Design and Synthesis of Water-Soluble Multifunctionalizable Thiol-Reactive Polymeric Supports for Cellular Targeting

Amino-Acid-Derived Anionic Polyacrylamides with Tailored Hydrophobicity-Physicochemical Properties and Cellular Interactions

Polyanions can internalize into cells via endocytosis without any cell disruption and are therefore interesting materials for biomedical applications. In this study, amino-acid-derived polyanions with different alkyl side-chains are synthesized via postpolymerization modification of poly(pentafluorophenyl acrylate), which is synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization, to obtain polyanions with tailored hydrophobicity and alkyl branching. The success of the reaction is verified by size-exclusion chromatography, NMR spectroscopy, and infrared spectroscopy. The hydrophobicity, surface charge, and pH dependence are investigated in detail by titrations, high-performance liquid chromatography, and partition coefficient measurements. Remarkably, the determined pK a-values for all synthesized polyanions are very similar to those of poly(acrylic acid) (pK a = 4.5), despite detectable differences in hydrophobicity. Interactions between amino-acid-derived polyanions with L929 fibroblasts reveal very slow cell association as well as accumulation of polymers in the cell membrane. Notably, the more hydrophobic amino-acid-derived polyanions show higher cell association. Our results emphasize the importance of macromolecular engineering toward ideal charge and hydrophobicity for polymer association with cell membranes and internalization. This study further highlights the potential of amino-acid-derived polymers and the diversity they provide for tailoring properties toward drug delivery applications.

Zwitterionic Polysulfobetaine Inhibits Cancer Cell Migration Owing to Actin Cytoskeleton Dynamics

Cell migration is an essential dynamic process for most living cells, mainly driven by the reorganization of actin cytoskeleton. To control actin dynamics, a molecular architecture that can serve as a nucleator has been designed by polymerizing sulfobetaine methacrylate. The synthesized zwitterionic polymer, poly(sulfobetaine methacrylate) (PZI), effectively nucleates the polymerization process of G-actin and substantially accelerates the rate of polymerization. Isothermal titration calorimetry (ITC) and bioinformatics analysis indicated binding between PZI and monomeric G-actin. Thus, in vitro actin dynamics was studied by dynamic light scattering (DLS), pyrene-actin polymerization assay, and total internal reflection fluorescence microscopy (TIRFM). Furthermore, a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore-containing monomeric unit was incorporated into the sulfobetaine zwitterionic architecture to visualize the effect of polymer in the cellular environment. The BODIPY-containing zwitterionic sulfobetaine polymer (PZI-F) successfully penetrated the cell and remained in the lysosome with minimal cytotoxicity. Confocal microscopy revealed the influence of this polymer on the cellular actin cytoskeleton dynamics. The PZI-F polymer was successfully able to inhibit the collective migration of the human cervical cancer cell line (HeLa cell) and breast cancer cell line (MDA-MB-231 cell), as confirmed by a wound healing assay. Therefore, polyzwitterionic sulfobetaine could be explored as an inhibitor of cancer cell migration.

Adamantoid Scaffolds for Multiple Cargo Loading and Cellular Delivery as β-Cyclodextrin Inclusion Complexes

There is huge demand for developing guests that bind β-CD and can conjugate multiple cargos for cellular delivery. We synthesized trioxaadamantane derivatives, which can conjugate up to three cargos per guest. 1 H NMR titration and isothermal titration calorimetry revealed these guests form 1 : 1 inclusion complexes with β-CD with association constants in the order of 103  M-1 . Co-crystallization of β-CD with guests yielded crystals of their 1 : 1 inclusion complexes as determined by single-crystal X-ray diffraction. In all cases, trioxaadamantane core is buried within the hydrophobic cavity of β-CD and three hydroxyl groups are exposed outside. We established biocompatibility using representative candidate G4 and its inclusion complex with β-CD (β-CD⊂G4), by MTT assay using HeLa cells. We incubated HeLa cells with rhodamine-conjugated G4 and established cellular cargo delivery using confocal laser scanning microscopy (CLSM) and fluorescence-activated cell sorting (FACS) analysis. For functional assay, we incubated HeLa cells with β-CD-inclusion complexes of G4-derived prodrugs G6 and G7, containing one and three units of the antitumor drug (S)-(+)-camptothecin, respectively. Cells incubated with β-CD⊂G7 displayed the highest internalization and uniform distribution of camptothecin. β-CD⊂G7 showed higher cytotoxicity than G7, camptothecin, G6 and β-CD⊂G6, affirming the efficiency of adamantoid derivatives in high-density loading and cargo delivery.

