Glucose-Driven Droplet Formation in Complexes of a Supramolecular Peptide and Therapeutic Protein.

Biology achieves remarkable function through processes arising from spontaneous or transient liquid-liquid phase separation (LLPS) of proteins and other biomolecules. While polymeric systems can achieve similar phenomena through simple or complex coacervation, LLPS with supramolecular materials has been less commonly shown. Functional applications for synthetic LLPS systems are an expanding area of emphasis, with particular focus on capturing the transient and dynamic state of these structures for use in biomedicine. Here, a net-cationic supramolecular peptide amphiphile building block with a glucose-binding motif is shown that forms LLPS structures when combined with a net-negatively charged therapeutic protein, dasiglucagon, in the presence of glucose. The droplets that arise are dynamic and coalesce quickly. However, the interface can be stabilized by addition of a 4-arm star PEG. When the stabilized droplets formed in glucose are transferred to a bulk phase containing different glucose concentrations, their stability and lifetime decrease according to bulk glucose concentration. This glucose-dependent formation translates into an accelerated release of dasiglucagon in the absence of glucose; this hormone analogue itself functions therapeutically to correct low blood glucose (hypoglycemia). These droplets also offer function in mitigating the most severe effects of hypoglycemia arising from an insulin overdose through delivery of dasiglucagon in a mouse model of hypoglycemic rescue. Accordingly, this approach to use complexation between a supramolecular peptide amphiphile and a therapeutic protein in the presence of glucose leads to droplets with functional potential to dissipate for the release of the therapeutic material in low blood glucose environments.

High-Affinity Host-Guest Recognition for Efficient Assembly and Enzymatic Responsiveness of DNA Nanostructures.

The combination of multiple orthogonal interactions enables hierarchical complexity in self-assembled nanoscale materials. Here, efficient supramolecular polymerization of DNA origami nanostructures is demonstrated using a multivalent display of small molecule host-guest interactions. Modification of DNA strands with cucurbit[7]uril (CB[7]) and its adamantane guest, yielding a supramolecular complex with an affinity of order 1010 m-1 , directs hierarchical assembly of origami monomers into 1D nanofibers. This affinity regime enables efficient polymerization; a lower-affinity ß-cyclodextrin-adamantane complex does not promote extended structures at a similar valency. Finally, the utility of the high-affinity CB[7]-adamantane interactions is exploited to enable responsive enzymatic actuation of origami nanofibers assembled using peptide linkers. This work demonstrates the power of high-affinity CB[7]-guest recognition as an orthogonal axis to drive self-assembly in DNA nanotechnology.

Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels.

Mechanical stimuli such as strain, force, and pressure are pervasive within and beyond the human body. Mechanoresponsive hydrogels have been engineered to undergo changes in their physicochemical or mechanical properties in response to such stimuli. Relevant responses can include strain-stiffening, self-healing, strain-dependent stress relaxation, and shear rate-dependent viscosity. These features are a direct result of dynamic bonds or noncovalent/physical interactions within such hydrogels. The contributions of various types of bonds and intermolecular interactions to these behaviors are important to more fully understand the resulting materials and engineer their mechanoresponsive features. Here, strain-stiffening in carboxymethylcellulose hydrogels cross-linked with pendant dynamic-covalent boronate esters using tannic acid is studied and modulated as a function of polymer concentration, temperature, and effective cross-link density. Furthermore, these materials are found to exhibit self-healing and strain-memory, as well as strain-dependent stress relaxation and shear rate-dependent changes in gel viscosity. These features are attributed to the dynamic nature of the boronate ester cross-links, interchain hydrogen bonding and bundling, or a combination of these two intermolecular interactions. This work provides insight into the interplay of such interactions in the context of mechanoresponsive behaviors, particularly informing the design of hydrogels with tunable strain-stiffening. The multiresponsive and tunable nature of this hydrogel system therefore presents a promising platform for a variety of applications.

