Site-selective in situ growth of fluorescent polymer-antibody conjugates with enhanced antigen detection by signal amplification

Two Birds with One Stone: A Novel Dithiomaleimide-Based GalNAc-siRNA Conjugate Enabling Good siRNA Delivery and Traceability

For the first time, a novel dithiomaleimides (DTM) based tetra-antennary GalNAc conjugate was developed, which enable both efficient siRNA delivery and good traceability, without incorporating extra fluorophores. This conjugate can be readily constructed by three click-type reactions, that is, amidations, thiol-dibromomaleimide addition and copper catalyzed azide-alkyne cycloaddition (CuAAC). And it also has comparable siRNA delivery efficiency, with a GalNAc L96 standard to mTTR target. Additionally, due to the internal DTMs, a highly fluorescent emission was observed, which benefited delivery tracking and reduced the cost and side effects of the extra addition of hydrophobic dye molecules. In all, the simple incorporation of DTMs to the GalNAc conjugate structure has potential in gene therapy and tracking applications.

Preparation of Temperature-Responsive Antibody–Nanoparticles by RAFT-Mediated Grafting from Polymerization

Herein, we report the preparation of temperature-responsive antibody–nanoparticles by the direct polymerization of N-isopropylacrylamide (NIPAAm) from immunoglobulin G (IgG). To this end, a chain transfer agent (CTA) was introduced into IgG, followed by the precipitation polymerization of NIPAAm in an aqueous medium via reversible addition–fragmentation chain transfer polymerization above the lower critical solution temperature (LCST). Consequently, antibody–polymer particles with diameters of approximately 100–200 nm were formed. Owing to the entanglement of the grafted polymers via partial chemical crosslinking, the antibody–nanoparticles maintained their stability even at temperatures below the LCST. Further, the dispersed nanoparticles could be collected by thermal precipitation above the LCST. Additionally, the antibody–nanoparticles formulation could maintain its binding constant and exhibited a good resistance against enzymatic treatment. Thus, the proposed antibody–nanoparticles can be useful for maximizing the therapeutic potential of antibody–drug conjugates or efficacies of immunoassays and antibody recovery and recycling.

Fluorogenic monomer activation for protein-initiated atom transfer radical polymerization

Fluorogenic atom transfer radical polymerization (ATRP) directly detects initiator-dependent polymer formation, as initially non-fluorescent polycyclic aromatic probe monomers reveal visible fluorescence upon polymerization in real time. Advancement of this initial proof-of-concept toward biodetection applications requires both a more detailed mechanistic understanding of probe fluorescence activation, and the ability to initiate fluorogenic polymerization directly from a biomolecule surface. Here, we show that simple monomer hydrogenation, independent of polymerization, reveals probe fluorescence, supporting the critical role of covalent enone attachment in fluorogenic probe quenching and subsequent fluorescence activation. We next demonstrate bioorthogonal, protein-initiated fluorogenic ATRP by the surface conjugation and characterization of protein-initiator conjugates of a model protein, bovine serum albumin (BSA). Fluorogenic ATRP from initiator-modified protein allows for real-time visualization of polymer formation with negligible background fluorescence from unmodified BSA controls. We further probe the bioorthogonality of this fluorogenic ATRP assay by assessing polymer formation in a complex biological environment, spiked with fetal bovine serum. Taken together, we demonstrate the potential of aqueous fluorogenic ATRP as a robust, bioorthogonal method for biomolecular-initiated polymerization by real-time fluorescence activation.

Synthesis of Disulfide-Bridging Trehalose Polymers for Antibody and Fab Conjugation Using a Bis-Sulfone ATRP Initiator

Antibodies and antigen binding fragments (FABs) are widely used as therapeutics and conjugated polymers can enhance the properties of these important biomolecules. However, limitations to the selectivity and stability of current conjugation methodologies can inhibit the exploration of new antibody-polymer conjugates. Herein, we describe a new strategy for the synthesis of these conjugates that forms a stable thioether bond and can be directly incorporated into an atom transfer radical polymerization (ATRP) initiator. Specifically, a bis-sulfone alkyl bromide initiator was synthesized and utilized in the activators generated by electron transfer (AGET) ATRP of ethylene glycol methacrylate and trehalose methacrylate to form the respective polymers. The trehalose polymer was then irreversibly inserted into the disulfide bonds of Herceptin and Herceptin FAB after mild reduction to form the conjugates with quantitative conversions as verified by Western Blot and mass spectrometry after cleavage of the polymer. The binding of the Herceptin and Herceptin Fab conjugates to the receptor was investigated by indirect ELISA (enzyme-linked immunosorbent assay) and the EC50's were 0.90 and 2.74 nM, respectively, compared to Herceptin (0.26 nM) and the Fab (0.56 nM). The conjugates were subjected to heating studies at a constant 75 °C, the temperature determined in a heat ramp to be the threshold of stability for the antibody and FAB; the trehalose polymer was found to considerably increase the thermal stability of both Herceptin and Herceptin Fab. This work provides a new way to prepare polymer-antibody/Fab conjugates utilizing bis-sulfone end groups installed by atom transfer radical polymerization of the functionalized initiators and a way to stabilize these important molecules by conjugation to trehalose polymers.

