Bifunctional ligands that target cells displaying the alpha v beta3 integrin

Selective Recruitment of Antibodies to Cancer Cells and Immune Cell-mediated Killing via In Situ Click Chemistry

Many current cancer immunotherapies function by redirecting immune system components to recognize cancer biomarkers and initiate a cytotoxic attack. The lack of a universal tumor biomarker limits the therapeutic potential of these approaches. However, one feature characteristic of nearly all solid tumors is extracellular acidity. This inherent acidity provides the basis for targeted drug delivery via the pH-low insertion peptide (pHLIP), which selectively accumulates in tumors in vivo due to a pH-dependent membrane insertion propensity. Previously, we established that we could selectively decorate cancer cells with antigen-pHLIP conjugates to facilitate antibody recruitment and subsequent killing by engineered effector cells via antibody-dependent cellular cytotoxicity (ADCC). Here, we present a novel strategy for opsonizing antibodies on target cell surfaces using click chemistry. We utilize pHLIP to facilitate selective tetrazine - trans-cyclooctene ligation of human IgGs to the cancer cell surface and induce ADCC. We demonstrate that our approach activates the primary ADCC signaling pathway via CD16a (FcγRIIIa) receptors on effector cells and induces the killing of cancer cell targets by engineered NK cells.

Small molecule- and peptide-drug conjugates addressing integrins: A story of targeted cancer treatment

Targeted cancer treatment should avoid side effects and damage to healthy cells commonly encountered during traditional chemotherapy. By combining small molecule or peptidic ligands as homing devices with cytotoxic drugs connected by a cleavable or non-cleavable linker in peptide-drug conjugates (PDCs) or small molecule-drug conjugates (SMDCs), cancer cells and tumours can be selectively targeted. The development of highly affine, selective peptides and small molecules in recent years has allowed PDCs and SMDCs to increasingly compete with antibody-drug conjugates (ADCs). Integrins represent an excellent target for conjugates because they are overexpressed by most cancer cells and because of the broad knowledge about native binding partners as well as the multitude of small-molecule and peptidic ligands that have been developed over the last 30 years. In particular, integrin αVβ3 has been addressed using a variety of different PDCs and SMDCs over the last two decades, following various strategies. This review summarises and describes integrin-addressing PDCs and SMDCs while highlighting points of great interest.

Assessing Squarates as Amine-Reactive Probes

Probes that covalently label protein targets facilitate the identification of ligand-binding sites. Lysine residues are prevalent in the proteome, making them attractive substrates for covalent probes. However, identifying electrophiles that undergo amine-specific, regioselective reactions with binding site lysine residues is challenging. Squarates can engage in two sequential conjugate addition-elimination reactions with amines. Nitrogen donation reduces the second reaction rate, making the mono squaramide a mild electrophile. We postulated that this mild electrophilicity would demand a longer residence time near the amine, affording higher selectivity for binding site lysines. Therefore, we compared the kinetics of squarate and monosquaramide amine substitution to alternative amine bioconjugation handles. The data revealed that N-hydroxy succinimidyl esters react 4 orders of magnitude faster, consistent with their labeling promiscuity. Squarate reactivity can be tuned by a substitution pattern. Electron-withdrawing groups on the vinylogous ester or amide increase reaction rates. Dithionosquarates react more rapidly than squarates, while vinylogous thioester analogs, dithiosquarates, react more slowly. We assessed squarate selectively using the UDP-sugar processing enzyme GlfT2 from Mycobacterium tuberculosis, which possesses 21 surface-exposed lysines. The reaction predominately modified one lysine proximal to a binding site to afford covalent inhibition. These findings demonstrate the selectivity of squaric esters and squaramides, which is a critical feature for affinity-based chemoproteomic probes.

