Design of lipid nanoparticles for the oral delivery of hydrophilic macromolecules

Abstract

Colloidal drug carriers prepared from solid triglycerides have been presented as a promising alternative to polymer nanoparticles. The present work is aimed at developing surface-modified lipid nanoparticles intended to encapsulate peptides within their structure and to study their physicochemical properties and in vitro stability in gastrointestinal fluids. The final goal is to explore their potential as oral delivery vehicles for macromolecules. The W/O/W multiple emulsion technique was originally applied and conveniently modified for the production of tripalmitin nanoparticles. This technique was selected because its makes the encapsulation of peptides feasible. Additionally, the surface of the particles could be modified through the incorporation of Poloxamer 188 or the lipid derivative PEG 2000-stearate into the formulation. This modification led to a reduction in the zeta potential values, varying from −34 mV for the non-coated particles to −20 mV for those prepared with PEG-stearate. Results of the stability of the nanoparticles in gastric and intestinal media indicate that the low pH of the gastric medium and the pancreatic enzymes in intestinal medium are responsible for the extensive aggregation and degradation of the non-coated lipid nanoparticles (80% degradation in 4 h). In contrast, PEG-stearate coated nanoparticles were more stable, as their polymer coating layer totally prevented aggregation in both media and significantly reduced pancreatin-induced degradation (40% approximately in 4 h). Preliminary studies showed that insulin, chosen as a model peptide, could be associated and released from PEG-stearate coated nanoparticles. Nevertheless, further work is required in order to optimize the release behavior of the entrapped peptide.

Introduction

The use of genetic engineering in the molecular design of new peptide drugs offers a promising approach for the treatment of several diseases. However, after nearly seven decades of therapeutic use, most protein drugs are still administered by invasive routes. Among the different alternatives investigated so far, the oral route continues to be a challenge as well as the most attractive way to administer proteins because of its unquestionable commercial potential.

Recently, the use of submicron-size particles as oral peptide delivery systems has attracted considerable pharmaceutical interest [1], [2], [3]. There are two essential features that justify the attention devoted to such systems, (i) their controlled release behavior that makes possible to bypass gastric and intestinal degradation of the encapsulated drug [4], [5] and (ii) their possible uptake and transport through the intestinal mucosa [6], [7], [8], [9]. Indeed, the uptake and transport of small particles across the small intestine is utterly accepted nowadays [7], [10], [11]. However, whether the extension of this phenomenon is enough to improve massive transport of poorly absorbable drugs across the intestine is still under discussion [12]. Future clarification of the parameters governing particle uptake will surely lead to the design of new and more efficient colloidal carriers.

A widely overlooked factor in the design of drug delivery systems for the oral route is their stability upon contact with gastrointestinal fluids. This consideration is especially important in carriers made from biodegradable materials, where colloidal size maximizes the surface available for enzymatic degradation [12], [13], [14], [15]. Moreover, the delicate stability of colloids can be seriously compromised under the drastic environmental conditions of the gastrointestinal tract. Indeed, this phenomenon of particle aggregation leading to sizes far larger than those capable of interacting with the intestinal mucosa has already been reported for PLA and PLGA nanoparticles [15]. One of the proposed strategies for protecting biodegradable particles from the effects of gastrointestinal fluids has been the formation of a coating with polymers such as polyvinyl alcohol [13], [14], Poloxamer [16], [17] or PEG [15].

Lipid nanoparticles are receiving increasing attention as alternative drug carriers to polymer nanoparticles [18], [19]. We have hypothesized that some advantages of these carriers for the oral delivery of peptides could be the stabilizing effect that lipids exert on proteins and the absorption promoting effect associated with these materials [20]. Nevertheless, a current limitation of solid lipid nanoparticles is the low loading capacity for hydrophilic macromolecular drugs [21]. Additionally, in the case of temperature sensitive molecules, i.e. proteins, a second drawback arises during the nanoparticle manufacturing process because of the high temperatures required to melt the lipid.

The final goal of our work is to develop new lipid nanoparticulate carriers that are suitable for oral administration of proteins. The specific aims in the work presented here were, first, to apply new technologies that avoid the necessity to melt the lipid for the preparation of peptide-loaded lipid nanoparticles and, second, to develop surface-modified nanoparticles and investigate their stability in gastrointestinal fluids. For this purpose, tripalmitin nanoparticles precipitated from multiple emulsions were prepared at room temperature, and the stability of various PEG-stearate and Poloxamer-coated nanoparticles in simulated digestive and intestinal fluids was evaluated in terms of particle aggregation and lipid degradation.