Multifunctional and Transformable 'Clickable' Hydrogel Coatings on Titanium Surfaces: From Protein Immobilization to Cellular Attachment

Multifunctionalizable hydrogel coatings on titanium interfaces are useful in a wide range of biomedical applications utilizing titanium-based materials. In this study, furan-protected maleimide groups containing multi-clickable biocompatible hydrogel layers are fabricated on a titanium surface. Upon thermal treatment, the masked maleimide groups within the hydrogel are converted to thiol-reactive maleimide groups. The thiol-reactive maleimide group allows facile functionalization of these hydrogels through the thiol-maleimide nucleophilic addition and Diels-Alder cycloaddition reactions, under mild conditions. Additionally, the strained alkene unit in the furan-protected maleimide moiety undergoes radical thiol-ene reaction, as well as the inverse-electron-demand Diels-Alder reaction with tetrazine containing molecules. Taking advantage of photo-initiated thiol-ene 'click' reactions, we demonstrate spatially controlled immobilization of the fluorescent dye thiol-containing boron dipyrromethene (BODIPY-SH). Lastly, we establish that the extent of functionalization on hydrogels can be controlled by attachment of biotin-benzyl-tetrazine, followed by immobilization of TRITC-labelled ExtrAvidin. Being versatile and practical, we believe that the described multifunctional and transformable 'clickable' hydrogels on titanium-based substrates described here can find applications in areas involving modification of the interface with bioactive entities.

Fabrication of Patterned Hydrogel Interfaces: Exploiting the Maleimide Group as a Dual Purpose Handle for Cross-Linking and Bioconjugation

Functional hydrogels that can be obtained through facile fabrication procedures and subsequently modified using straightforward reagent-free methods are indispensable materials for biomedical applications such as sensing and diagnostics. Herein a novel hydrogel platform is obtained using polymeric precursors containing the maleimide functional group as a side chain. The maleimide groups play a dual role in fabrication of functional hydrogels. They enable photochemical cross-linking of the polymers to yield bulk and patterned hydrogels. Moreover, the maleimide group can be used as a handle for efficient functionalization using the thiol-maleimide conjugation and Diels-Alder cycloaddition click reactions. Obtained hydrogels are characterized in terms of their morphology, water uptake capacity, and functionalization. Micropatterned hydrogels are obtained under UV-irradiation using a photomask to obtain reactive micropatterns, which undergo facile functionalization upon treatment with thiol-containing functional molecules such as fluorescent dyes and bioactive ligands. The maleimide group also undergoes conjugation through the Diels-Alder reaction, where the attached molecule can be released through thermal treatment via the retro Diels-Alder reaction. The antibiofouling nature of these hydrogel micropatterns enables efficient ligand-directed biomolecular immobilization, as demonstrated by attachment of streptavidin-coated quantum dots.

A modular and orthogonally reactive platform for fabrication of polymer–drug conjugates for targeted delivery

Increasing interest in utilization of polymeric systems in targeted drug delivery has necessitated fabrication of polymers that undergo facile functionalization with targeting groups and therapeutic agents in a modular and orthogonal fashion.

Orthogonal thiol-ene 'click' reactions: a powerful combination for fabrication and functionalization of patterned hydrogels

A combination of 'orthogonal' thiol-ene 'click' reactions is utilized for fabrication and functionalization of micro-patterned hydrogels. A furan-protected maleimide-containing parent copolymer is partially activated via the retro Diels-Alder reaction to obtain an 'orthogonally' functionalizable copolymer, where the different functional groups can be exploited for multi-functionalization or fabrication of functional hydrogels using combination of the nucleophilic and radical thiol-ene reactions.

