Studies of self-assembled substituted poly(acrylate) networks as potential sustained drug delivery systems and of fluorescent conjugated polymer nanoparticles in cell imaging

Abstract

Polymer networks are promising biomaterials for drug delivery as they have porous structures and are often biocompatible. The general aspects of the host-guest complexation capability of polymer networks containing cyclodextrins as well as their application in drug delivery are considered in Chapter 1. The introduction of cyclodextrins into polymer networks has the potential to improve drug loading capacity and modulate subsequent drug release behavior due to the host-guest complexation by cyclodextrins of drug molecules. Thus, Chapter 2 and Chapter 3 are concerned with new research on water soluble β-cyclodextrin, adamantyl and octadecyl substituted poly(acrylate) networks, respectively, as potential sustained drug delivery systems. In Chapter 2, research into self-assembled poly(acrylate) networks cross-linked through host-guest complexation between β-cyclodextrin, β-CD, substituents and adamantyl, AD, substituents as potential sustained drug delivery systems is described. A poly(acrylate) (PAA) 8.8% randomly substituted with β-CD through an ethyl tether, PAAβ-CDen, is synthesized as a host poly(acrylate). Poly(acrylate)s 3.3%, 3.0% and 2.9% randomly substituted with AD substituents, respectively, through ethyl, hexyl and dodecyl tethers in the PAAADen, PAAADhn and PAAADddn are synthesized as guest polymers. The host-guest complexation of PAAADen, PAAADhn, and PAAADddn by PAAβ-CDen in aqueous solution produce three self-assembled poly(acrylate) networks. These complexations are characterized by isothermal titration calorimetry, ITC, 2D NOESY ¹H NMR spectroscopy, and rheology. It is found that the length of the tether between the AD group and the poly(acrylate) backbone has a substantial influence on the complexation constants, KITC [ITC subscript], as well as the associated enthalpy change, ΔH, and entropy change, ΔS. The smallest and largest KITC [ITC subscript] occur for PAAADen with the shortest tether and PAAADddn with the longest tether, which coincides with the lowest and highest viscosities occurring for the aqueous PAAβ-CDen/PAAADen and PAAβ-CDen/PAAADddn networks. The complexation of three different dye molecules, acting as drug models, by the β-CD substituents in these networks is characterized by UV-Vis spectroscopy and 2D NOESY ¹H NMR studies. The results suggest that dye complexation by the β-CD substituents in the three poly(acrylate) networks is weaker by comparison with the complexation by native β-CD and PAAβ-CDen, as indicated by decreased complexation constants. The poly(acrylate) networks exhibit complexation-controlled dye release behavior, and thereby sustained dye release profiles. Thus, the three poly(acrylate) networks studied, which form hydrogels at higher concentrations, have substantial potential as sustained drug delivery systems. In Chapter 3, the complexation and release behavior of six dyes in a β-CD- and octadecyl-substituted poly(acrylate) network is explored to further extend the understanding of the host-guest complexation between β-CD substituents and guest molecules within a fourth polymer network system and its influence on the release of guest molecules from the polymer network. Thus, β-CD substituents are 9.3% randomly substituted onto poly(acrylate) through an ethyl tether to give PAAβ-CDen and octadecyl, C18, substituents are 3.5% randomly substituted onto poly(acrylate) to give PAAC18. The network forms through the host-guest complexation between the β-CD substituents and C18 substituents, and is characterized by a complexation constant of K = 1.13 × 10⁴ dm³ mol⁻¹, associated with ΔH = −21.55 kJ mol⁻¹ and TΔS = 1.59 kJ mol⁻¹. The complexation of the dyes by the β-CD substituents in the PAAβ-CDen/PAAC18 network is characterized by UV-Vis absorption and fluorescence spectroscopy and 2D NOESY ¹H NMR studies. The results suggest that the complexation of dyes by the β-CD substituents in the PAAβ-CDen/PAAC18 network is weaker by comparison with the complexation by native β-CD and PAAβ-CDen, as indicated by decreased complexation constants. The PAAβ-CDen/PAAC18 network exhibits complexation-controlled dye release behavior and thereby sustained dye release profiles. Thus, the PAAβ-CDen/PAAC18 network, or hydrogel at higher concentration, is a potential sustained drug delivery system. Conjugated polymer nanoparticles are promising fluorescent probes as a consequence of their high brightness and photostability. Chapter 1 introduces the general methods of preparing conjugated polymer nanoparticles and their wide ranges of biological applications. However, conjugated polymer nanoparticles exhibit large-scale aggregation and precipitation at the high ionic strengths encountered under physiological conditions, which presents an impediment to their biological applications. In seeking to address this issue, the research described in Chapter 4 and Chapter 5 addresses stabilization of conjugated polymer nanoparticles using hydrophobic linear alkyl group substituted poly(acrylate)s and bovine serum albumin and explores their deployment in cell imaging applications. In Chapter 4, the synthesis of hydrophobic linear alkyl group substituted poly(acrylate)s, PAACn, is described as is their employment as conjugated polymer nanoparticle stabilizers. (When n = 18, 16 and 10 the alkyl groups are octadecyl, hexadecyl and decyl, respectively.) The carboxylate groups of PAACn increase the surface charge of the conjugated polymer nanoparticles and thereby stabilize them in phosphate buffered saline, PBS. Nanoparticles of the green-yellow emitting conjugated polymer, F8BT, stabilized with PAACn, F8BT-PAACn, are prepared using a nano-precipitation method. In contrast to the significant aggregation with a negligible yield (~ 0%) of bare F8BT nanoparticles in PBS, high yields approaching 90% are observed for F8BT nanoparticles stabilized with PAAC18 at 1%, PAAC16 at 3%, and PAAC10 at 10% substitution. The F8BT-PAACn nanoparticles have small sizes ranging from 50 to 70 nm in diameter, highly negative surface charge and high colloidal stability over 4 weeks in PBS. These properties pave the way for the deployment of F8BT-PAACn nanoparticles in biological applications. Spectroscopic results indicate the PAACn has no adverse effect on the UV-Vis absorptivity and fluorescence brightness of F8BT-PAACn nanoparticles relative to bare F8BT nanoparticles. In addition, F8BT-PAACn nanoparticles are internalized by HEK 293 cells and exhibit negligible cytotoxicity. Thus, PAACn are versatile and robust stabilizing materials that facilitate the application of F8BT-PAACn nanoparticles as fluorescent probes in cell imaging.The research described in Chapter 5 shows that bovine serum albumin, BSA, stabilizes conjugated polymer nanoparticles in phosphate buffered saline, PBS, evidently due to the combined effects of the negatively charged surfaces arising from the BSA carboxylate groups and the steric effect of the bulk 3D structure of BSA. Three multicolored conjugate polymers, PDOF, F8BT, and MEHPPV, are employed to prepare their corresponding nanoparticles using a nano-precipitation method. In contrast to the significant aggregation with negligible yields (~ 0%) of bare conjugated polymer nanoparticles occurring in PBS, high yields approaching 100% are observed for conjugated polymer nanoparticles stabilized with BSA, CP-BSA nanoparticles, in PBS. These CP-BSA nanoparticles have small sizes ranging from 20 to 60 nm, negative surface charges and high colloidal stability. Spectroscopic results indicate the BSA has no adverse effect on the UV-Vis absorptivity and fluorescence brightness of the CP-BSA nanoparticles relative to bare conjugated polymer nanoparticles. These properties potentially pave the way for the deployment of these CP-BSA nanoparticles in biological applications.