The role of PEG on the stability in digestive fluids and in vivo fate of PEG-PLA nanoparticles following oral administration

https://doi.org/10.1016/S0927-7765(99)00157-5Get rights and content

Abstract

The aim of the present work was to evaluate if the presence of a polyethylenglycol (PEG) coating around PLA nanoparticles would affect their interaction with biological surfaces, following oral administration to rats. For this purpose, a model antigen, 125I-radiolabeled tetanus toxoid, was encapsulated in PLA and PLA-PEG nanoparticles by a modified water-in-oil-in-water solvent evaporation technique. Firstly, the stability of the nanoparticles in simulated gastrointestinal fluids was evaluated. Results showed an interaction between the nanoparticles and the enzymes of the digestive fluids, this interaction being considerably reduced by the PEG coating around the particles. On the other hand, the PLA forming the nanoparticles was found to be only slightly degraded (9% converted to lactate for PLA nanoparticles and 3% for PLA-PEG nanoparticles) and that the encapsulated tetanus toxoid remained mostly associated to the nanoparticles upon incubation in the digestive fluids for up to 4 h. Finally, the in vivo experiments showed that, after oral administration to rats, the levels of encapsulated radioactive antigen in the blood stream and lymphatics were higher for PLA-PEG nanoparticles than for PLA nanoparticles. In conclusion, the PLA-PEG nanoparticles have a promising future as protein delivery systems for oral administration.

Introduction

Many of the recent efforts aimed at maximizing the efficacy of drug therapy has been put into new drug delivery systems and, especially, into colloidal drug carriers. These systems, essentially represented by liposomes and nanoparticles, were first designed for parenteral use in order to protect labile drugs against environmental aggression and to achieve site-specific delivery of active molecules. However, as the oral route remains the most convenient and popular way of administration, much attention is presently being given to the oral application of these colloidal drug carriers systems. This research is highly stimulated by the increasing amount of macromolecular drugs and antigens (peptides, proteins and genes) which demand an adequate vehicle for their transport through mucosal surfaces.

Although the transport of nanoparticles across the intestinal mucosa has been a controversial issue, there is no doubt at present that this process occurs in animals [1], [2], [3], [4]. More precisely, it is currently accepted that nanoparticles are taken up by the M-cells of Peyer’s patches and the isolated follicles of the gut-associated lymphoid tissue (GALT) and also via the enterocytes [5], [6]. However, as the intensity of this transport and the characteristics of the particles which might influence it are not sufficiently known, their application to the oral administration of therapeutic macromolecules remains uncertain. Two major obstacles of obtaining therapeutically worthwhile levels of drugs have been identified so far: (i) the instability of the nanoparticles in the GI tract and (ii) the low intestinal uptake of the nanoparticles. There is evidence that biodegradable poly(lactic acid) (PLA) nanoparticles suffer a significant degradation in the intestinal fluids and that this degradation is affected by the surface composition of the particles [7], [8]. More specifically, following incubation of albumin coated PLA nanoparticles in simulated intestinal fluids, it was found that PLA was converted into lactate leading to an important mass loss of the polymer (50% reduction in 1 h) [8]. This observation led to the assumption that after oral administration of 14C-radiolabeled PLA nanoparticles to rodents, the radioactivity which crossed the intestinal barrier was partially related to monomers and oligomers as a consequence of the nanoparticles degradation [7], [9].

The aforementioned problems led us to hypothesize that a protective coating on the nanoparticles could be an interesting approach for the stabilization of nanoparticles in the GI tract and thus, for favouring the interaction and further transport of the intact nanoparticles through the intestinal barrier. In this respect, the polyethylenglycol (PEG)-coated PLA nanoparticles appear to be attractive protein carriers for oral administration. Indeed, since it is known that biodegradable nanoparticles degrade in the GI tract due to the enzymatic attack [7], [8], it could be presumed that the protein repellent properties of the PEG-coated nanoparticles would stabilize the nanoparticles in the digestive fluids. This assumption is supported by the observed fact that the PEG coating avoids plasma protein adsorption, thereby minimizing the interaction with phagocytic cells and increasing the blood circulation time [10], [11], [12], [13]. Another interesting feature of PEG, with respect to its application for mucosal drug administration, is related to its mucoadhesion promoting effect [14], [15], [16], [17], this behaviour being related to a mechanism of chain penetration across the polymer/mucosa interface. Therefore, we also hypothesized that PEG could improve the interaction of the nanoparticles with the intestinal mucosa. Furthermore, our hypothesis has been reinforced by our previous work aimed at investigating PLA and PLA-PEG nanoparticles as protein carriers for nasal administration [18]. Results showed that tetanus toxoid encapsulated in PLA-PEG nanoparticles reached the systemic circulation and the lymphatic system in a much greater proportion than in the case of PLA nanoparticles.

Based on these previous considerations, the aim of the present work was to investigate the influence of a PEG coating around poly(lactic acid) nanoparticles on their stability in the GI fluids and their ability to transport proteins through the intestinal mucosa. For this purpose, the stability of TT-loaded PLA and PLA-PEG nanoparticles in simulated digestive fluids was evaluated in terms of their aggregation, PLA degradation and release of the encapsulated macromolecule. As a second step, the in vivo absorption of TT encapsulated into PLA or PLA-PEG nanoparticles was evaluated.

Section snippets

Chemicals and animals

For the polymer synthesis, d,l-lactide was purchased from Aldrich (Milwakee, USA); monomethoxy polyethylenglycol (molecular weight (Mw) 5000 Da) and stannous octoate were obtained from Sigma Chemical (St Louis, USA). Purified tetanus toxoid ((TT), Mw 150 000 Da, 85–95% monomeric) dissolved in phosphate buffer saline, pH 7.4, was kindly donated by the Massachusetts Biological Laboratories (Boston, MA). Cholic acid (sodium salt, purity 99%), pepsin A from porcine stomach mucosa and pancreatin

Results and discussion

The search for the most suitable characteristics of the nanoparticles with respect to their use as protein carriers for mucosal administration led us to investigate the effect of a PEG coating around PLA nanoparticles in their stability and absorption of the associated protein. We hypothesized that the PEG coating would improve the stability of the nanoparticles in the GI tract by protecting them from enzyme attack. On the other hand, our previous studies aimed at exploring the potential of

Acknowledgements

This work was supported by a grant from the Spanish Government (CICYT: SAF 97-0169) and Pierre Fabre Iberica, S.A.

References (27)

  • H.O. Alpar et al.

    The transport of microspheres from the gastro-intestinal tract to inflammatory air pouches in the rat

    J. Pharm. Pharmacol.

    (1989)
  • P. Jani et al.

    The uptake and traslocation of latex nanospheres and microspheres after oral administration to rats

    J. Pharm. Pharmacol.

    (1989)
  • J. Kreuter et al.

    Quantitative and micro-autoradiographic study on mouse intestinal distribution of polycyanoacrylate nanoparticles

    Int. J. Pharm.

    (1989)
  • Cited by (0)

    View full text