RGD Peptidomimetic MMAE-Conjugate Addressing Integrin αVβ3-Expressing Cells with High Targeting Index

Small molecule-drug conjugates (SMDCs) mimicking the RGD sequence (-Arg-Gly-Asp-) with a non-peptide moiety require a pharmacophore-independent attachment site. A library of 36 sulfonamide-modified RGD mimetics with nM to pM affinity for integrin α_V β₃ was synthesized and analysed via DAD mapping. The best structure of the conjugable RGD mimetic was used and a linker was attached to an aromatic ring by Negishi cross-coupling. The product retained high affinity and selectivity for integrin α_V β₃ . The conjugable RGD mimetic was then attached to an enzymatically cleavable GKGEVA linker equipped with a self-immolative PABC and the antimitotic drug monomethyl auristatin E (MMAE). The resulting SMDC preferred binding to integrin α_V β₃ over α₅ β₁ in a ratio of 1 : 4519 (ELISA) and showed selectivity for α_V β₃ -positive WM115 cells over α_V β₃ -negative M21-L cells in the in vitro cell adhesion assay as well as in cell viability assays with a targeting index of 134 (M21-L/WM115).

Clearance of plasma PCSK9 via the asialoglycoprotein receptor mediated by heterobifunctional ligands

Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates plasma low-density lipoprotein cholesterol (LDL-C) levels by promoting the degradation of hepatic LDL receptors (LDLRs). Current therapeutic approaches use antibodies that disrupt PCSK9 binding to LDLR to reduce circulating LDL-C concentrations or siRNA that reduces PCSK9 synthesis and thereby levels in circulation. Recent reports describe small molecules that, like therapeutic antibodies, interfere with PCSK9 binding to LDLR. We report an alternative approach to decrease circulating PCSK9 levels by accelerating PCSK9 clearance and degradation using heterobifunctional molecules that simultaneously bind to PCSK9 and the asialoglycoprotein receptor (ASGPR). Various formats, including bispecific antibodies, antibody-small molecule conjugates, and heterobifunctional small molecules, demonstrate binding in vitro and accelerated PCSK9 clearance in vivo. These molecules showcase a new approach to PCSK9 inhibition, targeted plasma protein degradation (TPPD), and demonstrate the feasibility of heterobifunctional small molecule ligands to accelerate the clearance and degradation of pathogenic proteins in circulation.

Antibody Recognition of Cancer Cells via Glycan Surface Engineering

Stimulation of the body's immune system toward tumor cells is now well recognized as a promising strategy in cancer therapy. Just behind cell therapy and monoclonal antibodies, small molecule-based strategies are receiving growing attention as alternatives to direct immune response against tumor cells. However, the development of small-molecule approaches to modulate the balance between stimulatory immune factors and suppressive factors in a targeted way remains a challenge. Here, we report the cell surface functionalization of LS174T cancer cells with an abiotic hapten to recruit antibodies to the cell surface. Metabolic glycoengineering followed by covalent reaction with the hapten results in antibody recognition of the target cells. Microscopy and flow cytometry studies provide compelling evidence that metabolic glycoengineering and small molecule stimulators can be combined to direct antibody recognition.

Synthesis and Evaluation of a Non-Peptide Small-Molecule Drug Conjugate Targeting Integrin αVβ3

An integrin α_Vβ₃-targeting linear RGD mimetic containing a small-molecule drug conjugate (SMDC) was synthesized by combining the antimitotic agent monomethyl auristatin E (MMAE), an enzymatically cleavable Val-Ala-PABC linker with a linear conjugable RGD mimetic. The structure proposal for the conjugable RGD mimetic was suggested upon the DAD mapping analysis of a previously synthesized small-molecule RGD mimetic array based on a tyrosine scaffold. Therefore, a diversifying strategy was developed as well as a novel method for the partial hydrogenation of pyrimidines in the presence of the hydrogenolytically cleavable Cbz group. The small-molecule RGD mimetics were evaluated in an ELISA-like assay, and the structural relationships were analyzed by DAD mapping revealing activity differences induced by structural changes as visualized in dependence on special structural motifs. This provided a lead structure for generation of an SMDC containing the antimitotic drug MMAE. The resulting SMDC containing a linear RGD mimetic was tested in a cell adhesion and an in vitro cell viability assay in comparison to reference SMDCs containing cRGDfK or cRADfK as the homing device. The linear RGD SMDC and the cRGDfK SMDC inhibited adhesion of α_Vβ₃-positive WM115 cells to vitronectin with IC₅₀ values in the low µM range, while no effect was observed for the α_Vβ₃-negative M21-L cell line. The cRADfK SMDC used as a negative control was about 30-fold less active in the cell adhesion assay than the cRGDfK SMDC. Conversely, both the linear RGD SMDC and the cRGDfK SMDC are about 55-fold less cytotoxic than MMAE against the α_Vβ₃-positive WM115 cell line with IC50 values in the nM range, while the cRADfK SMDC is 150-fold less cytotoxic than MMAE. Hence, integrin binding also influences the antiproliferative activity giving a targeting index of 2.8.