Iterative Design Reveals Molecular Domain Relationships in Supramolecular Peptide-Drug Conjugates.

Supramolecular peptide-drug conjugates (sPDCs) are prepared by covalent attachment of a drug moiety to a peptide motif programmed for one-dimensional self-assembly, with subsequent physical entanglement of these fibrillar structures enabling formation of nanofibrous hydrogels. This class of prodrug materials presents an attractive platform for mass-efficient and site-specific delivery of therapeutic agents using a discrete, single-component molecular design. However, a continued challenge in sPDC development is elucidating relationships between supramolecular interactions in their drug and peptide domains and the resultant impacts of these domains on assembly outcomes and material properties. Inclusion of a saturated alkyl segment alongside the prodrug in the hydrophobic domain of sPDCs could relieve some of the necessity for ordered, prodrug-produg interactions. Accordingly, nine sPDCs are prepared here to iterate the design variables of amino acid sequence and hydrophobic prodrug-alkyl block design. All molecules spontaneously formed hydrogels under physiological conditions, indicating a less hindered thermodynamic path to self-assembly relative to previous prodrug-only designs. However, material studies on the supramolecular arrangement, formation, and mechanical properties of the resultant sPDC hydrogels as well as their drug release profiles showed complex relationships between the hydrophobic and peptide domains in the formation and function of the resulting assemblies. Together, these results indicate that sPDC material properties are intrinsically linked to holistic molecular design with coupled contributions from their prodrug and peptide domains in directing properties of the emergent materials.

Supramolecular Protein Stabilization with Zwitterionic Polypeptide-Cucurbit[7]uril Conjugates.

Protein aggregation is an obstacle for the development of new biopharmaceuticals, presenting challenges in shipping and storage of vital therapies. Though a variety of materials and methods have been explored, the need remains for a simple material that is biodegradable, nontoxic, and highly efficient at stabilizing protein therapeutics. In this work, we investigated zwitterionic polypeptides prepared using a rapid and scalable polymerization technique and conjugated to a supramolecular macrocycle host, cucurbit[7]uril, for the ability to inhibit aggregation of model protein therapeutics insulin and calcitonin. The polypeptides are based on the natural amino acid methionine, and zwitterion sulfonium modifications were compared to analogous cationic and neutral structures. Each polymer was end-modified with a single cucurbit[7]uril macrocycle to afford supramolecular recognition and binding to terminal aromatic amino acids on proteins. Only conjugates prepared from zwitterionic structures of sufficient chain lengths were efficient inhibitors of insulin aggregation and could also inhibit aggregation of calcitonin. This polypeptide exhibited no cytotoxicity in human cells even at concentrations that were five-fold of the intended therapeutic regime. We explored treatment of the zwitterionic polypeptides with a panel of natural proteases and found steady biodegradation as expected, supporting eventual clearance when used as a protein formulation additive.

Transient and Dissipative Host-Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel.

The transient self-assembly of molecules under the direction of a consumable fuel source is fundamental to biological processes such as cellular organization and motility. Such biomolecular assemblies exist in an out-of-equilibrium state, requiring continuous consumption of high energy molecules. At the same time, the creation of bioinspired supramolecular hydrogels has traditionally focused on associations occurring at the thermodynamic equilibrium state. Here, hydrogels are prepared from cucurbit[7]uril host-guest supramolecular interactions through transient physical crosslinking driven by the consumption of a reactive chemical fuel. Upon action from this fuel, the affinity and dynamics of CB[7]-guest recognition are altered. In this way, the lifetime of transient hydrogel formation and the dynamic modulus obtained are governed by fuel consumption, rather than being directed by equilibrium complex formation.

Multivalent Cucurbituril Dendrons for Cell Membrane Engineering with Supramolecular Receptors.

