Fast Diazaborine Formation of Semicarbazide Enables Facile Labeling of Bacterial Pathogens

Preclinical Evaluation of the Hsp70 Peptide Tracer TPP-PEG24-DFO[89Zr] for Tumor-Specific PET/CT Imaging

High precision in vivo PET/CT imaging of solid tumors improves diagnostic credibility and clinical outcome of patients. An epitope of the oligomerization domain of Hsp70 is exclusively exposed on the membrane of a large variety of tumor types, but not on normal cells, and thus provides a universal tumor-specific target. Here we developed a novel PET tracer TPP-PEG₂₄-DFO[⁸⁹Zr] based on the tumor cell-penetrating peptide probe TPP, which specifically recognizes membrane Hsp70 (mHsp70) on tumor cells. The implemented PEG₂₄ moiety supported tracer stability and improved biodistribution characteristics in vivo The K _d of the tracer ranged in the low nanomolar range (18.9 ± 11.3 nmol/L). Fluorescein isothiocyanate (FITC)-labeled derivatives TPP-[FITC] and TPP-PEG₂₄-[FITC] revealed comparable and specific binding to mHsp70-positive 4T1, 4T1⁺, a derivative of the 4T1 cell line sorted for high Hsp70 expression, and CT26 tumor cells, but not to mHsp70-negative normal fibroblasts. The rapid internalization kinetics of mHsp70 into the cytosol and the favorable biodistribution of the peptide-based tracer TPP-PEG₂₄-DFO[⁸⁹Zr] in vivo enabled a tumor-specific accumulation with a high tumor-to-background contrast and renal body clearance. The tumor-specific enrichment of the tracer in 4T1⁺ (6.2 ± 1.1%ID/g), 4T1 (4.3 ± 0.7%ID/g), and CT26 (2.6 ± 0.6%ID/g) mouse tumors with very high, high, and intermediate mHsp70 densities, respectively, reflected mHsp70 expression profiles of the different tumor types, whereas benign mHsp70-negative fibroblastic hyperplasia showed no tracer accumulation (0.2 ± 0.03%ID/g). The ability of our chemically optimized peptide-based tracer TPP-PEG₂₄-DFO[⁸⁹Zr] to detect mHsp70 in vivo suggests its broad applicability in targeting and imaging with high specificity for any tumor type that exhibits surface expression of Hsp70.Significance: A novel peptide-based PET tracer against the oligomerization domain of Hsp70 has potential for universal tumor-specific imaging in vivo across many tumor type. Cancer Res; 78(21); 6268-81. ©2018 AACR.

Versatile Bioconjugation Chemistries of ortho-Boronyl Aryl Ketones and Aldehydes

Biocompatible and bioorthogonal conjugation reactions have proven to be powerful tools in biological research and medicine. While the advent of bioorthogonal conjugation chemistries greatly expands our capacity to interrogate specific biomolecules in situ, biocompatible reactions that target endogenous reactive groups have given rise to a number of covalent drugs as well as a battery of powerful research tools. Despite the impressive progress, limitations do exist with the current conjugation chemistries. For example, most known bioorthogonal conjugations suffer from slow reaction rates and imperfect bioorthogonality. On the other hand, covalent drugs often display high toxicity due to off-target labeling and immunogenicity. These limitations demand continued pursuit of conjugation chemistries with optimal characteristics for biological applications. A spate of papers appearing in recent literature report the conjugation chemistries of 2-formyl and 2-acetyl phenylboronic acids (abbreviated as 2-FPBA and 2-APBA, respectively). These simple reactants are found to undergo fast conjugation with various nucleophiles under physiological conditions, showing great promise for biological applications. The versatile reactivity of 2-FPBA and 2-APBA manifests in dynamic conjugation with endogenous nucleophiles as well as conjugation with designer nucleophiles in a bioorthogonal manner. 2-FPBA/APBA conjugates with amines in biomolecules, such as lysine side chains and aminophospholipids, in a highly dynamic manner to give iminoboronates. In contrast to typical imines, iminoboronates enjoy much improved thermodynamic stability, yet are kinetically labile for hydrolysis due to imine activation by the boronic acid. Dynamic conjugations as such present a novel binding mechanism analogous to hydrogen bonding and electrostatic interactions. Implementation of this covalent binding mechanism has yielded reversible covalent probes of prevalent bacterial pathogens. It has also resulted in reversible covalent inhibitors of a therapeutically important protein Mcl-1. Such covalent probes/inhibitors with 2-FPBA/APBA warheads avoid permanent modification of their biological target, potentially able to mitigate off-target labeling and immunogenicity of covalent drugs. The dynamic conjugation of 2-FPBA/APBA has been recently extended to N-terminal cysteines, which can be selectively targeted via formation of a thiazolidino boronate (TzB) complex. The dynamic TzB formation expands the toolbox for site-specific protein labeling and the development of covalent drugs. On the front of bioorthogonal conjugation, 2-FPBA/APBA has been found to conjugate with α-nucleophiles under physiologic conditions with rate constant ( k₂) over 1000 M^-1 s^-1, which overcomes the slow kinetics problems and rekindles the interest of using the conjugation of α-nucleophiles for biological studies. With fast kinetics being a shared feature, this family of conjugation chemistries gives remarkably diverse product structures depending on the choice of nucleophile. Importantly, both dynamic and irreversible conjugations have been developed, which we believe will enable a wide array of applications in biological research. In this Account, we collectively examine this rapidly expanding family of conjugation reactions, seeking to elucidate the unifying principles that would guide further development of novel conjugation reactions, as well as their applications in biology.