Synthetic Antibody-Rhamnose Cluster Conjugates Show Potent Complement-Dependent Cell Killing by Recruiting Natural Antibodies

Monoclonal antibodies (mAbs) are one of the most rapidly growing drug classes used for the treatment of cancer, infectious and autoimmune diseases. Complement-dependent cytotoxicity (CDC) is one of the effector functions for antibodies to deplete target cells. We report here an efficient chemoenzymatic synthesis of structurally well-defined conjugates of a monoclonal antibody with a rhamnose- and an αGal trisaccharide-cluster to recruit natural anti-rhamnose and anti-αGal antibodies, respectively, to enhance the CDC-dependent targeted cell killing. The synthesis was achieved by using a modular antibody Fc-glycan remodeling method that includes site-specific chemoenzymatic Fc-glycan functionalization and subsequent click conjugation of synthetic rhamnose- and αGal trisaccharide-cluster to provide the respective homogeneous antibody conjugates. Cell-based assays indicated that the antibody-rhamnose cluster conjugates could mediate potent CDC activity for targeted cancer cell killing and showed much more potent efficacy than the antibody-αGal trisaccharide cluster conjugates for CDC effects.

Synthesis and biological evaluation of a monocyclic Fc-binding antibody-recruiting molecule for cancer immunotherapy

Antibody-recruiting molecules (ARMs) are bispecific molecules composed of an antibody-binding motif and a target-binding motif that redirect endogenous antibodies to target cells to elicit immune responses. To enhance the translational potential of ARMs, it is crucial to design antibody/target-binding motifs that have strong affinity and are easy to synthesize. Here, we synthesized a novel Fc-binding ARM (Fc-ARM) that targets folate receptor (FR)-positive cancer cells, Reo-3, using a recently developed monocyclic peptide 15-Lys8Leu, which binds strongly to the Fc region of an antibody. Reo-3 bound to the Fc region of the antibody with a K d of 5.8 nM, and recruited a clinically used antibody mixture to attack FR-positive IGROV-1 cells as efficiently as Fc-ARM2, in which a bicyclic Fc-binding peptide was used. These results indicate that 15-Lys8Leu, which can be synthesized readily, is suitable for various applications including the development of Fc-ARMs.

Multivalent Antibody-Recruiting Macromolecules: Linking Increased Binding Affinity with Enhanced Innate Immune Killing

Antibody-recruiting molecules (ARMs) are a novel class of immunotherapeutics. They are capable of introducing antibodies onto disease-relevant targets such as cancer cells, bacterial cells or viruses. This can induce antibody-mediated immune responses such as antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC) and antibody-dependent phagocytosis (ADCP), which can kill the pathogen. In contrast to the classic ARMs, multivalent ARMs could offer the advantage of increasing the efficiency of antibody recruitment and subsequent innate immune killing. Such compounds consist of multiple target-binding termini (TBT) and/or antibody-binding termini (ABT). Those multivalent interactions are able to convert low binding affinities into increased binding avidities. This minireview summarizes the current status of multivalent ARMs and gives insight into possible benefits, hurdles still to be overcome and future perspectives.

Efficient chemoenzymatic synthesis of fluorinated sialyl Thomsen-Friedenreich antigens and investigation of their characteristics

A set of fluorinated sialyl-T derivatives were efficiently synthesized using one-pot multi-enzyme (OPME) chemoenzymatic approach. The P. multocida α2-3-sialyltransferase (PmST1) involved in the synthesis showed extremely flexible donor and acceptor substrate specificities. These sialosides have been successfully investigated with stability towards Clostridium perfringens sialidase substrate specificity assay using ¹H NMR spectroscopy. Hydrolysis studies monitored by ¹H NMR clearly demonstrated that the fluorine substitution obviously reduced hydrolysis rate of Clostridium perfringens sialidase. To further investigate the fluorine influence, structure-dependent variation of sialoside-lectin binding was observed for MAL and different sialoside-immobilized surfaces. Subtle changes on the ligand of carbohydrate-binding protein were distinguished by SPR. These fluorinated sialyl-T derivatives obtained are valuable probes for further biological studies or antitumor drug design.

