Lipase degradation of Dynasan 114 and 116 solid lipid nanoparticles (SLN)—effect of surfactants, storage time and crystallinity

Abstract

In vivo drug release from solid lipid nanoparticles (SLN) takes place by diffusion and degradation of the lipid matrix. SLN with different degree of crystallinity were prepared to study the effect of crystallinity on the degradation velocity. These SLN were produced by using glycerides with different length of fatty acid chains and known differences in crystallisation velocity (Dynasan 114 and 116), and using stabilisers interfering differently with the crystallisation process of the lipid matrix (cholic acid sodium salt (NaCh), Poloxamer 407 (Plx 407)). NaCh disturbs the crystallisation process, Poloxamer shows little interference. The particles were characterised by photon correlation spectroscopy (PCS) and differential scanning calorimetry (DSC), degradation velocity was determined directly after production and during storage up to 4 weeks under different storage conditions using an especially developed assay based on the NEFA Test kit. After production, SLN with a lower crystallinity matrix (Dynasan 114 and 116, NaCh) degraded faster than higher crystalline particles (all SLN with Plx 407), and showed a decrease in degradation velocity with increasing crystallinity during storage. Fast crystallising particles made from Dynasan 116 stabilised with the non-interfering Plx 407 showed no change in the degradation velocity during storage. SLN produced with a higher crystalline lipid in combination with the crystallisation-disturbing NaCh (Dynasan 116, NaCh) required a ‘ripening time’ to reach sufficient crystallinity.

Introduction

Nanoparticles made from solid lipids (SLN) are a colloidal carrier system for topical, oral and parenteral administration of mainly lipophilic drugs (Siekmann and Westesen, 1992, Müller and Lucks, 1996, Cavalli et al., 1996, Heiati et al., 1997, Gasco, 1998). The system consists of lipid nanoparticles which are solid at room temperature. Solid lipid nanoparticles (SLN) are dispersed in an aqueous surfactant solution and are biodegradable, easy to produce (even in large scale) and biocompatible (Müller et al., 1995). SLN are of broad interest and recently it has been published a summary of nearly 10 years of research on SLN reviewing over 100 publications concerning SLN (Müller et al., 2000). SLN can be used to improve the bioavailability of drugs, e.g. cyclosporine A (Penkler et al., 1999), to protect sensitive drugs from decomposition (Jenning et al., 2000) and as a controlled release system for lipophilic drugs (zur Mühlen et al., 1998). As a new application SLN have been successfully tested for their capacity to be used as vaccine adjuvants (Müller et al., 1999, Müller and Olbrich, 1999, Olbrich et al., 2000). SLN are well tolerated when phagocytosed by phagocytic cells (Müller et al., 1988, Müller and Olbrich, 1999) making them interesting for i.v. applications.

SLN possess properties of emulsions and polymeric particles (solid matrix). The solid matrix offers the possibility to improve the stability against coalescence and the reduced mobility of incorporated drug molecules is a prerequisite for protecting them against chemical degradation and for a controlled drug release. Drug release can take place either by diffusion or by degradation of the lipid matrix which occurs mainly by enzymes like lipases and only to a very little extent by hydrolytic processes (Olbrich et al., 1998a). The nature of the lipid matrix and the surfactants have been shown to influence the biodegradation of SLN (Olbrich et al., 1997, Olbrich and Müller, 1999). Using triglycerides with long chain fatty acids and/or sterically hindering surfactants like Poloxamer 407 or Poloxamine 908 a delayed degradation takes place (Olbrich et al., 2000). When using short chain fatty acid triglycerides and/or degradation promoting surfactants as bile salts (e.g. cholic acid sodium salt) leads to a relatively fast degradation. The influence of particle size on degradation is different for certain surfactants. Poloxamer 407 stabilised Dynasan 114 SLN are more sensitive against size effects than cholic acid sodium salt stabilised Dynasan 114 SLN (Olbrich et al., 2000). However, no studies were performed to evaluate the influence of the physical state and degree of crystallinity of the lipid matrix on SLN degradation velocity, being the aim of this study. The degree of crystallinity of the particles after production is a function of the nature of the lipid, surfactants and stabilisers used and also can change during storage time of the particles. To elucidate the degradation properties of the SLN formulations an established lipase/colipase assay (Müller et al., 1995, Olbrich and Müller, 1999) was used.