Self-assembled short peptides: Recent advances and strategies for potential pharmaceutical applications

Self-assembled short peptides have intrigued scientists due to the convenience of synthesis, good biocompatibility, low toxicity, inherent biodegradability and fast response to change in the physiological environment. Therefore, it is necessary to present a comprehensive summary of the recent advances in the last decade regarding the construction, route of administration and application of self-assembled short peptides based on the knowledge on their unique and specific ability of self-assembly. Herein, we firstly explored the molecular mechanisms of self-assembly of short peptides, such as non-modified amino acids, as well as Fmoc-modified, N-functionalized, and C-functionalized peptides. Next, cell penetration, fusion, and peptide targeting in peptide-based drug delivery were characterized. Then, the common administration routes and the potential pharmaceutical applications (drug delivery, antibacterial activity, stabilizers, imaging agents, and applications in bioengineering) of peptide drugs were respectively summarized. Last but not least, some general conclusions and future perspectives in the relevant fields were briefly listed. Although with certain challenges, great opportunities are offered by self-assembled short peptides to the fascinating area of drug development.

Development of artesunate intelligent prodrug liposomes based on mitochondrial targeting strategy

Breast cancer is the leading cause of cancer-related deaths in women and remains a formidable therapeutic challenge. Mitochondria participate in a myriad of essential cellular processes, such as metabolism, and are becoming an ideal target for cancer therapy. Artemisinin and its derivatives have demonstrated multiple activities in the context of various cancers. Mitochondrial autophagy(mitophagy) is one of the important anti-tumor mechanisms of artemisinin drugs. However, the lack of specific tumor targeting ability limits the anti-tumor efficacy of artemisinin drugs. In this study, a GSH-sensitive artesunate smart conjugate (TPP-SS-ATS) was synthesized and liposomes (TPP-SS-ATS-LS) that target tumor cells and mitochondria were further prepared. The advantages of TPP-SS-ATS-LS targeting to the breast tumor were verified by in vivo and in vitro evaluations. In our study, the cytotoxicity was obviously enhanced in vitro and tumor growth inhibition rate was increased from 37.7% to 56.4% at equivalent artesunate dosage in breast cancer orthotopic implanted mice. Meanwhile, mitochondrial dysfunction, suppression of ATP production and respiratory capacity were detected in breast cancer cells. We further discovered that TPP-SS-ATS-LS inhibited tumor cells proliferation through mitophagy by regulating PHB2 and PINK1 expression_. These results provide new research strategies for the development of new artemisinin-based anti-tumor drugs.

Supramolecular Nanomedicines of In-Situ Self-Assembling Peptides

Nanomedicines provide distinct clinical advantages over traditional monomolecular therapeutic and diagnostic agents. Supramolecular nanomedicines made from in-situ self-assembling peptides have emerged as a promising strategy in designing and fabricating nanomedicines. In-situ self-assambly (SA) allows the combination of nanomedicines approach with prodrug approach, which exhibited both advantages of these strategies while addressed the problems of both and thus receiving more and more research attention. In this review, we summarized recently designed supramolecular nanomedicines of in-situ SA peptides in the manner of applications and design principles, and the interaction between the materials and biological environments was also discussed.

Delivery strategies of amphotericin B for invasive fungal infections

Invasive fungal infections (IFIs) represent a growing public concern for clinicians to manage in many medical settings, with substantial associated morbidities and mortalities. Among many current therapeutic options for the treatment of IFIs, amphotericin B (AmB) is the most frequently used drug. AmB is considered as a first-line drug in the clinic that has strong antifungal activity and less resistance. In this review, we summarized the most promising research efforts on nanocarriers for AmB delivery and highlighted their efficacy and safety for treating IFIs. We have also discussed the mechanism of actions of AmB, rationale for treating IFIs, and recent advances in formulating AmB for clinical use. Finally, this review discusses some practical considerations and provides recommendations for future studies in applying AmB for combating IFIs.