Unifying principles of bifunctional, proximity-inducing small molecules

Nature uses a variety of tools to mediate the flow of information in cells, many of which control distances between key biomacromolecules. Researchers have thus generated compounds whose activities stem from interactions with two (or more) proteins simultaneously. In this Perspective, we describe how these 'bifunctional' small molecules facilitate the study of an increasingly wide range of complex biological phenomena and enable the drugging of otherwise challenging therapeutic targets and processes. Despite their structural and functional differences, all bifunctional molecules employ Nature's strategy of altering interactomes and inducing proximity to modulate biology. They therefore exhibit a shared set of chemical and biophysical principles that have not yet been appreciated fully. By highlighting these commonalities-and their wide-ranging consequences-we hope to chip away at the artificial barriers that threaten to constrain this interdisciplinary field. Doing so promises to yield remarkable benefits for biological research and therapeutics discovery.

pH-Dependent Grafting of Cancer Cells with Antigenic Epitopes Promotes Selective Antibody-Mediated Cytotoxicity

A growing class of immunotherapeutics work by redirecting components of the immune system to recognize markers on the surface of cancer cells. However, such modalities will remain confined to a relatively small subgroup of patients because of the lack of universal targetable tumor biomarkers among all patients. Here, we designed a unique class of agents that exploit the inherent acidity of solid tumors to selectively graft cancer cells with immuno-engager epitopes. Our targeting approach is based on pHLIP, a unique peptide that selectively targets tumors in vivo by anchoring to cancer cell surfaces in a pH-dependent manner. We established that pHLIP-antigen conjugates trigger the recruitment of antibodies to the surface of cancer cells and induce cytotoxicity by peripheral blood mononuclear and engineered NK cells. These results indicate that these agents have the potential to be applicable to treating a wide range of solid tumors and to circumvent problems associated with narrow windows of selectivity.

Dual-receptor targeted strategy in nanoparticle design achieves tumor cell selectivity through cooperativity

Targeted liposomal nanoparticles are commonly used drug delivery vehicles for targeting cancer cells that overexpress a particular cell surface receptor. However, typical target receptors are also expressed at variable levels in healthy tissue, leading to non-selective targeting and systemic toxicity. Here, we demonstrated that the selectivity of peptide-targeted liposomes for their target cells can be significantly enhanced by employing a dual-receptor targeted approach to simultaneously target multiple tumor cell surface receptors. The dual-receptor targeted approach can be tuned to create cooperativity in binding only for the cancer cells, therefore leaving the healthy cells and tissue unharmed. We evaluated this strategy in a multiple myeloma disease model where the liposomes were functionalized with two distinct peptide antagonists to target VLA-4 and LPAM-1, two receptors with increasing relevance in multiple myeloma. By employing a multifaceted strategy to synthesize dual-receptor targeted liposomes with high purity, reproducibility, and precisely controlled stoichiometry of functionalities, we identified optimal design parameters for enhanced selectivity via systematic analysis. Through control of the liposomal formulation and valency of each targeting peptide, we identified that the optimal dual-receptor targeted liposome consisted of a peptide density of 0.75% VLA4pep and 1% LPAM1pep, resulting in an 8-fold and 12-fold increased cellular uptake over VLA-4 and LPAM-1 single targeted liposomes respectively. This formulation resulted in a cooperative ratio of 4.3 and enhanced uptake for myeloma cells that simultaneously express both VLA-4 and LPAM-1 receptors, but displayed no increase in uptake for cells that express only one or neither of the receptors, resulting in a 28-fold selectivity of the dual-targeted liposomes for cells displaying both targeted receptors over cells displaying neither receptor. These results demonstrated that through refined design and well-characterized nanoparticle formulations, dual-receptor targeted liposomes have the potential to improve cancer therapy by providing enhanced selectivity over conventional single-receptor targeted approaches.

Synthetic Immunotherapeutics against Gram-negative Pathogens

While traditional drug discovery continues to be an important platform for the search of new antibiotics, alternative approaches should also be pursued to complement these efforts. We herein designed a class of molecules that decorate bacterial cell surfaces with the goal of re-engaging components of the immune system toward Escherichia coli and Pseudomonas aeruginosa. More specifically, conjugates were assembled using polymyxin B (an antibiotic that inherently attaches to the surface of Gram-negative pathogens) and antigenic epitopes that recruit antibodies found in human serum. We established that the spacer length played a significant role in hapten display within the bacterial cell surface, a result that was confirmed both experimentally and via molecular dynamics simulations. Most importantly, we demonstrated the specific killing of bacteria by our agent in the presence of human serum. By enlisting the immune system, these agents have the potential to pave the way for a potent antimicrobial modality.