Plasmon-Triggered Hot-Spot Excitation on SERS Substrates for Bacterial Inactivation and in Situ Monitoring

Bacterial sensing and inactivating is one of the key steps to prevent bacterial propagation and transfer. Here, using Ag nanoparticle-grafted tungsten oxide films (WO₃/Ag), we developed a multifunctional platform that may act as a surface-enhanced Raman spectroscopy substrate for sensitively capturing and counting bacteria. Moreover, we demonstrated that the use of photon-triggered surface plasmon resonance of Ag on the WO₃ surface resulted in a significantly improved photocatalytic activity under visible light (638 nm). The photogenerated reactive oxygen species have been shown to be efficient in the inactivation of bacteria, and the bacteria inactivation process could be monitored in situ by Raman spectroscopy. On the basis of the obtained Raman results and fluorescence measurements of green fluorescence protein expressing bacteria, the active species triggered by hot spots was demonstrated to account for broken cell walls. The bacterial cell contents subsequently leaked out, leading to cell degradation. Potentially, our work may provide a promising strategy for capturing and monitoring the bactericidal process at low concentration and, specifically, may help in the investigation of related inactivation approaches and mechanisms.

Phage Display of Dynamic Covalent Binding Motifs Enables Facile Development of Targeted Antibiotics

Antibiotic resistance of bacterial pathogens poses an increasing threat to the wellbeing of our society and urgently calls for new strategies for infection diagnosis and antibiotic discovery. The antibiotic resistance problem at least partially arises from extensive use of broad-spectrum antibiotics. Ideally, for the treatment of infection, one would like to use a narrow-spectrum antibiotic that specifically targets and kills the disease-causing strain. This is particularly important considering the commensal bacterial species that are beneficial and sometimes even critical to the health of a human being. In this contribution, we describe a phage display platform that enables rapid identification of peptide probes for specific bacterial strains. The phage library described herein incorporates 2-acetylphenylboronic acid moieties to elicit dynamic covalent binding to the bacterial cell surface. Screening of the library against live bacterial cells yields submicromolar and highly specific binders for clinical strains of Staphylococcus aureus and Acinetobacter baumannii that display antibiotic resistance. We further show that the identified peptide probes can be readily converted to bactericidal agents that deliver generic toxins to kill the targeted bacterial strain with high specificity. The phage display platform described here is applicable to a wide array of bacterial strains, paving the way to facile diagnosis and development of strain-specific antibiotics.

Fluorogenic diazaborine formation of semicarbazide with designed coumarin derivatives

Bioorthogonal fluorogenic reactions serve as enabling tools in research and biotechnology. Herein we describe fluorogenic conjugations of semicarbazide with coumarin derivatives that incorporate a 2-acetylphenylboronic acid motif. These designed coumarins rapidly conjugate with semicarbazide to give diazaborine products with significantly enhanced fluorescence. To demonstrate potential applications of this fluorogenic reaction, we synthesized a semicarbazide-presenting amino acid d-Dap-Scz, which readily incorporates into the cell wall of Staphalococcus aureus and serves as a handle for conjugation with the coumarins. The fluorogenic conjugation of the coumarins to cell surface semicarbazide enables facile visualization of d-Dap-Scz treated bacteria.