Peptide-based supramolecular hydrogels for local drug delivery

Peptide-based supramolecular hydrogels have shown great promise as drug delivery systems (DDSs) because of their excellent biocompatibility, biodegradability, biological function, synthetic feasibility, and responsiveness to external stimuli. Self-assembling peptide molecules are able rationally designed into specific nanoarchitectures in response to the different environmental factors under different circumstances. Among all stimuli that have been investigated, utilizing inherent biological microenvironment, such as metal ions, enzymes and endogenous redox species, to trigger self-assembly endows such systems spatiotemporal controllability to transport therapeutics more accurately. Materials formed by weak non-covalent interactions result in the shear-thinning and immediate recovery behavior. Thus, they are injectable via a syringe or catheter, making them the ideal vehicles to deliver drugs. Based on the above merits, self-assembling peptide-based DDSs have been applied to treat various diseases via direct administration at the lesion site. Herein, in this review, we outline the triggers for inducing peptide-based hydrogels formation and serving as DDSs. We also described the advancements of peptide-based supramolecular hydrogels for local drug delivery, including intratumoral, subcutaneous, ischemia-related tissue (intramyocardial, intrarenal, and ischemic hind limb), and ocular administration. Finally, we give a brief perspective about the prospects and challenges in this field.

(Macro)molecular self-assembly for hydrogel drug delivery

Hydrogels prepared via self-assembly offer scalable and tunable platforms for drug delivery applications. Molecular-scale self-assembly leverages an interplay of attractive and repulsive forces; drugs and other active molecules can be incorporated into such materials by partitioning in hydrophobic domains, affinity-mediated binding, or covalent integration. Peptides have been widely used as building blocks for self-assembly due to facile synthesis, ease of modification with bioactive molecules, and precise molecular-scale control over material properties through tunable interactions. Additional opportunities are manifest in stimuli-responsive self-assembly for more precise drug action. Hydrogels can likewise be fabricated from macromolecular self-assembly, with both synthetic polymers and biopolymers used to prepare materials with controlled mechanical properties and tunable drug release. These include clinical approaches for solubilization and delivery of hydrophobic drugs. To further enhance mechanical properties of hydrogels prepared through self-assembly, recent work has integrated self-assembly motifs with polymeric networks. For example, double-network hydrogels capture the beneficial properties of both self-assembled and covalent networks. The expanding ability to fabricate complex and precise materials, coupled with an improved understanding of biology, will lead to new classes of hydrogels specifically tailored for drug delivery applications.

Advances in engineering of low molecular weight hydrogels for chemotherapeutic applications

Chemotherapy is the primary option for the treatment of cancer, inflammation, and infectious diseases. Conventional drug delivery poses solubility and bioavailability challenges, systemic toxicity, non-specific targeting, and poor accumulation of chemotherapeutic drugs at the desired site. Nanotechnology has led to the development of various nanomaterials that have decreased the toxicity and increased the accumulation of drugs at the target site. Systemic administration of nanomaterials causes burst release and non-specific targeting of chemotherapeutics, leading to off-target organ toxicity. Drug delivery based on low molecular weight hydrogels (LMWHs) provides a suitable alternative for drug delivery due to their ability to entrap chemotherapeutic drugs. Injectable and biodegradable LMWHs allow the administration of chemotherapeutics with minimal invasion, allow the sustained release of chemotherapeutic drugs for long periods, and reduce the challenges of immunogenicity and low drug entrapment efficiency. Herein, we summarize the advances in the engineering of LMWHs for controlled and prolonged delivery of chemotherapeutics for cancer, infectious diseases, and inflammatory disorders.

Advances in self-assembled injectable hydrogels for cancer therapy

Non-specific toxicity of chemotherapeutics and evolution of malignant tumors against them are major challenges for existing cancer chemotherapeutic regimens. Engineering of nanomaterials has attempted to minimize the toxicity of anticancer drugs, but systemic delivery of these nanomaterials still imposes many hurdles in their clinical use like burst release of chemotherapeutics and toxicity and immunogenicity associated with excipients of nanomaterials. However, there has been a surge in the development of natural and synthetic nanomaterials to deliver anticancer agents to the diseased (tumor) site as it can minimize the systemic circulation of anticancer drugs and reduce the toxicity-related challenges. Therefore, localized drug delivery is considered as the most effective way to deliver therapeutics but is further challenged by poor biodegradability, high immunogenicity, poor drug entrapment efficacy and inability to maintain sustained release of anticancer agents at the tumor site. This review maps out recent advancements in engineering of low molecular weight hydrogels derived from amino acid, fatty acyl, steroidal lipid and drug conjugated amphiphilic scaffolds. We have summarized the efforts for the development of molecular hydrogels in terms of biocompatibility, therapeutic potential and challenges associated with existing molecular hydrogels for cancer therapy.