The design and synthesis of an α-Gal trisaccharide epitope that provides a highly specific anti-Gal immune response

Carbohydrate antigens displaying Galα(1,3)Gal epitopes are recognized by naturally occurring antibodies in humans. These anti-Gal antibodies comprise up to 1% of serum IgG and have been viewed as detrimental as they are responsible for hyperacute organ rejections. In order to model this condition, α(1,3)galactosyltransferase-knockout mice are inoculated against the Galα(1,3)Gal epitope. In our study, two α-Gal trisaccharide epitopes composed of either Galα(1,3)Galβ(1,4)GlcNAc or Galα(1,3)Galβ(1,4)Glc linked to a squaric acid ester moiety were examined for their ability to elicit immune responses in KO mice. Both target epitopes were synthesized using a two-component enzymatic system using modified disaccharide substrates containing a linker moiety for coupling. While both glycoconjugate vaccines induced the required high anti-Gal IgG antibody titers, it was found that this response had exquisite specificity for the Galα(1,3)Galβ(1,4)GlcNAc hapten used, with little cross reactivity with the Galα(1,3)Galβ(1,4)Glc hapten. Our findings indicate that while homogenous glycoconjugate vaccines provide high IgG titers, the carrier and adjuvanting factors can deviate the specificity to an antigenic determinant outside the purview of interest.

The Double Four-Component Ugi Reaction between Squaramic or Squaramide Acids with Alkyl Isocyanides, Amines and Aromatic Aldehydes

Novel symmetrical squaramides have been synthesised through the double four-component Ugi reaction between squaramic or squaramide acids with alkyl isocyanides, amines and aromatic aldehydes in methanol under room temperature conditions without using additional reagents or catalysts.

D-amino acid mediated recruitment of endogenous antibodies to bacterial surfaces

The number of antibiotic resistant bacterial strains has been continuously increasing over the last few decades. Nontraditional routes to combat bacteria may offer an attractive alternative to the ongoing problem of drug discovery in this field. Herein, we describe the initial framework toward the development of bacterial d-amino acid antibody recruitment therapy (DART). DART represents a promising antibiotic strategy by exploiting the promiscuity of bacteria to incorporate unnatural d-amino acids and subsequently recruit antibodies to the bacterial surface. The conjugation of 2,4-dinitrophenyl (DNP) to various d-amino acids led to the discovery of a d-amino acid that specifically tags the surface of Bacillus subtilis and Staphylococcus aureus for the recruitment of anti-DNP antibodies (a highly abundant antibody in human serum). This system represents a novel strategy as an antibacterial therapy that targets planktonic Gram-positive bacteria.

Rhamnose glycoconjugates for the recruitment of endogenous anti-carbohydrate antibodies to tumor cells

Immunotherapy is a promising strategy for targeting tumors. One emerging approach is to harness the immune effector functions of natural antibodies to destroy tumor cells. Dinitrophenyl (DNP) and the galactose-α-1,3-galactose (αGal) epitope are two haptens that bind endogenous antibodies. One potential alternative is the deoxysugar L-rhamnose. We compared these candidates by using a biosensor assay to evaluate human sera for endogenous antibody concentration, antibody isotype distribution, and longevity of antibody-hapten interactions. Antibodies recognizing α-rhamnose are of equal or greater abundance and affinity as those recognizing αGal. Moreover, both rhamnose and αGal epitopes are more effective than DNP at recruiting the IgG antibody subtype. Exposure of tumor cells to rhamnose-bearing glycolipids and human serum promotes complement-mediated cytotoxicity. These data highlight the utility of α-rhamnose-containing glycoconjugates to direct the immune system to target cells.

