Improved Conversion Rates in Drug Screening Applications Using Miniaturized Electrochemical Cells with Frit Channels

The Combination of Electrochemistry and Microfluidic Technology in Drug Metabolism Studies

Drugs are metabolized within the liver (pH 7.4) by phase I and phase II metabolism. During the process, reactive metabolites can be formed that react covalently with biomolecules and induce toxicity. Identifying and detecting reactive metabolites is an important part of drug development. Preclinical and clinical investigations are conducted to assess the toxicity and safety of a new drug candidate. Electrochemistry coupled to mass spectrometry is an ideal complementary technique to the current preclinical studies, a pure instrumental approach without any purification steps and tedious protocols. The combination of microfluidics with electrochemistry towards the mimicry of drug metabolism offers portability, low volume of reagents and faster reaction times. This review explores the development of microfluidic electrochemical cells for mimicking drug metabolism.

Microfluidic Electrochemistry Meets Trapped Ion Mobility Spectrometry and High-Resolution Mass Spectrometry-In Situ Generation, Separation, and Detection of Isomeric Conjugates of Paracetamol and Ethoxyquin

Over the last 3 decades, electrochemistry (EC) has been successfully applied in phase I and phase II metabolism simulation studies. The electrochemically generated phase I metabolite-like oxidation products can react with selected reagents to form phase II conjugates. During conjugate formation, the generation of isomeric compounds is possible. Such isomeric conjugates are often separated by high-performance liquid chromatography (HPLC). Here, we demonstrate a powerful approach that combines EC with ion mobility spectrometry to separate possible isomeric conjugates. In detail, we present the hyphenation of a microfluidic electrochemical chip with an integrated mixer coupled online to trapped ion mobility spectrometry (TIMS) and time-of-flight high-resolution mass spectrometry (ToF-HRMS), briefly chipEC-TIMS-ToF-HRMS. This novel method achieves results in several minutes, which is much faster than traditional separation approaches like HPLC, and was applied to the drug paracetamol and the controversial feed preservative ethoxyquin. The analytes were oxidized in situ in the electrochemical microfluidic chip under formation of reactive intermediates and mixed with different thiol-containing reagents to form conjugates. These were analyzed by TIMS-ToF-HRMS to identify possible isomers. It was observed that the oxidation products of both paracetamol and ethoxyquin form two isomeric conjugates, which are characterized by different ion mobilities, with each reagent. Therefore, using this hyphenated technique, it is possible to not only form reactive oxidation products and their conjugates in situ but also separate and detect these isomeric conjugates within only a few minutes.

Label-Free Bioelectrochemical Methods for Evaluation of Anticancer Drug Effects at a Molecular Level

Cancer is a multifactorial family of diseases that is still a leading cause of death worldwide. More than 100 different types of cancer affecting over 60 human organs are known. Chemotherapy plays a central role for treating cancer. The development of new anticancer drugs or new uses for existing drugs is an exciting and increasing research area. This is particularly important since drug resistance and side effects can limit the efficacy of the chemotherapy. Thus, there is a need for multiplexed, cost-effective, rapid, and novel screening methods that can help to elucidate the mechanism of the action of anticancer drugs and the identification of novel drug candidates. This review focuses on different label-free bioelectrochemical approaches, in particular, impedance-based methods, the solid supported membranes technique, and the DNA-based electrochemical sensor, that can be used to evaluate the effects of anticancer drugs on nucleic acids, membrane transporters, and living cells. Some relevant examples of anticancer drug interactions are presented which demonstrate the usefulness of such methods for the characterization of the mechanism of action of anticancer drugs that are targeted against various biomolecules.

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

Reviewing the future of electrochemistry combined with infrared, Raman, and nuclear magnetic resonance spectroscopy as well as mass spectrometry.

Instrumentation and applications of electrochemistry coupled to mass spectrometry for studying xenobiotic metabolism: A review

The knowledge of metabolic pathways and biotransformation of xenobiotics, artificial substances foreign to the entire biological system, is crucial for elucidation of degradation routes of potentially toxic substances. Nowadays, there are many methods to simulate xenobiotic metabolism in the human body in vitro. In this review, the metabolism of various substances in the human body is described, followed by a summary of methods used for prediction of metabolic pathways and biotransformation. Above all, focus is placed on the coupling of electrochemistry to mass spectrometry, which is still a relatively new technique. This promising tool can mimic both oxidative phase I and conjugative phase II metabolism. Different experimental arrangements, with or without a separation step, and various applications of this technique are illustrated and critically reviewed.

Oxidation and adduct formation of xenobiotics in a microfluidic electrochemical cell with boron doped diamond electrodes and an integrated passive gradient rotation mixer

Reactive xenobiotic metabolites and their adduct formation with biomolecules such as proteins are important to study as they can be detrimental to human health. Here, we present a microfluidic electrochemical cell with integrated micromixer to study phase I and phase II metabolism as well as protein adduct formation of xenobiotics in a purely instrumental approach. The newly developed microfluidic device enables both the generation of reactive metabolites through electrochemical oxidation and subsequent adduct formation with biomolecules in a chemical microreactor. This allows us to study the detoxification of reactive species with glutathione and to predict potential toxicity of xenobiotics as a result of protein modification. Efficient mixing in microfluidic systems is a slow process due to the typically laminar flow conditions in shallow channels. Therefore, a passive gradient rotation micromixer has been designed that is capable of mixing liquids efficiently in a 790 pL volume within tens of milliseconds. The mixing principle relies on turning the concentration gradient that is initially established by bringing together two streams of liquid, to take advantage of the short diffusion distances in the shallow microchannels of thin-layer flow cells. The mixer is located immediately downstream of the working electrode of an electrochemical cell with integrated boron doped diamond electrodes. In conjunction with mass spectrometry, the two microreactors integrated in a single device provide a powerful tool to study the metabolism and toxicity of xenobiotics, which was demonstrated by the investigation of the model compound 1-hydroxypyrene.

Electrochemical Protein Cleavage in a Microfluidic Cell with Integrated Boron Doped Diamond Electrodes

Specific electrochemical cleavage of peptide bonds at the C-terminal side of tyrosine and tryptophan generates peptides amenable to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis for protein identification. To this end we developed a microfluidic electrochemical cell of 160 nL volume that combines a cell geometry optimized for a high electrochemical conversion efficiency (>95%) with an integrated boron doped diamond (BDD) working electrode offering a wide potential window in aqueous solution and reduced adsorption of peptides and proteins. Efficient cleavage of the proteins bovine insulin and chicken egg white lysozyme was observed at 4 out of 4 and 7 out of 9 of the predicted cleavage sites, respectively. Chicken egg white lysozyme was identified based on 5 electrochemically generated peptides using a proteomics database searching algorithm. These results show that electrochemical peptide bond cleavage in a microfluidic cell is a novel, fully instrumental approach toward protein analysis and eventually proteomics studies in conjunction with mass spectrometry.

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation-reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.