Biocompatible stabilizers in the preparation of PLGA nanoparticles: a factorial design study

Abstract

Poly(lactic-co-glycolic-acid) nanoparticles are often produced using the w/o/w emulsification solvent evaporation method. In most cases poly(vinyl alcohol) (PVA) is used as stabilizer of the emulsion. The goal of this study was to compare a series of polymers to PVA in a 2² full factorial design study. The influence of the concentration of PVA and the polymers tested on particle size and zeta potential value was evaluated before and after freeze-drying of the prepared particles. Nanoparticles were obtained with most polymers when they were used in combination with PVA. Leaving PVA out of the formulation, however, increased the size of the particles over 1 μm. Two exceptions are poloxamer and carbopol, which can be considered as valuable alternatives to PVA. Zeta potential values were usually slightly negative, the most extreme zeta potential values were measured when poloxamer and carbopol were employed. The use of gelatin type A made it possible to achieve positive values. The original 2² full factorial design study was further expanded to a central composite design for poloxamer and carbopol, in order to fit the measured data to a quadratic model and to calculate response surfaces.

Introduction

Because of their biodegradability and biocompatibility, polylactic acid and its copolymers with glycolic acid (PLGA) are widely employed for the preparation of sustained release preparations (Anderson and Shive, 1997). They are used for the production of implants, inserts and particulate systems. Especially micro- and nano-particles made of PLGA copolymers are widely investigated for the controlled release of classical drug molecules as well as peptides and proteins. The administration routes vary from parenteral (Das et al., 2000), oral (Coombes et al., 1997), dermatological (De Galon et al., 2001) pulmonary (O'Hara and Hickney, 2000) and nasal (Tobio et al., 1998) to ocular (Veloso et al., 1997, Moritera et al., 1992, Moritera et al., 1991).

Several methods were proposed for the preparation of PLGA microspheres, such as extrusion (Zhang et al., 1994), spray drying (O'Hara and Hickney, 2000) and supercritical fluid extraction (Kompella and Koushik, 2001). The technique mostly used, however, is the emulsification solvent evaporation method (O'Donnell and McGinity, 1997). It involves the solution of the PLGA polymer in an organic solvent, emulsifying the PLGA solution in a non solvent (mostly water) and precipitating the PLGA polymer as particles by evaporating the organic solvent. Lipophilic drugs are incorporated by dissolving them in the organic solvent along with PLGA. For hydrophilic drugs the w/o/w emulsification solvent evaporation is used, dissolving the drug into the inner water phase of the double emulsion.

In most cases, a stabilizer is added to the formulation in order to stabilize the emulsion formed during particle preparation. These stabilizers, however, can also influence the properties of the particles formed. The type and concentration of the stabilizer selected may affect the particle size. Being present at the boundary layer between the water phase and the organic phase during particle formation, the stabilizer can also be incorporated on the particle surface, modifying particle properties such as particle zeta potential and mucoadhesion (Scholes et al., 1999, Feng and Huang, 2001). Both size and zeta potential value are important physicochemical particle properties, as they determine the physical stability as well as the biopharmaceutical properties of the preparation. Drug release rate, biodistribution, mucoadhesion and cellular uptake can all be influenced by the type and concentration of the stabilizer used.

In the literature, poly(vinyl alcohol) (PVA) is the most popular stabilizer for the production of PLGA nanoparticles. In the present study other polymers were tested as stabilizers. These polymers were incorporated as such and in combination with PVA. The aim was to study how the use of other polymers in the preparation of nanoparticles would affect particle size and zeta potential value. The effect of the presence or absence of PVA during preparation as well as the effect of the concentration of the alternative stabilizers was evaluated. Considering the stability of PLGA particles, the effect of the freeze-drying process on the particle size and zeta potential was also studied.

The polymers evaluated as stabilizers in this study are cellulosic derivatives methylcellulose (MC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC) and hydroxypropylmethylcellulose (HPMC), as well as gelatin type A and B, carbomer and poloxamer. Some of these polymers have been reported as adjuvants in the preparation of PLGA particles, such as poloxamer (Scholes et al., 1999, De Rosa et al., 2000, O'Hara and Hickney, 2000, Couvreur et al., 1997), gelatin (Tobio et al., 1998, Arshady, 1991), HPMC (Gabor et al., 1999, Sansdrap and Moës, 1993), MC (Arshady, 1991) and carbopol (Wang et al., 1991). Moreover, HPMC, poloxamer and carbomer are interesting compounds because of their mucoadhesive properties (Takeuchi et al., 2001). Gelatin type A and B were both selected because of their difference in isoelectric point, resulting in different electrical charges. A difference in zeta potential of the particles produced with gelatin type A and B was thus expected.

Each polymer was compared to PVA using a 2² full factorial design. This rational methodology allows for the determination of the influence of the factors investigated and their interactions requiring a minimum of experiments. Moreover, the design was expanded to a central composite design, enabling the modelling of the responses as a function of the parameters investigated. This allows an estimation of the particle properties for a certain combination of polymer and PVA within the experimental region.