Integrated Microfluidics for Parallel Screening of an In Situ Click Chemistry Library

Advances of bioorthogonal coupling reactions in drug development

Currently, bioorthogonal coupling reactions have garnered considerable interest due to their high substrate selectivity and less restrictive reaction conditions. During recent decades, bioorthogonal coupling reactions have emerged as powerful tools in drug development. This review describes the current applications of bioorthogonal coupling reactions in compound library building mediated by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction and in situ click chemistry or conjunction with other techniques; druggability optimization with 1,2,3-triazole groups; and intracellular self-assembly platforms with ring tension reactions, which are presented from the viewpoint of drug development. There is a reasonable prospect that bioorthogonal coupling reactions will accelerate the screening of lead compounds, the designing strategies of small molecules and expand the variety of designed compounds, which will be a new trend in drug development in the future.

Azide-alkyne cycloadditions in a vortex fluidic device: enhanced "on water" effects and catalysis in flow

The Vortex Fluidic Device is a flow reactor that processes reactions in thin films. Running the metal-free azide-alkyne cycloaddition in this reactor revealed a dramatic enhancement of the "on water" effect. For the copper-catalyzed azide-alkyne cycloaddition, stainless steel or copper jet feeds were effective reservoirs of active copper catalyst.

Scale-up of microdroplet reactions by heated ultrasonic nebulization

Dramatically higher rates for a variety of chemical reactions have been reported in microdroplets compared with those in the liquid bulk phase. However, the scale-up of microdroplet chemical synthesis has remained a major challenge to the practical application of microdroplet chemistry. Heated ultrasonic nebulization (HUN) was found as a new way for scaling up chemical synthesis in microdroplets. Four reactions were examined, a base-catalyzed Claisen-Schmidt condensation, an oximation reaction from a ketone, a two-phase oxidation reaction without the use of a phase-transfer-catalyst, and an Eschenmoser coupling reaction. These reactions show acceleration of one to three orders of magnitude (122, 23, 6536, and 62) in HUN microdroplets compared to the same reactions in bulk solution. Then, using the present method, the scale-up of the reactions was achieved at an isolated rate of 19 mg min-1 for the product of the Claisen-Schmidt condensation, 21 mg min-1 for the synthesis of benzophenone oxime from benzophenone, 31 mg min-1 for the synthesis of 4-methoxybenzaldehyde from 4-methoxybenzyl alcohol, and 40 mg min-1 for the enaminone product of the Eschenmoser coupling reaction.

A Simple and Efficient Microfluidic System for Reverse Chemical Synthesis (5′-3′) of a Short-Chain Oligonucleotide Without Inert Atmosphere

Reverse DNA synthesis (5′-3′) plays diverse functional roles in cellular biology, biotechnology, and nanotechnology. However, current microfluidic systems for synthesizing single-stranded DNAs at a laboratory scale are limited. In this work, we develop a simple and efficient polydimethylsiloxane- (PDMS-) based microfluidic system for the reverse chemical synthesis of short-chain oligonucleotides (in the 5′-3′ direction) under ambient conditions. The use of a microfluidics device and anhydrous conditions effectively surpass the problem of moisture sensitivity during oligonucleotide synthesis. With optimized microfluidic synthesis conditions, the system is able to synthesize up to 21 bases-long oligonucleotides in air atmosphere. The as-synthesized oligonucleotides, without further purification, are characterized using matrix-assisted laser desorption ionization–time of flight (MALDI-TOF/TOF) mass spectroscopy (MS) supported by the denatured polyacrylamide gel electrophoresis (PAGE) analysis. This developed system is highly promising for producing the desired sequence at the nanomolar scale on-chip and on-demand in the near future.

Using Microfluidics and Imaging SAMDI-MS To Characterize Reaction Kinetics

Microfluidic platforms have enabled the simplification of biochemical assays with a significant reduction in the use of reagents, yet the current methods available for analyzing reaction products can limit applications of these approaches. This paper demonstrates a simple microfluidic device that incorporates a functionalized self-assembled monolayer to measure the rate constant for a chemical reaction. The device mixes the reactants and allows them to selectively immobilize to the monolayer at the base of a microfluidic channel in a time-dependent manner as they flow down the channel. Imaging self-assembled monolayers for matrix-assisted laser desorption/ionization mass spectrometry (iSAMDI-MS) is used to acquire a quantitative image representing the time-resolved progress of the reaction as it flowed through the channel. Knowledge of the surface immobilization chemistry and the fluid front characteristics allows for the determination of the chemical reaction rate constant. This approach widens the applicability of microfluidics for chemical reaction monitoring and establishes a label-free method for studying processes that occur within a dispersive regime.

Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

The striking finding that reaction acceleration occurs in confined-volume solutions sets up an apparent conundrum: Microdroplets formed by spray ionization can be used to monitor the course of bulk-phase reactions and also to accelerate reactions between the reagents in such a reaction. This Minireview introduces droplet and thin-film acceleration phenomena and summarizes recent methods applied to study accelerated reactions in confined-volume, high-surface-area solutions. Conditions that dictate either simple monitoring or acceleration are reconciled in the occurrence of discontinuous and complete desolvation as the endpoint of droplet evolution. The contrasting features of microdroplet and bulk-solution reactions are described together with possible mechanisms that drive reaction acceleration in microdroplets. Current applications of droplet microreactors are noted as is reaction acceleration in confined volumes and possible future scale-up.

Development of microfluidic platforms for the synthesis of metal complexes and evaluation of their DNA affinity using online FRET melting assays

Guanine-rich DNA sequences can fold into quadruple-stranded structures known as G-quadruplexes. These structures have been proposed to play important biological roles and have been identified as potential drug targets. As a result, there is increasing interest in developing small molecules that can bind to G-quadruplexes. So far, these efforts have been mostly limited to conventional batch synthesis. Furthermore, no quick on-line method to assess new G-quadruplex binders has been developed. Herein, we report on two new microfluidic platforms to: (a) readily prepare G-quadruplex binders (based on metal complexes) in flow, quantitatively and without the need for purification before testing; (b) a microfluidic platform (based on FRET melting assays of DNA) that enables the real-time and on-line assessment of G-quadruplex binders in continuous flow.

In situ click chemistry generation of cyclooxygenase-2 inhibitors

Cyclooxygenase-2 isozyme is a promising anti-inflammatory drug target, and overexpression of this enzyme is also associated with several cancers and neurodegenerative diseases. The amino-acid sequence and structural similarity between inducible cyclooxygenase-2 and housekeeping cyclooxygenase-1 isoforms present a significant challenge to design selective cyclooxygenase-2 inhibitors. Herein, we describe the use of the cyclooxygenase-2 active site as a reaction vessel for the in situ generation of its own highly specific inhibitors. Multi-component competitive-binding studies confirmed that the cyclooxygenase-2 isozyme can judiciously select most appropriate chemical building blocks from a pool of chemicals to build its own highly potent inhibitor. Herein, with the use of kinetic target-guided synthesis, also termed as in situ click chemistry, we describe the discovery of two highly potent and selective cyclooxygenase-2 isozyme inhibitors. The in vivo anti-inflammatory activity of these two novel small molecules is significantly higher than that of widely used selective cyclooxygenase-2 inhibitors.Traditional inflammation and pain relief drugs target both cyclooxygenase 1 and 2 (COX-1 and COX-2), causing severe side effects. Here, the authors use in situ click chemistry to develop COX-2 specific inhibitors with high in vivo anti-inflammatory activity.

In situ click chemistry generation of cyclooxygenase-2 inhibitors

Cyclooxygenase-2 isozyme is a promising anti-inflammatory drug target, and overexpression of this enzyme is also associated with several cancers and neurodegenerative diseases. The amino-acid sequence and structural similarity between inducible cyclooxygenase-2 and housekeeping cyclooxygenase-1 isoforms present a significant challenge to design selective cyclooxygenase-2 inhibitors. Herein, we describe the use of the cyclooxygenase-2 active site as a reaction vessel for the in situ generation of its own highly specific inhibitors. Multi-component competitive-binding studies confirmed that the cyclooxygenase-2 isozyme can judiciously select most appropriate chemical building blocks from a pool of chemicals to build its own highly potent inhibitor. Herein, with the use of kinetic target-guided synthesis, also termed as in situ click chemistry, we describe the discovery of two highly potent and selective cyclooxygenase-2 isozyme inhibitors. The in vivo anti-inflammatory activity of these two novel small molecules is significantly higher than that of widely used selective cyclooxygenase-2 inhibitors.

Traditional inflammation and pain relief drugs target both cyclooxygenase 1 and 2 (COX-1 and COX-2), causing severe side effects. Here, the authors use in situ click chemistry to develop COX-2 specific inhibitors with high in vivo anti-inflammatory activity.

