Folic Acid-Conjugated Brush Polymers Show Enhanced Blood-Brain Barrier Crossing in Static and Dynamic In Vitro Models Toward Brain Cancer Targeting Therapy

Over the past decades, evidence has consistently shown that treatment of central nervous system (CNS)-related disorders, including Alzheimer's disease, Parkinson's disease, stroke, multiple sclerosis, and brain cancer, is limited due to the presence of the blood-brain barrier (BBB). To assist with the development of new therapeutics, it is crucial to engineer a drug delivery system that can cross the BBB efficiently and reach target cells within the brain. In this study, we present a potentially efficient strategy for targeted brain delivery through utilization of folic acid (FA)-conjugated brush polymers, that specifically target the reduced folate carrier (RFC, SLC19A1) expressed on brain endothelial cells. Here, azide (N3)-decorated brush polymers were prepared in a straightforward manner coupling a heterotelechelic α-NH2, ω-N3-poly(2-ethyl-2-oxazoline) (NH2-PEtOx-N3) to N-acylated poly(amino ester) (NPAE)-based brushes. Strain-promoted azide-alkyne cycloaddition (SPAAC) 'click chemistry' with DBCO-folic acid (FA) yielded FA-brush polymers. Interestingly, while azide functionalization of the brush polymers dramatically reduced their association to brain microvascular endothelial cells (hCMEC/D3), the introduction of FA to azide led to a substantial accumulation of the brush polymers in hCMEC/D3 cells. The ability of the polymeric brush polymers to traverse the BBB was quantitatively assessed using different in vitro BBB models including static Transwell and microfluidic platforms. FA-brush polymers showed efficient transport across hCMEC/D3 cells in a manner dependent on FA composition, whereas nonfunctionalized brush polymers exhibited limited trafficking under the same conditions. Further, cellular uptake inhibition studies suggested that the interaction and transport pathway of FA-brush polymers across BBB relies on the RFC-mediated pathways. The potential application of the developed FA-brush polymers in brain cancer delivery was also investigated in a microfluidic model of BBB-glioblastoma. Brush polymers with more FA units successfully presented an enhanced accumulation into U-87 MG glioma cells following its BBB crossing, compared to controls. These results demonstrate that FA-modified brush polymers hold a great potential for more efficient delivery of future brain therapeutics.

Hydrophobicity Regulates the Cellular Interaction of Cyanine5-Labeled Poly(3-hydroxypropionate)-Based Comb Polymers

An in-depth understanding of the effect of physicochemical properties of nanocarriers on their cellular uptake and fate is crucial for the development of novel delivery systems. In this study, well-defined hydrophobic carboxylated poly(3-hydroxypropionate)-based comb polymers were synthesized. Two oligo(3-hydroxypropionate) (HP_n) of different degrees of polymerization (DP; 5 and 9) bearing α-vinyl end-groups were obtained by an hydrogen transfer polymerization (HTP)-liquid/liquid extraction strategy. 2-Carboxyethyl acrylate (CEA), representing the DP 1 analogue of HP_n, was also included in the study. (Macro)monomers were polymerized via reversible addition-fragmentation chain-transfer (RAFT) polymerization and fully characterized by ¹H NMR spectroscopy and size exclusion chromatography. All polymers were non-hemolytic and non-cytotoxic against NIH/3T3 cells. Detailed cellular association and uptake studies of Cy5-labeled polymers by flow cytometry and confocal laser scanning microscopy (CLSM) revealed that the carboxylated water-soluble PCEA, the polymer with the shortest side chain, efficiently targets mitochondria. However, increasing the side-chain DP led to a change in the intracellular fate. P(HP₅) was trafficked to both mitochondria and lysosomes, while P(HP₉) was exclusively found in lysosomes. Importantly, FLIM-FRET investigation of P(HP₅) provided initial insight into the mitochondria subcompartment location of Cy5-labeled carboxylated polymers. Moreover, intracellular uptake mechanism studies were performed. Blocking scavenger receptors by dextran sulfate or cooling cells to 4 °C significantly affected the cell association of hydrophobic carboxylated polymers with an insignificant response to membrane-potential inhibitors. In contrast, water-soluble carboxylated polymers' cellular association was substantially inhibited in cells treated with compounds depleting the mitochondrial potential (ΔΨ). Overall, this study highlights hydrophobicity as a valuable means to tune the cellular interaction of carboxylated polymers and thus will inform the design of future drug carriers based on Cy5-modified carboxylated polymers.

Nitrile-Functionalized Poly(2-oxazoline)s as a Versatile Platform for the Development of Polymer Therapeutics

In recent years, polymers bearing reactive groups have received significant interest for biomedical applications. Numerous functional polymer platforms have been introduced, which allow for the preparation of materials with tailored properties via post-polymerization modifications. However, because of their reactivity, many functional groups are not compatible with the initial polymerization. The nitrile group is a highly interesting and relatively inert functionality that has mainly received attention in radical polymerizations. In this Article, a nitrile-functionalized 2-oxazoline monomer (2-(4-nitrile-butyl)-2-oxazoline, BuNiOx) is introduced, and its compatibility with the cationic ring-opening polymerization is demonstrated. Subsequently, the versatility of nitrile-functionalized poly(2-oxazoline)s (POx) is presented. To this end, diverse (co)polymers are synthesized and characterized by nuclear resonance spectroscopy, size-exclusion chromatography, and mass spectrometry. Amphiphilic block copolymers are shown to efficiently encapsulate the hydrophobic drug curcumin (CUR) in aqueous solution, and the anti-inflammatory effect of the CUR-containing nanostructures is presented in BV-2 microglia. Furthermore, the availability of the BuNiOx repeating units for post-polymerization modifications with hydroxylamine to yield amidoxime (AO)-functionalized POx is demonstrated. These AO-containing POx were successfully applied for the complexation of Fe(III) in a quantitative manner. In addition, AO-functionalized POx were shown to release nitric oxide intracellularly in BV-2 microglia. Thus nitrile-functionalized POx represent a promising and robust platform for the design of polymer therapeutics for a wide range of applications.