Be squared: expanding the horizon of squaric acid-mediated conjugations

Squaric acid diesters can be applied as reagents to couple two amino-functional compounds. Consecutive coupling of two amines allows the synthesis of asymmetric squaric acid bisamides with either low molecular weight compounds but also biomolecules or polymers. The key feature of the squaric acid diester mediated coupling is the reduced reactivity of the resulting ester-amide after the first amidation step of the diester. This allows the sequential amidation and generation of asymmetric squaramides with high selectivity and in high yields. This article gives an overview of the well-established squaric acid diester mediated coupling reactions for glycoconjugates and presents recent advances that aim to expand this very versatile reaction protocol to the modification of peptides and proteins.

The Formation of Squaramic Acids and Squaryl Diamides by the Reaction of Squaric Acid with Alkyl Isocyanides

The reaction between squaric acid with one equivalent of alkyl isocyanides such as t-butyl isocyanide, cyclohexyl isocyanide and n-butyl isocyanide in ethanol at room temperature after 3h afforded the corresponding squaramic acid derivatives. Symmetric 2,4-squaryl diamides were obtained when the reaction of squaric acid was carried out with two equivalents of alkyl isocyanides under the same conditions. Asymmetric 2,4-squaryl diamides were obtained by reaction of squaramic acids with one equivalent of alkyl isocyanides.

A systematic analysis of peptide linker length and liposomal polyethylene glycol coating on cellular uptake of peptide-targeted liposomes

PEGylated liposomes are attractive pharmaceutical nanocarriers; however, literature reports of ligand-targeted nanoparticles have not consistently shown successful results. Here, we employed a multifaceted synthetic strategy to prepare peptide-targeted liposomal nanoparticles with high purity, reproducibility, and precisely controlled stoichiometry of functionalities to evaluate the role of liposomal PEG coating, peptide EG-linker length, and peptide valency on cellular uptake in a systematic manner. We analyzed these parameters in two distinct disease models where the liposomes were functionalized with either HER2- or VLA-4-antagonistic peptides to target HER2-overexpressing breast cancer cells or VLA-4-overexpressing myeloma cells, respectively. When targeting peptides were tethered to nanoparticles with an EG45 (∼PEG2000) linker in a manner similar to a more traditional formulation, their cellular uptake was not enhanced compared to non-targeted versions regardless of the liposomal PEG coating used. Conversely, reduction of the liposomal PEG to PEG350 and the peptide linker to EG12 dramatically enhanced cellular uptake by ∼9 fold and ∼100 fold in the breast cancer and multiple myeloma cells, respectively. Uptake efficiency reached a maximum and a plateau with ∼2% peptide density in both disease models. Taken together, these results demonstrate the significance of using the right design elements such as the appropriate peptide EG-linker length in coordination with the appropriate liposomal PEG coating and optimal ligand density in efficient cellular uptake of liposomal nanoparticles.

Studies related to Norway spruce galactoglucomannans: chemical synthesis, conformation analysis, NMR spectroscopic characterization, and molecular recognition of model compounds

Galactoglucomannan (GGM) is a polysaccharide mainly consisting of mannose, glucose, and galactose. GGM is the most abundant hemicellulose in the Norway spruce (Picea abies), but is also found in the cell wall of flax seeds, tobacco plants, and kiwifruit. Although several applications for GGM polysaccharides have been developed in pulp and paper manufacturing and the food and medical industries, attempts to synthesize and study distinct fragments of this polysaccharide have not been reported previously. Herein, the synthesis of one of the core trisaccharide units of GGM together with a less-abundant tetrasaccharide fragment is described. In addition, detailed NMR spectroscopic characterization of the model compounds, comparison of the spectral data with natural GGM, investigation of the acetyl-group migration phenomena that takes place in the polysaccharide by using small model compounds, and a binding study between the tetrasaccharide model fragment and a galactose-binding protein (the toxin viscumin) are reported.

Antibody-recruiting molecules: an emerging paradigm for engaging immune function in treating human disease

Synthetic immunology, the development of synthetic systems capable of modulating and/or manipulating immunological functions, represents an emerging field of research with manifold possibilities. One focus of this area has been to create low molecular weight synthetic species, called antibody-recruiting molecules (ARMs), which are capable of enhancing antibody binding to disease-relevant cells or viruses, thus leading to their immune-mediated clearance. This article provides a thorough discussion of contributions in this area, beginning with the history of small-molecule-based technologies for modulating antibody recognition, followed by a systematic review of the various applications of ARM-based strategies. Thus, we describe ARMs capable of targeting cancer, bacteria, and viral pathogens, along with some of the scientific discoveries that have resulted from their development. Research in this area underscores the many exciting possibilities at the interface of organic chemistry and immunobiology and is positioned to advance both basic and clinical science in the years to come.

