Self-assembling PEG-oligoesters: nanoparticle design for drug delivery

Abstract

Drug carrier systems can be used to overcome problems related to the proper formulation of a promising drug candidate, or to improve its efficacy and safety in vivo. In particular self-assembled nanosized polymeric vesicles and micelles are interesting for the (targeted) delivery of hydrophilic and hydrophobic drugs, respectively. Their small size and hydrophilic surface are attractive features to achieve long circulatory behaviour after intravenous admininstration, enabling extravasation and accumulation at tumour sites. Other favourable characteristics of these systems are their ease of preparation and their tailorability, as the use of (semi-)synthetic polymers offers the possibility to modulate the carriers properties and to control the release of the loaded drug. In this thesis the self-assembling properties of well-defined low molecular weight poly(ethylene glycol) (PEG)-b-oligoesters were studied in detail, and their suitability to design novel nanosized drug carriers was evaluated. It was demonstrated that mPEG-b-oligo(L-lactate)s (OLA) with monodisperse hydrophobic blocks formed nanoparticles with a hydrodynamic radius of 130-300 nm, whereas polydisperse mPEG-b-OLAs formed large aggregates. Further characterisation of the nanoparticles by static light scattering (SLS) indicated that they were highly hydrated nanoaggregates. When mPEG-b-OLAs are applied for the solubilisation of therapeutic proteins and drugs for intravenous administration, their relatively high critical aggregation concentration (CAC) necessitates the use of high amounts of mPEG-b-OLA, and systems with a lower CAC would be more attractive. Therefore the self-assembly of block oligomers with a more hydrophobic oligoester, mPEG-b-oligo(epsilon-caprolactone) (OCL), was investigated, which had much lower CACs than the mPEG-b-OLAs. Moreover, in water mPEG-b-OCL formed small and almost monodisperse oligomeric micelles with a hydrodynamic diameter of 8-15 nm, which may have attractive properties for drug delivery applications in terms of biodistribution and tumour penetration. Importantly, it was found that modification of the OCL block with an aromatic end group substantially improved the physical stability of these micelles, by an extensive reduction of the CAC and by improving their temperature sensitivity profile. Degradation studies revealed an estimated half-life of the hydrolytic cleavage of the ester bonds in these mPEG-b-OCL based micelles at physiological pH and temperature of several years. However, the presence of lipase accelerated the degradation and destabilisation of the mPEG-b-OCL micelles to half-lives of a few days to hours, indicating that degradation induced destabilisation and subsequent drug release from these mPEG-b-OCL micelles is feasible by the action of enzymes in vivo. The loading of these micelles with paclitaxel and docetaxel was evaluated, demonstrating that the presence of an aromatic end group is essential for the formation stable small-sized mPEG-b-OCL micelles loaded with 10% (w/w) of taxane, and their integrity of the was not affected by the presence of albumin. Their cytotoxic effect on C26 cells was comparable to that of the commercial formulations Taxol and Taxotere whereas the empty micelles, in contrast to Cremophor EL, were not toxic in the concentration range tested. In conclusion, the PEG-b-oligoesters studied in this thesis represent an interesting class of amphiphilic molecules, which possess attractive properties for pharmaceutical applications