Mapping of Enzyme Kinetics on a Microfluidic Device

A microfluidic platform or “microfluidic mapper” is demonstrated, which in a single experiment performs 36 parallel biochemical reactions with 36 different combinations of two reagents in stepwise concentration gradients. The volume used in each individual reaction was 36 nl. With the microfluidic mapper, we obtained a 3D enzyme reaction plot of horseradish peroxidase (HRP) with Amplex Red (AR) and hydrogen peroxide (H₂O₂), for concentration ranges of 11.7 μM to 100.0 μM and 11.1 μM to 66.7 μM for AR and H₂O₂, respectively. This system and methodology could be used as a fast analytical tool to evaluate various chemical and biochemical reactions especially where two or more reagents interact with each other. The generation of dual concentration gradients in the present format has many advantages such as parallelization of reactions in a nanoliter-scale volume and the real-time monitoring of processes leading to quick concentration gradients. The microfluidic mapper could be applied to various problems in analytical chemistry such as revealing of binding kinetics, and optimization of reaction kinetics.

A High-Throughput Platform for Formulating and Screening Multifunctional Nanoparticles Capable of Simultaneous Delivery of Genes and Transcription Factors

Simultaneous delivery of multiple genes and proteins (e.g., transcription factors; TFs) is an emerging issue surrounding therapeutic research due to their ability to regulate cellular circuitry. Current gene and protein delivery strategies, however, are based on slow batch synthesis, which is ineffective, poorly controlled, and incapable of simultaneous delivery of both genes and proteins with synergistic functions. Consequently, advances in this field have been limited to in vitro studies. Here, by integrating microfluidic technologies with a supramolecular synthetic strategy, we present a high-throughput approach for formulating and screening multifunctional supramolecular nanoparticles (MFSNPs) self-assembled from a collection of functional modules to achieve simultaneous delivery of one gene and TF with unprecedented efficiency both in vitro and in vivo. We envision that this new approach could open a new avenue for immunotherapy, stem cell reprogramming, and other therapeutic applications.

Poly(hydroxamic acid) functionalized copper catalyzed C–N bond formation reactions

Highly active khaya cellulose supported poly(hydroxamic acid) functionalized copper catalysts were synthesized and applied for the C–N bond formation reactions.

New organometallic Schiff-base copper complexes as efficient “click” reaction precatalysts

In the presence of sodium ascorbate, recyclable CuAAC precatalysts displayed high activity allowing the synthesis of a wide variety of 1,4-disubstituted 1,2,3-triazoles in high isolated yields.

A microfluidic live cell assay to study anthrax toxin induced cell lethality assisted by conditioned medium

It is technically challenging to investigate the function of secreted protein in real time by supply of conditioned medium that contains secreted protein of interest. The internalization of anthrax toxin is facilitated by a secreted protein Dickkopf-1 (DKK1) and its receptor, and eventually leads to cell lethality. To monitor the dynamic interplay between these components in live cells, we use an integrated microfluidic device to perform the cell viability assays with real-time controlled culture microenvironment in parallel. Conditioned medium, which contains the secreted proteins from specific cell lines, can be continuously pumped towards the cells that exposed to toxin. The exogenous DKK1 secreted from distant cells is able to rescue the sensitivity to toxin for those DKK1-knocked-down cells. This high-throughput assay allows us to precisely quantify the dynamic interaction between key components that cause cell death, and provide independent evidence of the function of DKK1 in the complex process of anthrax toxin internalization.

Pressure-accelerated azide-alkyne cycloaddition: micro capillary versus autoclave reactor performance

Pressure effects on regioselectivity and yield of cycloaddition reactions have been shown to exist. Nevertheless, high pressure synthetic applications with subsequent benefits in the production of natural products are limited by the general availability of the equipment. In addition, the virtues and limitations of microflow equipment under standard conditions are well established. Herein, we apply novel-process-window (NPWs) principles, such as intensification of intrinsic kinetics of a reaction using high temperature, pressure, and concentration, on azide-alkyne cycloaddition towards synthesis of Rufinamide precursor. We applied three main activation methods (i.e., uncatalyzed batch, uncatalyzed flow, and catalyzed flow) on uncatalyzed and catalyzed azide-alkyne cycloaddition. We compare the performance of two reactors, a specialized autoclave batch reactor for high-pressure operation up to 1800 bar and a capillary flow reactor (up to 400 bar). A differentiated and comprehensive picture is given for the two reactors and the three methods of activation. Reaction speedup and consequent increases in space-time yields is achieved, while the process window for favorable operation to selectively produce Rufinamide precursor in good yields is widened. The best conditions thus determined are applied to several azide-alkyne cycloadditions to widen the scope of the presented methodology.

