Poly(lactic acid-co-glycolic acid)–poly(ethylene glycol)–poly(lactic acid-co-glycolic acid) thermogel as a novel submucosal cushion for endoscopic submucosal dissection

Abstract

Endoscopic submucosal dissection (ESD) is a clinical therapy for early stage neoplastic lesions in the gastrointestinal tract. It is, however, faced with a crucial problem: the high occurrence of perforation. The formation of a submucosal fluid cushion (SFC) via a fluid injection is the best way to avoid perforation, and thus an appropriate biomaterial is vital for this minimally invasive endoscopic technique. In this study, we introduced an injectable thermogel as a novel submucosal injection substance in ESD. The hydrogel synthesized by us was composed of poly(lactic acid-co-glycolic acid)–poly(ethylene glycol)–poly(lactic acid-co-glycolic acid) (PLGA–PEG–PLGA) triblock copolymers. The polymer/water system was a low-viscosity fluid at room temperature and thus easily injected, and turned into a non-flowing gel at body temperature after injection. The submucosal injection of the thermogel to create SFCs was performed in both resected porcine stomachs and living minipigs. High mucosal elevation with a clear margin was maintained for a long duration. Accurate en bloc resection was achieved with the assistance of the thermogel. The mean procedure time was strikingly reduced. Meanwhile, no obvious bleeding, perforation and tissue damage were observed. The application of the thermogel not only facilitated the ESD procedure, but also increased the efficacy and safety of ESD. Therefore, the PLGA–PEG–PLGA thermogel provides an excellent submucosal injection system, and has great potential to improve the ESD technique significantly.

Introduction

The incidence of gastric cancer is high in many countries, especially in Eastern Asia. In Japan and Korea, early gastric cancer accounts for up to 50% of all of gastric cancer cases [1]. For early gastrointestinal neoplasms, endoscopic submucosal dissection (ESD) is now acknowledged as a preferred treatment modality [1], [2], [3]. A definite en bloc resection is believed to offer an accurate histological assessment and thus lower the risk of neoplastic recurrence. However, the use of electrocautery in ESD leads to a high risk of perforation [1], [2], [3]. To make mucosal excision easier and safer, the injection of a fluid into the submucosa to create a sufficient submucosal fluid cushion (SFC) between a lesion and the proper muscle layer is required.

A routine method to elevate mucosa involves a submucosal injection of normal saline solution (NS). It is, however, not easy to produce a high mucosal elevation and to maintain the desired height due to the rapid absorption of NS by the surrounding tissue. The low mucosal elevation makes operation difficult, and the electrocautery damage of the muscularis still results in perforation. Therefore, various substances, including glycerol, hyaluronic acid (HA) and hydroxypropyl methylcellulose, have been exploited to achieve sustained mucosal elevation and avoid perforation [4], [5], [6], [7]. These materials maintain the same condensed state before and after injection. In our opinion, a biomaterial system with in situ gelling after injection might be a better SFC. It would be ideal if the gelling was not triggered by any chemical reaction and the building blocks of the material were to have been approved by the US Food and Drug Administration. The present study introduces a temperature-sensitive polymer hydrogel as the submucosal injection agent in ESD. The polymer is composed of poly(ethylene glycol) (PEG) and poly(D,L lactic acid-co-glycolic acid) (PLGA), both of which have been applied clinically. The aqueous solution of the block copolymer with appropriate composition and concentration is a free-flowing sol at ambient temperature, and able to be transformed into a hydrogel at body temperature free of any chemical reaction.

Hydrogels are three-dimensional polymeric networks, which can absorb a significant amount of water but do not dissolve in water, resembling natural living tissues and presenting excellent biocompatibility [8], [9], [10], [11], [12], [13], [14]. In the past decade, in situ gel-forming polymers have attracted much attention as injectable biomaterials [15], [16], [17], [18], [19], [20], [21]. Some biodegradable block copolymers undergo a reversible sol–gel transition in water with increasing temperature, including PEG and PLGA [22], [23], PEG and poly(ε-caprolactone) [24], [25], PEG and poly(ε-caprolactone-co-lactide) [26], [27], PEG/polypeptide [28], [29] and poly(phosphazenes) [30]. These systems have been applied in sustained drug delivery, tissue engineering and the prevention of post-operative adhesion [25], [30], [31], [32], [33], [34], yet have never been tried as a submucosal injection agent in ESD, to the best of our knowledge.

Among those thermo-reversible hydrogels, the thermogelling PLGA–PEG–PLGA triblock copolymer system is a very promising biomedical material due to its convenient synthesis and good safety profile [23], [35], [36]. The gelation and degradation properties of PLGA–PEG–PLGA thermogels can be adjusted via block length, polymer concentration, sequence of PLGA segment and even end group [23], [37], [38], [39], [40]. To date, the biomedical applications of PLGA–PEG–PLGA thermogels have been focused on drug delivery [36], [41], [42], [43], [44], [45]. For instance, PLGA–PEG–PLGA formulation containing paclitaxel (an anticancer agent) can sustain the release of drug ex vivo for up to 50 days [36]. In clinical trials, this formulation (OncoGel) exhibited excellent efficacy against human esophageal cancer [46].

Herein, we tried the thermogelling PLGA–PEG–PLGA triblock copolymers as a submucosal injection substance to create SFCs, as schematically presented in Fig. 1. The current study assessed the feasibility, safety, durability and tissue biocompatibility of SFCs created with the PLGA–PEG–PLGA thermogel in both resected porcine stomachs and living minipigs.