Pyridine-2,6-dicarboxaldehyde-Enabled N-Terminal In Situ Growth of Polymer-Interferon α Conjugates with Significantly Improved Pharmacokinetics and In Vivo Bioactivity

Polymer-protein conjugates are a class of biohybrids with unique properties that are highly useful in biomedicine ranging from protein therapeutics to biomedical imaging; however, it remains a considerable challenge to conjugate polymers to proteins in a site-specific, mild, and efficient way to form polymer-protein conjugates with uniform structures and properties and optimal functions. Herein we report pyridine-2,6-dicarboxaldehyde (PDA)-enabled N-terminal modification of proteins with polymerization initiators for in situ growth of poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMA) conjugates uniquely at the N-termini of a range of natural and recombinant proteins in a mild and efficient fashion. The formed POEGMA-protein conjugates showed highly retained in vitro bioactivity as compared with free proteins. Notably, the in vitro bioactivity of a POEGMA-interferon α (IFN) conjugate synthesized by this new chemistry is 8.1-fold higher than that of PEGASYS that is a commercially available and Food and Drug Administration (FDA) approved PEGylated IFN. The circulation half-life of the conjugate is similar to that of PEGASYS but is 46.2 times longer than that of free IFN. Consequently, the conjugate exhibits considerably improved antiviral bioactivity over free IFN and even PEGASYS in a mouse model. These results indicate that the PDA-enabled N-terminal grafting-from method is applicable to a number of proteins whose active sites are far away from the N-terminus for the synthesis of N-terminal polymer-protein conjugates with high yield, well-retained activity, and considerably improved pharmacology for biomedical applications.

Multifunctional, histidine-tagged polymers: antibody conjugation and signal amplification

A polymer scaffold, with multiple reactive centres, was synthesised by RAFT polymerisation and conjugated to the antibody herceptin. A hexahistidine RAFT agent enabled simple purification of polymer–protein conjugates.

Tuning Butyrylcholinesterase Inactivation and Reactivation by Polymer-Based Protein Engineering

Organophosphate nerve agents rapidly inhibit cholinesterases thereby destroying the ability to sustain life. Strong nucleophiles, such as oximes, have been used as therapeutic reactivators of cholinesterase-organophosphate complexes, but suffer from short half-lives and limited efficacy across the broad spectrum of organophosphate nerve agents. Cholinesterases have been used as long-lived therapeutic bioscavengers for unreacted organophosphates with limited success because they react with organophosphate nerve agents with one-to-one stoichiometries. The chemical power of nucleophilic reactivators is coupled to long-lived bioscavengers by designing and synthesizing cholinesterase-polymer-oxime conjugates using atom transfer radical polymerization and azide-alkyne "click" chemistry. Detailed kinetic studies show that butyrylcholinesterase-polymer-oxime activity is dependent on the electrostatic properties of the polymers and the amount of oxime within the conjugate. The covalent coupling of oxime-containing polymers to the surface of butyrylcholinesterase slows the rate of inactivation of paraoxon, a model nerve agent. Furthermore, when the enzyme is covalently inhibited by paraoxon, the covalently attached oxime induced inter- and intramolecular reactivation. Intramolecular reactivation will open the door to the generation of a new class of nerve agent scavengers that couple the speed and selectivity of biology to the ruggedness and simplicity of synthetic chemicals.

Signal amplification in flow cytometry for cell surface antigen analysis

Signal enhancing systems have been introduced to enable detection of cell surface antigens by flow cytometry. Cell surface antigens are important targets that describe the function and lineage of cells. Although flow cytometry is an effective tool for analysing cell surface antigens, this technique has poor sensitivity, which prohibits the detection of many important antigens on cell membranes. Thus, signal amplification is essential for developing practical tools for evaluating cell surface antigens by flow cytometry. Using a bright fluorophore or fluorescent polymer incorporated into antibodies is a straightforward strategy to improve flow cytometry sensitivity but may affect the functional characteristics of the labelled antibody. In contrast, enzymatic signal amplification is a more practical and efficient strategy to improve sensitivity that should not affect antibody activity. Although enzymatic signal amplification still has a number of drawbacks, this approach is a promising strategy to analyse cell surface antigens.

Fluorogenic atom transfer radical polymerization in aqueous media as a strategy for detection

The development of novel approaches to signal amplification in aqueous media could enable new diagnostic platforms for the detection of water-soluble analytes, including biomolecules. This paper describes a fluorogenic polymerization approach to amplify initiator signal by the detection of visible fluorescence upon polymerization in real-time. Fluorogenic monomers were synthesized and co-polymerized by atom transfer radical polymerization (ATRP) in water to reveal increasing polymer fluorescence as a function of both reaction time and initiator concentration. Optimization of the fluorogenic ATRP reaction conditions allowed for the quantitative detection of a small-molecule initiator as a model analyte over a broad linear concentration range (pM to mM). Raising the reaction temperature from 30 °C to 60 °C facilitated sensitive initiator detection at sub-picomolar concentrations in as little as 1 h of polymerization. This method was then applied to the detection of streptavidin as a model biological analyte by fluorogenic polymerization from a designed biotinylated ATRP initiator. Taken together, these studies represent the first example of a fluorogenic ATRP reaction and establish fluorogenic polymerization as a promising approach for the direct detection of aqueous analytes and biomolecular recognition events.