Self-assembling Peptides in Current Nanomedicine: Versatile Nanomaterials for Drug Delivery

BACKGROUND

The development of modern nanomedicine greatly depends on the involvement of novel materials as drug delivery system. In order to maximize the therapeutic effects of drugs and minimize their side effects, a number of natural or synthetic materials have been widely investigated for drug delivery. Among these materials, biomimetic self-assembling peptides (SAPs) have received more attention in recent years. Considering the rapidly growing number of SAPs designed for drug delivery, a summary of how SAPs-based drug delivery systems were designed, would be beneficial.

METHOD

We outlined research works on different SAPs that have been investigated as carriers for different drugs, focusing on the design of SAPs nanomaterials and how they were used for drug delivery in different strategies.

RESULTS

Based on the principle rules of chemical complementarity and structural compatibility, SAPs such as ionic self-complementary peptide, peptide amphiphile and surfactant-like peptide could be designed. Determined by the features of peptide materials and the drugs to be delivered, different strategies such as hydrogel embedding, hydrophobic interaction, electrostatic interaction, covalent conjugation or the combination of them could be employed to fabricate SAPs-drug complex, which could achieve slow release, targeted or environment-responsive delivery of drugs. Furthermore, some SAPs could also be combined with other types of materials for drug delivery, or even act as drug by themselves.

CONCLUSION

Various types of SAPs have been designed and used for drug delivery following various strategies, suggesting that SAPs as a category of versatile nanomaterials have promising potential in the field of nanomedicine.

Novel "Carrier-Free" Nanofiber Codelivery Systems with the Synergistic Antitumor Effect of Paclitaxel and Tetrandrine through the Enhancement of Mitochondrial Apoptosis

Paclitaxel (Ptx), a type of microtubule depolymerization inhibitor, is one of the main components in gastric cancer chemotherapy. Some studies have demonstrated that tetrandrine (Tet), a bisbenzylisoquinoline alkaloid, has potential antitumor effects in several cancers. Aside from the direct anticancer effect, Tet is proved to synergistically enhance the antitumor effect of Ptx in gastric cancer. However, the application of the combinational strategy is limited by the poor solubility of both drugs. Nanodrug delivery systems including polymeric nanoparticles, self-assembled nanofibers, hydrogels, etc., hold the potential to meet the need. Here, a novel supramolecular nanomaterial, based on the concept of "carrier-free nanodrugs", is reported as a feasible platform for synergistic drug delivery. Ptx-SA-RGD is obtained through the conjugation of Ptx and the tumor-specific peptide RGD (arginine-glycine-aspartic acid) with succinic acid (SA) as a linker. Ptx-SA-RGD could self-assemble into Ptx nanofibers (P-NFs) with high drug-loading efficiency. Tet was then encapsulated into P-NFs to acquire novel Ptx and Tet coloaded self-assembled nanofibers (P/T-NFs). The uptake study shows the dynamic internalization of P/T-NFs by the gastric cancer cell line MGC-803. P/T-NFs significantly triggered the accumulation of reactive oxygen species (ROS) in gastric cancer cells MGC803 and further decreased the mitochondrial membrane potential, which led to the induction of mitochondrial apoptosis with superior cytotoxicity against free drugs. Moreover, P/T-NFs suppressed the expressions of p-STAT3 and p-JAK, initiated cytochrome-C release, and promoted caspase protein expression. Furthermore, P/T-NFs demonstrated the strongest tumor-delaying effect as well as the lowest toxicity. Therefore, self-assembled nanofibers of P/T-NFs demonstrated an increase of the mitochondrial apoptosis level and a stronger antitumor effect both in vitro and in vivo, which could be a potential way to enhance the clinical efficacy and reduce the side-effects of Ptx in gastric cancer.