Deformability and size-based cancer cell separation using an integrated microfluidic device

We present an integrated microfluidic device for cell separation based on the cell size and deformability by combining the microstructure-constricted filtration and pneumatic microvalves.

High throughput and multiplex localization of proteins and cells for in situ micropatterning using pneumatic microfluidics

We present a micropatterning method for protein/cell localization by using pneumatically controllable microstructures in an integrated microfluidic device.

Highly efficient click reaction on water catalyzed by a ruthenium complex

Reactivity of ruthenium-catalyzed click reaction has been enhanced greatly by using H₂O as the solvent.

Flow chemistry as a versatile tool for the synthesis of triazoles

This review surveys the continuous-flow strategies for the synthesis of triazoles by means of copper-catalyzed and catalyst-free cycloadditions between azides and various dipolarophiles.

Regioselective synthesis of triazoles via base-promoted oxidative cycloaddition of chalcones with azides in aqueous solution

A base-promoted oxidative cycloaddition of chalcones with azides was developed for the regioselective synthesis of trisubstituted triazoles under transition-metal-free conditions.

Digital optofluidics: LED-gated transport and fusion of microliter-sized organic droplets for chemical synthesis

Microdroplet-based organic syntheses have been developed as a valuable alternative to traditional bulk-based methods. However, unlike their water counterparts, organic microdroplets can prove challenging to manipulate. Here, we describe the first optical manipulation of discrete, nanoliter- to microliter-sized apolar droplets floating on a liquid surface to induce on-demand droplet fusion for organic synthesis. We demonstrate droplet transport on centimeter-scale distances at speeds of 0.1 to 1 mm·s(-1) with well-programmable, sequential or parallel, fusion events. Because our strategy is compatible with most usual hydrocarbon solvents, such droplets can be used as microcompartments for reagents. Organic reactions readily occur upon droplet fusion, as demonstrated with an ene reaction. With an LED as the sole power source, and without any fabrication step, optical setup, pump or electrode implementation, our method provides a robust and versatile way to place digital organic chemistry under optical control.

Industrial lab-on-a-chip: design, applications and scale-up for drug discovery and delivery

Microfluidics is an emerging and promising interdisciplinary technology which offers powerful platforms for precise production of novel functional materials (e.g., emulsion droplets, microcapsules, and nanoparticles as drug delivery vehicles- and drug molecules) as well as high-throughput analyses (e.g., bioassays, detection, and diagnostics). In particular, multiphase microfluidics is a rapidly growing technology and has beneficial applications in various fields including biomedicals, chemicals, and foods. In this review, we first describe the fundamentals and latest developments in multiphase microfluidics for producing biocompatible materials that are precisely controlled in size, shape, internal morphology and composition. We next describe some microfluidic applications that synthesize drug molecules, handle biological substances and biological units, and imitate biological organs. We also highlight and discuss design, applications and scale up of droplet- and flow-based microfluidic devices used for drug discovery and delivery.

An automated microfluidic multiplexer for fast delivery of C. elegans populations from multiwells

Automated biosorter platforms, including recently developed microfluidic devices, enable and accelerate high-throughput and/or high-resolution bioassays on small animal models. However, time-consuming delivery of different organism populations to these systems introduces a major bottleneck to executing large-scale screens. Current population delivery strategies rely on suction from conventional well plates through tubing periodically exposed to air, leading to certain disadvantages: 1) bubble introduction to the sample, interfering with analysis in the downstream system, 2) substantial time drain from added bubble-cleaning steps, and 3) the need for complex mechanical systems to manipulate well plate position. To address these concerns, we developed a multiwell-format microfluidic platform that can deliver multiple distinct animal populations from on-chip wells using multiplexed valve control. This Population Delivery Chip could operate autonomously as part of a relatively simple setup that did not require any of the major mechanical moving parts typical of plate-handling systems to address a given well. We demonstrated automatic serial delivery of 16 distinct C. elegans worm populations to a single outlet without introducing any bubbles to the samples, causing cross-contamination, or damaging the animals. The device achieved delivery of more than 90% of the population preloaded into a given well in 4.7 seconds; an order of magnitude faster than delivery modalities in current use. This platform could potentially handle other similarly sized model organisms, such as zebrafish and drosophila larvae or cellular micro-colonies. The device’s architecture and microchannel dimensions allow simple expansion for processing larger numbers of populations.