Intrinsic Green Fluorescent Cross-Linked Poly(ester amide)s by Spontaneous Zwitterionic Copolymerization

The spontaneous zwitterionic copolymerization (SZWIP) of 2-oxazolines and acrylic acid affords biocompatible but low molecular weight linear N-acylated poly(amino ester)s (NPAEs). Here, we present a facile one-step approach to prepare functional higher molar mass cross-linked NPAEs using 2,2'-bis(2-oxazoline)s (BOx). In the absence of solvent, insoluble free-standing gels were formed from BOx with different length n-alkyl bridging units, which when butylene-bridged BOx was used possessed an inherent green fluorescence, a behavior not previously observed for 2-oxazoline-based polymeric materials. We propose that this surprising polymerization-induced emission can be classified as nontraditional intrinsic luminescence. Solution phase and oil-in-oil emulsion approaches were investigated as means to prepare solution processable fluorescent NPAEs, with both resulting in water dispersible network polymers. The emulsion-derived system was investigated further, revealing pH-responsive intensity of emission and excellent photostability. Residual vinyl groups were shown to be available for modifications without affecting the intrinsic fluorescence. Finally, these systems were shown to be cytocompatible and to function as fluorescent bioimaging agents for in vitro imaging.

Tuning Cellular Interactions of Carboxylic Acid-Side-Chain-Containing Polyacrylates: The Role of Cyanine Dye Label and Side-Chain Type

Cellular uptake and intracellular targeting to specific organelles are key events in the cellular processing of nanomaterials. Herein, we perform a detailed structure-property relationship study on carboxylic acid-side-chain-bearing polyacrylates to provide design criteria for the manipulation of their cellular interactions. Redox-initiated reversible addition-fragmentation chain-transfer (RRAFT) polymerization of three tert-butyl-protected N-acylated amino ester-based acrylate monomers of different substitutions and degrees of polymerization (DPs) yielded defined and pH-responsive carboxylic acid-side-chain polymers upon deprotection (N-acetyl, DP 1: P(M₁); N-propionyl, DP 1: P(E₁), DP 2: P(E₂)). Flow cytometry studies revealed time-dependent cell association with P(E₂) > P(E₁) > P(M₁) at any given time point. Importantly, the type of cyanine dye used for labeling was found to significantly influence the cellular processing of the polymers. Changing the dye from Cy5 to its sulfonated version sulfoCy5 resulted in a much lower cellular association. Moreover, Cy5-labeled polymers were targeted to mitochondria, while sulfoCy5 modification caused a significant change in the cellular fate of polymers toward lysosome trafficking. This study highlights the importance of selecting a suitable dye but also demonstrates the possibilities for the rational design of organelle-specific targeting of carboxylated polyacrylates.

Carboxylated Cy5-Labeled Comb Polymers Passively Diffuse the Cell Membrane and Target Mitochondria

A detailed understanding of the cellular uptake and trafficking of nanomaterials is essential for the design of "smart" intracellular drug delivery vehicles. Typically, cellular interactions can be tailored by endowing materials with specific properties, for example, through the introduction of charges or targeting groups. In this study, water-soluble carboxylated N-acylated poly(amino ester)-based comb polymers of different degree of polymerization and side-chain modification were synthesized via a combination of spontaneous zwitterionic copolymerization and redox-initiated reversible addition-fragmentation chain-transfer polymerization and fully characterized by ¹H NMR spectroscopy and size exclusion chromatography. The comb polymers showed no cell toxicity against NIH/3T3 and N27 cell lines nor hemolysis. Detailed cellular association and uptake studies by flow cytometry and confocal laser scanning microscopy (CLSM) revealed that the carboxylated polymers were capable of passively diffusing cell membranes and targeting mitochondria. The interplay of pendant carboxylic acids of the comb polymers and the Cy5-label was identified as major driving force for this behavior, which was demonstrated to be applicable in NIH/3T3 and N27 cell lines. Blocking of the carboxylic acids through modification with 2-methoxyethylamine and poly(2-ethyl-2-oxazoline) or replacement of the dye label with a different dye (e.g., fluorescein) resulted in an alteration of the cellular uptake mechanism toward endocytosis as demonstrated by CLSM. In contrast, partial modification of the carboxylic acid groups allowed to retain the cellular interaction, hence, rendering these comb polymers a highly functional mitochondria targeted carrier platform for future drug delivery applications and imaging purposes.

A sequential native chemical ligation – thiol-Michael addition strategy for polymer–polymer ligation

Native Chemical Ligation (NCL) between cysteine-terminated polymers and functional thioesters has been employed to prepare functional (co)polymers. The retained thiol functionality at the NCL junction can be exploited for thiol-Michael addition.

Functional hydrophobic and hetero-grafted block comb polymers via a combination of spontaneous zwitterionic copolymerisation and redox-initiated RAFT polymerisation

Non-toxic hydrophobic and amphiphilic N-acylated poly(amino ester)-based comb polymers self-assemble into functional responsive nano-sized aggregates in aqueous solution.