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.

Amphiphilic Heterograft Copolymers Bearing Biocompatible/Biodegradable Grafts

The amphiphilic heterograft copolymers bearing biocompatible/biodegradable grafts [poly(2-methyl-2-oxazoline-co-2-pentyl-2-oxazoline)-g-poly(d-l-lactic acid)/poly(2-ethyl-2-oxazoline)] were synthesized successfully by the combination of cationic ring-opening polymerization and click chemistry via the ⟨"grafting to"⟩ approach. The challenge of this synthesis was to graft together hydrophobic and hydrophilic chains on a hydrophilic platform based on PMeOx. The efficiency of grafting depends on the chemical nature of the grafts and of the length of the macromolecular chains. The self-assembly of these polymers in aqueous media was investigated by DLS, cryo-TEM, and SANS. The results demonstrated that different morphologies were obtained from nanospheres and vesicles to filaments depending on the hydrophilic weight ratio in the heterograft copolymer varying from 0.38 until 0.84. As poly(2-ethyl-2-oxazoline) is known to be thermoresponsive, the influence of temperature rise on the nanoassembly stability was studied in water and in a physiological medium. SANS and DLS measurements during a temperature ramp allowed to show that nanoassemblies start to self-assemble in "raspberry like" primary structures at 50 °C, and these structures grow and get denser as the temperature is increased further. These amphiphilic heterograft copolymers may include hydrophobic drugs and should find important applications for biomedical applications which require stealth properties.

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.

Block copolymers comprising degradable poly(2-ethyl-2-oxazoline) analogues via copper-free click chemistry

We present the synthesis development of amphiphilic, degradable poly(2-ethyl-2-oxazoline) (PEtOx) analogue block copolymers in a modular fashion utilizing the strain-promoted azide–alkyne cycloaddition (SPAAC).

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.