pH-sensitive chitosan/alginate core-shell nanoparticles for efficient and safe oral insulin delivery

Abstract

Chitosan-alginate (CS/ALG) nanoparticles were prepared by formation of an ionotropic pre-gelation of an alginate (ALG) core entrapping insulin, followed by chitosan (CS) polyelectrolyte complexation, for successful oral insulin administration. Mild preparation process without harsh chemicals is aimed at improving insulin bio-efficiency in in vivo model. The nanoparticles showed an average particle size of 100–200 nm in dynamic light scattering (DLS), with almost spherical or sub-spherical shape and ∼85% of insulin encapsulation. Again, retention of almost entire amount of encapsulated insulin in simulated gastric buffer followed by its sustained release in simulated intestinal condition proved its pH sensitivity in in vitro release studies. Significant hypoglycemic effects with improved insulin-relative bioavailability (∼8.11%) in in vivo model revealed the efficacy of these core-shell nanoparticles of CS/ALG as an oral insulin carrier. No systemic toxicity was found after its peroral treatment, suggesting these core-shell nanoparticles as a promising device for potential oral insulin delivery.

Introduction

Sharp rise in prevalence of diabetes includes rapid population growth, aging, increase in obesity, physical inactivity and urbanization [1], [2] and finally reducing the quality of life. External insulin being the only effective treatment of diabetes demands success in terms of good patient compliance. Oral insulin is advantageous, with a very similar mechanism of action as of endogenous insulin enabling long-term glycemic control in diabetic patients [3], [4]. However, physicochemical barriers of gastrointestinal (GI) tract hampers the bioavailability of oral insulin [5]. Recently, polymeric nanoparticles have gained a lot of attention in oral insulin delivery [5] to improve insulin bioavailability in animal system without producing any systemic toxicity. Nanoparticles of both natural and synthetic biodegradable polymers, lipids and polysaccharides have been extensively studied over last few decades. Among them, chitosan (CS) and alginate (ALG) are most extensively studied in pharmaceutical research for controlled drug delivery. The CS, a linear polymer having (1-4)-linked 2-amino-2-deoxy-β-d-glucopyranose units on its structure, is a biodegradable, biocompatible, non-toxic and non-immunogenic mucoadhesive polymer, which offers certain advantages over other natural polymers in formulating nanoparticles for oral administration of drug molecules [6]. Cationic nature of CS (due to protonation of amine groups in acidic pH) facilitates effective encapsulation of biomolecules like proteins and drugs and prolongs the drug resident time in the GI tract [7]. Apart from the endowment of positive charge on the surfaces of nanoparticles, CS also elevates the contact time with intestinal epithelium enhancing permeation through tight junction of epithelial cells via paracellular transport [8]. Most importantly, CS is digested by chitosanase enzymes secreted by microorganisms at the intestine after oral ingestion [6].

ALG is another water-soluble natural linear polysaccharide, containing varying amounts of 1,4-linked β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues, extracted from brown seaweed. It is a very popular pH-responsive polymer due to its shrinkage in lower pH [9], enabling encapsulated drug retention in the stomach while protecting it against enzymatic deactivation. Properties of biodegradability, biocompatibility, low toxicity, low immunogenicity and good mucoadhesion [10], [11] facilitate its application in oral drug delivery. Micro- or nanoparticles of ALG can be formed either by physical or chemical crosslinking for the sustained drug release. As some crosslinkers are toxic in nature, physical crosslinking is usually preferred over chemical crosslinking [12]. Calcium, a divalent cation used for crosslinking of ALG, is reported to maintain the biological efficiency of the drug molecules [13], [14]. Again, CS/ALG nanoparticles were reported to be efficient in protecting insulin from the aggressive environment of the stomach and sustained insulin release was observed under intestinal milieu [15]. Furthermore, pH-sensitive, mucoadhesive CS/ALG nanoparticles showed significant hypoglycemic effects in rat model too [16].

Therefore, the main objective of our present investigation is to develop insulin-loaded core-shell CS/ALG nanoparticles with comparatively smaller size than those reported previously [15] to improve the insulin bioavailability after its oral administration. In the earlier report [15], no in vivo systemic toxicity was studied after oral delivery of insulin through these CS/ALG nanoparticles. In the present article, all these systemic toxicity were studied in detail in mice model.