Rationally engineered nanoparticles target multiple myeloma cells, overcome cell-adhesion-mediated drug resistance, and show enhanced efficacy in vivo

In the continuing search for effective cancer treatments, we report the rational engineering of a multifunctional nanoparticle that combines traditional chemotherapy with cell targeting and anti-adhesion functionalities. Very late antigen-4 (VLA-4) mediated adhesion of multiple myeloma (MM) cells to bone marrow stroma confers MM cells with cell-adhesion-mediated drug resistance (CAM-DR). In our design, we used micellar nanoparticles as dynamic self-assembling scaffolds to present VLA-4-antagonist peptides and doxorubicin (Dox) conjugates, simultaneously, to selectively target MM cells and to overcome CAM-DR. Dox was conjugated to the nanoparticles through an acid-sensitive hydrazone bond. VLA-4-antagonist peptides were conjugated via a multifaceted synthetic procedure for generating precisely controlled number of targeting functionalities. The nanoparticles were efficiently internalized by MM cells and induced cytotoxicity. Mechanistic studies revealed that nanoparticles induced DNA double-strand breaks and apoptosis in MM cells. Importantly, multifunctional nanoparticles overcame CAM-DR, and were more efficacious than Dox when MM cells were cultured on fibronectin-coated plates. Finally, in a MM xenograft model, nanoparticles preferentially homed to MM tumors with ∼10 fold more drug accumulation and demonstrated dramatic tumor growth inhibition with a reduced overall systemic toxicity. Altogether, we demonstrate the disease driven engineering of a nanoparticle-based drug delivery system, enabling the model of an integrative approach in the treatment of MM.

Small-molecule-modified surfaces engage cells through the αvβ3 integrin

Integrins play myriad and vital roles in development and disease. They connect a cell with its surroundings and transmit chemical and mechanical signals across the plasma membrane to the cell's interior. Dissecting their roles in cell behavior is complicated by their overlapping ligand specificity and shared downstream signaling components. In principle, immobilized synthetic peptides can mimic extracellular matrix proteins by supporting integrin-mediated adhesion, but most short peptide sequences lack selectivity for one integrin over others. In contrast, synthetic integrin antagonists can be highly selective. We hypothesized that this selectivity could be exploited if antagonists, when immobilized, could support cellular adhesion and activate signaling by engaging specific cell-surface integrins. To investigate this possibility, we designed a bifunctional (RGD)-based peptidomimetic for surface presentation. Our conjugate combines a high affinity integrin ligand with a biotin moiety; the former engages the α(v)β(3) integrin, and the latter allows for presentation on streptavidin-coated surfaces. Surfaces decorated with this ligand promote both cellular adhesion and integrin activation. Moreover, the selectivity of these surfaces for the α(v)β(3) integrin can be exploited to capture a subset of cells from a mixed population. We anticipate that surfaces displaying highly selective small molecule ligands can reveal the contributions of specific integrin heterodimers to cell adhesion and signaling.

Supramolecular complexing of membane Siglec CD22 mediated by a polyvalent heterobifunctional ligand that templates on IgM

We report the synthesis and in vitro evaluation of a multivalent homing device, a polymer which contains preordered pendant groups with dual specificity, a trisaccharide moiety, which is specific for the siglec CD22, and an antibody specific hapten, nitrophenol. The device efficiently attracts antihapten IgM to the surface of human lymphoma B cells as well as to CD22-conjugated magnetic beads by mediating the formation of a ternary complex on the surface of the target.

Squaramides: physical properties, synthesis and applications

Squaramides are remarkable four-membered ring systems derived from squaric acid that are able to form up to four hydrogen bonds. A high affinity for hydrogen bonding is driven through a concomitant increase in aromaticity of the ring. This hydrogen bonding and aromatic switching, in combination with structural rigidity, have been exploited in many of the applications of squaramides. Substituted squaramides can be accessed via modular synthesis under relatively mild or aqueous conditions, making them ideal units for bioconjugation and supramolecular chemistry. In this tutorial review the fundamental electronic and structural properties of squaramides are explored to rationalise the geometry, conformation, reactivity and biological activity.