The affinity possible from certain supramolecular motifs rivals that for some of the best-recognized interactions in biology. Cucurbit[7]uril (CB[7]) macrocycles, for example, are capable of achieving affinities in their binding to certain guests that rival that of biotin-avidin. Supramolecular host-guest recognition between CB[7] and certain guests has been demonstrated to spatially localize guest-linked agents to desired sites in vivo, offering opportunities to better exploit this affinity axis for applications in biomedicine. Herein, architectures of CB[7] are prepared from a polyamidoamine (PAMAM) dendrimer scaffold, installing a PEG-linked cholesterol anchor on the opposite end of the dendron to facilitate cell membrane integration. Cells are then modified with this dendritic CB[7] construct in vitro, demonstrating the ability to deliver a model guest-linked agent to the cell membrane. This approach to realize synthetic supramolecular "membrane receptors" may be leveraged in the future for in situ imaging or modulation of cell-based therapies or to facilitate a synthetic supramolecular recognition axis on the cell membrane.

Polymeric Microneedle Arrays with Glucose-Sensing Dynamic-Covalent Bonding for Insulin Delivery.

The ongoing rise in diabetes incidence necessitates improved therapeutic strategies to enable precise blood glucose control with convenient device form factors. Microneedle patches are one such device platform capable of achieving therapeutic delivery through the skin. In recent years, polymeric microneedle arrays have been reported using methods of in situ polymerization and covalent crosslinking in microneedle molds. In spite of promising results, in situ polymerization carries a risk of exposure to toxic unreacted precursors remaining in the device. Here, a polymeric microneedle patch is demonstrated that uses dynamic-covalent phenylboronic acid (PBA)-diol bonds in a dual role affording both network crosslinking and glucose sensing. By this approach, a pre-synthesized and purified polymer bearing pendant PBA motifs is combined with a multivalent diol crosslinker to prepare dynamic-covalent hydrogel networks. The ability of these dynamic hydrogels to shear-thin and self-heal enables their loading to a microneedle mold by centrifugation. Subsequent drying then yields a patch of uniformly shaped microneedles with the requisite mechanical properties to penetrate skin. Insulin release from these materials is accelerated in the presence of glucose. Moreover, short-term blood glucose control in a diabetic rat model following application of the device to the skin confirms insulin activity and bioavailability. Accordingly, dynamic-covalent crosslinking facilitates a route for fabricating microneedle arrays circumventing the toxicity concerns of in situ polymerization, offering a convenient device form factor for therapeutic insulin delivery.

Glucose-Fueled Peptide Assembly: Glucagon Delivery via Enzymatic Actuation.

Nature achieves remarkable function from the formation of transient, nonequilibrium materials realized through continuous energy input. The role of enzymes in catalyzing chemical transformations to drive such processes, often as part of stimuli-directed signaling, governs both material formation and lifetime. Inspired by the intricate nonequilibrium nanostructures of the living world, this work seeks to create transient materials in the presence of a consumable glucose stimulus under enzymatic control of glucose oxidase. Compared to traditional glucose-responsive materials, which typically engineer degradation to release insulin under high-glucose conditions, the transient nanofibrillar hydrogel materials here are stabilized in the presence of glucose but destabilized under conditions of limited glucose to release encapsulated glucagon. In the context of blood glucose control, glucagon offers a key antagonist to insulin in responding to hypoglycemia by signaling the release of glucose stored in tissues so as to restore normal blood glucose levels. Accordingly, these materials are evaluated in a prophylactic capacity in diabetic mice to release glucagon in response to a sudden drop in blood glucose brought on by an insulin overdose. Delivery of glucagon using glucose-fueled nanofibrillar hydrogels succeeds in limiting the onset and severity of hypoglycemia in mice. This general strategy points to a new paradigm in glucose-responsive materials, leveraging glucose as a stabilizing cue for responsive glucagon delivery in combating hypoglycemia. Moreover, compared to most fundamental reports achieving nonequilibrium and/or fueled classes of materials, the present work offers a rare functional example using a disease-relevant fuel to drive deployment of a therapeutic.