Structure-Activity Relationship of Peptide-Conjugated Chloramphenicol for Inhibiting Escherichia coli

Intravenous administration of a prodrug, chloramphenicol succinate (CLsu), is ineffective. Recently, we have shown that conjugation of diglycine of CLsu (CLsuGG) not only increases the antibiotic efficacy against Escherichia coli but also reduces adverse drug effects against bone marrow stromal cells. Here, we report the synthesis of structural analogues of CLsuGG and their activities against E. coli. These analogues reveal several trends: (i) except the water-insoluble analogues, the attachment of peptides to CLsu enhances the efficacy of the prodrugs; (ii) negative charges, high steric hindrance in the side chains, or a rigid diester decreases the activities of prodrugs in comparison to CLsuGG; (iii) dipeptides apparently increase the efficacy of the prodrugs most effectively; and so forth. This work suggests that conjugating peptides to CLsu effectively modulates the properties of prodrugs. The structure-activity relationship of these new conjugates may provide useful insights for expanding the pool of antibiotics.

Self-assembling peptide-based nanodrug delivery systems

Self-assembling peptide-based nanodrug delivery systems (NDDs), consisting of naturally occurring amino acids, not only share the advantages of traditional nanomedicine but also possess the unique properties of excellent biocompatibility, biodegradability, flexible responsiveness, specific biological function, and synthetic feasibility. Physical methods, enzymatic reaction, chemical reaction, and biosurface induction can yield versatile peptide-based NDDs; flexible responsiveness is their main advantage. Different functional peptides and abundant covalent modifications endow such systems with precise controllability and multifunctionality. Inspired by the above merits, researchers have taken advantage of the self-assembling peptide-based NDDs and achieved the accurate delivery of drugs to the lesion site. The present review outlines the methods for designing self-assembling peptide-based NDDs for small-molecule drugs, with an emphasis on the different drug delivery strategies and their applications in using peptides and peptide conjugates.

Cancer nanomedicine: focus on recent developments and self-assembled peptide nanocarriers

The applications of nanoparticulate drug delivery have received abundant interest in the field of cancer diagnosis and treatment.

Amphotericin B-conjugated polypeptide hydrogels as a novel innovative strategy for fungal infections

The present work is focused on the design and development of novel amphotericin B (AmB)-conjugated biocompatible and biodegradable polypeptide hydrogels to improve the antifungal activity. Using three kinds of promoting self-assembly groups (2-naphthalene acetic acid (Nap), naproxen (Npx) and dexamethasone (Dex)) and polypeptide sequence (Phe-Phe-Asp-Lys-Tyr, FFDKY), we successfully synthesized the Nap-FFDK(AmB)Y gels, Npx-FFDK(AmB)Y gels and Dex-FFDK(AmB)Y gels. The AmB-conjugated hydrogelators are highly soluble in different aqueous solutions. The cryo-transmission electron microscopy and scanning electron microscopy micrographs of hydrogels afford nanofibres with a width of 20-50 nm. Powder X-ray diffraction analyses demonstrate that the crystalline structures of the AmB and Dex are changed into amorphous structures after the formation of hydrogels. Circular dichroism spectra of the solution of blank carriers and the corresponding drug deliveries further help elucidate the molecular arrangement in gel phase, indicating the existence of turn features. The in vitro drug releases suggest that the AmB-conjugated hydrogels are suitable as drug-controlled release vehicles for hydrophobic drugs. The antifungal effect of AmB-conjugated hydrogels significantly exhibits the antifungal activity against Candida albicans. The results of the present study indicated that the AmB-conjugated hydrogels are suitable carriers for poorly water soluble drugs and for enhancement of therapeutic efficacy of antifungal drugs.

Release of small bioactive molecules from physical gels

Pharmaceutical drugs with low water solubility have always received great attention within the scientific community. The reduced bioavailability and the need of frequent administrations have motivated the investigation of new drug delivery systems. Within this context, drug carriers that release their payload in a sustained way and hence reduce the administration rate are highly demanded. One interesting strategy to meet these requirements is the entrapment of the drugs into gels. So far, the most investigated materials for such drug-loaded gels are derived from polymers and based on covalent linkages. However, over the last decade the use of physical (or supramolecular) gels derived from low molecular weight compounds has experienced strong growth in this field, mainly due to important properties such as injectability, stimuli responsiveness and ease of synthesis. This review summarizes the use of supramolecular gels for the encapsulation and controlled release of small therapeutic molecules.

Biodegradable pH-responsive hydrogels for controlled dual-drug release

pH-Responsive biodegradable hydrogels based on NIPAM/AA and a PLLA/PEG macro-crosslinker demonstrated pH mediated differential release of doxorubicin and tetracycline.

