceutical industry owing to its low cost, colon targeting, good biocompatibility and biodegradability (Chen, Liu, & Zhuo, 2005; Wang, 2011; Zhang, Xie, & Xin, 2005). Several modified KGM have been applied to overcome the drawbacks such as the broad molecular weight distribution and limited solubility in organic solvents (Chen et al., 2014), and employed in drug delivery system, e.g. carboxymethyl KGM grafted methoxy poly(ethylene glycol) (Xia et al., 2010) and amphiphilic cholesterol-modified carboxymethyl KGM conjugate (Wei et al., 2011). However, the relatively complex synthetic processes require activators that are difficult to be utterly removed, and don’t benefit intracellular triggered drug release. Thus, simpler synthesis procedures and functionalization for KGM-based micelles preparation may need to be explored.

Therefore, this study aimed to investigate novel pH-sensitive nano micelles, consisting of KGM based amphiphilic polymers (KGM-g-AHs) that are prepared via a simple two-step progress. The KGM-g-AH micelles with Schiff’s base as the “switch” are response to the intracellular endosomal/lysosomal pH, so as to achieve intracellular delivery, with curcumin as the reference drug. The curcumin loading capacity, stability, in vitro curcumin release behaviors, cell viability and cellular uptake were studied to evaluate the pH-sensitivity, controlled release property, safety and biocompatibility of KGM-g-AH micelles. Our proposed drug delivery mechanism is as follows (Scheme 1): KGM-g-AHs are firstly assembled with curcumin captured in the core to protect curcumin from degrading in neutral condition. After intravenous administration, KGM-g-AHs micelles could reach tumor sites through EPR effect and enter the cancer cells by endocytosis. Hydrolyzation of Schiff’s base in endosomal/lysosomal environment (pH 4.5–6.0) will lead to the degradation of the micelles, and result in intracellular curcumin release. This research may provide new options for cancer therapy and improvements of intracellular drug delivery.