Supramolecular "Click Chemistry" for Targeting in the Body.

The fields of precision imaging and drug delivery have revealed a number of tools to improve target specificity and increase efficacy in diagnosing and treating disease. Biological molecules, such as antibodies, continue to be the primary means of assuring active targeting of various payloads. However, molecular-scale recognition motifs have emerged in recent decades to achieve specificity through the design of interacting chemical motifs. In this regard, an assortment of bioorthogonal covalent conjugations offer possibilities for in situ complexation under physiological conditions. Herein, a related concept is discussed that leverages interactions from noncovalent or supramolecular motifs to facilitate in situ recognition and complex formation in the body. Classic supramolecular motifs based on host-guest complexation offer one such means of facilitating recognition. In addition, synthetic bioinspired motifs based on oligonucleotide hybridization and coiled-coil peptide bundles afford other routes to form complexes in situ. The architectures to include recognition of these various motifs for targeting enable both monovalent and multivalent presentation, seeking high affinity or engineered avidity to facilitate conjugation even under dilute conditions of the body. Accordingly, supramolecular "click chemistry" offers a complementary tool in the growing arsenal targeting improved healthcare efficacy.

Supramolecular Hydrogels via Light-Responsive Homoternary Cross-Links.

Host-guest physical cross-linking has been used to prepare supramolecular hydrogels for various biomedical applications. More recent efforts to endow these materials with stimuli-responsivity offers an opportunity to precisely tune their function for a target use. In the context of light-responsive materials, azobenzenes are one prevailing motif. Here, an asymmetric azobenzene was explored for its ability to form homoternary complexes with the cucurbit[8]uril macrocycle, exhibiting an affinity (Keq) of 6.21 × 1010 M-2 for sequential binding, though having negative cooperativity. Copolymers were first prepared from different and tunable ratios of NIPAM and DMAEA, and DMAEA groups were then postsynthetically modified with this asymmetric azobenzene. Upon macrocycle addition, these polymers formed supramolecular hydrogels; relaxation dynamics increased with temperature due to temperature-dependent affinity reduction for the ternary complex. Application of UV light disrupted the supramolecular motif through azobenzene photoisomerization, prompting a gel-to-sol transition in the hydrogel. Excitingly, within several minutes at room temperature, thermal relaxation of azobenzene to its trans state afforded rapid hydrogel recovery. By revealing this supramolecular motif and employing facile means for its attachment onto pre-synthesized polymers, the approach described here may further enable stimuli-directed control of supramolecular hydrogels for a number of applications.

Affinity-Directed Dynamics of Host-Guest Motifs for Pharmacokinetic Modulation via Supramolecular PEGylation.

Proteins are an impactful class of therapeutics but can exhibit suboptimal therapeutic performance, arising from poor control over the timescale of clearance. Covalent PEGylation is one established strategy to extend circulation time but often at the cost of reduced activity and increased immunogenicity. Supramolecular PEGylation may afford similar benefits without necessitating that the protein be permanently modified with a polymer. Here, we show that insulin pharmacokinetics can be modulated by tuning the affinity-directed dynamics of a host-guest motif used to non-covalently endow insulin with a poly(ethylene glycol) (PEG) chain. When administered subcutaneously, supramolecular PEGylation with higher binding affinities extends the time of total insulin exposure systemically. Pharmacokinetic modeling reveals that the extension in the duration of exposure arises specifically from decreased absorption from the subcutaneous depot governed directly by the affinity and dynamics of host-guest exchange. The lifetime of the supramolecular interaction thus dictates the rate of absorption, with negligible impact attributed to association of the PEG upon rapid dilution of the supramolecular complex in circulation. This modular approach to supramolecular PEGylation offers a powerful tool to tune protein pharmacokinetics in response to the needs of different disease applications.

Dynamic light scattering microrheology for soft and living materials.