Observation of the controlled assembly of preclick components in the in situ click chemistry generation of a chitinase inhibitor

The Huisgen cycloaddition of azides and alkynes, accelerated by target biomolecules, termed "in situ click chemistry," has been successfully exploited to discover highly potent enzyme inhibitors. We have previously reported a specific Serratia marcescens chitinase B (SmChiB)-templated syn-triazole inhibitor generated in situ from an azide-bearing inhibitor and an alkyne fragment. Several in situ click chemistry studies have been reported. Although some mechanistic evidence has been obtained, such as X-ray analysis of [protein]-["click ligand"] complexes, indicating that proteins act as both mold and template between unique pairs of azide and alkyne fragments, to date, observations have been based solely on "postclick" structural information. Here, we describe crystal structures of SmChiB complexed with an azide ligand and an O-allyl oxime fragment as a mimic of a click partner, revealing a mechanism for accelerating syn-triazole formation, which allows generation of its own distinct inhibitor. We have also performed density functional theory calculations based on the X-ray structure to explore the acceleration of the Huisgen cycloaddition by SmChiB. The density functional theory calculations reasonably support that SmChiB plays a role by the cage effect during the pretranslation and posttranslation states of selective syn-triazole click formation.

In situ click chemistry: a powerful means for lead discovery

Combinatorial chemistry and parallel synthesis are important and regularly applied tools for lead identification and optimisation, although they are often accompanied by challenges related to the efficiency of library synthesis and the purity of the compound library. In the last decade, novel means of lead discovery approaches have been investigated where the biological target is actively involved in the synthesis of its own inhibitory compound. These fragment-based approaches, also termed target-guided synthesis (TGS), show great promise in lead discovery applications by combining the synthesis and screening of libraries of low molecular weight compounds in a single step. Of all the TGS methods, the kinetically controlled variant is the least well known, but it has the potential to emerge as a reliable lead discovery method. The kinetically controlled TGS approach, termed in situ click chemistry, is discussed in this article.

Characterization of drug permeability in Caco-2 monolayers by mass spectrometry on a membrane-based microfluidic device

In this study, an integrated microfluidic device was developed for drug permeability assays with real-time online detection by a directly coupled mass spectrometer. The integrated microfluidic device contained two independent channels sandwiched by a semipermeable polycarbonate membrane for cell culture, and micro solid-phase extraction (SPE) columns for sample clean-up and concentration prior to mass spectrometry detection. Curcumin, a model drug, was delivered to an upper or bottom channel by a pressure-driven flow to mimic dynamic in vivo conditions, and it was forced to permeate into the other side channel. The concentration of curcumin permeated with time was directly detected by an electrospray ionization quadrupole time-of-flight mass spectrometer (ESI-Q-TOF MS) with high sensitivity after micro-SPE pretreatment. The total analysis time only needed about 30 min, and only 6 μL of the drug solution was required for each permeation experiment. The measured permeability of curcumin was consistent with the literature reported value. In addition, this technique offers the potential for parallelization and increasing throughput compared to conventional methods. Thus, the established platform provides a useful tool for drug permeability studies, which is crucial for drug discovery and development.

Probing the binding site of abl tyrosine kinase using in situ click chemistry

Modern combinatorial chemistry is used to discover compounds with desired function by an alternative strategy, in which the biological target is directly involved in the choice of ligands assembled from a pool of smaller fragments. Herein, we present the first experimental result where the use of in situ click chemistry has been successfully applied to probe the ligand-binding site of Abl and the ability of this enzyme to form its inhibitor. Docking studies show that Abl is able to allow the in situ click chemistry between specific azide and alkyne fragments by binding to Abl-active sites. This report allows medicinal chemists to use protein-directed in situ click chemistry for exploring the conformational space of a ligand-binding pocket and the ability of the protein to guide its inhibitor. This approach can be a novel, valuable tool to guide drug design synthesis in the field of tyrosine kinases.

Chip in a lab: Microfluidics for next generation life science research

Microfluidic circuits are characterized by fluidic channels and chambers with a linear dimension on the order of tens to hundreds of micrometers. Components of this size enable lab-on-a-chip technology that has much promise, for example, in the development of point-of-care diagnostics. Micro-scale fluidic circuits also yield practical, physical, and technological advantages for studying biological systems, enhancing the ability of researchers to make more precise quantitative measurements. Microfluidic technology has thus become a powerful tool in the life science research laboratory over the past decade. Here we focus on chip-in-a-lab applications of microfluidics and survey some examples of how small fluidic components have provided researchers with new tools for life science research.