Polymeric carriers for drug and gene delivery have been successfully used in clinical applications, and developing to achieve greater efficacy. A successful polymeric carrier must not only protect the therapeutics from degradation but also increase the pharmacokinetic and biopharmaceutical properties of the therapeutic agents. Furthermore, releasing the therapeutic agents from nanocarriers at the desired target sites and with the correct dosage is important to enhance patient outcomes. Numerous polymeric systems for drug and gene delivery that can release bioactive agents through response to external (e.g., light, electric or magnetic source, and ultrasound) or internal triggers (e.g., pH, enzyme, and redox) have been designed. However, release mechanism using these triggers is restrictive in terms of in vivo applications as there is limited accessibility of external stimuli to tissues or organs; and the efficiency of internal triggers are variable between cell lines and even within the same tissue or organ. Some carriers are not degradable, raising the important issue of toxicity due to accumulation in the body and healthy cells. In addition, the time of release is mostly uncontrollable. The main aim of this thesis is to synthesize and study of novel non-triggered and timed-release polymeric carriers, such as micelles and hydrogel, based on thermoresponsive PNIPAM and self-degradable PDMAEA. In particular, we developed the understanding of the self- assembly and disassembly properties of thermoresponsive PNIPAM copolymerized with the self- degradable PDMAEA and hydrophobic components. This is one of the first examples where the disassembly time can be controlled on-demand in a wide range of experimental conditions.
Initially, the self-catalysed hydrolysis of PDMAEA together with hydrophobic polymers were employed to finely tune the LCST and disassembly time of thermoresponsive PNIPAM and thus control the release time of oligo DNA (i.e., mimic of siRNA) from the polymer complex. The diblock thermoresponsive copolymers were synthesized by Reversible addition-fragmentation chain transfer (RAFT) polymerization, which consisted of a hydrophilic block (e.g., PDMA) for stabilization and a second thermoresponsive block with three components (e.g., NIPAM, DMAEA, and BA or Styrene) for self-assembly and disassembly. The copolymers were fully water-soluble below LCST and self-assembled into core-shell spherical particles with an average diameter of approximately 25 nm above LCST (e.g., 37 °C) and with a narrow particle size distribution. When the amount of acid groups from degradation of cationic DMAEA units was sufficiently high to increase LCST of the copolymer above 37 °C, the polymer nanoparticles sharply disassembled to unimers (i.e., the core of the copolymer became water soluble). These nanoparticles showed excellent binding to oligo DNA without any leakage until full disassembly to unimers. Interestingly, the disassembly time of the nanoparticles and consequently the release time of oligo DNA could be precisely controlled. These particles could be easily modified with folic acid to enhance cellular uptake by osteosarcoma cells.
In the next step, we studied the influence in the change in the number of self-degradable DMAEA and hydrophobic BA units in the copolymer composition on the disassembly time (tstart) and time from the start of disassembly to full unimer formation (tdegrade). The results showed a dependence of the tstart and tdegrade on the BA and DMAEA units. The mechanism of degradation was postulated to result from the degradation of the PDMAEA side groups to acrylic acid groups, which by increasing the LCST of the polymer to over 37 °C over time. Additionally, the polymer nanoparticles could be designed to self-assemble over a wide range of pHs and disassemble below a pH of 7.3. The polymer nanoparticles have potential for application in drug and gene delivery where timed controlled release is required.
The stabilization and self-disassembly of nanoparticles self-assembled from random thermoresponsive copolymers was further investigated in the presence of Sodium dodecyl sulphate (SDS) surfactant. Random copolymers of P(NIPAM-co-DMAEA-co-BA) and P(NIPAM-co- DMAEA-co-STY) were synthesized by RAFT polymerization. The polymers could self-assemble into nanoparticles stabilized by SDS with very narrow size distribution. The polymer particle size increased with the increase in SDS concentration, and these nanoparticles were stable under dilution. Increasing the amount of SDS, the LCST of copolymers were also increased due to more SDS molecules bound to PNIPAM molecules. In addition, the presence of SDS exhibited no influence on the self-degradation of DMAEA as well as self-disassembly characteristic of the polymer nanoparticles.
Based on the understanding of controllably timed-release polymer nanoparticle, non-triggered degradable polymeric hydrogel was further designed. Random copolymers of thermoresponsive PNIPAM, self-catalysed hydrolysis PDMAEA, hydrophilic PEGMEA, and hydrophobic PBA were synthesized via RAFT polymerization. The thermoresponsive copolymers could form stable gel at temperature higher than the gelation point (e.g., 37 °C) and then self-degraded into sol state after different times without the need of any trigger. Moreover, the gelation temperature and degradation time were dependent on the number of DMAEA units. Gold nanoparticles coated with PDMA were encapsulated in the hydrogel and subsequently released at different rates through the degradation of the hydrogels. This novel non-triggered release hydrogel may find potential in applications where controlled-release is beneficial.