Self-assembly hydrogels as multifunctional drug delivery of paclitaxel for synergistic tumour-targeting and biocompatibility in vitro and in vivo

Objectives

In this work, we designed the self-assembly peptide hydrogels to multiply therapeutic agents for improving anticancer effect and lowering adverse reaction of paclitaxel (PTX).

Methods

The folate (FA)-peptide-PTX hydrogels consist of self-assemble peptide hydrogel as nanoscale carrier, FA and RGD peptide as targeting moieties and paclitaxel as anticancer drug. The properties of hydrogels, such as morphology, size distribution, zeta potential and rheology, were investigated. Targeted specificity, biodistribution and anticancer effect were studied both in vitro and in vivo.

Key findings

Folate-peptide-PTX hydrogel nanoparticles were spherical in shape with hydrodynamic diameter of approximately 137.3 ± 15.2 nm. The hydrogels could only target monolayer cancer cells but also penetrated the nuclei of cells in vitro. The in-vivo real-time imaging further demonstrated that the hydrogels preferentially accumulated in tumour and sustained release. Compared to free paclitaxel, the FA-peptide-PTX hydrogels had higher anticancer effect and lower side effect.

Conclusions

The dual-targeted drug delivery possessed strong capability of synergistic targeted delivery, long-term drug release and better biocompatibility than paclitaxel both in vitro and in vivo. The results obtained demonstrated a high potential of the proposed drug delivery system in improving the therapeutic efficacy of paclitaxel.

Superior antitumor effect of extremely high drug loading self-assembled paclitaxel nanofibers

Recent studies focused on the nanodelivery system of paclitaxel (Ptx) to overcome the poor solubility and hypersensitivity of Ptx caused by the application of Cremophor EL as a solvent. Although many studies use different types of polymers as carriers to prepare Ptx-loaded polymeric nanoparticles, the relatively low loading efficiency of Ptx-loaded polymeric nanoparticles significantly limits its application. Here, we design and synthesize a simple conjugation of Ptx and succinic acid (Ptx-SA), which can self-assemble into nanofibers and become "carrier-free" with Ptx as the drug carrier. The highest loading efficiency of Ptx is 89.5% with a controlled release pattern. The cellular uptake study indicates the internalization of Ptx-SA nanofibers by A549 cells. The in vitro cytotoxicity test results indicate that Ptx-SA nanofibers were much more effective in inhibiting the proliferation of A549 cells than free Ptx, particularly at the lower working concentration. The clonogenic assay shows the enhanced effect of Ptx-SA in ameliorating the clonogenic abilities of A549 cells compared with the equivalent dose of free Ptx. Moreover, Ptx-SA significantly attenuates the expression of p-Akt and increases the expression of cleaved PARP and Caspase-3 compared to the equivalent dose of free Ptx, which demonstrates the enhanced apoptosis-inducing effect of Ptx-SA. The animal study demonstrates the superior antitumor effect of Ptx-SA compared to free Ptx. Therefore, the conjugation of Ptx with SA enables the self-assembly of Ptx-loaded nanofibers with stronger in vitro and in vivo antitumor effects, which is a promising method to improve the therapeutic efficacy of Ptx in treating lung cancer.

A Glycyrrhetinic Acid-Modified Curcumin Supramolecular Hydrogel for liver tumor targeting therapy

Curcumin (Cur), a phenolic anti-oxidant compound obtained from Curcuma longa plant, possesses a variety of therapeutic properties. However, it is suffered from its low water solubility and low bioavailability property, which seriously restricts its clinical application. In this study, we developed a glycyrrhetinic acid (GA) modified curcumin supramolecular pro-gelator (GA-Cur) and a control compound Nap-Cur by replacing GA with the naphthylacetic acid (Nap). Both compounds showed good water solubility and could form supramolecular gels by disulfide bond reduction triggered by glutathione (GSH) in vitro. Both formed gels could sustainedly release Cur in buffer solutions. We also investigated the cytotoxicity of pro-gelators to HepG2 cells by a MTT assay and determined the cellular uptake behaviours of them by fluorescence microscopy and LC-MS. Due to the over expression of GA receptor in liver cancer cells, our pro-gelator of GA-Cur showed an enhanced cellular uptake and better inhibition capacity to liver tumor cells than Nap-Cur. Therefore, the GA-Cur could significantly inhibit HepG2 cell growth. Our study provides a novel nanomaterial for liver tumor chemotherapy.