N,N'-diarylsquaramides: general, high-yielding synthesis and applications in colorimetric anion sensing

Zinc trifluoromethanesulfonate promotes efficient condensations of anilines with squarate esters, providing access to symmetrical and unsymmetrical squaramides in high yields from readily available starting materials. Efficient access to electron-deficient diaryl squaramides has enabled a systematic investigation of the colorimetric anion-sensing behavior of a p-nitro-substituted squaramide. Its behavior differs in dramatic and unexpected ways from that of structurally similar p-nitroaniline-based ureas, an effect that highlights the remarkable differences in acidity between the squaramide and urea functional groups. Computational studies illustrating the enhanced hydrogen bond donor ability and acidity of squaramides in comparison to ureas are presented.

A remote arene-binding site on prostate specific membrane antigen revealed by antibody-recruiting small molecules

Prostate specific membrane antigen (PSMA) is a membrane-bound glutamate carboxypeptidase overexpressed in many forms of prostate cancer. Our laboratory has recently disclosed a class of small molecules, called ARM-Ps (antibody-recruiting molecule targeting prostate cancer) that are capable of enhancing antibody-mediated immune recognition of prostate cancer cells. Interestingly, during the course of these studies, we found ARM-Ps to exhibit extraordinarily high potencies toward PSMA, compared to previously reported inhibitors. Here, we report in-depth biochemical, crystallographic, and computational investigations which elucidate the origin of the observed affinity enhancement. These studies reveal a previously unreported arene-binding site on PSMA, which we believe participates in an aromatic stacking interaction with ARMs. Although this site is composed of only a few amino acid residues, it drastically enhances small molecule binding affinity. These results provide critical insights into the design of PSMA-targeted small molecules for prostate cancer diagnosis and treatment; more broadly, the presence of similar arene-binding sites throughout the proteome could prove widely enabling in the optimization of small molecule-protein interactions.

RGD-xyloside conjugates prime glycosaminoglycans

Glycosaminoglycans (GAG) play decisive roles in various cardio-vascular & cancer-associated processes. Changes in the expression of GAG fine structures, attributed to deregulation of their biosynthetic and catabolic enzymes, are hallmarks of vascular dysfunction and tumor progression. The wide spread role of GAG chains in blood clotting, wound healing and tumor biology has led to the development of modified GAG chains, GAG binding peptides and GAG based enzyme inhibitors as therapeutic agents. Xylosides, carrying hydrophobic aglycone, are known to induce GAG biosynthesis in various systems. Given the important roles of GAG chains in vascular and tumor biology, we envision that RGD-conjugated xylosides could be targeted to activated endothelial and cancer cells, which are known to express α(v)β(3) integrin, and thereby modulate the pathological processes. To accomplish this vision, xylose residue was conjugated to linear and cyclic RGD containing peptides using click chemistry. Our results demonstrate that RGD-conjugated xylosides are able to prime GAG chains in various cell types, and future studies are aimed toward evaluating potential utility of such xylosides in treating myocardial infarction as well as cancer-associated thrombotic complications.

Profiling human serum antibodies with a carbohydrate antigen microarray

Carbohydrate antigen arrays (glycan arrays) have been recently developed for the high-throughput analysis of carbohydrate macromolecule interactions. When profiling serum, information about experimental variability, interindividual biological variability, and intraindividual temporal variability is critical. In this report, we describe the characterization of a carbohydrate antigen array and assay for profiling human serum. Through optimization of assay conditions and development of a normalization strategy, we obtain highly reproducible results with a within-experiment coefficient of variation (CV) of 10.8% and an overall CV (across multiple batches of slides and days) of 28.5%. We also report antibody profiles for 48 human subjects and evaluate for the first time the effects of age, race, sex, geographic location, and blood type on antibody profiles for a large set of carbohydrate antigens. We found significant dependence on age and blood type of antibody levels for a variety of carbohydrates. Finally, we conducted a longitudinal study with a separate group of 7 serum donors to evaluate the variation in anti-carbohydrate antibody levels within an individual over a period ranging from 3 to 13 weeks and found that, for nearly all antigens on our array, antibody levels are generally stable over this period. The results presented here provide the most comprehensive evaluation of experimental and biological variation reported to date for a glycan array and have significant implications for studies involving human serum profiling.