Site-Specific Insulin-Trehalose Glycopolymer Conjugate by Grafting From Strategy Improves Bioactivity

Insulin is an important therapeutic protein for the treatment of diabetes, but it is unstable and aggregates upon exposure to environmental stressors encountered during storage and transport. To prevent degradation of the protein in this manner and retain as much in vivo bioactivity as possible, a well-defined insulin-trehalose glycopolymer conjugate was synthesized. To accomplish this, a strategy was employed to site-specifically modify insulin with a polymerization initiator at a particular conjugation site; this also facilitated purification and characterization. Lysine of the B chain was preferentially modified by conducting the reaction at high pH, taking advantage of its higher nucleophilicity than the N-terminal amines. Trehalose monomer was polymerized directly from this macroinitiator to form a well-defined conjugate. Bioactivity of the site-specific conjugate was shown to be higher compared to the non-specific conjugate and the same as the analogous site-specific polyethylene glycol (PEG) conjugate as confirmed by the insulin tolerance test (ITT) in mice. The conjugated trehalose glycopolymer also stabilized insulin to heat as measured by high-performance liquid chromatography (HPLC).

A new construct of antibody-drug conjugates for treatment of B-cell non-Hodgkin's lymphomas

The aim of this study was to develop a new class of antibody-drug conjugates (ADCs) with the potential to not only enhance treatment efficacy but also improve tolerability for patients with B-cell lymphomas. Classic ADCs consist of monoclonal antibodies (mAbs) linked to drugs or toxins. They selectively deliver toxic moieties to tumor cells. As such, they greatly improve the therapeutic index compared to traditional chemotherapeutic agents. However, the therapeutic efficacy and safety of ADCs are dependent on linker stability and payload toxicity. Limited payload number on a single antibody (drug-to-antibody ratio, or DAR) has been driving investigators to use extremely toxic agents; however, even very low off-target binding of these ADCs may kill patients. Herein we report a new design of ADCs that consists of rituximab (RTX) and N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-epirubicin conjugates. The latter was selectively attached to RTX via reduced disulfide bonds. Such design allows the introduction of a large payload of drug on the antibody without adding attachment sites and without compromising the antigen-targeting ability. The binding of the new conjugate, namely RTX-P-EPI, to Ramos cells (with high CD20 expression) was confirmed. The cytotoxicity of RTX-P-EPI against Raji and Ramos cells was also determined. Interestingly, two-fold inhibition of cell proliferation was observed when using RTX-P-EPI compared with their equivalent physical mixture of RTX and P-EPI. Treatment of male SCID mice bearing subcutaneous Ramos B-cell lymphoma tumors demonstrated that RTX-P-EPI possessed superior efficacy when compared to combination of RTX with chemotherapy EPI (RTX+EPI) and P-EPI (RTX+P-EPI), whereas single RTX and a non-specific conjugate IgG-P-EPI only showed marginal effect. The conjugate RTX-EPI in which EPI was directly attached to RTX demonstrated much less antitumor activity compared with RTX-P-EPI. The results suggest that this new design possesses synergistic potential of immunotherapy combined with established macromolecular therapy; moreover, a conventional chemo-agent could be utilized to generate highly effective ADCs and to achieve lower risk of off-target toxicity.

Site-Selective Disulfide Modification of Proteins: Expanding Diversity beyond the Proteome

The synthetic transformation of polypeptides with molecular accuracy holds great promise for providing functional and structural diversity beyond the proteome. Consequently, the last decade has seen an exponential growth of site-directed chemistry to install additional features into peptides and proteins even inside living cells. The disulfide rebridging strategy has emerged as a powerful tool for site-selective modifications since most proteins contain disulfide bonds. In this Review, we present the chemical design, advantages and limitations of the disulfide rebridging reagents, while summarizing their relevance for synthetic customization of functional protein bioconjugates, as well as the resultant impact and advancement for biomedical applications.

Reversible Regulation of Thermoresponsive Property of Dithiomaleimide-Containing Copolymers via Sequential Thiol Exchange Reactions

The facile and efficient functionalization of thermoresponsive polymers based on sequential, reversible thiol-exchange reactions is reported. Well-defined dithiomaleimide-containing polymers have been synthesized via Cu(0)-mediated SET-LRP and characterized by ¹H NMR and size exclusion chromatography (SEC). The resulting thermosensitive copolymers were subsequently reacted with various thiols to demonstrate the applicability of the strategy, and the thiol-exchange reaction was found to be very fast and efficient. The cloud point of the prepared copolymers can be continually and reversibly tuned, and desirable functionality can be dynamically exchanged upon sequential addition of functional thiol reagents. Through the substitution by thioglucose, an ON-to-OFF switch for fluorescence of the copolymers along with the generation of a glycopolymer was achieved.