Folate-mediated chemotherapy and diagnostics: An updated review and outlook

Folate receptor (FR) is highly expressed in many types of human cancers, and it has been actively studied for developing targeted chemotherapy and diagnostic agents. Tremendous efforts have been made in developing FR-targeted nanomedicines and nanoprobes and translating them into clinical applications. This article provides a concise review on the latest development of folate-mediated nanomedicines and nanoprobes for chemotherapy and diagnostics with an emphasis on in vivo applications. The cellular uptake mechanisms, pharmacokinetics (PK), administration routes and major challenges in FR-targeted nanoparticles are discussed.

Peptide-drug conjugates as effective prodrug strategies for targeted delivery

Peptide-drug conjugates (PDCs) represent an important class of therapeutic agents that combine one or more drug molecules with a short peptide through a biodegradable linker. This prodrug strategy uniquely and specifically exploits the biological activities and self-assembling potential of small-molecule peptides to improve the treatment efficacy of medicinal compounds. We review here the recent progress in the design and synthesis of peptide-drug conjugates in the context of targeted drug delivery and cancer chemotherapy. We analyze carefully the key design features in choosing the peptide sequence and linker chemistry for the drug of interest, as well as the strategies to optimize the conjugate design. We highlight the recent progress in the design and synthesis of self-assembling peptide-drug amphiphiles to construct supramolecular nanomedicine and nanofiber hydrogels for both systemic and topical delivery of active pharmaceutical ingredients.

Enzymatic induction of supramolecular order and bioactivity

We showed in this study that enzymatic triggering is a totally different pathway for the preparation of self-assembling nanomaterials to the heating-cooling process. Because the molecules were under lower energy levels and the molecular conformation was more ordered during the enzymatic triggeration under mild conditions, nanomaterials with higher supramolecular order could be obtained through biocatalytic control. In this study, nanoparticles were obtained by an enzymatic reaction and nanofibers were observed through the heating-cooling process. We observed a distinct trough at 318 nm from the CD spectrum of a particle sample but not a fiber sample, suggesting the long range arrangement of molecules and helicity in the nanoparticles. The nanoparticles with higher supramolecular order possessed much better potency as a protein vaccine adjuvant because it accelerated the DC maturation and elicited stronger T-cells cytokine production than the nanofibers. Our study demonstrated that biocatalytic triggering is a useful method for preparing supramolecular nanomaterials with higher supramolecular order and probably better bioactivity.

Effects of paclitaxel (PTX) prodrug-based self-assembly peptide hydrogels combined with suberoylanilide hydroxamic acid (SAHA) for PTX-resistant cancer and synergistic antitumor therapy

Schematic illustration of PTX prodrug-based self-assembly peptide hydrogels encapsulated SAHA for drug combination.

AR, Yavvari PS, Srivastava A, Bajaj A. Interactions governing the entrapment of anticancer drugs by low-molecular-weight hydrogelator for drug delivery applications

We present the effect of size, charge, and hydrophobicity of anticancer drugs on their drug encapsulation efficacy in anl-alanine-based small-molecule hydrogelator.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

A supramolecular topical gel derived from a non-steroidal anti-inflammatory drug, fenoprofen, is capable of treating skin inflammation in mice

A new series of bioconjugates derived from a non-steroidal anti-inflammatory drug (NSAID), namely fenoprofen, has been synthesised by amidation with various biogenic molecules such as β-alanine, aminocaproic acid and tyramine with the aim of converting the NSAID into a supramolecular gelator for plausible biomedical applications. One such bioconjugate (2) showed gelation ability with methylsalicylate (MS) and 1% menthol in methyl salicylate (MMS) solvents. These gels were characterized by table top rheology, high resolution-transmission electron microscopy (HR-TEM) and dynamic rheology. Gelator 2 was found to be biostable both in proteolytic enzymes and in blood serum of BALB/c mouse under physiological conditions. It was also found to be biocompatible, as revealed by the methyl thiazolyldiphenyl tetrazolium bromide (MTT) assay in mouse macrophage RAW 264.7 and mouse myoblast C2C12 cells. The anti-inflammatory response (prostaglandin E2 assay, denoted PGE2 assay) of 2 was comparable to that of the parent drug fenoprofen calcium salt. Finally, a topical gel formulation of 2 displayed in vivo self-delivery application in treating imiquimod (IMQ) induced skin inflammation in BALB/c mice.