The Taming of the Maleimide: Fabrication of Maleimide-Containing 'Clickable' Polymeric Materials

Functional polymers are widely employed in various areas of biomedical sciences. In order to tailor them for desired applications, facile and efficient functionalization of these polymeric materials under mild and benign conditions is important. Polymers containing reactive maleimide groups can be employed for such applications since they provide an excellent handle for conjugation of thiol- and diene-containing molecules and biomolecules. Until recently, fabrication of maleimide containing polymeric materials has been challenging due to the interference from the highly reactive double bond. A Diels-Alder/retro Diels-Alder reaction sequence based strategy to transiently mask the maleimide group provides access to such polymeric materials. In this personal account, we summarize contributions from our group towards the fabrication and functionalization of maleimide-containing polymeric materials over the past decade.

Surface-Anchored Thiol-Reactive Soft Interfaces: Engineering Effective Platforms for Biomolecular Immobilization and Sensing

Fabrication of antibiofouling, specifically reactive polymeric coatings that undergo facile functionalization with thiol-bearing small molecules and ligands, yields effective platforms for biomolecular immobilization and sensing. Poly(ethylene glycol) (PEG)-based copolymers containing alkoxysilyl groups to enable surface-anchoring and furan-protected maleimide groups as latent thiol-reactive moieties as side-chains were synthesized. Reactive interfaces were obtained by coating these copolymers onto Si/SiO₂ or glass surfaces and activating the maleimide groups to their thiol-reactive forms via thermal treatment. A series of surfaces modified with copolymers containing varying amounts of maleimide groups were synthesized. Effectiveness of surface modification was probed using Fourier transform infrared spectroscopy, contact angle goniometry, ellipsometry and X-ray photoelectron spectroscopy. Facile surface modification through thiol-maleimide conjugation was established by attachment of a thiol-containing fluorescent dye, namely BODIPY-SH. It was demonstrated that these surfaces allow spatially localized modification through microcontact printing. Importantly, the extent of surface modification could be tuned by varying the initial composition of the copolymer used for coating. Using fluorescence microscopy, it was observed that increasing amount of fluorescent dye was attached onto surfaces fabricated with copolymers with increasing amount of masked maleimide groups. Thereafter, the thiol-maleimide conjugation was utilized to decorate these surfaces with biotin, a protein-binding ligand. It was observed that though these biotinylated surfaces were able to bind Streptavidin effectively, some nonspecific binding was observed on places that were not in conformal contact with the stamp during microcontact printing. This nonspecific binding was eliminated upon neutralizing the residual maleimide units on the printed surface using thiol-containing PEG. Notably, fluorescence analysis of Streptavidin immobilized onto biotinylated surfaces fabricated using varying amounts of maleimide demonstrated that the amount of immobilized protein could be tuned by varying surface composition. It can be envisioned that facile fabrication of these maleimide-containing polymeric surfaces, their effective functionalization in a tunable manner to engineer interfaces for effective immobilization or sensing of biomolecules in a spatially controlled manner would make them attractive candidates for various biotechnological applications.

Dendrons and Multiarm Polymers with Thiol-Exchangeable Cores: A Reversible Conjugation Platform for Delivery

Disulfide exchange reaction has emerged as a powerful tool for reversible conjugation of proteins, peptides and thiol containing molecules to polymeric supports. In particular, the pyridyl disulfide group provides an efficient handle for the site-specific conjugation of therapeutic peptides and proteins bearing cysteine moieties. In this study, novel biodegradable dendritic platforms containing a pyridyl disulfide unit at their focal point were designed. Presence of hydroxyl groups at the periphery of these dendrons allows their elaboration to multivalent initiators that yield poly(ethylene glycol) based multiarm star polymers via controlled radical polymerization. The pyridyl disulfide unit at the core of these star polymers undergoes efficient reaction with thiol functional group containing molecules such as a hydrophobic dye, namely, Bodipy-SH, glutathione, and KLAK sequence containing peptide. While conjugation of the hydrophobic fluorescent dye to the PEG-based multiarm polymer renders it water-soluble, it can be cleaved off the construct through thiol-disulfide exchange in the presence of an external thiol such as dithiothreitol. The multiarm polymer was conjugated with a thiol group containing apoptotic peptide to increase its solubility and cellular transport. In vitro cytotoxicity and apoptosis assays demonstrated that the resultant peptide-polymer conjugate had almost five times more apoptotic potential primarily through triggering apoptosis by disrupting mitochondrial membranes of human breast cancer cell line (MDA-MB-231) compared to naked peptide. The novel dendritic platform disclosed here offers an attractive template that can be modified to multiarm polymeric constructs bearing a "tag and release" characteristic.