We present a method for using dynamic light scattering in the single-scattering limit to measure the viscoelastic moduli of soft materials. This microrheology technique only requires a small sample volume of 12 µL to measure up to six decades in time of rheological behavior. We demonstrate the use of dynamic light scattering microrheology (DLSµR) on a variety of soft materials, including dilute polymer solutions, covalently-crosslinked polymer gels, and active, biological fluids. In this work, we detail the procedure for applying the technique to new materials and discuss the critical considerations for implementing the technique, including a custom analysis script for analyzing data output. We focus on the advantages of applying DLSµR to biologically relevant materials: breast cancer cells encapsulated in a collagen gel and cystic fibrosis sputum. DLSµR is an easy, efficient, and economical rheological technique that can guide the design of new polymeric materials and facilitate the understanding of the underlying physics governing behavior of naturally derived materials.

Kinetic Evolution in Metal-Dependent Self-Assembly of Peptide-Terpyridine Conjugates.

Nature realizes impressive structures and emergent functions through precisely organized non-covalent interactions, and this inspires the use of supramolecular motifs to engineer new materials. Herein, an amphiphilic peptide-terpyridine conjugate is reported that forms 1D nanostructures leading to hydrogels. Upon the addition of metal, a slow kinetic transition occurs, resulting in nanostructures which are dictated by the chosen metal binding to the terpyridine ligand. As such, bis-complex formation between terminal terpyridines redirects the assembly from peptide-driven 1D structures to an assortment of new nanostructures which evolve and appear over the course of weeks. Studies where pre-existing peptide structures are disrupted prior to metal addition yield these same structures right away, further confirming the kinetically labored pathway to their formation when beginning from an assembled state.

Temperature-Responsive Supramolecular Hydrogels by Ternary Complex Formation with Subsequent Photo-Cross-linking to Alter Network Dynamics.

Supramolecular hydrogels prepared from host-guest physical cross-linking of polymers have versatile utility in a number of applications. Routes to integrate stimuli-responsive features in these materials are intended to add another dimension to enhance their functionality. Herein, a guest which forms a homoternary complex with the cucurbit[8]uril macrocycle was appended to the ends of Pluronic F-127 polymers. This polymer undergoes temperature-responsive micelle formation, upon which CB[8] promotes their physical cross-linking via its host-guest interactions with the appended guests yielding a percolated hydrogel network. The particular guests used to form the homoternary complex can further be photo-dimerized to replace the physical host-guest interaction with a covalently bonded interaction. This change results in a reduction in hydrogel dynamics of roughly 2 orders of magnitude, yet temperature-responsive gelation and overall network architecture remain apparently unchanged. Hydrogels composed of micelles cross-linked by both supramolecular and photo-dimerized interactions support the injection and encapsulation of cells and enable inclusion and release of macromolecular payloads in vitro and in vivo. Thus, this approach points to a strategy to integrate external stimuli into supramolecular hydrogels through a combination of responsive polymers and light-directed supramolecular motifs.

Injectable Polymer-Nanoparticle Hydrogels for Local Immune Cell Recruitment.

The ability to engineer immune function has transformed modern medicine, highlighted by the success of vaccinations and recent efforts in cancer immunotherapy. Further directions in programming the immune system focus on the design of immunomodulatory biomaterials that can recruit, engage with, and program immune cells locally in vivo. Here, we synthesized shear-thinning and self-healing polymer-nanoparticle (PNP) hydrogels as a tunable and injectable biomaterial platform for local dendritic cell (DC) recruitment. PNP gels were formed from two populations of poly(ethylene glycol)-block-polylactide (PEG-b-PLA) NPs with the same diameter but different PEG brush length (2 or 5 kDa). PEG-b-PLA NPs with the longer PEG brush exhibited improved gel formation following self-assembly and faster recovery after shear-thinning. In all cases, model protein therapeutics were released via Fickian diffusion in vitro, and minor differences in the release rate between the gel formulations were observed. PNP hydrogels were loaded with the DC cytokine CCL21 and injected subcutaneously in a murine model. CCL21-loaded PNP hydrogels recruited DCs preferentially to the site of injection in vivo relative to non-CCL21-loaded hydrogels. Thus, PNP hydrogels comprise a simple and tunable platform biomaterial for in vivo immunomodulation following minimally invasive subcutaneous injection.

