Biodegradable thermosensitive polymers: synthesis, characterization and drug delivery applications

Abstract

The aim of the research described in this Thesis is to design polymeric micelles showing controlled instability due to "hydrophobic to hydrophilic" conversion of the core, and to demonstrate its utility as a drug delivery vehicle. For that purpose, a novel class of thermosensitive and biodegradable polymers, poly(N-(2-hydroxypropyl) methacrylamide mono/di lactate) (poly(HPMAm-mono/di lactate)), were synthesized. The cloud point (CP) of these polymers in water varied depending on the ratio of the copolymer composition. Since poly(HPMAm-dilactate), having a CP of 13 °C, is converted in time to poly(HPMAm-monolactate) (CP of 65 °C) or more hydrophilic pHPMAm, this polymer was supposed to be suitable for "hydrophobic to hydrophilic" conversion at body temperature. Therefore, amphiphilic AB block copolymers of poly(HPMAm-dilactate) and poly(ethylene glycol) (pHPMAmDL-b-PEG) were synthesized. These block copolymers formed polymeric micelles in water with a size of around 50 nm by rapidly heating an aqueous polymer solution from below to above the critical micelle temperature (CMT). By cryo-transmission electron microscopy analysis, it was shown that pHPMAmDL-b-PEG micelles have a spherical shape with a narrow size distribution. 1H NMR and static light scattering measurements demonstrated that the micelles have solid-like and dense core structures and that the hydrophobic core is stabilized with a hydrophilic PEG corona. Most importantly, the pHPMAmDL-b-PEG micelles showed controlled dissolution at body temperature as a result of the hydrophilization of the core due to the hydrolysis of the lactic acid side chain in the thermosensitive block, demonstrating that our concept of controlled instability can be achieved with pHPMAmDL-b-PEG micelles. Next, the loading of paclitaxel (PTX), a very hydrophobic cytostatic drug, into pHPMAmDL-b-PEG micelles was studied. Taking advantage of the thermosensitivity of pHPMAmDL-b-PEG, the loading was done by simple mixing of a small volume of a concentrated PTX solution in ethanol and an aqueous polymer solution and subsequent heating of the resulting solution above the CMT of the polymer. PTX could be almost quantitatively loaded in the micelles up to 2 mg/mL. Release of PTX was induced by the pH-dependent destabilization of the micelles at relatively high concentration of PTX, while dialysis against a large volume of water induced the release of PTX by diffusion. PTX-loaded micelles showed comparable cytotoxicity as Taxol (clinically used formulation of PTX in a 50:50 mixture of Cremophor EL and ethanol) against B16F10 cells. On the other hand, the empty micelles were far less toxic than the Cremophor EL vehicle, which is beneficial for in vivo applications. When administered intravenously into rats, pHPMAmDL-b-PEG micelles showed a relatively long blood circulation time with 20 % of the injected dose still circulating in the bloodstream after 24 hours. In contrast to empty micelles, PTX that was loaded into pHPMAmDL-b-PEG micelles was cleared quite rapidly after intravenous administration in mice. PTX-loaded pHPMAmDL-b-PEG micelles showed comparable in vivo antitumor efficacy as Taxol both after intravenous and intraperitoneal administration. In conclusion, the work presented in this Thesis indicates that pHPMAmDL-b-PEG polymeric micelles have promising features as vehicles for hydrophobic drugs owing to their controlled instability.