Dynamic Formation of Imidazolidino Boronate Enables Design of Cysteine-Responsive Peptides

We describe the dynamic and chemoselective conjugation between 2-formylphenylboronic acid and l-2,3-diaminopropionic acid yielding an imidazolidino boronate (IzB) complex. The IzB complex formation readily proceeds in biological milieu with little interference by common biomolecules except cysteine. We demonstrate the potential of this reversible conjugation for biological applications by creating "smart" peptides that specifically respond to cysteine in complex biological media. Specifically, the design and characterization of a fluorogenic sensor of cysteine is described.

Jose G, Shanbhag C, Kalia J. Rapid and reversible hydrazone bioconjugation in cells without the use of extraneous catalysts

Rapid, catalyst-free and reversible bioconjugation in mammalian cells.

Borylated oximes: versatile building blocks for organic synthesis

Herein, we demonstrate the synthesis and functionalization of α-boryl aldoximes from α-boryl aldehydes, with no sign of C-to-N boryl migration. Selective modification of the oxime functionality enables access to a wide range of borylated compounds, such as borylated heterocycles and N-acetoxyamides. By reducing the α-boryl aldoximes, MIDA deprotection yields the corresponding β-boryl hydroxylamines. As part of this study, we also demonstrate the utility of the boryl aldoxime motif in peptide conjugation.

Formation of hydrazones and stabilized boron-nitrogen heterocycles in aqueous solution from carbohydrazides and ortho-formylphenylboronic acids

A recent addition to the suite of fast bioorthogonal reactions combines hydrazines and hydroxylamines with ortho-carbonyl substituted phenylboronic acids. Carbohydrazides are easily incorporated into biomolecules, making them appealing substrates in these reactions. Here we show that simple alkyl carbohydrazides form a single product with ortho-formylphenylboronic acid in an organic solvent and in the solid state. The solution structures of the products formed from the carbohydrazides in buffered aqueous solution, however, are markedly different from those identified in the organic solvent and solid state. The reactants form a mixture of hydrazone and heterocyclic products, the relative composition of which varies with pH. The observed reversibility of bioconjugates using carbohydrazide can thus be explained by the reversibility of the boron-nitrogen bond in the heterocycle. In contrast, the inclusion of an α-amine into the carbohydrazide substrate yields a single product in which both nitrogens are bonded to boron. These tricyclic structures are the same in organic solvent, solid state and aqueous solution from pH 4 to pH 9. Bioconjugates formed with α-amino carbohydrazides are stable to SDS-PAGE, while those formed with simple alkyl carbohydrazides are not. We propose that the inclusion of an intramolecular stabilizing ligand into a carbohydrazide substrate is a generally applicable principle that may be exploited to form boronic acid-based bioconjugates with a defined structure and resistance to hydrolysis.

Oximes and Hydrazones in Bioconjugation: Mechanism and Catalysis

The formation of oximes and hydrazones is employed in numerous scientific fields as a simple and versatile conjugation strategy. This imine-forming reaction is applied in fields as diverse as polymer chemistry, biomaterials and hydrogels, dynamic combinatorial chemistry, organic synthesis, and chemical biology. Here we outline chemical developments in this field, with special focus on the past ∼10 years of developments. Recent strategies for installing reactive carbonyl groups and α-nucleophiles into biomolecules are described. The basic chemical properties of reactants and products in this reaction are then reviewed, with an eye to understanding the reaction's mechanism and how reactant structure controls rates and equilibria in the process. Recent work that has uncovered structural features and new mechanisms for speeding the reaction, sometimes by orders of magnitude, is discussed. We describe recent studies that have identified especially fast reacting aldehyde/ketone substrates and structural effects that lead to rapid-reacting α-nucleophiles as well. Among the most effective new strategies has been the development of substituents near the reactive aldehyde group that either transfer protons at the transition state or trap the initially formed tetrahedral intermediates. In addition, the recent development of efficient nucleophilic catalysts for the reaction is outlined, improving greatly upon aniline, the classical catalyst for imine formation. A number of uses of such second- and third-generation catalysts in bioconjugation and in cellular applications are highlighted. While formation of hydrazone and oxime has been traditionally regarded as being limited by slow rates, developments in the past 5 years have resulted in completely overturning this limitation; indeed, the reaction is now one of the fastest and most versatile reactions available for conjugations of biomolecules and biomaterials.