Supramolecular PEGylation of biopharmaceuticals.

The covalent modification of therapeutic biomolecules has been broadly explored, leading to a number of clinically approved modified protein drugs. These modifications are typically intended to address challenges arising in biopharmaceutical practice by promoting improved stability and shelf life of therapeutic proteins in formulation, or modifying pharmacokinetics in the body. Toward these objectives, covalent modification with poly(ethylene glycol) (PEG) has been a common direction. Here, a platform approach to biopharmaceutical modification is described that relies on noncovalent, supramolecular host-guest interactions to endow proteins with prosthetic functionality. Specifically, a series of cucurbit[7]uril (CB[7])-PEG conjugates are shown to substantially increase the stability of three distinct protein drugs in formulation. Leveraging the known and high-affinity interaction between CB[7] and an N-terminal aromatic residue on one specific protein drug, insulin, further results in altering of its pharmacological properties in vivo by extending activity in a manner dependent on molecular weight of the attached PEG chain. Supramolecular modification of therapeutic proteins affords a noncovalent route to modify its properties, improving protein stability and activity as a formulation excipient. Furthermore, this offers a modular approach to append functionality to biopharmaceuticals by noncovalent modification with other molecules or polymers, for applications in formulation or therapy.

Aromatic identity, electronic substitution, and sequence in amphiphilic tripeptide self-assembly.

The phenomenon of self-assembly in short peptides (2-4 amino acids) has been a source of curiosity, in part for its role in helping to better understand and predict how minimal sequences within proteins might contribute to the formation of larger structures or aggregates. Building on previous work in this field, here we investigate a family of amphiphilic tripeptides for their self-assembly and hydrogel formation. From a parent peptide, Ac-FID-NH2, which was previously shown to self-assemble into high aspect-ratio filaments and form hydrogels, we explored the significance of structural features or sequence variations on the observed self-assembly. This process entailed substituting key aromatic residues, altering the electronics of these aromatic drivers of assembly, and screening tripeptide constitutional isomers. This work more clearly elucidates the mechanisms and design parameters that govern the creation of materials from short peptide building blocks, as well as offering greater insight into the interactions between minimal segments of proteins that underlie their structure and aggregation.

Glucose-responsive insulin activity by covalent modification with aliphatic phenylboronic acid conjugates.

Since its discovery and isolation, exogenous insulin has dramatically changed the outlook for patients with diabetes. However, even when patients strictly follow an insulin regimen, serious complications can result as patients experience both hyperglycemic and hypoglycemic states. Several chemically or genetically modified insulins have been developed that tune the pharmacokinetics of insulin activity for personalized therapy. Here, we demonstrate a strategy for the chemical modification of insulin intended to promote both long-lasting and glucose-responsive activity through the incorporation of an aliphatic domain to facilitate hydrophobic interactions, as well as a phenylboronic acid for glucose sensing. These synthetic insulin derivatives enable rapid reversal of blood glucose in a diabetic mouse model following glucose challenge, with some derivatives responding to repeated glucose challenges over a 13-h period. The best-performing insulin derivative provides glucose control that is superior to native insulin, with responsiveness to glucose challenge improved over a clinically used long-acting insulin derivative. Moreover, continuous glucose monitoring reveals responsiveness matching that of a healthy pancreas. This synthetic approach to insulin modification could afford both long-term and glucose-mediated insulin activity, thereby reducing the number of administrations and improving the fidelity of glycemic control for insulin therapy. The described work is to our knowledge the first demonstration of a glucose-binding modified insulin molecule with glucose-responsive activity verified in vivo.