Self-healing hydrogels containing reversible oxime crosslinks

Self-healing oxime-functional hydrogels have been developed that undergo a reversible gel-to-sol transition via oxime exchange under acidic conditions. Keto-functional copolymers were prepared by conventional radical polymerization of N,N-dimethylacrylamide (DMA) and diacetone acrylamide (DAA). The resulting water soluble copolymers (P(DMA-stat-DAA)) were chemically crosslinked with difunctional alkoxyamines to obtain hydrogels via oxime formation. Gel-to-sol transitions were induced by the addition of excess monofunctional alkoxyamines to promote competitive oxime exchange under acidic conditions at 25 °C. The hydrogel could autonomously heal after it was damaged due to the dynamic nature of the oxime crosslinks. In addition to their chemo-responsive behavior, the P(DMA-stat-DAA) copolymers exhibit cloud points which vary with the DAA content in the copolymers. This thermo-responsive behavior of the P(DMA-stat-DAA) was utilized to form physical hydrogels above their cloud point. Therefore, these materials can either form dynamic-covalent or physically-crosslinked gels, both of which demonstrate reversible gelation behavior.

A supramolecular hydrogelator of curcumin

Here we report on the first supramolecular hydrogelator of curcumin and the evaluation of its inhibition capacity towards cancer cells and tumor growth.

Tools for identifying gelator scaffolds and solvents

Small molecule gelators are serendipitously discovered more often than they are designed. As a consequence, it has been challenging to develop applications based on the limited set of known materials. This synopsis highlights recent strategies to streamline the process of gelator discovery, with a focus on the role of unidirectional intermolecular interactions and solvation. We present these strategies as a series of tools that can be employed to help identify gelator scaffolds and solvents for gel formation. Overall, we suggest that this guided approach is more efficient than random derivatization and screening.

Enzymatic formation of a meta-stable supramolecular hydrogel for 3D cell culture

A meta-stable supramolecular hydrogel triggered by phosphatase allows separation of cells post culture by simply pipetting and centrifugation.

A supramolecular hydrogel based on carbamazepine

In this communication we report the first supramolecular hydrogel based on an antiepileptic drug carbamazepine (CBZ). CBZ plays a dual role of a drug molecule and an aromatic capping group in this self-delivery system.

Fibril aggregates of the poly(glutamic acid)–drug conjugate

Poly(glutamic acid)–doxorubicin conjugates form fibrillar aggregates in the aqueous solution.

A genetically modified protein-based hydrogel for 3D culture of AD293 cells

Hydrogels have strong application prospects for drug delivery, tissue engineering and cell therapy because of their excellent biocompatibility and abundant availability as scaffolds for drugs and cells. In this study, we created hybrid hydrogels based on a genetically modified tax interactive protein-1 (TIP1) by introducing two or four cysteine residues in the primary structure of TIP1. The introduced cysteine residues were crosslinked with a four-armed poly (ethylene glycol) having their arm ends capped with maleimide residues (4-armed-PEG-Mal) to form hydrogels. In one form of the genetically modification, we incorporated a peptide sequence ‘GRGDSP’ to introduce bioactivity to the protein, and the resultant hydrogel could provide an excellent environment for a three dimensional cell culture of AD293 cells. The AD293 cells continued to divide and displayed a polyhedron or spindle-shape during the 3-day culture period. Besides, AD293 cells could be easily separated from the cell-gel constructs for future large-scale culture after being cultured for 3 days and treating hydrogel with trypsinase. This work significantly expands the toolbox of recombinant proteins for hydrogel formation, and we believe that our hydrogel will be of considerable interest to those working in cell therapy and controlled drug delivery.

Linker-determined drug release mechanism of free camptothecin from self-assembling drug amphiphiles

We report here that the release mechanism of free camptothecin from self-assembling drug amphiphiles can be regulated by use of different linker groups. Our results highlight the significance of the linker group of drug amphiphiles on the drug release efficiency and their consequent in vitro efficacy.